handbook - refrigeration
TRANSCRIPT
REFRIGERATION
G u i d e B o o k 4
3E STRATEGY
REFRIGERATION
G u i d e B o o k 4
3E STRATEGY
ST
RA
TE
GY
EFFICIENCYENERGY
EARNINGS
EUROPEAN COMMISSION
Ne the r l an d s M in i s t e r y o f E c onom i c A f f a i r s
TSI
Technical Services International
M
YI N GRE ER A NL ES DA N
How
to sav
e ene
rgy a
nd mo
ney
H O W T O S A V E
E N E R G Y A N D M O N E Y
I N R E F R I G E R A T I O N
3E STRATEGY
HOW TO SAVE
ENERGY AND MONEY
IN REFRIGERATION
This booklet is part of the 3E strategy series. It provides advice on practical
ways of improving energy efficiency in industrial refrigeration applications.
Prepared for the European Commission DGXVII by:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
www.eri.uct.ac.za
This project is funded by the European Commission and co-funded by the
Dutch Ministry of Economics, the South African Department of Minerals
and Energy and Technical Services International (ESKOM), with the Chief
contractor being ETSU.
Neither the European Commission, nor any person acting on behalf of the
commission, nor NOVEM, ETSU, ERI, nor any of the information
sources is responsible for the use of the information contained in this
publication.
The views and judgements given in this publication do not necessarily
represent the views of the European Commission.
H O W T O S A V E
E N E R G Y A N D M O N E Y
I N R E F R I G E R A T I O N
3E STRATEGY
HOW TO SAVE
ENERGY AND MONEY
IN REFRIGERATION
This booklet is part of the 3E strategy series. It provides advice on practical
ways of improving energy efficiency in industrial refrigeration applications.
Prepared for the European Commission DGXVII by:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
www.eri.uct.ac.za
This project is funded by the European Commission and co-funded by the
Dutch Ministry of Economics, the South African Department of Minerals
and Energy and Technical Services International (ESKOM), with the Chief
contractor being ETSU.
Neither the European Commission, nor any person acting on behalf of the
commission, nor NOVEM, ETSU, ERI, nor any of the information
sources is responsible for the use of the information contained in this
publication.
The views and judgements given in this publication do not necessarily
represent the views of the European Commission.
QUICK 'CHECK-LIST' FOR SAVING ENERGY AND
MONEY IN REFRIGERATION SYSTEMS
This list is a selected summary of energy and cost savings opportunities outline in the text. Many more
are detailed in the body of the booklet. These are intended to be a quick 'checklist'.
EQUIPMENT MAINTENANCE (Chapter 3):
� Ensure that there is good and regular maintenance of all equipment.
� Avoid blockage of air flow through and around heat exchanges (e.g. evaporators and
condensers).
� Make sure that fouling of primary and secondary refrigeration circuits is kept to a minimum.
� Maintain isolation standards where appropriate.
EFFICIENT USE OF THE REFRIGERATION SYSTEM (Chapter 5):
� Keep operating hours to a minimum.
� Ensure that the cooling load is kept to a minimum.
� Avoid operating refrigeration plant under part-load conditions.
� Investigate the possibility of improving control functions.
� Reschedule production cycles to reduce peak electrical demand.
ALTERATIONS TO THE EXISTING PLANT (Chapters 3 and 5):
� Utilise waste heat where possible.
� Where appropriate, retrofit plant with more energy efficient components.
� Increase evaporator temperature to increase system COP.
� Reduce condensing temperature to increase system COP
� Upgrade automatic controls in refrigeration plants to provide accurate and flexible operation.
� Replace high-maintenance, centrifugal compressors with compressors selected for high
efficiency when operating at part load conditions.
� Upgrade insulation on primary and secondary refrigerant piping circuits.
REFRIGERANTS (Chapter 4):
� Review energy efficiency when replacing CFC with ozone benign refrigerants. (This might not
have an energy saving effect).
