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# Tutorial - Refrigeration and Heat Pump Cycles Notes | EduRev

## : Tutorial - Refrigeration and Heat Pump Cycles Notes | EduRev

``` Page 1

THERMODYNAMICS

TUTORIAL 5

HEAT PUMPS AND REFRIGERATION

On completion of this tutorial you should be able to do the following.

• Discuss the merits of different refrigerants.

• Use thermodynamic tables for common refrigerants.

• Define a reversed heat engine.

• Define a refrigerator and heat pump.

• Define  the coefficient of performance for a refrigerator and heat
pump.

• Explain the vapour compression cycle.

• Explain modifications to the basic cycle.

• Sketch cycles on a pressure - enthalpy diagram.

• Sketch cycles on a temperature - entropy diagram.

• Solve problems involving isentropic efficiency.

• Explain the cycle of a reciprocating compressor.

• Define the volumetric efficiency of a reciprocating compressor.

• Solve problems involving reciprocating compressors in refrigeration.

• Explain the ammonia vapour absorption cycle.

Page 2

THERMODYNAMICS

TUTORIAL 5

HEAT PUMPS AND REFRIGERATION

On completion of this tutorial you should be able to do the following.

• Discuss the merits of different refrigerants.

• Use thermodynamic tables for common refrigerants.

• Define a reversed heat engine.

• Define a refrigerator and heat pump.

• Define  the coefficient of performance for a refrigerator and heat
pump.

• Explain the vapour compression cycle.

• Explain modifications to the basic cycle.

• Sketch cycles on a pressure - enthalpy diagram.

• Sketch cycles on a temperature - entropy diagram.

• Solve problems involving isentropic efficiency.

• Explain the cycle of a reciprocating compressor.

• Define the volumetric efficiency of a reciprocating compressor.

• Solve problems involving reciprocating compressors in refrigeration.

• Explain the ammonia vapour absorption cycle.

1. INTRODUCTION

It is possible to lower the temperature of a body by use of the thermo-electric affect
(reversed thermo-couple or Peltier effect). This has yet to be developed as a serious
refrigeration method so refrigerators still rely on a fluid or refrigerant which is used in a
reversed heat engine cycle as follows.

Figure 1

Heat is absorbed into a fluid (this is usually an evaporator) lowering the temperature of
the surroundings. The fluid is then compressed and this raises the temperature and
pressure. At the higher temperature the fluid is cooled to normal temperature (this is
usually a condenser). The fluid then experiences a drop in pressure which makes it go
cold (this is usually a throttle valve) and able to absorb heat at a cold temperature. The
cycle is then repeated.

Various fluids or refrigerants are used in the reversed thermodynamic cycle.
Refrigerants such as air, water and carbon dioxide are used but most refrigerants are
those designed for vapour compression cycles. These refrigerants will evaporate at cold
temperatures and so the heat absorbed is in the form of latent energy. Let's look at the
properties of these and other refrigerants.
2
Page 3

THERMODYNAMICS

TUTORIAL 5

HEAT PUMPS AND REFRIGERATION

On completion of this tutorial you should be able to do the following.

• Discuss the merits of different refrigerants.

• Use thermodynamic tables for common refrigerants.

• Define a reversed heat engine.

• Define a refrigerator and heat pump.

• Define  the coefficient of performance for a refrigerator and heat
pump.

• Explain the vapour compression cycle.

• Explain modifications to the basic cycle.

• Sketch cycles on a pressure - enthalpy diagram.

• Sketch cycles on a temperature - entropy diagram.

• Solve problems involving isentropic efficiency.

• Explain the cycle of a reciprocating compressor.

• Define the volumetric efficiency of a reciprocating compressor.

• Solve problems involving reciprocating compressors in refrigeration.

• Explain the ammonia vapour absorption cycle.

1. INTRODUCTION

It is possible to lower the temperature of a body by use of the thermo-electric affect
(reversed thermo-couple or Peltier effect). This has yet to be developed as a serious
refrigeration method so refrigerators still rely on a fluid or refrigerant which is used in a
reversed heat engine cycle as follows.

Figure 1

Heat is absorbed into a fluid (this is usually an evaporator) lowering the temperature of
the surroundings. The fluid is then compressed and this raises the temperature and
pressure. At the higher temperature the fluid is cooled to normal temperature (this is
usually a condenser). The fluid then experiences a drop in pressure which makes it go
cold (this is usually a throttle valve) and able to absorb heat at a cold temperature. The
cycle is then repeated.

