Sharp JX-9400 Technical information

Ventilation and Airflow in Buildings
BUILDINGS | ENERGY | SOLAR TECHNOLOGY
Ventilation and Airflow in Buildings
Methods for Diagnosis and Evaluation
Claude-Alain Roulet
London . Sterling, VA
First published by Earthscan in the UK and USA in 2008
Copyright # Claude-Alain Roulet, 2008
All rights reserved
ISBN-13:
978-1-84407-451-8
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A catalogue record for this book is available from the British Library
Library of Congress Cataloging-in-Publication Data
Roulet, Claude-Alain.
Ventilation and airflow in buildings : methods for diagnosis and evaluation / ClaudeAlain Roulet.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-1-84407-451-8 (hardback)
ISBN-10: 1-84407-451-X (hardback)
1. Ventilation–Handbooks, manuals, etc. 2. Air flow–Measurement–Handbooks,
manuals, etc. I. Title.
TH7656.R68 2008
697.90 2–dc22
2007034790
The paper used for this book is FSC-certified and totally
chlorine-free. FSC (the Forest Stewardship Council) is
an international network to promote responsible management
of the world’s forests.
Contents
List of Figures and Tables
Preamble
Introduction
vii
xii
xiii
1
Airflow Rates in Buildings
Single-zone measurements
Application to buildings, multi-zone
Further interpretation of the flow matrix
Equations for volume flow rates
Summary of the various tracer gas methods
1
1
6
9
11
12
2
Airflow Rates in Air Handling Units
Measurement of the airflow rate in a duct
Airflow measurements at ventilation grilles
Airflow rate measurements in air handling units
Principle of the interpretation procedure
Node by node method
General method for ‘black box’ air handling unit
Planning tool
Example of application
Simple measurement using CO2 from occupants
Measurements in buildings with large time constants
Appropriate method for assessing the recirculation ratio
15
15
19
20
23
23
24
33
33
34
35
36
3
Age of Air and Ventilation Efficiency
Definitions
Measurement method
Practical interpretation of the concentration records
Error analysis
Example of application
Mapping the age of the air in rooms
39
39
42
45
46
47
49
4
Airtightness
Why check airtightness?
58
58
vi
Ventilation and Airflow in Buildings
Measurement methods
Determining the leakage coefficients
Corrections for standard conditions
Ways of expressing the airtightness
Airtightness of buildings
Measurement of airtightness of a duct or network
5
59
63
65
66
67
74
Measurements and Measures Related to Energy
Efficiency in Ventilation
Energy in buildings
Energy in air handling units
Heat exchangers
Energy for ventilation
Energy effects of indoor air quality measures
77
77
79
83
97
102
6
Contaminants in Air Handling Units
Filters
Ducts
Humidifiers
Rotating heat exchangers
Coils
Measurement protocols
Strategies to improve the performance of HVAC systems
108
108
109
110
111
113
113
125
7
Common Methods and Techniques
Expressing concentrations and flow rates
Tracer gas dilution techniques
Identification methods
Error analysis
Notes
132
132
134
147
153
165
References
166
Annexes
A
Unit Conversion Tables
B
Glossary
171
174
Index
187
List of Figures and Tables
Figures
0.1
0.2
0.3
0.4
0.5
0.6
1.1
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
Design and measured outdoor airflow rate per person in
12 buildings
xv
Relative difference between measured and design outdoor airflow
rate in 37 air handling units
xv
Comparison of design and measured recirculation rate in 34 air
handling units
xvi
Histogram and cumulated frequencies of measured recirculation
rates in air handling units designed without recirculation
xvi
Design and measured exfiltration ratios compared in 30 units
xvii
Ventilation efficiency in some ventilated areas
xvii
6
Records of CO2 concentration in an office room
Schematics of a supply and exhaust air handling unit
16
Location of the measurement points in circular and rectangular
ducts
18
Measuring the airflow rate in a duct with the tracer gas dilution
method
19
Schematics of a compensated flowmeter
20
Locations of tracer gas injection (arrows), and sampling points
for concentration measurements (Ci ) in a typical supply and
exhaust air handling unit.
21
Evolution of tracer gas concentration versus time
22
Example of multiple injection devices
23
The simplified network representing the air handling unit and
ducts
25
Measured airflow rates in a leaky air handling unit
33
Concentrations at locations shown in Figure 2.5 resulting from
injection of SF6 as tracer 1 and N2 O as tracer 2 in a leaky air
handling unit
34
Tracer gas concentrations in the supply duct, upstream (3) and
36
downstream (30 ) of the tracer gas injection port
Confidence interval of the recirculation ratio as a function of the
recirculation ratio itself for three assessment methods
38
viii
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
4.1
4.2
4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
Ventilation and Airflow in Buildings
Ventilation modes with typical airflow patterns and air
change efficiencies
Typical probability density curves for the age of the air
Typical probability curves for the age of the air
Record of tracer gas concentration in the exhaust duct during
the measurement of the age of air
Probability functions of the age of air, calculated from the
recorded concentration illustrated in Figure 3.4
Arrangement of the conference room and of its surroundings
Arrangement of the conference room after improvement
Room ventilation characteristics before and after improvement
Minimum design for a 2-D quadratic model
Experimental designs C3 (left) and composite centred (right)
Map of the age of the air at head level
Measured part of outdoor air that is not supplied by the system in
mechanically ventilated buildings, shown with uncertainty band
Airflow rates and pressure differences as measured in a real test,
together with power law and quadratic fits
Principle of the guarded zone technique applied to several
walls of a room
Logarithmic plot of airflow rates and pressure differences
Schematic of building airtightness test
Roof corner from inside
Roof corner under depressurization
Principle of the neutral height method for assessing leakage area
Location of tracer injection and sampling tubes for the
measurement of leakage airflow rates in a ventilation system
Two measurements providing, by difference, the duct leakage to
outside of the conditioned space
Psychrometric chart with constant relative humidity curves and
constant enthalpy lines
Paths in the psychrometric chart for heating and humidifying
outdoor air in winter to reach 208C and 50 per cent relative
humidity
Paths in the psychrometric chart for heating outdoor air in
winter or cooling it in summer to reach 208C and 50 per cent
relative humidity
Close view of a flat plate heat exchanger
Top half of a rotating heat exchanger
Relative position of fans and rotating heat exchangers
Schematics of an air handling unit, showing location of pressure
taps for pressure differential measurements
The simplified network representing the air handling unit and
ducts
Relative decrease of global heat recovery efficiency as a function
of exfiltration ratio exf and internal recirculation rate Rxs
41
41
42
44
44
48
49
49
55
56
57
59
61
63
65
68
69
70
73
75
76
80
81
82
84
85
86
88
89
93
List of Figures and Tables
5.10
5.11
5.12
5.13
5.14
5.15
5.16
5.17
5.18
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
6.15
6.16
6.17
7.1
7.2
7.3
Global heat recovery efficiency versus nominal heat exchanger
effectiveness measured in several units
Seasonal average coefficient of performance and specific net
energy saving of the tested units
Approximate figures for the efficiencies of various elements
needed to move the air in the ductwork
Installation of the differential manometer to measure the
pressure differential across the fan
Schematics of electric power measurement on a three-phase
motor
Front panel of the variable frequency controller
Fan efficiencies as a function of actual fan motor power
Air temperature increase as a function of pressure differential
across the fan
The HVAC system in the simulated building
Olfactive pollution of various new filters as a function of airflow
rate
Correlation between odour intensity and the mass of oil residues
in the tested ducts
Perceived air quality for the steam humidifier
Bacteria concentration at inner surface of a humidifier correlated
with the odour intensity
Some extract air is entrained in the supply airflow by the rotation
of the wheel
Schematics of the purging sector
Average VOC recirculation rates measured in the EPFL
laboratory unit, with and without a purging sector
The PAP meter
Recommended locations of small bottles in PAP meter
Schematics of an air handling unit showing location of VOC
injection and sampling points, Ci , for concentration analysis
Flash evaporation device for injecting the VOCs
EPFL wheel structure
SEM image of a new hygroscopic coating
Average VOC recirculation rates measured in the EPFL
auditorium (leaky) unit, with and without purging sector
Average VOC recirculation rates measured in the EPFL
laboratory unit, with and without purging sector
Transfer ratio as a function of the boiling point for three families
Recirculation rates for each chemical compound measured in
EPFL and EMPA units, in both cases without a purging
sector
Two strategies for injection and sampling
Significance limits and confidence interval
Normal (or Gaussian) distribution (left) and its probabillity
function (right)
ix
96
96
98
100
100
101
102
102
105
109
109
110
111
111
112
112
114
115
118
119
121
121
124
124
125
125
142
155
157
x
7.4
7.5
Ventilation and Airflow in Buildings
Student distribution for 1, 2 and 5 degrees of freedom compared
to the normal distribution
Confidence limit divided by standard deviation versus number
of measurements for various values of probability, P
158
159
Tables
1.1
1.2
1.3
2.1
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
5.1
5.2
5.3
5.4
6.1
6.2
6.3
6.4
6.5
Summary of different injection strategies
12
Summary of single-zone methods
13
Summary of multi-zone methods
13
Possible airflow rates in the network represented in Figure 2.8
25
Nominal time constant and room mean age of the air
corresponding to the probability curves shown in Figures 3.2
and 3.3
41
Minimum number of measurements needed to obtain the
coefficients of a kth degree polynomial empirical model
representing a variable in a two- and three-dimensional space
51
2-D, two-level full factorial design
54
2-D design changing one variable at a time
54
2-D full factorial design with three levels
55
Minimum 3-D design for assessing the coefficients of a linear
model
55
Full factorial design for assessing the coefficients of a linear
model with interactions
56
3-D centred star design
56
Condition number of MT M for some experimental designs and
three models
57
Humidity ratio and specific enthalpy of warm, humid air cooled
down and dried as shown in Figure 5.3
82
Measured airflow rates with experimental uncertainty band
(when available), total and specific fan power in audited units
95
Outdoor air efficiency, o , exfiltration and infiltration ratios exf
and inf , external and internal recirculation rates Re , Rxs and
Rie , heat recovery effectiveness "HR , global heat recovery efficiency
G , specific net energy saving, SNES in Wh/m3 , and coefficient of
performance, COP, of audited air handling
units
95
Uses of energy in buildings, energy saving measures and their
effects on indoor environment quality
104
PAP values and 2-propanone concentrations in PAP meters
used as milestones
116
List of VOCs used for contaminant transfer experiments
120
Characteristics of the air handling units used for the experiments 121
Pressure differentials in the units [Pa]
122
Climatic conditions in the units [8C]
122
List of Figures and Tables
6.6
6.7
6.8
6.9
6.10
6.11
6.12
6.13
6.14
7.1
7.2
7.3
7.4
7.5
7.6
7.7
7.8
VOC transfer rate in the experiments performed in both EPFL
units (%)
VOC transfer rate in the EMPA experiments
General IAQ strategies for HVAC systems
Checkpoints in HVAC units in visual inspection
IAQ strategies for filters
IAQ strategies for ducts
IAQ strategies for rotating heat exchangers
IAQ strategies for humidifiers
IAQ strategies for coils
Examples of coherent units
Properties of the gases most frequently used as tracers
Background concentration of some gases
Qualities of some tracer gases
Tracer gases most used in the mass spectrometer technique
Two-sided confidence limits TðP; N 2Þ for a Student
distribution
Data measured during a tracer gas experiment in two connected
rooms
Airflow rates [m3 /h], calculated from the data given in Table 7.7
xi
123
123
126
127
128
129
130
130
131
133
135
136
137
145
149
164
165
Preamble
This book includes information already published by the author in scientific
journals and in an Air Infiltration and Ventilation Centre (AIVC) technical
note (Roulet and Vandaele, 1991), now sold out. Part of the content of Chapters
2, 3 and 5 was also published by the author in a book edited by H. Awbi (Awbi,
2007).
Roulet, C.-A. and L. Vandaele, 1991, Airflow patterns within buildings: Measurement
techniques. AIVC Technical Note 34, AIVC, Bracknell, 265pp, order at
inive@bbri.be
Awbi, H., 2007, Ventilation Systems, Design and Performance, Taylor and Francis,
London, 522pp
Introduction
Why ventilate?
Without ventilation, a building’s occupants will initially be troubled by odours
and other possible contaminants and heat. Humidity may rise because of indoor
moisture sources such as the occupants, laundry, cooking and plants; thus
enhancing moisture hazards (for example, mould growth and condensation).
Oxygen will nevertheless not be missed until much later. The purpose of ventilation is to eliminate airborne contaminants, which are generated both by
human activity and by the building itself. These are:
.
.
.
.
.
bad odours, to which people entering the room are very sensitive;
moisture, which increases the risk of mould growth;
carbon dioxide (CO2 ) gas, which may induce lethargy at high concentrations;
dust, aerosols and toxic gases resulting from human activity, as well as from
the building materials (in principle, ‘clean’ materials should be chosen for
internal use, but this is not always possible);
excessive heat.
The airflow rate required to ensure good indoor air quality depends upon the
contaminant sources’ strengths and on their maximum acceptable concentration: the larger the contaminant sources’ strengths or the smaller the maximum
acceptable concentration, the greater the required ventilation rate is.
During the heating season in well-designed and clean buildings, the
occupants are the main source of contaminants (mostly odours and water
vapour). The airflow rate should then be between 22 cubic metres per hour
(m3 /h) per person, which limits the CO2 concentration to about 1000 parts
per million (ppm) above the outdoor concentration, and 54 m3 /h per person,
which limits the CO2 concentration to about 400 ppm above the outdoor
concentration – meaning that less than 10 per cent of people entering the
room will be dissatisfied by the odour (CEN, 2006). Airflow rates should be
much greater in poorly insulated buildings (where there is a risk of mould
growth and water vapour condensation), or in spaces where there is a particular source of contamination, including spaces where smoking is allowed.
In summer, the minimum airflow rate may be much greater than the
hygienic airflow rate in order to evacuate heat or provide cooling draughts.
xiv
Ventilation and Airflow in Buildings
However, when the outdoor temperature exceeds indoor temperatures, it may
be wise to reduce the ventilation rate, only allowing high levels of ventilation at
night when the outdoor temperature is low.
Ventilation is hence not only essential to ensure an acceptable indoor
air quality, but is also often used to improve thermal comfort. For this air
heating or cooling, air conditioning (including air humidity control) or
free cooling (increasing the outdoor airflow rate to cool down the building
fabric) are used. In order to achieve these goals, several conditions should be
met:
.
.
.
.
Airflow rates should be adapted to need: if too low, good air quality will
not be achieved, or draughts, noise and energy waste may result from an
excessive airing.
The air should be well distributed: ideally, the fresh air should reach any
occupied zones first and contaminated air should be quickly extracted.
The air supply should not decrease comfort. It should not cause complaints
about draughts, noise or poor air quality.
The air supplied by ventilation systems should be clean and, where
appropriate, should comply with the temperature and moisture
requirements.
In addition, to comply with a sustainable development policy, the ventilation
systems should be energy efficient and should perform as required using a
minimum amount of energy.
Why assess airflows in buildings?
The conditions listed above are most likely to be met when the building and its
ventilation system are not only well designed and built, but also well commissioned. Commissioning a ventilation system involves carrying out measurements to check that it performs as expected. When these conditions are not
met or when there are problems, measurements may help in finding the
causes of the problem and in fixing them.
In order to show the usefulness of measurements, some results from
investigations performed on several air handling units are shown below. It
should be emphasized that these ventilation units were not selected because
they had problems. The air handling units, located in different buildings,
were measured in several measurement sessions (Roulet et al., 1999). In
some units, the airflow rate was far from the design values, or there was
unexpected recirculation.
Outdoor airflow rate
The comparison of design and measured outdoor airflow rate per person in 12
buildings is shown in Figure 0.1. It can be seen that in several buildings the
airflow rate per person is larger than 50 m3 /h, and surpasses 200 m3 /h.
Introduction
xv
Measured [m3/(h · p)]
300
250
200
150
100
50
0
0
100
200
300
Design airflow rate [m3/(h · person)]
Figure 0.1 Design and measured outdoor airflow rate per person in
12 buildings
Source: Roulet et al., 1999.
Measured airflow rates differ from the design values in many ventilation
systems. Figure 0.2 shows the relative differences between measured and
design outdoor airflow rate in 37 air handling units, i.e.:
Measured airflow rate design airflow rate
Design airflow rate
These biases range from 79 per cent to þ67 per cent. Only 11 units are within
the 10 per cent range.
Recirculation rates
Some recirculation of air is often planned to distribute heat or cold without
conditioning too much outdoor air. This, however, decreases the global
indoor air quality, since the contaminants generated within the building are
recirculated throughout the whole building. Therefore, recirculation may not
be desirable. In any case, recirculation should be controlled. Design and
Percent bias
100%
50%
0%
-50%
-100%
Figure 0.2 Relative difference between measured and design outdoor
airflow rate in 37 air handling units
Source: Roulet et al., 1999.
xvi
Ventilation and Airflow in Buildings
100%
Measured
75%
50%
25%
0%
0%
25%
50%
75%
Design recirculation rate
100%
Figure 0.3 Comparison of design and measured recirculation rate in 34 air
handling units
Source: Roulet et al., 1999.
measured recirculation rates are compared in Figure 0.3. These are seldom the
same. Even worse: as shown in Figure 0.4, out of 27 units planned without
recirculation, 30 per cent have shown a recirculation rate of more than 20 per
cent.
Exfiltration
In supply and exhaust units, both airflow rates are either balanced, or the
supply flow rate is increased a little to make the building slightly overpressured.
When the envelope is not airtight, and when the balance between supply and
exhaust air is too large, air leaks through the envelope. This does not have
much influence on indoor air quality, but may strongly decrease the
efficiency of the heat recovery. In some buildings, as much as 100 per cent of
the supply air is lost in this way, as illustrated in Figure 0.5, which compares
50%
45%
40%
35%
30%
25%
20%
15%
10%
5%
0%
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0% 10% 20% 30% 40% 50%
Measured recirculation rate
Figure 0.4 Histogram and cumulated frequencies of measured
recirculation rates in air handling units designed without recirculation
Introduction
xvii
100%
Measured
75%
50%
25%
0%
-25%
-10%
0%
10%
Design exfiltration ratio
20%
Figure 0.5 Design and measured exfiltration ratios compared in 30 units
Source: Roulet et al., 2001a.
the design and measured exfiltration ratios, i.e. parts of the supply air leaking
through the building envelope in 30 units.
Ventilation efficiency
An efficient airing supplies the occupants with fresh air and blows polluted, old
air in unoccupied spaces. The ventilation efficiencies in several rooms, assessed
using the method described in Chapter 3, are illustrated in Figure 0.6.
Rather high ventilation efficiency, indicating piston-type ventilation, can be
seen in rooms 6 to 10, which are high auditoriums. The normal-height rooms 2
to 5 present a complete mixing, while room 1 shows poor ventilation efficiency,
partly explained by the fact that supply and exhaust are both located at the
ceiling in this room.
These few examples clearly illustrate the usefulness of measurements to
detect dysfunction.
Ventilation efficiency
100%
80%
60%
40%
20%
0%
1
2
3
4
5
6
7
8
9
10
Figure 0.6 Ventilation efficiency in some ventilated areas
Note: Dark bands are uncertainty bars. Note that in one unit the efficiency is below 50
per cent, indicating shortcuts and dead zones.
Source: Figure drawn from a table published in Roulet et al., 2001a.
xviii
Ventilation and Airflow in Buildings
When assess airflows in buildings?
Ventilation performance should be checked early to detect potential problems
and to optimize the overall performance of the ventilation system. This includes:
.
.
.
.
.
appropriate airflow rates;
negligible leakage and shortcuts;
high ventilation efficiency;
high fan efficiency;
clean air and so on.
This check should be performed:
.
.
.
when commissioning the ventilation system in order to control that the
system is built according to the specifications;
if there are indoor air quality problems to help in finding the causes;
before refurbishing the system in order to accurately know where there are
potential problems, which should be cured by the refurbishing.
Available methods to assess airflow rates and
related quantities
This section briefly presents the methods described in the book in order to
guide the user in the choice of the appropriate method. The section also
proposes adapted methods for different purposes and gives a general guideline
for planning measurements.
Airflow rates in buildings and in handling units
Chapter 1 describes in detail the method of using tracer gases for assessing
airflow rates between indoor and outdoor spaces, and between indoor spaces.
Such measurements may be useful to check if the ventilation is sufficient.
The method also allows checks to assess if the airflows follow defined paths
from room to room, as required in some buildings, such as laboratories handling dangerous substances.
Similar methods, described in Chapter 2, test if airflow rates in air handling
units correspond to the design values, and detect possible leakage or parasitic
airflows in such units. Such measurements are also useful to check the power
efficiency of fans (see Chapter 5, ‘Energy for ventilation’) and energy efficiency
of heat exchangers (Chapter 5, ‘Heat exchange efficiency’).
Age of air and ventilation efficiency
The longer the air stays at a given location, the larger will be its concentrations
of various contaminants. The age of the air, i.e. the time spent in the building
since the outdoor air entered it, can be measured using tracer gases. The
Introduction
xix
effectiveness of the ventilation in appropriately distributing the air in the ventilated space or in evacuating contaminants emitted at a given location can be
assessed using the methods described in Chapter 3.
When applied in the air handling unit, such measurements can be
performed simultaneously with measurements of airflow rates, thus reducing
the amount of work required. In a single measurement campaign, the mean
age of the air in the ventilated space, the efficiency of the ventilation system,
the supply, exhaust and recirculation flow rates can be measured as well as
the air leakage in both directions through the building envelope.
Airtightness
The building envelope should be reasonably airtight to ensure the efficiency of
a mechanical ventilation system, and this system itself should have airtight
ducts in order to distribute the air appropriately throughout the ventilated
space. Chapter 4 describes the measurement methods. The general methods
are described first and their application to the building envelope can be
found in Chapter 4, ‘Airtightness of buildings’, while the application to air
ducts or ductworks are in Chapter 4, ‘Measurement of airtightness of a duct
or network’.
Energy efficiency
To comply with a sustainable development policy and also to reduce the emission of greenhouse gases, energy efficiency of any system should be improved.
This can be achieved without the use of systems powered by non-renewable
energy. In our case, a mechanical ventilation system could be applied to an
appropriate building design, allowing natural ventilation. However, this is
not always possible, and in some cases – for example in cold or hot countries
where heat recovery on indoor air actually allows energy savings – may even
be counterproductive.
Therefore, the mechanical ventilation systems should be designed, built,
commissioned and maintained with the aim of ensuring good indoor air quality
at reduced energy use. Chapter 5 proposes various methods, checklists and
propositions to measure, check and improve the energy efficiency of ventilation
systems and components. They include:
.
.
.
.
efficiency of heat recovery (‘Heat exchange efficiency’);
effectiveness of heat exchangers(‘Heat exchangers’);
fan power efficiency (‘Energy for ventilation’);
energy effects of indoor air quality measures (‘Energy effects of IAQ
measures’).
Contaminants in air handling units
Unfortunately, in practice major sources of indoor air contaminants are
components in air handling units and ventilation systems (Bluyssen et al.,
xx
Ventilation and Airflow in Buildings
1995, 2003). This can, however, be avoided by appropriate design and maintenance. Chapter 5, ‘Energy effect of IAQ measures’, lists the sources and
causes of pollution in ventilation systems, proposes measurement protocols,
and sets out maintenance procedures and strategies to improve the quality of
the air delivered by mechanical ventilation systems.
Common techniques
The description of techniques and methods used for the measurements and
their interpretation is detailed in Chapter 7. This chapter includes:
.
.
.
.
the general description of tracer gas dilution techniques;
ways of expressing concentrations and flow rates;
mathematical identification methods;
error analysis.
1
Airflow Rates in Buildings
This chapter intends to help the reader to measure airflow rates and air change
rates in buildings and rooms, independently of a mechanical ventilation system.
It presents the techniques used to measure the airflow rate entering the
measured zone (single-zone measurements) and to measure inter-zone airflows
(multi-zone measurements).
A building zone or a zone is a space that can be considered as homogeneous
from the point of view of air quality or, more technically, a space in which each
tracer gas is homogeneously distributed. In practice, it is a room or a set of
adjacent rooms that have much larger airflow rates between them than to or
from other zones or the outdoor space.
The measurement techniques presented here are all based on the use of
tracer gases that are injected into the air and analysed in air samples after
mixing. More detailed information on the tracer gases themselves, on appropriate injection and sampling methods and on tracer gas analysers is given in
Chapter 7, ‘Tracer gas dilution techniques’.
Single-zone measurements
The tracer gas is injected in a space, mixed into the air and its concentration is
measured. Various strategies can be used for assessing the airflow rate entering
the space: recording and interpreting the concentration decay after having
stopped the injection, monitoring the tracer gas concentration when injecting
the gas at constant rate, or measuring the tracer gas flow rate required for
keeping its concentration constant. Airflow rates are obtained by interpreting
the evolution with time of either the tracer gas concentration or the injection
rate. The interpretation methods are based on the mass conservation of
tracer gas and of the air.
Mass conservation of tracer gas and air
The tracer gas injected in a building space is uniformly mixed into the air. The
conservation of the mass of tracer gas within a single zone in contact with the
2
Ventilation and Airflow in Buildings
outdoor environment is then:
dm
¼ I þ Co Qoi Ci Qio
dt
ð1:1Þ
where:
m
I
C
Q
i
o
is the mass of tracer gas in the zone (kg);
is the injection rate of the tracer gas (kg/s);
is the tracer gas mass concentration;
is the mass airflow rate (for example, Qio is airflow rate from indoor to
outdoors);
subscript for internal environment;
subscript for external environment.
In addition, the conservation of the mass of air gives:
Qoi ¼ Qio
ð1:2Þ
The mass of tracer in the zone is related to the mass of air M by:
ð1:3Þ
m ¼ Ci M
where Ci is the concentration of the tracer gas in the indoor air. Combining the
last three equations, we get:
M
dCi
¼ I þ Qio ðCi Co Þ
dt
ð1:4Þ
since M is very close to a constant if the temperature is constant. In principle,
this equation can directly provide the airflow rate:
dCi
dt
C
IM
Qio ¼
ð1:5Þ
writing C ¼ Ci Co .
This method is, however, very inaccurate, since very quickly the concentration may vary at random because of turbulence and non-homogeneities. It is
therefore better to take a time average by integrating it for a given period of
time:
ð t þ t
ð t þ t
ð t þ t
I
dCi
ð1:6Þ
dt M
Qio dt ¼
C
C
t
t
t
hence:
ð t þ t
t
Qio dt ¼
ð t þ t
t
I
dt M½lnðCðtÞÞ lnðCðt þ tÞÞ
C
or, dividing both members by t
I
M
CðtÞ
ln
hQio i ¼
C
t
Cðt þ tÞ
ð1:7Þ
ð1:8Þ
where the quantity between brackets h i is averaged over the time period t.
Airflow Rates in Buildings
3
This solution can be simplified, depending on the way the tracer is
injected.
Tracer decay, no injection
A suitable quantity of tracer gas is injected to achieve a measurable initial
concentration Ci;0 . At time t0 , this injection is stopped and I ¼ 0 afterwards.
From Equation 1.4, it can be found that the concentration decays with time
according to:
Q
ð1:9Þ
C ¼ Cðt0 Þ exp io t
M
The quantity
n ¼
M
Qio
ð1:10Þ
is called the nominal time constant of the measured zone. It is the ratio of the
mass of air contained in the zone to the mass airflow rate. It is also the time
needed to introduce a mass of new air equal to that contained in the zone.
Since I ¼ 0, Equation 1.8 becomes:
M
CðtÞ
ln
ð1:11Þ
hQio i ¼ t
Cðt þ tÞ
This equation allows easy calculation of the airflow rate from the measurement
of concentration at two instants. This method is called the decay method. It is a
direct measurement of the nominal time constant, and also provides an
unbiased estimate of the mean airflow rate.
Constant injection rate
If the injection rate is constant, the solution of Equation 1.4 is:
I
t
I
þ
C ¼ Cðt0 Þ
exp Qio
n
Qio
ð1:12Þ
Using identification technique (see Chapter 7 ‘Identification methods’), both n
and Qio (hence also M) can be obtained. This method is, however, of easy use
only when Qio is constant. In this case, the exponential term becomes negligible
after three or more time constants, and
C ¼
I
Qio
ð1:13Þ
Qio ¼
I
C
ð1:14Þ
or
The result is biased (underestimated) if the airflow rate is not constant.
4
Ventilation and Airflow in Buildings
Constant concentration
Using an electronic mass flow controller monitored by the tracer gas analyser,
the concentration of tracer gas can be maintained constant by varying the
injection rate in an appropriate way. In this case, the time derivative of the
concentration is zero and Equation 1.4 becomes very simple:
I þ Qio ðCi Co Þ ¼ 0
ð1:15Þ
hence:
Qio ¼
I
I
¼
ðCi Co Þ C
ð1:16Þ
This method provides an unbiased estimate of the airflow rate, even when it
varies with time.
Pulse injection
The method can also be used with the tracer injected as a short pulse at time t0 .
The injected mass M will result in a tracer gas concentration at sampling location that varies with time, starting from background concentration C0 , growing
and then decaying back to background concentration. Let C(t) be the tracer
concentration above background.The total mass of tracer gas passing at the
sampling location is then:
ð1
CðtÞQðtÞ dt
ð1:17Þ
M¼
t0
An approximation to infinite time can be good enough when the experiment
(and the integral) is stopped at time tf , when the concentration is close
enough to background.
Since both functions C(t) and Q(t) are positive and continuously derivable,
we can apply the integral mean value theorem (Axley and Persily, 1988), that is:
ð tf
ð1:18Þ
M ¼ QðÞ CðtÞ dt with t0 < < tf
t0
This means that there exists a time during the experiment, when the airflow
rate Q has a value satisfying the above equation. The knowledge of the injected
mass M and measurements of the integral of the concentration downwind can
then provide a value of Q.
The integral of the concentration can be calculated from the mass of
tracer m sampled downwind from the injection port by pumping the air at
known rate Qs;i through a tube. The sampled mass is related to concentration
by:
ð1
m¼
CðtÞQs ðtÞ dt
ð1:19Þ
0
Airflow Rates in Buildings
Applying again the integral mean value theorem, we get:
ð tf
m ¼ Qs;i ð 0 Þ CðtÞ dt with 0 < 0 < tf
5
ð1:20Þ
0
Combining Equations 1.18 and 1.20, we get:
m
Qi ðÞ ¼ Qs ð 0 Þ
M
ð1:21Þ
There are sampling pumps with controlled constant flow rate. These easily
enable Qs;i to be kept constant, and in most cases it is possible to keep the
airflow rate constant in supply and exhaust during the experiment.
Simple and cheap air change rate measurement using CO2
concentration decays
Method
CO2 generated by occupants can be used as a tracer gas, since it is easy and
cheap to measure. There are compact and light CO2 analysers on the market
that include a data logger. Peak value of the CO2 concentration during
occupancy is an indicator of the minimum airflow rate per person. Analysis
of the decays observed when the occupants leave the building provides the
nominal time constant of the ventilated space, which is directly dependent on
the outdoor airflow rate from the ventilation system and infiltration.
Depending on the state of the ventilation system during the decay, this
method provides either the total outdoor airflow rate provided by the system,
or the infiltration rate. When combined with a simple pressure differential
measurement, this method can also be used to check airtightness of building
envelopes.
Equivalent outdoor airflow rate
Air may enter into a measured zone not only directly from outdoors, but also
from neighbouring zones, whose CO2 concentration may differ from outdoor
air. These inter-zone airflows influence the CO2 concentration in the measured
zone, but can be measured only with complex and expensive techniques (see
Chapter 1, ‘Application to buildings, multi-zone’). The concept of equivalent
outdoor airflow rate is introduced to offset this inconvenience. It corresponds
to the outdoor airflow rate that would result in the same CO2 concentration
in the measured room without inter-zone airflows. In the following, this quantity is referred to as ‘outdoor airflow rate’.
Equivalent outdoor airflow rate per person
An adult person produces on average and for most of the time (i.e. when quiet
or doing light work with about a 100 W metabolic rate) about 20 litres per hour
(l/h) of CO2 . At steady state, and assuming that occupants are the only CO2
sources, the equivalent outdoor airflow rate per person, Qe , is related to CO2
6
Ventilation and Airflow in Buildings
Figure 1.1 Records of CO2 concentration in an office room
Source: Roulet and Foradini, 2002.
concentration C (Ci indoors and Co outdoors) by:
Qe ¼
S
Ci Co
ð1:22Þ
where S is the CO2 source strength, i.e. about 20 l/h. The equivalent outdoor
airflow rate per person can then be assessed during the periods of time when
steady state can reasonably be assumed, that is when the CO2 concentration
is constant.
Example of application
CO2 concentration was recorded every five minutes during several winter days
in an office room occupied by one person.
The evolution of CO2 concentration is shown in Figure 1.1. A base outdoor
concentration of about 600 ppm was determined from the minimum values at
the end of long decay periods (weekends). This base concentration is deducted
from the CO2 concentration to get the increase resulting from indoor sources.
On 19 November, a CO2 concentration of about 1500 ppm is observed. This
corresponds to an equivalent outdoor airflow rate at 22 m3 /(h person), obtained
by natural ventilation. Decay periods are selected in the record (rectangles in
Figure 1.1).They correspond to night or weekend periods, without occupancy,
when windows and doors are closed and the air change rate results from
infiltration only. The average nominal time constant from these five decays is
found to be 10 2 hours.
Application to buildings, multi-zone
Most buildings include several interconnected zones. In order to measure not
only the airflow between internal and external environments but also interzone flows, either several tracer gases should be used simultaneously (injecting
each of them in a different zone), or several experiments should be conducted
successively, injecting the tracer successively in the different zones, and
assuming that the measurement conditions, in particular the airflow pattern,
Airflow Rates in Buildings
7
do not change during the measurement campaign. This section describes ways
of interpreting the records of tracer gas injection rates and concentration in the
different zones to get the airflow rates between zones, as well as airflow to and
from outdoors. For more information on tracer gases and analysers, see
Chapter 7, ‘Tracer gas dilution techniques’.
Let us assume that there are N zones in the measured building, denoted by the
suffixes i and j, into which, in principle, N different tracers, denoted by the index
k, are injected. In principle, each zone receives only one tracer, but the equations
presented below allow the use of several gases in the same zone. No tracer is
injected in the outside air (zone 0), which is assumed to be of infinite volume.
However, the tracer concentration in that zone may differ from zero.
The multi-zone tracer gas theory is based on the conservation of the mass of
tracer gas and of air and on the following three assumptions:
1 In each zone, tracer concentrations are always homogeneous.
2 The atmospheric pressure is constant.
3 The injection of tracer gas does not change the density of air.
The first assumption is the weakest. In practice, homogeneous concentration
may only be achieved by the use of mixing fans, but these fans may affect
infiltration conditions.
The other two hypotheses are easily satisfied because the short-time relative
variations of atmospheric pressure are of the order of 0.01 per cent (daily
variations of the order of a per cent) and tracer gases are generally injected at
relatively low concentrations (104 or less).
Conservation of the masses of air and tracer gas k in zone i
In each zone, the rate of change of the air mass mi equals the sum of the
incoming flows minus the sum of the outgoing flows:
N
N
X
X
dmi
¼
Qij ð1 ij Þ Qji ð1 ij Þ
dt
j¼0
j¼0
Change
in mass
Incoming
flow rates
ð1:23Þ
Outgoing
flow rates
where ij is equal to one only when i ¼ j, and zero otherwise. The sum is then
over all terms for which i 6¼ j. Note that, in most cases, the left-hand side of
these equations is close to zero and can be neglected.
The conservation equation of the mass of tracer, k, in the zone, i, states that
the change of tracer mass within the zone is the sum of the mass of injected
tracer and the mass of tracer contained in the air entering the zone, minus
the mass of tracer contained in the outgoing air:
dmik
dt
Variation
¼
Iik
þ
N
X
Cjk Qij ð1 ij Þ Cik
j¼0
Injection
N
X
Qji ð1 ij Þ
j¼0
Inflow
Outflow
ð1:24Þ
8
Ventilation and Airflow in Buildings
where:
mik
Iik
Cjk
Cik
Qij
is
is
is
is
is
the
the
the
the
the
mass of tracer gas k in zone i;
injection rate of tracer gas k in (or just upwind of ) zone i;
concentration of tracer k in zone j;
concentration of tracer gas k in zone i;
airflow rate from node j to node i.
An extension of assumption 1 above is implicit in this equation, that is:
4 The airflow entering a zone does not modify the homogeneity of the concentration of tracer gases in that zone, i.e., an immediate and perfect mixing is
assumed.
If there are N tracers or N different sets of measurements using a single tracer
injected at various rates in the various zones, Equations 1.23 and 1.24 above
give a full set of N(N þ 1) equations. Therefore, this allows the N(N þ 1) flows
between all the zones, including the outdoor air as the zone zero to be determined.
There are two methods to transform this set of equations before solving. Since they
each have various advantages and disadvantages, they are both described below.
Global system of equations
The most common technique to be found in the literature (Sinden, 1978;
Sherman et al., 1980; Perera, 1982; Sandberg, 1984) is the following.
Let us express by Qii the sum of all the flows entering the zone i:
N
X
Qii ¼
Qij ð1 ij Þ
ð1:25Þ
j¼0
Using the above notation and taking apart the flows coming from outside,
Equation 1.24 becomes:
N
X
dCik
¼ Iik þ
Ckj Qij ð1 ij Þ þ C0k Qi0 Cik Qii
ð1:26Þ
Mi
dt
j¼1
Since any change in the outdoor level of tracer gas concentrations, C0k , will be
negligible, these levels are the base levels of tracer gas concentrations anywhere
else. In this case the tracer mass balances expressed in Equation 1.26 can be
written in a matrix form:
d
½M C þ Q C ¼ I
ð1:27Þ
dt
where each row of the N N matrices M C, Q C and I corresponds to a zone
and each column to a given tracer gas. More specifically:
M is a diagonal matrix whose elements are the masses of air contained in each
zone:
mi ¼ i Vi or M ¼ V
where is the diagonal matrix of the air densities in the zones, i , and V the
diagonal matrix of the volumes of the zones, Vi .
Airflow Rates in Buildings
9
C contains the differences in mass concentrations Cik C0k of gas k in zone i.
I is the matrix containing the mass flow rates Iik of the tracer, k, in zone i. In
usual measurements, this matrix is diagonal.
Q is the so-called flow matrix containing, the off-diagonal elements ( j 6¼ i)
being Qij , where Qij represents the mass flow rates from zone j to zone
i. The diagonal elements with j ¼ i contain the sum of the flows leaving
the zone i, as defined in Equation 1.25.
In Equation 1.26, i and j run from 1 to N and this system results in N 2
equations for the inter-zonal flows. The mass flows to and from outside are
given by Equation 1.23.
When steady state is reached for tracer gas concentration, Equation 1.27
becomes:
QC ¼ I
hence
Q ¼ I C 1
ð1:28Þ
This method looks very attractive, but has several disadvantages. First, the
number of tracer gases is limited, and it is therefore often impossible or at
least very impractical to have tracer gases injected in each zone. In this case,
the C-matrix is not square and cannot be inverted. In addition, this method
may give non-zero values to non-existent airflow rates, or even provide negative
airflow rates. For this reason, the node-by-node method, which allows writing
equations containing only significant airflow rates, was developed.
Zone by zone systems of equations
Another presentation of the same model is found in Roulet and Compagnon
(1989). It is obtained as follows.
Combining Equations 1.23 and 1.24, then taking into account that
mik ¼ mi Cik , and using:
dmik
dCik
dm
¼ mi
þ Cik i
dt
dt
dt
ð1:29Þ
we finally get:
mi
N
X
dCik
¼ Iik þ
ðCjk Cik ÞQij ð1 ij Þ
dt
j¼0
ð1:30Þ
For each zone i, these N equations give the N flows, Qij ( j ¼ 0; . . . ;
i 1; i þ 1; . . . ; N). The flows, Qji , are obtained from the same equations
applied to zone j and the remaining flows, Q0i , are given by Equations 1.23.
Further interpretation of the flow matrix
The final result of the measurements is the flow matrix Q defined above.
Further information can be deduced from this flow matrix, as shown in the
following discussion (Sandberg, 1984).
10
Ventilation and Airflow in Buildings
Properties of the flow matrix
The total outdoor airflow rate to each zone, i, is easily obtained by summing the
columns of the flow matrix:
Qi0 ¼
N
X
Qij
ð1:31Þ
j¼1
And the total exfiltration airflow rate from each zone, i, is the sum of the lines of
the flow matrix:
Q0i ¼
N
X
Qij
ð1:32Þ
i¼1
If there is no totally isolated chamber in the measured system, and if there is
some air exchange with outside (as is the case with any usual building), the
flow matrix determinant, jQj, is positive and Q has an inverse, Q1 .
The elements of this inverse Q1 are given by:
Aji
Wji ¼
ð1:33Þ
jQj
where Aji are the cofactors of the element Qij in Q.
Transfer of contaminants between zones
The basic equations applied to the case where a constant flow rate, Iik , of a
contaminant, k, is applied in each zone, i, leads to an equilibrium concentration
(for constant airflow rates) that is:
Cð1Þ ¼ Q1 I
ð1:34Þ
It follows that the equilibrium concentration in room, j, resulting from a
contaminant, k, released only in room, i, is:
Cjk ð1Þ ¼ Wji Iik
ð1:35Þ
and the non-diagonal elements of Q1 are hence the transfer indexes defined in
Sandberg (1984).
Using a simple inversion of the flow matrix, much information on the
possible spreading of contaminants can be obtained.
Age matrix and mean age of air
The matrix is defined as:
¼ Q1 M
ð1:36Þ
or, under the assumptions of constant, uniform temperature:
¼ q1 V
ð1:37Þ
Where q is the volume flow matrix and V a diagonal matrix with the volumes
Vii of room i on the diagonal. It is shown (Sandberg, 1984) that the row
Airflow Rates in Buildings
11
sums of the matrix are the mean age of air in the corresponding rooms:
hi i ¼ N
X
ð1:38Þ
ij
j¼1
This relation enables the measurement of the room mean age of air to be made,
even in rooms where there are several outlets or several ways for the air to leave
the room.
Equations for volume flow rates
All equations above are based on mass balance, and hence include mass airflow
rates and mass concentrations. However, for practical reasons, volume flow
rates and volume concentrations are of common use. Therefore, the basic
equations should be adapted as shown below.
The mass of the tracer k in the zone i is:
mik ¼ ik Vik ¼
i Vi Cik
ffi i Vi Cik ¼ ik Vi cik
1 Cik
ð1:39Þ
since Cik 1.
The tracer density is defined by ik ¼ mik =Vik where the volume, Vik , is
defined at atmospheric pressure, p. Using the perfect gas law for tracer k:
m
ð1:40Þ
pVik ¼ RTi ik
Mk
where R is the molar gas constant, R ¼ 8313.96 [J/(K kmole)], Mk the molar
mass of the tracer, k, and Ti is the absolute temperature of zone i. The density
of tracer k in zone i can be computed:
ik ¼
pMk
RTi
ð1:41Þ
This is also valid for the density of air, by simply omitting the suffix k and using
the average molecular weight (M ffi 29 g/mole) of the air.
