Copy of Thermo Lecture 6_ Air-Conditioning.pdf

Iswor Bajracharya, PhD
Department of Mechanical Engineering
Pulchowk Engineering Campus
Applied Thermodynamics ME504,
BME II/I
Air-Conditioning

4.1 Definition and scope of air-conditioning
4.2 Psychometrics and properties of air
4.3 Psychometric chart
4.4Various processes on psychometric chart and their analysis:
sensible heating , sensible cooling, cooling with dehumidification,
cooling with humidification, heating with dehumidification,
heating with humidification, adiabatic mixing of two streams of
moist air
4.5 Air-conditioning systems: DX system, all air system, all water
system, air-water system, merits and demerits of each system
4.6 Components of air-conditioning systems: Ducts, Fans, Grills,
Registers, Diffusers, Balancing dampers,Air filters,Air handling
units, Fan coil units, Humidifiers and Dehumidifiers
Outline
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2 Iswor Bajracharya/Air-Conditioning

Definition and Scope of Air-conditioning
Definition
 Air-conditioning is defined as the simultaneous control of
temperature, humidity, motion and air quality in a confined
enclosure.Thus it provide the following:
a) Control of air temperature at desired values at all times by heating or
cooling
b) Control of air humidity by humidification or dehumidification
c) Control of air movement at desirable velocity
d) Introduction of outside ventilation air as required.
e) Control of air quality (cleanliness ) by filtering, cleaning and purification of
air
f) Control of sound produced by system itself.
 Complete air-conditioning provides the simultaneous control
of these four parameters for both summer and winter.
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 Air-conditioning system may be classified as (1) Comfort air-
conditioning and (2) Industrial air-conditioning.
 The purpose of comfort air-conditioning is to provide an
environment which is comfortable to majority of the occupants.
It can be further subdivided as
a) Summer Air-Conditioning:The purpose is to reduce
the sensible heat and the humidity of air by cooling and
dehumidification.
b) Winter Air-Conditioning:The purpose is to increase
the sensible heat and humidity of the air by heating and
humidification.
c) Year Round Air-Conditioning: is used to control the
temperature and humidity of air throughout the air. 3/5/2016

Psychometrics and Properties of Air
 Psychrometry or Psychrometrics is the study of the properties
of the mixture of air and water vapor.
 Properties of mixture of air and water are called psychrometric
properties.
 Psychrometry is very important in air conditioning because in
air conditioning the human comfort conditions defined in
terms of temperature, humidity and air circulation/ventilation.
 Different psychrometric properties are:
1. Dry Air:
 Atmospheric air having 79% nitrogen and 21% oxygen by
volume is considered dry air. Its molecular weight is taken
as 29. Dry has no water vapor.
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2. Moist Air: It is the mixture of dry air and water vapor.
2. Moisture:Water vapor present in the air is known as moisture
and its quantity in air is an important factor in air-conditioning
system.
3. Dry BulbTemperature:
 The temperature of air measured by ordinary thermometer
is known as dry bulb temperature.
 Dry bulb temperature is an indication heat content in the air.
4. Wet BulbTemperature:
 The temperature of the air measured by the thermometer
when its bulb is covered with wet cloth and is exposed to the
flow of moving air is known as wet bulb temperature.
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Iswor Bajracharya/Air-Conditioning
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 The wet bulb temperature indicates the evaporation of water
from the thermometer bulb (evaporative effect of air) and the
cooling effect provided by that evaporation.
 Wet bulb temperature is lower than dry bulb temperature.
 The rate of evaporation from the wet bulb depends upon the
humidity of air and is reduced when the air contains more
water vapor.
 Wet bulb temperature is always in between dry bulb and dew
point temperature.
 By combining the dry bulb and wet bulb temperature in
a psychrometric chart the state of the humid air can be
determined

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5. Dew PointTemperature:
 It is the temperature at which the moisture present in the
are just begins to condense when the air is cooled.
 The dew point temperature corresponds to the saturation
temperature of water vapor in the mixture air and water
vapor (temperature at which air becomes completely
saturated).
 The dry bulb temperature, wet bulb temperature and dew
point will be same for the saturated air.
6. Specific Humidity or Humidity Ratio:
 It is the ratio of mass of water vapor to the mass of dry air.
 It is also expressed as mass of water vapor present per kg of
dry air

 The difference between the dry bulb and wet bulb temperature
is known as wet-bulb-depression.
 Mathematically,
where mv & ma are mass of water vapor and dry air respectively.
 The masses ma & mv are given by the expression as
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Where pa & pv are the partial pressure of dry air and water
vapor respectively. Ra & Rv are the gas constant for dry air and
water vapor. V is the volume of mixture, T is the temperature.

