Chapter 5 – Refrigeration and Air

conditioning

Refrigeration Definition, Refrigerant, Vapor Compression

Refrigeration/VAR, Properties of good refrigerant,
Performance characteristics of refrigeration system.

1
Refrigeration

BME _ Thermal Systems Applications _ Session 1 2

Contents of this session
1. Introduction to refrigeration
2. Definition
3. Refrigerant
4. Types of refrigerant
5. Vapor Compression Refrigeration/ VAR
6. Air Conditioning system
7. Properties of good refrigerant
8. Performance characteristics of a refrigeration system.

3
Introduction

BME _ Thermal Systems Applications _ Session 1 4

Definition
• Refrigeration is defined as a method of reducing the
temperature of a system below that of the surroundings and
maintain it at the lower temperature by continuously
abstracting the heat from it.

5
 Principle of Refrigeration

SURROUNDING
SYSTEM

Continuous Power to the system

6
Refrigerant
• In a refrigerator, a medium called refrigerant continuously extracts the heat
from the space within the refrigerator which is to be kept cool at
temperatures less than the atmosphere.

• Some examples are :

• Ammonia.
• Freon.
• Methyl Chloride.
• Carbon Dioxide.

7
Parts of a Refrigerator
• Evaporator.

• Condenser.

• Expansion Device.

• Circulating System

8
Parts of a Refrigerator

• Evaporator.

9
Parts of a Refrigerator

• Condenser.

BME _ Thermal Systems Applications _ Session 1 10

Parts of a Refrigerator

• Expansion device.

11
Parts of a Refrigerator

• Circulating system

12
Vapor Compression Refrigeration

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14
Vapor Compression Refrigeration

BME _ Thermal Systems Applications _ Session 1 15

Air Conditioner
Inside Outside
Air Filter

Wall or Window 16
 Properties of a Good Refrigerant:

Thermodynamic Physical Safe Working Other

Properties Properties Properties Properties

1. Boiling Point. 1. Toxicity. 1. COP.

2. Freezing Point 1. Specific 2. Flammability. 2. Odour.
3. Evaporator and Volume. 3. Corrosiveness. 3. Leak.
condenser 2. Specific Heat. 4. Chemical 4. Action with
Pressure. 3. Viscosity. Stability. lubricating oil.
4. Latent heat of
evaporation.

17
 Properties of a Good Refrigerant:

Thermodynamic An Ideal refrigerant should have low boiling temperature at

Properties atmospheric pressure.
An ideal refrigerant must have a very low freezing point
because the refrigerant should not freeze at low evaporator
temperatures.
1. Boiling Point. In order to avoid leakage of the atmospheric air and also to
2. Freezing Point enable detection of leakage of refrigerant, both the evaporator
3. Evaporator and and condenser pressure should be slightly above the
condenser atmospheric pressure.
Pressure.
4. Latent heat of The latent heat of vaporization must be very high so that a
evaporation. minimum amount of refrigerant will accomplish the desired
result.
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 Properties of a Good Refrigerant:

Physical The specific volume of a refrigerant must be very low. Lower

Properties the specific volume of the refrigerant at the suction of
compressor reduces the size of compressor.

A good refrigerant should have low specific heat when it is in

liquid state and high specific heat when it is vaporized. The low
specific heat of refrigerant helps in sub cooling of liquid and
1. Specific
high specific heat of vapor helps in decreasing the
Volume.
superheating of the vapor.
2. Specific Heat.
3. Viscosity. The viscosity of a refrigerant at both liquid and vapor states
must be very low as it improves the heat transfer and reduces
the pumping pressure.
19
 Properties of a Good Refrigerant:

Safe Working A good refrigerant should be non toxic.

Properties

A good refrigerant should be non flammable.

1. Toxicity. A good refrigerant must be non corrosive.

2. Flammability.
An ideal refrigerant must not decompose under operating
3. Corrosiveness.
conditions.
4. Chemical
Stability.