Introducing this in Equations 1.23 and 1.26 gives the set of balance equations to be used when handling volumes instead of masses. Equation 1.30
becomes:
N
X
ðcjk cik Þ
Vi dcik
i
¼ ik þ
qij ð1 ij Þ
Ti dt
Tk j ¼ 0
Tj
ð1:42Þ
where:
T is the absolute temperature of zone i or j, or of tracer k, depending on the
subscript;
cik is the volume concentration of tracer k in zone i;
iik is the volume injection rate of tracer k in zone i;
qij is the volume flow rate from zone j to zone i.
12
Ventilation and Airflow in Buildings
The air mass conservation (Equation 1.23) is rewritten as:
q0i ¼ Ti
N
N
X
qij ð1 ij Þ X
V dTi
qji ð1 ij Þ þ i
Ti dt
T
j
j¼0
j¼1
ð1:43Þ
These last two systems include N þ 1 equations for N þ 1 unknowns, qij , for
each zone i.
It should be noticed that Equation 1.43 can be simplified, and becomes
similar to Equation 1.23 if indoor and outdoor temperatures are close to each
other and if the internal temperature is constant. This means that, provided
such conditions are realized, the volume conservation equation can be used
instead of mass conservation.
Summary of the various tracer gas methods
The different tracer gas techniques can be broadly divided into two categories:
steady-state methods, which directly measure the flow rate, Q, and transient
methods, which measure the nominal time constant, n , or the air change rate,
n. The steady-state techniques are based on recording steady-state concentrations or concentrations integrated over a long time, while transient methods are
based on recording the change in tracer gas concentration. The different tracer
gas techniques and their properties are given in Tables 1.1 and 1.2.
If airflow varies with time, only the two-point decay and the constant
concentration methods give a correct estimate of the average flow. The constant
injection method underestimates the average flow rate if the integration time is
much longer than the period of flow variation.
Table 1.3 gives a summary of multi-zone methods. As far as single-zone
measurements are concerned, the following conclusions can be stated:
.
It appears that decay, pulse and step-up methods require the least measurement time and usually the least preparation. However, with the exception of
the two-point decay method, they give a biased estimate of a variable air
change rate. These biases remain small if the measurement period is limited
to times close to the nominal time constant.
Table 1.1 Summary of different injection strategies
Tracer injection strategy
Pulse injection
Decay
Constant injection rate
Constant concentration
Direct result
Cost
Qy
n or n
Qy
Q
Moderate
Moderate
Moderate
Relatively high
Note: y The volume has no influence only when the airflow rate, Q, is constant.
Airflow Rates in Buildings
13
Table 1.2 Summary of single-zone methods
Method name
Tracer injection
technique
Interpretation
method
Transient methods (tracer gas concentration changes)
(Simple) decay
Decay
Identification
Two-point decay
Decay
Integral
Step-up
Constant rate
Identification
Suited for
Unbiased Continuous
average
record
No
Yes
No
Steady-state methods (tracer gas concentration is nearly constant)
Pulse
Pulse
Integral
No
Constant injection Constant rate
Direct solution
Yes†
Long-term integral Any
Integral
No
Constant
Constant
concentration
concentration
Integral
Yes
No
(Yes)
No
(Yes)
No
No
Yes
Note: y Under condition.
Source: Sherman, 1990.
.
.
The long-term integral method, generally used with passive sources and
samplers, also gives a biased estimate of the average airflow rate. Since the
measurement time is larger, the bias may not be negligible. This technique,
however, provides an unbiased estimate of the average tracer concentration.
If the tracer is used to simulate a contaminant, such experiments are of great
interest for indoor air quality studies.
The constant concentration technique is accurate and gives an unbiased
estimate of the average airflow rate, but it requires the most technical
equipment.
Table 1.3 Summary of multi-zone methods
Tracer injection strategy
Unbiased average of
time-varying flow
Well suited for
continuous record
Single tracer (repeated measurements)
Pulse injection
Decay
Constant injection rate
Constant concentration
No
No
No
Yes
No
No
No
No
Multi-tracer
Pulse injection
Decay
Constant injection rate
Constant concentration
No
Yes*
No
Yes
No
No
Yes
Yes
Note: Only for a two-point estimate.
14
.
Ventilation and Airflow in Buildings
Constant injection used with long-term direct solution is simpler to use and
may give, under certain conditions, unbiased estimates of an average airflow
rate.
The two-point decay method, and more generally the multi-zone, transient
methods may lead to unacceptably large uncertainties if the measurement
time period is inappropriate. See Enai et al. (1990) for two-zone, two-tracer,
step-up and decay methods.
2
Airflow Rates
in Air Handling Units
Air handling units are designed to supply new air to the ventilated zone and to
extract vitiated air from this zone. Many other airflows may be found in such
units, as shown in Figure 2.1.
Measurements of airflow rates in ventilation systems are useful in order to
check if the air follows the expected paths and thus detect potential problems
early so they can be corrected, also allowing the optimization of the performance of the airflow system. Checks include examining if actual airflow rates
are close to the desired values and if leakages and short-circuits are negligible.
The checks should be performed:
.
.
.
when commissioning a new ventilation system in order to ensure that the
system is built according to the design specifications;
if there are indoor air quality problems to help finding their causes;
before refurbishing a ventilation system in order to accurately identify the
potential problems to be cured by the refurbishment.
Commissioning mechanical ventilation systems is paramount in order to ensure
that they function as designed. This includes not only the measurement of the
main airflow rates and pressure distributions, but also checks that there are no
excessive leakages or shortcuts. It should be noted that commissioning
protocols are available for most units mentioned in the Introduction that function as designed, while they are not available for units in which airflow rates are
not those designed or for units showing significant leakages or shortcuts.
This chapter proposes methods for measuring most airflow rates that occur
in ventilation systems.
Measurement of the airflow rate in a duct
Summary of measurement methods
Common methods used to measure airflow rates in ducts exploit well-known
relationships between flow rate and pressure drop across a restricted section
Ventilation and Airflow in Buildings
Fan
16
Extract air
Ventilated
space
Exhaust air
Outdoor air
Fan
Heating
Humidifaction
Cooling
Heat exchanger
Filter
Recirculation
dampers
Supply air
Figure 2.1 Schematics of a supply and exhaust air handling unit
Note: The main airflow paths are shown as solid arrows, and secondary or parasitic
airflow paths are shown as open arrows.
Source: Roulet et al., 2000a.
placed in the flow, for example a nozzle, Venturi or sharp-edged orifice (ISO,
2003). Alternatively, the air speed can be measured directly at a number of
points lying in a cross-section of the duct (a traverse), and the results integrated
along the traverse to give the volume flow rate (ISO, 1977). All of these
methods have the disadvantage that a long straight section of duct, both
upstream and downstream of the measurement point, is needed in order to
condition the flow. Moreover, the introduction of a restriction may significantly
change the airflow rate to be measured. Tracer techniques (ISO, 1978; Axley
and Persily, 1988; Riffat and Lee, 1990), which avoid these problems, employ
gas analysers and measure the dilution of a tracer gas introduced into the flow,
using equipment that is becoming increasingly common, robust and easy to use.
Orifice plate, nozzle and Venturi flowmeters
The change in pressure in a pipe with a section of restricted area can be
calculated by the Bernoulli law, provided there is neither friction nor compression. A relationship can hence be found between a pressure difference along the
flow line and the corresponding flow rate, which may itself be deduced from a
differential pressure measurement.
However, since there is a slight friction, the mass flow is:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2p
ð2:1Þ
Q ¼ Cd A
1 2
where:
Cd
A
p
is the discharge coefficient, taking account of friction losses,
is the smallest cross-section in the flow,
is the pressure difference between two taps properly located,
Airflow Rates in Air Handling Units
17
is the reduction ratio, which is the ratio of the smallest diameter to the
diameter of the pipe.
The flow may be restricted with an orifice plate, a nozzle or a Venturi tube. The
most sophisticated and expensive is the Venturi tube in which the discharge
coefficient is nearly 1 and constant for Re > 2 105 and higher than 0.94 if
Re > 50,000. Moreover, this device does not induce a large pressure drop in
the flow. At the other end of the spectrum is the simple and cheap orifice
plate, which induces a large pressure drop and shows discharge coefficients
that may be as low as 0.6, depending on the Reynolds number. The characteristics of the nozzle lie in between. The device that is most sensitive to
perturbation is the orifice, then the nozzle. The Venturi tube is the least
sensitive.
All these flowmeters should be mounted between two straight pipes, the
upstream pipe being up to 30 pipe diameters long, depending on the type of
perturbation upstream, and the downstream part at least 3 diameters long. If
a straightening vane 2 diameters long is installed upstream, the distance
between this vane and the flowmeter may be reduced down to 10 diameters.
The literature (for example, ASHRAE, 2001) provides detailed drawings of
such devices.
Velocity traverse
If the velocity of the air, v, is measured at enough points in the duct, the volume
airflow rate can be deduced by integration over the whole area, A, of the
cross-section as shown:
ð
ð2:2Þ
Q ¼ v dA
A
where is the density of the air.
Equipment to measure air speed
Provided the direction of flow is parallel to the duct, which may normally be
assumed, then velocity can be determined by the measurement of air speed
alone.
The air speed measurement devices should be small enough to enable them
to be easily inserted through small holes in duct walls. The most common
examples are hot wire or NTC anemometers, helix anemometers and Pitot
tubes.
The hot wire anemometer and NTC anemometers measure the temperature
drop of a heated wire or a heated resistor (with a negative temperature coefficient), which, in each case, is directly related to the temperature and speed of
the air flowing over it. The sensors are heated by an electric current and
measurements are made of the voltage drop, which depends on the temperature. The temperature of still air is taken into account by the use of a
reference sensor shielded from the flow. Such devices can measure speeds
18
Ventilation and Airflow in Buildings
0.316 R
0.548 R
0.707 R
0.837 R
0.949 R
Figure 2.2 Location of the measurement points in circular and rectangular ducts
Source: ASHRAE, 2001.
from 0.05 to 5 m/s, and are well suited for speeds of 1–5 m/s, which are typical in
ventilation ducts.
Helix anemometers measure the rotation speed of a small helix that is placed
perpendicular to the airflow. The pressure difference between the front and the
side of the Pitot tube is proportional to the square of the air speed. The helix
anemometer and the Pitot tube are most accurate for air speeds above 10 m/s,
and therefore may not be the best for measurement in ducts, where speeds as
low as 1 m/s can be measured.
Test procedure
The location of the measurement in the duct should be at least 8 diameters
downstream and 3 diameters upstream of any disturbances in the flow, such
as a bend in the duct or a change in cross-section. Flow-straightening vanes
located at 1.5 diameters upstream will improve the measurement accuracy.
Several measurements across the duct should be taken to enable integration.
It is advisable to notionally divide the duct section into sub-sections of equal
area and to take measurements at their centres. The ASHRAE Handbook of
Fundamentals (ASHRAE, 2001) proposes a division of 16 to 64 rectangular
sub-sections for a rectangular duct, and 20 annuli for cylindrical ducts (see
Figure 2.2). In the latter, any asymmetry in the flow may be taken into account
by taking measurements along two orthogonal directions.
Tracer gas dilution
Measurement of airflow rate in a duct is the simplest application of the tracer
gas dilution technique. It is illustrated in Figure 2.3. The tracer is injected at
a known constant flow rate, I. The air is analysed downstream, far enough
from the injection port to have a good mixing of the tracer into the air (see
‘Sampling points for concentration measurements’, below).
Airflow Rates in Air Handling Units
19
Figure 2.3 Measuring the airflow rate in a duct with the tracer gas
dilution method
Source: Awbi, 2007.
Assuming that no tracer is lost in between, the mass balance of the tracer gas
is, at steady state:
I ¼ ðC C0 ÞQ
ð2:3Þ
where:
C is the tracer concentration as obtained by analysis,
C0 the concentration upstream the injection port (if any),
Q the airflow rate in the duct, which is then:
Q¼
I
ðC C0 Þ
ð2:4Þ
This simple method assumes steady state: both airflow rate and injection flow
rate are constant and the concentration is recorded only when a constant
concentration is reached. This time is rather short, about five times the time
period needed for the first molecules of tracer gas to reach the analyser if the
air is not recirculated after delivery into a room. If there is recirculation, the
time needed to reach the steady state may be much longer, i.e. five times
the nominal time constant of the ventilated space. In this case, applying the
method described in ‘Measurement of airflow rate in a duct’ (above) will
reduce the time needed for measurements.
Note that, in a first approximation, mass balance can be replaced by volume
balance to get volume flow rates instead of mass flow rates. This approximation
remains valid as long as the difference between indoor and outdoor temperature
is smaller than 18C.
Airflow measurements at ventilation grilles
Ventilation inlet grilles should distribute the air in each room or space. In
supply and exhaust units, extract grilles should absorb an equivalent airflow
rate. Airflows should be balanced so that each room or space receives an
appropriate airflow rate. Measurements at each grille are essential to ensure
such a balance.
Ventilation and Airflow in Buildings
Airflow
20
Fan
Flowmeter
Differential
manometer
Figure 2.4 Schematics of a compensated flowmeter
Note: The differential manometer adjusts the fan so that there is no pressure drop
through the flowmeter.
The methods described in the section on ‘Measurement of airflow rate in a
duct’ (above) may of course be used to measure the airflow rates through grilles,
but specific instruments may be easier to use or bring a more accurate result.
Inflatable bag
This method consists in measuring (using a chronograph) the time required to
fill a plastic bag of a known volume when its opening is placed against the grid.
It is not very accurate and creates a counter-pressure that could perturb the
airflow, but the equipment is very cheap: a simple plastic bag, such as a rubbish
bag.
Flowmeter
Any type of flowmeter fixed to a box or cone adapted to the shape of the grille
may be used to measure the airflow rate through the grille, provided this
flowmeter does not require too large a pressure drop across it.
Compensated flowmeter
The compensated flowmeter (see Figure 2.4) is equipped with a fan, the speed
of which is adjusted so that the pressure differential through the instrument is
negligible during the measurement. This allows an accurate measure of the flow
rate going through the instrument without perturbing the measured grid or
duct. The fan itself could be the flowmeter, since the fan speed at zero pressure
differential is directly related to the airflow rate.
Airflow rate measurements in air handling units
Tracer gas injection ports
Injecting several tracer gases at various locations and analysing the air at other
appropriate locations allows the assessment of several airflow rates simultaneously.
Airflow Rates in Air Handling Units
C0
C4'
C7
C5
C6
21
C4
2
1
C1
C1'
3
C2
C3
C3'
4
Figure 2.5 Locations of tracer gas injection (arrows), and sampling points for
concentration measurements (Ci ) in a typical supply and exhaust air handling unit.
Source: Roulet et al., 1999.
In principle, the method described above in ‘Tracer gas dilution’ can be
applied to each branch of a duct network. However, this requires as many
tracer gas injections and air sampling measurements as there are airflow
rates. Experience has shown that the experiment can be made simpler, as
shown in Figure 2.5, where most practical and efficient injection locations are
indicated by arrows. In this figure, air sampling points for the required
tracer gas concentration measurements are also shown.
If several tracer gases are needed but not available, it is possible to use the
same tracer gas in several experiments, injecting the tracer successively at
different locations. In this case, care should be taken to ensure constant airflow
rates in the system. In particular, frequency controllers of the fans should be
blocked at a constant frequency. It is also recommended to start with injection
at location 2, then 3 and finally 1. This strategy shortens the time required
between two experiments to reduce the tracer gas concentration in the
system to a negligible level.
Two tracer gases or two successive measurements with one tracer gas allow
in most cases assessment of all primary and most secondary airflow rates:
.
.
tracer one injected in the main return air duct;
tracer two (or a second run with tracer one) injected in the outside air duct.
Additional injection ports may be useful to increase the accuracy. These are:
.
.
tracer three (or a third run with tracer one) injected in the main supply air
duct, allowing the direct and more accurate determination of supply airflow
rate;
tracer four (or a fourth run with tracer one) injected in the control room at
constant concentration to determine leakage from the control room into
the air handling unit.
The optimal tracer gas injection rate depends on the design airflow rate Qo in
the duct and on the required concentration, C, itself depending on the sensitivity of the tracer gas analyser. A good method is to adjust the tracer gas
injection flow rate on the basis of the outdoor airflow rate Q01 . If Ck is the
22
Ventilation and Airflow in Buildings
expected tracer gas concentration of tracer k:
Ik ¼ Ck Q01
ð2:5Þ
Sampling points for concentration measurements
Tracer gas concentrations are measured at several carefully chosen locations in
order to obtain enough information to determine all the wanted airflow rates. It
is important that there is a good mixing of tracer gas in the measured airflow.
For this, several criteria should be fulfilled. Practice has shown that sufficient
mixing is reached when the distance between injection ports and air sampling
location is at least:
.
.
10 diameters (or duct widths) in straight ducts;
5 diameters if there is a mixing element such as bends, droplet catcher or a
fan between injection ports and the air sampling location.
Proper mixing can be checked by looking at the variations of measured concentration with time, and when displacing the sampling location within the duct
(see Figure 2.6). If variations are large and random, change the sampling
and/or injection points, or use multiple injection ports until variations are
within the usual measurement noise.
If the minimum distances mentioned above cannot be achieved, use
multiple injection (see Figure 2.7) or install obstacles in the airflow to increase
the turbulence.
Turbulent flow may transport some tracer gas a little upwind of the
injection point. Therefore, the distance between sampling location upwind of
injection points and the injection nozzle should be at least one duct diameter
when there is no possibility of backward flow, and larger (3–5 duct diameters)
when backward airflow is suspected (for example close to T junctions).
When sampling, never use the tubes that were once used for injecting a pure
tracer gas, since some gas absorbed in the plastic of the tubes may be desorbed,
Concentration [ppm]
11.2
11.1
11.0
10.9
10.8
10.7
08:00
08:10
08:20
08:30
Time
08:40
08:50
09:00
Figure 2.6 Evolution of tracer gas concentration versus time
Note: The solid line indicates good mixing of the tracer gas; the broken line indicates
poor mixing.
Airflow Rates in Air Handling Units
23
Figure 2.7 Example of multiple injection devices
bringing additional tracer into the analyser, thus biasing the concentration
measurement. To avoid this, use different colours for injection and sampling
tubes.
Principle of the interpretation procedure
The ductwork is modelled by nodes connected by ducts. In principle, the same
equations as those used in Chapter 1, ‘Application to buildings, multi-zone’, to
assess airflow rates measurements in multi-zone buildings could be used, the
nodes being considered as zones. There is, however, an important difference
since, in many cases, the directions of inter-zone airflows are known, leading
to important simplifications in the system of equations.
In addition, tracer gas and air mass conservation equations can be written
for each node in the duct network, and this provides, in most cases, a
number of equations much larger than the number of unknown airflow rates
(see for example ‘Building the system of equations’, below). There are several
ways to use this peculiarity, which are described below for information. We
have nevertheless found that in practice the most robust system of equations
(the system that is the least sensitive to measurement uncertainties) can easily
be purpose-built for each type of air handling unit, as shown in ‘Simplest
way’, below.
Node by node method
The method presented in Chapter 1, ‘Zone by zone systems of equations’,
can also be applied to ductwork and air handling units. Airflow and tracer
24
Ventilation and Airflow in Buildings
gas conservation equations can be rearranged so as to obtain one system of
equations per node, giving all airflow rates entering in this node. At steady
state
Iik ¼
N
X
½Cjk Cik Qji
ð2:6Þ
j¼0
where:
Iik is the injection rate of tracer gas k in (or just upwind of ) node i,
Cjk is the concentration of tracer gas k in (or just downwind of ) node j,
Qji is the airflow rate from node j to node i.
‘Just upwind’ and ‘just downwind’ mean far enough from the node to ensure a
good mixing, but close enough to have no branching between the injection port
or sampling location and the node.
Each system can be rewritten in a matrix form:
~i
I~i ¼ C i Q
ð2:7Þ
where:
I~i
Ci
~i
Q
is the vector containing the tracer gas injection rate in the zone i,
is the matrix containing the concentrations differences, Cjk Cik , of tracer
k between zones j and i,
is the vector of airflow rates entering into zone i from zones j.
Airflow rates leaving the zones are determined by mass conservation equations
Qi0 ¼
N
N
X
X
½1 ij Qji ½1 ij Qij
j¼0
ð2:8Þ
j¼1
An application to a typical air handling unit is presented below.
General method for ‘black box’ air handling unit
In most cases, it is not practical to inject tracer gases and to sample the air
within the air handling unit. It is often much easier to find (or to bore) small
holes in duct walls to insert the injection and sampling tubes. Therefore, a
method for assessing airflow rates in air handling units using injection and
sampling ports located only outside the units is presented below.
Building the system of equations
The ducts, leakage and shortcut network in the air handling system seen from
outside, like a black box, are represented schematically in Figure 2.8.
Recirculation may be on purpose, or could result from leakage such as
that sometimes found in heat exchangers. It occurs anyway between nodes 6
and 2. Regarding indoor air quality, there is no difference whether the
Airflow Rates in Air Handling Units
25
2
Q60
Recirculation
AHU
6
Q61
1' Q12
1
Q04
Q46
4'
4
Q62
3'
Ventilated
space
Q40
0
1
3
AHU room
3
Figure 2.8 The simplified network representing the air handling unit and ducts
Note: Numbers in black circles represent the nodes of the network; boxes with arrows
are tracer gas injection locations; and numbered balloons are air sampling locations.
Arrows represent possible airflow rates.
Source: Awbi, 2007.
recirculated air passes through a leak between extract and supply parts of the
air handling unit or through a purpose-installed duct. Alternatively, ventilation units with heat exchangers seldom have recirculation ducts. Therefore,
the simplified network, as shown in Figure 2.8, is adapted for most
investigations.
The possible airflows are shown in Table 2.1.
Using four tracer gases as illustrated in Figure 2.8 and writing the conservation equations for them at the nodes gives the following system of equations:
Node 1, air inlet
0 ¼ ðC01 C11 ÞQ01 þ ðC61 C11 ÞQ61
ð2:9Þ
0 ¼ ðC02 C12 ÞQ01 þ ðC62 C12 ÞQ61
0 ¼ ðC03 C13 ÞQ01 þ ðC63 C13 ÞQ61
0 ¼ ðC04 C14 ÞQ01 þ ðC64 C14 ÞQ61
Table 2.1 Possible airflow rates in the network represented in Figure 2.8
0
Coming from node
0
1
2
4
6
7
Q40
Q60
Q70
1
Q01
Q61
Going into node
2
4
6
Q04
Q12
Q24
Q26
Q46
Q62
Q72
Q76
7
Q07
Q27
Q67
Note: The main airflows are in bold. The others are parasitic airflow rates that in principle should
be negligible.
26
Ventilation and Airflow in Buildings
Node 2, return
I11 ¼ ðC11 C31 ÞQ12 þ ðC61 C31 ÞQ62 þ ðC71 C31 ÞQ72
ð2:10Þ
0 ¼ ðC12 C32 ÞQ12 þ ðC62 C32 ÞQ62 þ ðC72 C32 ÞQ72
0 ¼ ðC13 C33 ÞQ12 þ ðC63 C33 ÞQ62 þ ðC73 C33 ÞQ72
0 ¼ ðC14 C34 ÞQ12 þ ðC64 C34 ÞQ62 þ ðC74 C34 ÞQ72
Node 4, vented space
0 ¼ ðC01 C41 ÞQ04 þ ðC31 C41 ÞQ24
ð2:11Þ
0 ¼ ðC02 C42 ÞQ04 þ ðC32 C42 ÞQ24
I43 ¼ ðC03 C43 ÞQ04 þ ðC32 C42 ÞQ24
0 ¼ ðC04 C44 ÞQ04 þ ðC34 C44 ÞQ24
Node 6, recirculation
0 ¼ ðC31 C61 ÞQ26 þ ðC41 C61 ÞQ46 þ ðC71 C61 ÞQ76
ð2:12Þ
I62 ¼ ðC32 C62 ÞQ26 þ ðC42 C62 ÞQ46 þ ðC72 C62 ÞQ76
0 ¼ ðC33 C63 ÞQ26 þ ðC43 C63 ÞQ46 þ ðC73 C63 ÞQ76
0 ¼ ðC34 C64 ÞQ26 þ ðC44 C64 ÞQ46 þ ðC74 C64 ÞQ76
Node 7, technical room
0 ¼ ðC01 C71 ÞQ07 þ ðC31 C71 ÞQ27 þ ðC61 C71 ÞQ67
ð2:13Þ
0 ¼ ðC02 C72 ÞQ07 þ ðC32 C72 ÞQ27 þ ðC62 C72 ÞQ67
0 ¼ ðC03 C73 ÞQ07 þ ðC33 C73 ÞQ27 þ ðC63 C73 ÞQ67
I74 ¼ ðC04 C74 ÞQ07 þ ðC34 C74 ÞQ27 þ ðC64 C74 ÞQ67
At each node, the entering mass of air equals the leaving mass. Taking account
of the possible airflows given in Table 2.1, we get, after some reorganization
and leaving main airflow rate at the left-hand side and parasitic airflow rates
at the right-hand side of each equation:
Node 0, outdoors
Node 1, inlet
Q01 Q60 ¼ Q40 Q04 þ Q70 Q07
Q01 Q12 ¼ Q61
ð2:14Þ
ð2:15Þ
Q12 Q24 þ Q62 ¼ Q26 þ Q27 Q72
ð2:16Þ
Node 4, vented space
Q24 Q46 ¼ Q40 Q04
ð2:17Þ
Node 6, recirculation
Q46 Q60 Q62 ¼ Q26 þ Q61 þ Q67 Q76 ð2:18Þ
Node 2, return
Node 7, technical room
Q07 Q70 ¼ Q72 Q27 þ Q76 Q67
ð2:19Þ
Airflow Rates in Air Handling Units
27
This system of 27 equations when combined with the system of Equation 2.9
can be solved in various ways to provide the six main airflow rates and
potentially ten parasitic flow rates. This global system of equations contains
more equations than unknowns. There are several ways to address this
situation, and we have found that some methods are better than others for
application to air handling units. Therefore we present the tested methods
below.
Least square solution
The system of equations from 2.9 to 2.19 is over-determined: there are 26
equations for calculating 16 airflow rates. In zones where I~i 6¼ 0, the system
could be solved by least square fit:
~i ¼ ½C T
Q
1 T ~
CT
i C i Ii
ð2:20Þ
T
where C is C transposed. The resulting flow vector is the one that best satisfies
the set of equations. However, the injection rate vector I~0 , back-calculated
~ and the measured conusing Equation 2.7 with the resulting flow vector Q
centration will not be equal to the actual one. This method always provides a
solution, but, depending on the condition of the system of equations, this
solution could be far from the reality.
At nodes where the tracer i is not injected, the system can only provide
linear combinations of airflow rates, as far as the determinant jC i j ¼ 0.
Eliminating some equations
Combining some of the equations of system 2.9 to 2.19 allows avoidance of the
measurement of some concentrations. A system having as many equations as
unknown airflow rates can be solved using:
~ ¼ C 1 I~
Q
ð2:21Þ
Experience showed that this interpretation method often leads to poorly conditioned systems of equations. Results are then very sensitive to slight changes of
input data.
Looking for the best conditioned system
A set of N equations (N being the number of unknown airflow rates, in this
case 16) can be selected to give the best accuracy, or the smallest sensitivity
to variations or errors of injection rates and concentrations. This set can be
theoretically selected by calculating the condition number (see Chapter 3,
‘Condition of the model matrix’) of all possible sets of equations extracted
from the full system, and taking the set with the smallest condition number.
This selection could be tedious: there are 13,037,895 sets of 16 equations that
can be extracted from the system 2.9!
28
Ventilation and Airflow in Buildings
Simplest way
A method providing all airflow rates with the simplest solutions – hence
probably the least sensitive to measurement errors – is given below.
The results are provided with their confidence intervals, calculated under
the assumption that random and independent errors affect the measurements
of tracer gas concentration and injection rates. In this case, the confidence
interval of any result, for example an airflow rate, is (see Chapter 7, ‘Error
analysis’):
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X @Q 2
ð2:22Þ
x2i
½Q Q; Q þ Q with Q ¼ TðP; 1Þ
@x
i
i
where:
TðP; 1Þ is the Student coefficient for having the actual value within the confidence interval with probability P,
xi
is for all variables (other airflow rates, tracer gas concentration and
injection rates) on which Q depends,
is for the standard deviation of the variable xi , assumed to be a
xi
random variable of mean xi .
The various airflow rates are then:
Intake airflow rate
Q12 ¼
I11
C10 1 C11
ð2:23Þ
with
Q12 ¼ TðP; 1Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2 þ I 2 ðC2 þ C2 Þ
ðC10 1 C11 Þ2 I11
11
11
10 1
ðC10 1 C11 Þ4
ð2:24Þ
Supply airflow rate
Q24 ¼
I43
C C33
ð2:25Þ
30 3
with
Q24 ¼ TðP; 1Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2 2
ðC30 3 C33 Þ2 I43
þ I43
ðC320 3 þ C33
Þ
ðC30 3 C33 Þ4
ð2:26Þ
Note that C6k ¼ C40 k , as long as there are no leakages into the air handling unit
between locations 40 and 6.
Extract airflow rate
Q46 ¼
I62
C40 2 C42
ð2:27Þ
Airflow Rates in Air Handling Units
29
with
Q46 ¼ TðP; 1Þ
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
2
ðC40 2 C42 Þ2 I52
þ I52
ðC420 2 þ C42
Þ
ðC40 2 C42 Þ4
ð2:28Þ
Outdoor airflow rate
Q01 ¼ Q12
C6k C1k
C6k C0k
ð2:29Þ
where k ¼ 1 or 2 is recommended.
Q01 ¼
TðP; 1Þ pffiffiffiffiffiffi
f01
ðC6k C0k Þ2
ð2:30Þ
where
f01 ¼ ðC6k C1k Þ2 ðC6k C0k Þ2 Q212
2
2
2
þ Q212 ½ðC6k C0k Þ2 C1k
þ ðC6k C1k Þ2 C0k
þ ðC1k C0k Þ2 C6k
External shortcut
Q61 ¼ Q12 Q01 ¼ Q12
Q61 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q212 þ Q201 ¼
C C11
1 61
C61 C01
¼ Q12
TðP; 1Þ pffiffiffiffiffiffi
f61
ðC61 C01 Þ2
C11 C01
C61 C01
ð2:31Þ
ð2:32Þ
where
f61 ¼ ðC61 C01 Þ2 ðC01 C11 Þ2 Q212
2
2
2
þ Q212 ½ðC61 C01 Þ2 C11
þ ðC11 C61 Þ2 C01
þ ðC01 C11 Þ2 C61
Recirculation flow rate
C7k C3k
C C1k
Q62 þ Q72
¼ Q12 3k
C6k C3k
C6k C3k
ð2:33Þ
Here again, it is recommended to use concentrations of tracer gas 2 (k ¼ 2).
Note that Q62 is aliased with Q72 , multiplied by a coefficient that is very
small for tracer 2, since only C62 differs significantly from zero.
Q62 ¼
TðP; 1Þ pffiffiffiffiffiffi
f62
ðC6k C3k Þ2
ð2:34Þ
where
f62 ¼ ðC6k C3k Þ2 ðC3k C1k Þ2 Q212
2
2
2
þ Q212 ½ðC6k C3k Þ2 C1k
þ ðC6k C1k Þ2 C3k
þ ðC1k C6k Þ2 C6k
30
Ventilation and Airflow in Buildings
The bias resulting from the alias with Q72 is not taken into account in the
confidence interval. If tracer 4 is used, we get:
Leakage to node 2
Q72 ¼
ðC32 C12 ÞðC64 C34 Þ ðC34 C14 ÞðC62 C32 Þ
ðC64 C34 ÞðC72 C32 Þ ðC74 C34 ÞðC62 C32 Þ
Q72 ¼
pffiffiffiffiffiffi
TðP; 1Þ
f72
ðC64 C34 ÞðC72 C32 Þ ðC74 C34 ÞðC62 C32 Þ
ð2:35Þ
ð2:36Þ
where
2
2
f72 ¼ ðC64 C34 Þ2 C12
þ ½C64 C14 þ Q72 ðC64 C74 Þ2 C32
2
þ ½C34 C14 þ Q72 ðC74 C34 Þ2 C62 þ Q272 ðC62 C34 ÞC72
2
2
þ ðC62 C32 Þ2 C14
þ ½C12 C62 þ Q272 ðC72 C62 Þ2 C34
2
2
þ ½C32 C12 Q72 ðC72 C32 Þ2 C64
þ Q272 ðC62 C32 ÞC74
The recirculation flow rate can then be determined separately:
Q62 ¼
ðC32 C12 ÞðC74 C34 Þ ðC34 C14 ÞðC72 C32 Þ
ðC74 C34 ÞðC62 C32 Þ ðC72 C32 ÞðC64 C34 Þ
Q62 ¼
pffiffiffiffiffiffi
TðP; 1Þ
f62
ðC74 C34 ÞðC62 C32 Þ ðC72 C32 ÞðC64 C34 Þ
ð2:37Þ
ð2:38Þ
where
2
2
þ ½C74 C14 þ Q62 ðC74 C64 Þ2 C32
f62 ¼ ðC74 C34 Þ2 C12
2
þ ½Q62 ðC74 C34 Þ2 C62 þ ½C14 C34 þ Q62 ðC64 C34 Þ2 C72
2
2
þ ðC62 C32 Þ2 C14
þ ½C12 C72 þ Q262 ðC62 C72 Þ2 C34
2
2
þ ½C32 þ C12 Q62 ðC62 C32 Þ2 C74
þ Q262 ðC72 C32 ÞC64
Infiltration flow rate
Q04 ¼
ðC33 C43 ÞQ24 þ I43
C43 C03
ð2:39Þ
TðP; 1Þ pffiffiffiffiffiffi
f04
ðC43 C03 Þ2
ð2:40Þ
with
Q04 ¼
where
2
f04 ¼ ðC43 C03 Þ2 ðC33 C43 Þ2 Q224 þ Q224 ðC43 C03 Þ2 C33
2
2
þ ½Q24 ðC03 C33 Þ þ I43 2 C43
þ ½Q24 ðC33 C33 Þ þ I43 2 C03
Airflow Rates in Air Handling Units
31
or
ðC3k C4k Þ
ðC C1k Þ ðC3k C4k Þ
¼ Q12 6k
ðC4k C0k Þ
ðC6k C3k Þ ðC4k C0k Þ
p
ffiffiffiffiffi
ffi
TðP; 1Þ
f04
¼
ðC4k C0k Þ2
Q04 ffi Q24
ð2:41Þ
Q04
ð2:42Þ
where
f04 ¼ ðC6k C3k Þ2 ðC3k C1k Þ2 Q212
2
2
2
þ Q212 ½ðC6k C3k Þ2 C1k
þ ðC6k C1k Þ2 C3k
þ ðC1k þ C6k Þ2 C6k
with k 6¼ 3 (recommended value: k ¼ 1).
Exfiltration flow rate
Q40 ¼ Q04 þ Q24 Q46
with
Q40 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q204 þ Q224 þ Q246
ð2:43Þ
ð2:44Þ
Inverse recirculation airflow rate through the air handling unit is:
Q26 þ ½Q27 Q72 ¼ Q62 Q24 þ Q12
with
Q26 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q262 þ Q224 þ Q212
ð2:45Þ
ð2:46Þ
Exhaust airflow rate
Q60 þ ½Q70 Q07 ¼ þQ04 Q40 þ Q01
with
Q60 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q204 þ Q240 þ Q201
ð2:47Þ
ð2:48Þ
If there is no exhaust duct (supply only systems), Q60 is zero, and Equation 2.47
can be used to get either an estimate of the net leakage rate between the technical room and outdoor environment, [Q70 Q07 ], or, if this is zero, to calculate
Q40 , or the net infiltration rate.
If Q60 ¼ 0 (supply only system):
Q04 Q40 þ ½Q70 Q07 ¼ Q01
ð2:49Þ
Leakage from technical room to node 6 can be assessed by:
Q76 ¼
ðC32 C62 ÞQ26 þ ðC42 C62 ÞQ46 þ I62
C72 C62
ð2:50Þ
ðC3k C6k ÞQ26 þ ðC4k C6k ÞQ46
C7k C6k
ð2:51Þ
or by
Q76 ¼
32
Ventilation and Airflow in Buildings
with k 6¼ 2 (k ¼ 4 is not recommended here).
Q76 ¼
TðP; 1Þ pffiffiffiffiffiffi
f76
C7k C6k
ð2:52Þ
where
2
2
2
f76 ¼ Q226 C3k
þ Q246 C4k
þ ðQ26 þ Q46 Q76 Þ2 C6k
2
þ ðC3k C6k Þ2 Q226 þ ðC4k C6k Þ2 Q246 þ ðk; 2Þ I6k
where: k ¼ 1, 2 or 3; and the delta function ðk; 2Þ ¼ 1 if k ¼ 2 and 0 if k 6¼ 2.
Leakage airflow rates to the technical room can be obtained from system and
equation (2.19).
Less than four tracer gases
If the tracer gas is injected at only one or two locations, the corresponding
equations should be deleted. In this case, some of the airflow rates cannot be
determined.
If only one tracer gas is injected at location 1, merely Q01 , Q12 and Q62 can
be measured. It is nevertheless interesting to assess three airflow rates in one
shot and with one tracer gas only!
When a tracer is injected just at location 2, Q45 and Q62 only can be
measured.
If tracer 4 is not used, leakage from the technical room into the unit cannot
be measured. Large leakage can nevertheless be detected from unexpected
dilution of the other tracer gases in the unit.
If tracer 3 is not used, Q24 , aliased with several leakage flow rates (its value
includes a linear combination of parasitic airflow rates), can nevertheless be
calculated from:
C C3k C7k C3k
Q24 þ Q26 þ Q27 Q72 1 þ 7k
C6k C3k C6k C3k
¼ Q12 þ Q62 ¼ Q12
C6k C1k
C6k C3k
ð2:53Þ
TðP; 1Þ pffiffiffiffiffiffi
f24
ðC6k C3k Þ2
ð2:54Þ
with
Q24 ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q212 þ Q262 ¼
where
f24 ¼ ðC6k C1k Þ2 ðC6k C3k Þ2 Q212
2
2
2
þ Q212 ½ðC6k C3k Þ2 C1k
þ ðC6k C1k Þ2 C3k
þ ðC1k C3k Þ2 C6k
In this equation, k ¼ 2 is recommended.
Airflow Rates in Air Handling Units
33
Planning tool
There are many types of air handling units, and, from our experience, each new
measurement poses new problems. It is hence impossible to provide a detailed
measurement protocol valid for all types. Therefore, a computer program was
developed that performs the following tasks:
.
.
.
.
.
Requests input data:
– characterization of the air handling unit (type, location, design airflow
rates, heat exchanger, position of fans with respect to heat exchanger,
etc.);
– tracer gas(es) used, injection location(s) and design concentration(s);
– characterization of building (approximate volume, number of occupants,
overpressurized or not, etc.);
– airflows that obviously cannot exist.
Evaluates the risk of poor tracer gas mixing from the distance between
injection and sampling locations and from the devices (fans, bends, filters,
dampers) placed in between.
Prepares a printed measurement protocol containing injection and sampling
locations, injection rates of tracer gases and a system of equation in
accordance with ‘Simplest way’ (above).
Requests measured tracer gas concentrations and actual injection flow rates
or reads them in a file.
Solves the system of equations and prepares a measurement report.
This piece of software is available on www.e4tech.com, in ‘Software and
Publications’.
Example of application
Sulphur hexafluoride was injected as tracer 1 and nitrous oxide as tracer 2 in
an air handling unit without planned recirculation, but equipped with a
rotating heat exchanger. Resulting concentrations are shown in Figure 2.10,
and measurement results in Figure 2.9.
440
Relief
Exhaust
3700
9400
Outside
8000
1000
11,000
Supply 6700
Figure 2.9 Measured airflow rates in a leaky air handling unit
Note: Design airflow rates were 13,000 m/h for both supply and return, and zero for
recirculation.
Source: Awbi, 2007.
34
Ventilation and Airflow in Buildings
100
N2O [ppm]
80
C3: Supply air
C4: Room air
C4¢: Exhaust
60
40
C6: Relief air
20
0
10:30
10:45
11:00
Time [h]
11:15
11:30
Figure 2.10 Concentrations at locations shown in Figure 2.5 resulting from
injection of SF6 as tracer 1 and N2 O as tracer 2 in a leaky air handling unit
Note: A shortcut through the heat exchanger dilutes exhaust air, thus decreasing the
relief air concentration. The presence of this tracer gas in supply air results from parasitic recirculation.
Source: Awbi, 2007.
Leaks in the heat exchanger, as well as in the return air channel, were
detected with this measurement. Measurement in three other identical units
in the same office did not show any shortcut. However, measured outdoor
airflow rates were between 55 and 66 per cent of the design value.
Simple measurement using CO2 from occupants
A special case is when only one tracer is injected in the ventilated space. This
could be the carbon dioxide emitted in the ventilated space by occupants.
That tracer gas is of great practical interest since it does not need any injection
system. In this case, Equation 2.33 can easily be solved. Assuming that there
is no inverse recirculation, and no leaks in the air handling unit, the global
recirculation rate is:
R¼
Q62
C C1k
¼ 3k
Q12 þ Q62 C4k C1k
ð2:55Þ
with
R ¼
TðP; 1Þ pffiffiffiffiffi
fR
ðC4k C3k Þ2
ð2:56Þ
where
2
2
2
fR ¼ ðC3k C4k Þ2 C1k
þ ðC4k C1k Þ2 C3k
þ ðC3k C1k Þ2 C4k
And the equivalent outdoor airflow rate per occupant is:
Q01 þ Q04 0:018½m3 =h
¼
Npersons
C4k C0k
ð2:57Þ
assuming that a person exhales 18 l/h of carbon dioxide and that occupants
are the only indoor sources of CO2 . Airflow rates are in m3 /(h person) if
Airflow Rates in Air Handling Units
35
concentrations are in volumetric ratios. It is not possible with only one tracer
injected into the ventilated space to differentiate between outdoor air from
mechanical ventilation and from infiltration.
Measurements in buildings with large time constants
Most methods are designed to measure units with recirculation ratios below 50
per cent. This is the case of the method proposed above. However, air handling
units designed to condition (heat or cool) spaces with large loads such as those
found in cold or tropical climates often present large recirculation ratios that
homogenize the concentrations, and large nominal time constants (ratio of
the ventilated volume to the outdoor airflow rate) that strongly increase the
time needed to get steady state in the supply duct (node 3) and in the room
(node 4). There is, however, a way to shorten the measurement time by
extrapolating the evolution of tracer gas concentration with time (Roulet and
Zuraimi, 2003), which is described below.