Various fluids or refrigerants are used in the reversed thermodynamic cycle.
Refrigerants such as air, water and carbon dioxide are used but most refrigerants are
those designed for vapour compression cycles. These refrigerants will evaporate at cold
temperatures and so the heat absorbed is in the form of latent energy. Let's look at the
properties of these and other refrigerants.
2
3

2. REFRIGERANTS

Refrigerants are given R numbers. Carbon dioxide, for example is R744. Some of them
are dangerous if released because they are either explosive or toxic. Toxic refrigerants
are placed in categories. Sulphur dioxide, for example, is classed as toxic group 1 which
means that death occurs after breathing it for 5 minutes.

In the past the most popular fluids have been ammonia (R717),fluorocarbons and halo-
carbons. The most popular of these is R12 or dichlorodifluoromethane (CF
2
Cl
2
).

The type of refrigerant used in a cycle is largely governed by the evaporation
temperature required and its latent capacity. Below is a list of some of them.

Refrigerant  R number Evaporation temp. Toxic group
at 1.013 bar.(
o
C)
C Cl
3
F  R11   24   5
C Cl
2
F
2
R12  -30   6
C ClF
3
R13  -82   6
C F
4
R14  -128   6
CH Cl
2
F  R21   9   4
CH Cl

F
2
R22  -40   5
CH F
3
R23  -84   5
C Cl
2
F C Cl

F
2
R113   47   4
C Cl
2
F C F
3
R114A   3   6
C Cl
2
F
2
C Cl

F
2
R114   3   6
C Cl
2
F
2
C

F
3
R115  -39

All the above are Halo-Carbons and Fluro-carbons which are non-flammable and may
be detected by a halide torch or electric cell sensor. Other refrigerants are shown below.

Ammonia is flammable and detected by going white in the presence of sulphur dioxide.
It has a strong characteristic pungent smell. Death occurs when breathed for 30 minutes.

NH
3
R717  -33   2

Carbon Dioxide is safe and non-toxic but it can suffocate.

CO
2
R744  -78

Sulphur Dioxide is highly toxic and does not burn.

Other refrigerants are in the Hydro-Carbon groups such as Propane, Butane and Ethane.
These are explosive. Because of the problems with damage to the ozone layer, new
refrigerants such as R134a have been developed and are now included in the
thermodynamic tables.

Now let's look at the use of thermodynamic tables for refrigerants.
Page 4

THERMODYNAMICS

TUTORIAL 5

HEAT PUMPS AND REFRIGERATION

On completion of this tutorial you should be able to do the following.

• Discuss the merits of different refrigerants.

• Use thermodynamic tables for common refrigerants.

• Define a reversed heat engine.

• Define a refrigerator and heat pump.

• Define  the coefficient of performance for a refrigerator and heat
pump.

• Explain the vapour compression cycle.

• Explain modifications to the basic cycle.

• Sketch cycles on a pressure - enthalpy diagram.

• Sketch cycles on a temperature - entropy diagram.

• Solve problems involving isentropic efficiency.

• Explain the cycle of a reciprocating compressor.

• Define the volumetric efficiency of a reciprocating compressor.

• Solve problems involving reciprocating compressors in refrigeration.

• Explain the ammonia vapour absorption cycle.

1. INTRODUCTION

It is possible to lower the temperature of a body by use of the thermo-electric affect
(reversed thermo-couple or Peltier effect). This has yet to be developed as a serious
refrigeration method so refrigerators still rely on a fluid or refrigerant which is used in a
reversed heat engine cycle as follows.

Figure 1

Heat is absorbed into a fluid (this is usually an evaporator) lowering the temperature of
the surroundings. The fluid is then compressed and this raises the temperature and
pressure. At the higher temperature the fluid is cooled to normal temperature (this is
usually a condenser). The fluid then experiences a drop in pressure which makes it go
cold (this is usually a throttle valve) and able to absorb heat at a cold temperature. The
cycle is then repeated.

Various fluids or refrigerants are used in the reversed thermodynamic cycle.
Refrigerants such as air, water and carbon dioxide are used but most refrigerants are
those designed for vapour compression cycles. These refrigerants will evaporate at cold
temperatures and so the heat absorbed is in the form of latent energy. Let's look at the
properties of these and other refrigerants.
2
3

2. REFRIGERANTS

Refrigerants are given R numbers. Carbon dioxide, for example is R744. Some of them
are dangerous if released because they are either explosive or toxic. Toxic refrigerants
are placed in categories. Sulphur dioxide, for example, is classed as toxic group 1 which
means that death occurs after breathing it for 5 minutes.

