air conditioning
Principles and Concepts
Air conditioning is the process whereby the condition of Air, as defined by its
temperature and moisture content, is changed. In practice other factors must also
be taken into account especially cleanliness; odor; velocity & distribution pattern.
Principles of Air- Conditioning:
Human comfort
Inevitably 'comfort' is a very subjective matter. The Engineer aims to ensure
'comfort' for most people found from statistical surveys .Most people (90%) are
comfortable when the air temperature is between 18-22°C and the %sat is
between 40-65%. This zone can be shown on the psychometric chart. And is
known as the comfort zone.
Outside air is quite likely to be at a different condition from the required comfort
zone condition. In order to bring its condition to within the comfort zone we may
need to do one or more of the following:-heat it; cool it; dehumidify it; humidify
it; or mix it.
Dry air mass flow
In order to use the psychometric chart for air-conditioning work we need to find
& use dry air mass flows. However, in practice air-flows are frequently measured
in terms of volume flow. In order to find dry air mass flow we need to use the
specific volume of the air.
Specific volume = volume/mass
The specific volume of the air is given from the
Psychometric chart in m³/kg of dry air, therefore the Mass flow will be in terms of
dry air mass flow.
Obviously the condition of the air must be known (Typically d.b. temp. & %sat) in
order to find the specific Volume.
Air heating
The heating process can be illustrated on the psychometric chart thus:
In the case of cooling, the mixture will firstly be sensibly cooled to the point of
saturation (called the dew point) then liquid water will precipitate if we cool
further. Because moisture is removed dehumidification is achieved.
The cooling/dehumidification process can be illustrated on the psychometric
chart thus:
The process of humidification allows the air to mix with extra water. A sufficient
contact time between the air and water will normally result in the air reaching
100%Saturation. The process is very close to the evaporation from a wet bulb. It
therefore follows a line of constant wet bulb Temperature.
Often, instead of exhausting 'stale' air completely some of it is filtered, deodorized and mixed with fresh incoming air. This conserves energy and narrows
the operating conditions for the air-conditioning system.
Heat Transfer
Heat is a form of energy. Every object on earth has some heat energy. The less
heat an object has, the colder we say it is. Cooling is the process of transferring
heat from one object to another. When an air-conditioning system cools, it is
actually removing heat and transferring it somewhere else. This can be
demonstrated by turning on a Spot Cooler and placing one hand in front of the
cold air nozzle and the other over the warm air exhaust. You will feel the action of
the transfer of heat.
Sensible and Latent Heat
There are two forms of heat energy: sensible heat and latent heat.
Sensible heat is the form of heat energy which is most commonly understood
because it is sensed by touch or measured directly with a thermometer. When
weather reporters say it will be 90 degrees, they are referring to sensible heat.
Latent heat cannot be sensed by touch or measured with a thermometer. Latent
heat causes an object to change its properties. For example, when enough latent
heat is removed from water vapor (steam or humidity), it condenses into water
If enough latent heat is removed from water (liquid), it will eventually freeze. This
process is reversed when latent heat is added.
Change of State
An object that changes from a solid to a liquid or liquid to vapor is referred to as a
change of state. When an object changes state, it transfers heat rapidly.
Moisture in the air is called humidity. The ability of air to hold moisture directly
relates to its temperature.
The warmer air is, the more moisture it is capable of holding. Relative humidity is
the percentage of moisture in the air compared to the amount of moisture it can
hold. A moisture content of 70°F air with 50% relative humidity is lower than 80°F
air with 50% relative humidity.
When the humidity is low, sweat evaporates from your body more quickly. This
allows you to cool off faster. High humidity conditions do not allow sweat to
evaporate as well because the air is at its maximum capacity.
Humidity is also a form of latent heat. When air contains more humidity, it has
more latent heat.
Refrigerants are substances used by air conditioners to transfer heat and create a
cooling effect. Air-conditioning systems use specially formulated refrigerants
designed to change state at specific temperatures providing optimum cooling.
Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for
hydrochlorofluorocarbon.This is currently the most common refrigerant used by
air-conditioning systems.
Many of the current forms of refrigerants used today are being phased out based
on concern for depletion of the ozone layer. Portables use R-22, which has been
deemed acceptable for use by the EPA until the year 2010. By that time, an
ozone-friendly refrigerant that can be easily substituted for R-22 will be readily
Psychometric Chart
The principles of psychometric chart apply to any physical system consisting of
gas-vapor mixtures. The most common system of interest, however, are mixtures
of water vapor and air because of its application in heating, ventilating, and airconditioning and meteorology.
Psychometric ratio
The psychometric ratio is an important property in the area of psychometrics as it
relates the absolute humidity and saturation humidity to the difference between
the dry bulb temperature and the adiabatic saturation temperature.
Mixtures of air and water vapor are the most common systems encountered in
psychometric. The psychometric ratio of air-water vapor mixtures is
approximately unity which implies that the difference between the adiabatic
saturation temperature and wet bulb temperature of air-water vapor mixtures is
small. This property of air-water vapor systems simplifies drying and cooling
calculations often performed using psychometric relationships.
A psychometric chart is a graph of the physical properties of moist air at a
constant pressure (often equated to an elevation relative to sea level). The chart
graphically expresses how various properties relate to each other, and is thus a
graphical equation of state. The thermo physical properties found on most
psychometric charts are:
Dry-bulb temperature (DBT) is that of an air sample, as determined by an
ordinary thermometer, the thermometer's bulb being dry. It is typically the
abscissa, or horizontal axis of the graph. The SI units for temperature are Celsius;
other units are Fahrenheit.
Wet-bulb temperature (WBT) is that of an air sample after it has passed through
a constant-pressure, ideal, adiabatic saturation process, that is, after the air has
passed over a large surface of liquid water in an insulated channel. In practice,
this is the reading of a thermometer whose sensing bulb is covered with a wet
sock evaporating into a rapid stream of the sample air. The WBT is the same as
the DBT when the air sample is saturated with water. The slope of the line of
constant WBT reflects the heat of vaporization of the water required to saturate
the air of a given relative humidity.
Dew point temperature (DPT) is that temperature at which a moist air sample at
the same pressure would reach water vapor saturation. At this saturation point,
water vapor would begin to condense into liquid water fog or (if below freezing)
solid hoarfrost, as heat is removed. The dew point temperature is measured easily
and provides useful information, but is normally not considered an independent
property. It duplicates information available via other humidity properties and the
saturation curve.
Relative Humidity (RH) is the ratio of the mole fraction of water vapor to the
mole fraction of saturated moist air at the same temperature and pressure. RH is
dimensionless, and is usually expressed as a percentage. Lines of constant RH
reflect the physics of air and water: they are determined via experimental
measurement. Note: the notion that air "holds" moisture, or that moisture
dissolves in dry air and saturates the solution at some proportion, is an erroneous
(albeit widespread) concept
Humidity Ratio (also known as Moisture Content, Mixing Ratio, or Specific
Humidity) is the proportion of mass of water vapor per unit mass of dry air at the
given conditions (DBT, WBT, DPT, RH, etc.). It is typically the ordinate or vertical
axis of the graph. For a given DBT there will be a particular humidity ratio for
which the air sample is at 100% relative humidity: the relationship reflects the
physics of water and air and must be measured. Humidity Ratio is dimensionless,
but is sometimes expressed as grams of water per kilogram of dry air or grains of
water per pound of air.
Specific Enthalpy symbolized by h, also called heat content per unit mass, is the
sum of the internal (heat) energy of the moist air in question, including the heat
of the air and water vapor within. In the approximation of ideal gasses, lines of
constant enthalpy are parallel to lines of constant WBT. Enthalpy is given in (SI)
Joules per kilogram of air or BTU per pound of air.
Specific Volume, also called Inverse Density, is the volume per unit mass of the air
sample. The SI units are cubic meters per kilogram of air; other units are cubic
feet per pound of dry air.
The versatility of the psychometric chart lies in the fact that by knowing three
independent properties of some moist air (one of which is the pressure), the
other properties can be determined. Changes in state, such as when two air
streams mix, can be modeled easily and somewhat graphically using the correct
psychometric chart for the location's air pressure or elevation relative to sea level.
For locations at or below 2000 ft (600 m), a common assumption is to use the sea
level psychometric chart.
How to read the chart
The most common chart used by practitioners and students alike is the "ω-t"
(omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as
the abscissa and the humidity ratios (ω) appear as the ordinates.
In order to use a particular chart, for a given air pressure or elevation, at least two
of the six independent properties must be known (DBT, WBT, RH, Humidity Ratio,
Specific Enthalpy, and Specific Volume). This gives rise to 15 possible
DBT : This can be determined from the abscissa
DPT : Follow the horizontal line from the point where the line from the horizontal
axis arrives at 100% RH, also known as the saturation curve.
WBT : Line inclined to the horizontal and intersects saturation curve at DBT point.
RH : Hyperbolic lines drawn asymptotically with respect to the saturation curve
which corresponds to 100% RH.
Humidity Ratio : Marked on Ordinate axis.
Specific Enthalpy : lines of equal values, or hash marks for, slope from the upper
left to the lower right.
Specific Volume : Equally spaced parallel family of lines.
Refrigerants are substances used by air conditioners to transfer heat and create a
cooling effect. Air-conditioning systems use specially formulated refrigerants
designed to change state at specific temperatures providing optimum cooling.