Writing the conservation equation of tracer gas 3 at node 4, in the ventilated
space, gives:
V
@C43
¼ I3 þ Q24 C23 þ Q04 C03 ðQ46 þ Q40 ÞC43
@t
ð2:58Þ
Because of the large recirculation ratio, it can be assumed that the concentration
is homogeneous in the ventilated space. Dividing this equation by the supply
airflow rate Q24 gives:
V @C43
I
¼ 3 þ C23 þ i C03 ð1 þ i ÞC43
Q24
Q24 @t
ð2:59Þ
where i is the infiltration ratio Q04 =Q24 . Using the definition of the nominal
time constant n , of the recirculation ratio R, and using the tracer gas conservation at node 2:
V
V Q01
¼
¼ n ð1 RÞ
Q24 Q01 Q24
and
C23 ¼ Rxs C43 þ ð1 RÞC03
ð2:60Þ
we get
n ð1 RÞ
@C43 m_ t3
¼
ð1 R þ i ÞðC43 C03 Þ
@t
m_ 24
ð2:61Þ
The steady-state concentration is:
1
¼
C43
I3
þ C03
Q24 ð1 R þ i Þ
and
1
C43 ðtÞ ¼ C43
ð1 et= Þ
ð2:62Þ
16
40
35
14
C3
30
12
C3¢
25
10
C3¢–C3
20
8
6
15
4
10
Tracer injection
5
0
0
2
4
6
2
8
10
Time (h)
12
14
16
0
Concentration difference [mg/m3]
Ventilation and Airflow in Buildings
Tracer gas concentration (mg/m3)
36
Figure 2.11 Tracer gas concentrations in the supply duct, upstream (3) and
downstream (30 ) of the tracer gas injection port
Note: Points are measured concentrations, while lines are exponential fits.
Source: Roulet and Zuraimi, 2003.
with
¼
n ð1 RÞ
1 R þ i
ð2:63Þ
The theoretical exponential can be fitted to the experimental points, as shown
in Figure 2.11 depicting an actual experiment. The concentration increase in
the supply duct, C30 C3 , quickly reaches its steady-state value, while a good
fit of the exponential can be obtained within two time constants, allowing the
determination of the steady-state concentration and time constant without
waiting for equilibrium that is reached after at least five time constants.
Appropriate method for assessing the
recirculation ratio
In Equations 2.33, 2.34 and 2.41 the concentration difference C6k C3k is at the
denominator, and these two concentrations are close to each other at steady
state when the recirculation ratio is high. This leads to a large confidence
interval of the calculated airflow rates. In units with large recirculation
ratios, it is better to inject the tracer gas at location 3 instead of location 2.
The supply airflow rate can then be determined with better accuracy, using:
Q24 ¼
I3
C30 3 C33
ð2:64Þ
Assuming that the confidence interval is the same for both concentrations, the
confidence interval is:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
Q
I
C
ffi TðP; 1Þ
þ2
ð2:65Þ
Q
I
C0 C
Airflow Rates in Air Handling Units
37
The recirculation airflow rate can then be calculated using:
Q62 ¼ Q24 Q12
ð2:66Þ
with
Q62 ¼ TðP; 1Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Q224 þ Q212 ffi TðP; 1Þ 1 þ ð1 RÞ2 Q
ð2:67Þ
assuming that the relative error Q=Q is the same for both airflow rates, and
taking into account that Q12 ¼ ð1 RÞQ24 . Note that, in this case, Q62
decreases when R increases.
The extract airflow rate Q46 cannot be assessed without injecting a tracer
gas in the extract duct. However, in air handling units that have no exhaust
duct (such as most units in Singapore and other tropical countries), Q60 ¼ 0,
hence Q46 ¼ Q62 , and Q40 ¼ Q01 þ Q04 .
The recirculation ratio is defined by:
R¼
Q62
Q62
¼
Q24 Q62 þ Q12
ð2:68Þ
Assuming that there is no leak in the air handling unit, it can be assessed using
three different methods:
Method A
R¼
C3k C10 k
C6k C10 k
ð2:69Þ
the subscript k being for any tracer gas except the one injected in the inlet duct.
The confidence interval is:
R ¼
TðP; 1Þ pffiffiffiffiffi
fR
ðC6k C10 k Þ2
ð2:70Þ
where
2
2
þ ðC3k C10 k Þ2 C6k
fR ¼ ðC3k C6k Þ2 C120 k þ ðC6k C10 k Þ2 C3k
ð2:71Þ
If we assume that the relative error is the same for all concentrations, and taking
into account that, for tracers injected at locations 2 and 3, C10 k ffi 0 and therefore
C3k ffi RC6k , we can get a simpler expression for the confidence interval of the
recirculation ratio:
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
TðP; 1ÞC
R ffi
ð2:72Þ
2ðR2 R þ 1Þ
C
The recirculation ratio can also be calculated using:
Method B
R¼
Q62
Q24
Ventilation and Airflow in Buildings
Confidence interval of R
38
15%
A
10%
B
C
5%
0%
0%
20%
40%
60%
80%
Recirculation ratio
100%
Figure 2.12 Confidence interval of the recirculation ratio as a function of the
recirculation ratio itself for three assessment methods
Note: For this figure, the relative confidence intervals of injection rate and concentrations are 5 per cent.
with
R ¼
Q pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ð1 þ RÞ
Q
ð2:73Þ
Or method C
R¼1
Q12
Q24
with
R ¼
pffiffiffi Q
2
ð1 RÞ
Q
ð2:74Þ
assuming that the relative error Q=Q is the same for both airflow rates, and
taking into account that Q12 ¼ ð1 RÞQ24 .
The accuracy of the three methods for determining R is compared in
Figure 2.12. Method B (Equation 2.73) should be preferred at low recirculation
ratios, while method C (Equation 2.74) is best at large recirculation ratios.
Method A could be applied where the other methods cannot be used. Note
that the relative error R=R becomes very large for small recirculation ratios.
It is interesting to see that some interpretation methods of the same
measurements provide more accurate results than others, and that the best
way depends on the unit measured. Therefore, care should be taken to select
the most appropriate method.
3
Age of Air and
Ventilation Efficiency
The airflow patterns should, in principle, be organized in order that new air
is brought to the head of the occupants, so that they get fresh, clean air, and
that contaminants be evacuated as quickly as possible, before being mixed
with indoor air. However, air, as any other fluid, always follows the easiest
path. This means that the airflow does not necessarily follow expected
patterns. Since air is transparent, unexpected airflow patterns are noticed
only when things go wrong. Depending on the airflow distribution in rooms
and for a given airflow rate, the concentration of contaminants in the occupied space may vary by a factor of two or more. Therefore, measurements
may be useful to check if the airflow patterns are as expected. Such measurements allow checks to ensure that:
.
.
.
the air change efficiency is as large as possible,
clean air is supplied to the right places,
air contaminants are quickly removed.
The measurement of the age of the air allows for the detection of possible
shortcuts and dead zones and for the checking of the general airflow pattern
in the room or in a building.
Definitions
The quantities defined below are explained in greater detail elsewhere
(Sandberg, 1984; Sutcliffe, 1990) and are only briefly described here.
Age of the air
Let us assume that the molecules of outdoor air start their indoor life when
entering the building or the ventilation system. These arrive at a given location, r, in a room after a time, r , that varies from one particle to the other. r
is called the residence time of the particle in the room, or its age. Note that
the air elements themselves, i.e. oxygen and nitrogen molecules, do not age.
40
Ventilation and Airflow in Buildings
However, the more time a small volume of air spends in a room, the more it
will be contaminated by pollutants.
Since there is a large number of air particles all taking different paths, we
define a probability density f(r ) that the age of particles arriving at a given
location is between and þ d and, a probability F(r ) that this age is
larger than . These two functions are, by definition, related by:
ð
dFðr Þ
ð3:1Þ
and
Fðr Þ ¼ 1 fðtr Þ dt
fðr Þ ¼ d
0
The local mean age of air at a point r is defined by the average age of all the
air particles arriving at that point:
ð1
ð1
r ¼
tfr ðtÞ dt ¼
Fr ðtÞ dt
ð3:2Þ
0
0
The room mean age of air hi is defined by the average of the ages of all the
air particles in the room.
Nominal time constant
The nominal time constant, n , of a ventilated zone, is the ratio of its volume,
V, to the supplied fresh volume airflow rate, q, (including infiltration), or
the ratio of the mass of air contained in the space, M, to the mass airflow
rate, Q:
V M
ð3:3Þ
n ¼ ¼
q
Q
Its inverse is the specific airflow rate or air change rate, n.
If the room or ventilated zone has a defined air exhaust, Sandberg (1984)
has shown that the nominal time constant is equal to the mean age of air at
this exhaust:
n ¼ e
ð3:4Þ
Air exchange efficiency
This efficiency expresses how the fresh air is distributed in the room. The
time, a , required on average to replace the air present in the space is twice
the room mean age of air (Sandberg and Sjo¨berg, 1983):
a ¼ 2hi
ð3:5Þ
At a given flow rate and zone volume, the shortest time required to replace
the air within the space is given by the nominal time constant. Therefore, the
air exchange efficiency, a , is calculated by:
ð3:6Þ
a ¼ n
2hi
The air exchange efficiency is equal to one for piston-type ventilation, where
the exhaust is reached at a time corresponding exactly to the nominal time
Age of Air and Ventilation Efficiency
41
Figure 3.1 Ventilation modes with typical airflow patterns and air
change efficiencies
Source: Roulet, 2004.
Table 3.1 Nominal time constant and room mean age of the air corresponding
to the probability curves shown in Figures 3.2 and 3.3
a
hi
Air exchange efficiency
Room mean age of the air
25%
1.44
50%
0.99
66%
0.76
80%
0.62
90%
0.55
99%
0.50
constant. In rooms with complete mixing, the room mean age of air equals
the nominal time constant, and the air exchange efficiency is 50 per cent.
Short-circuiting of air will also leave dead zones in the room, giving rise to
an efficiency that is lower than 0.5 (see Figure 3.1).
Table 3.1 shows the air exchange efficiency and the corresponding room
mean age of the air, assuming that the nominal time constant or the mean age
at exhaust is one hour.
Some typical probability density curves of the age of the air at the exhaust
are illustrated in Figure 3.2, the air exchange efficiency being used as
parameter. If the air is displaced like a piston, all air particles reach a given
location in the room at the same time, and they reach the exhaust at a time
f(τ) 4.0
η
3.5
25%
50%
66%
80%
90%
99%
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
0.5
1
1.5
2
2.5
τ
3
Figure 3.2 Typical probability density curves for the age of the air
Note: The parameter is the air exchange efficiency.
42
Ventilation and Airflow in Buildings
F(τ)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
η
25%
50%
66%
80%
90%
99%
100%
0
0.5
1
1.5
2
2.5
τ
3
Figure 3.3 Typical probability curves for the age of the air
Note: These theoretical curves are for illustration. Some of them, in particular at very
high efficiency, are not likely to be found in practice.
corresponding to the nominal time constant. The air exchange efficiency in
this very theoretical case is 100 per cent. At 99 per cent air exchange efficiency, there is already some spreading of the ages around an average still
equal to the nominal time constant. When probability density function
spreads out and the probability function smoothens, the mean age at exhaust
remains the same, but there are more young air particles and more aged ones.
In addition, the most probable age (the time of the maximum of the curve) is
reduced. At 66 per cent air change efficiency, this most probable age is
already half the nominal time constant. At 50 per cent efficiency or complete
mixing, the probability density of the age of the air at exhaust is an exponential: there are more young particles than old ones reaching the exhaust. With
this distribution, the most probable age is zero, but the mean age is still
equal to the nominal time constant. The last curve, with 25 per cent efficiency, represents a situation with a shortcut.
Figure 3.3 shows the corresponding probability curves for the age of the
air at exhaust, still with one hour as nominal time constant. This function
shows the percentage of particles at exhaust that are older than the value
given on the ordinate. At 100 per cent efficiency, all particles are exactly one
hour (one time constant) old. When the efficiency decreases, the function
progressively changes to become an exponential at 50 per cent efficiency or
complete mixing. When the efficiency decreases, there are less and less young
particles and more and more old ones at exhaust. At 25 per cent efficiency,
which is very poor, 10 per cent of the volume of the air at exhaust is older
than three time constants!
Measurement method
The air entering the room is marked with a gas (the tracer gas), and the
concentration of that tracer gas is monitored at the location of interest. This
Age of Air and Ventilation Efficiency
43
assumes that the tracer gas behaves the same as the air: no adsorption and
same buoyancy, which is the case if the tracer concentration is small. It can
be readily understood that if the air is marked at the inlet by a short pulse of
tracer gas, and if the tracer molecules follow the air molecules, they will
arrive at a given location at the same time as the air molecules. The time
spent between injection and the detection of most tracer gas molecules by the
analyser is the age of the tracer, hence the age of the air at the air sampling
location. The pulse technique is, however, not the only one and the following
three strategies can be used:
.
.
.
Step-down – a uniform concentration of tracer is achieved at the beginning
of the test, when the injection is stopped;
Step-up – the tracer is injected at the air inlet at a constant rate from the
starting time throughout the test;
Pulse – a short pulse of tracer is released in the air inlet at the starting
time.
The probability functions and the local mean ages at any point, r, can be
calculated from the time history of the net tracer concentration, Cr (t), which
is the measured concentration minus the background concentration. It was
shown, however, that for rooms with a single air inlet and a single air outlet,
the step-up method should be preferred, since it is the easiest to perform and
gives the best accuracy (Roulet and Cretton, 1992).
For the step-up technique, the tracer gas is injected at a constant rate into
the supply air in the outside air duct, starting at a known time t0 . It should
be ensured that the tracer and the air are fully mixed in the supply duct to
produce a steady concentration, C3 , at the air inlet. If C3 cannot be measured, the equilibrium concentration within the enclosure, C4 , may be used
instead. The notations for air sampling refer to Figure 2.5.
Tracer gas concentration at the locations where the age of air is looked for
is recorded. The sampling time interval should be short enough to record the
transient evolution of the concentration. It should then be much shorter than
the expected age of air. A good value is one tenth of the nominal time
constant, which can be estimated by dividing the ventilated volume by the
design airflow rate, or better, by the actual airflow rate if it is known.
One important location is in the exhaust duct, where Ce ¼ C6 is measured. Recording the evolution of the tracer gas concentration at this location
provides both the actual nominal time constant and the mean age of air in the
ventilated space. Injection rate is maintained constant until a steady state is
obtained. Depending on the air change efficiency, this may take up to four
time constants. An example of such a record is given in Figure 3.4.
When the concentration stabilizes at a value noted, C1 , the step-up
experiment is ended. However, it is recommended to continue recording the
concentration after having stopped the tracer gas injection, since this will
provide a second measurement of the age of the air, using the decay method.
For this purpose, the time when injection is closed should be noted, since
this time is the starting time of the decay experiment.
44
Ventilation and Airflow in Buildings
Concentration [ppm]
25
C∞
20
Step-up
15
Decay
10
5
to
Injection
0
09:00
09:10
09:20
09:30
to
09:40
09:50
10:00
10:10
10:20
Figure 3.4 Record of tracer gas concentration in the exhaust duct during the
measurement of the age of air
To interpret the recorded tracer gas concentrations and obtain the age of
air, the background (or supply) concentration should first be subtracted from
all measurements, and the elapsed time should be calculated by subtracting
the starting time from all time values. In the following formulae, the net
tracer gas concentration, Cr , is the difference between the concentration
measured at location r and the concentration of this gas in the outdoor air.
The probability function of the age of air can be calculated from the
concentration ratio:
Step-up FðÞ ¼ 1 Cðt t0 Þ
C1
Decay FðÞ ¼
Cðt t0 Þ
Cðt0 Þ
ð3:7Þ
Figure 3.5 shows the concentration ratio calculated from the recorded
concentration illustrated in Figure 3.4.
1.0
F(t)
0.0
Step-up
0.8
-0.2
Decay
-0.4
log[F(t)]
0.6
0.4
-0.6
-0.8
-1.0
Step-up
0.2
-1.2
Time [s]
0.0
0
200
400
600
800
1000
-1.4
Decay
0
200
400
Time [s]
600
800
1000
Figure 3.5 Probability functions of the age of air, calculated from the recorded
concentration illustrated in Figure 3.4
Note: Left is a linear scale and right is a logarithmic scale, showing an exponential decay
after 700 s.
Age of Air and Ventilation Efficiency
45
The local mean age of air at any location is the integral (or zero moment)
of the probability distribution:
ð1
Fr ðtÞ dt
ð3:8Þ
r ¼ 0 ¼
0
The first moment of the probability distribution is, by definition:
ð1
1 ¼
tFr ðtÞ dt
ð3:9Þ
0
If there is only one single exhaust, the room mean age of air can be deduced
from tracer concentration measurements in the exhaust duct, Ce ðtÞ:
ð1
tFe ðtÞ dt
1
hi ¼
¼ ð01
ð3:10Þ
0 e
Fe ðtÞ dt
0
In this case, the nominal time constant of the ventilated space, n , which is
the ratio of the space volume and the volumetric airflow rate, is equal to the
mean age of air at the exhaust:
ð1
Fe ðtÞ dt
ð3:11Þ
n ¼ e ¼
0
Therefore, the air exchange efficiency, a , can be assessed directly by
measuring the evolution of the concentration at the exhaust:
2 0
a ¼ n ¼ e ¼
ð3:12Þ
2hi 2hi
21 e
Practical interpretation of the concentration records
In practice, the moments in the above formulae are calculated numerically,
on the basis of discrete recorded values of the concentration and time. A
simple way to calculate these moments uses the trapezium integration
method, with the general formulation of:
ð tN
N
1
X
1
fðtÞ dt ffi
ð3:13Þ
ð f þ fj þ 1 Þt
2 j
0
j¼0
where fj is for fðtj Þ and t for tj þ 1 tj .
Approximating the variation of the concentration during each time step
by a straight line, the two integrals defined above can be estimated by
summing finite elements:
ð1
1
F0 þ FN NX
þ
Fe ðtÞ dt ¼
Fj t þ "0 ðN; d Þ
ð3:14Þ
0 ¼
2
0
j¼1
1 ¼
ð1
0
tFe ðtÞ dt ¼
1
NFN NX
þ
jFj t2 þ "1 ðN; d Þ
2
j¼1
ð3:15Þ
46
Ventilation and Airflow in Buildings
where:
Fj
is the probability distribution at time t ¼ j t,
Step-up case Fj ¼ 1 Cðt0 þ jtÞ
Cð1Þ
Decay case Fj ¼
Cðt0 þ j tÞ
Cðt0 Þ
ð3:16Þ
N
is the last measurement integrated using the trapezium method,
"n ðN; d ) is the rest of the integral, evaluated using an exponential fit on the last
measurements (see below).
The number of measurements, N, could be large enough to ensure that the
sum of the terms for j > N are negligible, or, in other words, that CN is very
close to the steady-state value. In this case, the remaining parts, "n ðN; d ),
are negligible. In practice, however, the measurement can be stopped before
reaching the steady state. In this case, the tail in the integral of the moments
is not measured, but is estimated. As shown in Figure 3.5, this tail is, in
most cases, exponential of the form:
Step-up:
CðtÞ ¼ C1 ð1 et=c Þ
Decay:
CðtÞ ¼ Cð0Þet=c
Therefore, for time larger than tN ¼ N t, it can be assumed that:
t t
Fðt > tN Þ ¼ FN exp N
d
ð3:17Þ
ð3:18Þ
where d is a time constant determined by a fit on the last measurements in
the exponential part. In this case, the remaining part, "n ðN; d Þ, of the
moments are:
ð1
ð1
t t
dt ¼ FN d
"0 ðN; d Þ ¼
Fe ðtÞ dt ¼ FN
exp N
d
tN
tN
ð1
ð3:19Þ
"1 ðN; d Þ ¼
tFe ðtÞ dt ¼ FN d ðtN þ d Þ
tN
The time required for reaching an exponential decay depends not only on the
nominal time constant of the room, but also on the ventilation system. The
decay will be exponential from the beginning of the test where complete
mixing occurs. In case of displacement ventilation, the decay should be very
sharp after a time equal to the age of air.
Error analysis
Using, mutatis mutandis Equation 2.22, the confidence interval of Fð) is:
bFj Fj ; Fj þ Fj c
Age of Air and Ventilation Efficiency
47
with
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
Ce ð jt þ t0 Þ 2
Ce ð jt þ t0 Þ
Ce ð1Þ
Fj ¼ TðP; 1Þ
þ
Ce ð jt þ t0 Þ
Ce2 ð1Þ
Then, the confidence intervals of the moments are:
qffiffiffiffiffiffi
qffiffiffiffiffiffi
and
1 ¼ TðP; 1Þ f1
0 ¼ TðP; 1Þ f0
ð3:20Þ
ð3:21Þ
with
f 0 ¼
t
2
þ
2
ððF0 Þ2 þ ðFN Þ2 Þ þ ðtÞ2
N
1
X
Fj2
j¼1
F0 þ FN
þ
2
N
1
X
2
Fj
ðtÞ2 þ ð"0 Þ2
ð3:22Þ
j¼1
and
f 1 ¼
t
2
þ
2
ððF0 Þ2 þ ðFN Þ2 Þ þ ðtÞ2
N
1
X
j2 Fj2
j¼1
1
NFN NX
þ
jFj
2
j¼1
2
ðtÞ2 þ ð"1 Þ2
ð3:23Þ
in which
"0 ¼ TðP; 1Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
d2 FN
þ FN
ðd Þ2
ð3:24Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
2
ðtN þ d Þ2 d2 FN
þ ðtN þ 2d Þ2 FN
ðd Þ2
ð3:25Þ
and
"1 ¼ TðP; 1Þ
where d is the confidence interval resulting from the exponential fit.
Finally we get:
n ¼ e ¼ 0 ðFe Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
hi ¼
ð1 ðFe ÞÞ2 ð0 ðFe ÞÞ2 þ ð1 ðFe ÞÞ2
ð0 ðFe ÞÞ2
ð3:26Þ
ð3:27Þ
and
a ¼
1 ðFe Þ
ð20 ðFe ÞÞ2
ð3:28Þ
Example of application
The ventilation system of a 60-seat conference room was retrofitted to
improve indoor air quality. The old, mixing-type installation was replaced by
48
Ventilation and Airflow in Buildings
Air inlets
Exhaust grilles
Conference
room
Hall
Office
Figure 3.6 Arrangement of the conference room and of its surroundings
Source: Roulet et al., 1998.
a displacement ventilation system, as shown in Figure 3.6. The conference
room is 8 m by 10 m wide and 3 m high. It is completely embedded in an old,
massive building. Its walls, floor or ceiling have no contact with the outdoor
environment. It has no windows, but leaky entrance doors leading to a hall.
The mechanical displacement ventilation system includes two low-velocity
air inlets, 1 m high, put at the floor level against one wall of the room. The
air, slightly colder than the room temperature, is introduced at low speed
close to the ground through three inlets. This cold air spreads on the ground
like a lake, and should go up faster where there are heat sources like occupants. This asymmetric disposition does not allow a uniform distribution of
fresh air in the room, but was first adopted for practical reasons, the building
layout not allowing an optimum location of air inlets and outlets. Five
exhaust grilles are located in the ceiling. Since the building owner was
interested in assessing the actual performance of the new system, age of air
measurements were performed in order to check if the airflow pattern in the
room was as expected.
First, age of air measurements were performed in the room as it was. It
was found that the air was poorly distributed in the room because of the
asymmetrical disposition of inlet grilles. In addition the doors were found to
be leaky, and much air from the ventilation system was leaving the room
quickly after entering it, thus reducing the purging effect. The indoor air
quality was, however, good, since the ventilation rate was very high (eight
minutes nominal time constant, or 15 outdoor air change per hour, or 60 m3 /h
per occupant at full occupancy!).
On the basis of these results, improvements were brought to the system.
The leakages in the room envelope were sealed, a door was added, a new
inlet was added on the wall opposite to the existing inlets, and an exhaust
grille was added in the ceiling as shown in Figure 3.7.
A second measurement campaign was performed, showing a significant
improvement of the ventilation efficiency (see Figure 3.8). The air change
Age of Air and Ventilation Efficiency
49
Air inlets
Added air inlet
Exhaust grilles
Added exhaust grille
Added door
Conference
room
Hall
Office
Figure 3.7 Arrangement of the conference room after improvement
Source: Roulet et al., 1998.
100%
1000
800
700
Mean age of air
Nominal time constant
Air change efficiency
90%
80%
70%
600
60%
500
50%
400
40%
300
30%
200
20%
100
10%
Air change efficiency
Age and time constant [s]
900
0%
0
Initial values
After improvement
Figure 3.8 Room ventilation characteristics before and after improvement
Source: Roulet et al., 1998.
efficiency was doubled, and the mean age of air was maintained despite a
reduction of the ventilation rate – and of energy use – by a factor two.
Mapping the age of the air in rooms
This chapter demonstrates how to assess the age of air at some location and
on the average in a room. It may nevertheless be interesting to map this
quantity in a room, in order to check, for example, if the occupants have
the best possible air, or to look for dead zones. Davidson and Olsson (1987)
have already generated such maps using computer codes, and some
qualitative representations have been drawn from measurements (Valton,
1989). Since the measurement of the age of the air at a given location is not
50
Ventilation and Airflow in Buildings
straightforward, takes time and has its cost, the theory of experimental
design (Box et al., 1978) may help in providing a maximum of information
through a minimum of measurements.
Minimum number of measurements
A map of any scalar variable, v, in a three-dimensional room is in principle
obtained by measuring the variable at each node of a network and interpolating between these nodes. Such measurements are, however, very expensive
and may be unfeasible. If only five values are taken on each axis, at least 125
measurements are required, meaning analysis of the air every few minutes at
125 locations over a couple of hours. Therefore, it makes sense to apply a
method that needs a minimum number of measurements points. This
minimum number depends on the objective of the mapping experiment, or
more precisely on the required mapping details. Since the interpolation
between measurement points needs a model, the mapping network
indeed depends on the empirical model chosen to represent the map of the
variable, v.
Any infinitely derivable function (as v is assumed to be) can be developed
in a Taylor series around a given point. This gives a polynomial, which can
be approximated by its k þ 1 first terms, k being the degree of the polynomial. In the following, models of degree 1 and 2 are considered. If a linear
model is adopted (degree 1), such as:
X
bi xi
ð3:29Þ
v¼aþ
i
where xi are the three coordinates of the measured point, only four measurements are needed to obtain a set of coefficients (a; bi ). If more measurements
are made, the coefficients may be obtained by a least square fit procedure
provided there is no (or negligible) uncertainty on the coordinates. If their
coordinates differ for the other points, these supplementary measurement
points give information on the validity of the model used.
If the linear model does not appear to be valid, higher degree models may
be used. For example, a quadratic model:
X
X
X
bi xi þ
bij xi xj þ
bii x2i
ð3:30Þ
v¼aþ
i
i 6¼ j
i
that contains ten coefficients, can be chosen. Such a model may already fit
many practical situations and present minimal and maximal value(s). To
determine its coefficients, measurements taken at ten locations are the
minimum necessary.
An intermediate model is the interactions model:
X
X
bi xi þ
bij xi xj
ð3:31Þ
v¼aþ
i
i 6¼ j
Age of Air and Ventilation Efficiency
51
Table 3.2 Minimum number of measurements needed to obtain the coefficients of
a kth degree polynomial empirical model representing a variable in a two- and
three-dimensional space
Model
dimensions
2
3
Linear
Interaction
Quadratic
Cubic
4th degree
3
4
4
7
6
10
10
20
15
35
for which seven coefficients must be determined. Table 3.2 summarizes the
minimum number of measurements needed.
Location of the measurement points
An important issue is the appropriate location of measurement points. The
set of measurement points is called an experimental design. There are many
possible experimental designs, but they do not give the expected results
with the same accuracy. For example, it is obvious that to fit a linear
model in one dimension only (the straight line modelled by y ¼ ax þ b), the
location of the two measurement points (the minimum number) that gives
the best accuracy for the coefficients a and b is at the extremities of the
experimental domain, i.e. at the minimum and maximum possible values of
the variable, x.
For more sophisticated models or in a larger number of dimensions, the
locations of the best sampling points are not so obvious. However, several
tools exist for planning such experiments, which are found in the literature
(Fedorov, 1972; Box et al., 1978; Bandemer and Bellmann, 1979; Feneuille
et al., 1983; Aeschlimann et al., 1986) and are applied below.
In experiments to determine the age of the air, points close to walls do not
represent the inner volume, and the sampling points should not be located too
close to walls or in the corners of the room. In the following, the ‘room’ or the
‘experimental domain’ is a volume that is smaller than the actual measured
space by about 20 per cent in each direction.
Let us take a coordinate system in such a rectangular volume using as the
unit, for each direction, the half-length of that domain in that direction.
Three numbers, included in the interval [1; þ1], locate any point in the
‘room’.
The experimental design can be represented by a rectangular matrix with
three columns (one for each coordinate) and as many lines as measurement
points. A general condition is that in order to obtain the coefficients of a
polynomial of degree k, each of the variables x, y and z shall take at least
k þ 1 values in the experimental design, which should have at least k þ 1
levels on each axis.
52
Ventilation and Airflow in Buildings
The criteria described below are used to establish the most efficient
design.
Criteria for location of the measurement points
The model matrix M
First, let us look at the method used to obtain these coefficients. For each
point, the model is applied, replacing the xi by their values given by the
experimental design. A system of equations (one equation for each location)
is obtained this way, which can be written in a matrix notation:
V ¼ MA
ð3:32Þ
where:
V
(v1 ; v2 ; . . . ; vn ) is a vector containing the measured quantities at the n
locations.
M is a matrix, each line of which corresponding to one location. Its first
column is filled with ones and corresponds to a constant in the model.
The next three columns may contain the coordinates of the locations if
the model contains linear terms. The next three columns may contain
the products of these coordinates two by two (for example, x1 x2 ; x1 x3 ;
x2 x3 ) in case of interaction terms and, for a quadratic model, the next
three columns contain the squares of the coordinates. Other models will
produce other matrices.
A is a vector containing the coefficients (e.g. a, bi , bij (i 6¼ j) and bii ) of the
model.
In the general case, M is rectangular and the least square fit procedure is
used:
A ¼ ðMT MÞ1 M0 V
ð3:33Þ
T
where M is the transposed matrix of M. This equation is also valid if M is
a square matrix, but reduces to the simpler equation:
A ¼ M1 V
ð3:34Þ
In any case, a matrix should be inverted and the determinant of this matrix
should not be zero! Since this determinant can be calculated before making
the measurements, it is a first criterion for the choice of the experimental
design: it should be significantly different from zero.
Variance of the calculated response
If the coefficients are known, an estimate ve of the value of the variable v can
be obtained at each location in the enclosure:
ve ¼ AT r
where r is the vector (1, x1 ; x2 ; x3 ).
ð3:35Þ
Age of Air and Ventilation Efficiency
53
If 2 is the experimental variance of the measured variable v, the variance
(ve ) of the estimated variable is:
2
2 ðve Þ ¼ rT ðMT MÞ1 r2
ð3:36Þ
A variance function can be defined:
VF ¼
N 2
ðve Þ ¼ NrT ðMT MÞ1 r
2
ð3:37Þ
where N is the number of measurements. VF depends on the experimental
design (M and N) and on the location r and can hence be calculated before
doing any measurement.
If VF depends only on the distance to the origin (or the modulus of r),
the experimental design is said to be isovariant by rotation. If VF is a
constant within the experimental domain, the design gives a uniform accuracy. A good experimental design should have a small variance function, as
constant as possible.
If ðMT MÞ1 is diagonal, the design is orthogonal. In this case, the
variance function is minimum.
Condition of the model matrix
The condition number of the matrix M plays an important role on the
upper bound of the relative errors on the result (see Chapter 7, ‘Upper
bound of the errors’). This condition multiplies the experimental errors and
transfers these errors into the result A. It should therefore be as small as
possible. This number depends on the experimental design and on the model
chosen but does not depend on the results of the measurements. Hence it can
be calculated before doing any measurement and constitutes one more
criterion, which is relatively easy to compute, for the choice of the best
experimental design. This is a much better criterion than the determinant of
MT M.
Expendability of the experimental design
It may be interesting that the measurements performed to obtain the coefficients of a first-degree polynomial are not lost and could be used with
other measurements to expand the polynomial to a higher degree. Some
designs are expandable that way, some others are not.
Examples of experimental designs
Several experimental designs for mapping parallelepiped volumes or a
rectangular area were examined (Roulet et al., 1991). Linear, interaction and
quadratic models were tested. Several of these designs were found to be
unusable (singular matrix or too large a condition number for the quadratic
model). Only good examples are given below.
54
Ventilation and Airflow in Buildings
As mentioned above, the experimental domain is about 20 per cent
smaller than the measured space, samples of air being taken at least 0.1 times
the characteristic enclosure dimension from the walls.
Factorial designs
A factorial design for k dimensions and l levels is obtained by dividing the
experimental domain (for example, the interval [1; 1]) on each axis into l
equidistant levels. The complete factorial design contains all the points
obtained by the l k combination of the l possible values of the k coordinates.
The number of points in a full factorial design is hence l k . If l and k are
greater than 2, the full factorial designs often have many more points than
the minimum required, and are therefore seldom used. However, partial
factorial designs can be obtained by selecting the necessary number of
measurement points from the full design. Some examples are given below.
The 2-D, two-level full factorial design (see Table 3.3) is optimal for a
linear model, providing the coefficients of that model with the best accuracy.
If, for economical reasons, one point is omitted, the confidence intervals of
the coefficients are twice that based on four measurement points.
Table 3.3 2-D, two-level full factorial design
No
x
y
1
2
3
4
1
1
1
1
1
1
1
1
It is very important to note that the frequently used design consisting of
changing one variable at a time (see Table 3.4) is less accurate than the former.
Table 3.4 2-D design changing one variable at a time
No
x
y
1
2
3
4
1
0
1
0
0
1
0
1
Adding a fifth point at the centre (0,0) of the 2-D, two-level full factorial
design allows assessing the coefficient of the interaction term b12 , without loss
of accuracy. The following two points:
No
x
y
6
7
1
1
0
0
Age of Air and Ventilation Efficiency
55
Figure 3.9 Minimum design for a 2-D quadratic model
can be added to obtain a minimum design for a quadratic model, which has a
condition number of 6.3 (see Figure 3.9).
The 2-D full factorial design with three levels shown in Table 3.5 has a
better condition number (4.4) for a quadratic model.
Table 3.5 2-D full factorial design with three levels
No
x
y
No
x
y
1
2
3
4
5
1
0
1
1
0
1
1
1
0
0
6
7
8
9
1
1
0
1
0
1
1
1
3-D designs
In three dimensions, the four point design of Table 3.6 is best for a linear
model.
Table 3.6 Minimum 3-D design for assessing the coefficients of a linear model
No
x
y
z
1
2
3
4
1
1
1
1
1
1
1
1
1
1
1
1
It can be expanded to a full factorial design (see Table 3.7), which is
appropriate for an interaction model.
Also at three dimensions, the star design shown in Table 3.8 is less
accurate and requires more work than the minimum design of Table 3.6.
However, combining the centred star design with the full factorial design
of Table 3.7 gives a so-called composite centred design, suitable for a quadratic model, having a condition number of 4.4. If fewer points are wanted,
56
Ventilation and Airflow in Buildings
Table 3.7 Full factorial design for assessing the coefficients of a linear model
with interactions
No
x
y
z
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Table 3.8 3-D centred star design
No
x
y
z
9
10
11
12
13
14
15
1
0
0
1
0
0
0
0
1
0
0
1
0
0
0
0
1
0
0
1
0
the points 8, 5 and 2 can be deleted (in that order) giving finally a design
having 12 points and a condition number of 4.8. Finally, deleting two more
points (3 and 15) gives the design C3, which has six points in the centre of
the faces and four points at opposite corners (see Figure 3.10).
The condition number of MT M calculated using the absolute value norm
T
|M M| for these designs and three models is given in Table 3.9.
There are numerous other possibilities that can be imagined or found in
the literature.
Figure 3.10 Experimental designs C3 (left) and composite centred (right)
Age of Air and Ventilation Efficiency
57
Table 3.9 Condition number of MT M for some experimental designs and
three models
Experimental design
Number of
points
Quadratic
model
Interactions
model
Linear
model
3
4
5
6
9
–
–
–
6.3
4.4
–
–
1.0
1.0
1.0
2.0
1.0
1.0
1.0
1.0
4
8
10
15
–
–
4.3
4.4
–
1.0
3.2
1.0
1.0
1.0
1.0
1.0
2-D Designs
2-level part factorial
2-level full factorial
Centred 2-level factorial
Minimum for quadratic
3-level full factorial
3-D Designs
2-level half factorial
2-level full factorial
C3
Composite centred
Example of application
The age of air was mapped in the conference room after the improvement,
first in the empty room, and then with ten occupants sitting around a Ushaped table. These measurements were performed at the head level of
sitting persons, using the nine-sampling-point full factorial design shown in
Table 3.5. The results are shown in Figure 3.11. In the unoccupied room,
the air is older at the middle left, where there is only one air inlet. When the
room is occupied, the air is younger in the middle of the room, where the
occupants are.
600
500
400
300
200
100
0
4
800–1000
600–800
400–600
200–400
0–200
1000
-5
-1
0
X
-2 -4
800
-5
600
400
-1
200
3
2
Age of air [s]
Age of air [s]
500–600
400–500
300–400
200–300
100–200
Y
0
4
3
2
Y
0 -2 -4
X
Figure 3.11 Map of the age of the air at head level
Note: Left is the unoccupied room, and right is the room occupied by ten persons sitting
around a conference table.
4
Airtightness
Why check airtightness?
Controlled airflows, having adequate flow rate and passing at the appropriate
locations are essential for good indoor air quality. Leakage, allowing the
uncontrolled air to follow inappropriate paths, should therefore be reduced
as much as possible. This requires an airtight building envelope and airtight
ductwork: building and ductwork airtightness is a prerequisite for efficient
natural or mechanical ventilation. Envelope leakage is not an appropriate way
for airing buildings.
In buildings with hybrid ventilation, the indoor air quality is controlled
partly by a mechanical ventilation system and partly by a natural ventilation
design. The share between these two systems could be seasonal (natural ventilation in mild seasons and mechanical ventilation with heat recovery during hot
or cold seasons) or spatial: small rooms with an external wall with natural ventilation, and large rooms or rooms located inside the building with mechanical
ventilation. In any case, both ventilation systems should be controlled.
While Figure 0.5 shows the exfiltration ratios (i.e. the part of the supplied
air leaving the building by another way than the exhaust duct) for several buildings, Figure 4.1 shows the infiltration ratios (i.e. the part of outdoor air that is
not supplied by the mechanical ventilation unit) measured in 11 spaces that are
equipped with full mechanical (not hybrid) ventilation. Out of these ten spaces,
seven have an infiltration rate significantly different from zero, and in two of
them more than 30 per cent of the outdoor air is not controlled!
Exfiltration has a negative effect on heat recovery, since the heat in the air
leaving the building through leakage cannot be recovered (see Chapter 5,
‘Effect of leakages and shortcuts on heat recovery’). In cold climates, warm
and humid indoor air going through the external envelope through leaks encounters increasingly colder surfaces. The water vapour of this air eventually
condenses on the coldest surfaces within the cracks, thus creating dramatic
condensation problems at leakage locations. Infiltration has a negative effect on
indoor air quality, since infiltrated air is neither filtered, dried, cooled nor heated.
A survey performed by Carrie et al. (1997) in France and Belgium has
shown that, on the average, 40 per cent of the supplied air is lost through ductwork leakage before reaching the user. This obviously reduces the effective
Airtightness
59
Infiltration ratio
60%
50%
40%
30%
20%
10%
0%
1
2
3
4
5
6
7
8
9
10
11
Figure 4.1 Measured part of outdoor air that is not supplied by the system
in mechanically ventilated buildings, shown with uncertainty band
ventilation rate. Maintaining air quality requires an increase in supply air,
leading to energy wastage.
Checking the airtightness of a building envelope or a duct network should
therefore be performed at each commissioning of a building or ventilation
system.
Measurement methods
The airtightness of the envelope of the measured object is in fact expressed by
its permeability to air, which is the relationship between the leakage airflow rate
and the pressure differential through the envelope of the object. This relationship can be either expressed by a mathematical expression (see Equations 4.1
and 4.3) or by an equivalent leakage area (see Equation 4.19).
One internationally standardized way to assess this permeability requires
the maintenance of a pressure differential between the interior and the exterior
of the object with a fan, and the measurement of the airflow rate needed to
maintain this pressure differential (ISO, 1998). Other, simpler but less accurate
methods applicable to buildings are described below in ‘The stack effect
method’. They use the stack effect to create the pressure differential and openings and the location of the neutral level (the level where the indoor–outdoor
pressure differential is zero) to estimate the leakage area.
The fan pressurization method
Before measurements are taken, all purpose-installed openings (doors,
windows, ventilators, etc.) should be closed, and the mechanical ventilation
system switched off (unless used for pressurization). The ventilation grilles
should either be sealed or all ventilation dampers should be closed. It may be
necessary to seal chimneys and flues but these can be sealed later as part of the
test if desired. Clean up open fires to avoid dispersion of ash in the rooms.
The pressure differential is created by a fan installed in an opening of the
envelope of the object, or by the ventilation fans themselves. It is measured
with a sensitive manometer (range 0–100 Pa (Pascals) or 10 mm water
60
Ventilation and Airflow in Buildings
column) and the airflow rate through the fan is measured using any of the
following methods:
.
.
.
The airflow rate through a fan depends on the pressure differential and its
rotation speed. Measuring these two quantities allows assessment of the
airflow rate from the characteristic curve of the fan. Blower doors use this
method.
A suitable airflow meter such a nozzle or a sharp-edged orifice is installed in
the airflow circuit (see Figure 4.5).
The tracer gas dilution technique, as described in Chapter 2, ‘Tracer gas
dilution’.
These measurements are repeated for several pressure differentials, ranging
from a few Pascals to about 60 Pa, or even more for some cases.
The minimum pressure is limited by the noise of the pressure differential,
for example, the random pressure variations resulting from wind and stack
effect. Therefore, measurements should be performed when there is no wind
and the minimum pressure differential is in practice twice the natural
pressure differential. The maximum pressure is limited by the resistance of
the object by practical limits such as fan airflow rate combined with the
object’s leakage. Note that 100 Pa is a pressure that can result from 40 km/h
wind velocity.
Since fan pressurization is subject to the disturbing influence of natural
pressure fluctuations created by the wind, most measurements are made at
pressure differentials far above those created by natural forces. This may
lead to inaccuracy if the results are extrapolated to lower pressure differentials.
Two general models are used to characterize air permeability. The power
law, fully empirical, reflects the fact that leakage is a combination of various
cracks and openings that may be arranged in parallel and series network:
q ¼ Cpn
ð4:1Þ
where:
q
p
n
C
is the
is the
is the
is the
volume airflow rate through the leakage site (m3 /s);
pressure difference across the leakage site (Pa);
flow exponent (0:5 < n < 1);
airflow coefficient (m3 s1 Pan ).
Since the airflow may be either laminar or turbulent, and the airflow rate is
proportional to the pressure differential in laminar flows and to its square
root in turbulent flows, Etheridge proposes a quadratic law, that expresses
that the flow is a mix of laminar and turbulent flow arranged in parallel (Etheridge and Sandberg, 1996):
p ¼ aq2 þ bq
ð4:2Þ
where:
a and b are coefficients representing respectively the turbulent and laminar
parts of the quadratic law (Pa s/m3 and Pa s2 /m6 ).
Airtightness
61
Airflow rate [m3/h]
1000
800
600
400
Measured points
Power law
Quadratic law
200
0
0
10
20
Pressure [Pa]
30
40
Figure 4.2 Airflow rates and pressure differences as measured in a real
test, together with power law and quadratic fits
Inverting this relationship gives the airflow rate resulting from a pressure
differential:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
b þ b2 4ap
ð4:3Þ
q¼
2a
By fitting one of these two models on the measured points on a [q; p] diagram,
the coefficients and the exponent of either of the above models can be assessed
(see Figure 4.2).
