Chapter 6 Psychrometry & Air-Conditioning

AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech.
PSYCHROMETRY &
AIR-CONDITIONING
Prof. Aniket Suryawanshi
Asst. Prof. Automobile Engg. Dept.
R. I. T. Rajaramnagar

Outline
• Psychrometry terms and Psychrometry relations.
• Psychrometry charts and Psychrometry processes.
• Air conditioning components and equipment.
• Air conditioning systems and its controls.
• Human comfort and factors affecting on A. C.
• Load estimation, duct design for automobiles.

Dry-Bulb Temperature (DBT): It is the temperature of air measured by an ordinary
thermometer.
It is denoted by – Tdb
Wet-Bulb Temperature (WBT): It is the temperature measured by a thermometer when
the bulb is covered by a wetted wick and exposed to a current of rapid moving air.
It is denoted by – Twb
Relative Humidity (RH): It is the ratio of the partial pressure of water vapor in the
mixture to the saturated partial pressure at the dry bulb temperature.
It is expressed in percentage.
Specific Humidity (Humidity Ratio): It is the ratio of the mass of water vapour per unit
mass of dry air in the mixture of vapour and air.
It is expressed as grams of water per kg of dry air.
Psychrometric Terms

Dew Point Temperature (DPT): It is the temperature to which air must be cooled at
constant pressure in order to cause condensation of any of its water vapour. It is equal to
steam table saturation temperature corresponding to the actual partial pressure of water
vapour in the air.
It is denoted by – Tdp
Specific Enthalpy: It is the is the measure of the total thermal energy in moist air.
It is expressed as energy per unit weight of air (kJ/kg of dry air).
Specific Volume: It represents the space occupied by a moist air.
It is expressed as volume per unit weight of dry air. (m3/kg of dry air).
Psychrometric Terms

Atmospheric air: Air in the atmosphere containing some water vapor (or moisture).
Dry air: Air that contains no water vapor.
Saturated air: The air saturated with moisture.
Water vapor in air behaves as if it existed alone and
obeys the ideal-gas relation Pv = RT. Then the
atmospheric air can be treated as an ideal-gas mixture:
Pa =Partial pressure of dry air
Pv = Partial pressure of vapor (vapor pressure)
Note: For saturated air, the vapor pressure is equal to the saturation pressure of
water.
Water vapor in the air plays a major role in human comfort. Therefore, it is an
important consideration in air-conditioning applications.
DRY AND ATMOSPHERIC AIR

Psychrometric Relations
Absolute or Specific Humidity (Humidity Ratio): The mass of water vapor present in
a unit mass of dry air.

Relative Humidity: The ratio of the amount of moisture the air holds (mv) to the maximum
amount of moisture the air can hold at the same temperature (mg).
Degree of Saturation: The ratio of actual humidity ratio to the humidity ratio of saturated
air at the same temperature and total pressure.

The enthalpy of moist (atmospheric) air is
expressed per unit mass of dry air, not per unit
mass of moist air.
Specific Enthalpy:

Example 1
A 5-m x 5-m x 3-m room contains air at 25°C and 100 kPa at a relative
humidity of 75 percent.
Determine:
a) the partial pressure of dry air,
b) the specific humidity,
c) the enthalpy per unit mass of the dry air,
d) the mass of the dry air and water vapor in the room.

Example 1…. cntd

Question:
In cold weather, condensation frequently occurs on the inner surfaces of the windows due to
the lower air temperatures near the window surface. Consider a house, that contains air at
20°C and 75 percent relative humidity. At what window temperature will the moisture in the
air start condensing on the inner surfaces of the windows?
Analysis:
Example 2
The saturation pressure of water at 20°C is Psat 2.3392 kPa …..(Table A–4).

