air conditioning cycles, condenser, vapour compression refrigration system.

1. ME 6404 – THERMAL ENGINEERING
UNIT – V – AIR CONDITIONING
Presented by
P.SELVAN,
Assistant Professor,
Dept. of Mechanical Engineering,
Kamaraj College of Engineeriechnology,
Virudhunagar

2. COMPRESSOR
• Superheated Vapor:
– Enters as low press, low temp vapor
– Exits as high press, high temp vapor
• Temp: creates differential (DT) promotes heat transfer
• Press: Tsat allows for condensation at warmer temps
• Increase in energy provides the driving force to circulate
refrigerant through the system

3. 3
COMPRESSORS
There is a large variety of compressors.
Some of variations are:
The compressor manufacturer
Piston, vane, or scroll type
The piston and cylinder arrangement
How the compressor is mounted
Style and position of ports
Type and number of drive belts
Compressor displacement
Fixed or variable displacement

4. 4
COMPRESSOR OPERATION
The compressor increases the refrigerant
pressure about five to ten times. This
increases the temperature so heat can
leave the refrigerant in the condenser.
Out/Discharge: High
Pressure, about 200 psi
& High Temperature,
above ambient
In/Suction: Low
Pressure, about 30 psi
& Low Temperature,
close to freezing

5. 5
PISTON COMPRESSORS
This two-cylinder compressor
uses a crankshaft to move the
pistons up and down.
Refrigerant flow is controlled
by the suction and discharge
reeds in the valve plate.
Reed Valve Plate
Piston
Connecting Rod
Crankshaft
Shaft Seal

6. 6
SCOTCH YOKE COMPRESSORS
A Scotch yoke compressor has two pairs of
pistons that are driven by a slider block on
the crankshaft. The pistons are connected
by a yoke.
Pistons Yoke
Discharge Reed
Suction Reed

7. 7
SCROLL COMPRESSORS
Fixed
Scroll
Orbiting
Scroll
Shaft Seal
Clutch Assembly
The orbiting scroll is driven by the
crankshaft and moves in a small
circular orbit. The fixed scroll
remains stationary

8. 8
SWASH PLATE COMPRESSORS
Pistons
Swash Plate
Shaft Seal
Clutch Assembly
The swash plate is mounted at an
angle onto the drive shaft. It drives
three double-ended pistons. Two sets
of reeds control the refrigerant flow in
and out of the cylinders,
Reed Plate

9. 9
VANE COMPRESSORS
Rotor
Vane
Shaft Seal
Discharge
Reed
The rotor is driven by the clutch and
driveshaft. The vanes move in and out of
the rotor to follow the outer wall to
pump refrigerant.

10. 10
WOBBLE PLATE COMPRESSORS
Wobble Plate Bearing Angle/Drive Plate
Piston
The wobble plate does not rotate; it just wobbles, being driven by the angled drive plate that
does rotate. Variable displacement, wobble plate compressors can change the angle of the drive
plate, and this changes piston stroke and compressor displacement. Most wobble plate
compressors have 5 to 7 pistons.

11. 11
CONDENSER OPERATION
Hot, high pressure gas is pumped from the compressor to enter the condenser.
The gas gives up its
heat to the air passing through the condenser. Removing heat from the hot gas causes it
to change state and become liquid.

12. 12
CONDENSER TYPES
Condensers A and C are round
tube, serpentine condensers.
Condenser B is an oval/flat tube,
serpentine condenser.
Condenser D is an oval/flat tube,
parallel flow condenser.
Flat tube condensers are more
efficient.

13. 13
SERPENTINE CONDENSER
Refrigerant flows from the
upper inlet to the bottom
outlet through two tubes.
These tubes wind back and
forth though the condenser.

14. 14
PARALLEL FLOW CONDENSER
Refrigerant flows from the upper inlet to the
bottom outlet through groups of parallel
tubes. Some carry refrigerant from the right
to the left, and others move it back to the
right side.