Portables use a refrigerant called R-22 or HCFC-22. HCFC stands for
hydrochlorofluorocarbon.This is currently the most common refrigerant used by
air-conditioning systems.
Many of the current forms of refrigerants used today are being phased out based
on concern for depletion of the ozone layer. Portables use R-22, which has been
deemed acceptable for use by the EPA until the year 2010. By that time, an
ozone-friendly refrigerant that can be easily substituted for R-22 will be readily
In the refrigeration cycle, a heat pump transfers heat from a lower temperature
heat source into a higher temperature heat sink. Heat would naturally flow in the
opposite direction. This is the most common type of air conditioning. A
refrigerator works in much the same way, as it pumps the heat out of the interior
into the room in which it stands.This cycle takes advantage of the universal gas
law PV = nRT, where P is pressure, V is volume, R is the universal gas constant, T is
temperature, and n is the number of moles of gas (1 mole = 6.022×1023
In the refrigerator, the cycle is continuous. In the following example, provided
that the refrigerant being used is pure ammonia, which boils at -27 degrees F. This
is what happens to keep the refrigerator cool:
• The compressor compresses the ammonia gas. The compressed gas heats
up as it is pressurized (orange).
• The coils on the back of the refrigerator let the hot ammonia gas dissipate
its heat. The ammonia gas condenses into ammonia liquid (dark blue) at
high pressure.
• The high-pressure ammonia liquid flows through the expansion valve.
Expansion valve can be considered as a small hole. On one side of the hole
is high-pressure ammonia liquid. On the other side of the hole is a lowpressure area (because the compressor is sucking gas out of that side).
• The liquid ammonia immediately boils and vaporizes (light blue), its
temperature dropping to -27 F. This makes the inside of the refrigerator
• The cold ammonia gas is sucked up by the compressor, and the cycle
The Refrigeration Cycle
A=Inside the refrigerator
C=Expansion Valve
Heat pump and refrigeration cycle
Thermodynamic heat pump and refrigeration cycles are the models for heat
pumps and refrigerators. The difference between the two is that heat pumps are
intended to keep a place warm and refrigerators designed to cool it. Technically a
refrigerator cycle is also a heat pump cycle.
A heat pump is when heat is removed from a low-temperature space or source
and rejected to a high-temperature sink with the help of external mechanical
The inverse of the heat pump cycle is the thermodynamic power cycle. In the
power cycle, heat is supplied from a high-temperature source to the heat engine,
part of the heat being used to produce mechanical work and the rest being
rejected to a low-temperature sink. This satisfies the second law of
thermodynamics. A heat pump describes the changes that take place in the
refrigerant as it alternately absorbs and rejects heat as it circulates through a
refrigerator. It is also applied to HVACR work, when describing the "process" of
refrigerant flow through an HVACR unit, whether it is a packaged or split system.
Heat naturally flows from hot to cold. Work is applied to cool a living space or
storage volume by pumping heat from a lower temperature heat source into a
higher temperature heat sink. Insulation is used to reduce the work and energy
required to achieve and maintain a lower temperature in the cooled space. The
operating principle of the refrigeration cycle was described mathematically by
Sadi Carnot in 1824 as a heat engine.
The most common types of heat pump systems use the reverse-Rankine vaporcompression refrigeration cycle although absorption heat pumps are used in a
minority of applications.
Heat pump can be classified as:
• Vapor cycle,
• Gas cycle, and
• Stirling cycle
Vapor cycle refrigeration can be classified as:
• Vapor compression refrigeration
• Gas absorption refrigeration
Vapor-compression cycle
The vapor-compression cycle is used in most household refrigerators as well as in
many large commercial and industrial refrigeration systems. The following Figure
provides a schematic diagram of the components of a typical vapor-compression
refrigeration system.
Thermodynamics of the cycle can be analyzed on a diagram .In this cycle, a
circulating refrigerant such as Freon enters the compressor as a vapor. From point
1 to point 2, the vapor is compressed at constant entropy and exits the
compressor superheated. From point 2 to point 3 and on to point 4, the
superheated vapor travels through the condenser which first cools and removes
the superheat and then condenses the vapor into a liquid by removing additional
heat at constant pressure and temperature. Between points 4 and 5, the liquid
refrigerant goes through the expansion valve (also called a throttle valve) where
its pressure abruptly decreases, causing flash evaporation and auto-refrigeration
of, typically, less than half of the liquid.
That results in a mixture of liquid and vapor at a lower temperature and pressure
as shown at point 5. The cold liquid-vapor mixture then travels through the
evaporator coil or tubes and is completely vaporized by cooling the warm air
(from the space being refrigerated) being blown by a fan across the evaporator
coil or tubes. The resulting refrigerant vapor returns to the compressor inlet at
point 1 to complete the thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle,
and does not take into account real-world effects like frictional pressure drop in
the system, slight thermodynamic irreversibility during the compression of the
refrigerant vapor, or non-ideal gas behavior (if any).
Vapor absorption cycle
In the early years of the twentieth century, the vapor absorption cycle using
water-ammonia systems was popular and widely used but, after the development
of the vapor compression cycle, it lost much of its importance because of its low
coefficient of performance (about one fifth of that of the vapor compression
cycle). Nowadays, the vapor absorption cycle is used only where waste heat is
available or where heat is derived from solar collectors.
The absorption cycle is similar to the compression cycle, except for the method of
raising the pressure of the refrigerant vapor. In the absorption system, the
compressor is replaced by an absorber which dissolves the refrigerant in a
suitable liquid, a liquid pump which raises the pressure and a generator which, on
heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some
work is required by the liquid pump but, for a given quantity of refrigerant, it is
much smaller than needed by the compressor in the vapor compression cycle. In
an absorption refrigerator, a suitable combination of refrigerant and absorbent is
used. The most common combinations are ammonia (refrigerant) and water
(absorbent), and water (refrigerant) and lithium bromide (absorbent).
Gas cycle
When the working fluid is a gas that is compressed and expanded but doesn't
change phase, the refrigeration cycle is called a gas cycle. Air is most often this
working fluid. As there is no condensation and evaporation intended in a gas
cycle, components corresponding to the condenser and evaporator in a vapor
compression cycle are the hot and cold gas-to-gas heat exchangers in gas cycles.
The gas cycle is less efficient than the vapor compression cycle because the gas
cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As
such the working fluid does not receive and reject heat at constant temperature.
In the gas cycle, the refrigeration effect is equal to the product of the specific heat
of the gas and the rise in temperature of the gas in the low temperature side.
Air is by nature the safest and cheapest refrigerant. Environmental concerns
about ozone depletion, global warming and increasingly stringent legislation have
renewed interest in alternative refrigeration technologies.
Air cycle systems have specific advantages that apply to all potential applications:
• The working fluid (air) is free, environmentally benign, totally safe and nontoxic.
• Air cycle equipment is extremely reliable, thereby reducing maintenance
costs and system down-time.
• The performance of an air cycle unit does not deteriorate as much as that
of a vapor-compression unit when operating away from its design point.
• When operating in a refrigeration cycle, an air cycle unit can also produce
heat at a useful temperature.
The use of air as a refrigerant is based on the principle that when a gas expands
isentropically from a given temperature, its final temperature at the new pressure
is much lower. The resulting cold gas, in this case air, can then be used as a
refrigerant, either directly in an open system, or indirectly by means of a heat
exchanger in a closed system. The efficiency of such systems is limited to a great
extent by the efficiencies of compression and expansion, as well as those of the
heat exchangers employed. Originally, slow speed reciprocating compressors and
expanders were used. The poor efficiency and reliability of such machinery were
major factors in the replacement of such systems with vapor compression
equipment. However, the development of rotary compressors and expanders
greatly improved the isentropic efficiency and reliability of the air cycle. Advances
in turbine technology, together with the development of air bearings and ceramic
component offer further efficiency. Combining this with newly available compact
heat exchangers with greatly improved heat transfer characteristics makes
competition with many existing vapor compression, and certainly liquid nitrogen
systems, quite feasible.
Environmental control in buildings
Until recently the use of air cycle has been largely restricted to aircraft cabin air
conditioning systems. A recent trial has demonstrated the advantages that air
cycle technologies can offer to passenger train air conditioning systems. An
important conclusion of this trial was that air cycle train air conditioning systems
will have lower overall life cycle ownership costs than comparable vapour
compression systems. The successful demonstration of these units in Germany’s
ICE2.2 high speed trains by Normalair-Garrett Ltd. led to the company receiving
the Engineering Council’s Environmental Award for Engineers in 1996.
Studies carried out by the Buildings Research Establishment (BRE) and frperc have
demonstrated that air cycle systems in buildings would have a number of
advantages. These include •
Lamination of the need to use environmentally damaging CFC, HCFC or
other alternative refrigerants in building air conditioning systems
Use of high grade heat recovery from air cycle cooling systems resulting in
lower energy consumption
Improved reliability and reduced maintenance compared with conventional
Maintenance of near full load efficiency at part load conditions
No susceptibility to refrigerant leakage
Food freezing system
Currently frperc are working on the design, construction and installation of an air
cycle fluidized bed freezer for food freezing. The air cycle plant will operate with
air as the refrigerant delivering it to the freezer bed at -75°C.