20
 Properties of a Good Refrigerant:

Other The COP of a refrigerator should be high so that the

Properties energy spent in refrigeration will be less.

A good refrigerant should be odorless.

The refrigerants must be such that any leakage can be

1. COP. detected by simple tests.
2. Odour.
3. Leak.
A good refrigerant must not react with the lubricating oil.
4. Action with
lubricating oil.

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PERFORMANCE CHARACTERISTICS OF
REFRIGERATION SYSTEM

22
Refrigeration Effect

• In a refrigeration system, the rate at which the heat is

absorbed in a cycle from the interior space to be cooled is
called refrigeration effect.

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Capacity of Refrigeration
• The capacity of refrigeration is expressed in terms of tons of
refrigeration which is the unit of refrigeration.
A ton of refrigeration is defined as the quantity of heat
absorbed in order to form one ton of ice in 24 hrs when the initial
temperature is 0⁰C.
One (American) ton = 2000 pounds
In SI System,
1 ton of Refrigeration = 210 kJ/min
= 3.5 kW

24
Ice Making Capacity

Ice making capacity is defined as the capacity of the refrigerating

system to make ice beginning from water at room temperature
to solid ice.

It is usually specified in kJ/Hr.

25
Coefficient of Performance

The COP of a refrigeration system is defined as the ratio of heat

absorbed in a system to the work supplied.
𝑄
𝐶𝑂𝑃 =
𝑊
Q = Heat absorbed or removed.
W = Work Supplied.

26
Relative Coefficient of Performance

The ratio of actual COP to the Theoretical COP is known as

Relative COP

𝐴𝑐𝑡𝑢𝑎𝑙 𝐶𝑂𝑃
𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐶𝑂𝑃 =
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝐶𝑂𝑃

27
Chapter 6 – Thermal Systems Applications

Session 2 – Pumps, blowers and compressors, Positive

displacement machines, Turbo machines, Lobe pump,
Gear pump, Scroll pump, Screw pump, Vane pump
and Centrifugal compressor
 Topic Learning Outcomes:

1. Explain the working principle of Refrigeration system

2 Describe the working principle of Air conditioning system

3 Outline the working of a pump & blower

4 Describe the functioning of an air compressor

BME _ Thermal Systems Applications _ Session 2 29

 Content
1. Definition
2. Pump, Blower and compressor
3. Positive displacement machines
4. Dynamic or Turbo Machines
5. Gear Pump
6. Screw Pump
7. Scroll Pump
8. Lobe Pump
9. Vane Pump
10. Centrifugal Compressor

30
 Definition
A pump is a device that moves fluids (liquids
or gases), or sometimes slurries, by
mechanical action, typically converted from
electrical energy into Hydraulic energy.

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PUMPS

32
The purpose of a pump is to add energy to
a fluid, resulting in an increase in fluid
pressure, not necessarily an increase of
fluid speed across the pump.

The purpose of a turbine is to extract

energy from a fluid, resulting in a
decrease of fluid pressure, not
necessarily a decrease of fluid speed
across the turbine.
33
A fan is a gas pump with relatively low pressure rise and high flow rate.
Examples include ceiling fans, house fans, and propellers.

A blower is a gas pump with relatively moderate to high pressure rise and
moderate to high flow rate. Examples include centrifugal blowers and
squirrel cage blowers in automobile ventilation systems, furnaces, and
leaf blowers.

A compressor is a gas pump designed to deliver a very high pressure rise,

typically at low to moderate flow rates. Examples include air compressors
that run pneumatic tools and inflate tires at automobile service stations,
and refrigerant compressors used in heat pumps, refrigerators, and air
conditioners.
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Difference between Fan, Blower and Compressor

ΔP = Rise in pressure
V = Discharge rate
35
Pumps and turbines in which energy is supplied or extracted by a
rotating shaft are properly called turbo machines or dynamic
machines. In dynamic machines, there is no closed volume;
instead, rotating blades supply or extract energy to or from the
fluid.

