Institute of Technology
School of Mechanical Engineering
Refrigeration and Air-conditioning
(MEng 5212)
Part II: Chapter 5
Design and Selection of Air conditioning
equipments
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Introduction
In order to maintain required comfort inside the conditioned
space, energy has to be either supplied or extracted from the
conditioned space.
The energy in the form of sensible as well as latent heat has to
be supplied to the space in winter and extracted from the
conditioned space in case of summer.
An air conditioning system consists of an air conditioning plant
and a thermal distribution system.
cont...
• As shown in the figure, the air conditioning (A/C) plant acts
either as a heat source (in case of winter systems) or as a heat
sink (in case of summer systems).
• Air, water or refrigerant are used as media for transferring
energy from the air conditioning plant to the conditioned
space.
• A thermal distribution system is required to circulate the
media between the conditioned space and the A/C plant.
• Another important function of the thermal distribution system
is to introduce the required amount of fresh air into the
conditioned space so that the required Indoor Air Quality
(IAQ) can be maintained.
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Air conditioning equipments:
 Equipments are:
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Cooling Coil
Heating Coil
Humidifier
Dehumidifier
Filters
Dampers
Supply Fan
Diffusers
Return Fan
Heating and Cooling Coils
 The cooling coils are fed from a source of cooling, such as
chillers, a cooling tower, or DX unit. The refrigerant enters
the cooling coils, absorbs heat from the air passing the coils,
and returns to the source to desorb the heat that it tooks.
 Cooling coils are used either of the water or the direct
expansion type depending on the media flowing through the
tubes.
 The heating coils are fed from a heat source such as a boiler
or electricity. The heat from these coils is transferred to the
air passing over it.
 Heating coils may be used with hot water or steam as the
heat transfer media whilst frost pre-heaters usually have
electric heating elements.
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Types of Heating and Cooling Coils
a) Fin & Tube Heat Exchangers
 Fin and tube heat exchangers are used extensively for heating
and cooling air.
 They consist of one or more rows of finned tubes connected to
headers and mounted within a sheet metal casing with flanged
ends suitable for duct mounting.
 The heating elements are normally manufactured with copper
tubes / aluminium tubes, with the extended surfaces, or fins,
being of aluminium or sometimes copper.
 The most common type of finning arrangements are the
rectangular fins.
 A heating coil is shown below.
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Fig. Heating coil
cont...
 In Water Coils, hot or chilled water or brine circulates through the
tubes of the coil either emitting or absorbing sensible heat as the
air flows over the fins attached to the outside surfaces.
 Usually the flow of water and air are in opposite directions to
each other, this being known as counter-flow heat exchanger. This
configuration gives maximum heat transfer.
b) Direct Expansion Coils (Evaporator Coils)
• In the direct expansion coil (DX),
or evaporator, a refrigerant
evaporates inside the tubes of the
coil, as shown below.
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cont...
 Latent heat is absorbed by the air stream from the refrigerant as
the refrigerant evaporates.
 With this type of coil, as with steam, there is no distinction made
between parallel and counter-flow since the surface temperature
is more uniform owing to the refrigerant in the tubes boiling at a
constant temperature.
 When DX coils are used they becomes the evaporator of the
refrigeration cycle, and may be termed either ‘dry’ or ‘flooded’.
 Dry DX coil: only a sufficient quantity of refrigerant is
introduced to operate in the predominantly vapour state.
 Flooded DX coil: most of the coil is filled with liquid refrigerant
and although this is more efficient, it is not used so much in airconditioning since the additional refrigerant is expensive.
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Cont...
 Evaporator coils come in a variety of shapes and sizes, depending
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on the type of installation, the amount of cooling capacity needed,
and the manufacturer. They are constructed of aluminium finned
copper tubing.
The copper tubing runs perpendicular to the aluminium fins,
making U-turns back and forth until the desired coil size is
achieved.
Added cooling capacity without an increase in length and width is
accomplished by adding more rows of copper tubing.
All evaporator coils must have drain pan to collect the water that
condenses as the air flowing across the coil cools.
The water can drain away by gravity or pump.
The cooling effect that takes place inside the coil require a pressure
drop in the refrigerant. This drop can be accomplished in a number
of ways: capillary tube, piston or orifice, or thermostatic expansion
valve.
c) Chilled water coils
• Chilled water coils also
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known as hydronic cooling
coils are typically used to
cool or remove moisture
from air streams.
The air to be cooled moves
through the fins, and either
water or an ethylene or
propylene glycol solution
move through the tubes.
Chilled water coils typically
consist of between 3-12 rows
of tubes.
Heat Transfer in Cooling Coils
 Chilled-water cooling coils are finned-tube heat exchangers
consisting of rows of tubes (usually copper) that pass through
sheets of formed fins (usually aluminium).
 As air passes through the coil and contacts the cold fin surfaces,
heat transfers from the air to the water flowing through the tubes.
 The following equation quantifies the heat-transfer process:
• The log-mean temperature difference (LMTD)
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Improving Coil Performance
 Lowering the supply air temperature reduces the amount of air
required for sensible cooling and saves fan energy.
 Increase LMTD is to supply the coil with colder water.
 Increase U: Recall that turbulent flow reduces the film resistance
to heat transfer. Choosing a fin configuration with a more
pronounced waveform and/or adding turbulators inside the coil
tubes will improve the heat-transfer coefficient.
 Increase A: Any additional increase in heat-transfer capacity must
be achieved by physically increasing the available surface area;
that is, by:
 Adding rows
 Adding fins
 Increasing the physical size of the coil (which will increase the
initial costs of the coil, air handler, and airside accessories).
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Humidifiers
 Humidifiers add moisture to the air.