G u i d e B o o k E s s e n t i a l s
HOW TO SAVE
ENERGY AND MONEY
IN REFRIGERATION
ACKNOWLEDGEMENTS
Other titles in the 3E strategy series:
HOW TO SAVE ENERGY AND MONEY: THE 3E STRATEGY
HOW TO SAVE ENERGY AND MONEY IN ELECTRICITY USE
HOW TO SAVE ENERGY AND MONEY IN BOILERS AND FURNACES
HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
HOW TO SAVE ENERGY AND MONEY IN STEAM SYSTEMS
HOW TO SAVE ENERGY AND MONEY INSULATION SYSTEMS
Copies of these guides may be obtained from:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
Tel No: +27 (0) 21 650 3892
Fax No: +27 (0) 21 686 4838
E-mail: [email protected]
Website: http://www.3e.uct.ac.za
The Energy Research Institute would like to acknowledge the following for their contribution
in the production of the guide:
� Energy Technology Support Unite (ETSU), UK, for permission to use information
from the ‘’Energy Efficiency Best Parctice’’ series of handbooks.
� Energy Conservation Branch, Department of Energy, Mines and Resources, Canada,
for permission to use information from the ‘’Energy Management’’ series of manuals.
� TLV Co, Ltd, for permission to use figures from their set of handbooks on steam.
� Wilma Walden for graphic design work ([email protected]).
� Doug Geddes of South African Breweries for the cover colour photography.
QUICK 'CHECK-LIST' FOR SAVING ENERGY AND
MONEY IN REFRIGERATION SYSTEMS
This list is a selected summary of energy and cost savings opportunities outline in the text. Many more
are detailed in the body of the booklet. These are intended to be a quick 'checklist'.
EQUIPMENT MAINTENANCE (Chapter 3):
� Ensure that there is good and regular maintenance of all equipment.
� Avoid blockage of air flow through and around heat exchanges (e.g. evaporators and
condensers).
� Make sure that fouling of primary and secondary refrigeration circuits is kept to a minimum.
� Maintain isolation standards where appropriate.
EFFICIENT USE OF THE REFRIGERATION SYSTEM (Chapter 5):
� Keep operating hours to a minimum.
� Ensure that the cooling load is kept to a minimum.
� Avoid operating refrigeration plant under part-load conditions.
� Investigate the possibility of improving control functions.
� Reschedule production cycles to reduce peak electrical demand.
ALTERATIONS TO THE EXISTING PLANT (Chapters 3 and 5):
� Utilise waste heat where possible.
� Where appropriate, retrofit plant with more energy efficient components.
� Increase evaporator temperature to increase system COP.
� Reduce condensing temperature to increase system COP
� Upgrade automatic controls in refrigeration plants to provide accurate and flexible operation.
� Replace high-maintenance, centrifugal compressors with compressors selected for high
efficiency when operating at part load conditions.
� Upgrade insulation on primary and secondary refrigerant piping circuits.
REFRIGERANTS (Chapter 4):
� Review energy efficiency when replacing CFC with ozone benign refrigerants. (This might not
have an energy saving effect).
G u i d e B o o k E s s e n t i a l s
HOW TO SAVE
ENERGY AND MONEY
IN REFRIGERATION
ACKNOWLEDGEMENTS
Other titles in the 3E strategy series:
HOW TO SAVE ENERGY AND MONEY: THE 3E STRATEGY
HOW TO SAVE ENERGY AND MONEY IN ELECTRICITY USE
HOW TO SAVE ENERGY AND MONEY IN BOILERS AND FURNACES
HOW TO SAVE ENERGY AND MONEY IN COMPRESSED AIR SYSTEMS
HOW TO SAVE ENERGY AND MONEY IN STEAM SYSTEMS
HOW TO SAVE ENERGY AND MONEY INSULATION SYSTEMS
Copies of these guides may be obtained from:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
Tel No: +27 (0) 21 650 3892
Fax No: +27 (0) 21 686 4838
E-mail: [email protected]
Website: http://www.3e.uct.ac.za
The Energy Research Institute would like to acknowledge the following for their contribution
in the production of the guide:
� Energy Technology Support Unite (ETSU), UK, for permission to use information
from the ‘’Energy Efficiency Best Parctice’’ series of handbooks.