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Where is the universal gas constant, Ma & Mv are the molecular
weight of dry air and water vapor respectively.
Where p is the total pressure of the mixture.

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7. Absolute Humidity: The weight of water vapor present in
unit volume of air.
8. Relative Humidity: It is the ratio of actual mass of water
vapor in given volume to the mass of water vapor in the same
volume if the air is saturated at the same temperature and
pressure. Mathematically,

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Therefore, the relative humidity can be also defined as the ratio
of the partial pressure of water vapor in a given volume of
mixture to the partial pressure of water vapor in the same
volume if it is saturated at the same temperature.
9. Enthalpy of Moist Air:The enthalpy of moist air is the sum
of the enthalpy of dry air and the enthalpy of water vapor
associated in the same dry air.

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Psychrometer
 A psychrometer is a device which is used for measuring dry bulb
and wet bulb temperatures simultaneously.
 It may be classified as follows:
1. Laboratory psychrometer
2. Sling psychrometer
3. Aspirating psychrometer
4. Continuous recording psychrometer
 Dry bulb temperature can be measured using a normal
thermometer freely exposed to the air.
 Wet bulb temperature can be measure by wrapping a
thermometer bulb by wet cloth and then exposing to the flow
of air.

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 As the air is passed over a thermometer covered with wet wick
or cloth, the moisture contained in the wick tends to evaporate.
The cooling effect provided by evaporation lowers the
temperature measured by thermometer to the rate at which
evaporation occurs.
 For the wet bulb, there is a dynamic equilibrium between heat
gained because the wet bulb is cooler than the surrounding air
and heat lost because of evaporation.
 The wet bulb temperature is the temperature of an object that
can be achieved through evaporative cooling, assuming good air
flow and that the ambient air temperature remains the same.

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Figure:Wet bulb and dry bulb
thermometer Figure: Laboratory Psychrometer

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Figure: Sling Psychrometer

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Psychometric chart
 A psychrometric chart is a graphical representation of various
important properties of air and their inter-relations.
 In the chart, the dry bulb is taken along abscissa and specific
humidity as ordinate to the right hand side of the chart.
 Psychrometric chart contains the following:
1. Dry bulb temperature line on x- axis, Specific humidity line
on y- axis, and dew point, or 100% relative humidity curve
2. Different values of dry bulb temperature along the x- axis
3. Different values of specific humidity along the y- axis
4. Different curves of relative humidity. 100% being farthest
from dry bulb temperature axis, and 0% being on the axis
itself

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5. Enthalpy lines runs diagonally of the chart at an angle of 30
°C to the horizontal.
6. Wet bulb temperature, almost coincidental lines as the
enthalpy values also runs diagonally at an angle of 30°C to
the horizontal lines.
7. Specific volume lines which is more slanted than enthalpy
lines runs diagonally at an angle of 60°C from dry bulb
temperature axis.

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Figure: Schematic of Psychrometric Chart
Dry BulbTemperature (° C)
Humidity
Ratio
(kg/kg
of
dry
air)

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SomeTips for Psychrometric Chart
 If dry bulb and wet bulb temperatures are known, relative
humidity and dew point temperature can be read from the chart.
 If dry bulb and relative humidity are known, wet bulb
temperature can be known from the chart.
 If wet bulb and relative humidity are known, dry bulb
temperature can be read from the chart.
 If wet bulb and relative humidity are known, the dew point can
be read from the chart.
 If dry bulb and relative humidity are known, dew point can be
read from the chart.


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Psychrometric Process
 In order to condition the air for the human comfort, certain
process are carried out on the outside air. These processes
which affect the psychrometric properties of air are called
Psychrometric Processes. The major psychrometric processes
are:
1. Sensible heating or cooling
2. Cooling with dehumidification
3. Cooling with humidification
4. Heating and dehumidification
5. Heating and humidification
6. Mixing of air streams

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Sensible Heating or Cooling
 Heating or cooling of air without addition or subtraction of
moisture is called sensible heating or cooling.
 Heating can be achieved by passing air over the heating coil like
electric heater or steam coils and cooling can be done by passing
air over the cooling coil like evaporating coil of refrigerant cycle.
AIR IN HOT
AIR
OUT
Steam
Heating Coil
AIR
IN
COLD
AIR OUT
Refrigerant
Cooling Coil
Figure: Sensible heating and cooling

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DBT
w
1 2
3
Sensible
Heating
Sensible
Cooling
Total sensible head gained by air is the sum of the heat gained by
dry air and water vapor. So the heat gained by air can be written
as:
(T1>T3)
For 1 kg of air entering into the system, w
is very small and therefore ma ≈ 1 kg.