 These humidifiers inject steam from a central boiler source directly
into the space or distribution duct, boiler treatment chemicals
discharged into the air system may compromise indoor air quality.
 Residential Humidifiers: Residential humidifiers designed for
central air systems depend on airflow in the heating system for
evaporation and distribution.
 Pan Humidifiers: capacity varies with temperature, humidity, and
airflow. Vapor is introduced into the air by evaporation.
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Pan- humidifiers
 The main part is a tank of water heated by low-pressure steam or
forced hot water where a water temperature of 200°F (93°C) or
higher is maintained.
 The evaporative-type humidifier is fully automatic, the water level
being controlled by means of a float control.
 In operation, when the relative humidity drops below the humiditycontrol setting, the humidifier fan blows air over the surface of the
heated water in the tank.
 The air picks up moisture. The air is blown to the space to be
humidified. When the humidity control is satisfied, the humidifier
fan stops.
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Industrial Humidifiers
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Dehumidifier
 Dehumidifiers remove moisture from the air.
 There are three ways to remove moisture from air:
 By cooling it to condense the water vapor,
 By increasing its total pressure which also causes condensation or
 By passing the air over a desiccant, which pulls moisture from the air
through differences in vapor pressures.
 Cooling-based Dehumidification
➢ When air is chilled below its dew
point temperature, moisture
condenses on the nearest surface.
➢ The air has been dehumidified by
the process of cooling and
condensation.
➢ The amount of moisture removed
depends on how cold the air can be
chilled the lower the temperature,
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Desiccant Dehumidifiers
 Instead of cooling the air to condense its moisture,
desiccants attract moisture from the air by creating an area
of low vapor pressure at the surface of the desiccant.
 The pressure exerted by the water in the air is higher, so the
water molecules move from the air to the desiccant and the
air is dehumidified.
 There are five typical equipment configurations for
desiccant dehumidifiers:
 Liquid spray-tower
 Solid packed tower
 Rotating horizontal bed
 Multiple vertical bed
 Rotating Honeycombe
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Liquid spray-tower
• Spray-tower dehumidifiers function much like an air washer, except instead
of water, the units spray liquid desiccant into the air being dried, which is
called the process air.
• The desiccant absorbs moisture from the air and falls to a sump.
• The liquid is sprayed back into the air, and continues to absorb moisture until
a level control indicates it should be dried out and re-concentrated.
• Then part of the solution is drained off and circulated through a heater.
• The warm desiccant is sprayed into a second airstream, called the
reactivation air. Moisture leaves the desiccant and moves to the air.
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Solid packed tower
• In
the
packed
tower
dehumidifier, solid desiccants
like silica gel or molecular sieve
is loaded into a vertical tower.
• Process air flows through the
tower, giving up its moisture to
the dry desiccant.
• After the desiccant has become
saturated with moisture, the
process air is diverted to a second
drying tower, and the first tower
is heated and purged of its
moisture
with
a
small
reactivation airstream.
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Rotating horizontal bed
• In this device, dry, granular
desiccant is held in a series of
shallow, perforated trays that
rotate continuously between
the process and reactivation
airstreams.
• As the trays rotate through
the process air, the desiccant
adsorbs moisture.
• Then the trays rotate into the
reactivation airstream, which
heats the desiccant, raising its
vapor pressure and releasing
the moisture into the air.
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Multiple vertical bed
• In recent years manufacturers
have combined the better features
of packed tower and rotating
horizontal bed designs in an
arrangement that is well-suited to
atmospheric
pressure
dehumidification applications, yet
can achieve low dew points.
• The single or double tower is
replaced by a circular carousel
with eight or more towers that
rotate by means of a ratcheting
drive system between the process
and reactivation air streams.
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Rotating Honeycombe
• Another dehumidifier design uses a
rotating Honeycombe wheel to
present the desiccant to the process
and reactivation airstreams.
• This is sometimes called a DEW
(Desiccant Wheel) dehumidifier.
• The finely divided desiccant is
impregnated into the semi-ceramic
structure, which in appearance
resembles corrugated cardboard that
has been rolled up into the shape of a
wheel.
• The wheel rotates slowly between
the
process
and
reactivation
airstreams.
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Filters
 Filters also play an important part in keeping conditioned space
free from larger particles that could damage it.
 They filter dust, pollen, pet dander, and other small particles out
of the air.
 These particles are trapped in the filter to prevent them from
being recirculated throughout conditioned space.
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Types of filters
 The different types of filters available for residential AC systems.
 Fiberglass Air Filters
 Pleated Air Filters
 High-efficiency particulate air Filters
 UV Filters
 Fiberglass Air Filters: are disposable and the least expensive type
of filter. They do cause less strain on HVAC systems when
drawing in air and keep large particles out of your system.
 Pleated Air Filters: are made of cotton or polyester folds. They
are slightly more expensive than fiberglass filters, but they are
more effective with filtering dust and other small particles like
pollen, mould spores, and pet dander.
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cont...
 HEPA Filters: If you have allergies or other respiratory problems,
high-efficiency particulate air (HEPA) filters are recommended.
 While they are more expensive than fiberglass or pleated air
filters, they are the most effective at screening up to 99.97% of
dust, pollen, mould, pet dander, viruses, bacteria, and other
irritants out of the air.
 UV Filters: are used in air cleaners that may be built into your AC
system. Using ultraviolet light, these filters kill viruses, bacteria,
and other microorganisms in the air that passes through them.
 However, they are not so efficient at removing dust and other
contaminants, but they are great if you have concerns over indoor
air quality that an air cleaner can address.
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End of Part II, Chapter 5
Next Lecture
Chapter 6: Air distribution system
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