36
In positive-displacement machines, fluid is directed into a closed volume.
Energy transfer to the fluid is accomplished by movement of the
boundary of the closed volume, causing the volume to expand or
contract, thereby sucking fluid in or squeezing fluid out, respectively.

37
Positive Displacement Pumps

Lobe Pump

Source: https://www.youtube.com/watch?v=mEF3qh-hH-I

Source: https://gfycat.com/decimalsoftcanvasback
38
 Gear Pump

Source: http://bestanimations.com/Science/Gears/Gears4.html
Source: http://processprinciples.com/2012/07/gear-pumps/

39
 Scroll Pump

Source:
http://www.gentecsys.com/Knowledge/KB04_comp_tec
Source: https://gfycat.com/discover/compresor-gifs
h.htm

40
 Screw Pump or Cavity pump

Source:
https://en.wikipedia.org/wiki/Archimed
es%27_screw Source: https://empoweringpumps.com/leistritz-screw-
pump-applications-in-pipelines-refineries-and-chemical-
plants/

41
 Vane pump

Source: https://en.wikipedia.org/wiki/Rotary_vane_pump
Source: https://makeagif.com/gif/rotary-
vane-pump-animation-FwVfPQ

42
Dynamic machines:
Input
Centrifugal pump

43
Source: https://gfycat.com/fataleducatedcolt

44
CENTRIFUGAL PUMP

45
CENTRIFUGAL COMPRESSOR

46
Chapter 5. Thermal Engineering-2

Thermal systems Applications

Refrigeration systems, Air conditioning systems, pumps, blowers and

compressors, and their working principles and specifications.

Study material for Chapter 5:

Chapter 5:
Turbines and Internal combustion engines are power developing thermal
systems where as Refrigeration, air conditioning systems , pumps, blowers
and compressors are power consuming devices.

______________________________________________________________

Pumps:
There are two broad categories of turbomachinery, pumps and turbines. The
word pump is a general term for any fluid machine that adds energy to a
fluid. Some authors call pumps energy absorbing devices since energy is
supplied to them, and they transfer most of that energy to the fluid, usually
via a rotating shaft (Fig. 5.1a). The increase in fluid energy is usually felt as
an increase in the pressure of the fluid. Turbines, on the other hand, are
energy producing devices—they extract energy from the fluid and transfer
most of that energy to some form of mechanical energy output, typically in
the form of a rotating shaft (Fig.5.1b).

Figure 5.1: (a) A pump supplies energy to a fluid, while (b) a turbine extracts energy
from a fluid.

1
The fluid at the outlet of a turbine suffers an energy loss, typically in the
form of a loss of pressure. An ordinary person may think that the energy
supplied to a pump increases the speed of fluid passing through the pump
and that a turbine extracts energy from the fluid by slowing it down. This is
not necessarily the case. Consider a control volume surrounding a pump
(Fig. 5.2).

Figure 5.2 : For the case of steady flow, conservation of mass requies that the mass
flow rate out of the pump must equal the mass flow rate into the pump; for
incompressible flow with equal inlet and outlet cross-secional areas (Dout = Din) , we
conclude that Vout =Vin , Pout >Pin.

We assume steady conditions. By this we mean that neither the mass flow
rate nor the rotational speed of the rotating blades changes with time. (The
detailed flow field near the rotating blades inside the pump is not steady of
course, but control volume analysis is not concerned with details inside the
control volume.) By conservation of mass, we know that the mass flow rate
into the pump must equal the mass flow rate out of the pump. If the flow is
incompressible, the volume flow rates at the inlet and outlet must be equal
as well. Furthermore, if the diameter of the outlet is the same as that of the
inlet, conservation of mass requires that the average speed across the outlet
must be identical to the average speed across the inlet. In other words, the
pump does not necessarily increase the speed of the fluid passing through
it; rather, it increases the pressure of the fluid. Of course, if the pump were
turned off, there might be no flow at all. So, the pump does increase fluid
speed compared to the case of no pump in the system. However, in terms of
changes from the inlet to the outlet across the pump, fluid speed is not
necessarily increased. (The output speed may even be lower than the input
speed if the outlet diameter is larger than that of the inlet.)

