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 
©D.J.Dunn  www.freestudy.co.uk 
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 
©D.J.Dunn  www.freestudy.co.uk 
3 
©D.J.Dunn  www.freestudy.co.uk 
 
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 
©D.J.Dunn  www.freestudy.co.uk 
3 
©D.J.Dunn  www.freestudy.co.uk 
 
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 
©D.J.Dunn  www.freestudy.co.uk 
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 
©D.J.Dunn  www.freestudy.co.uk 
3 
©D.J.Dunn  www.freestudy.co.uk 
 
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 
©D.J.Dunn  www.freestudy.co.uk 
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 
©D.J.Dunn  www.freestudy.co.uk 
 
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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