Fluidized beds have a number of useful characteristics. Heat and mass transfer
rates to and within the bed are high and there is a good uniformity of treatment
of the particles to yield high quality individually quick frozen products. Freezing
food faster can increase turnover on an existing footprint, reduce the freezing
cost and produce a higher quality of frozen food. Freezing food with an air cycle
refrigeration plant has two advantages;
• The air can replace toxic, inflammable or environmentally unfriendly
refrigerants and replace it with a safe and replaceable refrigerant
• It is capable of producing freezing temperatures far colder than vapor
compression plant for less energy consumption, size and cost. Freezing
temperatures as low as those produced by cryogenic refrigeration are
possible but without the high running costs and energy consumption
inherent in such systems.
CFC free heat pumps
The objective of the project is to develop heat pump systems, to be used in
existing as well as new buildings, using air as the environmentally benign working
fluid to improve the primary energy ratio of heating and cooling systems. To
improve the efficiency of air cycle systems the (isentropic) efficiency of the
rotating equipment (expanders and compressors) is crucial. High efficiency
equipment is available in other application fields such as pressurized air systems
and energy recovery systems .
The need for heating and cooling in buildings:
The prime requirement in respect of the indoor climate in a building is that room
temperature should be at a comfortable level, regardless of the weather
conditions outside. In addition, the indoor air must be acceptably clean, lighting
and acoustic conditions must be good etc.Nevertheless, the first and foremost
condition for a building to be usable at allies that the indoor temperature
inacceptable. As soon as the ambient temperature is lower than the
Indoor temperature, heat flows out from the building through its boundary
surfaces (the building envelope). At the same time, the building also loses heat
through air infiltration, i.e. the inward leakage of outdoor air into the building
through gaps and cavities in walls, roofs, doors and windows. Bearing in mind the
fact that the indoor temperature in most buildings is maintained at a little over20
°C, this means, throughout most of the year, the building is losing heat to its
The internal heat generation in commercial premises and some industrial
buildings, on the other hand, is often relatively great. In combination with the fact
that construction standards have been developed and improved, so that buildings
are nowadays well insulated and airtight, this means that the heat losses through
the building envelope are small. If we consider new office buildings, department
stores, hospitals and similar buildings within the commercial premises and
industrial sector, we find that heat deficits usually occur only during the night and
at weekends, while there is nearly always ahead surplus during working hours.
Such buildings require only simple heating systems to meet the modest heat
deficits, as opposed to the considerably more extensive systems needed in order
to deal with the substantial heat surpluses, and to prevent the indoor
temperature becoming unacceptably high during working hours.
In general terms, the greater the heat surplus, and therefore the greater the
capacity of the cooling system, the more difficult it is to produce an indoor
climate that is good in all respects. It is therefore always important to attempt to
design the building in general so that there will be only a low heat surplus.
Comfort cooling
The surplus heat that has to be removed from buildings in order to maintain the
indoor temperature below some previously determined maximum permissible
temperature is referred to as the cooling requirement. In other words, the cooling
requirement of the building is exactly the same as its heat.
The climate control system in building has to maintain both the thermal climate
and the air quality. Maintaining the thermal climate consists primarily of keeping
the temperature of the indoor air within given limits. Maintaining the air quality
consists of controlling the ‘cleanliness’ of the indoor air by supplying a sufficient
quantity of outdoor air to ventilate the interior of the building. Maintenance of air
quality sometimes also includes ensuring that given
Concentrations of particles and/or gases are not exceeded.
The cooling system must be able to deal with variations in the cooling
requirement, whether over the day or over the year. The two basic types of all-air
cooling systems are the constant air flow system and the variable air flow system,
Although there are also combinations of the two methods. The need for comfort
cooling arises, therefore, when requirements in respect of the thermal climate
also include requirements in respect of maximum permissible indoor
temperatures. In general, HVAC (Heating, Ventilation and Air-Conditioning)
systems used in order actively to cool buildings can be divided up into three main
_ all-air cooling systems
_ all-water cooling systems
_ combined systems
(With cooling supplied both by air and by water)
All-air cooling systems
The design air flow rate in these systems, and thus the necessary sizes of
ventilation ducts, is determined by the design cooling requirement. In other
words, it is the thermal requirements, and not the air quality requirements, that
determine the necessary air flow rate. In existing buildings, it is normally both
difficult and expensive to replace the ventilation duct system. If the existing ducts
cannot transport sufficiently large air quantities to meet the cooling
requirements, all-water-cooling systems will usually be installed in connection
with Conversion or modernization.
Constant air volume systems (CAV systems)
In such systems, the temperature of the air supplied to the building can vary, but
the air flow rate is kept constant. Such systems are referred to as Constant Air
Volume (CAV) systems. It is the rooms having the greatest cooling requirement
that normally determine the supply air temperature delivered by the central air
conditioning unit: the air may, if necessary, be heated before supplied to other
rooms. Although a CAV system supplies air at a constant flow rate, the fans are
sometimes powered by two-speed motors, running at the lower speed when the
building cooling requirement falls. The air flow rate is then reduced in proportion
to the fan speed.
Variable air volume systems (VAV systems)
The air flow rate to each room is varied as necessary, but with maintenance of a
constant supply temperature, i.e. the supply temperature does not change even if
the load changes. However, the supply air temperature is normally changed in
step with the time of year, as a function of the ambient temperature.
Systems of this type are referred to as Variable Air Volume (VAV) systems.
The air flow to each individual room is controlled by dampers in some form of box
(VAV-box) in the immediate vicinity of the supply point to the room, while the
central supply and exhaust air fans are controlled by variable inlet vanes or by
adjustable speed drive controlled motors, usually of the frequency-inverter type.
The control system normally maintains a constant static pressure in the supply air
duct. The flow rate varies from a maximum, during the hottest days, down to
perhaps 20 % of maximum flow rate during the coldest days of the year, when the
purpose of the air is only to maintain the air quality.
All-water cooling systems
Systems of this type supply all-water cooling to the individual rooms, with the
ventilation system designed purely to maintain the air quality. Systems of this
type are often chosen in connection with conversion or renovation projects.
There is usually space above the false ceilings to install the water pipes needed
for distribution of cold water throughout the building.
Combined systems
All-air and all-water cooling systems can be combined in many ways. One such
need for a combined system is if all-air cooling is used, but the cooling
requirement is so great that an all-air cooling system alone is not capable of
dealing with it satisfactorily, as such high air flow rates would be required that
draughts would be unavoidable. It is also possible to combine all-air cooling
systems so that certain parts of the building, or certain rooms, are cooled by a
VAV system, while other parts of the building are cooled by a CAV system.
Cooling supply devices
Cooling can be supplied to a room in a number of different ways. The following
are brief descriptions of how chilled beams, cooling panels, fan coil units and
induction units operate. Fan coil units and induction units are normally positioned
below windows in the outside walls.
Chilled beams
These are units which, by natural convection from a finned heat exchanger, cool
the air in the room. They may also be combined with the supply air terminal
device in order to provide both functions and, in many cases, to increase the
cooling capacity of the baffle. Some chilled beams can also incorporate a heating
Cooling panels
Cooling panels can be hung from the ceiling. Cold water flows through an
aluminium plate, which transfers heat from the air to the cold water. The panel
cools the warm room air and also cools the room surfaces by low-temperature
radiation. These panels are produced in a number of versions, e.g. for mounting
flat against the ceiling, hanging, or for integration in a false ceiling. Most of their
cooling capacity is provided by radiation.
Fan coil units
These are units by which both heating and cooling can be supplied to a room
(although not at the same time).
A fan coil unit incorporates a fan which circulates the room air through the unit, in
which the air is either heated or cooled as required. The two heat exchangers
(heating and cooling) are supplied with hot or cold water from a central unit in
the building. This type of room cooler unit can meet the highest cooling
requirements, but it also has the highest noise level.
Induction units
These are units by which both heating and cooling can be supplied to a room
.When in use, the ventilation air for the room is supplied through the induction
unit. It flows through a nozzle with high velocity, which therefore has the effect of
inducing air from the room through the heating or cooling heat exchangers.
The term air conditioning most commonly refers to the cooling and
dehumidification of indoor air for thermal comfort. In a broader sense, the term
can refer to any form of cooling, heating, ventilation or disinfection that modifies
the condition of air.[1] An air conditioner (AC or A/C in North American English,
aircon in British and Australian English) is an appliance, system, or mechanism
designed to stabilize the air temperature and humidity within an area (used for
cooling as well as heating depending on the air properties at a given time) ,
typically using a refrigeration cycle but sometimes using evaporation, most
commonly for comfort cooling in buildings and transportation vehicles.
The concept of air conditioning is known to have been applied in Ancient Rome,
where aqueduct water was circulated through the walls of certain houses to cool
them. Similar techniques in medieval Persia involved the use of cisterns and wind
towers to cool buildings during the hot season. Modern air conditioning emerged
from advances in chemistry during the 19th century, and the first large-scale
electrical air conditioning was invented and used in 1902 by Willis Haviland
Air conditioning applications:
Air conditioning engineers broadly divide air conditioning applications into
comfort and process.
Comfort applications aim to provide a building indoor environment that remains
relatively constant in a range preferred by humans despite changes in external
weather conditions or in internal heat loads.