The dry- and the wet-bulb temperatures of atmospheric air at 1 atm (101.325 kPa) pressure
are measured with a sling psychrometer and determined to be 25°C and 15°C, respectively.
Determine
(a) the specific humidity,
(b) the relative humidity, and
(c) the enthalpy of the air.
Example 3

Example 3…. cntd

• The air conditioning processes
are best understood by studying
Psychrometry.
• Psychrometry is the study of
atmospheric air and its
associated water vapor.
• A Psychrometric chart
represents physical and thermal
properties of moist air in a
graphical form.
Psychrometry Charts

Psychrometry – Dry Bulb Temp

Psychrometry – Wet Bulb Temp

Psychrometry – Saturation Line

Psychrometry – Relative Humidity

Psychrometry – Humidity Ratio

Psychrometry – Dew Point Lines

Psychrometry – Specific Enthalpy

Psychrometry – Specific Volume

A room contains humid air Tdb = 25°C and Twb =20°C.
Calculate:
(a) Relative Humidity
(b) Dew Point Temperature (Tdp)
(c) Humidity Ratio
(d) Specific Volume
(e) Specific Enthalpy
Example 4

57. 5 KJ/kg
of d. a.
0.865 m3/kg
of d. a.
0.0128
kg/ kg
of d. a.
17.8 °C

Psychrometry – Processes

Heating with Humidification
Problems with the low relative humidity resulting from simple heating can be eliminated
by humidifying the heated air. This is accomplished by passing the air first through a
heating section and then through a humidifying section.

AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech.29
The atmospheric air at Tdb1 = 25°C and Twb1 =12°C is flowing at a rate of 100
m3/min through a duct. The dry saturated steam at 100°C is injected into the
air stream at a rate of 72 kg/hr. calculate the specific humidity, DBT, WBT,
relative humidity and enthalpy of air leaving the duct.
Example 5

34.1 KJ/kg
of d. a.
0.848 m3/kg
of d. a.
0.034 kg/
kg of d.
a.
Example 5

34.1 KJ/kg
of d. a.
0.848 m3/kg
of d. a.
0.034 kg/
kg of d. a.
0.0135 kg/
kg of d. a.
61.33 KJ/kg
of d. a.
0.866 m3/kg
of d. a.
DBT=27°C
Example 5

Cooling with Dehumidification
The specific humidity of air remains constant during a simple cooling process, but its
relative humidity increases. If the relative humidity reaches undesirably high levels, it
may be necessary to remove some moisture from the air, that is, to dehumidify it. This
requires cooling the air below its dew-point temperature.

Example 6
Air at 40°C DBT and 60 % RH is cooled to 25°C DBT. It is achieved by
cooling and dehumidification. Air flow rate is 40 m3/min. Using
psychrometric chart
Calculate:
a) The dew point temperature
b) Mass of water drained out per hr.
c) Capacity of cooling coil
d) By-pass factor
If the apparatus dew-point temperature is 20°C, find the by pass factor of
coil.

0.925 m3/kg
of d. a.
0.0285 kg/
kg of d. a.
113 KJ/
kg of d. a.
30.6 °C
71 KJ/ kg
of d. a.
0.018 kg/
kg of d. a.
Example 6

Evaporative Cooling
Water in a porous jug left in an open,
breezy area cools as a result of
evaporative cooling.
In desert (hot and dry) climates, we can
avoid the high cost of conventional cooling
by using evaporative coolers, also known as
swamp coolers.
As water evaporates, the latent heat of
vaporization is absorbed from the water body
and the surrounding air. As a result, both the
water and the air are cooled during the
process.
This process is essentially identical to
adiabatic saturation process.

Air Conditioning
• Air conditioning (often referred to as A/C or AC) is the process of altering the
properties of air (primarily temperature and humidity) to more comfortable
conditions, typically with the aim of distributing the conditioned air to an occupied
space to improve thermal comfort and indoor air quality.
• Air conditioning is defined as a process which cools (or heat) , clean, circulates,
freshen air, and controls its moisture content simultaneously.
• Most of the air conditioning, particularly in India and such other countries, is about
cooling : i.e. removal of heat from an enclosed space.
• In the most general sense, air conditioning can refer to any form of technology that
modifies the condition of air (heating, cooling, (de-) humidification, cleaning,
ventilation, or air movement).

Requirement of Air Conditioning
• Comfort and health of occupants
• Needs of certain industrial processes
• Efficient working of commercial premises/offices
Comfort air conditioning : - Aim at giving max human comfort to
occupants/users of conditioned space
Industrial air conditioning : - The conditioning creates, controls and
maintains such an environment inside the conditioned space, that it
would suit best to the needs of industry.