Reductive sealing
Global permeability measurement does not provide information on the location
of leakage. Reductive sealing aims to quantify the proportion of the total air
leakage that is attributable to different components or groups of components.
One method to locate leakage is to seal components, areas, rooms or zones
suspected of leakage and to perform a new pressurization test. The difference
is the leakage of the sealed leaks.
The fan pressurization equipment is set up and the dwelling prepared, in
the usual way (see ‘The fan pressurization method’, above). An air leakage
test is then carried out, either pressurizing or depressurizing. A component
or group of components is then chosen (for example, all chimneys and flues)
and sealed with self-adhesive tape, polyethylene sheet, inflatable bladder or
modelling clay, as appropriate. It is important that care is taken to make a
good seal on all components. For example, special attention should be given
to corners and around the fasteners.
The air leakage test is then repeated, the difference between this test and the
first test being a measure of the air leakage attributable to the component or
group of components that were sealed.
Further components can then be chosen and the process continued. The
difference of leakage flow rate at each pressure between two successive tests
is the leakage of the components sealed between these tests. Leakage coefficients for each group of sealed components can be calculated from these leakage
62
Ventilation and Airflow in Buildings
flow rates at each pressure using the method described in section ‘Determining
the leakage coefficients’, below.
Components that can usually be sealed include: chimneys, flues; ventilation
openings, external doors (other than the one used to mount the pressurization
fan), wooden ground floors or roof, cracks between walls and floors, pipe and
cable entry or exit points, and any other obvious cracks and openings (for
example, gaps between a window frame and the wall into which it is fixed).
The last air leakage test in this process gives a measure of the background air
leakage, i.e. the remaining air leakage not sealed during the previous tests. It is
quite common for this to constitute more than half of the overall air leakage,
even where all of the most obvious air leakage paths have been sealed.
When all the components required have been sealed and air leakage tests
carried out, the pressurization fan can be reversed and the air leakage tests
repeated as the components are progressively unsealed in the reverse order to
that in which they were sealed.
Best results are usually obtained by sealing groups of components (for
example, all openable windows in the dwelling) because the leakage through
an individual component (for example, a single openable window) can be too
small for the pressurization fan to resolve.
Multi-zone fan pressurization method
The technique can be extended to multi-zone pressurization, aiming to assess
the air permeability of not only the envelope of an object (for example, a
building) but also of its internal partitions (Fu¨rbringer and Roulet, 1991).
The building zones are represented by nodes of a network linked by partitions.
One of the zones is the outdoor air. To assess the coefficients of all partitions
requires the measurement of many inter-zone airflow rates and pressure differentials. An appropriate design of the experiment aiming to assess the required
coefficients, and only these, will considerably reduce the work required.
For example, two fans and a control system allow assessment of the leakages
of many parts of a building. To avoid the tedious work of sealing with plastic
foil all building parts that should not be measured, the airflows through these
parts are inhibited by maintaining a zero pressure difference across them.
The measuring fan with its flowmeter equipment is installed in a wall or
door of the room containing the element to be measured. Another larger fan
is installed in a door or a window of the building (or the dwelling) containing
the room (see Figure 4.3).
In order to get the leakage characteristics for a given element, the pressure
in the room should be varied step by step from 10 Pa up to 60 or 70 Pa. The
guarding pressure should be varied simultaneously to maintain a zero average
pressure difference between the room and the building. This pressure difference actually varies between 1 and þ1 Pa. A fit through several measurements
provides the airflow rate corresponding to a zero pressure difference.
Several experiments are necessary to measure the other walls of the room
by simply opening or closing various doors and windows. When a set of
Airtightness
63
Figure 4.3 Principle of the guarded zone technique applied to several
walls of a room
Note: At the left, only the external wall is measured, while at the right, the right partition
wall is included in the measurement.
experiments is performed, enough equations can be written to compute the
airflows through the various measured parts for each pressure step.
For that purpose, the pressure steps should be the same in each experiment,
for example, 10, 30, 50 and 70 Pa. In order to get accurate pressure steps, the
fans speeds are automatically controlled. Since even this control cannot be
good enough to get accurate measurements (because of external and random
influences such as temperature or wind fluctuations), the data should be automatically selected and recorded only when the following conditions are
fulfilled:
.
.
The pressure in the room is equal, within a predefined tolerance, to the
predefined value of the pressure for each step.
The pressure difference between the room and the guarding zone is smaller
(in absolute value) than a predefined small value.
Finally, for each pressure step and each configuration, several values are measured and averaged to minimize the effect of random noise.
Determining the leakage coefficients
Density corrections
It is the airflow through the fan, qm , which is measured and the airflow q
through the leak that is needed to calculate the leakage coefficients. In pressurization experiments, the air blown by the fan comes from outside while the air
leakage comes from inside. In depressurization experiments, the opposite is the
case, but in both cases the temperature of the airflows may not be the same.
During the measurements, the mass of air is conserved and:
q
ð4:4Þ
hence
q¼ m m
m qm ¼ q
where m and are the densities of the air going respectively through the fan
and through the leaks. Since the density of the air is inversely proportional to
64
Ventilation and Airflow in Buildings
its absolute temperature, and as long as the pressure differential remains small
with respect to the atmospheric pressure:
q ¼ qm
T
Tm
ð4:5Þ
where T and Tm are the absolute temperatures of the air going respectively
through the measured elements and through the fan or the airflow-measuring
device. This assumes that the variations of air moisture do not significantly
change the density. Before any further analysis, Equation 4.5 should be used
to correct the measured flows for density if the indoor–outdoor temperature
difference is larger than a few degrees. Note that a difference of 108C will
induce a bias of 3 per cent in the airflow rate if this correction is not performed.
Two measurement points
If measurements are performed at two pressures only, for example at the lowest
accurately measurable pressure differential and at the maximum acceptable
one, results of measurements are p1 , q1 and p2 , q2 . The coefficients of the
power law are then:
n¼
log q1 log q2
log p1 log p2
and
q1
q
¼ 2n
n
p1 p2
ð4:6Þ
p1 q22 p2 q21
q1 q2 ðq2 q1 Þ
ð4:7Þ
C¼
The coefficients of the quadratic law are:
a¼
p1 q2 p2 q1
q1 q2 ðq2 q1 Þ
and
b¼
More than two measurement points
More than two measurements may be useful for testing the fitness of the model
and to increase the accuracy of results. In this case, the least square fit method
can be applied to get the coefficients of the power law. For this, Equation 4.1
can be linearized by taking the logarithm of both sides:
log Q ¼ log C þ n log p
ð4:8Þ
This expresses a linear relationship between log Q and log p (see Figure 4.4):
y ¼ a þ nx
ð4:9Þ
with:
y ¼ log Q
a ¼ log C
ð4:10Þ
x ¼ logðpÞ
An appropriate fitting technique (see Chapter 7, ‘Identification methods’) can
be used to identify the parameters, a and b, and the corresponding confidence
Airtightness
65
log(airflow rate)
5.5
5.0
slope n
4.5
log(C)
4.0
3.5
0
1
2
3
Log(pressure difference)
4
Figure 4.4 Logarithmic plot of airflow rates and pressure differences
Note: The slope of the best-fit line is an estimate of n and its ordinate at origin is an
estimate of logðCÞ.
intervals. If the coefficients a and b are known, the airflow coefficient C and the
exponent n are calculated using:
C ¼ expðaÞ
and
n¼b
ð4:11Þ
The Etheridge model in Equation 4.2 can be rewritten, dividing by the airflow
rate q:
p
¼ a þ bq
q
ð4:12Þ
that is again a linear model:
y ¼ a þ bx
ð4:13Þ
with
y¼
p
q
and
x¼q
ð4:14Þ
In this case, the linear fit directly provides the coefficients a and b of the
Etheridge model.
The measurement points can also be interpreted using the inverse problem
theory (Tarantola, 1987), taking into account a priori knowledge such that the
exponent n is between 0.5 and 1. Fu¨rbringer et al. (1994) propose such a
method, which has the advantage of providing a clear view of the error margins
of the coefficients.
Corrections for standard conditions
Coefficients obtained from measurements that are performed under different
atmospheric conditions should be corrected to reduce them to standard
conditions, for example 208C and 101,300 Pa.
66
Ventilation and Airflow in Buildings
Using the subscript o for these standard conditions and no subscript for the
measurement conditions, then:
ð2n 1Þ ð1 nÞ
¼
C
ð4:15Þ
Co
o
o
where is the viscosity (kg s1 m1 ), the density of air and n the power law
exponent. The variation of the air density is:
pTo
¼
o p o T
ð4:16Þ
and the variation of the viscosity is given by the following approximation as a
function of the absolute temperature, T:
pffiffiffiffi
1:458 106 T
¼
ð4:17Þ
110:4
1þ
T
hence
110:4
sffiffiffiffiffiffi
1þ
T
17:1 þ 0:047
To
¼
ð4:18Þ
ffi
110:4 17:1 þ 0:047o
o
To
1þ
T
where is the temperature in degrees Celsius. The approximation given in the
second part of Equation 4.18 can be used between 108C and 408C.
Since the correction is small and if the temperatures and pressures are
known with a reasonable accuracy, the additional errors introduced by this
correction are negligible.
Ways of expressing the airtightness
For practical reasons, permeability is often characterized by one figure only.
Some information is of course lost when one figure is used to represent the permeability instead of two. The following ways are commonly used for this issue.
Airflow rate at conventional pressure
The airflow rate at a given, conventional pressure, is calculated from Equations
4.1 or 4.3 depending on which parameters are available. The conventional
pressure is usually 1, 4, 10 or 50 Pa, depending on the standard used or on
the local uses.
50 Pa corresponds to a pressure differential commonly used for measurements and therefore at a pressure range for which the leakage rate is measured
accurately. It does not, however, correspond to a typical pressure differential
across building envelopes, which is closer to 4 Pa. Airflow rate at 1 Pa is the
coefficient C in Equation 4.1. 10 Pa is a compromise between accuracy obtained
at high pressures and actual, lower pressures.
Airtightness
67
Virtual air change rate
By dividing the airflow rate at conventional pressure by the internal volume of
the tested enclosure gives a virtual leakage air change rate at that pressure. For
this figure, 50 Pa is the most used pressure difference, and the figure is then
noted n50; in [h1 ]. This value is less than 1 h1 in airtight buildings but,
depending on the climate and building habits, buildings may have figures
larger than 10 h1 . This figure does not indeed characterize the quality of the
envelope, since it depends on the volume of the enclosure. It provides an
indication of the importance of infiltration in relation to building ventilation.
Specific leakage rate
The airflow rate at conventional pressure divided by the area of the envelope of
the tested enclosure provides a figure characterizing this envelope. For such
application, the most common pressure differential is 4 Pa, and this parameter
is then v4 , or specific leakage rate at 4 Pa. It is expressed in m3 /(h m2 ). It is also
the average air velocity through the envelope. This figure is less than 1, even
0.5 m3 /(h m2 ) for airtight envelopes.
Equivalent leakage area
An equivalent leakage area, i.e. the area of a circular hole with sharp edges that
would have the same airflow rate at a given pressure differential, is:
rffiffiffi
ð4:19Þ
pðn 1=2Þ
AL ¼ C
2
The uncertainty of the leakage area resulting from uncertainties on coefficients
C and n is:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
2
C
ð4:20Þ
þ ðn ln pÞ2
AL ¼ A2L
C
Specific equivalent leakage area
The equivalent leakage area can be divided by the area of the envelope of the
tested enclosure to provide a specific leakage area. At 4 Pa, this ratio, expressed
in cm2 /m2 , is close to the specific leakage rate expressed in m3 /(h m2 ).
Airtightness of buildings
The main reason for conducting building airtightness measurements is to
characterize the leakage of the building envelope in the absence of climatic or
other variable parameters influencing the results. Therefore the building (or
part of the building or a particular component) is pressurized or depressurized
68
Ventilation and Airflow in Buildings
in order to create a pressure difference large enough to minimize influences
from wind and temperature differences on the results. This pressure differential is built up and maintained by means of a fan, forcing airflow through the
envelope or component to be evaluated. This amplified airflow can be put in
evidence by both qualitative (visualization) as well as quantitative (measurement of the airflow for a given pressure difference) techniques in order to
assess the leakage locations, areas and characteristics.
External fan
The technique involves replacing an external door with a panel containing a
powerful, variable speed fan. Several commercial blower doors are now
available. These can be adjusted to fit snugly into any domestic doorframe.
Airflow through the fan creates an artificial, uniform static pressure within
the building. Internal and external pressure taps are made and a manometer
is used to measure the induced pressure differential across the building
envelope. It has become common practice to test buildings up to a pressure
difference of 50 Pa.
Some means must also be provided to enable the volumetric flow rate
through the fan to be evaluated. The aim of this type of measurement is to
relate the pressure differential across the envelope to the airflow rate required
to produce it (see ‘Determining the leakage coefficients’, above).
The general configuration for a pressurization/depressurization test is
shown in Figure 4.5. The measurement procedure will depend upon the
purpose of the test and the exact equipment used.
The airflow required to produce a given pressure difference under pressurization (airflow in) will not necessarily be identical to the flow required to
produce the same pressure differential under depressurization (airflow out).
This difference is mainly due to the fact that certain building elements can
act as flap valves. For example, some types of window will be forced into
their frames under pressurization while the reverse will be true for evacuation.
This implies that the actual leakage area of the building envelope will be a function of the type of test conducted. Hence, ideally, the fan and flow measuring
mechanism must be reversible.
Figure 4.5 Schematic of building airtightness test
Airtightness
69
The overall airtightness of the structure and the size of the available fan
govern the maximum volume of enclosure that may be pressurized. Even if
large fans are available, in large leaky structures it may be possible to only
achieve a limited range of pressure differentials. Several researchers have
used trailer mounted fans with maximum flow capacities of about 25 m3 /s to
examine buildings with volumes as large as 50,000 m3 .
Internal fan
Because of the size and cost of trailer-mounted equipment and the inherent difficulties of transportation and required manpower, other techniques have been
developed for the examination of large buildings. One method is to create the
required pressure differential using the building’s existing air handling system.
This technique relies on the building possessing a suitable mechanical ventilation
system, which can be adjusted to meet the needs of the measurement. Essentially,
the supply fans are operated while all return and extract fans are turned off and
return dampers closed (or exhaust ducts sealed) so that the air supplied to the
building can only leave through the leakage sites.
The analysis of measurement results proceeds along the same lines as that
for small buildings, but because of the large building volume it may not be
possible to achieve a pressure difference of 50 Pa.
Leakage visualization
Leakage can be visualized using infrared imaging, using a camera able to see far
infrared radiation emitted by any surface. When reducing the internal pressure,
outdoor air enters the building through leakage. Outdoor air, with a temperature that should differ from indoor air, changes the temperature of surfaces
close to the leaks, thus making them visible, as in Figures 4.6 and 4.7 showing
the connection between two walls and the roof of a wooden building. In this
Figure 4.6 Roof corner from inside
Source: Roulet, 2004.
70
Ventilation and Airflow in Buildings
Figure 4.7 Roof corner under depressurization
Source: Roulet, 2004.
case, the airtightness is not good enough and cold air enters the inhabited space
through cracks between wooden panels.
The stack effect method
This simple and easy-to-install method to estimate the air leakage distribution
in tall buildings is based on the pressure distribution induced in buildings by
the stack effect (Tamura and Wilson, 1966). Three parts can be estimated
separately: the ground floor, the top floor and the remaining floors.
The basic idea is to pressurize the building with the stack effect, and to plan
three different experiments where two airflows can be measured to get three
independent equations for the three different leakages that will be estimated
(Hakajiwa and Togari, 1990).
For this measurement method, the building should be tall and the temperature difference between indoors and outdoors should be large enough, in such a
way that the pressure difference between inside and outside induced by the
stack effect is larger than the pressure caused by the wind. Therefore, calm
weather should be preferred and the mechanical ventilation system switched
off. The pressure difference resulting from buoyancy is proportional to the
product of the indoor–outdoor temperature difference and maybe the stack
height. It reaches 30 Pa if the product of the height and the temperature difference is 700 Km.
The leakage of the building is divided into three parts:
.
.
.
leakage through the ground level including the entrance door (suffix g);
leakage through the top level including the roof (suffix t);
leakage through the remaining floors (suffix r).
If the building has all its internal doors open as well as the staircase and the lift
shaft, and if the temperature does not vary too much throughout the building,
Airtightness
71
there is a priori only one neutral plane at the height z0 . The neutral plane is the
generally horizontal plane in the building or part of it where the indoor–
outdoor pressure differential is zero. Its height depends on the size and position
of the ventilation and leakage openings. It is such that the airflows going in and
out of the building are balanced.
The pressure difference, p, caused by the stack effect at any height, z, in a
given building configuration is then:
ðz
ðzÞg dz
ð4:21Þ
pðzÞ ¼
z0
where ðzÞ is the difference between the densities of indoor and outdoor air at
height z, and g ¼ 9:81 m/s2 is the acceleration due to gravity.
If the temperatures are homogeneous, Equation 4.21 gives:
pðzÞ ¼ gðz z0 Þ
ð4:22Þ
Using the law of perfect gases to express the air density, we get:
Mp 1
1
gðz z0 Þ
pðzÞ ¼
R Ti Te
ð4:23Þ
where:
Ti and Te
M
p
R
are the indoor and outdoor air absolute temperatures,
is the average molar mass of the air, i.e. 0.029 kg/mole,
is the atmospheric pressure,
is the constant for perfect gases, i.e. 8.31396 J/mole K.
The leakage of the building is represented by the usual power law:
Q ¼ C pn
ð4:24Þ
Assuming that the exponent n is the same for every leak, there are three
unknowns, the leakage coefficients, Cg , Cr and Ct . To estimate these coefficients,
three measurements are performed, where the pressure differences, the temperatures at various heights in the building and some airflows are measured. A first
relationship is given by the conservation of mass with a closed envelope. The
two other equations are obtained by mass conservation with a large opening at
the bottom and at the top of the building. In these cases, the airflows through
these openings are measured. The relations are as follows:
1 All openings closed – in this case, the neutral plane is somewhere at midheight of the building and, by conservation of the mass of air, we have:
ð zrt
dðr Cr pðzr Þn Þ
ð4:25Þ
g Cg pðzg Þn ¼ t Ct pðzt Þn þ
zrb
where g , r and t are the densities of the air at the ground level, the
remaining floors and at the top level in order to have the proper mass flow.
2 Entrance door open – the airflow through the open entrance door (or any
other large opening on the ground level), Qg , is measured, either by
72
Ventilation and Airflow in Buildings
measuring the air speed at several locations and integrating over the whole
opening or using a tracer gas.
ð zn
n
dðr Cr pðzr Þn Þ ¼ 0
ð4:26Þ
g Qg þ t Ct pðzt Þ þ
zrb
3 Windows open at the top level – the airflow through these windows, Qt , is
measured. We have similarly:
ð zrt
n
dðr Cr pðzr Þn Þ ¼ 0
ð4:27Þ
g Cg pðzg Þ þ t Qt þ
zrb
The neutral plane is now at the top level.
Assuming that n is 0.6 or two-thirds, which are the most probable values, the
system of three equations above can be solved to estimate Cg , a global Cr and
Ct . If the temperatures are not uniform inside or outside, Equation 4.21
should be used instead of Equation 4.22. The system is then more complex
but can still be solved. The most important condition to observe during the
measurement is the absence of wind.
The main advantage of the method is that it does not require the use of
sophisticated equipment. As a minimum, the required equipment is:
.
.
.
.
wind velocity meter, 0–5 m/s, for measuring the airflow rate in the openings;
differential manometers, 0–50 Pa;
air temperature thermometers;
length measuring device as long as the building is tall.
This equipment can be completed by more differential manometers and more
thermometers, used to verify the linearity of the pressure distribution through
the building.
Neutral height method
A simple variant of the stack effect method offers in many cases a good estimate
of the leakage area and determines if specifications are met or exceeded (Van der
Maas et al., 1994). It is also based on the determination of the neutral height
and the equipment necessary is only an airflow direction detector (small
smoke generator such as a cigarette, incense stick or small flame) and a yardstick. With no wind, this method even allows the leakage characterization of
a single storey building.
The measurement should be performed with the mechanical ventilation
system switched off, preferably on a cold day without wind and when the
building is heated. In these conditions, the airflow through the building results
from buoyancy only. The method can also be used, mutatis mutandis, in cooled
buildings in a warm climate.
The principle is to determine the position of the neutral height in an outside
opening such as a door. The airflow direction detector is moved from bottom to
Airtightness
73
Figure 4.8 Principle of the neutral height method for assessing
leakage area
Note: The left shows no leak (except the test opening), while on the right, the leakage is
above the test opening.
Source: Roulet, 2004.
top of the opening to observe the flow direction. The neutral level is located
between the ingoing and outgoing flow directions. Sensitivity can be increased
or decreased by reducing or increasing the width of the opening.
In an airtight building, the neutral height will be located at about midheight of the measuring opening (see Figure 4.8, left). Cold air enters the
building through the lower half of the opening, and warm air leaves the
building through the upper part.
Using the Bernoulli equation and mass conservation, it can be shown that
the mass flow rate through the upper or lower half of the opening is:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
gH T
ð4:28Þ
QA ¼ Cd o A
3
T0
where (Van der Maas et al., 1994):
Cd
o
is the discharge coefficient of the opening (for example, 0.6),
is the density of air outdoors (assumed to be the cold zone at temperature
To ),
A
is the area of the opening of with W and height H,
g
is the gravitational acceleration (about 9.81 m/s),
T is the indoor–outdoor temperature difference.
If some air enters or leaves the enclosure through another opening or through
leaks, part of the incoming air will leave or enter the building through it and will
not, therefore, pass through the upper or lower part of the test opening. The
neutral height rises or goes down to balance the two airflows (see Figure 4.8,
right). Using mass conservation, the net mass flow rate is given, assuming
74
Ventilation and Airflow in Buildings
that Ti > To , by:
1=2 3=2
To
z 3=2
zn
Q ¼ QA
1 n
ffi QA ð1 aÞ3=2 a3=2
Ti
H
H
with
a¼
ð4:29Þ
zn
H
where zn is the height of the neutral level.
The opening area between this neutral level and the mid-height of the
opening is close to the equivalent leakage area. If the leakage is small, the
sensitivity can be increased by reducing the width of the opening, for
example by partly closing the door.
A neutral height below the mid-height of the test opening means that most
of the leakage area is below the opening. If the other leakages or openings are
larger than the test opening, the neutral height will not be found within the
test opening, even when this is wide open. In this case, a walk through the
building is necessary to identify and, if possible, to close or seal these large
openings.
The equivalent area measured this way is the difference between the equivalent areas of the openings or leakage areas located above and below the opening.
Therefore, it is useful to make this measurement at two test openings located at
the bottom and the top of the building.
Measurement of airtightness of a duct or network
To ensure that fresh air reaches the ventilated space, thus ensuring acceptable
air quality, and to avoid energy waste when the air is either heated or cooled, the
duct network should be airtight. Significant energy may be wasted, for
example, where leaky ductwork passes through an unheated space such as an
attic, basement or crawl space. As an example, it was found that ductwork is
the most significant source of leakage in western US houses, together with fireplaces (Dickerhoff et al., 1982). Modera (1989) confirms these findings, but
some houses were nevertheless found to be acceptable.
Several techniques allow for the checking of the airtightness of air ductwork. In Finland, the airtightness of the ventilation system has to be checked
when commissioning the system (NBCF, 1987), but in most countries,
measurements are seldom carried out. ASTM (2003) provides guidance to
perform such tests. Some measurement methods are described below.
Pressurization method
The principle of this method is a combination of those principles described in
Chapter 2, ‘Measurement for airflow rate in a duct’ and Chapter 4, ‘The fan
pressurization method’, above. All intakes, supply terminals, exhaust and
extract terminals connected to the system should be carefully sealed, for
Airtightness
Seal
75
Seal
Sampling
Tracer A
Tracer B
Seal
Seal
Sampling
Figure 4.9 Location of tracer injection and sampling tubes for the
measurement of leakage airflow rates in a ventilation system
Source: Roulet and Vandaele, 1991.
example, using plastic sheeting and adhesive tape. Inflated balloons are also
well suited to seal circular ducts.
Tracer gas injection and air sampling tubes are installed at appropriate
points in the main supply or exhaust ducts, as shown in Figure 4.9, to quantify
any residual flow rate resulting from leakage.
The system fans (or a fan added at one register if required) are used to
pressurize the supply side and depressurize the exhaust side of the network.
Since all inlet and exhaust grilles or ducts are sealed, the flow, qL , through
the fan(s) results from leakage, and is measured as described in Chapter 2,
‘Measurement of the airflow rate in a duct’, together with the pressure difference, p, between the inside and outside of the ducts. The flow rate is the
sum of all leaks downstream of the measurement point in pressurized ducts,
and upstream for depressurized ducts.
A series of measurements is made at different fan speeds, and the coefficients
of Equations 4.1 or 4.2 are determined, and the relationship is subsequently
used to calculate the leakage rate at the service pressure difference.
Flow rate difference method
If a duct is very leaky, the leakage can be obtained by measuring the difference
between the flow rates at two locations along the flow. Since additional pressure
drop should be avoided, tracers are recommended to measure the airflow rates.
One tracer should be injected at a point upstream of the first location, and a
second tracer injected at the first location. The concentrations of each tracer
are measured after the second, downstream location, at a distance where a
good mixing is achieved (see Chapter 2, ‘Sampling points for concentration
measurements’). If steady flows can be assumed, two sequential measurements
using a single tracer at each point may be used instead.
For depressurized ducts, only one tracer is necessary since it is diluted by
the air entering the duct through leakage. The tracer gas is injected at the
76
Ventilation and Airflow in Buildings
upstream end of the duct and its concentration is measured at both ends to give
the flow rate at each.
The leakage of the whole supply or exhaust network may be determined by
measuring the difference between the airflow rate in the main duct (close to the
fan) and the sum of all the flow rates at the individual inlet or extract terminals.
For this purpose, the main airflow rate through the fan may be measured with a
tracer, and the flow rates at the terminals may be determined by one of the
methods described in Chapter 2, ‘Airflow measurements in air handling
units’, the most appropriate being the compensated flowmeter (see Chapter 2,
‘Compensated flowmeter’).
With this method, the leakage flow rate is the difference of two large
numbers. Therefore, it is not the best one for tight or only slightly leaky
ducts. Because of its ease of use, it can nevertheless be used for diagnosis
purposes, to detect if the ductwork is very leaky or not.
Differential building pressurization
The methods described above measure the leakage of the air duct system. From
the point of view of energy saving, however, it may be useful to measure the air
leakage to or from outside only, and not leakages between the system and the
interior of the building.
For this purpose, the duct system is assumed to be a part of the envelope and
the duct leakage is obtained by difference. In a first test, the closed building is
pressurized after sealing the outdoor air intake and exhaust of the building,
with all the registers and returns open (see Figure 4.10, left). In a second
test, all registers and returns are sealed (see Figure 4.10, right). The difference
in airflow rate between the two tests, for each pressure difference, results from
duct leakage to the outside.
The major advantage of this method is that it needs little equipment, in
addition to that required for envelope leakage measurements. However, it is
prone to inaccuracy since the duct leakage is again obtained as the difference
between the measurements of two large airflow rates.
Seal
Seal
Seal
Conditioned space
Seal
Conditioned space
Figure 4.10 Two measurements providing, by difference, the duct leakage
to outside of the conditioned space
5
Measurements and Measures
Related to Energy Efficiency
in Ventilation
Energy in buildings
Energy uses and indoor environment quality
Energy is used in buildings for many purposes such as:
.
.
.
.
.
.
.
heating and cooling;
drying and humidifying;
ventilation (moving the air);
hot water supply;
lighting;
building systems such as lifts, escalators, communication networks;
cooking, washing, leisure, producing goods and services.
According to the Rio Declaration, sustainable buildings should take account of
environmental, economical and social factors. This includes, among others, low
energy use, good indoor environment quality and health. The three factors have
equal importance: a building cannot be good if it fails in one of them. Ventilation plays a large role in these issues by ensuring a good indoor air quality. In
mechanically ventilated buildings, ventilation uses energy to move the air and,
in many cases, to condition it.
In some case, especially when appropriate studies are not performed, there
may be a conflict between strategies to reduce energy use and to improve
indoor environment quality. However, studies and existing high performance
buildings show that it is possible to realize healthy, comfortable and energy
efficient buildings. Basic recommendations to reach these objectives are
(Roulet, 2004):
.
.
.
Prefer passive methods to active ones wherever possible.
Think about user comfort, needs and behaviour.
Adapt the building and its systems to its environment.
78
Ventilation and Airflow in Buildings
Passive and active ways to get high quality buildings.
Passive ways are architectural and constructive measures that naturally provide
a better indoor environment quality without or with much less energy use.
Examples are:
.
.
.
.
.
improving winter thermal comfort with thermal insulation, passive solar
gains, thermal inertia and controlled natural ventilation;1
improving summer thermal comfort with thermal insulation, solar protection, thermal inertia and appropriate natural ventilation;
ensuring indoor air quality by using low-emitting materials and controlled
natural ventilation;
providing controlled daylighting;
protecting from outdoor noise with acoustical insulation adjusting the
reverberation time for a comfortable indoor acoustics.
Passive means are often cheap, well accepted by the occupants, use very little or
no energy, and are much less susceptible to break down than active means.
However, they often depend on meteorological conditions and therefore
cannot always fulfil their objectives. They should be adapted to the location
and therefore need creativity and additional studies from the architect, and a
design error may have dramatic consequences.
Active (or technological) ways improve the indoor environment quality by
mechanical actions, using energy to complement the passive ways or even to
compensate for low building performance. Examples are:
.
.
.
.
.
heating boilers and radiators for winter comfort;
artificial cooling by air conditioning or radiant panels for summer comfort;
mechanical ventilation;
artificial lighting;
actively diffusing background music or noise to cover the ambient noise.
Active ways, when appropriately designed, built and maintained, are perfectly
adapted to needs. Flexible and relatively independent of meteorological conditions, they allow for the correction of architectural errors. However, the
required technology is often expensive, uses a lot of energy and may break
down. Furthermore, active means require a higher maintenance input.
Passive ways are preferred, but cannot always fulfil the comfort objectives.
Therefore, the appropriate strategy is to use them as much as reasonably
possible and to compensate for their insufficiencies with active systems,
which will then be of lesser importance. This strategy often allows more
freedom in choosing the type and location of active systems.
The passive way of ensuring indoor air quality is of course natural ventilation,
but also reduction of pollutant emission indoors by an appropriate choice of
building materials and furniture. The corresponding active way is mechanical
ventilation wherever necessary, including heating, cooling, humidification or
dehumidification. An appropriate design of the ventilation systems, a careful
commissioning of new systems and conscientious maintenance guarantee good
Measurements and Measures Related to Energy Efficiency in Ventilation
79
indoor air quality at a lower energy cost. Once again, measurements may help in
commissioning and diagnosing failures.
Energy in air handling units
Energy for heating and cooling buildings
Heating and cooling aim to keep a quasi steady and comfortable temperature
indoors, despite variations of the outdoor temperature, and taking into account
the solar radiation heating the building fabric, mainly through windows, and
internal heat gains from occupants and their activities. The amount of energy
needed for this depends on the following parameters:
.
.
.
.
.
the climate, which is the imposed boundary condition;
the opaque parts of the building envelope, the function of which is to protect
the indoor environment against the weather. Reinforced thermal insulation
and good airtightness are essential for this purpose;
the transparent parts of this envelope, ensuring daylighting and view, but
also allowing the solar radiation to heat the indoor environment. This passive
solar heating is welcome in the cold season, but induces overheating in the
warm season. Therefore, transparent parts of the envelope should be
equipped with mobile and efficient solar protection to control the passive
solar heating and daylighting;
the thermal inertia (thermal mass) of the building fabric, which naturally
stabilizes the indoor temperature;
the internal gains resulting from occupants’ activities, which contribute to
heating in the cold season but add to the cooling load in hot season.
It can easily be seen that heating and cooling energy needs depend mainly on
the building design and its location. These energy needs may be satisfied by
different systems, including air conditioning. The systems should be energy
efficient, i.e. satisfy the needs at a minimum energy cost. Among the numerous
heating and cooling systems, we only consider here using air as a medium. It
should be mentioned that, because of its low density, air is a poor heat carrier:
one litre of water carries, in practice, 7–15 times more heat than one cubic metre
of air!2
Energy for air conditioning
Buildings are primarily ventilated for the purpose of removing the pollutants
generated within them. The air leaving the buildings has the characteristics
(temperature, humidity, chemical composition) of the indoor air. It is replaced
at the same mass airflow rate by air coming from outdoors, which also has its
own characteristics. Air conditioning is giving or taking heat and water
vapour to or from the outdoor air entering the building to obtain the required
indoor air temperature and humidity. This needs energy.
80
Ventilation and Airflow in Buildings
30
Sat.
90 kJ/kg
Water content [g/kg]
25
90%
80
80%
70
20
70%
60
60%
50
15
50%
40
40%
30
10
30%
20
5
20%
10
10%
0
0
10
20
30
ºC
Figure 5.1 Psychrometric chart with constant relative humidity curves
and constant enthalpy lines
Note: It is shown that air at 208C and 50 per cent relative humidity contains about
7.5 g of water vapour per kilogram. Its enthalpy is 39 kJ/kg and its dew point is close
to 108C.
Figure 5.1, a psychrometric chart for air, shows several characteristics of
humid air. The curves show the water content of air as a function of its
temperature for various relative humidities. The water content cannot be
greater than that shown by the saturation curve. Air with a relative humidity
0 < ’ < 1 has a water content ’ times that of the saturated air.
Energy is needed for heating or cooling the air as well as for evaporating
water in it or condensing water for drying it. Taking as a reference dry air at
08C, the specific enthalpy or energy needed to heat and humidify 1 kg of air
to reach the temperature and humidity ratio x is:
h ¼ cda þ ðL þ cw Þx
ð5:1Þ
where:
cda is the specific heat capacity of dry air, about 1006 J/(kgK);
cw is the specific heat capacity of water vapour, about 1805 J/(kgK);
L is the latent heat of evaporation, i.e. the heat required to evaporate 1 kg of
water, about 2,501,000 J/kg;
x is the humidity ratio, i.e. the mass of water vapour per kilogram of dry
air.
This humidity ratio, x, is related to the water content, !, which is the mass
concentration of water vapour in moist air of Figure 5.1, by:
x¼
!
1!
and
!¼
x
1þx
ð5:2Þ
Measurements and Measures Related to Energy Efficiency in Ventilation
81
30
Sat.
90 kJ/kg
Water content [g/kg]
25
90%
80
80%
70
20
70%
60
60%
50
15
50%
40
40%
30
10
30%
20
5 10
20%
Humidifying
Heating
10%
0
0
10
20
30
°C
Figure 5.2 Paths in the psychrometric chart for heating and humidifying
outdoor air in winter to reach 208C and 50 per cent relative humidity
Figure 5.2 illustrates the paths of temperature and water content of air for
heating and humidifying outdoor air in winter, at 08C and 80 per cent relative
humidity, in order to get 50 per cent relative humidity at 208C.
A part of the energy required for heating and humidifying the air in winter
is brought from free sources such as solar radiation or metabolic activity of
occupants. All the electricity used for lighting and other appliances that are
not part of the heating system end up as heat, in most cases released into
indoor air. Plants and occupants, as well as activities such as cooking and
drying laundry add water vapour to indoor air. When this is not enough to
reach a comfortable indoor climate, the complement is provided by a heating
system. In this case, about 0.34 Wh is needed to heat or cool 1 m3 of air by
18C, as long as the air is humidified by ‘free sources’. This value is therefore
used in models calculating the energy for heating buildings. If a humidifier is
used, it will take 2.5 kJ (about 0.7 Wh) per gram of water vapour generated.
This heat is taken in the indoor environment if the humidifier does not generate
water vapour but water droplets (spray humidifiers).
The paths in the psychrometric chart for cooling and drying summer
outdoor air from 308C and 70 per cent relative humidity down to 208C and
50 per cent relative humidity are shown in Figure 5.3. Note that, for drying
the air, it should first be cooled down at the dew point temperature corresponding to the required specific humidity, and then reheated to the required
indoor temperature. It should also be noticed that it is impossible to cool the
air below its dew point without drying it.
Hot and humid outdoor air cools down and eventually dries on contact with
cold surfaces, on which excess water vapour condenses. If these surfaces are not
cooled down, such as the building fabric or furniture, their temperature rises
and cooling stops after a while. However, the air temperature rises more
82
Ventilation and Airflow in Buildings
30
Sat.
90 kJ/kg
Water content [g/kg]
25
90%
80
70
20
15
80%
Cooling
60
70%
50 Drying
60%
50%
40
40%
30
10
30%
20
5 10
20%
Reheating
10%
0
0
10
20
30
°C
Figure 5.3 Paths in the psychrometric chart for heating outdoor air in
winter or cooling it in summer to reach 208C and 50 per cent relative
humidity
slowly if the air is in contact with massive structures that were cooled down
before, for example, by strong airing during the cool night.
Mechanical cooling is needed to keep the surfaces in contact with the air
cold, and to get continuous air drying and cooling. Warm, humid air is first
cooled down when passing through a refrigerated heat exchanger (horizontal
‘cooling’ line in Figure 5.3) until it reaches its dew point. Then it is dried by
losing the water that condenses on the heat exchanger (‘drying’ curve) until
it reaches the required specific humidity, at a new lower dew point. It should
then be reheated to the required temperature.
Numerical values for this process are given in Table 5.1. The largest change
in enthalpy is when drying, since 2500 J should be withdrawn from the heat
exchanger to condense each gram of water.
The energy required to reheat the dry, cold air can be provided by various
means:
Table 5.1 Humidity ratio and specific enthalpy of warm, humid air cooled down
and dried as shown in Figure 5.3
Process
Cooling
Drying
Heating
Temperature
(8C)
Relative
humidity
’ (%)
Humidity
ratio
x (g/kg)
Specific
enthalpy
h (J/(kg K))
Enthalpy
increase
h (J/(kg K))
30.0
23.9
9.3
20.0
70
100
100
50
18.8
18.8
7.3
7.3
78,756
71,815
27,571
38,724
6941
44,244
11,153
Measurements and Measures Related to Energy Efficiency in Ventilation
.
.
.
83
From the indoor environment, heat loads and solar gains. This way,
common in tropical climates, saves the investment of the heating system,
and heating energy is free. It has, however, the disadvantage of blowing
cold air into the occupied spaces, often leading to draughts. In such systems,
recirculation is often very large and temperature control is obtained by
varying the supply airflow rate.
Heat provided to a warm heat exchanger by the heat pump used to cool down
the chilled water. This heat pump provides cooling water at temperatures
higher than indoor temperature. This water or a part of it can be circulated
into the warm heat exchanger without any running cost. The investment is
limited to pipes connecting the chiller condenser to the warm heat exchanger
and to a control valve.
Heat provided to a warm heat exchanger by a separate heating system. This
is expensive both in investment and running costs and should not be used.
Measurement of energy for heating, cooling, humidifying or
dehumidifying air
The amount of energy needed to increase the temperature and humidity of a
known volume of air depends only on the start and final values of temperature
and humidity ratios. Using Equation 5.1:
Q ¼ hV ¼ Vcda þ ðL þ cw Þx
ð5:3Þ
Therefore, measuring the airflow rate (according to Chapter 2, ‘Measurement
of airflow in a duct’) through the heating coils and humidifier (if any), as well as
air temperature and moisture upwind and downwind of these elements, allows
for the calculation of the heating and humidifying power.
This is not that simple for cooling and dehumidifying. The measurement of
airflow rate is the same, but air temperature and humidity should be measured
before and after each of the processes mentioned in Table 5.1:
.
.
Cooling and dehumidification – measurements in outdoor air and after the
cooling coils provide the power taken from the chilled water. This power
may also be obtained by measuring the chilled water flow rate in the cooling
coil and its temperature increase.
Reheating – measurements before and after the heating coil give the power
provided by the reheating system. This power can also be calculated from
measurements of the heating water flow rate in the heating coil and its
temperature decrease.
Heat exchangers
The purpose of heat exchangers is to transfer heat from water to air (heating
coils) or vice versa (cooling coils). This heat should be transferred in the
most efficient way possible, without transferring contaminants. The diagnosis
should characterize the performance of the exchanger.
84
Ventilation and Airflow in Buildings
To improve energy efficiency, mechanical ventilation systems are often
equipped with heat recovery for recovering the heat contained in exhaust air.
This heat is in most cases given back to supply air. Such heat recovery exchangers are efficient during both cold and hot seasons, saving heating and cooling
energy. Some of these heat exchangers also transfer humidity, thus decreasing
the energy used to humidify or dehumidify the air.
As shown in Figures 0.3, 0.4 and 0.5, air handling units may have parasitic
shortcuts and leakages. Such leakages have often been observed in buildings
(Presser and Becker, 1988; Hanlo, 1991; Fischer and Heidt, 1997; Roulet
et al., 1999). They can dramatically decrease the efficiency of ventilation and
heat recovery (Roulet et al., 2001). Moreover, leakage in a building’s envelope
allows indoor air to escape outdoors without passing through the heat recovery
system. In addition, these units use electrical energy for fans, which may, in
some cases, exceed the saved heat. The influence of these various phenomena
on the real energy saving is addressed in this chapter.
Types of heat exchangers
Water-to-air heat exchangers are in most cases made out of finned tubes in
which the water circulates. The fins increase the exchange area between the
exchanger surface and the air.
The heat exchangers most commonly used for heat recovery are plate heat
exchangers, rotating heat exchangers and heat pipes. Most common air-to-air
exchangers are plate heat exchangers, in which the exhaust air is blown in
several channels limited by plates made of glass, metal or plastic (see Figure
5.4). The other side of these plates is in contact with inlet air, so that heat
Figure 5.4 Close view of a flat plate heat exchanger
Measurements and Measures Related to Energy Efficiency in Ventilation
85
Figure 5.5 Top half of a rotating heat exchanger
can be transferred from the warm side to the other. The heat recovery efficiency
of these exchangers ranges from 60 to 80 per cent, depending on the type and
size. A variant of this exchanger is the heat pipe exchanger, in which heat pipes
are used to transport heat from warm to cold air. The air leakage between both
sides of such heat exchangers should be zero.
Rotating heat exchangers
Rotating heat exchangers are used in larger systems (see Figure 5.5). A disc
with a porous structure (honeycomb, corrugated metallic foils) allowing the
air to flow easily through it, is placed so as to have half of its area in the exhaust
duct, and the other half in the supply duct. This disc rotates slowly and is
heated in the warmer air, where air moisture may also condense on the surface
of the porous structure. It is then cooled in colder, dryer air, also evaporating
here the condensed water. This way, sensible and latent heat contained in warm
air is given to cold air, and the heat recovery efficiency may reach 90 per cent. A
gasket and a purging sector limit contamination from exhaust air to fresh air,
without eliminating it completely (see Chapter 6, ‘Contaminant transport in
rotating heat exchangers’).
A small leakage can be accepted in a rotating heat exchanger, resulting in a
recirculation rate of less than 4 per cent. Reduced leakage is achieved by carefully installing the rotating heat exchanger, and by balancing the air pressure
between both sides of the exchanger. To achieve this, supply and exhaust
fans should not be on the same side of the heat exchanger (see Figure 5.6).