In the past the most popular fluids have been ammonia (R717),fluorocarbons and halo-
carbons. The most popular of these is R12 or dichlorodifluoromethane (CF
2
Cl
2
).

The type of refrigerant used in a cycle is largely governed by the evaporation
temperature required and its latent capacity. Below is a list of some of them.

Refrigerant  R number Evaporation temp. Toxic group
at 1.013 bar.(
o
C)
C Cl
3
F  R11   24   5
C Cl
2
F
2
R12  -30   6
C ClF
3
R13  -82   6
C F
4
R14  -128   6
CH Cl
2
F  R21   9   4
CH Cl

F
2
R22  -40   5
CH F
3
R23  -84   5
C Cl
2
F C Cl

F
2
R113   47   4
C Cl
2
F C F
3
R114A   3   6
C Cl
2
F
2
C Cl

F
2
R114   3   6
C Cl
2
F
2
C

F
3
R115  -39

All the above are Halo-Carbons and Fluro-carbons which are non-flammable and may
be detected by a halide torch or electric cell sensor. Other refrigerants are shown below.

Ammonia is flammable and detected by going white in the presence of sulphur dioxide.
It has a strong characteristic pungent smell. Death occurs when breathed for 30 minutes.

NH
3
R717  -33   2

Carbon Dioxide is safe and non-toxic but it can suffocate.

CO
2
R744  -78

Sulphur Dioxide is highly toxic and does not burn.

Other refrigerants are in the Hydro-Carbon groups such as Propane, Butane and Ethane.
These are explosive. Because of the problems with damage to the ozone layer, new
refrigerants such as R134a have been developed and are now included in the
thermodynamic tables.

Now let's look at the use of thermodynamic tables for refrigerants.
4
3. TABLES

The section of the fluid tables devoted to refrigerants is very concise and contains only
two superheat temperatures. The layout of the tables is shown below.

15K                30 K
t p
s
v
g
h
f
h
g
s
f
s
g
h s h s

t is the actual temperature in degrees Celsius.
p
s
is the saturation pressure corresponding to the temperature.
It follows that if the refrigerant is wet or dry saturated, it must be at temperature t and
pressure p
s
. If the refrigerant has 15 degrees of superheat, then the actual temperature is
t+15 and the properties are found under the 15 K heading. Similarly if it has 30 K of
superheat, its actual temperature is t+30.

For example, R12 at 2.191 bar and 20
o
C must have 30 K of superheat since its
saturation temperature would is -10
o
C. From the 30 K columns we find that h=201.97
kJ/kg and s = 0.7695 kJ/kg K.

When dealing with liquid refrigerant, take the properties as h
f
and s
f
at the given
temperatures. The pressures are never very high so the pressure term will not cause
much error.
Page 5

THERMODYNAMICS

TUTORIAL 5

HEAT PUMPS AND REFRIGERATION

On completion of this tutorial you should be able to do the following.

• Discuss the merits of different refrigerants.

• Use thermodynamic tables for common refrigerants.

• Define a reversed heat engine.

• Define a refrigerator and heat pump.

• Define  the coefficient of performance for a refrigerator and heat
pump.

• Explain the vapour compression cycle.

• Explain modifications to the basic cycle.

• Sketch cycles on a pressure - enthalpy diagram.

• Sketch cycles on a temperature - entropy diagram.

• Solve problems involving isentropic efficiency.

• Explain the cycle of a reciprocating compressor.

• Define the volumetric efficiency of a reciprocating compressor.

• Solve problems involving reciprocating compressors in refrigeration.

• Explain the ammonia vapour absorption cycle.

1. INTRODUCTION

It is possible to lower the temperature of a body by use of the thermo-electric affect
(reversed thermo-couple or Peltier effect). This has yet to be developed as a serious
refrigeration method so refrigerators still rely on a fluid or refrigerant which is used in a
reversed heat engine cycle as follows.

Figure 1

Heat is absorbed into a fluid (this is usually an evaporator) lowering the temperature of
the surroundings. The fluid is then compressed and this raises the temperature and
pressure. At the higher temperature the fluid is cooled to normal temperature (this is
usually a condenser). The fluid then experiences a drop in pressure which makes it go
cold (this is usually a throttle valve) and able to absorb heat at a cold temperature. The
cycle is then repeated.

Various fluids or refrigerants are used in the reversed thermodynamic cycle.