The four atmospheric condition which affect human comforts are-
Temperature
Humidity
Factors affecting Human Comfort
Temperature control
 Effective temp = 200C to 230C in summer
 Effective Temp = 180C to 220C in winter
 For Indian conditions 250C with 60% relative humidity to 300C with 45 %
relative humidity, with air velocity not exceeding 10 m/min
 During summer dehumidification is done so that relative humidity is 40 to 50%
 During winter humidification is done so that relative humidity is 50 to 60 %
 High velocity of conditioned air may cause greater temperature difference
between outside and inside.
 Velocity = 6 to 9 m/sec is desirable
Air should be free from odour, toxic gases, bacteria and other micro-organisms
Air movements
Air purity

1. Filtration
2. Heating (in winter)
3. Cooling (in summer)
4. Humidification
5. Dehumidification
6. Air circulation or distribution
Essentials of A. C. Systems

Air Conditioning Systems Equipment's
1. Filter
2. Washer
3. Fan
4. Duct
5. Radiator
6. Convector

• Should be capable of removing dust, ash, smoke, bacteria etc
• Should be capable of holding moderate amount of dust without affecting
the efficiency
• Should offer low resistance to flow of air
• Workable under sufficient range of velocities.
• Should afford easy cleaning manually or mechanically
1. Filtration
• Heating is necessary in winter so as to compensate heat loss from room.
• Pre heating of incoming air may be done by passing over air furnaces or
coils through which hot water is circulated.
2. Heating (in winter)
• Cooling of incoming air is necessary in summer.
• Principle of mechanical refrigeration is used.
3. Cooling (in summer)

• Humidification or addition of water is necessary in winter when
air because of low temp has less humidity.
• Incoming air is passed through pans of water or wet cloth strips
4. Humidification
5. Dehumidification
• In this process certain amount of water is extracted from air.
• This is done in summer where incoming air is cooled.
• Adsorbents can be used to adsorb excess moisture from air

6. Air circulation or distribution

Air Conditioning Cycle

Air Conditioning Component

a) Window A. C.
b) Spit A. C.
c) Central A. C.
d) Unitary or “packed” A. C.
Classification based on equipment's arranged:
Air Conditioning Systems

Classification based on major function:- (Purpose)
a) Comfort A.C.: Creating atmospheric conditions conductive to human health,
comfort & efficiency. A.C. in home, offices, stores, restaurants, theatres, hospitals etc.
are of these types.
b) Industrial A.C.: The purpose of these system is to control atmospheric conditions
primarily for proper conduct of research & manufacturing e.g. electronics &
computer industries, paper mills, textile mills, candy factories, photo processing etc.

a) Winter A.C.: system: These systems, when properly designed & installed
maintain indoor atmospheric conditions for winter comfortable, namely
temperature, humidity. Air purity & air movements, when outside temperature is
very cold.
b) Summer A.C.: Maintains indoor condition comfortable when outside
atmospheric conditions are not comfortable, but it is very hot & dry.
c) All year round A.C.: Maintains indoor temperature comfortable throughout
the year, while outside temperature may be very cold or hot.
Classification based on season of the year :

Window Air Conditioning Systems
• These types of AC are designed to be fitted
in window sills.
• A single unit of Window Air Conditioner
houses all the necessary components,
namely the compressor, condenser,
expansion valve or coil, evaporator and
cooling coil enclosed in a single box.
• Since a window AC is a single unit, it takes
less effort to install as well as for
maintenance.
• This is the most commonly used air
conditioner for single rooms.
Advantages
• Single unit air conditioner
• Less effort needed for installation
• Costs lesser in comparison to other varieties

Split Air Conditioning Systems
 These are kits of 2 units, one internal and another external.
 The indoor unit installed inside a room intakes warm air and
throws in cold air.
 The outdoor unit on the other hand is installed out of the
house. It contains the compressor and is linked to the
internal unit via drain pipes and electric cables. This external
unit throws out the warm air.
 Split air conditioners are used for small rooms and halls,
usually in places where window air conditioners cannot be
installed. However, these days many people prefer split air
conditioner units even for places where window air
conditioners can be fitted.
 The split air conditioner takes up a very small space of your
room, looks aesthetically cool and makes very little noise.
Advantages
• Internal unit takes up less space for installation
• Usually more silent than window ACs
• Minimally affect your home decor
• Can be installed in room with no windows