86
Ventilation and Airflow in Buildings
Figure 5.6 Relative position of fans and rotating heat exchangers
Placing both fans on the same side results in a large pressure differential
through the rotating heat exchanger, thus increasing leaks. A parasitic
recirculation rate as large as 40 per cent was measured by the author in such
a unit!
Most rotating heat exchangers are equipped with a purging chamber,
located between inlet and exhaust air ducts, on the warm side of the wheel
(see Chapter 6, ‘Contaminant transport in rotating heat exchangers’).
Glycol heat exchanger
When exhaust and inlet ducts are not side by side, heat can be transported by a
hydraulic circuit with two heat exchangers. The fluid (generally a glycol–water
mix) is heated by the air–liquid heat exchanger located in one of the ducts, then
pumped to the other exchanger to give heat to the cold air.
Heat pump
In exhaust only systems, the recovered heat cannot be given to outdoor air, but
to the hydraulic heating system or to a hot water boiler. For this, the temperature of the hot side of the recovery system is increased using a heat pump,
whose cold source is the exhaust air.
Heat exchange efficiency
The efficiency of heat recovery exchangers has two aspects: the energy (or
enthalpy) efficiency and the temperature efficiency.
The first is the ratio of the enthalpy flow delivered to the supply air by the
enthalpy flow in exhaust air:
E ¼
Hdownwind; supply Hupwind; supply
Hupwind; exhaust Houtdoor air
ð5:4Þ
If supplied air upwind of the heat exchanger (inlet air) has the same characteristics as that of the outdoor air, Houtdoor air may be replaced by Hupwind; supply .
The enthalpy of air is determined by its temperature and moisture content
(Equation 5.3). Therefore, measurement of temperature and moisture content
of air upwind and downwind of both sides of the heat exchanger allows the
determination of the enthalpy efficiency of the heat exchanger itself.
Measurements and Measures Related to Energy Efficiency in Ventilation
87
The enthalpy flow, H, is the product of mass airflow rate and specific
enthalpy, h:
H ¼ Qh
ð5:5Þ
where is the density of air.
At ambient temperature, a numerical expression of Equation 5.3 for air is:
h ¼ 1004:5 þ xð2;500;000 þ 1858:4Þ
ð5:6Þ
where:
is the air temperature,
x is the humidity ratio, that is the mass of water vapour per kg dry air.
The humidity ratio can be calculated from water vapour partial pressure, p, and
atmospheric pressure, pa :
x¼
0:62198p
pa p
ð5:7Þ
The water vapour partial pressure is calculated from relative humidity, ’ by:
ð5:8Þ
p ¼ ’ps
where ps is the water vapour pressure at saturation, which depends on the
temperature:
22:5 if < 0
ps ¼ 610:5 exp
ð5:9Þ
273 þ if > 0
ps ¼ 610:5 exp
17:27 237:3 þ ð5:10Þ
The humidity ratio can also be derived from mass concentration of water, Cw ,
or volume concentration, cw :
x¼
Cw
cw
¼
1 Cw 1 cw
ð5:11Þ
Also interesting, and much simpler to assess, is the efficiency or effectiveness, or
temperature efficiency of the heat exchanger, which reveals how well a heat
exchanger performs. This efficiency is simply calculated from temperature
measurements in both circuits of the heat exchanger:
Hot side:
";h ¼
hot; in hot; out
hot; in cold; in
ð5:12Þ
Cold side:
";c ¼
cold; out cold; in
hot; in cold; in
ð5:13Þ
When the mass flows multiplied by the specific heats are equal on both sides the
efficiency will also be equal.
88
Ventilation and Airflow in Buildings
Exhaust air
6
Extract air
4
- +
Outdoor air
1
3
Supply air
Figure 5.7 Schematics of an air handling unit, showing location of
pressure taps for pressure differential measurements
Leakage through heat exchangers
Some heat exchangers let some air leak between both air channels. This is in
most cases not expected, since there are very few air handling units equipped
with both recirculation and a heat exchanger. In addition, some air is entrained
by the rotation of the wheel in rotating heat exchangers. The amount of air
transferred this way can be measured with tracer gases (see Chapter 2, ‘Airflow
measurements at ventilation grilles’), and the leakage flow rate is one of the
results of the measurement of airflow rates in the air handling unit.
As mentioned in Chapter 2, the global recirculation rate can easily be
checked by measuring the concentration of a tracer injected in the ventilated
space, such as the carbon dioxide exhaled by occupants. Assuming that there
is no inverse recirculation and no leaks in the air handling unit, the global
recirculation rate is:
R¼
Csupply Coutdoor
Cexhaust Coutdoor
ð5:14Þ
If no recirculation is expected, but a significant recirculation rate is observed, it
may be the result of leakage through the heat exchanger. If more information is
required, in particular to check whether it is the exchanger or another part of
the air handling unit that leaks, additional measurements could be performed,
as described in Chapter 2, ‘Airflow rate measurements in air handing units’.
Pressure differential measurements are useful to explain leakage. In addition, these are easier to perform than leakage measurements and can readily
bring information for a diagnosis. Pressure differentials should be measured
between the following locations (see Figure 5.7):
.
.
Between one and six on one hand, and three and four on the other hand.
These pressure differentials drive the leakage direction. They should be
zero or slightly positive, so that a possible leakage flow goes from supply
to exhaust, and not the contrary.
Between one and three on one hand, and four and six on the other hand.
These pressure differentials increase with clogging. Compare them with
the nominal pressure differential given by the factory for the actual airflow
Measurements and Measures Related to Energy Efficiency in Ventilation
89
rates. If these pressure differentials are significantly larger than the nominal
values, the wheel should be cleaned.
Indication on how to measure pressure differentials is given in ‘Measurement of
pressure differences’, below.
A word of caution: there should be no fan between the pressure taps used to
measure the pressure differentials!
Effect of leakages and shortcuts on heat recovery
Definitions of global heat recovery efficiency
Building leakage and shortcuts within the ventilation system may significantly
reduce the effectiveness of the heat recovery as shown below (Roulet et al.,
2001).
Consider the airflows in the ventilation unit schematically presented in
Figure 5.8. Outdoor air enters the inlet grille and is blown through the heat
recovery system, where it is either heated or cooled. When heat recovery is
not needed, for example to bring free cooling during the night, plate heat
exchangers are bypassed or the wheel is stopped in rotating heat exchangers.
Then, after additional heating or cooling when required, the outdoor air
enters the supply duct to be distributed into the ventilated space. As the
envelope is not perfectly airtight, the supply air is mixed with infiltration air
in the ventilated space. A part of the air may also be lost by exfiltration. The
extract air flows through the other part of the heat recovery system where it
is either cooled (if inlet air should be warmed up) or heated (if fresh outdoor
air should be precooled). The air is then blown to the outside through the
exhaust duct to the atmosphere.
If the exhaust and inlet grilles are not well located, it is possible that a part of
this exhaust air re-enters the inlet grille, resulting in an external recirculation
rate. Leakage through the heat recovery system may also result in an internal
recirculation rate, from inlet to exhaust, or from extract to supply.
Re
o
re
e
a
Rie
i
x
Rxs
HR
rs
s
Ventilated
space
inf
AHU
exf
Figure 5.8 The simplified network representing the air handling unit
and ducts
Note: o: outdoor air; i: inlet grille; s: supply air; x: extract air; e: exhaust air; a:
atmosphere; HR: heat recovering exchanger; Re : external recirculation; Rie : inlet to
extract recirculation; Rxs : extract to supply recirculation; inf: infiltration; exf: exfiltration. Arrows represent considered airflow rates.
Source: Roulet et al., 2001.
90
Ventilation and Airflow in Buildings
In simplified methods to calculate heating (or cooling) demand of buildings,
ventilation heat loss, V , is often calculated by (CEN, 1999, 2007):
V ¼ m_ ðhx ho Þð1 G Þ
ð5:15Þ
where:
m_ is the mass flow rate of outdoor air in kg/s,
hx is the specific enthalpy of extract air, which is considered as representative
of the average indoor air,
ho is the specific enthalpy of outdoor air,
G is the global efficiency of the heat recovery system.
This global efficiency, G , is the efficiency of the whole system, including of the
ventilated building and its ventilation equipment. It should not be confused
with the nominal efficiency of the heat recovery unit itself, "HR . This efficiency,
defined in ‘Heat exchange efficiency’, above, is measured at the factory with
balanced intake and exhaust airflow rates (m_ re ¼ m_ rs ) and is:
"HR ¼
hrs hi hx hre x re
¼
ffi
hx ho
hx ho
x o
ð5:16Þ
where the signification of subscripts can be seen in Figure 5.8, and h are specific
enthalpies of the air in J/kg. As a first approximation, only sensible heat is
considered, and the temperatures at the same locations can be used. As
shown below, this replacement leads to optimistic results when the air handling
unit has parasitic recirculation or when the building has infiltration or exfiltration.
Global heat recovery efficiency
Without heat recovery, the heat loss of the building, L , resulting from these
airflow rates is the sum of extract heat flow and exfiltration heat loss, or the
heat necessary to bring outdoor air to indoor climate conditions:
L ¼ ðm_ x þ m_ exf Þðhx ho Þ ¼ ðm_ s þ m_ inf Þðhx ho Þ
ð5:17Þ
The heat recovered by the exchanger is:
R ¼ m_ re ðhx hre Þ ¼ m_ rs ðhrs hi Þ
ð5:18Þ
since, in a first approximation, all the heat taken from extract air is given to
supply air. The global heat recovery efficiency of the system is then:
G ¼
R
m_ re ðhx hre Þ
m_ re
¼
"
¼
L ðm_ x þ m_ exf Þðhx ho Þ ðm_ x þ m_ exf Þ HR
ð5:19Þ
It can readily be seen that this global efficiency is not equal to the nominal
efficiency of the heat recovery system, "HR . An expression giving G as a function of the outdoor airflow, exfiltration and recirculation rates can be derived
from Equation 5.19 by taking account of mass conservation at the nodes of
the system.
Measurements and Measures Related to Energy Efficiency in Ventilation
91
We have mentioned above the following recirculation rates:
External
Re ¼
m_ i m_ o m_ e m_ a
¼
m_ e
m_ e
Rie ¼
Inlet to exhaust
m_ i m_ rs m_ e m_ re
¼
m_ i
m_ i
Rxs ¼
Extract to supply
m_ s m_ rs m_ x m_ re
¼
m_ x
m_ x
ð5:20Þ
ð5:21Þ
ð5:22Þ
The mass flow balance for the whole building is:
m_ a þ m_ exf ¼ m_ o þ m_ inf
ð5:23Þ
Combining this equation with the definition of the external recirculation rate,
we get:
m_ e ¼
1
ðm_ þ m_ inf m_ exf Þ
1 Re o
ð5:24Þ
Then, writing the mass flow rate balance at node 1 (see Figure 5.8), we get:
m_ i ¼ m_ o þ Re m_ e ¼
m_ o þ Re ðm_ inf m_ exf Þ
1 Re
ð5:25Þ
The mass balance at node 2 gives:
m_ rs ¼ ð1 Rie Þm_ i ¼
ð1 Rie Þ
½m_ þ Re ðm_ inf m_ enf Þ
ð1 Re Þ o
ð5:26Þ
From mass balances at nodes 3 and 4:
m_ s ¼ m_ rs þ Rxs m_ x
ð5:27Þ
and
m_ s ¼ m_ x þ m_ exf m_ inf
ð5:28Þ
we get
m_ x ¼
¼
1
½m_ þ m_ inf m_ exf 1 Rxs rs
m_ o ð1 Rie Þ þ ð1 Re Rie Þðm_ inf m_ exf Þ
ð1 Rxs Þð1 Re Þ
ð5:29Þ
Mass balance at node 5 gives:
m_ re ¼ m_ x ð1 Rxs Þ
ð5:30Þ
Therefore:
G ¼
m_ x ð1 Rxs Þ
" ¼ x re "HR
m_ x þ m_ exf HR
ð5:31Þ
92
Ventilation and Airflow in Buildings
where:
x ¼
m_ x
m_ x þ m_ exf
ð5:32Þ
is the extraction efficiency, i.e. that part of the air leaving the ventilated volume,
which is extracted through the air handling unit, and
re ¼ 1 Rxs ¼
m_ re
m_ x
ð5:33Þ
is the air recovery efficiency, or that part of the extract air that passes through
the heat recovery unit.
Looking at Equation 5.32, it seems at first glance that the global heat
recovery efficiency depends only on extract and exfiltration airflow rates.
However, the purpose of ventilation is to provide fresh, outdoor air in the ventilated volume. Let us see how Equation 5.32 is changed when the fresh airflow
rate taken at inlet grille is used as a reference.
Fresh air entering the air handling unit is m_ o . Because of external recirculation, this air is mixed with exhaust air into the inlet duct. A part, Rie , of this mix
is recirculated to the exhaust duct. All the fresh, outdoor air that enters the
building through the air handling unit is found in m_ rs , which is, from the definition of Rie and using Equation 5.25:
m_ rs ¼ ð1 Rie Þm_ i ¼ ð1 Rie Þðm_ o þ Re m_ e Þ
ð5:34Þ
Since m_ e is no longer fresh, the only part of m_ rs that is fresh is m_ o ð1 Rie Þ.
Therefore the total fresh airflow rate entering the ventilated space in building
is:
m_ ¼ m_ o ð1 Rie Þ þ m_ inf
ð5:35Þ
which means that
m_ o ð1 Rie Þ ¼ m_ m_ inf
ð5:36Þ
replacing in Equation 5.32 m_ x by its value given by Equation 5.29, and taking
into account the above relation, gives finally:
G ¼
b1 exf Re Rie ðinf exf Þcð1 Rxs Þ
"
1 Re Rie ðinf exf Þ exf ½Re þ Rxs ð1 Re Þ HR
ð5:37Þ
where
inf ¼
m_ inf
m_
and
exf ¼
m_ exf
m_
ð5:38Þ
are respectively the infiltration and exfiltration ratios. In other terms, the
extraction efficiency is:
x ¼
1 exf Re Rie ðinf exf Þ
1 Re Rie ðinf exf Þ exf ½Re þ Rxs ð1 Re Þ
ð5:39Þ
Measurements and Measures Related to Energy Efficiency in Ventilation
Global efficiency
1.0
93
Recirculation
Rxs
0.8
0.0
0.6
0.2
0.4
0.4
0.2
0.6
0.0
0.0
0.8
0.2
0.4
0.6
0.8
Exfiltration ratio γexf
1.0
Figure 5.9 Relative decrease of global heat recovery efficiency as a
function of exfiltration ratio exf and internal recirculation rate Rxs
Source: Roulet et al., 2001.
which depends on all parasitic airflow rates. When there is no external recirculation (Re ¼ 0), Equation 5.37 simplifies to:
G ¼
ð1 exf Þð1 Rxs Þ
"HR ¼ x "HR
1 Rxs exf
ð5:40Þ
and infiltration has no effect. In this case, exfiltration through the envelope and
internal recirculation from extract to supply ducts have the same effect, since
both drive air away from the heat recovery device. The extraction efficiency
in Equation 5.40 is illustrated in Figure 5.9, which indeed represents the
relative reduction of heat recovery resulting from exfiltration and internal
recirculation.
Global efficiency G equals the effectiveness "HR only if there is no exfiltration, and neither external nor extract-to-supply recirculation. Otherwise, G is
smaller than "HR .
The inlet to exhaust recirculation, as well as the infiltration ratio, have only
a small effect on heat recovery efficiency, but reduce the amount of fresh air
supplied by the unit to the ventilated space. In order to get the same amount
of fresh air, the supply airflow rate should be increased. Fresh air efficiency
can be defined by:
o ¼
m_ m_ inf m_ o ð1 Rie Þ
¼
m_ s
m_ s
ð5:41Þ
This recirculation obviously results in an increased consumption of electric
energy for the fans, which is approximately proportional to the cube of
the airflow rate, without delivering more fresh air. However, such parasitic
recirculation is often not noticed, and hence can lead to an undiscovered
reduction of indoor air quality.
94
Ventilation and Airflow in Buildings
Net energy saving and performance index
Heat recovery systems recover thermal energy but use electric energy for the
fans. The net energy saving should therefore take into account the primary
energy needed to produce electricity and the fact that the losses of the fans
heat the air. The net energy saving per cubic metre of supplied outdoor air
(SNES in Wh/m3 ) averaged over a heating period is:
SNES ¼ o
G L þ fan ð fr fp Þ
m_
ð5:42Þ
where:
L ¼ m_ cðx o Þ is the ventilation heat loss, based on average internal and
external temperature during the heating season;
is the part of the fan power recovered as heat in the supply
fr
air. This factor fr is close to one for supply fans and zero
for exhaust fans;
is a production factor, accounting for the fact that the
fp
production of 1 kWh of electric energy requires much more
primary energy.
A net gain in thermal or primary energy is achieved by the heat recovery system
only when SNES is positive. Otherwise the system even wastes energy.
By analogy with heat pumps, a coefficient of performance, COP, is defined
by the ratio of recovered heating power and used electric power:
COP ¼
G L þ fr fan
fan
ð5:43Þ
This COP is defined without taking account of the production factor, fp , as is
usually the case for heat pumps.
Examples of application
Airflow rates and heat exchanger efficiencies were measured in ten large units
and three small, wall-mounted room ventilation units. The main characteristics
of these units are summarized in Table 5.2.
Recirculation ratios and efficiencies measured in these units are given in
Table 5.3 and illustrated in Figure 5.10. The specific net energy saving
(SNES) and COP are calculated with a 16 K indoor–outdoor average temperature difference during 210 days, a recovery factor for fans, fr ¼ 0:5 (taking
account that there are two fans in these units, one of them in the supply
duct) and a production factor, fp ¼ 3:55, which is the average for low-voltage
electricity in Europe according to Frischtknecht et al. (1994).
Major leakages have been observed in several buildings. In four of them,
infiltration represents a significant part of the outdoor air, and in four of
them, most of the air leaves the building through the envelope instead of
passing the heat recovery unit. Significant internal recirculation is observed
Measurements and Measures Related to Energy Efficiency in Ventilation
95
Table 5.2 Measured airflow rates with experimental uncertainty band (when
available), total and specific fan power in audited units
Airflow rates (m3 /h)
Unit Outdoor air
1
2
3
4
5
6
7
8
9
10
1900 100
2530 80
2380 70
2200 300
5000 200
15,000 2000
11,000 400
16,000 1000
9000 1000
14,300 600
a
b
c
25
42
74
Supply air
Fan power
Extract air
Exhaust air
W
Wh/m3
2070 70
1790 40
1600 200
990
2900 200
1860 50
1500 200
850
2480 70
1930 40
1830 50
1800
3400 100
3240 90
2000 2000 1800
5400 100
6000 700 5500 700
3710
16,400 700 11,000 1000 10,000 3000 11,800
11,600 200 10,000 300 9500 900
8180
17,400 700 13,400 600 12,000 2000 9760
10,000 2000 1970 90
1000 3000 3800
16,200 400
3420 70
1000 1000 7970
36
75
87
34
74
87
24
41
74
13
27
32
0.27
0.19
0.42
0.33
0.34
0.45
0.39
0.33
0.35
0.45
0.22
0.24
0.20
Table 5.3 Outdoor air efficiency, o , exfiltration and infiltration ratios exf
and inf; external and internal recirculation rates Re , Rxs and Rie , heat
recovery effectiveness "HR , global heat recovery efficiency G , specific net
energy saving, SNES in Wh/m3 , and coefficient of performance, COP, of
audited air handling units
o
exf
inf
1
2
3
4
5
6
7
8
9
10
97%
92%
100%
68%
98%
97%
100%
97%
95%
93%
16%
47%
29%
77%
8%
43%
14%
25%
97%
91%
0%
6%
9% 20%
7%
0%
76% 55%
17%
0%
8%
0%
0%
4%
0%
0%
49%
0%
18% 100%
a
b
c
74%
57%
68%
8%
2%
0%
Unit
0%
0%
0%
Re
0%
0%
0%
Rxs
Rie
x
"HR
G
7%
5%
5%
1%
7%
6%
2%
0%
0%
6%
0%
0%
0%
0%
0%
0%
0%
0%
0%
0%
86%
59%
72%
31%
92%
61%
87%
77%
10%
18%
70%
70%
90%
30%
80%
90%
80%
70%
50%
50%
56%
1.55
39%
1.35
62%
1.18
9% 0.05
69%
1.92
52%
0.69
68%
1.45
54%
1.17
5% 0.37
8% 0.92
6.5
8.0
5.2
3.3
6.7
4.5
5.5
5.5
1.8
1.5
0% 94%
4% 99%
25% 100%
63%
80%
90%
40%
44%
55%
6.2
6.8
8.2
33%
44%
39%
SNES COP
1.37
2.21
2.69
Ventilation and Airflow in Buildings
Global recovery efficiency
96
100%
Large units
Small units
80%
60%
40%
20%
0%
0%
20%
40%
60%
80%
100%
Heat exchanger efficiency
Figure 5.10 Global heat recovery efficiency versus nominal heat exchanger
effectiveness measured in several units
8
7
6
5
4
3
2
1
0
Good
Poor
Unacceptable
COP
in the three small units, and external recirculation above 20 per cent is
measured in three large units. These leakages significantly affect heat recovery
efficiencies, which drop from nominal values of between 50 and 90 per cent
down to actual values ranging between 5 and 69 per cent. On average, the
nominal heat recovery effectiveness "HR is 70 per cent, but the global, real
efficiency is only 43 per cent. In the best case, 80 per cent heat recovery
effectiveness is reduced by 15 per cent down to a 69 per cent real efficiency.
Only 8 units out of 13 have a net energy saving larger than 1 Wh/m, as
shown in Figure 5.11. Note that 1 Wh allows heating one cubic metre of air
by about 38C. Negative specific net energy savings are observed in three
units, where the heat recovery uses more energy than it saves! The coefficient
of performance of good units can be much larger than those of a heat pump
used for heating buildings, but is rather small in three units. A COP of less
than 2.5 indicates that the heat recovery is less efficient than heating the air
with a gas boiler with 75 per cent efficiency (Ruyssevelt, 1987).
–1
Large units
Small units
0
1
2
SNES [Wh/m3]
3
Figure 5.11 Seasonal average coefficient of performance and specific net
energy saving of the tested units
Measurements and Measures Related to Energy Efficiency in Ventilation
97
Best net energy savings in large units (7 and 8 in Table 5.2) are 80,000–
90,000 kWh per winter season, but unit 10 actually wastes as much energy.
Small units (a, b and c) save between 80 kWh and 350 kWh during an entire
season. From an energy and economic lifetime analysis perspective, such
units are disadvantageous.
Note that these results are obtained when the heat recovery is functioning.
Annual average efficiency may even be less due to reduced operation time
(Drost, 1993).
Energy for ventilation
The energy to move the air is the product of a force by a displacement. The
force is the pressure, p, exerted on the section area, A, of the duct, and the
displacement is the path, l, of the air during a time interval, t. But A l is
the volume of air displaced during this time interval. The energy to move a
volume V of air is hence:
Em ¼ pA l ¼ pV
ð5:44Þ
Taking a time derivative of the above equation provides the mechanical power,
m , needed to get an airflow rate, q:
dEm
dV
¼ p
¼ pq
ð5:45Þ
dt
dt
The mechanical power delivered by a fan is the product of the volume airflow
rate, Q, delivered by the fan, and the pressure differential, p, across the fan.
The mechanical power required to move the air through a ductwork is also the
product of the volume airflow rate through the ductwork, and the pressure
difference between the main supply and main exhaust ducts. Since the pressure
difference is proportional to the square of the airflow rate, the mechanical
power for ensuring a given airflow rate into a ductwork is proportional to the
cube of the airflow rate! Increasing the airflow rate in a room by 10 per cent
requires 33 per cent more fan power and doubling the airflow rate requires a
power eight times larger if the ductwork is not adapted to this new airflow rate.
m ¼
Why check fan power and related quantities?
The electrical energy needed to move the air depends on the properties of the
air distribution system and of the fan. For a given nominal power, efficiencies
varying by a factor two or more were measured (see ‘Examples of application’,
below). Assessing the fan efficiency and the specific power (in Joules or
Watt-hours per cubic metre of transported air) is part of a comprehensive
energy diagnosis of a mechanical ventilation system.
Poor fan efficiency not only wastes expensive electric energy, but also
hinders efficient cooling. The cooling power of the air blown by the fan is:
cool ¼ cQ ¼ cq ð5:46Þ
98
Ventilation and Airflow in Buildings
where:
is the density of air,
c
is the heat capacity of air,
is the temperature difference between exhaust air and supply air.
The kinetic energy given to the air by the fan is, sooner or later, degraded into
heat by viscosity and friction on the surfaces of ducts, room walls and furniture.
The kinetic energy of the air leaving the room to the outside is very small when
compared to that of the air just after passing through the fan, especially in units
with large recirculation ratios. Since the fan motor is in the airflow, its heat loss
is also delivered to the air. Therefore, nearly all the energy given to the fan ends
as heat in the indoor air. This corresponds to a heating power equal to the
electric power consumed by the fan motor, e . Hence:
heat ¼ e ¼
q p
f
ð5:47Þ
For air conditioning, the heating power should be small when compared to the
cooling power. Therefore, the ratio:
cool
c ¼ f
heat
p
ð5:48Þ
should be as large as possible. This means that the fan efficiency should be as
close as possible to one (or 100 per cent). In addition, the pressure differential
should be as small as possible.
Another way to look at this issue is to calculate the air temperature increase
resulting from heat loss:
heat ¼
e
p
¼
cq f c
ð5:49Þ
This should be as small as possible, so again, the fan efficiency should be large
and the pressure differential should be at a minimum.
The energy losses of fans are shared between the elements of the chain
linking the electrical network to the aeraulic ductwork (see Figure 5.12). In
this chain, the fan is often the worst culprit. It is not, however, simple to
assess the efficiency of each element, and we will concentrate on the measurement of the efficiency of the whole chain, by measuring on the one hand the
Converter
Motor
Transmission
Fan
Ductwork
η = 0.95
η = 0.8
η = 0.95
η = 0.6
Sizes the fan
Figure 5.12 Approximate figures for the efficiencies of various elements
needed to move the air in the ductwork
Measurements and Measures Related to Energy Efficiency in Ventilation
99
consumption of electrical energy by the fan motor, and on the other hand the
kinetic energy given to the air in duct.
The fan efficiency is the ratio of useful power, m , to the electrical power
consumed by the fan motor, e :
f ¼
m q p
¼
e
e
ð5:50Þ
Measuring the airflow rate, q, and the pressure differential, p, across the fan
provides the kinetic power of the air, and measuring in addition the electric
power used by the fan allows for the assessment of fan efficiency.
The uncertainty band resulting from uncertainties on the measured quantities is:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðqÞ 2
ðpÞ 2
e 2
þ
þ
ð5:51Þ
f ¼ f
e
q
p
Measurement of airflow rate
Airflow rates through both supply and exhaust fans can be assessed by tracer
gas measurements, as described in Chapter 2, ‘Airflow measurements at
ventilation grilles’. Depending on the units used when interpreting the results,
these measurements may provide either volume airflow rates, qv , or mass
airflow rates, Qm . These are related by:
Qm ¼ q
where is for the density of the air, which can be calculated by:
p
p½Pa
M
½kg=m3 ¼
ffi 3:46 103
T½K
RT
ð5:52Þ
ð5:53Þ
where:
p
T
M
R
is the
is the
is the
is the
atmospheric pressure (average 101,300 Pa at sea level),
absolute temperature,
average molar mass of the air mixture (about 28.8 g/mole),
molar gas constant ¼ 8.31396 J/(moleK) .
Then, if mass airflow rates are taken from tracer gas measurements, the airflow
through the fans should first be converted into volume airflow rate.
Airflow rate through the fan can also be assessed from the fan speed, the
pressure differential across the fan, and the fan characteristics – provided by
the factory – which give the airflow rate from the fan speed and pressure
differential.
Measurement of pressure differences
The pressure differential is measured with a differential manometer with a
range of 200–500 Pa (20–50 mm water column).
100
Ventilation and Airflow in Buildings
q
265 Pa
Dp
Figure 5.13 Installation of the differential manometer to measure the
pressure differential across the fan
The two ports of this manometer are connected to pressure taps located on
both sides of the fan (see Figure 5.13). Care should be taken to avoid too much
dynamic pressure on these taps. It is advisable to install the pressure taps
perpendicular to the airflow, preferably close to the air duct wall, and at
locations where the air velocity is about the same on both sides of the fan so
that the dynamic pressure, if any, is the same on both sides. If the pressure
varies significantly when moving one of the pressure taps, this means that the
dynamic pressure has an effect.
On most air handling units, a differential pressure switch is installed to
check the function of the fan. This switch is connected by two pipes to taps
installed in the ducts before and after the fan. These taps can be used to connect
the differential manometer, but the safety switch should be either disabled or
short-circuited. Otherwise, the fan motor will stop as soon as the pipes of the
pressure switch are disconnected.
Measurement of electric power
The electric power used by the fan motor is measured with a wattmeter, which
should be wired to the fan according to Figure 5.14. This instrument measures
Voltage
inputs
U
V
W
Current
inputs
13.3 kW
U
V
W
R
S
T
Figure 5.14 Schematics of electric power measurement on a
three-phase motor
Measurements and Measures Related to Energy Efficiency in Ventilation
101
simultaneously the r.m.s. voltage, U, between phase and neutral point, the
r.m.s. current, I, running into each motor coil and the phase shift, ’, between
voltage and current. The power is calculated by:
e ¼
3
X
Uj Ij cosð’j Þ
ð5:54Þ
j¼1
the sum being on all three phases.
In very small air handling units, the fan motor may be single phase, and
e ¼ UI cosð’Þ
ð5:55Þ
To measure the current an ampere-meter must be installed in the circuit. An
easy way is to use clamp-on ampere-meters. A measuring clamp is installed
around each wire leading to the motor. This clamp contains a transformer
that gives a current proportional to the current running through the closed
clamp. Care should be taken not to install the clamp around the two-wire or
four-wire cable. The measured current will be zero in this case, whatever the
power used by the motor.
In some air handling units, the fan is controlled by a variable frequency
controller (see Figure 5.15). Such devices are often equipped with a screen
Figure 5.15 Front panel of the variable frequency controller
102
Ventilation and Airflow in Buildings
100%
Efficiency
80%
60%
40%
20%
0%
0
2,500
5,000
Measured power [W]
7,500
Temperature increase [K]
Figure 5.16 Fan efficiencies as a function of actual fan motor power
2.5
2.0
1.5
1.0
0.5
0.0
0
500
1,000
Pressure differential [Pa]
1,500
Figure 5.17 Air temperature increase as a function of pressure differential
across the fan
on which the frequency, the voltage, the current and the fan motor power can
be displayed.
Examples of application
Fan efficiencies were measured on several fans of various units. Figure 5.16
represents the measured fan efficiencies versus their measured used electric
power. It shows a general improvement of the fan efficiency when fan motor
power increases. However, the dispersion is huge and large differences can
be observed for each power class. For example, efficiencies range from 30 per
cent to more than 60 per cent for 3 kW fans, and from 10 per cent to 35 per
cent for small fans. Figure 5.17 illustrates the fact mentioned above (see
‘Why check fan power and related quantities?’) that the air temperature
increases when the pressure differential is large. The dispersion results from
variations in fan efficiencies.
Energy effects of indoor air quality measures
In the 1970s, after the oil crises, measures were hastily taken in many buildings
to reduce their energy use. These measures were planned with only two
objectives: energy efficiency and return on investment, without taking care of
Measurements and Measures Related to Energy Efficiency in Ventilation
103
indoor environment quality and health or paying attention to possible damages
to buildings. If a decrease of thermal comfort was implicitly accepted, cases of
mould growth, increased indoor pollution and health hazards were not
expected but often observed. Since then, the idea that saving energy in buildings decreases the indoor environment quality still prevails.
Of course, some energy conservation opportunities such as low internal
temperature or too low ventilation rates may degrade the indoor environment.
These should therefore either be avoided, or accepted only in case of emergency
and for a limited period of time.
Some other energy saving measures should be used only in conjunction with
others. For example, retrofitting windows in poorly insulated dwellings leads to
a risk of mould growth, and improving the envelope airtightness without taking
care of ensuring and controlling a minimum ventilation rate may decrease
indoor air quality.
Table 5.4 lists, in the first column, various uses of energy in buildings.
Known ways to save energy are presented in the second column, and effects
of these energy saving measures on comfort or indoor environment quality
are presented in the third column. It can readily be seen that there are many
cases where energy saving measures, when well designed and executed,
improve indoor environment quality.
Several recommendations, resulting from experience and recent surveys
performed within European projects (Bluyssen et al., 1995; Roulet et al.,
2005) are given below.
Method
The method used to scientifically support these recommendations is described
in detail in Jaboyedoff et al. (2004). A typical office building equipped with full
air conditioning and cold ceiling, including heat recovery, was simulated using
an appropriate computer program. The heating, ventilation and air conditioning (HVAC) system is shown in Figure 5.18.
The three-storey building is of heavy construction and well insulated with
low-e, clear glazing. The office rooms are oriented south with 53 per cent glazed
area. The internal temperature can be controlled using either air conditioning
or hydronic heating and cooling.
Numerous simulations were performed for different climates: northern
(Oslo), central (London and Zu¨rich) and southern Europe (Rome), and the
following variants were calculated:
.
.
.
.
.
.
.
outdoor air supply: 15–50 m3 /hour and per person;
relative humidity of supply air to room: 50 per cent and no humidification;
efficiency of heat recovery: 0 (no heat recovery), 0.75 and 0.85;
infiltration: 0.5 and 1.0 air changes per hour;
set point for cooling: 248C, 288C and no cooling;
ventilation: 24 hours a day or during working hours only (7 am to 7 pm);
natural ventilation – using windows – instead of mechanical ventilation.
104
Ventilation and Airflow in Buildings
Table 5.4 Uses of energy in buildings, energy saving measures and their effects on
indoor environment quality
Energy use
Energy saving measures
Impact on indoor
environment
Compensation of
transmission heat
loss in winter
Better, thicker insulation
IR reflective by low emissivity
coated
Improves comfort and health
by preventing mould growth
Compensation of
ventilation heat
loss in winter
Lower ventilation rate
Needs a reduction of indoor
pollution sources to maintain
indoor air quality
Less draughts, less noise
Generally improves indoor air
quality in winter
Winter heating in
general
Improve solar gains with larger,
well-oriented windows
Improve the use of gains by
better insulation and good
thermal inertia
If windows are poor: cold
surfaces
Overheating if poor solar
protection
If well designed: good visual
contact with outdoor
environment, excellent
summer and winter comfort
Elimination of heat
gains during warm
season
Use passive or ‘free’ cooling
Very comfortable in
appropriate climates and
buildings
Better indoor air quality and
comfort
Should be kept within comfort
zone
Internal
temperature
control
Comfortable set-point
temperature, improved control
Avoids over- and under-heating
Ventilation
(moving the air)
Natural ventilation
Reduce airflow rate
Increase duct size
Efficient fans
Best where applicable
Possible only where
overventilated
Less noise
Less noise
Humidification
Switch it off
No health effect in most cases
Lighting
Use daylighting
Comfortable light, with limited
heat gains when well controlled
Comfort depends on the quality
of light. Reduced heat load
Limit the ventilation rate to the
required level
Use heat recovery on exhaust
air
Use efficient, wellcommissioned and maintained
cooling systems
Higher internal temperature
Use efficient artificial lighting
Measurements and Measures Related to Energy Efficiency in Ventilation
105
Cold ceiling
Fan
Heating
Humidifaction
Cooling
Heat exchanger
Filter
Office space
Radiators
Figure 5.18 The HVAC system in the simulated building
For each variant, the effect of the following changes in design and operation was
simulated:
.
.
.
.
.
.
.
.
.
with 50 per cent recirculation instead of no recirculation;
with or without heating or cooling supply air;
effect of ventilation strategies on heating demand;
effect of airtightness on heating demand;
with and without a droplet catcher with 20 Pa pressure drop downwind of
the cooling coil;
with an improved filter with 150 Pa pressure drop;
changing used filters at a pressure drop of 180 Pa instead of 250 Pa;
with rotating or flat plate heat exchanger: efficiency 0.85 and 0.75;
effect of increase of pressure difference through ductwork: 1600 Pa versus
1000 Pa.
Simulation results
The main results of these simulations are summarized below. These conclusions, in particular the numerical values, are valid for the building and the
system simulated. However, the order of magnitude and general tendencies
are likely to be valid for more general situations.
Recirculation
The electric energy used by cooling and fans decreases by about 40 per cent
(27 per cent for Rome, 43 per cent for Zu¨rich and 50 per cent for Oslo) if no
recirculation is used, compared to 50 per cent recirculation. No heat recovery
was used in these cases.
Heating
The energy use for heating mainly depends on climate and internal gains. Except
for heat recovery and time schedule of operation (working hours/24 hours
106
Ventilation and Airflow in Buildings
per day), ventilation strategies have a minor influence on heating energy demand.
The air may be either heated by coils in the supply air or by radiators in the room.
The energy use for heating does not change significantly in all climates.
The tightness of the building envelope has a large influence, up to a factor of
two, on the heating energy need. When high infiltration occurs, humidity is also
reduced in winter.
Cooling
Cold ceilings are more effective than air conditioning. For the same airflow rate
and same comfort conditions, more energy was required for cooling using air
conditioning than with the hydronic cooling ceiling.
Lowering the set point for cooling from 268C to 248C causes an augmentation of the cooling demand of the zones by a factor of three to eight, depending
on the geographic location.
A droplet catcher downwind of a cooling coil has a negligible effect on
energy demand, but may be essential to avoid humidifying downstream filters
or acoustic dampers, changing them to biotopes.
Filters
Using a two-stage filter system instead of an old F7 filter leads to an increase
in electric power use for fans of 10–15 per cent, depending on the pressure
difference over the system.
Again, depending on the pressure difference over the system, earlier
replacement of a filter results in a decrease in fan power consumption by 2–3
per cent.
Humidification
In the northern (Oslo) and central locations (London and Zu¨rich), humidifying the supply air at 30 per cent minimum relative humidity requires about
20–25 per cent more energy for ventilation than without humidification. In
the southern climate (Rome), the increase is only 3 per cent, mainly because
humidification is seldom required. In all climates, humidifying the supply air
at 30 per cent increases the total heating energy need by 5–10 per cent, while
this need almost doubles if the relative humidity is set at 50 per cent.
Heat recovery
Without heat recovery, the heating energy use for ventilation is 70–140 per cent
more than with medium efficiency (50 per cent) heat recovery.
Heat recovery with high efficiency (75 per cent) – such as those achieved by
well-installed rotating heat exchangers in airtight buildings – leads to a reduction of the heating energy demand for ventilation by about 30 per cent,
compared to medium efficiency (50 per cent) heat recovery. That means that
the 3 per cent reduction in efficiency caused by installing a purging sector in
Measurements and Measures Related to Energy Efficiency in Ventilation
107
a rotating heat exchanger (see Chapter 6, ‘Rotating heat exchangers’), has a
negligible effect on energy demand.
Infiltration or exfiltration through a leaky building envelope strongly
reduces the efficiency of heat recovery (see ‘Effect of leakages and shortcuts
on heat recovery’, above). With a heat recovery efficiency of 75 per cent, the
heating energy demand for cold and mild climates (Oslo, Zu¨rich, London) is
approximately 3–5 per cent higher with an efficiency of 0.75 than with 0.85.
For warm climates (Rome), this number is approximately 15 per cent higher,
however, with low absolute values.
Ductwork
An increase of pressure difference from 1000–1600 Pa, caused by air velocity,
length, curves, duct wall smoothness and deposits in the ducts, leads to an
increase in electric power use of 60 per cent. The increase in total electric
power depends on the geographic location and ranges from 25–55 per cent.
Notes
1 Natural ventilation can be controlled by installing (automatically or manually) adjustable vents in an airtight building envelope.
2 These ratios take not only density and heat capacity into account, but also practical
temperatures.
6
Contaminants in
Air Handling Units
The purpose of mechanical ventilation systems is to supply appropriate
amounts of clean air and to evacuate vitiated air. However, in field audits it
was seen that ventilation systems often host contaminant sources and are, in
the worst cases, the main source of air pollution in buildings (Fanger, 1988;
Bluyssen et al., 1995, 2000b). Components in the mechanical ventilation
system may considerably pollute the passing air. The main sources and reasons
for pollution in a ventilation system vary considerably depending on the type
of construction, use and maintenance of the system. This chapter summarizes
the results of these field audits, and proposes methods to detect the sources of
contaminants and strategies to avoid these.
Filters
Filters are one of the main sources of sensory pollution in ventilation systems
(Bluyssen et al., 2000a). Some new filters may also influence the perceived air
quality negatively. The filter material has a significant influence on the starting
pollution effect of new filters (see Figure 6.1).
When filters get older, i.e. are in use for some time, the emission of odours
first decreases, but increases again later, when the filter gets loaded. The reason
for this emission after the filter is in use for some time remains unclear,
however.
Micro-organisms may not be the only pollution source on a filter, but it is
important to keep filters dry, since wet media filters are perfect supports for
microbial growth and microbes may also emit dangerous pollutants and bad
odours. Filters may be moistened either by snow, rain or fog entering the
outdoor air inlet, or by water droplets spread by some humidifiers or found
in airflows downstream of the cooling coils.
Environmental conditions such as airflow (amount or intermittent/continuous) and temperature do not have a significant influence on the pollution of
downwind air.
Contaminants in Air Handling Units
109
16
Cassette
Cellulose
Glass fibre
Odour intensity
14
12
10
8
6
4
2
0
0
500
1000
Airflow rate [m3/h]
1500
Figure 6.1 Olfactive pollution of various new filters as a function of
airflow rate
Source: Bluyssen et al., 2000a, 2003.
Ducts
The duct material and the manufacturing process has the biggest effect on the
perceived air quality (Bjo¨rkroth et al., 2000). Depending on the machinery used
in the manufacturing process, new spiral wound ducts, flexible ducts and other
components of the ductwork might contain small residual amounts of processing oil. The oil layer is very thin and invisible, but it emits an annoying
odour. Aluminium ducts score the best with respect to perceived air quality.
Plastic ducts seem a feasible solution, but some flexible plastic ducts are very
smelly.
Oil residues are the dominating sensory pollution source in new ducts.
The sensory assessments showed a clear correlation between the total mass of
oil residues (average surface density surface area) and the perceived air
pollution (see Figure 6.2).
7
Odour intensity
6
5
4
3
1 m/s
3 m/s
5 m/s
2
1
0
0.0
0.5
1.0
1.5
Mass of oil residue in duct [g]
Figure 6.2 Correlation between odour intensity and the mass of
oil residues in the tested ducts
Source: Bjo¨rkroth et al., 2000.
110
Ventilation and Airflow in Buildings
The effect of airflow on the perceived air quality from ducts was relatively
small and is probably insignificant in normal applications. Increasing the
airflow rate in the duct does not, surprisingly, reduce the odour intensity: the
additional airflow rate certainly dilutes the evaporated oil but the increased
air velocity also evaporates more oil.
Emissions from dust/debris accumulated in the ducts during construction
(mostly inorganic substances) seem to be less important. No simple correlation
was observed between the amount of accumulated dust and odour emissions.
However, the organic dust accumulated during the operation period may
produce more severe odour emissions. When dust has accumulated on the
inner surface of the ducts, the relative humidity of the air in the ducts has a
larger effect on the odour emissions of ducts without oil residues than ducts
with oil residues.