� Energy Conservation Branch, Department of Energy, Mines and Resources, Canada,
for permission to use information from the ‘’Energy Management’’ series of manuals.
� TLV Co, Ltd, for permission to use figures from their set of handbooks on steam.
� Wilma Walden for graphic design work ([email protected]).
� Doug Geddes of South African Breweries for the cover colour photography.
T a b l e o f c o n t e n t s
AUDITING (Chapter 5)
Refrigeration efficiency is usually expressed as the coefficient of performance (COP), defined as:
COP =
Once the system performance has been established it is useful to identify the contribution of each plant
component to the total system power input. Suitable electricity submeters can be installed for this purpose. The
main contributors are normally:
� compressors (typically 65%);
� condenser pumps (typically 5%);
� condenser fans (typically 10%);
� evaporator pumps (typically 15%);
� lights (typically 5%).
The next stage is to divide the total cooling load amongst the various process requirements. This should allow
the loads that significantly affect costs to be highlighted.
Cooling effect (kW)Power input to compressor (kW)
3E STRATEGY
1. INTRODUCTION...................................................................................................................................................................1
1.1 Purpose.....................................................................................................................................................................................1
2. THE REFRIGERATION PROCESS...................................................................................................................................2
2.1 The vapour compression cycle ...................................................................................................................................2
2.2. Reverse Carnot Cycle.....................................................................................................................................................4
2.2.1 Coefficient of Performance...............................................................................................................................4
2.3 Theoretical Vapour Compression Cycle ...............................................................................................................5
2.3.1 Model Coefficient of Performance................................................................................................................6
2.3.2 Practical Considerations .....................................................................................................................................7
2.4 Absorption Cycle............................................................................................................................................................11
2.5 Special Refrigeration Systems ...................................................................................................................................13
2.6 Variations on the simple Carnot circuit................................................................................................................13
2.6.1 Suction/liquid heat exchanger.......................................................................................................................13
2.7 Multiple evaporator circuits .......................................................................................................................................14
2.7.1 Multiple compressor Systems .......................................................................................................................15
2.7.2 Cascade Systems .................................................................................................................................................17
2.7.3 Heat Pump Systems ...........................................................................................................................................18
3. EQUIPMENT ............................................................................................................................................................................20
3.1 Compressors.....................................................................................................................................................................20
3.1.1 Types of compressor housing .......................................................................................................................20
3.1.2 Hermetic and semi-hermetic compressors ...........................................................................................20
3.1.3 Open compressors ............................................................................................................................................20
3.1.4 Reciprocating compressors............................................................................................................................21
3.1.5 Screw compressors............................................................................................................................................21
3.1.6 Scroll compressors .............................................................................................................................................22
3.1.7 Compressor performance data ...................................................................................................................22
3.1.8 Capacity control...................................................................................................................................................22
3.2 Evaporators........................................................................................................................................................................23
3.2.1 Direct expansion .................................................................................................................................................23
3.2.2 Flooded.....................................................................................................................................................................24
3.2.3 Oil control in evaporators...............................................................................................................................25
3.2.4 Energy efficient operation of evaporators ..............................................................................................27
3.2.5 Defrosting................................................................................................................................................................27
T a b l e o f c o n t e n t s
AUDITING (Chapter 5)
Refrigeration efficiency is usually expressed as the coefficient of performance (COP), defined as:
COP =
Once the system performance has been established it is useful to identify the contribution of each plant
component to the total system power input. Suitable electricity submeters can be installed for this purpose. The
main contributors are normally:
� compressors (typically 65%);
� condenser pumps (typically 5%);
� condenser fans (typically 10%);
� evaporator pumps (typically 15%);
� lights (typically 5%).
The next stage is to divide the total cooling load amongst the various process requirements. This should allow
the loads that significantly affect costs to be highlighted.