The purpose of a pump is to add energy to a fluid, resulting in an increase

in fluid pressure, not necessarily an increase of fluid speed across the
pump.

An analogous statement is made about the purpose of a turbine:

The purpose of a turbine is to extract energy from a fluid, resulting in a

decrease of fluid pressure, not necessarily a decrease of fluid speed across
the turbine.

Fluid machines that move liquids are called pumps, but there are several
other names for machines that move gases (Fig. 5.3).

2
A fan is a gas pump with relatively low pressure rise and high flow rate.
Examples include ceiling fans, house fans, and propellers.
A blower is a gas pump with relatively moderate to high pressure rise and
moderate to high flow rate. Examples include centrifugal blowers and
squirrel cage blowers in automobile ventilation systems, furnaces, and leaf
blowers.
A compressor is a gas pump designed to deliver a very high pressure rise,
typically at low to moderate flow rates. Examples include air compressors
that run pneumatic tools and inflate tires at automobile service stations,
and refrigerant compressors used in heat pumps, refrigerators, and air
conditioners.

Figure 5.3: When used with gases, pumps are called fans, blowers or compressors,
depending on the relative values of pressure rise and volume flow rate.

Pumps and turbines in which energy is supplied or extracted by a rotating

shaft are properly called turbomachines, since the Latin prefix turbo means
“spin.” Not all pumps or turbines utilize a rotating shaft, however. The
hand-operated air pump you use to inflate the tires of your bicycle is a
prime example (Fig. 5.4 a). The up and down reciprocating motion of a
plunger or piston replaces the rotating shaft in this type of pump, and it is
more proper to call it simply a fluid machine instead of a turbomachine. An
old-fashioned well pump operates in a similar manner to pump water
instead of air (Fig. 5.4 b). Nevertheless, the words turbomachine and
turbomachinery are often used in the literature to refer to all types of pumps
and turbines regardless of whether they utilize a rotating shaft or not.

3
Figure 5.4: Not all pumps have a rotating shaft; (a) energy is supplied to this manual
tyre pump by the up and down motion of a person’s arm to pump air; (b) a similar
mechanism is used to pump water with an old –fashioned well pump.

Fluid machines may also be broadly classified as either positive-

displacement machines or dynamic machines, based on the manner in
which energy transfer occurs. In positive-displacement machines, fluid is
directed into a closed volume. Energy transfer to the fluid is accomplished
by movement of the boundary of the closed volume, causing the volume to
expand or contract, thereby sucking fluid in or squeezing fluid out,
respectively. Your heart is a good example of a positive-displacement pump
(Fig. 5.5a). It is designed with one-way valves that open to let blood in as
heart chambers expand, and other one-way valves that open as blood is
pushed out of those chambers when they contract. An example of a positive-
displacement turbine is the common water meter in your house (Fig. 5.5b),
in which water forces itself into a closed chamber of expanding volume
connected to an output shaft that turns as water enters the chamber. The
boundary of the volume then collapses, turning the output shaft some more,
and letting the water continue on its way to your sink, shower, etc. The
water meter records each 360° rotation of the output shaft, and the meter is
precisely calibrated to the known volume of fluid in the chamber.

Figure 5.5: (a) The human heart is an example of a positive displacement pump;
blood is pumped by expansion and contraction of heart chambers called ventricles.
(b) The common water meter in your house is an example of a positive displacement
turbine; water fills and exits a chamber of known volume for each revolution of the
output shaft.