Humidifiers
Odour intensity
The main reasons for pollution from humidifiers are: disinfecting additions, old
water in tanks or dirty tanks, microbiological growth, stagnant water in the tank
when the humidifier is off, and desalinization and demineralization devices and
agents (Mu¨ller et al., 2000).
Humidifiers only pollute the air significantly if the humidifier is not used or
maintained in the prescribed way. Investigations make it clear that periodical
cleaning of humidifiers and the use of fresh water are paramount for a good
air quality.
Under normal conditions, it was found for all humidifiers that airflow has no
influence on the odour intensity caused by humidifiers (see Figure 6.3).
A relation was found between perceived air quality and the concentration of
bacteria on the inside of the humidifier (see Figure 6.4). The odour intensity
increases with increasing number of bacteria. This was not the case for other
locations in an HVAC system. A similar correlation could not be found for
fungi.
10
9
8
7
6
5
4
3
2
1
0
0
1 on
2 on
3 on
1 off
2 off
3 off
500
1000
1500
2000
Airflow rate [m3/h]
Figure 6.3 Perceived air quality for the steam humidifier
Source: Mu¨ller et al., 2000.
Contaminants in Air Handling Units
111
Odour intensity
6
5.5
5
4.5
4
3.5
200
300
400
500
600
Surface concentration of bacteria
[thousands of CFU/cm2]
700
Figure 6.4 Bacteria concentration at inner surface of a humidifier
correlated with the odour intensity
Note: CFU ¼ colony forming unit.
Source: Mu¨ller et al., 2000.
Rotating heat exchangers
Rotating heat exchangers are not themselves sources of contaminants, but they
may transfer contaminants from exhaust to supply air with entrained air, and
through possible leakage around the wheel at the separation wall. Leakage from
exhaust to supply was measured by the author in several units, and found to be
negligible in most cases (see Chapter 5, ‘Leakage through heat exchangers’).
A part of the extract air is indeed entrained to the supply duct by the
rotation of the wheel, as shown in Figure 6.5. All the exhaust air contained
in a sector of the wheel is entrained back into the supply air.
This is avoided by installing a purging sector, which returns this vitiated air
back to the exhaust duct (see Figure 6.6). This chamber covers a sector of about
58, in which the outdoor air passes through the wheel, makes a 1808 turn in the
purging chamber, passes back in the wheel and finally leaves the air handling
unit by the exhaust air duct. This cleans the wheel from contaminants accumulated when passing in the extract air, before entering the outdoor air. Note that
this device functions properly only when the sense of rotation of the wheel is
Supply air
Rotation air
Extract air
Figure 6.5 Some extract air is entrained in the supply airflow by the
rotation of the wheel
112
Ventilation and Airflow in Buildings
Exhaust air
Outdoor air
Extract air
Supply air
Figure 6.6 Schematics of the purging sector
Note: A part of the outdoor air cleans the porous structure and then is sent back to the
exhaust air.
such that a sector of it that contains exhaust air passes first through the purging
chamber. The author has seen wheels turning the wrong way!
In addition, contaminants can be transferred from exhaust to supply ducts
by adsorption–desorption. This was confirmed by measurements with volatile
organic compounds (Andersson et al., 1993; Roulet et al., 2000) and perceived
air quality (Pejtersen, 1996). For example, measurements performed by the
author according to the protocol described in ‘Contaminant transport in
rotating heat exchangers’, below, gave the transfer rates illustrated in Figure
6.7. This figure shows transfer rates with and without a purging sector.
Leakage and entrained air would result in the same recirculation rate for all
chemical compounds, this rate being close to zero for the unit giving the results
of Figure 6.7. This is obviously not the case.
70%
60%
50%
40%
30%
20%
10%
0%
n-
D
ec
an
e
1Bu
ta
no
l
1H
ex
an
ol
Ph
en
ol
1H
ex
an
Be
al
nz
al
1,
de
6hy
D
ic
de
hl
or
oh
ex
an
D
ip
e
ro
py
le
th
er
Li
m
on
en
e
m
-X
yl
en
e
M
es
ity
le
ne
No purging sector
With purging sector
Figure 6.7 Average VOC recirculation rates measured in the EPFL
laboratory unit, with and without a purging sector
Source: Roulet et al., 2000.
Contaminants in Air Handling Units
113
Figure 6.7 shows that certain categories of volatile organic compounds
(VOCs) are easily transferred by a sorption transfer mechanism. Among the
tested VOCs, those having the highest boiling point were best transferred. The
largest transfer rate in a well-installed unit was found for phenol (30 per cent).
Leakage and pollutant transfer can be avoided or at least strongly reduced
through proper installation of the wheel, good maintenance of the gasket,
proper installation of a purging sector, and by maintenance of a positive
pressure differential from supply to exhaust duct at wheel level.
Coils
Laboratory tests (Bluyssen et al., 2003) show that heating and cooling coils
without condensing or stagnating water, are components that have small contributions to the overall odour intensity of the air. On the contrary, cooling coils
with condensed water in the pans are microbial reservoirs and amplification
sites that may be major sources of odours to the inlet air.
Measurement protocols
HVAC systems are in general low sources of measurable chemical pollutants.
When searched for, most pollutants are below the detection limits of
common analysers, and chemical analyses can be successful only in very
polluted systems. They are therefore not discussed here.
No standard procedure exists for microbiological measurements in ventilation systems. The techniques used are air sampling with impactors, gluing
airborne microbes (mould, yeast, bacteria) on appropriate culture media, or
simply exposing these culture media in open Petri dishes or on films lying or
glued on the inner walls of ducts or units. The main problem is ensuring
reproducible samples.
Only two methods are presented here: the measurement of sensory pollution and the assessment of contaminant transfer.
Sensory pollution
Principle of the method
Since the nose is the most sensitive instrument to detect pollutants, the
measurement protocol to assess the pollution resulting from ventilation systems
or components is mainly focused on measuring the sensory pollution effect,
evaluated by a trained panel of people (Bluyssen, 1990; Elkhuizen et al., 1995).
A panel of 12–15 subjects is selected and trained to give a value to the odour
intensity. To evaluate air quality at a given place, each panel member smells the
air – after having refreshed his or her nose in pure, fresh air – and gives a value
to the odour intensity. The final value is the average over the panel.
114
Ventilation and Airflow in Buildings
Selecting the panel
The subjects are selected from a group of at least 50 applicants of ages ranging
from 18 to approximately 35 years old. There is no restriction on distribution of
gender. Participants should abstain from smoking and drinking coffee for at
least one hour before any test. Also, they are asked not to use perfume,
strong smelling deodorants or make-up, and not to eat garlic or other spicy
food the day before the tests and on the day of the tests.
The selection is based on the quantitative assessment of the concentration of
a reference gas by smelling. The reference gas is 2-propanone, which is easy to
measure and to produce in various concentrations in the air. Passive evaporation creates known concentration of this gas in air, and this air is presented
to the human nose at a constant airflow coming out of a so-called PAP meter,
which consists of a 3 l jar made of glass covered with a plastic cap, a fan and
a diffuser (see Figure 6.8). The cap has two holes; in one of them the fan is
placed to suck the air through the jar and to blow it into a glass cone that
diffuses the exhausted air. The angle of the cone is at 88, to avoid mixing
with room air. The diameter of the top of the cone is 8 cm, convenient to situate
the nose in the middle. The small fan should produce at least 0.9 l/s, several
times more than the airflow during inhalation. The person therefore inhales
exclusively air from the jar, undiluted by room air.
Ø24 mm
Cap (plastic)
3 litre jar
240 mm
Fan
Ø140 mm
Figure 6.8 The PAP meter
Source: Bluyssen, 1990.
430 mm
Cone (top Ø80 mm)
Contaminants in Air Handling Units
115
Bottle
Jar
Fan
Figure 6.9 Recommended locations of small bottles in PAP meter
Source: Bluyssen, 1990.
The 2-propanone gas is evaporated in the PAP by placing one or more 30 ml
glass bottles filled with 10 ml of 2-propanone and making different holes in the
caps of these bottles. The concentration (in parts per million) of 2-propanone
obtained with one small bottle is about three times the diameter of the hole
in millimetres. Placing one or more bottles enables the production of different
2-propanone concentrations. The actual 2-propanone concentrations should be
measured by a suitable calibrated analyser. The position of the small bottles in
the PAP meter is of great importance. Recommended positions are illustrated
in Figure 6.9.
The steady-state concentration of 2-propanone in the top of the diffuser
depends on the level of the liquid in the small bottles (which is standardized
at 10ml), the location of the small bottles in the jar of the PAP meter, the
ambient temperature (standardized at 228C), and the size of the holes in the
caps of the small bottles through which the 2-propanone diffuses. To get steady
state, it is recommended to condition the whole apparatus and the small bottles
with 2-propanone the day before any test. One hour before the test, place the
bottles in position in the jar, leave the over-caps off and activate the fan. After
30 minutes a steady-state level with less than 3 per cent variation should be
reached.
The space where the sensory panel is trained has to fulfil certain criteria.
Preferable is a space that has:
.
.
.
.
.
temperature control;
100 per cent outdoor ventilation;
a filtration unit (for example, active carbon);
a Teflon layer on walls, floor and ceiling;
displacement ventilation (from floor to ceiling) or local exhaust.
Acceptable is a space that is empty (no smoking) and has:
.
walls, floor and ceiling covered with a Teflon layer or cleaned with a nonsmelling agent;
116
Ventilation and Airflow in Buildings
Table 6.1 PAP values and 2-propanone concentrations in PAP meters used as milestones
Value
1 (no odour)
2
5
10
20
.
.
Concentration [ppm]
<1
5
19
42
87
mechanical air supply with filtered air;
mixing ventilation with a certain minimum ventilation rate.
During the tests, the background level of 2-propanone should not be more than
1 ppm.
Five different 2-propanone concentrations generated by five PAP meters
are used as milestones for the training. These milestones have PAP values
corresponding to the concentrations in Table 6.1 of 2-propanone in the top
of the cone.
The equation for calculating the values is:
PAP value ¼ 0:84 þ 0:22 2 propanone concentration (ppm)
ð6:1Þ
Once the milestones are calibrated, it is important to keep the same PAP meters
with the same bottles at the same locations in the jar, since changing these parameters may change the 2-propanone concentration in the cone.
For selecting the panel, eight additional PAP meters with five concentrations ranging from 0–40 ppm and three with 5–90 ppm are prepared. The applicants are asked to assess these eight different concentrations of 2-propanone
using the milestones as the reference. They will be instructed to have at least
two inhalations of unpolluted air in between each PAP sniff. The question
asked of the applicants is:
How strong is the air that you perceive? Give a number on a scale from 1
to 20, but refer this number always to the numbers on the milestones 1, 2,
5, 10 and 20. One is equal to no smell (you perceive nothing), 20 is equal
to extreme strong smell.
The applicants are allowed to go back and forth between the eight different
unknown concentrations and the milestones as much as they need, and note
the given value for each unknown PAP meter. The dozen subjects with the
least sum of differences between given and actual values are selected.
Training procedure
The 12–15 subjects are trained for three to five days in smaller groups of three
or four people. Each day, they receive approximately one hour of intensive
training. In the first two days, the panel is trained to assess perceived air
Contaminants in Air Handling Units
117
pollution of concentrations of 2-propanone unknown to them by making
comparison with the milestones. On the third, fourth and fifth days, training
includes 2-propanone concentrations and other sources of pollution. Since
the pollutants have different characters to 2-propanone it is of great importance
that the subjects understand that they are exposed to the intensity by
comparing the intensity of the milestones. Up to 12 different concentrations
of pollutants are presented each training day.
After the evaluation of each unknown concentration of 2-propanone, the
panel member is given the correct answer and their performance is discussed
with the experiment leader. The panel members write their votes on a form
where it is possible to follow their performance during the training. Panel
members are submitted to a test similar to the selection test at the end of the
third and fifth training days. If they do not succeed, they should either quit
the panel or train for two additional days.
Experimental day
On each experimental day, the panel members are retrained for approximately
15–20 minutes per group of three to four people. During this training the panel
members are exposed to two or three different concentrations of 2-propanone
and two different materials, on which they receive feedback.
Also on each experimental day, the panel members are exposed to six
different concentrations of 2-propanone corresponding to the values 1, 3, 7,
12, 16 and 19. The concentrations of 2-propanone should be measured just
before the sensory assessments of each group of panel members. These exposures make it possible to compare different sensory panels and to calculate
performance factors.
The panel members are placed in a well-ventilated waiting room. During
each round of assessments, one by one the subjects assess the intensity of the
perceived air pollution of the air sample (from a material in a PAP meter, in
a walk-in climate chamber, air from a ventilation system and so on) by
making comparison with the intensity of the milestones. The panel members
are allowed to go back and forth between the milestones and the polluted air
sample. The members write down their assessment on a voting sheet, which
is handed to the experiment leader before making the next assessment.
The time between assessments or each panel member should not be less
than three minutes. An experienced panel member can assess an air sample
within 30–45 seconds. With a panel of 12 subjects the time between assessments
for a panel member will be approximately nine minutes, and for a group of four
panel members three minutes.
Contaminant transport in rotating heat exchangers
Principle of the method
A heated VOC mix is injected in the extract duct (star in Figure 6.10), in such a
way that the VOCs are well evaporated and mixed into the air at location C4
118
Ventilation and Airflow in Buildings
Exhaust air
C6
Extract air
C4
- +
Outdoor air
C1
Supply air
C3
Figure 6.10 Schematics of an air handling unit showing location of VOC
injection and sampling points, Ci , for concentration analysis
Source: Roulet et al., 2000.
of Figure 6.10. The concentration of these VOCs in the air is analysed at the
four locations shown in Figure 6.10. Locations C6 and C3 should be far
enough from the rotating heat exchanger to ensure a good mixing after
sorption and desorption.
If there is no transfer at all, C3 ¼ C1 ¼ C0 , the outdoor concentration of
each compound, and C6 ¼ C4 . If some VOCs are transferred, there should
be, at steady state, equilibrium between the amount of VOC taken in extract
air and the amount transferred to supply air:
Qe ðC4 C6 Þ ¼ Qs ðC3 C1 Þ
ð6:2Þ
The transfer rate can be calculated as the ratio of the mass flow rate of VOC
delivered into supply air, and the mass flow rate of VOC in extract air:
R¼
Qs ðC3 C1 Þ
Qs ðC3 C1 Þ
¼
Q e C4
Qe C6 Qs ðC3 C1 Þ
ð6:3Þ
Injection technique
Injection could be performed in two ways:
1 At a constant flow rate – for example, by evaporating the mix placed in an
open pan. Interpretation is performed as described in Chapter 1, ‘Constant
injection rate’, or Chapter 2, ‘Tracer gas dilution’. The problem is that the
injection rate, I, is rather difficult to control for each component in the mix.
2 Pulse injection – sampling is started first. Soon after the sampling starts, a
known mass of VOC mix is injected, either with a spray or, better, by flash
evaporation (see Figure 6.11). This injection does not need to be very short,
but it should be shorter than the sampling time. Sampling is continued after
the end of the injection for more than two air changes, ensuring that VOC
concentration is returned down to background level. Interpretation is
performed as described in Chapter 1, ‘Pulse injection’.
The amount injected is such that the resulting concentration values are clearly
above the concentration outdoors, but below the saturation limit of the
samplers.
Contaminants in Air Handling Units
Stainless steel
turnings
119
Syringe
Hot air blower (200 °C)
Ø 6 mm copper tube
Figure 6.11 Flash evaporation device for injecting the VOCs
Source: Roulet et al., 2000.
Air sampling and analysis
Air at the four locations is sampled with a pump through small tubes filled with
an adsorbing medium (for example, activated charcoal, TENAX). The sampling
rate is about 0.1 l of air per minute. VOCs accumulate in the compound by
absorption, as long as the medium is not saturated.
The sampling tube is then hermetically sealed and taken to the laboratory
for further analysis. The VOCs are desorbed by heating the tubes and they
are stored in a cold trap. The content of the trap is then injected in the
column of a gas chromatograph. A flame ionization detector (FID) is used to
detect and measure the amount of each compound, while a mass spectrograph
is used to help identify the compound (Mogl et al., 1995). An FID analyser can
also be used on site to sample and analyse the air but, if pulse injection is used,
this analysis should be performed at periodical and short intervals at the four
locations shown in Figure 6.10.
Which VOCs?
The ‘natural’ concentration of VOCs is often (and hopefully!) not large enough
to provide accurate measurement of transfer rate. Therefore, a mix of various
VOCs should be injected in the extract duct in order to obtain concentrations
that are significantly larger than concentrations in outdoor air. Criteria to
determine the VOCs’ mixture are:
.
.
.
.
.
The compounds are selected on their occurrence in buildings. Sources are
paints, paper, solvents, carpets and human emissions.
They are representative of the different organic families with the characteristic of their functional group and saturation degree. The result is different
boiling point and polarity.
They are easy to analyse and the results are significant. The concentrations
chosen must be under the saturation limit of the TENAX sampler.
The compounds should be easy to manipulate
They should not give a long-lasting and bad smell in the room and should
present an acceptable toxicity at the measured concentrations.
Lists of VOCs found most often in office buildings, as well as proposals for
VOC cocktails can be found in the literature (Brown et al., 1994; Hodgson,
1995; Maroni et al., 1995; Molhave et al., 1997; Wolkoff et al., 1997; van der
120
Ventilation and Airflow in Buildings
Table 6.2 List of VOCs used for contaminant transfer experiments
Class
Alkanes
Compound
(current names)
n-decane
Alcohols
n-butanol
1-hexanol
phenol
Haloalcanes 1,6-dichlorohexane
Aldehydes
hexanal (caproaldehyde)
benzaldehyde
Cyclic and
4-isopropenyl 1-methyl
aromatic
cyclohexene (limonene)
hydrocarbons 1,3-dimethylbenzene
(m-xylene)
1,3,5-trimethylbenzene
(mesitylene)
Ethers
dipropylether
Formula
C10 H22
CH3 (CH2 )3 OH
CH3 (CH2 )5 OH
C6 H5 OH
Cl(CH2 )6 Cl
CH3 (CH2 )4 CHO
C6 H5 CHO
C10 H16
Sources
Paints and associated
supplies
Adhesives
Solvent
Boiling
point
[ oC]
174
117
158
182
203
128
179
177
1,3-(CH3 )2 C6 H4
Solvent, adhesives
Paper, paints
Adhesives
Perfumed waxes and
cleaners
Solvent, fuel
139
1,3,5-(CH3 )3 C6 H3
Solvent, fuel
165
CH3 (CH2 )2 O(CH2 )2 CH3 Paints and associated
supplies
91
Wal et al., 1998). These lists can inspire a selection of compounds to include in
the mix. However, the total number of compounds should not be too large in
order to keep the analysis at a practical level. Therefore, the list in Table 6.2,
which includes most significant VOCs for several classes of organic
compounds, was found suitable after careful selection.
It should be noted that acetic and butyric acids were chosen in a first step,
but eliminated after the initial experiments for two reasons. First, because they
are poorly selected by the columns used in the gas chromatograph, and second,
because butyric acid has an awful smell.
Example of application
Experiments were performed on three small to medium air handling units,
ventilating an auditorium and a laboratory at the EPFL,1 and a building at
the EMPA.2 The characteristics of these units, measured using tracer gas
dilution technique, are summarized in Table 6.3.
Both EPFL units are equipped with a rotating heat exchanger, whose wheel
has a diameter of 785 mm, is 200 mm thick and rotates at 11 rpm. It is made of
thin aluminium sheet, sinusoidally corrugated with a wavelength of 4.2 mm and
peak-to-peak amplitude of 1.9 mm. A flat foil is placed between corrugated foils
(see Figure 6.12). The aluminium sheets are treated to have a hygroscopic
surface (see Figure 6.13)
The purging sector is a rectangle 100 mm high and 40 mm thick. The
purging sector was correctly used in the laboratory unit. The auditorium
Contaminants in Air Handling Units
121
Table 6.3 Characteristics of the air handling units used for the experiments
EPFL
Quantity
Outdoor airflow rate
Supply airflow rate
Extract airflow rate
Exhaust airflow rate
Recirculation flow rate
Recirculation rate
Room mean age of air
EMPA
Unit
Auditorium
Laboratory
Akademie
3
1900 100
2070 70
1790 40
1600 200
130 50
7% 4%
15 1
2530 80
2900 200
1860 50
1500 200
100 200
5% 11%
10 1
7000 1000
9500 120
5790 60
3000 1600
1500 900
26% 16%
20 1
m /h
min
unit, however, was found with the wheel turning in the wrong way, making the
purging sector inactive.
The measured ventilation efficiency in the EMPA academy is about 70 per
cent, showing displacement ventilation. The rotating heat exchanger is made of
Hexcore, a honeycomb consisting of a synthetic fibre material (Nomex). It has
no hygroscopic coating. The wheel has a diameter of 1580 mm, is 140 mm thick
2 mm
Figure 6.12 EPFL wheel structure
10 mm
Figure 6.13 SEM image of a new hygroscopic coating
122
Ventilation and Airflow in Buildings
Table 6.4 Pressure differentials in the units [Pa]
Auditorium unit
Filters
Across
wheel
in
Inlet – supply
88 5
Extract – exhaust 107 5
Laboratory unit
out
80 5
110 5
EMPA unit
in
out
in
out
85 5
60 5
82 5
67 5
97 2
94 1
109 3
88 1
125 5
54 5
30 5
21 5 230 2 283 5
72 5 125 5 125 5 137 5 423 2 475 5
Between Cold side
supply and Warm side
exhaust
and rotates at 5 rpm. The diameter of one honeycomb cell is 1.5 mm. The wheel
has no purging sector and the fans are not in their ideal positions. This explains
the high recirculation flow rate of about 15 per cent.
Pressure differentials were measured in all units just before or after the
experiments. These are shown in Table 6.4. It can be seen that a negative
pressure between supply and exhaust ducts allows vitiated air to pass from
exhaust to supply through possible leakage. This is especially remarkable in
the EMPA unit, where the fans are both on the same side of the wheel.
The climatic conditions just before or after the experiments are shown in
Table 7.2.
Results
Experiments were performed several times in various conditions. In order to
avoid adsorption in filters, these were taken out during experiments. Experiments were performed with the rotating heat exchanger turning in the correct
direction, i.e. with an active purging sector, and in the wrong direction, thus
suppressing the effect of the purging sector.
Table 6.5 Climatic conditions in the units [8C]
Unit
Expt.
Dew point
Inlet
Supply
Extract
Exhaust
Auditorium unit
Laboratory unit
EMPA
Aþ1 Aþ2 Aþ3 A1 A2 Lþ1 Lþ2 L1 L2
Temperature
Inlet
4.4
Supply
20.0
Extract 18.0
Exhaust 10.0
0.1
1.1
2.3
0.9
0.1
1.1
2.3
0.9
3.8
3.8
4.6
3.8
2.5
0.9
0.2
1.6
3.8
3.8
4.6
3.8
2.5
0.9
0.2
1.6
1.8
11.7
11.2
12.1
4.4
20.0
18.0
10.0
9.7
20.4
20.2
13.0
4.4
20.0
18.0
10.0
9.7
20.4
20.2
13.0
15.2
21.0
22.4
18.1
13.1
22.2
22.2
15.5
15.2
21.0
22.4
18.1
13.1
22.2
22.2
15.5
19.5
24.0
24.6
20.7
Contaminants in Air Handling Units
123
Table 6.6 VOC transfer rate in the experiments performed in
both EPFL units (%)
Auditorium unit
Purging sector
Experiment no.
1,6-Dichlorohexane
1-Butanol
1-Hexanal
1-Hexanol
Benzaldehyde
Dipropylether
Limonene
Mesitylene
m-Xylene
n-Decane
Phenol
With
Laboratory unit
Without
With
Without
Aþ1 Aþ2 Aþ3 A1 A2 Lþ1 Lþ2 L1 L2
26
9
15
36
10
10
11
5
4
8
51
16
10
14
32
12
13
10
9
8
10
44
38
29
23
45
31
13
13
20
13
25
54
4
16
15
30
18
5
16
15
10
16
50
58
42
29
54
49
22
14
31
20
42
71
8
11
17
16
8
5
3
7
9
8
32
17
9
3
20
10
5
0
9
9
10
37
34
34
38
41
28
24
14
28
28
27
64
47
39
34
62
39
9
13
28
27
29
64
Results of all experiments are shown in Tables 6.6 and 6.7.
Evidence for adsorption
Leakage and entrained air would result in the same recirculation rate for all
chemical compounds, this rate being about 7 per cent for the auditorium
unit, close to zero for the laboratory unit, and about 25 per cent for the
EMPA unit. Recirculation rates of most VOCs are larger than that.
The differences among compounds shown in Figures 6.14 and 6.15 can be
explained only by a physico-chemical behaviour such as adsorption. In all
Table 6.7 VOC transfer rate in the EMPA experiments
Experiment no.
Sampling time
1-Hexanal
1-Hexanol + m-Xylol
Benzaldehyde
Dipropylether
Limonene
Mesitylene
n-Decane
Phenol
1 (%)
2 (%)
3 (%)
91 mins
74
51
85
15
35
46
46
94
91 mins
57
39
63
25
22
29
30
57
45 mins
92
59
100
53
47
54
54
101
Average
(%)
Standard
Deviation
74
49
83
39
35
43
43
84
18
10
19
20
13
13
12
24
124
Ventilation and Airflow in Buildings
70%
60%
50%
40%
30%
20%
10%
0%
n-
D
ec
an
e
1Bu
ta
no
1l
H
ex
an
ol
Ph
e
1- nol
H
ex
Be
an
nz
al
1,
al
6d
D
e
h
ic
yd
hl
e
or
oh
ex
D
an
ip
e
ro
py
le
th
er
Li
m
on
en
e
m
-X
yl
en
M
e
es
ity
le
ne
No purging sector
With purging sector
Figure 6.14 Average VOC recirculation rates measured in the EPFL
auditorium (leaky) unit, with and without purging sector
Source: Roulet et al., 2000.
experiments, the smallest recirculation rates are for limonene. They are close to
the rates that could be expected from leakage or entrained air.
At the other extreme, it is clear that phenol, hexanal and dichlorohexane
present a strong adsorption, which cannot be completely removed in the
purging sector.
More evidence for adsorption is the dependence on boiling point shown
in Figure 6.16. The transfer ratio for each type of compound increases with
the boiling point of the compound, as can be expected for adsorption or
condensation.
It is often claimed that non-hygroscopic wheels have a lower transfer ratio
than hygroscopic wheels. Alternatively, it is also said among rotating heat
exchanger specialists that non-hygroscopic wheels become hygroscopic with
time (Ruud and Carlsson, 1996). It should be noticed that the recirculation
rates measured in similar conditions (without purging sector) in a unit with a
hygroscopic wheel (EPFL laboratory) and with a non-hygroscopic wheel
(EMPA unit) are clearly correlated (see Figure 6.17). The line in this figure
is a least square fit line, with a correlation coefficient of R ¼ 0:79, and slope
70%
60%
50%
40%
30%
20%
10%
0%
nD
ec
an
e
1Bu
ta
no
1l
H
ex
an
ol
Ph
en
ol
1H
ex
Be
an
nz
al
1,
al
6de
D
hy
ic
hl
de
or
oh
ex
D
an
ip
e
ro
py
le
th
er
Li
m
on
en
e
m
-X
yl
en
M
e
es
ity
le
ne
No purging sector
With purging sector
Figure 6.15 Average VOC recirculation rates measured in the EPFL
laboratory unit, with and without purging sector
Contaminants in Air Handling Units
125
Transfer ratio
70%
60%
50%
Alcohols
6 C chains
6 C cyclic
40%
30%
20%
10%
0%
0
50
100
150
Boiling point [°C]
200
250
Figure 6.16 Transfer ratio as a function of the boiling point for
three families
Note: The filled square corresponds to dichlorohexane.
100%
EMPA
80%
y = 1.09x + 0.22
R2 = 0.63
60%
40%
20%
0%
0%
20%
40%
60%
80%
EPFL laboratory
Figure 6.17 Recirculation rates for each chemical compound measured in
EPFL and EMPA units, in both cases without a purging sector
unity. The average difference between recirculation rates is 22 per cent, close to
the recirculation rate resulting from internal leakage in the EMPA unit.
This supports the hypothesis that the surface of the clean new wheel has not
much influence on the adsorption properties of aged wheels. There are,
however, new developments of rotating heat exchangers transferring latent
heat and water vapour, without transferring chemical compounds such as
odours (Okano et al., 1999; Seibu Giken Co., 1999).
Strategies to improve the performance of
HVAC systems
Indoor air quality strategies for an optimal performance of HVAC systems and
their components can be divided in two categories:
.
Strategies that affect the HVAC system while in use – operation, maintenance and replacement strategies;
126
Ventilation and Airflow in Buildings
Table 6.8 General IAQ strategies for HVAC systems
Design
Operation
Prevent pollution from outdoor air coming into the system
Select appropriate filtering system
Discontinuous sources: ventilate mainly
Locate outdoor air intake at a clean site, when the source intensity is small
far from potential pollution sources
Continuous sources: use an appropriate
filtering system
Prevent pollution
Avoid recirculation
Install appropriate filtering system in
extract duct. Active charcoal absorbs
odours
Install possibility for switching of
recirculation system at certain hours
from recirculation
Suppress recirculation if possible
Discontinuous indoor air sources: no
recirculation at certain hours
Continuous indoor air sources: use an
appropriate filtering system
System settings/operation strategies
Room temperature 208C (lower air temp. Start system before official business hours
improves perceived air quality)
to purge the building (for example, two
Room humidity at 30–60% (lower room
nominal time constants earlier)
humidity improves perceived air quality,
Switch off during certain periods during
but lower than 30 per cent might affect
the day, when nobody is present (meeting
health negatively).
rooms, for example)
.
Strategies that affect the design of the HVAC system – design principles and
innovative design strategies.
For both of these categories, IAQ strategies were defined to prevent HVAC
systems and components from being and becoming a source of pollution
(Bluyssen et al., 2003). These are summarized in Tables 6.8 to 6.14.
Contaminants in Air Handling Units
127
Table 6.9 Checkpoints in HVAC units in visual inspection
Component
passed
fail
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
h
Humidifier
Access door exists, but closed airtight
Clean water, no visible microbial growth or oil film
Drop separator and drip pan are clean (if applicable)
Drainage with siphon; clean
Humidifier automatically shuts down if the air handling unit is off
The humidifier runs dry when the HVAC unit is shut down
h
h
h
h
h
h
h
h
h
h
h
h
Ducts
Access door not blocked
No condensed water or microbial growth in the ducts
No pieces of construction materials, mineral wool etc in the ducts
No metal powder in the ducts
Amount of dust under limit value
h
h
h
h
h
h
h
h
h
h
Outdoor chamber
Access door: exists but closed
Chamber is clean (no dirt, leaves, loose insulation materials etc.)
Appropriate water drainage
Filters
Access door: closed airtight
Pressure difference meter is installed and operational
Chamber is clean and dry (no water on the bottom of the chamber)
No broken filters or visible leaks
Filters bags do not touch on the bottom of the chamber when the unit is
not in operation
Filters are clean (commissioning) or not too dirty/smelly (100% of the back
side is not dark)
Filters are of class F7 or better (fine filters)
Coils
Access door: exists, but closed airtight
Fins are clean and not damaged
No visible microbial growth (cooling coils)
Drip pan and drainage (cooling coils)
Rotating heat exchanger
Access doors on both sides
No significant leakage between supply and exhaust channels
Purging sector is installed on the warm side of the wheel
Wheel rotates in correct direction (wheel material passes in front of the
purging sector after having left the exhaust part of the unit)
F5 or better class filters are installed upwind of the wheel in both ducts
Pressure difference between supply and exhaust is positive
Wheel stops automatically when heat recovery is smaller than the power
needed to turn the wheel
128
Ventilation and Airflow in Buildings
Table 6.10 IAQ strategies for filters
Design
Operation
Prevent filters from becoming a source of pollution
Select a low-polluting new filter
Change filter on time: depending on the
Condition (bake) new filters before use
situation, traffic and other loads once in
Use another filtering method such as
1–12 months, but in general every six
electrostatic filtering or apply a two or
months for highly polluted areas (town)
more phase filtering system
and one year for low-polluting areas
Keep the filter dry (by proper layout of air (countryside)
intake section, heating of supply air with
Check pollution effect regularly in
pre-heater or heating filters for a certain sensory, chemical and biological terms
period)
Minimize micro-organisms (UV radiation)
Avoid snow penetrating in the HVAC
system by proper layout of air intake
section
Keep filter bags from lying on the bottom
of the filter chamber when the HVAC unit
is not in operation (preventing filters
absorbing water due to eventual rain or
snow penetration)
Prevent outdoor air from bypassing the filter
Make certain the filter frame and sealing
Make certain the filter frame and sealing
seat have no leaks
seat have no leaks
Prevent outdoor air impurities from
passing the filter itself: proper choice of
filter type
Contaminants in Air Handling Units
129
Table 6.11 IAQ strategies for ducts
Design
Operation
Prevent ducts from becoming a source of pollution
Use duct processed without oil, and which Inspect ducts at least once a year and
does not emit pollutants itself (label or
clean if necessary
smell)
Interior surfaces should be smooth, avoid
sharp edges or self-tapping screws in
ducts
Prevent dust accumulating during operation or debris from construction
Keep duct ends closed until in operation Inspect ducts at least once a year and
Keep accessories packed in closed boxes clean if necessary
Remove packaging just before installation Check filter system that provides clean air
Prior to first operation, check all parts in to duct at least once a year
contact with the airflow for cleanliness,
and reclean if necessary
Install a filter system upwind of the ducts
Prevent condensation points
Add insulation material outside the ducts
Prevent condensation from humidifiers
Other recommendations
Limit flexible air ducts (difficult to clean)
Check location and service openings,
Avoid sealant with high emission and do
especially in spaces with suspended
not attach tapes or tags
ceilings Very often service openings in
Install service openings
ducts are useless because there is no
Install stiffeners and other fittings in such a opening in the suspended ceiling or there
way that deposits of dirt are prevented
are cables under the service opening
and cleaning can be carried out
130
Ventilation and Airflow in Buildings
Table 6.12 IAQ strategies for rotating heat exchangers
Design
Operation
Select a wheel equipped with purging
sector and install it with the purging
sector on the warm side of the wheel
Supply and exhaust fans should be located
and sized so that a positive pressure
difference of about 200 Pa is achieved
between supply and exhaust ducts at the
wheel level
Avoid hygroscopic wheels when
contamination is an important concern
Change wheels if they are warped
Install filters in both channels upwind of
the heat exchanger
If pressure on supply side is negative
compared to exhaust side then change
pressure hierarchy
If the rotation of wheel is in the wrong
direction then change to proper direction:
the wheel should pass from exhaust to
supply ducts in front of the purging sector
Clean dirty wheels according to
instructions of the manufacturer, with
either compressed air, vacuum cleaner or
pressurized water
Check that the wheel control stops the
wheel when no heat can be recovered
Table 6.13 IAQ strategies for humidifiers
Design
Operation
Prevent pollution from water, water tanks and devices/agents to disinfect, demineralize
and/or desalinate the water
Remove oil residue before use of
If there is an oil film on water surface,
humidifier to prevent an oil film on water drain water and clean humidifier
Take care using disinfecting material
immediately
Use UV as germicide
Change water every week
De-ionization cartridges to prepare soft
Clean tank (not possible for steam: keep
water might emit VOCs
them dry/empty when not in use)
Install demineralization device in disperser Clean humidifier regularly every six
to keep the oscillator circuit board free
months (dry or wet)
from mineral precipitation for as long as
Desalinization must take place with an
possible
agent that does not smell
Use a control system with which
Humidifier should automatically shut
Check operation of control system
down when the HVAC system is off
regularly
New water is added when the water
temperature exceeds 208C (spray nozzle
and evaporative humidifiers, ultrasonic
humidifiers)
Other recommendations
Do not use porous wet material
Prevent condensation from steam
humidifier
Contaminants in Air Handling Units
131
Table 6.14 IAQ strategies for coils
Design
Operation
Prevent water reservoirs and material of
Keep outlet of drain free at the lowest
point of the drain pan (include angle)
Water/condensation should not stay too
long in reservoirs: change system design
Remove oil before installation
Prevent corrosion by selecting the proper
material
Do not place any adsorbing material
behind cooling coil
coils from becoming a source of pollution
Maintain on time
Water collection reservoirs: remove
water regularly, clean
Check for visible growth of moulds on coil
surface
Prevent water droplets from being produced
Place a droplet catcher behind cooling coil
Notes
1 E´cole Polytechnique Fe´de´rale de Lausanne (Swiss Federal Institue of Technology of
Lausanne).
2 Swiss Federal Research Institute for Materials.
7
Common Methods
and Techniques
Expressing concentrations and flow rates
Coherent units
When using equations, such as Equation 2.7, to model ventilation systems,
coherent units should be used to get the correct results. Some examples are
given in Table 7.1, and Annex A gives conversion tables. If the analysers and
tracer gas flowmeters do not provide coherent units, the measured data
should be converted to coherent units before further interpretation.
Corrections for density changes
Note that Equation 2.7 is essentially a mass conservation equation, and is therefore exact only when mass flow rates and mass concentrations are used.
However, volume concentration and volume flow rate can be used as long as
the density of air does not change too much along its flow.
The perfect gas law can model air as well as the tracer gases at ambient temperature:
pV ¼ nRT
ð7:1Þ
where:
p
is the
V
is the
n
is the
R ¼ 8:31396 J/(mole K) is the
T
is the
pressure of the gas,
occupied volume,
number of moles of gas,
molar gas constant,
absolute temperature.
The density of a mixture
of gases, can then be calculated. Let Mi be the molar
P
mass of the gas, i , that is the mass of Nav ¼ 6.02486 1023 molecules. The
density is obtained by multiplying Equation 7.1 by M and dividing it by V:
n M
p
m M
¼
ð7:2Þ
¼ ¼
V
V
RT
Common Methods and Techniques
133
Table 7.1 Examples of coherent units
Airflow rate
kg/s
m3 /h
m3 /h
Injection rate
Concentration
kg/s
m3 /h
cm3 /h
Mass concentration
Volume concentration
ppm
with
P
i M i ni
M¼ P
i ni
ð7:3Þ
¼ 28:96 g/mole.
being the average molar mass of the mixture. For dry air, M
The relative change in density is then:
p T
M
¼ þ
p
T
M
ð7:4Þ
As long as the tracer gas is present only in trace concentration, the temperature
has the largest effect on the density.
Conversion formulae for concentration
The conversion between units of tracer (or contaminant) concentration
requires the knowledge of the densities of tracer (or contaminant) and air, or
of their molecular masses. Therefore, formulae are given below instead of
tables. Concentration of a gas in a mixture can be expressed in several ways:
The mass concentration, Cm , is the ratio of the mass of the considered gas, x, and
the mass of the mixture:
m
Cm ¼ P x
i mi
ð7:5Þ
The molar concentration, CM , is the ratio of the number of molecules, or of
moles, of the considered gas to the total number of molecules, or moles, in
the mixture:
n
CM ¼ P x
i ni
ð7:6Þ
The volume concentration, CV , is the ratio of the volume of the considered gas at
the considered temperature and pressure and the volume of the mixture at the
same temperature and pressure. It is also the ratio of the partial pressure of the
considered gas and the total pressure:
CV ¼
Vx p x
¼
V
p
ð7:7Þ
134
Ventilation and Airflow in Buildings
In a mixture, every gas, x, occupies the whole volume, V:
V¼
nx RT
px
ð7:8Þ
The relations between mass concentration, Cm , molar concentration, CM , and
volume concentration, Cv of component x, are then:
m
M n
M
M
Cm ¼ P x ¼ P x x ¼ x CM ¼ x CV
M
M
i mi
i Mi ni
ð7:9Þ
Molar and volume concentration are the same, since at a given pressure and
temperature, one mole of gas always occupies the same volume.
Tracer gas dilution techniques
Tracer gas dilution techniques are among the most efficient to assess airflow
patterns within buildings and air handling systems. They consist of ‘colouring’
or marking the air with a tracer gas, i.e. a gas that mixes well with the air and is
easy to analyse in trace amounts. Non-toxic tracer gases may be useful to simulate the behaviour of contaminants having similar densities.
The concentration of tracer gas is analysed when or where the tracer is
well mixed with the air. The evolution of the measured concentration
depends on both the injection flow rate and the airflow rate that dilutes the
tracer. Interpreting this concentration evolution provides airflow rates, age of
the air, ventilation efficiency, leakage flow rates and so on.
Applications of these techniques are presented in Chapters 1, 2 and 3. The
technique itself, i.e. the tracer gases, the injection techniques and the analysers
are presented here.
Properties of tracer gases
A tracer gas used for airflow measurements in buildings should ideally have the
following properties:
1 be easily analysable, preferably at low concentrations to reduce cost and side
effects such as density changes or toxicity;
2 have low background concentration, allowing the use of low concentration
in measurements;
3 be neither flammable nor explosive at practical concentrations, for obvious
safety reasons;
4 be non-toxic at the concentration used, for obvious health reasons in
inhabited buildings;
5 have a density close to the air density (i.e. a molecular weight close to
29 g/mole) to ensure easy mixing;
6 not be absorbed by furnishings, decompose or react with air or building
components;
7 should be cheap in the quantity required for measurement.
Common Methods and Techniques
135
Table 7.2 Properties of the gases most frequently used as tracers
Tracer name
Chemical
formula
Molecular Density/air MAC
weight
@NTP
[ppm]
MDCy
[]
Helium
Neon
He
Ne
4
20
0.14
0.69
–
–
>6 106
>20 1012
Carbon dioxide
Nitrous oxide
CO2
N2 O
44
44
1.53
1.53
5000
25
3 106
50 109
Sulphur hexafluoride SF6
146
5.10
1000
Freon R11
Freon R12
Freon R13
Freon R22
Freon R111
Freon R112
Freon R113
Freon R114
Freon R115
Halon 1211
Halon 1301
137
120
104
86
220
203
187
171
154
165
149
4.74
4.17
3.60
2.99
7.60
7.03
5.90
5.90
5.31
5.53
4.99
1000
1000
1000
1000
1000
1000
1000
1000
1000
?
?
CFCl3
CF2 Cl2
CF3 Cl
CHF2 Cl
CCl3 –CCl2 F
CCl2 F–CCl2 F
CCl2 F–CClF2
CClF2 CClF2
CClF2 CF3
CF2 BrCl
CF3 Br
IR
IR
0:1 1012 ECD (IR)
1 1012
50 109
50 109
20 109
50 109
50 109
50 109
0:5 109
10 1012
ECD
ECD
ECD
ECD
ECD
ECD
ECD
ECD
ECD
ECD
ECD
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
(IR)
Liquid @NTP
Perfreons
PB
C6 F6
Perfreobenzene
PMB
CF3 C6 F5
Perfluoromethylbenzene
PMCH
CF3 C6 F11
Perfluoro-methyl-cyclohexane
PDCH
CF3 CF3 C6 F10
Perfluoro-dimethyl-cyclohexane
PMCP
CF3 C5 F9
Perfluoro-methyl-cyclopentane
PDCB
CF3 CF3 C4 F6
Perfluoro-dimethyl-cyclobutane
Analyser
(besides
MS)
186
(6.4)
ECD
236
(8.1)
ECD
350
(12.1)
–
1014
ECD
400
(13.8)
–
1014
ECD
300
(10.3)
–
1014
ECD
300
(10.3)
–
1014
ECD
Note: MAC ¼ maximum acceptable concentration for health safety; y MDC ¼ minimum
detectable concentration using the best available analyser. The useful concentration is about 100
times larger; IR: Infrared absorption spectrograph or photo-acoustic detector; ECD: Gas
chromatography and electron capture detector.