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By-Pass Factor
 When by-pass circuit is used, some fraction of air will be by-
passed from this by-pass circuit.
 If 1 kg of air is passed to the system, assuming that B kg will be
by-passed, the remaining (1-B) kg of air will be passing over the
heating or cooling coil.
 If T1 is the inlet temperature of the air and T3 is the
temperature of the heating coil, then the temperature of the air
coming out from the system will be less thanT3, sayT2 (T2<T3)

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Air Out
atT2
1 kg ofAir In
atT1
B
(1-B) (1-B) kg
Air atT3
B kg atT1
Steam In Steam Out
ma = Mass of dry air
w = Mass of water vapor
Cpa =Specific heat of dry air
Cpv = Specific heat of water vapor
Cpm = Mean specific heat
Balancing enthalpies, we can write:
B is known as by-pass factor of coil.

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Cooling with Dehumidification
 In this process, air temperature is reduced along with removal
of water vapor from air.
 The process of removal of water vapor form the air is called
dehumidification of air.
 The dehumidification of air is only possible if the air is cooled
below the dew point temperature of the air.
 If the air is cooled to below dew point, the moisture will drop
out of the air in the form of condensate.
 Figure shows the cooling with dehumidification where the
moist air entering is passed over cooling coil having cool
refrigerant inside. Water vapor present in moist air gets
condensed and drop out.

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 This can be show on a psychrometric chat as air is sensibly
cooled until dew point temperature is reached (point 1to 2) and
then is further cooled to a lower temperature along the
saturation curve down (point 2 to 3).
 Point 3 (T3) is called apparatus dew point (ADP) of the cooling
coil.
 In ideal condition, the temperature of the air coming out of the
cooling coil will be T3. No cooling coil is 100 % efficient , so the
condition of air coming out of coil be represented by point 4.
 The psychrometric process from state point 1 to 2 to 3 may be
shown as a straight line for simplicity as shown with a yellow line

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Moist Air
T1,w1,p1
mw
Cooling Coil
(T3>T4<T1,
w4<w1)
1 3
Cold &
dehumidified Air
T4,w4,p1
Figure: Cooling with dehumidification process
4
Total heat removed from the air is given by
Where,QL = Latent Heat
QS = Sensible Heat
The ratio QS/QT is called sensible heat factor (SHF).

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Moist
Air
T1,w1,
p1
mw
Cooling Coil
Steam In Steam Out
Air
T2,w2,p1
(T2<T1,
w2<w1)
Cold &
dehumidified
Air
T3,w2,p1
(T3>T2<T1,
w2<w1)
1 2 3

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Figure: Cooling with dehumidification
DBT
w
1
2
3
4
T1
T2
T3 T4
∆w (Amount of
condensation)
h1
h1’
h4
h3
∆TDB

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Cooling with Humidification
 The process of adding water vapor in the unsaturated air is
called humidification of air.
 If unsaturated air is passed through a spray chamber of water,
the part of water will be evaporated and is carried with the air
which increases the specific humidity and decreases the dry
bulb temperature.
 The arrangement for this process is shown in the figure and the
psychrometric chart shows this process.
 As show in the psychrometric process, there is no change in
enthalpy. So this process is also called adiabatic humidification.
 The heat required for the evaporation of water particle is taken
from the air causing decrease in BDT of air.

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∆w
Supply of
ColdWater
w3>w1,
T2>T3<T1
Complete humidification of air is
not possible. So the final state
will be the point 3 (below point
2).The efficiency of spray
chamber is given by an
effectiveness, defined as:
DBT
w
1
2
3
T1
T2 T3
h
∆TDB

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Heating with Humidification
 If unsaturated air is passed through a spray chamber of steam,
the part of the steam is carried with the air which increases
the specific humidity and increases the dry bulb temperature.
 The arrangement for this process is shown in the figure and
the psychrometric chart shows this process.
Supply of
Steam
w3>w1,
T2<T3>T1
Figure: Heating with humidification

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∆w
Complete humidification of air is
not possible. So the final state will
be the point 3 (below point 2).
The efficiency of spray chamber is
given by an effectiveness, defined
as:
DBT
w
1
2
3
T1 T2
T3
h1
∆TDB
h3
h2

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Heating with Dehumidification
 If air is passed over a solid absorbent surface or liquid
absorbent spray, simultaneous heating and dehumidification is
accompanied.
 The part of the water vapor will be absorbed by the absorbent
material reducing the vapor content of air (dehumidification)
and will be condensed out of the air.
 Consequently, the latent heat of condensation is liberated
which causes the sensible heating of air and therefore total heat
of the air remains constant.
 Therefore, it follows a path along a constant enthalpy line as
shown in the psychrometric chart.