In dynamic machines, there is no closed volume; instead, rotating blades

supply or extract energy to or from the fluid. For pumps, these rotating
blades are called impeller blades, while for turbines, the rotating blades are
called runner blades or buckets. Examples of dynamic pumps include
enclosed pumps and ducted pumps (those with casings around the blades
such as the water pump in your car’s engine), and open pumps (those
without casings such as the ceiling fan in your house, the propeller on an
airplane, or the rotor on a helicopter). Examples of dynamic turbines include

4
enclosed turbines, such as the hydroturbine that extracts energy from water
in a hydroelectric dam, and open turbines such as the wind turbine that
extracts energy from the wind (Fig. 5.6).

Figure 5.6: A wind turbine is a good example of a dynamic machine of the open type;
air turns the blades, and the output shaft drives an electric generator.

Figure 5.7: Centrifugal monoblock pump is a good example of a dynamic machine of

a closed type; motor drives the shaft carrying the blades which transfers the energy
to the Liquid being pumped.

Positive displacement Pumps

Examples of Positive displacement pumps:

Lobe pump, gear pump, scroll pump, cavity pump/ conveyor, Peristaltic
Pump, Reciprocating pump,

5
Figure 5.8: Lobe pump Figure 5.9: Gear pump

Figure 5.10: Scroll pump Figure 5.11: Cavity pump

Figure 5.12:360 Degree Peristaltic Pump Figure5.13: Reciprocating pump

6
 Figure 5.14: Piston Pump and plunger pump

Dynamic machines: Centrifugal pump

Figure 5.15: The pumping system

Centrifugal pumps basically consist of a stationary pump casing and an

impeller mounted on a rotating shaft. The pump casing provides a pressure
boundary for the pump and contains channels to properly direct the suction
and discharge flow. The pump casing has suction and discharge
penetrations for the main flow path of the pump and normally has small
drain and vent fittings to remove gases trapped in the pump casing or to
drain the pump casing for maintenance.

Figure 5.16 is a simplified diagram of a typical centrifugal pump that shows

the relative locations of the pump suction, impeller, volute, and discharge.
The pump casing guides the liquid from the suction connection to the
center, or eye, of the impeller. The vanes of the rotating impeller impart a
radial and rotary motion to the liquid, forcing it to the outer periphery of the
pump casing where it is collected in the outer part of the pump casing called

7
the volute. The volute is a region that expands in cross-sectional area as it
wraps around the pump casing. The purpose of the volute is to collect the
liquid discharged from the periphery of the impeller at high velocity and
gradually cause a reduction in fluid velocity by increasing the flow area. This
converts the velocity head to static pressure. The fluid is then discharged
from the pump through the discharge connection.

Figure :5.16: Cut sectional view of a centrifugal pump.

Important point to be note: Pumps handle liquids and compressors

handle gases; there are no machines which can handle both liquid
and gases.

Compressors:
Air compressor:
The purpose of an air compressor is to provide a continuous
supply of pressurized air.

Air compressors of various designs are used widely throughout DOE

facilities in numerous applications. Compressed air has numerous uses
throughout a facility including the operation of equipment and portable
tools. Three types of designs include reciprocating, rotary, and
centrifugal air compressors.

Centrifugal air compressor:

The centrifugal compressor, originally built to handle only large volumes of
low pressure gas and air (maximum of 40 psig), has been developed to
enable it to move large volumes of gas with discharge pressures up to 3,500
psig. However, centrifugal compressors are now most frequently used for
medium volume and medium pressure air delivery. One advantage of a
centrifugal pump is the smooth discharge of the compressed air. The
centrifugal force utilized by the centrifugal compressor is the same force

8
utilized by the centrifugal pump. The air particles enter the eye of the
impeller, designated D in Figure 15.17. As the impeller rotates, air is thrown
against the casing of the compressor. The air becomes compressed as more
and more air is thrown out to the casing by the impeller blades. The air is
pushed along the path designated A, B, and C in Figure 5.17. The pressure
of the air is increased as it is pushed along this path. Note in Figure 5.17
that the impeller blades curve forward, which is opposite to the backward
curve used in typical centrifugal liquid pumps. Centrifugal compressors can
use a variety of blade orientation including both forward and backward
curves as well as other designs.