Item 5 is important mainly if the concentration is relatively high (for example,
0.1 per cent or higher). For this reason and to achieve also items 3, 4 and 7,
items 1 and 2 are essential.
Table 7.2 shows properties of tracers that have been used, together with
appropriate detection methods. Note that the mass spectrometer (MS) can
136
Ventilation and Airflow in Buildings
Table 7.3 Background concentration of some gases
Gas
Formula
Rural concentration
Water vapour
Argon
H2 O
Ar
20 103
9.3 103
Carbon dioxide
Helium
Methane
CO2
He
CH4
350 106
5.24 106
1.48 106
Nitrous oxide
Ozone
Nitrogen oxides
N2 O
O3
NOx
315 109
35 109
3 109
Methyl chloride
Freon R12
Freon R11
Carbon tetrachloride
Chloroform
Neon
CH3 Cl
CCl2 F2
CCl3 F
CCl4
CHCl3
Ne
630 1012
305 1012
186 1012
135 1012
20 1012
18 1012
Sulphur hexafluoride
Halon 1301
PDCH or Perfluoro-dimethyl-cyclohexane
PMCH or Perfluoro-methyl-cyclohexane
PMCP or Perfluoro-methyl-cyclopentane
PDCB or Perfluoro-dimethyl-cyclobutane
SF6
CF3 Br
CF3 CF3 C6 F10
CF3 C6 F11
CF3 C5 F9
CF3 CF3 C4 F6
850 1015
750 1015
22 1015
4.5 1015
3.2 1015
0.34 1015
Source: Dietz et al., 1983.
potentially analyse any tracer. Table 7.3 shows their background concentrations in outdoor air.
It can be seen in Tables 7.2 and 7.4 that no tracer complies with all the
requirements. Moreover, because of possible interferences in the analyser
used for multi-tracer experiments, the use of a specific tracer may forbid the
use of several other interesting tracers.
A comparative experiment of the mixing of different tracers (SF6 , N2 O and
He) was performed to study the effect of density (Niemela¨ et al., 1990). This
study shows that differences may occur when the tracer is at concentrations
higher than 10 per cent, for example, where it is not well mixed with air at
the injection location. However, density effect is not a major cause of error
for tracer gas measurements, and mixing can be improved (see ‘Mixing tracer
gases’, below).
Indeed, each tracer gas has some inconveniences: helium is too light and
requires an expensive mass spectrometer for analysis; neon is expensive; CO2
is very cheap to obtain and to analyse but has a large background; N2 O
interferes with water vapour; SF6 has a strong greenhouse effect; freons and
halons destroy the ozone layer; and perfreons are adsorbed in furniture.
Common Methods and Techniques
137
Table 7.4 Qualities of some tracer gases
Name
Compliance with the quality
Low
No
No
Ease BackNo fire
Low Density
hazard toxicity close to reactivity of ground local cost
use conc. sources
air
Helium
Neon
Carbon dioxide
Nitrous oxide
SF6
Freons R11, R12, R13
Freons R111 to R115
Halon BCF
Halon R13B1
Perfreons (PFT)
þþ
þþ
þþ
y
y
y
y
y
þþ
þþ
þþ
–
–
þ
þ
þ
þ
þ
þþ
—
þþ
þ
þ
–
–
—
—
—
—
þþ
þþ
–
–
þ
þ
þ
þþ
þþ
—
þ
þþ
þþ
þ
þþ
þ
þ
þ
þ
þ
–
þ
—
þ
þþ
þ
þ
þþ
þþ
þþ
þ
þþ
—
þ
þþ
þ
þ
þ
þ
þþ
þ
—
þþ
þ
–
þ
þ
–
þ
þ
Note: þþ Very good for that property; þ Good; – Not so good; — Poor; Is not combustible but
a strong oxidant at high concentration and temperature; y Is not combustible but decomposes in a
flame, producing toxic chemicals.
Mixing tracer gases
Perfect mixing of tracer gas in the air of the measured zone or in the measured
duct is essential when determining the airflow rates, but not for experiments to
determine the age of the air. We found that in a closed, quiet, isothermal room,
it may take several hours to mix a tracer gas into the air. In a 60 m3 room, this
time is shortened to less than half an hour if a 100 W heat source (such as a quiet
person or a light bulb) is present. A small 20 W fan like those used to ventilate
the power supplies of computers reduces the mixing time down to five minutes.
Several methods can be used to improve or accelerate the mixing of tracer
gases. The most widely used method is to inject the tracer upwind of a
mixing fan, which can be a small 20 W cooling fan used in the electronic
industry. Alternatives include portable oscillating fans. This method works
perfectly but changes the thermal gradients in the measured zone, and may
affect the air exchange rates. It should not be used during the measurement
of the age of the air.
Mixing fans are not necessary if the injection nozzles are located at the
locations where natural convection or mechanical ventilation provides significant air currents. Moreover, a continuous injection flow rate greatly assists
the attainment of a uniform tracer gas concentration and is therefore preferred
to pulse injection.
Quick mixing with the air around the injection port is obtained if the
velocity of the tracer gas at the injection nozzle is large enough to create a
turbulent jet (Silva and Afonso, 2004). For this purpose, the flow controlling
valve and nozzle should be at the end of the injection tube, with the tube
138
Ventilation and Airflow in Buildings
maintained under pressure. It is noted, however, that these two conditions
complicate the experimental arrangement since control leads must extend to
the end of the injection tube, and the system, under pressure, will be more
sensitive to leaks.
In buildings with a large internal volume it may be necessary to discharge
large amounts of tracer. If this is the case, then the following method may be
used: the operator works out a zigzag or circular path through the area that
will give good coverage of the building. The time taken to walk along the
path is noted. The amount of gas required to dose the area is evaluated (from
knowledge of the building volume and the required initial concentration),
and the gas flow rate needed to discharge that volume of gas in the time
taken to walk along the path is calculated. The gas cylinder is set to discharge
the gas at the required rate and the operator walks along the path carrying the
discharging cylinder. Some mixing of air and tracer will occur as a consequence
of the movement of the operator through the building.
Mixing of tracer gas in ducts and air handling units is described in Chapter
2, ‘Tracer gas injection ports’.
Tracer density problem
If the tracer gas density differs significantly from that of the air, that is, if
its molecular weight differs much from 29g/mole, the concentrated tracer
rises or falls (depending on its density) directly as it leaves the injection duct.
Injection jets help to avoid this, while diffusers make the phenomenon worse,
since they lower the injection speed of the concentrated tracer.
Since there is no non-toxic tracer gas with the density of air (apart from
ethane, which is explosive, the closest are highly toxic carbon monoxide,
hydrogen cyanide and nitrous oxide!), a simple approach is to dilute the
tracer in air at about 1 :10 or more, and to use this diluted mixture. This not
only adjusts the density, but also increases the injection flow rate – thus helping
mixing – and makes the problem of flow control easier in relatively small rooms
or for small concentrations.
In any case, the tracer concentration should not exceed a value that significantly changes the density of air. A proposed limit is:
Clim 3 104
air
102
tracer Mtracer
ð7:10Þ
where M is the molecular mass of the tracer gas, in grams per mole. The factor
3 104 corresponds to a change in density for a temperature variation of 0.1 K
in pure air. Such changes are very unlikely to have a significant effect on the
airflows in a space. With common tracer gases, this limit concentration is
much larger than the concentrations commonly used.
Several experiments have shown that if the tracer is properly injected, the
errors caused by the tracer density are negligible in relation to other sources
of error (Sandberg and Blomqvist, 1985).
Common Methods and Techniques
139
Sampling methods
Samples of air containing tracer gases need to be taken for analysis. There are
several sampling methods, each one being adapted to a particular purpose.
Grab sampling using hand pumps and bags is very cheap, easy to install
and needs few materials in the field. This method can be used for decay
measurements in no more than a few zones, and for constant emission
provided conditions remain constant.
The passive sampling technique, which relies on adsorbing the tracers on a
porous material, is used to sample the air continuously in such a way that the
amount of tracer collected is proportional to the dose. An advantage of the
passive (and also of active adsorbing) sampling is that, because of the storage
in the adsorbing material, very tiny concentrations can be detected. The passive
samplers and emitters are the only testing material and can be sent for analysis
by mail.
The above methods are most suitable for a small number of measurements.
For continuous monitoring of variable airflow rates in several zones over a long
period of time, sampling networks using tubes and pumps are recommended.
Such a sampling network is made of pipes returning from each zone to the
analyser, and one or more pumps to draw the air–tracer mixture through
these pipes.
Grab sampling
This technique does not require expensive equipment to be used on the measurement site. The tracer gas is initially injected into the space and allowed to
mix with the air. Because this whole process is designed to be as simple as
possible, rudimentary injection techniques are usually employed: releasing
the tracer from a syringe, a plastic bag or a plastic bottle has shown itself to
be adequate for the purpose.
When required, the air in the space is sampled using syringes, flexible
bottles, air bags or chemical indicator tubes (see ‘Chemical indicator tubes’,
below). The sample taken in this manner is intended to give an instantaneous
picture of the tracer concentration at that time, hence the actual time taken
to take the sample should be kept as short as possible.
After further defined periods of time, more samples can be taken. A
minimum of two samples are required to evaluate the average air change rate
between the sampling times, but often more are taken to ensure accuracy.
The time interval between samples or the absolute time that samples are
taken must also be recorded. Air samples are then returned to the laboratory
for analysis.
Passive sampling
These sampling devices are metallic or glass tubes a few millimetres in
diameter, partly filled with a given quantity of adsorbing material, such as
activated charcoal. For transport and storage, these tubes are sealed with
140
Ventilation and Airflow in Buildings
tight caps. Properly used passive samplers adsorb all the tracers that are in the
air entering the sampler. They are used to obtain a quantity of tracer that is
nearly proportional to the dose (that is, the time integral of the concentration)
received during the measurement time.
Passive (or diffusive) sampling is initiated by opening one end of the tube
for hours, days or weeks. Since the tracer reaching the adsorbent is adsorbed,
there is a concentration gradient between the absorbent and the entrance of
the tube. This leads to a diffusive flow of tracer, proportional to the concentration gradient.
Active sampling can be carried out with the same tubes by pumping the
air through the tube. This technique is mainly used to achieve quick
sampling. To ensure that the entire tracer contained in the air is trapped on
the adsorbent, care must be taken not to sample too large a volume of air or
not to pump the air through the adsorbent too fast, otherwise ‘break through’
will occur.
Networks, pumps and pipes
A star-type pipe network may be used in conjunction with valves and pumps to
periodically collect samples of air in the monitored zones and to direct them to
the analyser, one after the other.
Any small, airtight air pump is suitable to pump the sampled air to the
analyser. Its model and size is chosen for a low working pressure, and with a
flow large enough to flush the content of the pipe between two analysers.
The working pressure is determined by the pressure drop through the sampling
tubes and the analyser, and is usually less than 1000 Pa.
The tubing must be airtight and should not significantly adsorb or absorb
the tracer gases. For these reasons, polyvinyl chloride (PVC) pipes should be
avoided, as well as Teflon if freons or PFTs are used. Suitable materials are
nylon and polyethylene. Metallic pipes can also be used, but they are more
difficult to install. Never use tubes that have contained pure tracers, such as
pipes once used for injection, since the small amounts of tracer gas absorbed
in the plastic material will contaminate the sampled air. Such tubes should
be marked or coloured and used exclusively for injection.
The inner diameter of these tubes may range from a few millimetres to 1 cm.
Smaller pipes lead to larger pressure drops and need stronger pumps, whereas
larger pipes need larger flows to flush the content of the pipe in a reasonable
time. To avoid large pressure drops and noise, the average speed in the tubes
should not exceed 5 m/s. The choice of tube size is influenced by the overall
length of tubing required, i.e. by the size of the building. Some indications
are given below.
For common tracer gases at temperatures commonly found in buildings, the
pressure drop, p, for a length, L, of pipe of diameter, D, and volume flow rate,
q, is given by:
p
q
ffi 733 106 4
L
D
ð7:11Þ
Common Methods and Techniques
141
As an example, for an airflow rate of 100 l/h, that is 28 106 m3 /s, the
minimum pipe inner diameter will be 2.6 mm to have an air speed of 5 m/s.
In this case, the pressure drop will be 420 Pa/m, which may be too large in
most buildings. A pipe with a 4 mm inner diameter will have an air speed of
2.2 m/s and a pressure drop of 80 Pa/m, which allows for 12 m long pipes
with a pump allowing 1000 Pa under pressure at 10 l/h.
Injection and sampling port locations
To ensure the best possible mixing of tracer with air in the measured zone, the
tracer gases should be injected at the locations where natural convection or
mechanical ventilation provides significant air currents. Examples are ventilation inlets and the bottom of heating devices.
Sampling locations should be kept away from injection points, but at
locations which are representative of the air in the zone or where mixing can
be reasonably assumed to be good. Ventilation exhaust grilles are generally
good locations.
If there is a convective loop in the measured room, it is convenient to place
injection and sampling points on this loop but at two opposite points. However,
the points should not be placed near a door or a window that can be opened
during the measurement.
In a two-storey building with an open staircase, the upstairs tracer injection
points should be placed close to the staircase, while the sampling points should
be near the outside walls.
To obtain a more representative sample, or to inject at several locations in
a zone, the sampling or injection pipe may be connected to a mixing box or
manifold, from which several pipes, of the same length and diameter, go to
various locations in the zone.
Injection and sampling sequence
Multi-zone tracer gas active measurement methods generally use only one
analyser and often single bottles of each gas. This requires the zones to be
scanned in sequence. There are several ways to plan these sequences, the two
extremes being sequential or simultaneous operation (see Figure 7.1).
In sequential operation, one zone at a time, the sample tube for a given zone
is pre-purged with a fresh sample prior to analysis. After this, some tracer gas is
injected and the injection pipe is subsequently purged and the cycle repeated in
another zone.
The simultaneous operation applies mainly to the constant concentration
method. The air from zone i is pre-purged, while the air from the preceding
zone, i-1, is analysed. At the same time, the amount of tracer to be injected
to zone i-2 (already analysed) is calculated and delivered while the injection
pipe of zone i-3 (already injected) is purged. This strategy is much more
complex to control but is fast.
Purge
injection
Tracer gas
Analysis
Pre-purge
Tracer gas
Analysis
Purge
Ventilation and Airflow in Buildings
Purge
142
Figure 7.1 Two strategies for injection and sampling
Note: Left is one zone at a time; right is time shared.
Remember, however, that, in order to achieve good mixing, it is advantageous to inject the tracer continuously whenever possible. A series of short
pulses spread evenly over time can simulate this continuous injection.
The air used to purge the pipes should ideally come from the measured zone
and be returned to the same zone. However, if the flow rate in the sampling
pipes is small, compared to the ventilation rate (which is the common case),
it is reasonable to purge the injection tubes with outside air and to exhaust
samples to the outside.
To ensure that the analysed air has a concentration averaged over the time
between two measurements; it is possible to continuously pump the air from
each zone into an inflatable bag that is then periodically emptied into the analyser.
Tracer gas analysers
Objective of the analysis
Measurement of the concentration of tracer in the air is the basic parameter
needed for the interpretation or control of the concentration itself. Several
analysing principles are available for such measurements. For any analyser,
the salient features to consider are the following:
.
.
.
Sensitivity – it is desirable to minimize the quantity of tracer gas used, not
only from the point of view of cost, but also toxicity, fire or other hazards
that may be relevant. The more sensitive the analyser, the lower is the
required working concentration.
Selectivity – the analyser should not be sensitive to other gases usually
present in indoor air, for example, nitrogen, oxygen, water vapour, carbon
dioxide, argon and so on.
Speed – the time needed for the analysis must be considered, especially
if several locations are to be sampled in sequence. The analysis time depends
on the type of instrument and varies from milliseconds for mass spectrometers
Common Methods and Techniques
.
143
up to several minutes for gas chromatographs or multi-tracer infrared
analysers. Faster analysis will enable more frequent sampling of each zone
and hence provide more detailed data. Frequent sampling (for example,
every five to ten minutes) is essential for the constant concentration technique
to maintain accurate control of concentration.
Accuracy – last but not least, the accuracy of the concentration measurement
directly influences the accuracy of the results.
There are several principles employed for analysis for tracer gas concentrations
that differ in the gases analysed, the range of concentration detected, accuracy,
speed, ease of use and cost. These principles are discussed and specific
examples are given below.
Infrared absorption spectrometry
Any polyatomic gas molecule exhibits vibration modes, which are excited by
infrared radiation. The wavelengths of the infrared radiation corresponding
to molecular vibration frequencies1 are absorbed in proportion to the number
of tracer gas molecules present in the infrared beam. This absorption is used
as a measure of the concentration of tracer gas molecules in the path between
an infrared source and detector. This technique is referred to as absorption
spectroscopy.
Infrared absorption spectrometers may be either dispersive or non-dispersive
types, and both are in common use. Dispersive spectrometers use a diffraction
grating that reflects the electromagnetic waves of a light beam into different
directions, each direction corresponding to a given wavelength. The instrument
is tuned by the operator to a narrow band of wavelengths specific to the gas of
interest and any convenient infrared detector measures the absorption. Some
modern instruments use tuneable laser diodes that emit the appropriate
wavelength.
In non-dispersive devices, all the infrared radiation present in the absorption
bands of the tracer is used. The infrared light beam is sent through both a reference channel containing nitrogen or pure air and an analysis channel that
contains a sample of room air. A chopper is used to alternately pass the radiation
from each channel to an analysis chamber containing a pure sample of the tracer
gas. This gas heats and cools in response to the modulated beam. The heated gas
expands through a measuring channel into an expansion chamber. The resulting
alternating flow through the measuring channel is measured by a highly sensitive
gas flow detector, which transmits an electric signal.
Characteristics of infrared absorption spectrometers:
.
.
.
Analysable tracers – N2 O, SF6 , halon 1301, CO2 . Other detectable gases,
such as H2 O, benzene, alkenes and so on, are not suitable as tracers.
The sensitivity depends on the analysed gas. For example, full-scale deviation is 200 ppm for N2 O, or 20 ppm for SF6 and CF3 Br.
Interfering gases – care must be taken to eliminate the effects of other gases
absorbing at similar frequencies (cross-sensitivity), particularly water
144
.
.
Ventilation and Airflow in Buildings
vapour and CO2 present at high concentrations in the air. Filters are used to
minimize the effect but humidity should be measured simultaneously to
some tracers, such as N2 O, to allow for corrections.
Analysis time – 10–50 s.
Accuracy – 1 per cent of full scale if the zero drift is controlled.
Photo-acoustic detector
This analyser is also an infrared absorption spectrometer, but uses a different
detector. An infrared radiation beam is first chopped then optically filtered to
leave only frequencies that are absorbed by the tracer of interest. This beam
then enters a gas-tight chamber containing the air sample. As above, the
sample is heated and cooled in phase with the chopping frequency, creating
sound waves in the chamber. This is the photo-acoustic effect. Microphones
detect these sound waves.
Characteristics of analysers with photo-acoustic detector:
.
.
.
.
.
Analysable tracers – N2 O, SF6, CO2 , freons F11, F112, 113 and 114, halons
(one filter for each tracer). Other detectable gases are not suitable as tracers,
such as H2 O, benzene, alkenes and so on.
Sensitivity – the detection limit depends on the tracer but is typically
0.05 ppm, and the dynamic range is 105 . The lowest full-scale range may
then be 2 ppm but 10 ppm is recommended with usual tracers. The sensitivity for N2 O, CO and CO2 drops strongly when these gases are diluted
in dry nitrogen, as is often the case for calibration gases. Adding a special
‘Nafion’ tube in the sampling circuit allows for the moistening of the mixture
in order to recover the normal high sensitivity.
Interfering gases – several gases (which are not necessarily present in the air)
may interfere with each tracer. Therefore, filters and tracers should be
chosen in accordance with manufacturer’s specifications.
Analysis time – 30 s for one gas, 105 s for five gases and H2 O.
Accuracy – 1 per cent of full scale.
Mass spectrometry
The pressure of the air sample is first lowered to about 105 Pa by pumping it
through a capillary tube. The molecules of the sample are then ionized, accelerated to a given velocity and passed into a mass spectrometer.
The classical mass spectrometer curves the trajectory of the ions with a
strong magnetic field. The radius of curvature depends on the velocity and
charge-to-mass ratio of the ion; only those having the appropriate combination
will pass a slit placed in front of the detector.
The most suitable spectrometer is, however, the quadrupole mass spectrometer, which is currently used in vacuum processes to analyse the residual
gases. The gases entering into the analyser are ionized and the positive ions
are separated by directing them axially between two pairs of rods creating an
electric field at variable radio frequency. The ions follow a helicoidal path in
Common Methods and Techniques
145
Table 7.5 Tracer gases most used in the mass spectrometer technique
Mass
Comments
127
51
7.6% of mass 127 peak. Interferes with Freon 22
Freon R22
þ
CHF
CHClFþ
CHClþ
2
51
69
85
Interferes with SF6
2.1% of peak 51. Interferes with R14 and R13B1
1.5% of peak at mass 51. Interferes with R12
Freon R12
CClFþ
2
CFþ
3
85
69
Freon R13B1
Gas
Ions
SFþ
5
SFþ
SF6
Freon R12B2þ CF2 Brþ
n-Butane
He
C4 H þ
10
C3 H þ
7
þ
He
120
Not commonly available
58
43
Flammable above 2% concentration
4
5.24 ppm background concentration
Ne
Ne
þ
20
Expensive, 18 1012 background concentration
Ar
Arþ
40
Background of 1%. Not a tracer but a good
reference
Note: Mass-to-charge ratio of the most common isotopes singly charged.
Source: Sherman and Dickerhoff, 1989.
this field. Only ions having a charge-to-mass ratio that corresponds to a given
radio frequency reach an orifice at the end of that path and pass into an electron
multiplier, whose signal is proportional to the number of incoming ions.
Such instruments deliver a signal for each molecule having a given chargeto-mass ratio. A given molecule may give several signals at different radio
frequencies, since ionization may often break the molecule into several ions.
For example, water vapour gives a peak not only at mass 18 (H2 Oþ ), but also
þ
at mass 17 (HOþ ), 16 (Oþ ), 2 (Hþ
2 ) and 1 (H ). The electric current is proportional to the concentration, but the sensitivity depends on the analysed gas.
Characteristics of mass spectrometers:
.
.
.
.
.
Analysable tracers – any tracer that can be distinguished from the normal
components of air. Confusion may occur if the molecule or a part of it has
the same charge-to-mass ratio as components of air. Examples are shown
in Table 7.5.
Sensitivity – 2 106 for tracers with low background concentration.
Interfering gases – any gas present in the sample may interfere with another,
but it is nevertheless possible to analyse up to seven tracers without there
being too much interference from the gases in the air or between the tracers
themselves.
Analysis time – a few milliseconds.
Accuracy – 1 per cent.
146
Ventilation and Airflow in Buildings
Gas chromatography
A puff of the sampled air is injected into a separating (chromatographic)
column, a tube in which adsorbent material is packed. This column is heated
and the pulse of sample is pushed with a flow of inert carrier gas. The various
components of the sample pass through the column at various speeds according
to their affinity for the adsorbent material. At the end of the column, the
components emerge in sequence and can be quantitatively detected with a suitable detector. Both flame ionization detectors (FIDs) and electron capture
detectors (ECDs) have been used for tracer gas analysis.
In the FID, a pair of polarized electrodes collects the ions produced
when organic compounds are burned into an hydrogen flame, and the
current produced is amplified before measurement. This detector is rugged,
reliable, easy to maintain and operate, and is by far the most used in gas
chromatography, since it combines a good sensitivity to organic compounds
(limit of detection of about 109 g) with a good linearity within a range of up
to 107 .
However, organic compounds are not very good tracer gases and the ECD,
which is much more sensitive to halogenated2 compounds, is the most common
in tracer gas analysis. In this detector, a radioactive nickel cathode emits
electrons, which are received on an anode. Halogens capture these electrons,
lowering the received current and thereby indicating the tracer concentrations.
ECDs are popular since they can measure halogenated tracers to exceptionally
low concentrations.
Characteristics of gas chromatographs:
.
.
.
.
.
Analysable tracers (with ECD) – any halogenated compound like SF6 ,
freons, perfluorocarbons or perfluorocycloalkanes.
Sensitivity – from ppb (109 ) range for SF6 down to 1014 for the PFTs.
Interfering gases – H2 O, O2 (oxygen traps and desiccators are used to
suppress these effects).
Analysis time – a few minutes but can be lowered down to 20 seconds by
shortening and back flushing the column, if high selectivity is not needed.
Accuracy – depends on the quality of the calibration, but can be 2 per cent of
reading.
Chemical indicator tubes
This is a single shot method to estimate the air change in a single zone by the
decay or constant injection technique with some tracers.
Detector tubes are glass tubes packed with a selective solid absorbent, which
gives a colour reaction to some gases. The tubes used are sensitive to CO2 in the
0.01–0.30 per cent range. Tubes as supplied by the manufacturer are sealed at
both ends. To make a measurement the seals are broken, one end of the tube
(the correct end is indicated on the tube) is inserted into a pair of specially
designed hand bellows, the other end being left open to sample the air tracer
mixture.
Common Methods and Techniques
147
By making the prescribed number of strokes of the hand-held bellows, the
correct amount of air is drawn through the tube. This enables the tracer gas
evaluation to be made. The glass tube has graduation marks on it, and the
length of the discolouration caused by the reaction indicates the concentration
of tracer in the room air. Detector tubes can only be used once and must be
discarded after each sample taken.
This single shot method is not very accurate but it is cheap and easy to
operate. Therefore, it is suitable for a rough first estimate of the air change
rate. The interpretation of the result is performed using the integral decay
method (see Chapter 1, ‘Pulse injection’).
Characteristics of chemical indicator tubes:
.
.
.
.
Analysable tracers – CO2 , H2 O and many toxic gases that are not useable as
tracers.
Sensitivity – 0.01–0.3 per cent range for CO2 .
Analysis time – one minute.
Accuracy – 5 per cent or 10 per cent of full scale.
Calibration of the analysers
Any analyser should be periodically calibrated by analysing standard samples,
which are mixtures of the tracers in air or other inert gas. The calibration
mixtures containing N2 O and CO2 must be moistened when a photo-acoustic
detector is used. To transfer the calibration mixture from the containers to
the analyser, never use valves or tubes that were previously used with pure
or high concentration tracers.
During the measurements, it is recommended to periodically sample and
analyse the outside air as a convenient zero reference, even if no tracer is
expected in the outdoor air.
Identification methods
Identification is to assess the values of some parameters in formulae from values of
variables involved in these formulae. There are several identification techniques.
Only some of them are presented below. The rationale of the presented methods
can be found in the literature, so only the final formulae are presented here.
Linear least square fit
The problem is the following: given N pairs of data (x; y), find the straight
line:
y ¼ a þ nx
ð7:12Þ
fitting these points at the best. That means that the coefficients a and n should
be such that the sum of the ‘distances’ of the measured points to the line is a
minimum.
148
Ventilation and Airflow in Buildings
Linear least square fit of the first kind
Such methods are used to find the coefficients of leakage models of Equations
4.1 or 4.2 in fan pressurization (see Chapter 4, ‘The fan pressurization
method’).
The regression of the first kind assumes that the abscissa, xi , of each
measurement is well known and that the distribution of the ordinates around
the regression line is Gaussian with a constant standard deviation. This
method is very commonly used but it should be emphasized that the above
hypotheses are not verified in the case of permeability tests because the
values of xi are measured estimates.
The regression line of the first kind minimizes the sum of the square of the
residual ordinates (vertical distances):
SSRy ¼
N
X
½ yi ða þ nxi Þ2
ð7:13Þ
1
Its coefficients can be calculated using the following relationships. First
compute the estimates of the averages:
x ¼
N
1X
x
N i¼1 i
N
1X
y
y ¼
N i¼1 i
ð7:14Þ
and the estimates of the variances:
s2x ¼
N
1 X
ðx xÞ2
N 1 i¼1 i
s2y ¼
N
1 X
ð y yÞ2
N 1 i¼1 i
sxy ¼
ð7:15Þ
N
1 X
ðx xÞð yi yÞ
N 1 i¼1 i
Then the best estimates of the coefficients a and n, according the above
hypotheses, are:
n¼
sxy
s2x
ð7:16Þ
a ¼ y n
x
The slope given by Equation 7.16 is valid if the xi are exactly known, and the
minimized distance is the sum of the square of the vertical distances between
the measured points and the regression line.
Common Methods and Techniques
149
Confidence in the coefficients
The variances on the linear coefficients of the regression of the first kind are
usually estimated using the following relations, which assume that the dispersion around the line is Gaussian with a constant standard deviation and is
the result of the measurement errors:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
u N
2
u1 X
1 sy nsxy
x2
ð7:17Þ
and
sa ¼ sn t
sn ¼
N2
sx
N i¼1 i
If T(P; ) is the significance limit of the two-sided Student distribution for a
probability, P, for degree of freedom, , then the confidence levels on the
coefficients are:
Ia ¼ sa TðP; N 2Þ
ð7:18Þ
In ¼ sn TðP; N 2Þ
ð7:19Þ
This means that with a probability, P, the coefficient, a, lies in the interval
½a Ia ; a þ Ia and the same for n.
The estimate of the variance around the regression line at abscissa x is:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðN 1Þ 2
sy ðxÞ ¼ sn
sx þ ðx xÞ2
ð7:20Þ
N
and the confidence interval in the estimate of y using the regression line for any
x is:
Iy ðxÞ ¼ sy ðxÞTðP; N 2Þ
ð7:21Þ
The values of the two-sided Student distribution are given in Table 7.6. The
relation (Equation 7.17) can be used to obtain the confidence intervals for a0
and n0 in the second kind regression if the roles of x and y are permuted.
Table 7.6 Two-sided confidence limits TðP; N 2Þ for a Student distribution
TðP; N 2Þ for probability P ¼
N2
1
2
3
4
5
6
7
8
9
10
0.8
0.9
0.95
0.99
0.995
0.999
3.078
1.886
1.638
1.533
1.476
1.440
1.415
1.397
1.383
1.372
6.3138
2.9200
2.3534
2.1318
2.0150
1.9432
1.8946
1.8595
1.8331
1.8125
12.706
4.3027
3.1825
2.7764
2.5706
2.4469
2.3646
2.3060
2.2622
2.2281
63.657
9.9248
5.8409
4.6041
4.0321
3.7074
3.4995
3.3554
3.2498
3.1693
127.32
14.089
7.4533
5.5976
4.7733
4.3168
4.0293
3.8325
3.6897
3.5814
636.619
31.598
12.924
8.610
6.869
5.959
5.408
5.041
4.781
4.5787
150
Ventilation and Airflow in Buildings
Regression of the second kind
When there are uncertainties in both axes, there is no reason to emphasize the x
axis, and the same procedure can be followed commuting the roles of x and y.
Generally another regression line is obtained, which is given by:
y0 ¼ a0 þ n0 x
ð7:22Þ
with another pair of coefficients:
n0 ¼
s2y
sxy
0
ð7:23Þ
0
a ¼ y n x
This regression line minimizes the sum of the square of the residual abscissae:
N X
y0 a0 2
xi ð7:24Þ
SSRx ¼
n0
i¼1
If the two lines are close to each other or, in other words, if:
n0 n
and therefore
a0 a
ð7:25Þ
then it can be said that there is a good correlation between the two physical
quantities x and y. The correlation coefficient defined by:
n
sxy
R¼
ð7:26Þ
hence
R2 ¼ 0 s s
n
x y
is a measure of the interdependence of x and y. It is not a measure of the quality
of the fit or of the accuracy of the estimates of the coefficients, since jRj ¼ 1 for
any fit based on only two sets of points. The estimates of the errors on a and n
are calculated in ‘Confidence in the coefficients’, above.
Orthogonal regression
If the two regressions of the second kind are calculated and different results are
obtained, the problem is to choose the coefficients: which pair is the closest to
the reality? Since each pair of coefficients is obtained assuming that one variable
is exactly known, it is likely that the best set is neither of them and instead lies in
between, but where?
There are several answers to that question, none of them being really satisfactory. One recipe is to take an average slope:
n ¼
n þ n0
2
ð7:27Þ
or a weighted average slope:
n ¼
"y n þ "x n0
2
ð7:28Þ
Common Methods and Techniques
151
where "y and "x are the experimental errors on y and x respectively and
deduce a corresponding value of a using Equation 7.16. This recipe does not
show clearly which quantity is minimized by the fit.
Another more physical way is so-called ‘orthogonal’ regression. It minimizes the real (orthogonal) distance between the measured points and the
regression line drawn with the scales on the axes inversely proportional to
the experimental errors, using variables weighted by the experimental errors:
Yi ¼
yi
"y
and
Xi ¼
xi
"x
ð7:29Þ
Writing the regression line with these coordinates:
Y ¼ A þ X
ð7:30Þ
the minimized residual is:
SSR? ¼
N
X
ðYi Xi AÞ2
i¼1
2 þ 1
ð7:31Þ
The slope in original (x, y) coordinates is given by the following relations. Let
us define:
n ¼
s2y s2x
2sxy
ð7:32Þ
If x and y are respectively the estimated standard deviations of the abscissas,
xi , and ordinates, yi , the slope is given by:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
n? ¼ x n n2 þ 1
ð7:33Þ
y
Between the two possibilities, the sign of n? must be chosen equal to the sign of
n given by Equation 7.16. This sign is always positive in pressurization tests.
The coefficient, a, is obtained with the relation of Equation 7.16.
Bayesian identification
In section ‘Linear least square fit’, above, it was said that the usual regression
techniques for the identification of the coefficients can strictly be used only
when one of the variables is well controlled. In this case only, the relations
given in ‘Confidence in the coefficients’, can be used to get a good estimate of
the confidence intervals of the obtained coefficients.
When there are uncertainties on both axes, for example in pressurization
tests, these methods are not strictly valid, since they do not give any information on the relation between the two coefficients and their uncertainties. If
several measurements of the same leak are performed, several pairs of
coefficients, C and n, for the relation:
Q ¼ C pn
ð7:34Þ
152
Ventilation and Airflow in Buildings
will be obtained, and a correlation between C and n will be found: the larger C
values correspond to the smaller n and vice versa. A good identification
technique should give the most likely couple of coefficients together with the
probability density fðC; nÞ. Such a technique exists (Tarantola, 1987) and is
summarized below.
Identification of the model parameters
Let us put in a vector, z, both measured data and model parameters that have to
be determined and assume that this vector is a random variable with a normal
distribution in k-fold space (k ¼ number of parameters and data):
fðzÞ ¼ A exp½ 12 ðz zp ÞT C 1
z ðz zp Þ
ð7:35Þ
where:
zp
Cz
is the a priori vector, z, containing the measured values and reasonable
estimates of the parameters to be identified,
is the covariance matrix between the elements of z. Its diagonal elements
are the variances of the measured quantities and a priori estimated
variances of the parameters. These latter variances are generally large,
since the parameters are generally not known before the measurements.
The components of the vector, z, are linked by a mathematical model or a set of
equations that can be written:
ðzÞ ¼ 0
ð7:36Þ
For example, if a linear relationship is assumed between two measured variables, x and y, the set of equations:
yi ¼ a þ bxi
can be written
0
1 x1
B
B 1 x2
B
B B
B
@ 1
xn
ð7:37Þ
in a matrix form:
1
y1
1
C0
y2 C a
CB
C
C
C@ b A ¼ 0
C
A 1
ð7:38Þ
yn
Generally, the proposed model is not exact and it may be assumed that it has a
normal distribution:
gðzÞ ¼ B exp½ 12 ðzÞT C 1
T ðzÞ
ð7:39Þ
where CT is the covariance matrix of the model. If the model is exact, this
distribution is a Dirac distribution:
gðzÞ ¼ ½ðzÞ
ð7:40Þ
Combining the prior knowledge contained in the distribution fðzÞ with the
model described with the distribution gðzÞ gives a new distribution containing
Common Methods and Techniques
the a posteriori information. This new distribution:
h
i
T 1
ðzÞ ¼ C exp 12 fðzÞT C 1
T ðzÞ þ ðz zp Þ C z ðz zp Þg
153
ð7:41Þ
From this distribution, the z vector presenting the maximum likelihood can be
found. It is the vector, z, that minimizes the exponent:
T 1
ðzÞT C 1
T ðzÞ þ ðz zp Þ C z ðz zp Þ
ð7:42Þ
This most probable vector contains the identified model parameters and the
most probable values of the measured quantities. Practically, this vector is
found using numerical methods looking for the minimum of the exponent
given above. More references on such methods are Mitchell and Kaplan
(1969) and Nelder and Mead (1965).
Error analysis
This method allows us to obtain the a posteriori estimate, Czi , of the covariance matrix of the distribution, (z). For that purpose, the model, (z), is
linearized around the most probable vector, zs . The a posteriori covariance
matrix is then:
1
1 1
T
1
C iz ¼ ðF T
¼ Cz CzF T
s CT F s þ Cz Þ
s ðF s C z F s þ C T Þ F s C z
ð7:43Þ
where Fs is a matrix having the dimension N M, with M ¼ Nn þ n þ N, N
being the number of measurements and n the number of parameters to be
identified. Fs contains the derivatives of the model, (z), evaluated at the
point, zs :
0
1
@1
@1
B @z1
@zM C
B
C
Fs ¼ B C
ð7:44Þ
@ @
A
@
n
N
@z1
@zM zs
Error analysis
Purpose of the error analysis
The accuracy of any measurement depends on the conditions in which the
measurement is done, on the quality of the measuring instrument and on the
skill of the people making the measurement. Measurements cannot be perfect,
accuracy cannot be infinite, and any measurement result includes some uncertainty. That means that the result is not absolute, but it is always possible to
state that the actual value is contained, with a given probability, within some
confidence limits, or vice versa, that the probability that the actual value is
outside some limits is lower than a certain value. Since this confidence interval
may be large, there is no sense in giving the result of a measurement without
any information on its reliability.
154
Ventilation and Airflow in Buildings
Generally, an instrument does not directly give the required information. In
most cases, several measurements are combined to obtain the needed value. For
example, in tracer gas measurements, several concentrations, gas flows, time
and volume measurements are combined in equations that are solved to get
the airflow rates. The errors accompanying the measured values propagate
through the interpretation formulae and finally give a probable error on the
final result.
In this chapter, some methods for estimating the error on the result are
presented. Note that only the instrumental and random errors are treated
here. Bias caused by misuse of the instruments or by a lack of precautions is
not discussed here.
Definitions
Let x be the result of a measurement. If several measurements of the same
physical quantity are made, the results, xi , of these measurements will not be
all equal, but nearly all of them will be within some interval. The confidence
interval with a probability, P, has this probability to include the actual value.
In practice, about NP results out of a large number, N, of measured values
of the same quantity should be included in the confidence interval.
The confidence interval or the probable error can be expressed by two
ways:
1 The absolute error is expressed in the same units as the physical quantity:
Measurement ¼ x x ½unit
ð7:45Þ
and the confidence interval goes from x x to x þ x.
2 The relative error is the ratio of the absolute error to the measured value:
" ¼ x=x
ð7:46Þ
which can be expressed in per cent by multiplying " by 100.
The inverse relation is:
x ¼ x"
ð7:47Þ
The results should always be given with their confidence interval (or with an
estimate of the possible error) and with the unit used. The digits in the results
should all be significant:
Correct: length ¼ 420 10 mm or 420 mm within 2 per cent;
Not coherent: length ¼ 421:728 9:511 mm or 421.728 mm
within 2.255 per cent.
A few statistics
Error analysis cannot be done well without using some basic statistical theory.
There are simplified methods, which unfortunately often give too large an error
Common Methods and Techniques
155
0.5
0.4
0.3
0.2
-3
-2
-1
0.1
Confidence
interval
0
0
1
2
x-<x>
3
s
Figure 7.2 Significance limits and confidence interval
domain. The statistical method allows one to obtain more information on the
reliability of the results.
Because of random reading errors and uncontrolled perturbations, the test
values will follow a given distribution. We can model such distributions by
treating x as a stochastic variable.
The probability density function, fðXÞ, of the variable x is the probability to
find x between X and X þ dx.
Its integral F(X) is the probability of having x < X:
ðX
fðxÞ dx
ð7:48Þ
FðXÞ ¼ probðx < XÞ ¼
1
The lower significance limit is the value Xi for which FðXi Þ ¼ p, where p is a
given probability. The upper significance limit is the value Xs for which
FðXs Þ ¼ 1 p.
The confidence interval [Xi ; Xs ] is the range between the lower and the upper
significance limit (see Figure 7.2). The probability to find x in this interval is
P ¼ 1 2p.
Average
If the same importance is given to all the results, an estimate of the average, ,
of the variable, x, based on N measurements is calculated by:
P
x
hxi ¼ i i ffi ð7:49Þ
N
where the sum runs over these N measurements (i ¼ 1; . . . ; N).
If we give more importance to some measurements than to the others, a
weight, wi , can be attributed to each value, xi , and the weighted average is
calculated by:
P
wx
hxi ¼ Pi i i
ð7:50Þ
i wi
156
Ventilation and Airflow in Buildings
Variance and standard deviation
A figure representing the importance of the scattering around the average value
is the mean square deviation or variance:
P 2
P 2
2
ðx Þ Nhxi2
i ðxi hxiÞ
¼ i i
ð7:51Þ
Sx ¼
ðN 1Þ
ðN 1Þ
The square root of Sx is the estimate, sx , of the standard deviation, x :
pffiffiffiffiffiffi
ð7:52Þ
sx ¼ Sx ffi x
The larger the number of measurements, the better the estimate.
Covariance
An estimate sxy of the covariance xy of two random variables x and y, of which
N measurements xi and yi were done, is calculated by:
P
P
ðx y Þ Nhxih yi
i ðxi hxiÞð yi h yiÞ
¼ i i i
ð7:53Þ
Sx ¼
ðN 1Þ
ðN 1Þ
This figure gives the tendency of two quantities to vary together. If these two
variables are totally independent, the covariance will be zero. The covariance of
a quantity with itself is the variance, already defined in Equation 7.51.
Statistical distributions
There are numerous probability distributions with a mathematical model. It is
not the place here to present all of them. They can be found in the specialized
handbooks such as Bevinton (1969), Diem and Lentner (1970) and Box et al.
(1978). The two most used distributions, which are also used afterwards to
estimate the confidence intervals, are presented below.
The probability density function of the normal or Gaussian distribution (see
Figure 7.3, left) is:
2
1
c
x
where
c¼
ð7:54Þ
fðcÞ ¼ pffiffiffiffiffiffi exp 2
2
where is the average and the standard deviation of the variable x.
The probability of the normal distribution (see Figure 7.3, right) is:
1
c
ð7:55Þ
FðcÞ ¼ 1 þ erf pffiffiffi
2
2
where the error function erf(x) is:
ð
2 x
erfðxÞ ¼ pffiffiffi expð2 Þ d
0
with erfðxÞ ¼ erfðxÞ.