The highest performance for tasks performed by people seated in an office is
expected to occur at 72 °F (22 °C) Performance is expected to degrade about 1%
for every 2 °F change in room temperature.[6] The highest performance for tasks
performed while standing is expected to occur at slightly lower temperatures. The
highest performance for tasks performed by larger people is expected to occur at
slightly lower temperatures. The highest performance for tasks performed by
smaller people is expected to occur at slightly higher temperatures. Although
generally accepted, some dispute that thermal comfort enhances worker
productivity, as is described in the Hawthorne effect.
Comfort air conditioning makes deep plan buildings feasible. Without air
conditioning, buildings must be built narrower or with light wells so that inner
spaces receive sufficient outdoor air via natural ventilation. Air conditioning also
allows buildings to be taller since wind speed increases significantly with altitude
making natural ventilation impractical for very tall buildings. Comfort applications
for various building types are quite different and may be categorized as:
• Low-Rise Residential buildings, including single family houses, duplexes, and
small apartment buildings
• High-Rise Residential buildings, such as tall dormitories and apartment
• Commercial buildings, which are built for commerce, including offices,
malls, shopping centers, restaurants, etc.
• Institutional buildings, which includes hospitals, governmental, academic,
and so on.
• Industrial spaces where thermal comfort of workers is desired.
In addition to buildings, air conditioning can be used for comfort in a wide variety
of transportation including land vehicles, trains, ships, aircraft, and spacecraft.
Process applications aim to provide a suitable environment for a process being
carried out, regardless of internal heat and humidity loads and external weather
conditions. Although often in the comfort range, it is the needs of the process
that determine conditions, not human preference. Process applications include
• Hospital operating theatres, in which air is filtered to high levels to reduce
infection risk and the humidity controlled to limit patient dehydration.
Although temperatures are often in the comfort range, some specialist
procedures such as open heart surgery require low temperatures (about 18
°C, 64 °F) and others such as neonatal relatively high temperatures (about
28 °C, 82 °F).
• Clean rooms for the production of integrated circuits, pharmaceuticals, and
the like, in which very high levels of air cleanliness and control of
temperature and humidity are required for the success of the process.
• Facilities for breeding laboratory animals. Since many animals normally only
reproduce in spring, holding them in rooms at which conditions mirror
spring all year can cause them to reproduce year round.
• Aircraft air conditioning. Although nominally aimed at providing comfort for
passengers and cooling of equipment, aircraft air conditioning presents a
special process because of the low air pressure outside the aircraft.
• Data processing centers
• Textile factories
• Physical testing facilities
• Plants and farm growing areas
• Nuclear facilities
• Chemical and biological laboratories
• Mines
• Industrial environments
• Food cooking and processing areas
In both comfort and process applications the objective may be to not only control
temperature, but also humidity, air quality, air motion, and air movement from
space to space.
Humidity control
Refrigeration air conditioning equipment usually reduces the humidity of the air
processed by the system. The relatively cold (below the dew point) evaporator
coil condenses water vapor from the processed air, (much like an ice cold drink
will condense water on the outside of a glass), sending the water to a drain and
removing water vapor from the cooled space and lowering the relative humidity.
Since humans perspire to provide natural cooling by the evaporation of
perspiration from the skin, drier air (up to a point) improves the comfort
provided. The comfort air conditioner is designed to create a 40% to 60% relative
humidity in the occupied space. In food retailing establishment’s large open
chiller cabinets act as highly effective air dehumidifying units.
Some air conditioning units dry the air without cooling it, and are better classified
as dehumidifiers. They work like a normal air conditioner, except that a heat
exchanger is placed between the intake and exhaust. In combination with
convection fans they achieve a similar level of comfort as an air cooler in humid
tropical climates, but only consume about a third of the electricity. They are also
preferred by those who find the draft created by air coolers discomforting.
Energy use
It should be noted that in a thermodynamically closed system, any energy input
into the system that is being maintained at a set temperature (which is a standard
mode of operation for modern air conditioners) requires that the energy removal
rate from the air conditioner increase. This increase has the effect that for each
unit of energy input into the system (say to power a light bulb in the closed
system) requires the air conditioner to remove that energy. In order to do that
the air conditioner must increase its consumption by the inverse of its efficiency
times the input unit of energy. As an example presume that inside the closed
system a 100 watt light bulb is activated, and the air conditioner has an efficiency
of 200%. The air conditioners energy consumption will increase by 50 watts to
compensate for this, thus making the 100 W light bulbs utilize a total of 150 W of
Portable air conditioners
A portable air conditioner or portable A/C is an air conditioner on wheels that can
be easily transported inside a home or office. They are currently available with
capacities of about 6,000 to 60,000 BTU/h (1800 to 18 000 watts output) and
with and without electric resistance heaters. Portable air conditioners come in
three forms, split, and hose and evaporative:
A split system has an indoor unit on wheels connected to an outdoor unit via
flexible pipes, similar to a permanently fixed installed unit.
Hose systems Air-to-Air and Monoblock are vented to the outside via air ducts. A
function of all cooling that use a compressor, is to create water as it cools the air.
The "monoblock" version collects the water in a bucket or tray and stops when
full. The Air-to-Air version re-evaporates the water and discharges it through the
ducted hose and can hence run continuously.
A single duct unit draws air out of the room to cool its condenser. This air is then
replaced by hot air from outside or other rooms, thus reducing efficiency.
However, modern units run on approximately 1 to 3 ratio i.e., to produce 3 kW of
cooling this will use 1 kW of electricity.
Air cooled portable air conditioners are compressor-based refrigerant system that
uses air to exchange heat, similar to a car or typical household air conditioner.
With this type of system the air is dehumidified as it is cooled.
Evaporative air conditioners do not have a compressor or condenser. Instead,
liquid water is poured in and released as vapor. Because they do not have a
condenser which needs cooling, they do not need hoses or pipes, allowing them
to be truly portable.
As a rule of thumb, 400 square feet (37 m²) can be cooled per 12,000 BTU/h (3.5
kW or one ton of air conditioning) by a refrigerative air conditioner. However,
other factors will affect the total heat load. Evaporative air conditioners use much
less energy.
Types of air conditioner equipment
• Window and through-wall units
Many traditional air conditioners in homes or other buildings are single
rectangular units used to cool an apartment, a house or part of it, or part of a
building. For an example, see the photos to the right. Hotels frequently use PTAC
systems, which combine heating into the same unit. Air conditioner units need to
have access to the space they are cooling (the inside) and a heat sink; normally
outside air is used to cool the condenser section. For this reason, single unit air
conditioners are placed in windows or through openings in a wall made for the air
conditioner; the latter type includes portable air conditioners.
Window and through-wall units have vents on both the inside and outside, so
inside air to be cooled can be blown in and out by a fan in the unit, and outside air
can also be blown in and out by another fan to act as the heat sink. The controls
are on the inside.
Evaporation coolers
In very dry climates, so-called "swamp coolers" are popular for improving comfort
during hot weather. This type of cooler is the dominant cooler used in Iran which
has the largest number of units than anywhere else in the world, hence some
referring to "swamp coolers" as Persian coolers. An evaporative cooler is a device
that draws outside air through a wet pad, such as a large sponge soaked with
water. The sensible heat of the incoming air, as measured by a dry bulb
thermometer, is reduced. The total heat (sensible heat plus latent heat) of the
entering air is unchanged. Some of the sensible heat of the entering air is
converted to latent heat by the evaporation of water in the wet cooler pads. If the
entering air is dry enough, the results can be quite comfortable. These coolers
cost less and are mechanically simple to understand and maintain.
There is a related, more complex process called absorptive refrigeration which
uses heat to produce cooling. In one instance, a three-stage absorptive cooler first
dehumidifies the air with a spray of salt-water or brine. The brine osmotically
absorbs water vapor from the air. The second stage sprays water in the air,
cooling the air by evaporation. Finally, to control the humidity, the air passes
through another brine spray. The brine is reconcentrated by distillation. The
system is used in some hospitals because, with filtering, a sufficiently hot
regenerative distillation removes airborne organisms.
Absorptive chillers
Some buildings use gas turbines to generate electricity. The exhausts of these are
hot enough to drive an absorptive chiller that produces cold water. The cold
water is then run through radiators in air ducts for hydronic cooling. The dual use
of the energy, both to generate electricity and cooling, makes this technology
attractive when regional utility and fuel prices are right. Producing heat, power,
and cooling in one system is known as trigeneration.
Central air conditioning
Central air conditioning, commonly referred to as central air (US) or air-con (UK),
is an air conditioning system which uses ducts to distribute cooled and/or
dehumidified air to more than one room, or uses pipes to distribute chilled water
to heat exchangers in more than one room, and which is not plugged into a
standard electrical outlet.
With a typical split system, the condenser and compressor are located in an
outdoor unit; the evaporator is mounted in the air handling unit (which is often a
forced air furnace). With a package system, all components are located in a single
outdoor unit that may be located on the ground or roof.
Central air conditioning performs like a regular air conditioner but has several
added benefits:
• When the air handling unit turns on, room air is drawn in from various parts
of the house through return-air ducts. This air is pulled through a filter
where airborne particles such as dust and lint are removed. Sophisticated
filters may remove microscopic pollutants as well. The filtered air is routed
to air supply ductwork that carries it back to rooms. Whenever the air
conditioner is running, this cycle repeats continually.