Figure: 15.17: Simplified centrifugal compressor

There may be several stages to a centrifugal air compressor, as in the

centrifugal pump, and the result would be the same; a higher pressure
would be produced. The air compressor is used to create compressed or high
pressure air for a variety of uses. Some of its uses are pneumatic control
devices, pneumatic sensors, pneumatic valve operators, pneumatic motors,
and starting air for diesel engines.

Refrigeration system:

Refrigeration Systems:
Introduction:
One of the major application area of thermodynamics is refrigeration, which
is the transfer of heat from a lower temperature region to a higher
temperature region. Devices that produce refrigeration are called
refrigerators, and the cycles on which they operate are called refrigeration
cycles. The most frequently used refrigeration cycle is the vapor-compression
refrigeration cycle in which the refrigerant is vaporized and condensed
alternately and is compressed in the vapor phase. For large scale cooling
needs, the more economical and desirable system is vapour-absorption
refrigeration system where the thermal energy can be directly used as a

9
source of energy instead of using electrical energy as a major source of
energy.

Refrigeration: The art of producing and maintaining the temperature in an

enclosed space below that of the surrounding temperature by continuously
extracting the heat from it, is known as refrigeration. In order to maintain the
low temperature in the refrigerated space, it is necessary to remove heat
continuously equal to the amount of heat leaking into the enclosed space
and reject the same to the surrounding atmosphere at higher temperature.
We all know from experience that heat
flows in the direction of decreasing
temperature, that is, from high-
temperature regions to low-temperature.
This heat-transfer process occurs in
nature without requiring any devices.
The reverse process, however, cannot
occur by itself. The transfer of heat from
a low-temperature region to a high-
temperature one requires special devices
called refrigerators. Refrigerators are
cyclic devices, and the working fluids
used in the refrigeration cycles are
called refrigerants.
A refrigerator is shown schematically in
Fig.1a. Here QL, is the magnitude of the
heat removed from the refrigerated
space at temperature TL, QH is the
magnitude of the heat rejected to the
warm space at temperature TH, and
Wnet,in is the net work input to the
refrigerator.
Figure : The objective of a refrigerator
is to remove heat (QL) from the cold QL and QH represent magnitudes and
medium; the objective of a heat thus are positive quantities. Another
pump is to supply heat (QH) to a device that transfers heat from a low-
warm medium. temperature medium to a high-
temperature one is the heat pump.

Refrigerators and heat pumps are essentially the same devices; they differ in
their objectives only. The objective of a refrigerator is to maintain the
refrigerated space at a low temperature by removing heat from it.
Discharging this heat to a higher-temperature medium is merely a necessary
part of the operation, not the purpose. The objective of a heat pump,
however, is to maintain a heated space at a high temperature. This is
accomplished by absorbing heat from a low-temperature source, such as
well water or cold outside air in winter, and supplying this heat to a warmer
medium such as a house (Fig. 1 b).

Principle of Refrigeration: It is based on the second law of

thermodynamics, which states that heat can be made to flow from a body at
10
lower temperature to a body at higher temperature with the help of external
energy source. Hence it is also called as a reversed heat engine.

The performance of refrigerators and heat pumps is expressed in terms of

the coefficient of performance (COP), defined as

Thus,
Coefficient of performance of refrigeration system is defined as a ratio of
refrigerating effect to the input work required to produce the effect.

Coefficient of performance of Heat Pump is defined as a ratio of heating effect

to the input work required to produce the effect.

These relations can also be expressed in the rate form by replacing the
. . .
quantities QL, QH, and Wnet,in by Q L , Q H and W net .in , respectively. Notice that
both COPR and COPHP can be greater than 1.

As QH = QL + Wnet.in , it implies that

COP HP = COPR + 1

for fixed values of QL and QH. This relation implies that COPHP >1 since COPR
is a positive quantity.

