Studies on New Air Purification and Air Quality Control System of

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The 2nd International Symposium on Aircraft Airworthiness (ISAA 2011)
Studies on New Air Purification and Air Quality Control
System of Airliner Cabin
HU Tao a*, LIU Menga, PANG Lipinga, WANG Juna
a
School of Aeronautics Science and Engineering, Beihang University, Xueyuan Rd. 37, Beijing 100191, China
Abstract
Airworthiness standards of China and America both give the maximum permissible concentrations of CO, CO2 and
O3 in airliner cabin. The general specifications of aircraft engine (GJB241-87, GJB242-87) also have the maximum
permissible concentrations of pollutants in the engine bleed air used for the air conditioning system. During
airworthiness certification flight tests, the concentrations of these harmful pollutants in cabin are required to be tested
and validated. According to these requirements, combined with active and passive control strategies, a new cabin air
quality (CAQ) control system was presented based on nano-photocatalytic oxidation (NPCO) purification technique.
A CAQ dynamic model was established and used to do simulation analysis of the CAQ control strategies. Results
showed that the CAQ control system based on new NPCO device can well handle the CAQ problems in current
airliner cabins, and the new CAQ control strategies may help to reduce the CAQ change amplitude, and to improve
the CAQ as well.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Airworthiness
Technologies Research Center NLAA, and Beijing Key Laboratory on Safety of Integrated Aircraft and
Propulsion Systems, China Open access under CC BY-NC-ND license.
Keywaords: Airworthiness; Nano-photocatalytic oxidation (NPCO); Cabin air quality (CAQ); Control strategies
1. Introduction
The airliner cabin is small and crowded, and needs to bleed air from outside to adjust the cabin
pressure, temperature and other environmental parameters during flight. The metabolisms of passengers
* Corresponding author. Tel.: +86-10-8233-9310; fax: +86-10-8231-6654.
E-mail address: hutaosy@gmail.com.
1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license.
doi:10.1016/j.proeng.2011.10.039
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and crews, equipment operation, material release, and the bled air from external environment, make the
composition of cabin air very complicated. Besides the external atmosphere components, the cabin air
also contains inorganic and organic contaminants.
Nomenclature
A
the UV intensity influence constant, mW/cm2
C
the concentration of pollutant inside the cabin, mg/L
C0
the initial concentration of the pollutant, mg/L
CB
the pollutant concentration in the fresh air, mg/L
Ce
the pollutant concentration after purification, mg/L
D
the impeller diameter of the fan, m
I0
the initial UV intensity, mW/cm2
k1
the adsorption equilibrium constant, L/mg
km
the catalytic reaction constant, mg/(L•min)
k2, k3 & k5
linear regression constants, dimensionless
k4
linear regression constant, L/mg
k6
linear regression constant, (L/mg)2
M
the amount of pollutant that is emitted from pollution sources, mg/min
n
fan speed, r/min
Qf
amount of fresh air supply for the cabin, L/min
Qr
recirculated air amount from the cabin, L/min
r avg
the average catalytic rate, mg/(L•min)
RH
relative humidity (%) of the cabin air, dimensionless
V
the cabin volume, L
Vbed
the catalyst bed volume, L
Vp
the volume of air purifiers, L
ε
the UV absorption coefficient, cm2/g
δ
the catalyst layer thickness, cm
ρb
the catalyst density, g/cm3
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According to statistic reports, the passengers and crews have long complained the air quality problems
of airliners which have caused fatigue, dizziness, headache, ear disease, eye dryness, throat pain, and
occasionally nervous system disorder even loss [1]. To save energy, the future airliners will require a
higher cabin air recycling rate, which may make the CAQ problems even worse [2].
Airliner CAQ directly relates to the health, comfort and safety of the crews and passengers. Light
pollution of cabin air can reduce the crew working efficiency, and heavy one may lead to destructive
accident. Consequently, during the flight, it has an important practical significance to keep the man-made
environment of cabin clean and comfortable [3].
Currently, the dispositions of harmful substances in cabin air are usually dilution by fresh air bled from
outside, using high efficiency particulate air (HEPA) filters, ozone converter and activated carbon (AC)
adsorption units. HEPA filters can not effectively remove organic contaminants, and their airflow
resistance is large; AC unit volume is relatively large which is not suitable for aircraft with very limited
space, and it has regeneration and other problems. All of these problems require introducing a new air
quality control technology to solve.