Outdoor unit:
• This unit houses important components of the air
conditioner like the compressor, condenser coil
and also the expansion coil or capillary tubing.
• This unit is installed outside the room or office
space which is to be cooled.
• The compressor is the maximum noise making
part of the air conditioner, and since in the split
air conditioner, it is located outside the room, the
major source of noise is eliminated.
• In the outdoor unit there is a fan that blows air
over the condenser thus cooling the compressed
Freon gas in it.
• This gas passes through the expansion coil and
gets converted into low pressure, low
temperature partial gas and partial liquid Freon
fluid.
There are two main parts of the split air conditioner.

Indoor unit:
• It is the indoor unit that produces the cooling
effect inside the room or the office.
• This is a beautiful looking tall unit usually white
in color, though these days a number of stylish
models of the indoor unit are being launched.
• The indoor unit houses the evaporator coil or the
cooling coil, a long blower and the filter.
• After passing from the expansion coil, the
chilled Freon fluid enters the cooling coil.
• The blower sucks the hot, humid and filtered air
from the room and it blows it over the cooling
coil.
• As the air passes over cooling coil its
temperature reduces drastically and also loses
the excess moisture.
• The cool and dry air enters the room and
maintains comfortable conditions of around 25-
27 degree Celsius as per the requirements.

• Central air conditioning is used for cooling big buildings, houses, offices, entire
hotels, gyms, movie theaters, factories etc.
• If the whole building is to be air conditioned, HVAC engineers find that putting
individual units in each of the rooms is very expensive making this a better
option.
• A central air conditioning system is comprised of a huge compressor that has
the capacity to produce hundreds of tons of air conditioning.
• Cooling big halls, malls, huge spaces, galleries etc. is usually only feasible with
central conditioning units
• central air conditioner unit is an energy moving or converted machines that are
designed to cool or heat the entire house.
• It does not create heat or cool. It just removes heat from one area, where it is
undesirable, to an area where it is less significant.
• Central air conditions has a centralize duct system.
• It uses AC refrigerant (we may know it as Freon) as a substance to absorb the
heat from indoor evaporator coils and rejects that heat to outdoor condenser
coils or vice verse.
Central Air Conditioning Systems

• The chilled water types of central air conditioning
plants are installed in the place where whole large
buildings, shopping mall, airport, hotel, etc.
comprising of several floors are to be air conditioned.
• While in the direct expansion type of central air
conditioning plants, refrigerant is directly used to
cool the room air; in the chilled water plants the
refrigerant first chills the water, which in turn chills
the room air.
• In chilled water plants, the ordinary water or brine
solution is chilled to very low temperatures of about
6 to 8 degree Celsius by the refrigeration plant.
• This chilled water is pumped to various floors of the
building and its different parts.
• In each of these parts the air handling units are
installed, which comprise of the cooling coil, blower
and the ducts. The chilled water flows through the
cooling coil. The blower absorbs return air from the
air conditioned rooms that are to be cooled via the
ducts.
Central Air Conditioning Systems

• The window and split air conditioners are
usually used for the small air conditioning
capacities up to 5 tons.
• The central air conditioning systems are used
for where the cooling loads extend beyond 20
tons.
• The packaged air conditioners are used for the
cooling capacities in between these two
extremes.
• The packaged air conditioners are available in
the fixed rated capacities of 3,5, 7, 10 and 15
tons.
• These units are used commonly in places like
restaurants, telephone exchanges, homes,
small halls, etc.
Packaged Air Conditioning Systems

Winter Air Conditioning Systems
In winter outside temperature is low hence heating of air is needed for comfort.
 Cycle of operations consists of:
• Air cleaning
• Air heating
• Humidification
• Air distribution/circulation

Summer Air Conditioning Systems
In summer outside temp is more and hence cooling of air is required
 Cycle of operations consists of
• Air cleaning
• Air cooling
• Dehumidification
• Air distribution/circulation

Air Conditioning Controlling Systems
1. Manually Controlled Systems
2. Automatic Controlling Systems
3. Semi-Automatic Controlling Systems

Duct Design
Sizes of Ducts 1. Max. Air Velo. without Undue Noise.
2. Max. Air Velo. without Excessive Frictional Loss.
Large Ducts ↓ Frictional Loss.
↑ Space / Investment.
Economic Balance is required…!!!