The cooling capacity of a refrigeration system—that is, the rate of heat

removal from the refrigerated space—is often expressed in terms of tons of
refrigeration. The capacity of a refrigeration system that can freeze 1 ton
(2000 lbm) of liquid water at 0°C (32°F) into ice at 0°C in 24 h is said to be 1
ton.
One ton of refrigeration is equivalent to 211kJ/min.

The cooling load of a typical 200-m2 residence is in the 3-ton (10-kW) range.

Refrigerator: A machine used to remove heat continuously from a

refrigerated space and to reject it to the atmosphere.

Refrigerant: It is the working fluid (liquids or gases) used in refrigerators.

Examples: Ammonia (NH3), Methyl Chloride (CH3Cl), Freon-12(CCl2F2), Freon-
13, Freon-22, Carbon-dioxide (CO2), Sulphur dioxide (SO2), Brine, Air, water,
etc.

Ice-Making Capacity: It is the capacity of a refrigerating system to make ice,

starting from water at room temperature.

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Relative COP: It is defined as the ratio of actual COP to the theoretical COP.

1) Vapor-Compression Refrigeration System:

Parts of a Vapor-compression Refrigeration system:

The essential elements required to accomplish the refrigerating process and
make up the refrigerator system are:
1. Evaporator 2. Compressor 3. Condenser 4. Expansion valve

1. Evaporator: It is also called as the cooling unit, chilling unit or freezing

unit. Evaporator is the part of a refrigerator where substance which are
to be cooled are kept and the liquid refrigerant is evaporated by the
absorption of heat from the refrigerator cabinet. It consists of metal tube
in the form of coil kept in the refrigerated space. The purpose of this coil
is to provide more surface area over which the medium can come in
contact and at the same time, passage through which refrigerant can
flow. The refrigerant in the form of liquid enters the evaporator, absorbs
heat from the medium and will gradually change from liquid to vapour.

2. Compressor: The refrigerant from the evaporator is drawn at low pressure

to the compressor, through suction valve and delivers it to the condenser
through exhaust valve at high pressure and temperature. The main
objective of the compressor is to increase the pressure of the working
fluid to higher pressure, so that, corresponding to this high pressure the
saturation temperature of the refrigerant should be slightly higher than
the atmospheric temperature. This is necessary to reject heat in the
condenser and to condense the refrigerant to saturated liquid.
Refrigeration compressors are usually either rotary or reciprocating type
and are driven by an electric motor.

3. Condenser: A condenser is an appliance in which the heat from the

refrigerant is rejected to another medium, usually the atmospheric air or
water. It is made of either finned tubing or tubing interlaced with wire, to
increase the heat transfer area.
The two main functions of the condenser are:
(i) It transfers the latent heat of evaporation, which was absorbed by the
refrigerant in the evaporator and the heat developed due to
compression, to the surrounding air.
(ii) It condenses the refrigerant vapour to a refrigerant liquid so that it
can be reused in the refrigeration cycle.

4. Expansion Valve: An expansion valve serves as a device to reduce the

pressure and temperature of the liquid refrigerant before it passes to the
evaporator. The liquid refrigerant form the condenser is passed through
an expansion valve where it reduces its pressure and temperature.

Working of Vapour Compression Refrigeration system:

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1-2 Isentropic compression in a compressor
2-3 Constant-pressure heat rejection in a condenser
3-4 Throttling in an expansion device
4-1 Constant-pressure heat absorption in an evaporator

Fig: Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.

The refrigerant at low temperature and low pressure passing through the
evaporator coils absorbs the latent heat of evaporation from the substances
to be cooled and gets evaporated. Thus the temperature of the freezing
chamber gets lowered. The evaporated low pressure refrigerant is drawn by
compressor and compresses it to high pressure, so that corresponding to
that high pressure, the saturation temperature of the refrigerant is higher
than the temperature of the cooling medium (ambient air or water) in the
condenser. Thus the high-temperature and high pressure vapour rejects
heat to the cooling medium and gets condensed to saturated liquid in the
condenser. At the exit of the condenser the saturated liquid refrigerant is
ready to expand to low pressure and temperature. The high pressure,
approximately room temperature liquid refrigerant flows to the throttle valve
(expansion valve or a capillary tube) in which it expands to a low pressure
and then ducted to the evaporator to repeat the cycle.