• Because the central air conditioning unit is located outside the home, it
offers a lower level of noise indoors than a free-standing air conditioning
Thermostats control the operation of HVAC systems, turning on the heating or
cooling systems to bring the building to the set temperature. Typically the heating
and cooling systems have separate control systems (even though they may share
a thermostat) so that the temperature is only controlled "one-way". That is, in
winter, a building that is too hot will not be cooled by the thermostat.
Thermostats may also be incorporated into facility energy management systems
in which the power utility customer may control the overall energy expenditure.
In addition, a growing number of power utilities have made available a device
which, when professionally installed, will control or limit the power to an HVAC
system during peak use times in order to avoid necessitating the use of rolling
Equipment capacity
Air conditioner equipment power in the U.S. is often described in terms of "tons
of refrigeration". A "ton of refrigeration" is defined as the cooling power of one
short ton (2000 pounds or 907 kilograms) of ice melting in a 24-hour period. This
( Residential "central air"
systems are usually from 1 to 5 tons (3 to 20 kW) in capacity.
The use of electric/compressive air conditioning puts a major demand on the
nation's electrical power grid in warm weather, when most units are operating
under heavy load.
Health implications
Air conditioning has no greater influence on health than heating—that is to say,
very little—although poorly maintained air-conditioning systems (especially large,
centralized systems) can occasionally promote the growth and spread of
microorganisms, such as Legionella pneumophila, the infectious agent
responsible for Legionnaire's disease, or thermophilic actinomycetes.Conversely,
air conditioning (including filtration, humidification, cooling, disinfection, etc.) can
be used to provide a clean, safe, hypoallergenic atmosphere in hospital operating
rooms and other environments where an appropriate atmosphere is critical to
patient safety and well-being. Air conditioning can have a positive effect on
sufferers of allergies and asthma.
In serious heat waves, air conditioning can save the lives of the elderly. Some local
authorities even set up public cooling centers for the benefit of those without air
conditioning at home.
Properly maintained air-conditioning systems do not cause or promote illness,
despite superstitions that air-conditioning is unconditionally dangerous to one's
health. As with heating systems, the advantages of air conditioning generally far
outweigh the disadvantages.
The internal section of the same unit.
A modern Americool window airconditioner internal section
External section of a typical AC
Air Conditioning Units
How Air –Conditioners work
Air conditioners and refrigerators work the same way. Instead of cooling just the
small, insulated space inside of a refrigerator, an air conditioner cools a room, a
whole house, or an entire business. Air conditioners use chemicals that easily
convert from a gas to a liquid and back again. This chemical is used to transfer
heat from the air inside of a home to the outside air.
The machine has three main parts. They are a compressor, a condenser and an
evaporator. The compressor and condenser are usually located on the outside air
portion of the air conditioner. The evaporator is located on the inside the house,
sometimes as part of a furnace.
• The working fluid arrives at the compressor as a cool, low-pressure gas
called Freon. The compressor squeezes the fluid. This packs the molecule of
the fluid closer together. The closer the molecules are together, the higher
its energy and its temperature.
• The working fluid leaves the compressor as a hot, high pressure gas and
flows into the condenser. If you looked at the air conditioner part outside a
house, look for the part that has metal fins all around. The fins act just like
a radiator in a car and help the heat go away, or dissipate, more quickly.
• When the working fluid leaves the condenser, its temperature is much
cooler and it has changed from a gas to a liquid under high pressure. The
liquid goes into the evaporator through a very tiny, narrow hole. On the
other side, the liquid's pressure drops. When it does it begins to evaporate
into a gas.
• As the liquid changes to gas and evaporates, it extracts heat from the air
around it. The heat in the air is needed to separate the molecules of the
fluid from a liquid to a gas.
• The evaporator also has metal fins to help in exchange the thermal energy
with the surrounding air.
• By the time the working fluid leaves the evaporator, it is a cool, low
pressure gas. It then returns to the compressor to begin its trip all over
• Connected to the evaporator is a fan that circulates the air inside the house
to blow across the evaporator fins. Hot air is lighter than cold air, so the hot
air in the room rises to the top of a room.
• There is a vent there where air is sucked into the air conditioner and goes
down ducts. The hot air is used to cool the gas in the evaporator. As the
heat is removed from the air, the air is cooled. It is then blown into the
house through other ducts usually at the floor level.
• This continues over and over and over until the room reaches the
temperature you want the room cooled to. The thermostat senses that the
temperature has reached the right setting and turns off the air conditioner.
As the room warms up, the thermostat turns the air conditioner back on
until the room reaches the temperature.
A-Expansion Valve
Schematic diagram of an air-conditioner
Window AC Units
A window air conditioner unit implements a complete air conditioner in a small
space. The units are made small enough to fit into a standard window frame. It
• A compressor
• An expansion valve
• A hot coil (on the outside)
• A chilled coil (on the inside)
• Two fans
• A control unit.
The fans blow air over the coils to improve their ability to dissipate heat (to the
outside air) and cold (to the room being cooled).
Most air conditioners have their capacity rated in British thermal units (BTU).
Generally speaking, a BTU is the amount of heat required to raise the
temperature of one pound (0.45 kg) of water 1 degree Fahrenheit (0.56 degrees
Celsius). Specifically, 1 BTU equals 1,055 joules. In heating and cooling terms, 1
"ton" equals 12,000 BTU.
A typical window air conditioner might be rated at 10,000 BTU. For comparison, a
typical 2,000-square-foot (185.8 m2) house might have a 5-ton (60,000-BTU) air
conditioning system, implying that you might need perhaps 30 BTU per square
foot. (Keep in mind that these are rough estimates. To size an air conditioner for
your specific needs, contact an HVAC contractor.)
The energy efficiency rating (EER) of an air conditioner is its BTU rating over its
wattage. For example, if a 10,000-BTU air conditioner consumes 1,200 watts, its
EER is 8.3 (10,000 BTU/1,200 watts). Obviously, you would like the EER to be as
high as possible, but normally a higher EER is accompanied by a higher price.
Let's say that you have a choice between two 10,000-BTU units. One has an EER
of 8.3 and consumes 1,200 watts, and the other has an EER of 10 and consumes
1,000 watts. Let's also say that the price difference is $100. To understand what
the payback period is on the more expensive unit, you need to know:
• Approximately how many hours per year you will be operating the unit
• How much a kilowatt-hour (kWh) costs in your area
Let's say that you plan to use the air conditioner in the summer (four months a
year) and it will be operating about six hours a day. Let's also imagine that the
cost in your area is $0.10/kWh. The difference in energy consumption between
the two units is 200 watts, which means that every five hours the less expensive
unit will consume 1 additional kWh (and therefore $0.10 more) than the more
expensive unit.
Assuming that there are 30 days in a month, you find that during the summer you
are operating the air conditioner:
4 mo. x 30 days/mo. x 6 hr/day = 720 hours
[(720 hrs x 200 watts) / (1000 watts/kW)] x $0.10/kWh = $14.40
Split-system AC Units
A split-system air conditioner splits the hot side from the cold side of the system,
like this:
The cold side, consisting of the expansion valve and the cold coil, is generally
placed into a furnace or some other air handler. The air handler blows air through
the coil and routes the air throughout the building using a series of ducts. The hot
side, known as the condensing unit, lives outside the building.
The unit consists of a long, spiral coil shaped like a cylinder. Inside the coil is a fan,
to blow air through the coil, along with a weather-resistant compressor and some
control logic. This approach has evolved over the years because it is low-cost, and
also because it normally results in reduced noise inside the house (at the expense
of increased noise outside the house). Besides the fact that the hot and cold sides
are split apart and the capacity is higher (making the coils and compressor larger),
there is no difference between a split-system and a window air conditioner.
• In warehouses, businesses, malls, large department stores and the like, the
condensing unit normally lives on the roof and can be quite massive.
Alternatively, there may be many smaller units on the roof, each attached
inside to a small air handler that cools a specific zone in the building.
• In larger buildings and particularly in multi-story buildings, the split-system
approach begins to run into problems. Either running the pipe between the
condenser and the air handler exceeds distance limitations (runs that are
too long start to cause lubrication difficulties in the compressor), or the
amount of duct work and the length of ducts becomes unmanageable.
Chilled-water and Cooling-tower AC Units
In a chilled-water system, the entire air conditioner lives on the roof or behind the
building. It cools water to between 40 and 45 F (4.4 and 7.2 C). This chilled water is
then piped throughout the building and connected to air handlers as needed. There is
no practical limit to the length of a chilled-water pipe if it is well-insulated.
A-Expansion valve
C-Heat Exchanger
D-Chilled water to the building
Cooling Towers
In all of the systems described earlier, air is used to dissipate the heat from the
outside coil. In large systems, the efficiency can be improved significantly by
using a cooling tower. The cooling tower creates a stream of lowertemperature water. This water runs through a heat exchanger and cools the
hot coils of the air conditioner unit. It costs more to buy the system initially,
but the energy savings can be significant over time (especially in areas with
low humidity), so the system pays for itself fairly quickly.
Cooling towers come in all shapes and sizes. They all work on the same
• Generally, the water trickles through a thick sheet of open plastic mesh.
• Air blows through the mesh at right angles to the water flow.
• The evaporation cools the stream of water.
• Because some of the water is lost to evaporation, the cooling tower
constantly adds water to the system to make up the difference.