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 Commercially used solid absorbent materials are silica gel,
activated alumina and for the liquid absorbents, such as
solutions of organic salts or inorganic compounds like ethylene,
glycol.
DBT
w
1
2
3
T1 T2
T3
h
∆TDB
∆w

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Adiabatic mixing of two streams of moist air
 Adiabatic mixing of two air streams is one in which there is no
heat interaction (without addition or rejection of heat or
moisture) during mixing.
 Mixing of air streams is done frequently in air conditioning to
maintain the required psychrometric properties. Figure shows
adiabatic mixing of two streams entering at (1) and (2) and
leaving at (3).

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 If dry air entering be ma1 & ma2 and water vapor flow rate be w1 &
w2 with streams (1) and (2), h1 and h2 be associated heat of the air
stream respectively at steady state, then mass balances and
energy balances can be given as under:
……….. (1)
……….. (2)
……….. (3)
Since
Substituting these values into equation (2)

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……….. (4)
Substituting the value ma3 = (ma1+ ma2) into equation (3), we get
……….. (5)
From equation (4) and (5), we get ,
……….. (6)

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 Equation (6) is a straight line equation joining point 1 and 2 on
which the final state point 3 lies in between.
DBT
w
1
2
3
T2 T1
T3
h1
∆TDB
h3
h2
(w1-w3)
(w3-w2)

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Figure: Different Psychrometric Process

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Air-conditioning systems
Air Conditioning System
Ducted
Non-Ducted
SplitType Window
Floor
Mounted
Ceiling
Mounted
Wall
Mounted
PackagedType
Air
Cooled
Water
Cooled
DX
system
ChilledWater
System
Vapor Compression Unit
Reciprocating Unit (Air &
Water Cooled)
Centrifugal
Unit (Water
cooled Only) ScrewType Unit (Air,
Water Cooled)
Vapor
Absorption Unit
HotWater
Fired
Direct
Fired
Steam
Fired

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Air-conditioning systems
Air conditioning systems are also classified based upon the type of
fluid used either for heating or cooling.
1. Direct Expansion (DX) System
2. All Air System
3. AllWater System (Hydronic System)
4. Air-Water or Combined System

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DX System
 A direct expansion air conditioning (DX) system uses a
refrigerant vapor to directly cool the supply air to an occupied
space.
 DX systems (both packaged and split) directly cools the air
supplied to the building because the evaporator is in direct
contact with the supply air.
 DX systems can come equipped with all the components in the
unit (packaged system) intended for installation on the rooftop
or by the side of a building; or it may have some components
installed inside the building and some outside (split system). DX
systems require a ventilation fan to distribute the cool air and
resupply/re-circulate it.

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 This system is generally adopted for private residences, offices
and groups of offices in a single zone.
Advantages
 DX systems are less expensive to install, and uses less space in
mechanical and electrical rooms than centralized cooling
systems.
 DX systems can be expanded in an incremental fashion to
match changing building requirements.
 Packaged Systems have standardized operating performances
per unit, allowing more precise system sizing.
 Packaged Systems generally require less ventilation, and do
not require dedicated condensate lines.

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 Packaged Systems occupy less space than comparable split
systems
 Split Systems tend to be larger allowing for fewer units, and
therefore less maintenance costs than a comparable Packaged
system
 Split Systems have lower noise levels because the compressor
unit is located further away from the cooling load area
 Split Systems may allow vertical duct shafts to be smaller in
size

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Figure: DX System

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All Air System
 In this system, air is conditioned at the central plant and that
conditioned air is distributed to each space through ducting.
 The major components needed for this system are:
a) Supply Air Fan: Supply fan is necessary to distribute or supply
air from the central plant through duct and diffuser to the
room.
b) Cooling Coil: Cooling coil cools the air and dehumidifies the
air in summer. Chilled water or refrigerant enters the cooling
coil from the cooling source.
c) Heating Coil: After dehumidification, air may be excessively
cooled. The heating coil reheats the dehumidified air during
summer.Also heats the air during winter season.