ð7:56Þ
Common Methods and Techniques
0.5
1
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.2
0
0
-3
-2
-1
157
0
x – <x>
s
1
2
3
-3
-2
-1
0
x – <x>
s
1
2
3
Figure 7.3 Normal (or Gaussian) distribution (left) and its probabillity
function (right)
The confidence interval [c; c] of the normal distribution is obtained by
solving the equation:
pffiffiffi
P ¼ erfðc= 2Þ
ð7:57Þ
for a given value of P.
If the normalized variable:
x
c¼
ð7:58Þ
is calculated using the estimate, s, of the standard deviation (based on N þ 1
measurements) instead of the exact value, , (which is not known in practice),
then this estimate of the normalized variable:
x
ð7:59Þ
t¼
s
has a probability density function following the Student distribution (see Figure
7.4):
Nþ1
t2 N þ 1=2
2
t
ð7:60Þ
fðt; NÞ ¼ pffiffiffiffiffiffiffi 1 þ
N
N
N
2
where the gamma function ðx=2Þ is:
if x is even:
ðx=2Þ ¼ ðx=2 1Þðx=2 2Þ 3 2 1
if x is odd:
ðx=2Þ ¼ ðx=2 1Þðx=2 2Þ 1=2
ð7:61Þ
If n is large, the Student distribution tends to the normal distribution.
The confidence interval ½T; T where T ¼ TðP; Þ of the Student distribution cannot be expressed analytically. It can be found in Table 7.6 (or in
158
Ventilation and Airflow in Buildings
0.4
Normal
5
2
1
0.3
0.2
0.1
0
–4
–3
–2
–1
0
1
2
3
4
Figure 7.4 Student distribution for 1, 2 and 5 degrees of freedom
compared to the normal distribution
more detail in statistical tables such as Zwillinger (2003)) and in most
mathematical software packages.
Confidence interval of the Gaussian distribution
Assuming that a measurement, xi , is a combination of the ‘true’ value and a
random error, ei , we have:
xi ¼ þ ei
ð7:62Þ
By measuring xi , we expect to find the best estimate of . This can be done by
performing N > 1 measurements and computing their average. This average x
is the estimate of the ‘true’ value :
P
x
ð7:63Þ
ffi hxi ¼ i i
N
Let us recall that the confidence interval is the interval that has a given
probability, P (for example, 95 per cent), to contain the ‘true’ value. In other
terms, the probability of it being wrong, that is that the ‘true’ value being
outside this confidence interval, is the error probability p ¼ 1 P.
What we need now is precisely to give the confidence interval around x that
will contain with a fair probability. The value of this confidence interval
depends on the probability distribution of the measured values. In principle,
a reasonable distribution function should be chosen, adjusted on the measurements and the validity of this adjustment should be tested with the 2 test.
In most cases, however, and mainly when the number of the measurements
is large, a normal distribution with a mean and a standard deviation can be
assumed for the results of the measurements. Under this assumption, the
confidence limit of the ‘true’ value is given by:
s
ð7:64Þ
Ic ¼ pffiffiffiffiffi TðP; N 1Þ
N
where s is the estimate of , and TðP; N 1Þ is the confidence interval of the
Student distribution with N 1 degrees of freedom. These are shown in
Figure 7.5.
Confidence limit/standard deviation
Common Methods and Techniques
159
5
4
P = 99.9%
3
99%
2
90%
1
0
50%
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Number of measurements
Figure 7.5 Confidence limit divided by standard deviation versus number
of measurements for various values of probability, P
Hence, we can state:
¼ hxi Ic
ð7:65Þ
P is the probability that the confidence interval contains the ‘true’ value. P is
chosen a priori, in practice between 0.9 and 0.99, depending on the degree of
confidence needed. The higher the probability, the broader is the confidence
interval ½Ic ; Ic .
Note that the confidence interval of the ‘true’ value stabilizes to a value
close to the standard deviation if more than seven measurements are performed.
Error analysis
What is the problem?
If several measurements are combined to obtain the needed results, the errors
should also be combined the proper way to get the resultant error. In other
words, the problem is the following.
Suppose that we need several results y1 ; y2 ; . . . ; yj ; . . . ; yM , each of them
depending on measurements of several variables x1 ; x2 ; . . . ; xj ; . . . ; xN :
yj ¼ fj ðx1 ; x2 ; . . . ; xj ; . . . ; xN Þ
ð7:66Þ
Here, j ( j ¼ 1 to M) enumerates the various results (for example, M different
airflow rates) and i (i ¼ 1 to N) enumerates the variables on which the results
depend (for example, the tracer gas concentrations and flow rates or pressures
and conductances).
If the measurements, xi , each have an absolute error, xi , what are the
errors, yj , on the results, yj ?
Most simple error analysis
The simplest rule, which is taught everywhere, is the following: the error y on
the result is estimated by replacing, in the total differential df of the function f,
160
Ventilation and Airflow in Buildings
the infinitely small increments dxi by the absolute error xi and by summing the
absolute values:
X @fj
ð7:67Þ
x
yj ¼
@x i i
i
If only arithmetical operations are used, the rules simplify to the following:
.
.
If the result is obtained by adding or subtracting the measurements, the
absolute error on the result, y, is the sum of the absolute errors, x, of
each measurement.
If the result is obtained by multiplying or dividing measured data, the
relative error on the result, y=y, is the sum of the relative errors, x=x, on
the measurements.
Estimate of the variance
The simplest method illustrated above is very rough, since it overestimates the
confidence interval by supposing that all the errors in the measurements pull
the result in the same direction, which is highly improbable. A statistical interpretation is then needed to take account of randomly distributed errors.
If the variances, sxi , and the covariances, sxi;xj , of the measurements are
known or estimated, the covariances on the results, syk; yl , is, in a first approximation:
X X @fk @fl
syk; yl ¼
s
ð7:68Þ
@xi @xj xi;xj
i
j
The variance of a given result is then:
X X @fk @fk
ðsyk Þ2 ¼
s
@xi @xj xi;xj
i
j
ð7:69Þ
and if the measured variables are independent (that is if sxi;xj ¼ 0 when i 6¼ j ),
this simplifies to:
X @fk 2
ðsxi Þ2
ð7:70Þ
ðsyk Þ2 ¼
@x
i
i
The corresponding confidence intervals are then easily obtained by multiplying
these results by the Student coefficient T(P; 1).
Linear equations systems
To interpret the results of an experience, we often should solve a system of
equations such as:
Ay ¼ x
ð7:71Þ
where components of the vector x and the coefficients in the matrix A (which are
the results of the measurements) are perturbed by random errors that can be
Common Methods and Techniques
161
represented by a vector x and a matrix A. The question is: which is the
resulting error y on the vector y, which is the vector containing the final results?
If the matrix A and the vector x were known, we could write:
ðA þ AÞð y þ yÞ ¼ x þ x
ð7:72Þ
and, taking Equation 7.71 into account, we could solve:
y ¼ ðA þ AÞ1 ðx AyÞ
ð7:73Þ
This equation can be used many times in a Monte-Carlo process, varying each
time all the components of A and x at random, according to their probability
density function. This provides a series of vectors y from which an estimate
of the probability density functions of the components can be calculated.
However, this procedure is time consuming and, assuming a normal distribution of the measurement methods, simpler methods are available, which are
described next.
Complete error analysis
The requested final result is calculated using:
X
hence
yi ¼
ij xj
y ¼ A1 x
ð7:74Þ
j
where the coefficients ij are those of the inverse matrix A1 . The error
calculated with the most simple (or the differential) method will then be:
X @yi
X @yi
þ
x
a
yi ¼
k
kl @x
@a
k
k
kl
k;l
X @ij
X
jik xk j þ
x
a
¼
@a j kl kl
k
k;l
ð7:75Þ
But since
@ij
¼ ik lj
@akl
ð7:76Þ
we get finally:
X
X X
yi ¼
jik xk j þ
ik jl xj kl k
kl
ð7:77Þ
j
If the variances and covariances s2xk;xl , s2aki;amn and s2aki;xm are known, the
covariance of the results s2yi; yj is well estimated using a first order Taylor’s
expansion (Bevinton, 1969). We get:
X @yi @yj
X @yi @yj
s2akl amn þ
s2xk xl
s2yi ; yj ¼
@a
@a
@x
@x
kl
mn
k
j
klmn
kl
X @yi @yj
@yj @yi 2
sakl xm
þ
þ
ð7:78Þ
@a
@x
@a
kl
m
kl @xm
klmn
162
Ventilation and Airflow in Buildings
the partial derivatives are computed as above and we get finally:
X
X
ik yl jm yn s2akl amn þ
ik jl s2xk xl
s2yi ; yj ¼
klmn
þ
X
kl
ðik yl jm jk yl im Þs2akl xm
ð7:79Þ
klmn
which simplifies, if the variables are independent (that is, if the covariances are
zero, which is not always the case):
X
X
ik jk y2ln s2aki akl þ
ik jk s2xk xk
ð7:80Þ
s2yi ; yj ¼
kl
kl
Upper bound of the errors
The vector y contains a large number of data, but it is helpful to represent the
error by a single value. To obtain such a single value, the following definitions,
which can be found in the specific mathematical literature (for example, Deif,
1986) are used.
Vectorial norms and matrix norms
The norm jxj of a vector x is any operation of the n-fold real space Rn in the
ensemble of real numbers R satisfying:
jxj 0 and jxj ¼ 0 if and only if x ¼ 0
jcxj ¼ jcj jxj for any c 2 R
ð7:81Þ
jx þ yj jxj þ j yj
For example, the Euclidian norm that corresponds best to the standard
deviation:
ffiffiffiffiffiffiffiffiffiffiffiffiffi
qX
x2i
ð7:82Þ
jxj2 ¼
complies
P with the relations in Equation 7.81, but there are many others, like
xi or jxj1 ¼ maxðjxi jÞ.
jxj1 ¼
The norm jAj of a matrix A is any application NðAÞ ) jAj 2 R satisfying:
jAj 0 and jAj ¼ 0 if and only if A ¼ 0
jcAj ¼ jcj jAj for any c 2 R
jA þ Bj jAj þ j Bj
ð7:83Þ
jA Bj jAj j Bj
The matrix norm jAj is consistent with the vectorial norm jxj if:
jAxj jAj jxj for any x
ð7:84Þ
Common Methods and Techniques
and the matrix norm is subordinated to the vectorial norm jxj if:
jAxj
for any x 6¼ 0
jAj ¼ max
jxj
163
ð7:85Þ
The subordinated norm is the smallest matrix norm compatible with the
norm jxj.
For example, the norm jAj2 , defined as:
pffiffiffiffiffi
jAj2 ¼ 1
ð7:86Þ
where 1 is the largest eigenvalue of AH A (AH ¼ hermitic conjugate or transpose of the complex conjugate matrix) is subordinated to the Euclidian norm
jxj2 but the Frobisher norm:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
X X ffi
a2i
a2j
ð7:87Þ
jAjF ¼
i
j
is consistent with the Euclidian norm but not subordinated to it.
Finally, the following norms, which lead to faster calculations, are often
used:
X
jxi j
ð7:88Þ
jxj1 ¼
i
and the corresponding norm for the matrix A:
jAj1 ¼ max jAj j
ð7:89Þ
where Aj are the column vectors of A.
Calculation of the upper bound
From the norms of the experimental errors y and A, it is possible to calculate
an upper limit to the norm of the resulting error x by the use of the following
relation.
If jIj ¼ 1 (it is true for jIj2 ), then the norm of the relative error is:
jxj
jAj jA1 j
jyj jAj
þ
ð7:90Þ
jxj
jAj
1 jAj jA1 j j yj
The quantity:
condðAÞ ¼ jAj jA1 j
ð7:91Þ
is of great importance in this calculation. It is the so-called condition number of
the matrix A related to the used norm. This number indicates how nearly
singular the matrix is.
If the spectral norm jAj2 is used, we get the smallest possible condition
number, which is:
pffiffiffiffiffiffiffiffiffiffi
ð7:92Þ
cond2 ðAÞ ¼ jAj2 jA1 j2 ¼ 1 n
164
Ventilation and Airflow in Buildings
where 1 and n are respectively the largest and the smallest eigenvalues of
AH A. This number is the spectral condition number.
Constant absolute error
If the absolute error is constant:
A ¼ e1 and y ¼ y1
ð7:93Þ
where 1 and 1 are respectively a matrix of order N and an N-component vector
with all elements equal to 1 (they are not the identity matrix and the unit
vector).
It is easy to see that:
pffiffiffiffiffi
and
j1j2 ¼ N
ð7:94Þ
j1j2 ¼ N
because the eigenvalues of 1 are N and 0N 1 , and those of 1H 1 ¼ 12 are N 2 and
0N 1 . It follows that:
pffiffiffiffiffi
and
jyj2 ¼ N y
ð7:95Þ
jAj2 ¼ eN
and
jxj
cond2 ðAÞ
jxj
1 eNjA1 j
pffiffiffiffiffi
N y Ne
þ
j yj
jAj
ð7:96Þ
Constant relative error
In this case, if e and " are the constant relative errors on A and y:
A ¼ eA
and
y ¼ "y
ð7:97Þ
and, from the definitions of the norms:
jAj ¼ ejAj
and
jyj ¼ "y
ð7:98Þ
and we get, for any norm satisfying jIj ¼ 1:
jxj
condðAÞ
ð" þ eÞ
jxj
1 condðAÞ
ð7:99Þ
assuming that e condðAÞ < 1, that is that A þ dA is regular.
Example: A measurement with two tracer gases at constant concentration in
two zones gives the results in Table 7.7.
Table 7.7 Data measured during a tracer gas experiment in two connected rooms
Tracer concentration 1
Tracer concentration 2
Injection rate, tracer 1
Injection rate, tracer 2
Zone 1
Zone 2
10 ppm
6.46 ppm
2:65 104 m3 /h
0 m3 /h
2.29 ppm
10 ppm
0 m3 /h
3:6 104 m3 /h
Common Methods and Techniques
165
Table 7.8 Airflow rates [m3 /h], calculated from the data given in Table 7.7
Flow going to
Flow coming from
Outdoors
Zone 1
Zone 2
Outdoors
Zone 1
Zone 2
–
21.4
22.2
11.0
–
20.1
32.6
9.7
–
From which, using the method described in Chapter 1, ‘Zone by zone
systems of equations’, we get the airflow rates to and from each zone [m3 /h]
shown in Table 7.8.
Let us suppose that the error on the injection rate is 5 per cent and if the
relative error on the concentration is 2 per cent. What is the probable error
on the airflows? Using the most simple method, we get a relative error of 9
per cent.
Using the spectral norm, we get relative errors of 12 per cent for zone 1 and
7 per cent for zone 2. These are upper limits that are easily calculated, but there
is more information than the simple method allows, since we can see the difference in quality of the measurements between the two zones.
Notes
1 The wavelength, and the frequency, f, of an electromagnetic wave such as light,
infrared or radio waves are related by f ¼ c, where c is the velocity of light
(3 1010 m/s).
2 Halogenated compounds are compounds containing fluorine, chlorine bromine and
iodine in their molecule.
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Annex A
Unit Conversion Tables
Introduction
SI units are used throughout this book. Non-SI units are, however, of general
use in air infiltration and ventilation, like the air change rate in l/hour or US
units. To expedite the unit’s translations, some tables are given below. Only
physical quantities which are of general use in air infiltration and ventilation
measurement techniques are listed.
The figures given in the tables are multiplying factors transforming values
expressed in units of the first column into values expressed in the first row
units. Example: 1 cm ¼ 0.01 m.
Multiples and sub-multiples
Multiples
Sub-multiples
Prefix
Symbol
Factor
Prefix
Symbol
Factor
peta
tera
giga
mega
kilo
hecto
deca
P
T
G
M
k
h
da
1015
1012
109
106
103
102
101
femto
pico
nano
micro
milli
centi
deci
f
p
n
m
m
c
d
1015
1012
109
106
103
102
101
Length
Name
Symbol
1
1
1
1
1
m
cm
in
ft
yd
metre
centimetre
inch
foot
yard
m
cm
in
ft
yd
1
0.01
0.0254
0.3048
0.9144
100
1
2.54
30.48
91.44
39.37008
0.3937008
1
12
36
3.28084
0.0328084
1/12
1
3
1.093613
0.01093613
1/36
1/3
1
172
Ventilation and Airflow in Buildings
Area
Name
1
1
1
1
1
m2
cm2
sq in
sq ft
sq yd
1
104
6.4516 104
0.092903
0.836127
104
1
6.4516
929.0304
8361.27
1550
0.3937008
1
144
1296
10.7639
0.0328084
1/144
1
9
1.19599
0.01093613
1/1296
1/9
1
Symbol
square metre
square centimetre
square inch
square foot
square yard
2
m
cm2
sq in
sq ft
sq yd
Volume
Name
1 cubic metre
1 litre
1 millilitre
1 cubic yard
1 cubic foot
1 cubic inch
m3
Symbol
3
m
l
ml
cu yd
cu ft
cu in
ml cm3
l
1
0.001
0.000001
0.76455551
0.02831687
196.6 106
1000
1
0.001
764.5555
28.31687
0.196645
6
10
1000
1
764555.5
28316.87
196.6449
cu yd
cu ft
cu in
1.30795
1.308 103
1.308 106
1
0.037037
257.2 106
35.31464
0.035315
35.32 106
27
1
6.944 103
5085.308
5.085308
0.005085
3888
144
1
Note: the millilitre is equal to the cubic centimetre.
Mass
Name
Symbol
1 kilogram
1gram
1 pound
1 ounce
1 grain
kg
g
lb
oz
gr
kg
g
lb
oz
gr
1
0.001
0.45359229
0.02834952
64.79 106
1000
1
453.5923
28.34953
0.064799
2.204623
2.205 103
1
0.0625
142.9 106
35.27396
0.03527396
16
1
2.2857 103
15,432
15.43
700
437
1
Time
Name
Symbol
1
1
1
1
1
s
min
h
d
yr
second
minute
hour
day
year
s
min
h
d
yr
1
60
3600
86,400
31,556,926
1/60
1
60
1440
526103
1/3600
1/60
1
24
8765
1/86400
1/1440
1/24
1
365.25
31,688 109
1.90133 106
114.08 106
2.73791 103
1
Unit Conversion Tables
173
Pressure
Name
Symbol
1 Pascal
1 millibar
1 mm water column
Pa
mbar
mm
H2 O
1 inch water column
in H2 O
1 pound per square inch lb/in2
or psi
Pa
mbar
mm H2 O in H2 O
psi
1
100
9.81
0.01
1
0.0981
0.102
10.2
1
0.004
0.422
0.0393
145.037 106
14.5037 103
1.42 103
249
6894.76
2.5
68.9476
25.4
703
1
27.7
36 103
1
Volume flow rate
Symbol
m3 /s
l/min
m3 /h
cu ft/s
cu ft/min
cu ft/h
m3 /s
l/min
m3 /h
cu ft/s
cu ft/min
cu ft/h
1
16.667 106
277.78 106
0.02831687
471.95 106
7.87 106
60,000
1
16.666667
1699.0122
28.316870
0.47194784
3600
0.06
1
101.9407335
1.699012225
0.028316870
35.3146
588.58 106
0.00980962
1
0.01666667
277.78 106
2118.878
0.0353146
0.58857728
60
1
0.01666667
127132.693
2.11887822
3
3600
60
1
Mass flow rate
Symbol
kg/s
kg/min
kg/h
lb/s
lb/min
lb/h
kg/s
kg/min
kg/h
lb/s
lb/min
lb/h
1
0.01666667
277:78 106
0.45359229
0.00755987
125:1 106
60
1
0.01666667
27.2155375
0.45359229
0.00755987
3600
60
1
1632.932
27.21554
0.4535923
2.204623
0.03674372
612:4 106
1
0.01666667
277:8 106
132.27738
2.204623
0.03674372
60
1
0.01666667
7,936.643
132.27738
2.204623
3600
60
1
Annex B
Glossary
Items in italics are additional entries in the glossary.
Age of the air (or age of a contaminant)
Average time period since the fresh air (or a contaminant) entered the room or
the building. This age depends on the location in the building. The room mean
age of air is the average of the age over the whole room.
Air change performance
Coefficient defined by ASHRAE, which is the double of the air exchange
efficiency.
Air change rate (or specific airflow rate)
The ratio of the volumetric rate at which air enters (or leaves) an enclosed space
divided by the volume of that space. Often this is expressed in air changes per
hour. Its inverse is the nominal time constant.
Air exchange efficiency
Efficiency of the ventilation to change the air in a room. It is half the ratio of the
nominal time constant and the room mean age of air.
Air exchange rate
General term relating to the rate of airflow between one space and another. This
can be between various internal zones of a building or between the building and
the atmosphere.
Air exfiltration
The uncontrolled leakage of air out of a building.
Airflow coefficient
Coefficient in the air leakage characteristics, which has the dimension of an
airflow. This coefficient multiplies the pressure differential at a power exponent.
Airflow rate
The mass or volume of air moved in unit of time. (The transport may be within
an enclosure or through an enclosing envelope.)
Air infiltration
The uncontrolled inward air leakage through cracks and interstices in any
building element and around windows and doors of a building (i.e.,
Glossary
175
adventitious openings), caused by pressure effects of the wind and/or the
effect of differences in the indoor and outdoor air density.
Air infiltration characteristic
The relationship between the infiltration airflow rate into a building and the
parameters that cause the movement.
Air leakage
Airflow rate through a component of the building envelope, or the building
envelope itself, when a pressure difference is applied across the component.
Air leakage characteristic
An expression that describes the air leakage rate of a building or component.
This may be:
.
.
.
the air leakage flow rate at a reference pressure difference across the component or building envelope;
the relationship between flow rate and the pressure difference across the
building envelope or component;
the equivalent leakage area at a reference pressure difference across the
component or building envelope.
Airtightness
A general descriptive term for the leakage characteristics of a building.
Analyser
Instrument used to measure the concentration of a tracer gas or a contaminant in
a sample of air.
Anemometer
Any instrument measuring the air speed or the air velocity.
Background concentration
Concentration of a gas in outdoor air.
Background leakage
Leakage of air through a building envelope that is not accounted for by obvious
measurable gaps.
Balanced fan pressurization
Technique utilizing two or more blower doors to evaluate the leakage of individual internal partitions and external walls of multi-zone buildings. Technique
involves using the fans to induce a zero pressure difference across certain
building components, thus eliminating their leakage from the measurement.
Balanced ventilation
Ventilation systems in which fans both supply and extract air from the enclosed
space, the supply and extract flow rates being equal.
Blower door (or fan door)
A device that fits into a doorway for supplying or extracting a measured flow
rate of air to or from a building. It is normally used for testing for air leakage
by pressurization or depressurization.
176
Ventilation and Airflow in Buildings
Building component
General term for any individual part of the building envelope. Usually applied
to doors, windows and walls.
Building envelope
The total of the boundary surfaces of a building, through which heat (or air) is
transferred between the internal spaces and the outside environment.
Calibration
Operation where the output of a measuring device is compared with reference
standards, to accurately quantify the results provided by the measuring device.
Capacitance pressure transducer
A device with a metal diaphragm sensing element acting as one plate of a capacitor. When pressure is applied it moves with respect to a fixed plate, changing
the thickness of the dielectric between. The resulting signal is monitored using
a bridge circuit.
Cell
Volume in a room limited by a theoretical or physical surface, in which the
physical quantities of interest can be considered as homogeneous. A room
can be divided in several cells.
Chemical indicator tubes (or Dra¨ger1 tubes)
Glass tubes containing an adsorbing material that changes colour in the
presence of a specific gas.
Compensated flowmeter
Airflow rate measuring instrument in which a fan compensates the pressure
drop required by the measuring device.
Component leakage
The leakage of air through the building envelope or internal partitions, which is
directly attributable to flow through cracks around doors, windows and other
components.
Concentration
Ratio expressing the amount of a chemical component in a mixture. This ratio
may be expressed in terms of mass, of volume or of number of molecules. In air,
it can also be the ratio of the mass of component divided by the volume of air.
Condition number
Number expressing how much the errors in measured data are enlarged when
transmitted, through the interpreting equations, to the final results.
Conductance
Generally, any path allowed for air between two zones. Also the ratio of the flow
rate through a path to the pressure differential across that path.
Connected space
A space in a building adjacent to the measurement space with which significant
exchange of air may take place, thus increasing the effective volume of the space.
Glossary
177
Constant concentration technique
A method of measuring ventilation rate whereby an automated system injects
tracer gas at the rate required to maintain the concentration of tracer gas at a
fixed, predetermined level. The ventilation rate is proportional to the rate at
which the tracer gas must be injected.
Constant injection rate technique
A method of measuring ventilation rate whereby tracer is emitted continuously
at a uniform rate. The equilibrium concentration of tracer gas in air is
measured.
Contaminant
An unwanted airborne constituent that may reduce the acceptability of the air
quality.
Contaminant removal effectiveness
See ventilation efficiency.
Continuity equation (or mass balance)
Mathematical expression relating to the conservation of matter, an example of
which is the equation equating the flow of tracer gas into a space with the flow of
tracer gas out of a space. this particular equation is the basis for evaluating air
exchange rates from tracer gas measurement.
Damper
Adjustable plate in a duct for controlling the flow rate.
Decay rate technique
A method for measuring ventilation rate whereby a quantity of tracer gas is
released and the decrease in concentration measured as a function of time.
Deduction method
Multi-fan testing method in which the pressure differential between two zones of
a building is changed step by step in order to obtain the leakage characteristics of
building elements in these zones.
Density
Ratio of the mass of a quantity of matter to its volume.
Depressurization
Term used to describe fan pressurization when a static under-pressure is
created within the building.
Differential pressure
See pressure differential.
Discharge coefficient
A dimensionless coefficient relating the mean flow rate through an opening to
an area and the corresponding pressure difference across the opening.
Displacement flow (or piston flow)
With this type of flow, incoming outdoor air displaces internal air without mixing.
178
Ventilation and Airflow in Buildings
Distribution effectiveness
Ratio of the average tracer gas or contaminant concentration to the concentration
that could be reached, at equilibrium, in the same zone or building with the
same tracer or contaminant sources. Also the ratio of the contaminant or
tracer turnover time to the room mean age of air. It is the inverse of the relative
contaminant removal effectiveness.
Door panel
Panel adapted to a door or a window on which the pressurization fan is
mounted.
Draught gauge
Inclined u-tube manometer.
Dra¨ger1 tubes
See chemical indicator tubes.
Effective volume
The volume of the interior building (or room) in which mixing occurs.
Efficiency of the ventilation system
Ratio of the fresh air provided by the ventilation system to an enclosure to the
total amount of air entering the room, including infiltration.
Electron capture detector
An instrument, which uses a weak beta source to generate electrons in an
ionization chamber, subjected to a pulsed voltage, thus generating a current.
Electron-capturing material in the sample reduces the number of electrons in
the chamber and thus the current. This reduction can be calibrated in terms
of tracer gas concentration; hence the concentration of tracer gas in an air
sample can be evaluated.
Envelope (of a building)
See building envelope.
Equivalent leakage area
The equivalent amount of orifice area that would pass the same quantity of air
as would pass collectively through the building envelope at a specified reference
pressure difference.
Experimental design
The way an experiment is planned, or, more precisely, a list of values of
controlled parameters at which measurements should be performed to obtain
the required results.
Extract ventilation
A mechanical ventilation system, in which air is extracted from a space or
spaces, thus creating an internal negative pressure. Supply air is drawn
through adventitious or intentional openings.
Glossary
179
Fan pressurization
General term applied to any technique involving the production of a steady
static pressure differential across a building envelope or component. Often
referred to as dc pressurization.
Flame ionization detector
Detector used in conjunction with a gas chromatograph, in which the change in
ionic current caused in a hydrogen–air flame by a tracer or contaminant is
detected. This detector is sensitive to organic compounds.
Flow coefficient
In the power function approach this parameter is used in conjunction with the
flow exponent to quantify flow through an opening.
Flow equation
Equation describing the airflow rate through a building (or component) in
response to the pressure difference across the building (or component).
These equations are usually power law or quadratic law in form.
Flow exponent
In the power function approach, this parameter characterizes the type of flow
through a component (n ¼ 1 represents laminar flow, n ¼ 0:5 represents turbulent flow). For most flow paths, n takes a value between these extremes.
Fortuitous leakage
Uncontrolled air leakage through a building envelope due to the natural action
of wind and temperature, i.e., air infiltration.
Gas chromatography
A process by which gases can be separated from one another. Used in this
application to separate tracer gases from each other and from the constituents
of air, thus allowing individual analyses to be performed.
Gasometer
Instruments to measure volumes of any gas.
Grab sampling method
Any tracer gas method where air/tracer samples are obtained from a building
and analysed afterwards in a laboratory.
Guard zone technique
Dual fan pressurization technique used to measure the leakage characteristics
of a building part. One fan is used to pressurize a guarding zone, surrounding
the guarded zone in which the other fan just maintains a zero pressure differential between these zones. The measured building part is the only unguarded
part.
Hot wire anemometer
Anemometer in which the temperature of a heated wire exposed to the wind
determines the air velocity.
180
Ventilation and Airflow in Buildings
Indoor air pollution
Pollution occurring indoors from any source, i.e., from outside as well as inside
the building.
Infrared gas analyser
Instrument used to determine tracer gas concentrations by determining the
transmission of infrared radiation at an absorption frequency through a fixed
path length.
Inter-zonal airflow
General term applied to the process of air exchange between internal zones of a
building.
Leakage area
See equivalent leakage area.
Leakage characteristics
Equation relating the airflow rate through a leak and the pressure differential
across this leak. This relation involves the flow coefficient and the flow exponent.
Leakage path
A route by which air enters or leaves the building or flows through a
component.
Leakage site
A point on the outer or inner surfaces of a building envelope or an internal wall
where a leakage path emerges.
Leeward
Downwind side of any object.
Manometer
A device for measuring pressure employing the principle of displacement of
liquid levels in a liquid-filled u-tube. The limbs of the ‘u’ may be vertical,
inclined (draught gauge) or curved.
Mass balance
See continuity equation.
Mass flow controller
Device controlling the flow rate of a gas by means of a valve controlled
according the measurement of the mass flow rate.
Mass spectrometry
Technique allowing the quantitative measurement of amounts of different
gases, based on the separation of the ionized gas molecules according their
mass-to-charge ratio.
Mechanical ventilation
Ventilation by means of one or more fans.
Glossary
181
Mixing
The degree of uniformity of distribution of outdoor air or foreign material in a
building.
Mixing fan
Small electric fan used to aid the mixing of room air and tracer gas before and/or
during a measurement.
Multiple tracer gas technique
General term applied to measurement methods using two or more tracer gases.
These methods are often used to evaluate inter-zonal airflows.
Multi-zone
A building or part of a building comprising a number of zones or cells.
Natural ventilation
Ventilation using only purpose-provided openings and the natural motive
forces of wind and temperature difference.
Nominal time constant
The ratio of the volume of an enclosed space divided by the volumetric rate at
which air enters (or leaves) that space. Its inverse is the air change rate.
Normalized leakage area
Equivalent leakage area expressed per unit building envelope area.
Orifice plate
A device for assessing gas flow by measuring the pressure drop across an orifice
in the flow line.
Outdoor air
Air from free atmosphere that is generally assumed to be sufficiently uncontaminated to be used for ventilation.
Passive sampling
Method of sampling tracer gas in a building by the process of passive
diffusion.
Passive tracer source
Small container injecting continuously a small flow of tracer (mostly PFT
tracers) by passive diffusion through its cover cap.
Perfluorocarbon tracers (or PFT)
Tracer gases composed of a family of perfluoroalkylcycloalkanes, i.e., cyclic
organic compounds in which the hydrogen atoms are all replaced by fluorine
atoms. These tracers can be analysed in trace amount because the background
concentration is low and the electron capture detector is very sensitive to them.
Photo-acoustic detector
Tracer gas analyser in which the alternate expansion and contraction of the
gas sample irradiated with a chopped beam of convenient wavelength is
detected with a microphone.
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Ventilation and Airflow in Buildings
Piston-type ventilation
See displacement flow.
Pitot tube
Anemometer measuring the difference between the pressure in a tube facing
the flow, in which the flow is stopped, and the pressure along a side of the
tube.
Pollutant removal effectiveness
See ventilation efficiency.
Pollution migration
Descriptive term for the movement of indoor air pollutants throughout a
building.
Pollution source
Any object, usually within a building, that produces a substance that will
contaminate the internal environment.
Power law
Flow equation in which the airflow rate through the building envelope is proportional to a power of the pressure differential.
Ppm
Unit for expressing volume concentration, which is a part per million (106 ) or
a cubic centimetre per cubic metre.
Pressurization
Airtightness measuring technique using a fan to pressurize the measured
volume at a constant pressure. See also fan pressurization.
Pressure differential
Usual term for the difference in pressure across a building envelope or component, whether caused by natural or artificial means.
Pressure tap
Point at which pressure is measured.
Pulse injection technique
Tracer gas measuring technique in which the tracer is injected in a short pulse.
Purging flow rate
Part of the airflow rate, which effectively removes the contaminants out of
the location of interest. It is the product of the airflow rate and the ventilation
efficiency.
Purpose-provided openings
Openings in the building envelope for the specific purpose of supplying or
extracting ventilation air.
Quadratic law
Flow equation in which the pressure differential is related to the airflow rate by a
quadratic polynomial.
Glossary
183
Reductive sealing method
A method of determining the leakage of specific building components by
pressurizing the building and recording the leakage changes as components
are sealed successively. When all the major outlets and component cracks are
sealed, the remainder is the background leakage.
Relative contaminant removal effectiveness
Ratio of the concentration that could be reached, at equilibrium, in the same
zone or building with the same tracer or contaminant sources, to the average
tracer or contaminant concentration. Also the ratio of the room mean age of air
to the contaminant or tracer turnover time. It is the inverse of the distribution
effectiveness.
Residence time
See age of the air.
Residual gas analyser
See mass spectrometry.
Retrofit
The process of reducing energy loss in a building by physical means, for
example, reducing excess air infiltration by obstructing flow through cracks
and openings.
Reynolds number
Ratio of the inertial force to the friction force. It is also the ratio of the velocity
of a fluid to its dynamic viscosity, multiplied by a typical dimension, for
example, the duct diameter.
Room
Volume of a building limited by building elements. In ventilation technique,
this concept keeps its usual meaning. A room may be divided in several cells
and several rooms may be combined in a zone.
Room mean age of air
Average of the mean age of air over the whole room.
Sample container
Container used to obtain a sample of air/tracer mixture from a measured
building. The sample is usually returned to a laboratory for analysis.
Short-circuiting
A direct flow path between an air supply point and an air extract point, i.e., air
flows along the shortest path, without mixing.
Single tracer gas technique
General term applied to any method using only one tracer gas. These methods
are usually used to evaluate air change rate.
Single zone
Any case where a building or part of a building is considered to be a single wellmixed space.
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Ventilation and Airflow in Buildings
Site analysis
Applied to any tracer gas measurement technique where tracer gas concentrations
and air exchange rates are determined directly at the measurement building.
Smoke leak visualization
A method of detecting leaks in the building fabric by pressurizing the building
and using smoke to trace the paths followed by the leaking air.
Specific airflow rate (or air change rate)
The ratio of the volumetric rate at which air enters (or leaves) an enclosed space
divided by the volume of that space. Its inverse is the nominal time constant.
Specific leakage area
Equivalent leakage area expressed per unit floor area.
Stack effect
Pressure differential across a building envelope caused by differences in the
density of the air due to an indoor–outdoor temperature difference.
Step injection technique
Tracer gas measurement technique in which the tracer is injected at constant
rate, starting from a given time.
Supply ventilation
A system in which air is supplied to a space(s) so creating an internal positive
pressure. Air leaves the building through adventitious or purpose-provided
openings.
Tachometer
Instrument for measuring velocity or speed of rotation, used to evaluate the
speed of fans, this in turn is used to calibrate the fan in terms of airflow.
Often used in blower doors.
Thermography
The process of converting the heat emitted from an object into visible pictures.
Used to indicate and represent the temperature distribution over part of a
building envelope.
Tracer gas
A gas used at low concentration, together with an analyser, to determine airflow
rates or other related quantities.
Tracer gas analyser
Any instrument used to evaluate the concentration of tracer gas in a sample of
air.
Tracer gas injection
Any process by which tracer gas is released into a space.
Tracer gas sampling
Any process by which tracer gas or air containing tracer gas is sampled for
analysis.
Glossary
185
Turnover time of a contaminant
Ratio of the mass of contaminant contained in an enclosure to the mass flow rate
of the contaminant source in this enclosure.
Ventilation
The process of supplying and removing air by natural or mechanical means to
and from any space.
Ventilation efficiency
An expression describing the ability of a mechanical (or natural) ventilation
system to distribute the outdoor air in the ventilated space.
Ventilation energy
Energy loss from a building due to ventilation.
Venturi tube
Duct with a restricted section, which allows the measurement of the flow rate
through the pressure differential between the restricted and the normal section.
Windward
Upwind side of any object.
Zone
Part of a building, which is considered as a single volume for the experiment
performed, or the physical quantity of interest. A zone may contain several
rooms.
Index
absolute error, 154
active ways, 78
adsorption, 123
age matrix, 10
age of the air, 39, 42, 174
air
age of, 39, 42, 174
change
efficiency, 40
leakage rate, 67
performance, 174
rate, 1, 174
conditioning, 79
exchange efficiency, 174
exchange rate, 174
handling unit, 20
airflow rates in, 15
energy in, 79
permeability, 59
speed, 17
airflow
assessment, xiv
coefficient, 60, 174, 179
meter, 20
rate, xiii, 1, 174
equivalent outdoor, 5
exfiltration, 31
exfiltration, xvi
exhaust, 31
extract, 28
in air handling unit, 15
in duct, 15
infiltration, 30
intake, 28
inter-zone, 6
minimum, xiii
outdoor, xiv, 29
per person, 5
recirculation, xv, 29, 37
supply, 28
volume, 11
airtightness, 58, 66, 175
of buildings, 67
of ducts, 74
analyser, 142, 175, 184
anemometer, 17, 175, 179
average, 155
bag flow meter, 20
balanced ventilation, 175
Bayesian identification, 151
blower door, 175
building
airtightness, 67
envelope, 176
high quality, 78
chemical indicator tubes, 146, 176
chromatograph, 145, 179
CO2, 5, 34
coefficient
airflow, 60, 174, 179
discharge, 177
leakage, 60, 64
of performance, 94
coil, 131
contamination, 113
compensated flowmeter, 20, 176
computer program, 32
concentration, 133, 176
condition number, 53, 163, 176
conditions standard, 65
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Ventilation and Airflow in Buildings
conductance, 176
confidence interval, 28, 149, 154,
155
conservation equation, 1, 7, 177
constant concentration, 3, 12, 176
constant injection rate, 3, 12, 43,
177
contaminant, 108, 177
in heat exchangers, 117
transfer, 10
cooling, 79, 105
coil, 83
covariance, 156
decay method, 3, 12, 43, 177
density, 177
correction for, 132
of tracer gas, 138
detector tubes, 146, 176
discharge coefficient, 177
displacement flow, 177
duct, 15, 129
airtightness, 74
contamination, 109
efficiency
air exchange, 174
of fans, 97
of heat exchangers, 86
of heat recovery, 90
of ventilation, xvii, 39, 178, 185
electric power of fan, 100
electron capture detector, 146, 178
energy, 77
and well-being, 102
saving, 94, 104
ventilation, 97
envelope of a building, 176
equivalent
leakage area, 67, 178
outdoor airflow rate, 5
error analysis, 153
exfiltration, xvi, 31, 174
exhaust, 31
experimental design, 51
extract airflow rate, 28
extract ventilation, 178
fan
efficiency, 97
electric power, 100
pressurization, 59, 179
balanced, 175
filter, 106, 128
contamination, 108
flame ionization detectors, 146, 179
flow
coefficient, 60, 179
exponent, 60, 179
matrix, 9
meter, 20
compensated, 20, 176
gamma function, 157
gas chromatograph, 145, 179
Gaussian distribution, 156
glycol heat exchanger, 86
grab sampling, 139, 179
grille, 19
heat exchange efficiency, 86
heat exchanger, 83, 130
contaminants in, 117
heat pump, 86
heat recovery, 106
effect of leakage on, 89
efficiency, 90
heating, 79, 105
coil, 83
helix anemometers, 18
high quality buildings, 78
hot wire anemometers, 17, 179
humidifier, 130
contamination by, 110
humidity ratio, 87
humidification, 106
identification method, 147
indoor
air quality strategies, 125
environment, 104
infiltration, 30, 175
infrared absorption, 143, 180
injection, 118, 184
location, 141
sequence, 141
intake airflow rate, 28
inter-zone airflows, 6
leakage, 58, 175
Index
air change rate, 67
and heat recovery, 89
area, 67, 178
characteristics, 180
coefficients, 60, 64
heat exchangers, 88
visualization, 69
least square fit, 27, 147
manometer, 180
mapping experiments, 49
mass
conservation, 1, 7, 177
spectrometer, 144, 180
matrix
age, 10
flow, 9
norm, 162
mechanical power, 97
minimum airflow rate, xiii
mixing, 137, 181
model matrix, 52
multi-zone, 181
airflow rates measurements, 6
pressurization, 62
network, 140
neutral height, 72
node by node, 23
nominal time constant, 3, 40, 181
normal distribution, 156
nozzle, 16
NTC anemometers, 17
optimal performance, 125
orifice plate, 16, 181
outdoor air, 181
outdoor airflow rate, xiv, 29
passive sampling, 139, 181
passive ways, 78
performance, 125
permeability, 59
photo-acoustic, 144, 182
pipes, 140
piston flow, 177
Pitot tube, 18, 182
planning
of experiments, 49
tool, 32
plate heat exchanger, 84
power law, 60, 182
pressure difference, 99
and energy use, 107
pressurization, 177, 179, 182
multi-zone, 62
single zone, 59
probability function, 155
psychrometric chart, 79
pulse injection, 4, 12, 43, 182
pumps, 140
quadratic law, 60, 183
recirculation, xv, 29, 34, 37, 88, 105
large-ratio, 36
reductive sealing, 61
regression, 148
relative error, 154
rotating heat exchangers, 85
contamination by, 111
sampling, 22, 119, 185
location, 141
methods, 139
sequence, 141
sealing, 61
sensory pollution, 113
significance limit, 155
single zone, 1
specific leakage rate, 67
stack effect, 70, 184
standard conditions, 65
standard deviation, 156
step-down, 43
step-up, 43, 184
Student distribution, 157
supply airflow rate, 28
system of equations (errors in), 160
temperature efficiency, 87
thermography, 69, 184
time constant, 35
nominal, 40
tracer gas, 1, 18, 134, 184
analyser, 142, 184
constant concentration, 3, 12, 176
constant injection, 3, 12, 43, 177
decay method, 3, 12, 43, 177
density, 138
189
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Ventilation and Airflow in Buildings
tracer gas (Continued)
injection, 21
mixing of, 137
pulse injection, 4, 12, 182
sampling, 22
techniques, 12
trained panel, 113
transfer of contaminants, 10
units, 132
conversion tables, 171
variance, 52, 156, 160
vectorial norm, 162
velocity traverse, 17
ventilation, xiii, 185
balanced, 175
efficiency, xvii, 39, 178, 185
energy, 97
grilles, 19
system, 15
Venturi tube, 16, 185
visualization of air leakage, 69
VOC, 119
volume flow rates, 11
water vapour pressure, 87
well-being, 102
zone, 1, 9, 185