Air-Distribution Systems
There are various types of air-distribution systems, like fans, filters, ductwork,
outlets, dampers etc.
HVAC is an initialism/acronym that stands for "heating, ventilating, and air
conditioning". HVAC is sometimes referred to as "climate control" and is
particularly important in the design of medium to large industrial and office
buildings such as sky scrapers and in marine environments such as aquariums,
where humidity and temperature must all be closely regulated whilst maintaining
safe and healthy conditions within.
Heating, ventilating, and air conditioning is based on the basic principles of
thermodynamics, fluid mechanics, and heat transfer The invention of the
components of HVAC systems goes hand-in-hand with the industrial revolution,
and new methods of modernization, higher efficiency, and system control are
constantly introduced by companies and inventors all over the world.
The three functions of heating, ventilating, and air-conditioning are closely
interrelated. All seek to provide thermal comfort, acceptable indoor air quality,
and reasonable installation, operation, and maintenance costs. HVAC systems can
provide ventilation, reduce air infiltration, and maintain pressure relationships
between spaces. How air is delivered to, and removed from spaces is known as
room air distribution.
In modern buildings the design, installation, and control systems of these
functions are integrated into one or more HVAC systems. For very small buildings,
contractors normally "size" and select HVAC systems and equipment. For larger
buildings where required by law, "building services" designers and engineers,
such as mechanical, architectural, or building services engineers analyze, design,
and specify the HVAC systems, and specialty mechanical contractors build and
commission them. In all buildings, building permits for, and code-compliance
inspections of the installations are the norm.
HVAC systems use ventilation air ducts installed throughout a building that supply
conditioned air to a room through rectangular or round outlet vents, called
"diffusers"; and ducts that remove air through return-air "grilles.
Heating systems may be classified as central or local. Central heating is often used
in cold climates to heat private houses and public buildings. Such a system
contains a boiler, furnace, or heat pump to heat water, steam, or air, all in a
central location such as a furnace room in a home or a mechanical room in a large
building. The system also contains piping or ductwork to distribute the heated
fluid, and radiators to transfer this heat to the air. The term radiator in this
context is misleading since most heat transfer from the heat exchanger is by
convection, not radiation. The radiators may be mounted on walls or buried in the
floor to give under-floor heat.
In boiler fed or radiant heating systems, all but the simplest systems have a pump
to circulate the water and ensure an equal supply of heat to all the radiators. The
heated water can also be fed through another heat exchanger inside a storage
cylinder to provide hot running water.
Forced air systems send heated air through ductwork. During warm weather the
same ductwork can be reused for air conditioning. The forced air can also be
filtered or put through air cleaners. Most ducts cannot fit a human being (as they
do in many films) since this would require a greater duct-structural integrity and
create a potential security liability.
Heating can also be provided from electric, or resistance heating using a filament
that glows hot when you cause electricity to pass through it. This type of heat can
be found in electric baseboard heaters, portable electric heaters, and as backup
or supplemental heating for heat pump (or reverse heating) system.
The heating elements (radiators or vents) should be located in the coldest part of
the room and typically next to the windows to minimize condensation. Popular
retail devices that direct vents away from windows to prevent "wasted" heat
defeat this design parameter. Drafts contribute more to the subjective feeling of
coldness than actual room temperature. Therefore, rather than improving the
heating of a room/building, it is often more important to control the air leaks.
The invention of central heating is often credited to the ancient Romans, who
installed a system of air ducts called "hypocaust" in the walls and floors of public
baths and private villas. The ducts were fed with hot air from a central fire.
Generally, these heated by radiation; a better physiologic approach to heating
than conventional forced air convective heating.
Ventilating is the process of "changing" or replacing of air in any space to remove
moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes
both the exchange of air to the outside as well as circulation of air within the
building. It is one of the most important factors for maintaining acceptable indoor
air quality in buildings. Methods for ventilating a building may be divided into
mechanical/forced and natural types. Ventilation is used to remove unpleasant
smells and excessive moisture, introduce outside air, and to keep interior building
air circulating, to prevent stagnation of the interior air.
Air-Handling unit
HVAC Systems Design and Safety
Heating, ventilating and air-conditioning (HVAC) systems can play several roles to
reduce the environmental impact of buildings. The primary function of HVAC
systems is to provide healthy and comfortable interior conditions for occupants.
Well-designed, efficient systems do this with minimal non-renewable energy and
air and water pollutant emissions. Cooling equipment that avoids
chlorofluorocarbons and hydro chlorofluorocarbons (CFCs and HCFCs) may
eliminate a major cause of damage to the ozone layer.
However, even the best HVAC equipment and systems cannot compensate for a
building design with inherently high cooling and heating needs. The greatest
opportunities to conserve non-renewable energy are through architectural design
that controls solar gain, while taking advantage of passive heating, day lighting,
natural ventilation and cooling opportunities. The critical factors in mechanical
systems' energy consumption - and capital cost - are reducing the cooling and
heating loads they must handle.
Air Change per Hour (ACH)
The number of times per hour that the volume of a specific room or building is
supplied or removed from that space by mechanical and natural ventilation.
Air handler, or air handling unit (AHU)
Central unit consisting of a blower, heating and cooling elements, filter racks or
chamber, dampers, humidifier, and other central equipment in direct contact with
the airflow. This does not include the ductwork through the building.
British thermal unit (BTU)
Any of several units of energy (heat) in the HVAC industry, each slightly more than
1 kJ. One BTU is the energy required to raise one pound of water one degree
Fahrenheit, but the many different types of BTU are based on different
interpretations of this “definition”. In the United States the power of HVAC
systems (the rate of cooling and dehumidifying or heating) is sometimes
expressed in BTU/hour instead of watts.
A device that removes heat from a liquid via a vapor-compression or absorption
refrigeration cycle. This cooled liquid flows through pipes in a building and passes
through coils in air handlers, fan-coil units, or other systems, cooling and usually
dehumidifying the air in the building. Chillers are of two types; air-cooled or
water-cooled. Air-cooled chillers are usually outside and consist of condenser coils
cooled by fan-driven air. Water-cooled chillers are usually inside a building, and
heat from these chillers is carried by recirculating water to outdoor cooling
A device that controls the operation of part or all of a system. It may simply turn a
device on and off, or it may more subtly modulate burners, compressors, pumps,
valves, fans, dampers, and the like. Most controllers are automatic but have user
input such as temperature set points, e.g. a thermostat. Controls may be analog,
or digital, or pneumatic, or a combination of these.
Fan-coil unit (FCU)
A small terminal unit that is often composed of only a blower and a heating
and/or cooling coil (heat exchanger), as is often used in hotels, condominiums, or
A component in the basic refrigeration cycle that ejects or removes heat from the
system. The condenser is the hot side of an air conditioner or heat pump.
Condensers are heat exchangers, and can transfer heat to air or to an
intermediate fluid (such as water or an aqueous solution of ethylene glycol) to
carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as
with cooling towers).
Constant air volume (CAV)
A system designed to provide a constant air volume per unit time. This term is
applied to HVAC systems that have variable supply-air temperature but constant
air flow rates. Most residential forced-air systems are small CAV systems with
on/off control.
A plate or gate placed in a duct to control air flow by introducing a constriction in
the duct.
A component in the basic refrigeration cycle that absorbs or adds heat to the
system. Evaporators can be used to absorb heat from air (by reducing
temperature and by removing water) or from a liquid. The evaporator is the cold
side of an air conditioner or heat pump.
A component of an HVAC system that adds heat to air or an intermediate fluid by
burning fuel (natural gas, oil, propane, butane, or other flammable substances) in
a heat exchanger.
Fresh air intake (FAI)
An opening through which outside air is drawn into the building. This may be to
replace air in the building that has been exhausted by the ventilation system, or
to provide fresh air for combustion of fuel.
A facing across a duct opening, usually rectangular is shape, containing multiple
parallel slots through which air may be delivered or withdrawn from a ventilated
Heat load, heat loss, or heat gain
Terms for the amount of heating (heat loss) or cooling (heat gain) needed to
maintain desired temperatures and humidities in controlled air. Regardless of
how well-insulated and sealed a building is, buildings gain heat from warm air or
sunlight or lose heat to cold air and by radiation. Engineers use a heat load
calculation to determine the HVAC needs of the space being cooled or heated.
Blades, sometimes adjustable, placed in ducts or duct entries to control the
volume of air flow. The term may also refer to blades in a rectangular frame
placed in doors or walls to permit the movement of air.
Makeup air unit (MAU)
An air handler that conditions 100% outside air. MAUs are typically used in
industrial or commercial settings, or in once- through (blower sections that only
blow air one-way into the building), low flow (air handling systems that blow air
at a low flow rate), or primary-secondary (air handling systems that have an air
handler or rooftop unit connected to an add-on makeup unit or hood) commercial
HVAC systems.
Packaged terminal air conditioner (PTAC)
An air conditioner and heater combined into a single, electrically-powered unit,
typically installed through a wall and often found in hotels.
Roof-top unit (RTU)
An air-handling unit, defined as either "recirculating" or "once-through" design,
made specifically for outdoor installation. They most often include, internally,
their own heating and cooling devices. RTUs are very common in some regions,
particularly in single-story commercial buildings.