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d) Duct work: Air is distributed to each space through duct and is
also returned from each space through duct. Ducts are
arranged such that the system takes in some outside ventilation
air, the rest being return air recirculated from the rooms.
e) Preheat Coil: This coil is required in cold climates (below
freezing point) to temper air so that chilled water inside
cooling coils cannot freeze up. This coil is not required when
DX cooling coils are used.
f) Filter: Filters are required to clean the air.
g) All air system is applied to comfort or process air-conditioning
in a single zone or multiplicity of zones like hospitals, hotels,
schools, stores etc.

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Figure:All air system air conditioning

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All-Water System
 In all water system, hot or chilled water is supplied to each space
from the central plant. Terminal units such as fan-coil units are
installed in each room which heat or cool the room air.
Ventilation air can be brought through the outside wall and
terminal units.
 No air is distributed from the central plant.
 The advantage of this system is that it can be used where there is
space limitation in buildings because it does not require air
ducts.
 It can be also used in existing building with minimum
interference.

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 Considerably less expensive than all-air system for commercial
use particularly in high rise building.
 The multiplicity of fan coil units mean that a great deal of
maintenance works and costs.
 Control of ventilation air quantities is not precise with the small
fans in the units and also the control of humidity is limited.

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HW /CHW
Generator
Outside
Air ConditionedAir
Conditioned
Space
Other Rooms
Figure:All-water system

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Air-Water System
 It is the combination of air and water system and therefore, it
distributes both chilled or hot water and conditioned air from a
central system to each room.
 This system utilizes the best features of all-air system and all-
water system. Most of the energy is carried in the water and the
air quantities distributed are only enough for ventilation.
Therefore, the duct size and ceiling height required is small.
 Air is supplied at high velocities and therefore, no fan or motor
is required in this type of unit.

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 Large number of equipments and components are used in air
conditioning system to
 Clean and purify the air,
 Heat or cool the air,
 Humidify or dehumidify the air, to
 Supply and distribute the air,
 Control the air volume
 For achieving the above purposes, following equipments are
used in air-conditioning systems:
Air filters, Humidifiers & Dehumidifiers, Ducts, Fans, Air
Handling units, Dampers, Grills, Registers, Diffusers.
Components of Air-Conditioning System

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Components of Air-Conditioning System
Air Cleaning
 The level of oxygen will be reduced in the recirculated air in the
air conditioning system. So, outside air is mixed in the system
air but this outside air may contain dust, bacteria, odors and
other pollutants. Therefore, it is necessary to remove all these
harmful ingredients before mixing in the air conditioning
system.This is done by cleaning and filtering the air.
 The removal of impurities provides the following advantages:
1. Improves the quality of air.
2. Protects machines and equipments from corrosion.
3. Better health to the occupants.
4. Elimination of dust

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Methods of Air Cleaning
 Dust particles are removed by air filtering using Air Filters. The
use of particular type of filters depends upon the nature and size
of dust particles.
 Bacteria and germs are removed by ultraviolet lights.
Types of Air-Filters
Filters are classified as:
1. Dry Filters
2. Viscous-Filters
3. Wet Filters
4. Electric Filters

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1. Dry Filters:
 Dry filters are available in two forms as cleanable filters and
throw-away filters.
 Usually made of cloth, paper, or glass wool.These filters
trap the dust in the air when passes through these filters.
 Dry filters are capable of collecting 99% or more dusts up to
0.5 micron size.
 The dry filters have limited dust holding capacity . So it
cannot be used where the concentration of dust in the air is
higher than 2.5 grams per 1000 m3 of air.
 This filter can be used for all ordinary applications.

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 Dry filters are expensive thanViscous filter but have better
efficiency.
 Efficiency of dry filter is as good as electric filters.
Figure: Dry Filters

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2. Viscous Filters:
 These filters are made of glass wool, plastic fiber in the
form of pad and is impregnated with viscous oil.
 These filters can be washed in gasoline and can be reused
again.
 Low cost and costless maintenance.
 Low filtration efficiency and not suitable for application
where fine dust is a problem.
 So these filters are used as pre-filter for the dry filters
which are costly.