15. 15
HEAT EXCHANGERS
 Condensers have to move heat from the refrigerant to the air.
 Evaporators must move heat from air to the refrigerant.
 Both require a lot of contact area for both air and refrigerant.
 Both require free movement of air and refrigerant.

16. 16
RECEIVER DRYERS
A receiver dryer is mounted in the liquid line of a
TXV system. It is used to:
•to store a reserve of refrigerant.
•hold the desiccant bag that removes water from
the refrigerant.
•filter the refrigerant and remove debris particles.
•provide a sight glass so refrigerant flow can be
observed.
•provide a location for switch mounting.
Barb
Connections,
Note Sight
Glass
Male Flare
Connections
Male O-ring
Connections,
Note Switch

17. PROBLEM-1
An ideal vapor-compression refrigeration cycle operates at steady state
with Refrigerant 134a as the working fluid. Saturated vapor enters the
compressor at 2 bar, and saturated liquid exits the condenser at 8 bar.
The mass flow rate of refrigerant is 7 kg/min. Determine
a)the compressor power, in kW
b)the refrigerating capacity, in tons
c)the coefficient of performance

18. Analyzing vapor-Compression
Refrigeration Systems

19. Ideal Refrigeration Cycle
An ideal cycle has no irreversibilities within the evaporator and
condenser, and there are no frictional pressure drops. Compression
is isentropic. The T-s diagram is shown on the next slide.
Process 1-2s: Isentropic compression of the refrigerant;
Process 2s-3: Heat transfer from refrigerant to outside air, at
constant pressure;
Process 3-4: Throttling process to a two-phase mixture at lower
pressure;
Process 4-1: Heat transfer to the refrigerant as it flows at constant
pressure through the evaporator;

20. Analyzing vapor-Compression
Refrigeration Systems
As the refrigerant passes through the evaporator, the heat
transfer per unit mass of refrigerant flowing is:
A ton of refrigeration is equal to 200 Btu/min or 211 kJ/min.
Work done by compressor per unit mass flow of refrigerant is
flowmasstrefrigeranismhh
m
Qin



);( 41 −=
);( 12 hh
m
Wc
−=



21. Analyzing vapor-Compression
Refrigeration Systems
Heat rejected by the refrigerant:
Expansion valve:
Coefficient of performance:
);( 32 hh
m
Qout
−=


34 hh =
;
)(
)(
12
41
hh
hh
m
W
m
Q
c
in
−
−
==




β

22. SOLUTION
Let us first get the properties at each state in the cycle.
State 1: p1 = 2 bar, sat vapor. h1 = 241.30 kJ/kg, s1 = 0.9253
kJ/kg.K
State 2: p2 = 8 bar, s2 = s1, h2 = 269.92 kJ/kg
State 3: p3 = 8 bar, sat. liquid, h3 = 93.42 kJ/kg
State 4: Throttling process, h4 = h3 = 93.42 kJ/kg
a)The compressor power is:
CW m h h kg s kJ kg kW( ) ( / )( . . ) / .= − = − =2 1
7
269 92 241 30 3 34
60
 

23. Solution
b) The refrigerating capacity is
c)The coefficient of performance is
inQ m h h kg kJ kg tons kJ
tons
( ) ( /min)( . . ) / / / /min
.
= − = −
=
1 4 7 241 30 93 42 211
4 91
 
h h
h h
( ) ( . . )
.
( ) ( . . )
− −
β = = =
− −
1 4
2 1
241 30 93 42
5 17
269 92 241 30

24. • Problem-2
• A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapor-compression refrigeration cycle
between 0.12 and 0.7 MPa. The mass flow rate of the refrigerant
is 0.05 kg/s. Show the cycle on a T-s diagram with respect to
saturation lines. Determine:
• a) the rate of heat removal from the refrigerated space,
• b) the power input to the compressor,
• c) the rate of heat rejection to the environment, and
• d) the coefficient of performance.