Variable air volume (VAV) system
An HVAC system that has a stable supply-air temperature, and varies the air flow
rate to meet the temperature requirements. Compared to CAV systems, these
systems waste less energy through unnecessarily-high fan speeds. Most new
commercial buildings have VAV systems.
Thermal zone
A single or group of neighboring indoor spaces that the HVAC designer expects
will have similar thermal loads. Building codes may require zoning to save energy
in commercial buildings. Zones are defined in the building to reduce the number
of HVAC subsystems, and thus initial cost. For example, for perimeter offices,
rather than one zone for each office, all offices facing west can be combined into
one zone. Small residences typically have only one conditioned thermal zone, plus
unconditioned spaces such as unconditioned garages, attics, and crawlspaces, and
unconditioned basements.
The selection of hot and chilled water coils will have a substantial impact on the
fan energy use.
Thin coil design
Traditional AHU design specifies coil sizes assuming a face velocity of between
400 and 500 feet per minute. A new design technique called low face velocity,
high coolant velocity or LFV/HCV has been researched at the University of
Adelaide, Australia. This technique uses a "thin" coil design that is roughly half the
number of tubes in depth as in conventional designs but double the coil face area.
The net result is a face velocity in the range of 150 to 200 feet per minute (FPM)
with much higher heat transfer efficiency and lower pressure drop than in
conventional designs. Because the coil's pressure loss is proportional to the
velocity at a square rate, face velocity reduction can result in pressure drops of
one-fourth or less compared to the equivalent, traditionally designed coil.
Preheat coils
A preheat coil is commonly used to control condensation inside the HVAC system
for laboratories that use 100 percent outside air or when the outside air
temperature falls below freezing. If a heating coil is used downstream, the
preheat coil should become inactive to save energy when outdoor temperatures
reach 45 degrees F. Preheat coils are also used to warm the outside air stream,
assuring better air stream mixing and providing free humidification.
A damper is a valve or plate that stops or regulates the flow of air inside a duct,
chimney, VAV box, air handler, or other air handling equipment. A damper may be
used to cut off central air conditioning (heating or cooling) to an unused room, or
to regulate it for room-by-room temperature and climate control. Its operation
can be manual or automatic. Manual dampers are turned by a handle on the
outside of a duct. Automatic dampers are used to regulate airflow constantly and
are operated by electric or pneumatic motors, in turn controlled by a thermostat
or building automation system.
In a chimney flue, a damper closes off the flue to keep the weather (and birds and
other animals) out and warm or cool air in. This is usually done in the summer,
but also sometimes in the winter between uses. In some cases, the damper may
also be partly closed to help control the rate of combustion. The damper may be
accessible only by reaching up into the fireplace by hand or with a wood poker, or
sometimes by a lever or knob that sticks down or out. On a wood burning stove or
similar device, it is usually a handle on the vent duct as in an air conditioning
system. Forgetting to open a damper before beginning a fire can cause serious
smoke damage to the interior of a home, if not a house fire.
Opposed blade dampers in a mixing duct
Dampers must be installed in places where airflow needs to be controlled and/or
blocked. Dampers located directly behind an outlet tend to be noisy. A better
location is in the final branch near the connection to the trunk duct. Wherever a
balancing or volume damper is located, it should be accessible. Lay-in ceiling tiles
provide good access; in a fixed ceiling, an access door is needed. Dampers should
not be installed in hood exhaust systems even if the exhaust duct passes through
a firewall. Use the UL approved alternative -- a properly supported, heavy-gauge
steel, unobstructed duct.
Dampers have to withstand the maximum static pressure in a system. The
maximum static pressure is the maximum that can be experienced in a system,
not simply the pressure introduced by the fan during normal operation. Maximum
static pressure usually occurs when all dampers in a system are closed except
those on one flow path.
Automated zone dampers
A zone damper (also known as a Volume Control Damper or VCD) is a specific type
of damper used to control the flow of air in an HVAC heating or cooling system. In
order to improve efficiency and occupant comfort, HVAC systems are commonly
divided up into multiple zones. For example, in a house, the main floor may be
served by one heating zone while the upstairs bedrooms are served by another. In
this way, the heat can be directed principally to the main floor during the day and
principally to the bedrooms at night, allowing the unoccupied areas to cool down.
Zone dampers as used in home HVAC systems are usually electrically powered. In
large commercial installations, vacuum or compressed air may be used instead. In
either case, the motor is usually connected to the damper via a mechanical
• Cost.
• Power consumption.
• Zone dampers are not 100% reliable. The motor-to-open/motor-to-closed
style of electrically operated zone dampers aren't "fail safe" (that is, they
do not fail to the open condition). However, zone dampers that are of the
"Normally Open" type are fail-safe, in that they will fail to the open
• No inherent redundancy for the furnace. A system with zone dampers is
dependent upon a single furnace. If it fails, the system becomes completely
• The system can be harder to design, requiring both “SPDT” thermostats
(and relays) and the ability of the system to withstand the fault condition
whereby all zone dampers are closed simultaneously.
Fire dampers
Fire dampers are fitted where ductwork passes through fire compartment walls /
fire curtains as part of a fire control strategy. In normal circumstances, these
dampers are held open by means of fusible links. When subjected to heat, these
links fracture and allow the damper to close under the influence of the integral
closing spring. The links are attached to the damper such that the dampers can be
released manually for testing purposes. The damper is provided with an access
door in the adjacent ductwork for the purpose of inspection and resetting in the
event of closure.
Ducts are used in heating, ventilation, and air conditioning (HVAC) to deliver and
remove air. These needed airflows include, for example, supply air, return air, and
exhaust air. Ducts also deliver, most commonly as part of the supply air,
ventilation air. As such, air ducts are one method of ensuring acceptable indoor
air quality as well as thermal comfort.
A duct system is often called ductwork. Planning ('laying out'), sizing, optimizing,
detailing, and finding the pressure losses through a duct system is called duct
Duct materials
Like modern steel food cans, at one time air ducts were often made of tin, like 'tin
cans' were made for food. Tin is more corrosion resistant than plain steel, but is
also more expensive. With improvements in mild steel production, and its
galvanization to resist rust, steel 'sheet metal' has replaced tin in ducts as well as
food cans..
Galvanized steel
Ducts are still most often made of galvanized steel. Various fittings allow
transitioning between the various shapes and sizes. A "tee" connection, for
example, is where the air flow can be divided into two or more downstream
branches. Many factory-made shapes and sizes are available but galvanized steel
can easily be cut and bent to form additional shapes when required. Steel ducts
are commonly wrapped or lined with fiberglass thermal insulation, both to reduce
heat loss or gain through the duct walls and water vapor from condensing on the
exterior of the duct when the duct is carrying cooled air. Insulation, particularly
duct liner, also reduces duct-borne noise. Both types of insulation reduce
'breakout' noise through the ducts' sidewalls.
Polyurethane duct board (Preinsulated aluminum ducts)
While as mentioned above, galvanized steel is still very common, always more
rectangular ducts are being manufactured from “duct board”, thanks to the fact
that custom or special shapes and sizes of ducts can easily be shop or field
fabricated. In addition to the fact that ducts made with “duct board” do not need
any further insulation. Among the various types of rigid polyurethane foam panels
available, a new water formulated panel stands out. In this particular panel, the
foaming process is obtained through the use of water instead of the CFC, HCFC,
HFC and HC gasses. The foam panels are then coated with aluminum sheets on
either side, with thicknesses that can vary from 50 micrometres for indoor use to
200 micrometres for external use in order to guarantee the high mechanical
characteristics of the duct. The ducts construction starts with the plotting of the
single pieces on the panel. The pieces are then cut from the panel (with a 45° cut
as explained below), bent if necessary in order to obtain the different fittings, and
finally closed through an operation of gluing, pressing and taping. Having
obtained the various duct sections, they can easily be installed by using an
invisible aluminum flange system.
Fiberglass duct board (Preinsulated non metallic ductwork)
Also the fiberglass panels provide built-in thermal insulation and the interior
surface absorbs sound, helping to provide quiet operation of the HVAC system.
The duct board is formed by sliding a specially-designed knife along the board
using a straightedge as a guide; the knife automatically trims out a "valley" with
45° sides; the valley does not quite penetrate the entire depth of the duct board,
providing a thin section that acts as a hinge. The duct board can then be folded
along the valleys to produce 90° folds, making the rectangular duct shape in the
fabricator's desired size. The duct is then closed with staples and special
aluminum or similar 'metal-backed' tape. Commonly available duct tape should
not be used on air ducts, metal, fiberglass, or otherwise, that are intended for
long-term use; the adhesive on so called 'duct tape' dries and releases with time.
Flexible tubing
Flexible ducts, known as flex, have a variety of configurations, but for HVAC
applications, they are typically flexible plastic over a metal wire coil to make
round, flexible duct. Most often a layer of fiberglass insulation covers the duct,
and then a thin plastic layer protects the insulation. Flexible duct is very
convenient for attaching supply air outlets to the rigid ductwork. However, the
pressure loss through flex is higher than for most other types of ducts. As such,
designers and installers attempt to keep their installed lengths (runs) short, e.g.,
less than 15 feet or so, and to minimize turns. Kinks in flex must be avoided.