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Figure:Viscous Filters

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3. Wet Filters:
 This is the combination of water spray and any other type of
filter like dry or viscous filter.
 Dust particles are first wetted by water spray and then
passed through the filters.
 Due to the additional weight of water, dust particles will fall
down.
Figure:Wet Filter

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4. Electronic Filter
 Air is passed through the pair of oppositely charged
conductors of very high voltage ( 8000 to 15,000 V) and
becomes ionized. Both positive and negative ions are formed.
 This ionized air is further passed through the collection
chamber which consists of a set of metal plates. These metal
plates connected to the of positive and negative terminals of
DC supply.
 Positive ions are attracted by negative plates and negative ions
are attracted by positive plates.
 Collector plates are cleaned periodically by washing with hot
water spray.

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 The advantage: low cost in the long run, low maintenance cost,
requires small installation space, filter is effective for small
particles like smoke or mist.
 Disadvantage: Danger of electrocution, protection is required,
pre-filter is necessary to reduce load on this filter.

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Humidifiers
 Humidification of air is done by using one of the four methods:
1. Injecting the steam
2. Atomizing the water
3. Evaporating the water
4. By air washing
 Water treatment is necessary before it is used for humidifying
the air in air conditioning system.

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Humidifying by Steam Injection
 In this system, humidification is done by injecting the steam
into the air in the duct of air-handling unit (AHU). The steam
condenses to very fine mist and evaporates almost instantly into
vapor as air is passed. This process does not raise the
temperature to a appreciable amount.
 If a central plant steam is not available, a steam generator must
be installed to produce steam for humidification.
 Advantage: if extra capacity steam is available, the initial cost of
this system is low.
 It does not carry any harmful impurities.
 Compact in size.

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 Drawbacks: It is not used for comfort air conditioning as it
carries odors which are unpleasant.
 If not properly located, it may increase the temperature of air.
Figure: Steam Humidifier

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AtomizationType Humidifier
 An effective humidification can be achieved by using
compressed air to draw water by aspiration from a supply tank
and to blow it in the form of fine mist into the duct carrying
the air.
 This humidifier does not add heat to the air. The heat required
for the evaporation of water is taken from the air by causing
decrease in DBT of the air.
 Atomizing humidifier capacity range from 5 to 50 kg/hr.
 This humidifier is noisy due to high velocity of air and therefore
preferred for industrial building where compressed air is
readily available.

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Figure:Atomization type humidifier

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ImpactType Humidifier
 This is most practical type of humidifier. It consists of water
nozzle through which water is issued against a hard target
which produces a fine spray.
 Air is forced into the chamber to pick-up the water by
evaporation from the spray.
 Eliminators are placed in the path of air-stream to prevent the
water droplets carried with air.

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Figure: Impact type humidifier

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Pan and CoilType Humidifier
 This system consists of a pan which carries water and heating
element. The heating device may be electric coil, a steam coil or
a hot water.
 The heating coil warms the water and evaporates it which will
mix in the air.
Figure: Pan and CoilType Humidifier

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SprayType AirWasher
 Most effective and commonly type of humidifier.
Figure:Air-washer humidifier

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Dehumidifier
SprayType Dehumidifier
 This type of humidifier is used extensively in large installation
where the duct system carries the air from a central unit to
various rooms. The required temperature of the spray as per
DBT of air can be controlled by adjusting flow of refrigerant.

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Figure: Spray type humidifier

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Fans and Blowers
 Fans and blowers are used to distribute the conditioned air
through the equipments and ductwork to various rooms.
 When the quantity of air handled is considerably large, then it is
know as blower. The word fan is used for small air handling
capacity.
 There are different types of fans: axial flow type, centrifugal flow
type also known as radial flow type.
 Axial flow type fans are not used in duct air-conditioning as they
cannot develop high pressures. Centrifugal type is used for the
duct system as they can develop considerable pressure compared
to axial fans.
 Fan is selected based on the volume of air required and system
resistance.

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Grills and Registers
 Grills and registers are used to supply the conditioned air
properly to an air-conditioned room.
 It is a decorative covering for an outlet of conditioned air or
intake of air from the air-conditioned room. A grill provided
with damper is known as register.
 Grills and registers can be located at ceiling or at wall.

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Ducts
 Ducts are used through which conditioned air is delivered
to the room and is also used for the return air.
 Galvanized mild steel sheet is used for the fabrication of
duct. Usually duct is fabricated in rectangular or circular
shape.
 For insulation purpose, duct is covered with fiber glass.

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Air Handling Unit (AHU)
 The Air Handling Unit is the combination of air filters, heating
or cooling coils, humidifier/dehumidifier, mixing chamber,
and casing.
 AHU supplies the conditioned air to the room and air is
returned to the AHU again from the room.

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END

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