25. Solution
Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c)
4.06

26. Problem-3
Consider an ideal refrigeration cycle which uses R-12 as
the working fluid. The temperature of the refrigerant in
the evaporator is –20°C and in the condenser it is 40°C.
The refrigerant is circulated at the rate of 0.03kg/s.
Determine the coefficient of performance and the
capacity of the plant in rate of refrigeration.
For each control volume analyzed, the thermodynamic
model is the R-12 tables. Each process is SSSF with no
change in kinetic or potential energy.

27. Control volume: Compressor.
Inlet state: T1 known, saturated vapor; state fixed.
Exit state: P2 known(saturation pressure at T3).
At T3=40°C
2 1
2 1
cw h h
s s
= −
=
2
1
1 2
2
2
2 1
0.9607
178.61 /
0.7082
50.8
211.38 /
32.77 /
g
o
c
P P MPa
h kJ kg
s s
T C
h kJ kg
w h h kJ kg
= =
=
= =
=
=
= − =

28. Control volume: Expansion valve.
Inlet state: T3 known, saturated liquid; state fixed.
Exit state: T4 known.
Control volume: Evaporator.
Inlet state: State 4 known.
Exit state: State 1 known.
3 4 74.53 /h h kJ kg= =
1 4 104.08 /
3.18
3.12
L
L
c
q h h kJ kg
q
w
Capacity kW
β
= − =
= =
=

29. • Problem-3
Consider a 300 kJ/min refrigeration system that operates on an ideal
vapor-compression refrigeration cycle with refrigerant-134a as the
working fluid. The refrigerant enters the compressor as saturated
vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-
s diagram with respect to saturation lines, and determine the:
a) quality of the refrigerant at evaporator inlet,
b) coefficient of performance, and
c) power input to the compressor.

30. PSYCHROMETRY & AIR PROCESSES
Psychrometry is the science dealing with the physical laws of
air – water mixtures.
When designing an air conditioning system, the temperature and
moisture content of the air to be conditioned, and the same
properties of the air needed to produce the desired air
conditioning effect, must be known.
Once these properties are known, the air conditioning task can
be determined.

31. COMPOSITION OF AIR
ATMOSPHERIC AIR

Atmospheric air is not completely dry but a mixture of dry air and
water vapor.

In atmospheric air, the content water vapor varies from 0 to 3% by
mass.

The processes of air-conditioning and food refrigeration often
involve removing water from the air (dehumidifying), and adding
water to the air (humidifying).

32. THERMAL PROPERTIES OF MOIST AIR
(1) Dry bulb temperature t:

Dry bulb temperature is the temperature of the air, as measured by an
ordinary thermometer.

The temperature of water vapor is the same as that of the dry air in
moist air.

Such a thermometer is called a dry-bulb thermometer in
psychrometry, because its bulb is dry.
(2) Wet bulb temperature tWB:

Wet bulb temperature is thermodynamic adiabatic temperature in an
adiabatic saturation process, and measured by a wet bulb
thermometer.

The wet bulb temperature will be discussed in the next paragraph.

33. THERMOMETERS
Dry Bulb
Wet Bulb

34. (3) Dew point temperature tDP:

When the unsaturated moist air is
cooled at constant vapor pressure
or at constant humidity ratio, to a
temperature, the moist air becomes
saturated and the condensation of
moisture starts, this temperature is
called dew point temperature of the
moist air.

Condensation occurs at the Dew Point
Temperature

35. (4) Relative humidity Ф:
 Relative humidity is defined as the ratio of the mole fraction of the water
vapor in a given moist air to the mole fraction of water vapor in a saturated
moist air at the same temperature and the same atmospheric pressure.
 Relative humidity is usually expressed in percentage (%).
 From the ideal gas relations, relative humidity can be expressed as
 ---- the mole fraction of the water vapor in moist air;
 ---- mole fraction of water vapor in a saturated moist
air at the same temperature;
satw
w
satw
w
P
P
x
x
,,
==φ
wx
satwx ,
Relative
Humidity
(%)
Amount of moisture that a given amount
of air is holding
=
Amount of moisture that a given
amount of air can hold