NPCO technology is developed in 1970s as a new air pollution control technology, and has caused
much attention due to its high efficiency, small size, and wide range of applications.
Concerning the existing cabin air quality problems, a new cabin air purification system was presented
based on NPCO technique. Combined with active and passive control strategies, a new cabin air quality
(CAQ) control method was introduced for this system. And finally, the CAQ control system was modeled
and simulation analyzed.
2. CAQ standards and guidelines
Table 1. Limits on pollutants concentrations in aircraft cabin air given by different organizations [1]
Organizations
FAA
Name of standards
14 CFR 25
ASHRAE
ASHRAE 62-1999
EPA NAAQS
40 CFR 50
OSHA PEL
29 CFR 1910
ACGIH TLV
ACGIH 1999
CCAR
CCAR-25[4]
CCAR
Air pollutants (ppm)
O3
a
0.1; 0.25(3h)
0.005
CO2
CO
NOx
5000
50
—
0.055(annual
average)
0.055(annual
average)
5
700 above ambient
0.12(1h)
0.08(8h)
0.1
0.05(T)b
0.08(S)c
0.1(light)
0.1; 0.25(3h)
3
—
5000
9(8h)
35(1h)
9(8h)
35(1h)
50
5000(T)
30000(S)
25(T)
5000
50
3(T)
5(S)
—
3
MH7005-1995[5]
0.13mg/m
0.15kPa (partial pressure)
10mg/m
—
GJB241, GJB242 COSTIND
0.1
5
5000,Max. not over 30000 50
1987[6, 7]
a: Recommended maximum exposure time, 3h;
b: T=time-weighted average concentration in a normal 8-h workday and a 40-h workweek, to which nearly all workers may be
repeatedly exposed, day after day, without adverse effect (ACGIH 1999);
c: STEL=short-term exposure level is a 15-min TWA exposure that should not be exceeded at any time during the workday
(ACGIH 1999).
Many guidelines and standards established for air quality in aircraft cabins are applicable to routine
exposures in other indoor and outdoor environments. Table 1 lists contaminants that may be encountered
under routine conditions in aircraft and the exposures recommended or legally established by various
organizations. The organizations include FAA, the American Society of Heating, Refrigerating and Air-
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Conditioning Engineers (ASHRAE), the Environmental Protection Agency (EPA), OSHA, and the
American Conference of Governmental Industrial Hygienists (ACGIH). FAA is the only organization
with regulatory authority to establish standards for the aircraft cabin environment. ASHRAE has
committees that provide guidelines on exposure in indoor environments, including those of aircraft. EPA
promulgates national ambient air-quality standards (NAAQSs) for outdoor air. OSHA establishes
permissible occupational exposure limits (PELs), and ACGIH recommends threshold limit values (TLVs)
to protect worker health. Because the limits established by OSHA and ACGIH are intended for
application in workplaces populated by healthy adults of working age, they are not intended to apply in
situations where infants, children, the elderly, or those with medical conditions might be exposed; these
subpopulations, included among aircraft passengers, are addressed by EPA’s NAAQSs.
Table 1 also shows the limits of CO, CO2 and O3 concentrations given by the airworthiness standards
of China (CCAR-25), hygienic standard on cabin of civil passengers' aircraft and helicopter (MH70051995), and China general specifications for aircraft engines (GJB241-87, GJB242-87).
The units of pollutants limits in MH7005-1995 are different with others. If we take the cabin pressure
81kPa, the temperature 25
℃, tointo
them
convert
unit of ppm, they will be 0.083ppm, 1852ppm, and
10.92ppm, respectively, which is lower than others. However, other standards and guidelines give more or
less the same limits including the airworthiness standards of China (CCAR-25) and America (14 CFR 25),
which reflects the agreements of China airworthiness standards with the international ones.