Refrigerants such as air, water and carbon dioxide are used but most refrigerants are
those designed for vapour compression cycles. These refrigerants will evaporate at cold
temperatures and so the heat absorbed is in the form of latent energy. Let's look at the
properties of these and other refrigerants.
2
3

2. REFRIGERANTS

Refrigerants are given R numbers. Carbon dioxide, for example is R744. Some of them
are dangerous if released because they are either explosive or toxic. Toxic refrigerants
are placed in categories. Sulphur dioxide, for example, is classed as toxic group 1 which
means that death occurs after breathing it for 5 minutes.

In the past the most popular fluids have been ammonia (R717),fluorocarbons and halo-
carbons. The most popular of these is R12 or dichlorodifluoromethane (CF
2
Cl
2
).

The type of refrigerant used in a cycle is largely governed by the evaporation
temperature required and its latent capacity. Below is a list of some of them.

Refrigerant  R number Evaporation temp. Toxic group
at 1.013 bar.(
o
C)
C Cl
3
F  R11   24   5
C Cl
2
F
2
R12  -30   6
C ClF
3
R13  -82   6
C F
4
R14  -128   6
CH Cl
2
F  R21   9   4
CH Cl

F
2
R22  -40   5
CH F
3
R23  -84   5
C Cl
2
F C Cl

F
2
R113   47   4
C Cl
2
F C F
3
R114A   3   6
C Cl
2
F
2
C Cl

F
2
R114   3   6
C Cl
2
F
2
C

F
3
R115  -39

All the above are Halo-Carbons and Fluro-carbons which are non-flammable and may
be detected by a halide torch or electric cell sensor. Other refrigerants are shown below.

Ammonia is flammable and detected by going white in the presence of sulphur dioxide.
It has a strong characteristic pungent smell. Death occurs when breathed for 30 minutes.

NH
3
R717  -33   2

Carbon Dioxide is safe and non-toxic but it can suffocate.

CO
2
R744  -78

Sulphur Dioxide is highly toxic and does not burn.

Other refrigerants are in the Hydro-Carbon groups such as Propane, Butane and Ethane.
These are explosive. Because of the problems with damage to the ozone layer, new
refrigerants such as R134a have been developed and are now included in the
thermodynamic tables.

Now let's look at the use of thermodynamic tables for refrigerants.
4
3. TABLES

The section of the fluid tables devoted to refrigerants is very concise and contains only
two superheat temperatures. The layout of the tables is shown below.

15K                30 K
t p
s
v
g
h
f
h
g
s
f
s
g
h s h s

t is the actual temperature in degrees Celsius.
p
s
is the saturation pressure corresponding to the temperature.
It follows that if the refrigerant is wet or dry saturated, it must be at temperature t and
pressure p
s
. If the refrigerant has 15 degrees of superheat, then the actual temperature is
t+15 and the properties are found under the 15 K heading. Similarly if it has 30 K of
superheat, its actual temperature is t+30.

For example, R12 at 2.191 bar and 20
o
C must have 30 K of superheat since its
saturation temperature would is -10
o
C. From the 30 K columns we find that h=201.97
kJ/kg and s = 0.7695 kJ/kg K.

When dealing with liquid refrigerant, take the properties as h
f
and s
f
at the given
temperatures. The pressures are never very high so the pressure term will not cause
much error.
4. VAPOUR COMPRESSION CYCLES

4.1 THE BASIC CYCLE

Refrigeration/heat pump cycles are similar to heat engine cycles but they work in
reverse and are known as reversed heat engine cycles. A basic vapour cycle consists of
isentropic compression,
constant pressure cooling,
isentropic expansion and
constant pressure heating.
You may recognise this
as a reverse of the
Rankine cycle or even the
reverse of a Carnot cycle.
The heating and cooling
will involve evaporation
and condensing. Let's
consider the cycle first
conducted entirely with
wet vapour.

Figure 2

The basic principle is that the wet vapour is compressed and becomes dryer and warmer
in the process. It is then cooled and condensed into a wetter vapour at the higher
pressure. The vapour is then expanded. Because of the cooling, the expansion back to
the original pressure produces a fluid which is much colder and wetter than it was
before compression. The fluid is then able to absorb heat at the cold temperature
becoming dryer in the process and is returned to the original state and compressed
again. The net result is that heat is absorbed at a cold temperature and rejected at a
higher temperature.
Work is needed to
drive the compressor
but some of it is
returned by the
turbine.
5

The thermodynamic
cycle for refrigerators
is often shown on a
pressure – enthalpy
diagram (p – h) and
professional charts
are available but not
used in the
Engineering Council
exams. Figure 3
shows the basic cycle.
Figure 3
```
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