36. (5) Degree of Saturation μ:
 Degree of saturation is defined as the ratio of the
humidity ratio of moist air w to the humidity ratio of
saturated moist air wsat at the same temperature and
atmosheric pressure.
(6) Humidity ratio (Moisture Content) w:
 The humidity ratio is the mass kg of water vapor
interspersed in each kg of dry air.
 It should be noted that the mass of water refers only to
the moisture in actual vapor state, and not to any
moisture in the liquid state, such as dew, frost, fog or
rain.
 The humididy ratio, like other several properties to be
studied- enthalpy and specific volume-is based on 1kg
of dry air.
w
satw
sat PB
PB
w
w
−
−
×== ,
φµ

37. (7) Specific Volume/Moist Volume v:
 Specific volume of moist air v , m3
/kgdry is defined as the total volume of
the moist air (dry air and water vapor mixture) per kg of dry air.
(8) Specific Enthalpy:
 Specific enthalpy of moist air h (kJ/kgdry) is defined as the total
enthalpy of the dry air and water vapor mixture per kg of dry air.
 Enthalpy values are always based on some datum plane.
 Usually the zero value of the dry air is chosen as air at 0℃, and the
zero value of the water vapor is the saturated liquid water at 0℃.

38. ADIABATIC SATURATION
PROCESS
An unsturated air, which has dry bulb
temperature t1, humidity ratio w1 and
enthalpy h1, flows through a spray of
water, as shown in Figure.

The spray can provide enough surface
area so that the air leaves the spray
chamber in equilibrium with the water,
with respect to both the temperature
and the vapor pressue.

In order to perpetuate the process, it is
necessary to provide makeup water
(w2-w1) to compensate for the amount
of water evaporated into the air.

The temperature of the makeup water
is the same as that in the sump.

If the device is adiabatic, then the
process is called adiabatic saturation
process.

39. THERMODYNAMIC WET BULB
TEMPERATURE

In an adiabatic saturation process, the temperature of the water
in the sump is called the thermodynamic wet bulb temperature
tWB of the air , or simply the wet bulb temperature of the air.

An adiabatic saturation process is a constant wet bulb
temperature process, the wet bulb temperatures of air at the
inlet and the outlet are same, and they are equal to tWB.

That is 21 ⋅⋅ = WBWB tt

40. MEASUREMENT OF WET BULB
TEMPERATURE

A simple wet bulb thermometer used to measure the wet bulb temperature
is based on the principle of the adiabatic saturation process.

It is an ordinary thermometer being wrapt with a cloth sleeve of wool or
flannel, around its bulb.

The cloth sleeve should be clean and free of oil and thoroughly wet with
clean fresh water.

The water in the cloth sleeve evaporates as the air flows at a high velocity
(≥2.5m/sec).
 The evaporation, which takes the heat from the thermometer bulb, lowers
the temperature of the bulb.
 The thermometer indicates approximately the wet bulb temperature tWB.

The difference between the dry-bulb and wet-bulb temperatures is called
the wet-bulb depression.

If the air is saturated, evaporation cannot take place, and the wet-bulb
temperature is the same as the dry-bulb temperature; and the wet-bulb
depression equals zero.

41. PSYCHROMETRIC CHART

44. PSYCHROMETRIC CHART

A psychrometric chart graphically
represents the thermodynamic properties
of moist air and it is very useful in
presenting the air conditioning processes.

The psychrometric chart is bounded by
two perpendicular axes and a curved line:
• 1) The horizontal ordinate axis
represents the dry bulb temperature
line t, in℃
• 2) The vertical ordinate axis
represents the humidity ratio line w,
in kgw/kgdry.air
• 3) The curved line shows the
saturated air, it is corresponding to
the relative humidity Ф=100% .