3. New CAQ control system
There are mainly three methods to control airliner cabin pollutants: (1) dilution by fresh air from
outside; (2) removal by air purification devices; (3) combination of the above two methods. To address
the deficiencies of commonly used HEPA/AC compound purification units, a new air purification system
(Fig. 1) based on NPCO technology was proposed. This system uses a general filter to replace HEPA one
to reduce the airflow resistance, so that the power and volume of the recycling fan can be reduced, which
will lead to the aircraft weight reducing and energy saving during flight. A NPCO purifier is applied to
replace the AC adsorption unit, since it can not only effectively remove organic pollutants, but also can
clear the cabin microorganisms. It has many advantages, such as high efficiency, small size, light weight,
low airflow resistance, and wide application, which make it very ideal for purification of airliner cabin
environment.
Recirculated air
Recirculated air
Fresh air
Normal
filter
HEPA filter
Normal air
purification
system
New air
purification
system
NPCO purifier
AC adsorber
Engine
bleed air
Ozone
Converter
Cabin
exhaust
Airliner cabin
Fig. 1. Cabin air purification process diagram
Air mixing
unit
Environ.
Control
System
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Polluted
air
Purified air
Photocatalyst layers
UV lights
Fig. 2. Internal structure of a NPCO air purifier
3.1. NPCO air purifier
As shown in Fig. 2, the new nano-TiO2 photocatalystic air purifier is composed of TiO2 photocatalyst
covered multi-layer honeycomb structures to ensure that air pollutants can fully contact with the
photocatalyst layers at the surface areas of the structures. To improve efficiency of the photocatalytic
oxidation, a group of UV lamps are installed between two honeycomb structures so that the photons of
UV can be efficiently used.
This unique structure and compound mode purifier has the following characteristics: a) UV light
directly shines to the TiO2 photocatalytic layers without shading, which makes the UV light fully utilized;
b) compact structure design makes the purifier small size and light weight, which is suitable for aircraft
installation; c) unique honeycomb structures provide a great numbers of small channels for the gas flow so
that the airflow resistance is very small which can help the aircraft to reduce energy consumption.
3.2. Comparisons of two purifiers
According to the data provided by [8, 9], a detailed comparative performance analysis between NPCO
purifiers and HEPA/AC units was made.
Fig. 3 and 4 respectively show the weight and volume of the two types of purifiers while dealing with
the same amount of air and achieving the same cleanness targets, from which we can see that NPCO
devices are much smaller and lighter than HEPA/AC units, and this effect becomes more obvious for
higher air amount disposing. Consequently, NPCO devices are much more suitable than HEPA/AC units
for aircraft cabin air purification.
NPCO purification devices
HEPA / AC units
System volume (ft 3 )
System weight (kg)
NPCO purification devices
HEPA / AC units
Air volume (CFM)
Fig. 3. Weight comparisons of the two types of purifiers
Air volume (CFM)
Fig. 4. Volume comparisons of the two types of purifiers
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System LCC ($)
NPCO purification devices
HEPA / AC units
Air volume (CFM)
Fig. 5. LCC comparisons of the two types of purifiers
With the world economy development, the intense competition in aviation industry and the serious
energy issues, a wider range of quality standards must be fully considered for the airliner designs, and one
of these quality standards is "Life Cycle Cost (LCC)" which means the total investment costs from the
design stage until the end of the product life. It is an evaluation method from the system engineering
perspective point of view. Fig. 5 shows LCC comparison of these two purifiers [8], from which we can
find that the LCC of NPCO device is only about half of that of HEPA/AC units at the same air amount
disposing. This is a considerable saving of running costs for airliners.
Moreover, according to the experimental results of [9], NPCO devices have a better combined effect in
low humidity environment. During the airliner cruising, the average relative humidity of cabin is usually
10% to 20%, or even below 10% in some cases, which is more suitable for application of NPCO
technology.
4. New CAQ control system simulations
Based on the above new air purification system, new combination of active and passive CAQ control
strategies were presented. The CAQ change and dynamic characteristics model was established by
applying lumped parameter method. The dynamic characteristics of the cabin pollutants and the CAQ
control strategies were simulation studied based on this model.
4.1. New CAQ control strategies
There are some deficiencies in current airliner cabin pollutants control. For example, increasing
amount of fresh air will increase energy consumption and the aircraft performance penalty, and may bring
some new pollutants into the cabin from the outside air; the cabin air purification devices can not remove
all contaminants (such as CO2, etc.), and they also has the initial investments and running costs problems.