Cooling effect (kW)Power input to compressor (kW)
3E STRATEGY
1. INTRODUCTION...................................................................................................................................................................1
1.1 Purpose.....................................................................................................................................................................................1
2. THE REFRIGERATION PROCESS...................................................................................................................................2
2.1 The vapour compression cycle ...................................................................................................................................2
2.2. Reverse Carnot Cycle.....................................................................................................................................................4
2.2.1 Coefficient of Performance...............................................................................................................................4
2.3 Theoretical Vapour Compression Cycle ...............................................................................................................5
2.3.1 Model Coefficient of Performance................................................................................................................6
2.3.2 Practical Considerations .....................................................................................................................................7
2.4 Absorption Cycle............................................................................................................................................................11
2.5 Special Refrigeration Systems ...................................................................................................................................13
2.6 Variations on the simple Carnot circuit................................................................................................................13
2.6.1 Suction/liquid heat exchanger.......................................................................................................................13
2.7 Multiple evaporator circuits .......................................................................................................................................14
2.7.1 Multiple compressor Systems .......................................................................................................................15
2.7.2 Cascade Systems .................................................................................................................................................17
2.7.3 Heat Pump Systems ...........................................................................................................................................18
3. EQUIPMENT ............................................................................................................................................................................20
3.1 Compressors.....................................................................................................................................................................20
3.1.1 Types of compressor housing .......................................................................................................................20
3.1.2 Hermetic and semi-hermetic compressors ...........................................................................................20
3.1.3 Open compressors ............................................................................................................................................20
3.1.4 Reciprocating compressors............................................................................................................................21
3.1.5 Screw compressors............................................................................................................................................21
3.1.6 Scroll compressors .............................................................................................................................................22
3.1.7 Compressor performance data ...................................................................................................................22
3.1.8 Capacity control...................................................................................................................................................22
3.2 Evaporators........................................................................................................................................................................23
3.2.1 Direct expansion .................................................................................................................................................23
3.2.2 Flooded.....................................................................................................................................................................24
3.2.3 Oil control in evaporators...............................................................................................................................25
3.2.4 Energy efficient operation of evaporators ..............................................................................................27
3.2.5 Defrosting................................................................................................................................................................27
1
3E STRATEGY
3.3 Expansion devices...........................................................................................................................................................28
3.3.1 Thermostatic expansion valves ....................................................................................................................28
3.3.2 Float valve systems..............................................................................................................................................30
3.4 Condensers........................................................................................................................................................................32
3.4.1 Air-cooled condensers.....................................................................................................................................32
3.4.2 Water-cooled condensers .............................................................................................................................32
3.4.3 Evaporative condensers...................................................................................................................................33
3.4.4 Loss of condenser efficiency due to air in system ................................................................................38
4. REFRIGERANTS .....................................................................................................................................................................35
4.1 Desirable Characteristics ............................................................................................................................................35
4.2 Common Refrigerants - Vapour Compression Cycles................................................................................38
4.3 Common Refrigerants - Absorption Cycle........................................................................................................38
4.4 Brines and Secondary Coolants...............................................................................................................................38
5. ENERGY MANAGEMENT OPPORTUNITIES ....................................................................................................39
5.1 Housekeeping Opportunities...................................................................................................................................39
5.1.1 General maintenance ........................................................................................................................................39
5.1.2 Plant operation .....................................................................................................................................................40
5.1.3 Instrumentation....................................................................................................................................................40
5.1.4 Trouble shooting .................................................................................................................................................42
5.1.5 Housekeeping Worked Examples..............................................................................................................42
5.2 Low Cost Opportunities.............................................................................................................................................45
5.2.1 Low Cost Worked Examples........................................................................................................................46
5.3 Retrofit Opportunities..................................................................................................................................................47
APPENDIX 1: GLOSSARY OF TERMS............................................................................................................................49
APPENDIX 2: ENERGY, VOLUME AND MASS CONVERSION FACTORS ............................................57
APPENDIX 3: EXAMPLE OF MEASURING COP DIRECTLY..........................................................................58
Throughout history, humans have used various
forms of refr igerat ion. S imple cool ing
arrangements, such as those provided by iceboxes The following summarizes the purpose of this
and root cellars, allowed long term storage of guide.
perishable foods. These, and other simple
techniques, though largely supplanted by � Introduce the subject of Refrigeration and
mechanical refrigeration equipment, are still used Heat Pumps as used in the Industrial,
by campers, cottagers and people in remote or less Commercial and Institutional Sectors.