• Air should be conveyed as directly as possible to save space, power and material
• Sudden changes in directions should be avoided. When not possible to avoid
sudden changes, turning vanes should be used to reduce pressure loss
• Diverging sections should be gradual. Angle of divergence ≤ 200
• Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4
• Air velocities should be within permissible limits to reduce noise and vibration
• Duct material should be as smooth as possible to reduce frictional losses
General Rules for Duct Design

Steps in Duct Design
Step 1 : Layout the Most Convenient System of placing the ducts.
Step 2 : Calculating the Air Requirement at each Duct Outlet.
Step 3 : Determining the Sizes of these Outlet Branches.
Step 4 : Calculate size of each duct by :
a) Assumed Velocity Method.
b) Constant Pr. Drop Method / Equal Friction Method.
c) Balanced Pressure Loss Method.
d) Static Regain Method.
Step 5 : Maximum Resistance α Fan Static Pressure.

Air Conditioning Load Estimation

An Air conditioning system is to be designed for a restaurant with the following data:
Outdoor design condition = DBT=400C, WBT =280C
Inside design conditions = DBT=250C, RH =50%
Solar heat gain through walls, roof, floor =5.87 kW
Solar heat gain through glass =5.52 kW
No. of occupants =25
Sensible heat gain per person=58 W
Latent heat gain per person = 60 W
Internal lightening load =15 lamps of 100 W & 10 CFL tubes of 80 W
Sensible heat gain from other resources = 11.60 kW
infiltration air =15 m3/min
If fresh 25 % air and 75% recirculated air is mixed and passed through the conditioner
coil find the following:
i) Dew point temperature.
ii) Condition of supply air to the room.
iii) Amount of air required in m3/hr.
iv) Capacity of conditioning plant.
Assuming By-pass factor = 0.2
Example on Cooling Load Estimation

WO = 0.0191
Wi = 0.0098
O
I

kW1.50
1000
6025
occupantstodueloadLH
kW1.45
1000
5825
occupantstodueloadSH






0.7871
8.47531.33
31.33
RLHLRSHL
RSHL
RSHF 




kW8.4756.9751.5LoadHeatLatentRoom
kW31.334.5911.62.301.455.525.87LoadHeatSensibleRoom
kW2.31000/10)x(8015)x(100loadlighteningtodueloadSH
kW6.9750.0098)-x(0.019115x50aironinfiltratitodueloadSH
Wi)-(Wo/min)(mrateflowairx50aironinfiltratitodueloadLH
kW4.5925)-x(4015x0.0204aironinfiltratitodueloadSH
Ti)-(To/min)(mrateflowairx0.0204aironinfiltratitodueloadSH
3
3







C0
3 28.75
25x0.7540x0.25Tix0.75Tox0.25
T 




11

WO = 0.0191
Wi = 0.0098
O
I
3
hO = 90
hi = 51
DPT = 10.5 4
h4 = 37
h3 = 60.5
W4 = 0.0088
W3 = 0.0121
Twb4 = 130C
Tdb4 = 14.40C
Tdb3 = 28.750C
Tdbo = 400C
Tdbi = 250C

/hrm8693.11requiredairTotal
/minm144.88requiredairTotal
kW31.33)T-(Tix/min)(mrequiredairTotalx0.0204RSHLorRSCL
airtotalbyabsorbedHeatorLoadHeatSensibleRoom
3
3
4
3



TR19.845kW69.4537)-(60.5x0)(8693.11/6x0.0204RSHLorRSCL
)h-(hx/min)(mrequiredairTotalx0.0204coilcoolingofCapacity 43
3



Thank You !

Chapter 6 Psychrometry & Air-Conditioning

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to Chapter 6 Psychrometry & Air-Conditioning

Similar to Chapter 6 Psychrometry & Air-Conditioning (20)

More from ANIKET SURYAWANSHI

More from ANIKET SURYAWANSHI (10)

Recently uploaded

Recently uploaded (20)

Chapter 6 Psychrometry & Air-Conditioning