As a result, new CAQ control strategies were presented with the following characteristics:
 The requirements of the CO2 concentration, the organic and the inorganic pollutants concentrations are
taken into consideration simultaneously. When the main pollution source of the cabin environment is
the human bodies, the CO2 concentration is the reliable indicator to determine the CAQ. Consequently,
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the concentration level of CO2 in the cabin can be selected as the control parameters of the active
control.
Recirculated air
Fresh air
Engine
bleed air
Ozone
converter
Environ.
control
system
Qf
Cabin
Air mixing
unit
Pressure
controller
C
C
•
M
Qr
Recirculation
fan
Qf
Ce
NPCO device
Filter
C
Fig. 6. Schematic diagram of new airliner CAQ control system
 The combined active and passive control strategies were adopted. As shown in Fig. 6, by changing the
fan speed, the air recycling rate is adjusted to active control CO2 concentration, and PID control is used
here. Moreover, to simplify the control strategies and reduce the change frequency of the fan speed,
when CO2 concentration is lower than the specified value, the system is paused to control it. The cabin
organic pollutants and bacteria are cleared by NPCO device through passive control. The dust and
particulate pollutants in the cabin air is removed by the filter installed in the recycling loop. O3 is
mainly cleared by the ozone converter installed in the engine bleed pipelines.
 To guarantee the health and safety of passengers, the CAQ control system is also designed with
automatic alarm system for CO and O3. As the air supplied to the cabin comes from the engine
compressor (not the turbine), CO is mainly from the cabin. Therefore, when CO concentration is
exceeded, the system gives alarm and takes the maximum fresh air operation mode. O3 mainly comes
from the outside air, so the minimum fresh air mode will be used if ozone concentration is exceeded.
4.2. Dynamic analysis and modelling
In order to study the dynamic characteristics of the CAQ control system, it can be divided into two
lumped parts: NPCO part and the controlled object (the cockpit) part. And the recycling fan model can
also be established according to the fan laws.
The following basic assumptions [10, 11] are applied to simplify the CAQ dynamic model.
 The total air flow rate Q supplied to the cabin is a constant;
 Pollutant gases evenly spread throughout the whole cabin (i.e. lumped parameter treatment). The
pollutants’ concentrations in the cabin are equal to those at the cabin air outlet;
 The leakage of cabin air is zero.
 Concerning the controlled object (the cabin), according to the mass conservation principle, the gas
concentration dynamic equations inside the cabin can be given by:
•
dC
V
= M + Q f C B − Qr (C − Ce ) − Q f C
(1)
dt
Q = Q f + Qr
(2)
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where CB is the pollutant concentration in the fresh air, and it is zero for organic pollutants. Ce is the
pollutant concentration after purification, and it is equal to C for CO2.
NPCO reaction usually follows the L-H kinetic model [12]. According to literature [13], the dynamic
equation of NPCO purification process is given by:
Vp
dCe
= r avgVbed
dt
(3)
where the average catalytic rate r avg is calculated by:
r avg =
(
)
k1k m Ce k3 + k 4C0 + k 6C0 (k5 + k 2 RH )
× [ln ( A + I 0 exp(− εδρb )) − ln ( A + I 0 )]
εδρb
1 + k1Ce
2
(4)
The parameters in equation (4) are obtained from literature [13] as listed in Table 2. Nano-TiO2 is used
as the catalyst, and the thickness of the catalyst layer is 0.003cm.
Table 2. Parameters’ values of the NPCO device model [13]
Parameter
k1
k2
k3
k4
k5
k6
km
A
I0
ε
δ
ρb
Value
0.01477
-0.0156
5854
-23.41
11.35
0.0341
196.4
0.00098
0.0002
9.63
0.003
3.894
For the recycling fan, the following formula can be obtained from the fan laws:
Qr
= Constant
nD 3
(5)
Eqs. (1)-(5) constitute the dynamic model of the cabin air quality (CAQ) control system. Analysis of
this model shows that the change of the pollutant source in the controlled object (i.e. the cabin) is the
basic disturbance factor of this control system; recirculation fan speed and NPCO device have a main
impact on the system control quality.
4.3. Simulation and result analysis
4.3.1 Simulation example
Here we took a modern jet with a capacity of 170 passengers and 6 crews as an example to discuss the
specific applications of the simulation model. The jet needs a total air supply amount of 6100kg/h for the
cabin.