developed areas.� Make building owners and operators
aware of the potential energy and cost Mechanical refrigeration systems were first built in
savings available through the implemen-the late nineteenth century, but did not become
tation of Energy Management Oppor-commonplace until the 1940s. Although
tunities.mechanical refrigeration provides benefits such as
refrigerated storage independent of season or · Provide methods of calculating the potential
cl imate, and better l iving and working energy and cost savings, using simple worked
environments, the energy costs related to examples.
operation of these systems are significant. This
guide examines refrigeration and heat pump
systems and identifies where energy consumption
and costs may be reduced.
1.1 PURPOSE
1. INTRODUCTION
1
3E STRATEGY
3.3 Expansion devices...........................................................................................................................................................28
3.3.1 Thermostatic expansion valves ....................................................................................................................28
3.3.2 Float valve systems..............................................................................................................................................30
3.4 Condensers........................................................................................................................................................................32
3.4.1 Air-cooled condensers.....................................................................................................................................32
3.4.2 Water-cooled condensers .............................................................................................................................32
3.4.3 Evaporative condensers...................................................................................................................................33
3.4.4 Loss of condenser efficiency due to air in system ................................................................................38
4. REFRIGERANTS .....................................................................................................................................................................35
4.1 Desirable Characteristics ............................................................................................................................................35
4.2 Common Refrigerants - Vapour Compression Cycles................................................................................38
4.3 Common Refrigerants - Absorption Cycle........................................................................................................38
4.4 Brines and Secondary Coolants...............................................................................................................................38
5. ENERGY MANAGEMENT OPPORTUNITIES ....................................................................................................39
5.1 Housekeeping Opportunities...................................................................................................................................39
5.1.1 General maintenance ........................................................................................................................................39
5.1.2 Plant operation .....................................................................................................................................................40
5.1.3 Instrumentation....................................................................................................................................................40
5.1.4 Trouble shooting .................................................................................................................................................42
5.1.5 Housekeeping Worked Examples..............................................................................................................42
5.2 Low Cost Opportunities.............................................................................................................................................45
5.2.1 Low Cost Worked Examples........................................................................................................................46
5.3 Retrofit Opportunities..................................................................................................................................................47
APPENDIX 1: GLOSSARY OF TERMS............................................................................................................................49
APPENDIX 2: ENERGY, VOLUME AND MASS CONVERSION FACTORS ............................................57
APPENDIX 3: EXAMPLE OF MEASURING COP DIRECTLY..........................................................................58
Throughout history, humans have used various
forms of refr igerat ion. S imple cool ing
arrangements, such as those provided by iceboxes The following summarizes the purpose of this
and root cellars, allowed long term storage of guide.
perishable foods. These, and other simple
techniques, though largely supplanted by � Introduce the subject of Refrigeration and
mechanical refrigeration equipment, are still used Heat Pumps as used in the Industrial,
by campers, cottagers and people in remote or less Commercial and Institutional Sectors.
developed areas.� Make building owners and operators
aware of the potential energy and cost Mechanical refrigeration systems were first built in
savings available through the implemen-the late nineteenth century, but did not become
tation of Energy Management Oppor-commonplace until the 1940s. Although
tunities.mechanical refrigeration provides benefits such as
refrigerated storage independent of season or · Provide methods of calculating the potential
cl imate, and better l iving and working energy and cost savings, using simple worked
environments, the energy costs related to examples.
operation of these systems are significant. This
guide examines refrigeration and heat pump
systems and identifies where energy consumption
and costs may be reduced.
1.1 PURPOSE
1. INTRODUCTION
The majority of refrigeration systems are driven by � The temperature at which refrigerant boils
a machine, which compresses and pumps varies with its pressure; the higher the
refrigerant vapour around a sealed circuit. Heat is pressure, the higher the boiling point;
absorbed and rejected through heat exchangers. � When refrigerant liquid boils, changing its
These systems work on what is called a vapour state to a gas, it absorbs heat from its
compression cycle. surroundings;
� Refrigerant can be changed back from a gas
There are other types of plant which can be used to to a liquid by cooling it, usually by using air
obtain a cooling effect, such as absorption cycle or water.
systems, but these are not in common use and are
only economically viable where there are large Note: In the refrigeration industry the term
supplies of waste heat. evaporation is used instead of boiling. Also, if a gas is
heated above its boiling point it is said to be
superheated and if liquid is cooled below its
condensing temperature it is sub-cooled.