Flexible duct is normally not used on the negative pressure portions of HVAC duct
The objectives of good duct design are occupant comfort, proper air distribution,
economical heating and cooling system operation, and economical duct
installation. The outcome of the duct design process will be a duct system (supply
and return plenums, ducts, fittings, boots, grilles, and registers) that
• Provides conditioned air to meet all room heating and cooling loads.
• Is properly sized so that the pressure drop across the air handler is within
manufacturer and design specifications.
• Is sealed to provide proper air flow and to prevent air from entering the house
or duct system from polluted zones.
• Has balanced supply and return air flows to maintain a neutral pressure in the
• Minimizes duct air temperature gains or losses between the air handler and
supply outlets, and between the return register and air handler.
Supply ducts deliver air to the spaces that are to be conditioned. The two most
common supply duct systems for residences are the trunk and branch system and
the radial system because of their versatility, performance, and economy.
The spider and perimeter loop systems are other options.
In the trunk and branch system, a large main supply trunk is connected directly to
the air handler or its supply plenum and serves as a supply plenum or an
extension to the supply plenum. Smaller branch ducts and run outs are connected
to the trunk. The trunk and branch system is adaptable to most houses, but it has
more places where leaks can occur. It provides air flows that are easily balanced
and can be easily designed to be located inside the conditioned space of the
house. There are several variations of the trunk and branch system. An extended
plenum system uses a main supply trunk that is one size and is the simplest and
most popular design. The length of the trunk is usually limited to about 24 feet
because otherwise the velocity of the air in the trunk gets too low and air flow
into branches and run outs close to the air handler becomes poor. Therefore, with
a centrally located air handler, this duct system can be installed in homes up to
approximately 50 feet long. A reducing plenum system uses a trunk reduction
periodically to maintain a more uniform pressure and air velocity in the trunk,
which improves air flow in branches and run outs closer to the air handler.
Similarly, a reducing trunk system reduces the cross-sectional area of the trunk
after every branch duct or run out, but it is the most complex system to design.
A spider system is a more distinct variation of the trunk and branch system. Large
supply trunks (usually large-diameter flexible ducts) connect remote mixing boxes
to a small, central supply plenum. Smaller branch ducts or run outs take air from
the remote mixing boxes to the individual supply outlets. This system is difficult to
locate within the conditioned space of the house.
In a radial system, there is no main supply trunk; branch ducts or run outs that
deliver conditioned air to individual supply outlets are essentially connected
directly to the air handler, usually using a small supply plenum. The short, direct
duct runs maximize air flow. The radial system is most adaptable to single-story
homes. Traditionally, this system is associated with an air handler that is centrally
located so that ducts are arranged in a radial pattern. However, symmetry is not
mandatory, and designs using parallel runouts can be designed so that duct runs
remain in the conditioned space (e.g., installed above a dropped ceiling).
A perimeter loop system uses a perimeter duct fed from a central supply plenum
using several feeder ducts. This system is typically limited to houses built on slab
in cold climates and is more difficult to design and install.
Return ducts remove room air and deliver it back to the heating and cooling
equipment for filtering and reconditioning. Return duct systems are generally
classified as either central or multiple-room return.
A multiple-room return system is designed to return air from each room supplied
with conditioned air, especially those that can be isolated from the rest of the
house (except bathrooms and perhaps kitchens and mechanical rooms). When
properly designed and installed, this is the ultimate return duct system because it
ensures that air flow is returned from all rooms (even with doors closed),
minimizes pressure imbalances, improves privacy, and is quiet. However, design
and installation costs of a multi-room return system are generally higher than
costs for a central return system, and higher friction losses can increase blower
A central return system consists of one or more large grilles located in central
areas of the house (e.g., hallway, under stairway) and often close to the air
handler. In multi-story houses, a central return is often located on each floor. To
ensure proper air flow from all rooms, especially when doors are closed, transfer
grilles or jumper ducts must be installed in each room (undercutting interior
doors to provide 1 inch of clearance to the floor is usually not sufficient by itself).
Transfer grilles are through-the-wall vents that are often located above the
interior door frames, although they can be installed in a full wall cavity to reduce
noise transmission. The wall cavity must be well sealed to prevent air leakage.
Jumper ducts are short ducts routed through the ceiling to minimize noise
Locating the air handler unit and air distribution system inside the conditioned
space of the house is the best way to improve duct system efficiency and is highly
recommended. With this design, any duct leakage will be to the inside of the
house. It will not significantly affect the energy efficiency of the heating and
cooling system because the conditioned air remains inside the house, although air
distribution may suffer. Also, ducts located inside the conditioned space need
minimal insulation (in hot and humid climates), if any at all. The cost of moving
ducts into the conditioned space can be offset by smaller heating and cooling
equipment, smaller and less duct work, reduced duct insulation, and lower
operating costs.
There are several methods for locating ducts inside the
conditioned space.
• Place the ducts in a furred-down chase below the ceiling (e.g., dropped ceiling in
a hallway), a chase furred-up in the attic, or other such chases. These chases must
be specially constructed, air-sealed, and insulated to ensure they are not
connected to unconditioned spaces.
• Locate ducts between the floors of a multi-story home (run through the floor
trusses or joists). The exterior walls of these floor cavities must be insulated and
sealed to ensure they are within the conditioned space. Holes in the cavity for
wiring, plumbing, etc., must be sealed to prevent air exchange with
unconditioned spaces.
• Locate ducts in a specially-constructed sealed and insulated crawlspace (where
the walls of the crawlspace are insulated rather than the ceiling). Ducts should not
be run in exterior walls as a means of moving them into the conditioned space
because this reduces the amount of insulation that can be applied to the duct and
the wall itself. A supply outlet is positioned to mix conditioned air with room air
and is responsible for most of the air movement within a room. Occupant comfort
requires that supply register locations be carefully selected for each room. In cold
climates, perimeter floor outlets that blanket portions of the exterior wall (usually
windows) with supply air are generally preferred. However, in today’s better
insulated homes, the need to locate outlets near the perimeter where heat loss
occurs is becoming less important. In hot climates, ceiling diffusers or high wall
outlets that discharge air parallel to the ceiling are typically installed. In moderate
climates, outlet location is less critical. Outlet locations near interior walls can
significantly reduce duct lengths (decreasing costs), thermal losses (if ducts are
located outside the conditioned space), and blower requirements. To prevent
supply air from being swept directly up by kitchen, bathroom, or other exhaust
fans, the distance between supply registers and exhaust vents should be kept as
large as possible. The location of the return register has only a secondary effect
on room air motion. However, returns can help defeat stratification and improve
mixing of room air if they are placed high when cooling is the dominant spaceconditioning need and low when heating is dominant. In multi-story homes with
both heating and cooling, upper-level returns should be placed high and lowerlevel returns should be placed low. Otherwise, the location of the return register
can be determined by what will minimize duct runs, improve air circulation and
mixing of supply air, and impact other considerations such as aesthetics.
In designing the air distribution the following recommendations before finalizing
the design should be considered:
• Design the air distribution system to be located inside the conditioned space of
the house to the greatest extent possible. Do not locate ducts in exterior walls.
• The entire air distribution system should be “hard” ducted, including returns
(i.e., building cavities, closets, raised-floor air handler plenums, platform returns,
wall stud spaces, panned floor joists, etc., should not be used).
• In two-story and very large houses, consider using two or more separate heating
and cooling systems, each with its own duct system. In two-story homes, for
example, upper stories tend to gain more heat in summer and lose more heat in
winter, so the best comfort and performance is often achieved by using separate
systems for the upper and lower stories.
• Consider supply outlet locations near interior walls to reduce duct lengths.
• Locate supply outlets as far away from exhaust vents as possible in bathrooms
and kitchens to prevent supply air from being swept directly up by the exhaust
• Consider installing volume dampers located at the takeoff end of the duct
rather than at the supply register to facilitate manual balancing of the system
after installation. Volume dampers should have a means of fixing the position of
the damper after the air distribution system is balanced.
• When using a central return system, include (a) a return on each level of a multistory house, (b) a specification to install transfer grilles or jumper ducts in each
room with a door (undercutting interior doors to allow 1 inch of clearance to the
floor is usually not sufficient), and (c) if at all possible, a return in all rooms with
doors that require two or more supply ducts.
• Specify higher duct insulation levels in ducts located outside the conditioned
space than those specified by the 2000 International Energy Conservation Code,
especially when variable-speed air handling equipment is being used. Lower air
flows provided by variable-speed heating and cooling systems to improve
operating efficiency increase the resident time of air within the air distribution
system, which in turn increases thermal losses in the winter and thermal gains in
the summer. Attic insulation placed over ducts helps where it is possible.
• Specify that all duct joints must be mechanically fastened and sealed prior to
insulation to prevent air leakage, preferably with mastic and fiberglass mesh.
Consider testing of ducts using a duct blower to ensure that the air distribution
system is tight, especially if ducts are unavoidably located in an unconditioned
space. A typical requirement is that duct leakage (measured using a duct blower
in units of cubic feet per minute when the ducts are pressurized to 25 Pascals)
should not exceed 5% of the system air flow rate.
Principles of Air-Conditioning.
Psychometric Chart
Refrigeration Cycle
Vapor Compression cycle
Vapor absorption cycle
Air cycle
Comfort cooling
Cooling supply devices
Air conditioning
• Application
• Types of AC units
• Central air-conditioning
• Window AC units
• Air distribution systems
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