CAAC airworthiness standards (CCAR-25) and the U.S. FAA airworthiness standards (14 CFR 25)
both set the maximum permissible concentration of CO2 to 5000ppm, so the CO2 control indicator is also
set to 5000ppm here.
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4.3.2 Analysis of simulation results
CO2 concentration (ppm)
30% fresh air
50% fresh air
75% fresh air
100% fresh air
0
20
40
60
80
100
Time (min)
110
120
130
Fig. 7. Simulation results of CO2 concentration profiles under different air recirculation rates
Fig. 7 shows the CO2 concentration changes with the flight profiles predicted by the model simulations
under different cabin air recirculation rates, from which it can be found that the CO2 concentration
increases with the increase of cabin air recirculation rates during stable cruise; the cabin CO2
concentration also increases during the takeoff, climbing and landing period, which might be due to the
model assumptions that the environmental control system does not work during the takeoff and landing
period so that the passengers’ breathing makes the increase of cabin CO2 concentration.
noNPCO
NPCO
Benzene concentration (mg/L)
Benzene concentration (mg/L)
noNPCO
NPCO
Time (min)
Fig. 8. Benzene concentration profiles for air recirculation
rate of 50%
Time (min)
Fig. 9. Benzene concentration profiles for cabin air
recirculation rate of 75%
We assume that the cabin suffers a sudden unclear contamination of benzene and its concentration in
the cabin reaches the maximum limit of 0.1mg/L during the airliner cruise, then the purifiers begin to
work. Fig. 8 and 9 show benzene contamination profiles under different cabin air recirculation rates after
that. Fig. 10 presents the profiles of benzene contamination when the cabin suffers a step benzene
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pollution from 0 to 200mg/s during the airliner cruise. In the figures, NPCO and noNPCO represent the
controlled object concentration profiles from simulations using NPCO devices and without airpurification devices, respectively.
From Fig. 8 and 9 we can see that the use of NPCO air purification device can make the benzene
concentration dropping down more quickly than that of no air purification device (noNPCO), and this
effect becomes much stronger for higher air recirculation rate cases. As shown in Fig. 10, when the cabin
suffers a step pollution, the final benzene concentration and the increase duration are both lower by using
the NPCO device than no purification device. The simulation results of Fig. 8 to 10 suggest that using
NPCO devices can effectively restrain the concentration change amplitude of organic pollutants in the
cabin, and play a positive role to shorten the duration of the transition process, so that the control effect of
organic pollutants is significantly improved. And as the increase of the air recirculation rate, the inhibitory
effect of this passive control will be also enhanced.
Without control
With control
CO2 concentration (ppm)
Benzene concentration (mg/L)
noNPCO
NPCO
Time (min)
Fig. 10. Benzene concentration curves with a step
pollution
0
20
40
60
80
Time (min)
100
120
140
160
Fig. 11. CO2 concentration curves with a step pollution
Fig. 11 shows the simulation results of the system active control when the cabin suffers a step CO2
pollution from 0 to 2710mg/s, from which it can be found that the active control strategies described
above in part 4 show excellent performance to quickly and accurately make the system returning to the
given conditions. Moreover, the PID controller in use has a simple structure and high reliability.
5. Conclusions
Cabin air quality (CAQ) of airliner is directly related to the safety, health and comfort of passengers
and crew members. Concerning the problems of current CAQ control system, a high efficient NPCO
purification device has been introduced to replace the general HEPA/AC unit. Compared with HEPA/AC
unit, the new NPCO purification device has low pressure drop, low life cycle cost, low volume and weight;
they can not only clear the volatile organic contaminants (VOCs) in the cabin, but also kill bacteria and
viruses in the cabin air, which can help to reduce the amount of fresh air supplied to the cabin in order to
save energy and improve the competitiveness of the airliners. A new CAQ control technology combined
with active and passive control methods has been presented. The dynamic characteristics model of CAQ
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has been established using lumped parameter method, and it has been applied to conduct CAQ simulation
studies.
Simulation results show that the new control technology can effectively solve the current CAQ
problems. The NPCO device, used as the passive controller, shows an important effect on restraining the
pollutant concentration change magnitude and shortening the duration of the transition process. The active
control strategy that the cabin CO2 concentration is controlled by changing the fan speed to adjust the air
recirculation rate performs very well with characteristics of simpleness, rapidity and stabilization.
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