45. PROPERTIES IN PSYCHROMETRIC CHART

The psychrometric chart incorporates seven parameters and properties.
They are dry bulb temperature t , relative humidity Ф , wet bulb
temperature tWB, dew point temperature tDP , specific volume v, humidity ratio
w and enthalpy h.
a. Dry-bulb temperature t is shown along the bottom axis of
the psychrometric chart.
The vertical lines extending upward from this axis are
constant- temperature lines.
b. Relative humidity lines Ф are shown on the chart as curved lines
that move upward to the left in 10%
increments.
The line representing saturated air ( Ф= 100% ) is the
uppermost curved line on the chart. And the line of Ф = 0% is a
horizontal ordinate axis itself.
c. Wet-bulb temperature tWB : On the chart, the constant wet-bulb lines
slope a little upward to the left, and the wet bulb temperature is read
following a constant wet-bulb line from the state-point to the
saturation line.

46. PROPERTIES IN PSYCHROMETRIC CHART
d. Dew point temperature tDP : This temperature is read by following a
horizontal line from the state-point to the saturation line.
e. Specific volume v: It is shown from the constant-volume lines
slanting upward to the left.
f. Humidity ratio w: it is indicated along the right-hand axis of the
chart.
g. Enthalpy h: It is read from where the constant enthalpy line crosses
the diagonal scale above the saturation curve. The constant enthalpy
lines, being slanted lines, are almost coincidental as the constant
wet-bulb temperature lines.

Only two properties are needed to characterize the moist air because
the point of intersection of any two properties lines defines the state-
point of air on a psychrometric chart.

Once this point is located on the chart, the other air properties can be
read directly.

47. AIR CONDITIONS ON THE
PSYCHROMETRIC CHART
wet bulb
dew point
30
Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)
HumidityRatio(grains/lbofdryair)HumidityRatio(grains/lbofdryair)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
40
60
80
100
120
140
160
180
200
220
humidity ratio
dry bulb
relative humidity

48. AIR CONDITIONS ON THE
PSYCHROMETRIC CHART
• There is one more property of air that is displayed on the psychrometric chart
—specific volume. It is defined as the volume of one Kg of dry air at a specific
temperature and pressure. As one pound of air is heated it occupies more
space—the specific volume increases.
14.0
13.0
13.5
14.5
specific volume lines
(cubic feet / pound of dry air)
30
Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)
HumidityRatio(grains/lbofdryair)HumidityRatio(grains/lbofdryair)
25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110
20
40
60
80
100
120
140
160
180
200
220

49. COMFORT ZONE
The specifications for human comfort


Relative humidity:

Air flow rate:
dry-bulb temperature
humidityratio
wet-bulb temperature
80°F
[26.7°C
]
70°F
[21.2°C
]
60 %
RH
40 % RHA
CtC 00
2722 ≤≤
%60%40 ≤≤ φ
sec/25.0 m≈

50. AIR HANDLING PROCESSES
1. Sensible Heating
2. Sensible Cooling
3. Humidification
4. Dehumidification
5. Cooling-Dehumidification
6. Heating-Humidification

51. SENSIBLE COOLING
• The sensible cooling happens when the air is cooled without altering the specific
humidity. During this process, the relative humidity of the air will increase.
• The sensible cooling can only take place under the condition when the temperature
of the cooling coil is not below the dew point temperature of the air being
processed.

52. SENSIBLE HEATING
• The sensible heating is similar to sensible cooling, but with the dry bulb temperature
increasing.
• It should be noted that there should be no water within the heating system because the
evaporation of the water will increase the specific humidity of the air.

53. HUMIDIFICATION
• In humidification process the moisture is added to air without changing the
dry-bulb temperature, the air condition moves upward along a dry-bulb
temperature line.

54. DEHUMIDIFICATION
• In dehumidification process the moisture is removed from the air without
changing its dry-bulb temperature, the air condition moves downward along
a dry-bulb temperature line.

55. CHANGE IN DRY BULB
TEMPERATURE AND MOISTURE

56. COMBINED PROCESSES

57. HEAT AND MASS TRANSFER
BETWEEN MOIST AIR AND SOLID
SURFACE

There are a lot of heat exchangers in refrigeration and air conditioning
systems.