13
 The expansion valve lowers the pressure
and temperature of the refrigerant, at
the same time evaporates the refrigerant
partly. Thus the refrigerant entering the
evaporator will be a wet vapour and at a
very low temperature of around -20°C.
To maintain the evaporator temperature
with the desired limits, the motor driving
the compressor is controlled by a
thermostat switch.

Dichloro-difluro methane, popularly

known as Freon-12, or R-12 is the most
commonly used refrigerant.

Figure: An ordinary household

refrigerator.

Refrigerants commonly used in Practice:

The most commonly used refrigerants are:

1. Ammonia:- In vapour absorption refrigerators
2. Carbon-dioxide:- In marine refrigerators
3. Sulphur dioxide:- in household refrigerators
4. Methyl chloride:- in small scale refrigeration and domestic
refrigerators
5. Freon-12:- In domestic vapour compression refrigerators
6. Freon-22:- in Air-conditioners

Properties of a good Refrigerant

The desirable properties of an ideal refrigerant can be grouped into four

main types.
1) Thermodynamic properties
(a) Boiling point
(b) Freezing point
(c) Evaporator and condenser pressure
(d) Latent heat of Evaporation
2) Physical Properties
(a) Specific Volume
(b) Specific Heat
(c) Viscosity
3) Safe working Properties
a) Toxicity
b) Flammability

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 c) Corrosiveness
d) Chemical Stability
4) Other properties
a) COP
b) Odour
c) Leak
d) Action with Lubricating Oil

1) Boiling point: An ideal refrigerant must have low boiling temperature

at atmospheric pressure.

2) Freezing point: An ideal refrigerant must have a very low freezing point
because the refrigerant should not freeze at low evaporator
temperatures.

3) Evaporator and condenser Pressure: In order to avoid leakage of the

atmospheric air and also to enable the detection of the leakage of the
refrigerant, both the evaporator and condenser pressures should be
slightly above the atmospheric pressure.

4) Latent heat of Evaporation: The latent heat of evaporation must be

very high so that a minimum amount of refrigerant will accomplish
the desired result; in other words, it increases the refrigeration effect.

5) Specific volume: The specific volume of the refrigerant must be very

low. The lower specific volume of the refrigerant at the suction of the
compressor reduces the size of the compressor.

6) Specific heat of liquid and vapour: A good refrigerant must have low
specific heat when it is in liquid state and high specific heat when it is
vaporised. The low specific heat of the refrigerant helps in sub-cooling
of the liquid and high specific heat of the vapour helps in decreasing
the superheating of the vapour. Both these desirable properties
increase the refrigeration effect.

7) Viscosity: The viscosity of a refrigerant at both the liquid and vapour

states must be very low as it improves the heat transfer and reduces
the pumping pressure.

8) Toxicity: A good refrigerant should be non-toxic, because any leakage

of the refrigerant poisons the atmosphere if it is toxic.

9) Corrosiveness: A good refrigerant should be non-corrosive to prevent

the corrosion of the metallic parts of the refrigerators.

10)Chemical Stability: An ideal refrigerant must not decompose under

operating conditions.

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11) Coefficient of Performance: The coefficient of performance of a
refrigerant must be high so that the energy spent in refrigeration will
be less.

12) Odour: A good refrigerant must be odourless, otherwise some

foodstuff such as meat, butter, etc. loses their taste.

13) Leakage tests: The refrigerant must be such that any leakage can be
detected by simple tests.

14) Action with lubricating oil: A good refrigerant must not react with the
lubricating oil used in lubricating the parts of the compressor.

Air conditioning Systems:

Refer K.R.Gopalkrishna text book.

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