Moist air makes heat transfer and/or mass transfer with the solid
surface of the heat exchangers.

If the solid surface is dry during the process, there is heat transfer
only.
However, if the solid surface is wetted, there are both heat transfer
and mass transfer.
1. Sensible heat transfer between moist air and solid surface
 The sensible heat transfer rate dqsensible, KW, from the wetted surface to the
moist air is )( assensible ttdAdq −⋅⋅= α

58. HEAT AND MASS TRANSFER
BETWEEN MOIST AIR AND SOLID
SURFACE
2. Mass transfer between moist air and solid surface
 The mass transfer rate of water vapor dm, kgw/s, from the wetted
surface to the moist air is
3. Latent heat transfer between moist air and solid surface
 The latent heat transfer rate dqlatent , KW, from the wetted surface to the
moist air is
)( as wwdADdm −⋅⋅=
fgaslatent hwwdADdq )( −⋅⋅=

61. Process: from one state to another

64. Human comfort zone

65. FORCED VENTILATION

66. EVAPORATIVE COOLING
Fogging

67. EVAPORATIVE COOLING
Pad and Fan system

68. APPROACH TO RELEASE THERMAL
STRESS

69. TYPES OF AIR CONDITIONERS
1. Room air conditioners
 Window type air conditioner
 Split type air conditioner
1. Central air conditioning systems
2. Heat pumps
3. Evaporative coolers

70. TYPES OF AIR CONDITIONERS
• Room air conditioners use the standard compressor cycle and are sized to cool
just one room. To cool an entire house, several room units are necessary.
• Central air conditioning systems also operate on the compressor cycle
principle and are designed to cool the entire house. The cooled air is distributed
throughout the house using air ducts, which may be the same ducts that are
used by the heating system.
• Heat pumps, described in the heating section, use the compressor cycle, but it is
reversible. In the summer, the heat pump transfers heat from indoors to
outdoors. In the winter, the heat pump transfers heat from outdoors to indoors.
Heat pumps may be powered by electricity or natural gas.
• Evaporative coolers, also called "swamp coolers", do not use the compressor
cycle. Instead, they cool air by blowing it over a wet surface. You have
experienced this phenomenon when you get out of a swimming pool while a
breeze is blowing. As water evaporates, it absorbs heat from the air. Evaporative
cooling systems depend on the ability of air to absorb moisture, and so they only
work in dry climates such as the Southwest U.S.

71. TYPES OF AIR CONDITIONERS

72. WINDOW TYPE AIR CONDITIONER

73. WINDOW TYPE AIR CONDITIONER
• Window type air conditioners cool rooms rather than the entire home.
• Less expensive to operate than central units
• Their efficiency is generally lower than that of central air conditioners.
• Can be plugged into any 15- or 20-amp, 115-volt household circuit that
is not shared with any other major appliances
• It is a packaged AC contains all components in single unit.

74. SPLIT TYPE AIR CONDITIONER
• Split type air conditioners is like
window type AC but it contains
two units such as indoor and
outdoor units .
• An outdoor metal cabinet contains
the condenser and compressor,
and an indoor cabinet contains the
evaporator and expansion device.

75. 75
CENTRAL AIR CONDITIONING
• Circulate cool air through a system of supply and
return ducts. Supply ducts and registers (i.e.,
openings in the walls, floors, or ceilings covered by
grills) carry cooled air from the air conditioner to
the home.
• This cooled air becomes warmer as it circulates
through the home; then it flows back to the central
air conditioner through return ducts and registers

76. CENTRAL AIR CONDITIONING
SYSTEMS
• Outside air is drawn in,
filtered and heated before it
passes through the main air
conditioning devices. The
colored lines in the lower part
of the diagram show the
changes of temperature and of
water vapor concentration
(not RH) as the air flows
through the system.

77. TOTAL AIR CONDITIONING
• Variable fresh air mixer and dust and pollutant filtration.
• Supplementary heating with radiators in the outer rooms and individual mini
heater and
• Humidifier in the air stream to each room.