To enable the refrigerant to be condensed it has to
be compressed to a higher pressure, and it is at this
point that energy has to be used to drive the
machine that performs this task. The machine is Heat can only flow naturally from a hot to a colder called a compressor and it is usually driven by an body. In refrigeration system the opposite must electric motor.occur. This is achieved by using a substance called a
refrigerant, which absorbs heat and hence boils or The operation of a simple refrigeration system is evaporates at a low pressure to form a gas. This gas shown in Figure 1. The diagram shows the is then compressed to a higher pressure, such that it refrigerant pressure (bars) and its heat content transfers the heat it has gained to ambient air or (kJ/kg).water and turns back into a liquid (condenses). In
this way heat is absorbed, or removed, from a low The refrigeration cycle can be broken down into temperature source and transferred to one at a the following stages:higher temperature.
1 - 2 Low pressure liquid refrigerant in the There are a number of factors, which make the evaporator absorbs heat from its operation of the vapour compression cycle surroundings, usually air, water or some possible:
2.1 THE VAPOUR
COMPRESSION
CYCLE
other process liquid. During this process it cooling for this process is usually achieved
changes its state from a liquid to a gas, and by using air or water. A further reduction in
at the evaporator exit is sl ight ly temperature happens in the pipe work and
superheated. liquid receiver (3b - 4), so that the
refrigerant liquid is sub-cooled as it enters
2 - 3 The superheated vapour enters the the expansion device.
compressor where its pressure is raised.
There will also be a big increase in 4 - 1 The high pressure sub-cooled liquid passes
temperature, because a proportion of the through the expansion device, which both
energy put into the compression process is reduces its pressure and controls the flow
transferred to the refrigerant. into the evaporator.
3 - 4 The high pressure superheated gas passes It can be seen that the condenser has to be capable
from the compressor into the condenser. of rejecting the combined heat inputs of the
The initial part of the cooling process (3 - evaporator and the compressor; i.e. (1 - 2) + (2 - 3)
3a) desuperheats the gas before it is then has to be the same as (3 - 4). There is no heat loss or
turned back into liquid (3a - 3b). The gain through the expansion device.
2 3
2. THE REFRIGERATION
PROCESS
Figure 1: Single stage vapour compression circuit and pressure enthalpy diagram (source: ETSU)
The majority of refrigeration systems are driven by � The temperature at which refrigerant boils
a machine, which compresses and pumps varies with its pressure; the higher the
refrigerant vapour around a sealed circuit. Heat is pressure, the higher the boiling point;
absorbed and rejected through heat exchangers. � When refrigerant liquid boils, changing its
These systems work on what is called a vapour state to a gas, it absorbs heat from its
compression cycle. surroundings;
� Refrigerant can be changed back from a gas
There are other types of plant which can be used to to a liquid by cooling it, usually by using air
obtain a cooling effect, such as absorption cycle or water.
systems, but these are not in common use and are
only economically viable where there are large Note: In the refrigeration industry the term
supplies of waste heat. evaporation is used instead of boiling. Also, if a gas is
heated above its boiling point it is said to be
superheated and if liquid is cooled below its
condensing temperature it is sub-cooled.
To enable the refrigerant to be condensed it has to
be compressed to a higher pressure, and it is at this
point that energy has to be used to drive the
machine that performs this task. The machine is Heat can only flow naturally from a hot to a colder called a compressor and it is usually driven by an body. In refrigeration system the opposite must electric motor.occur. This is achieved by using a substance called a
refrigerant, which absorbs heat and hence boils or The operation of a simple refrigeration system is evaporates at a low pressure to form a gas. This gas shown in Figure 1. The diagram shows the is then compressed to a higher pressure, such that it refrigerant pressure (bars) and its heat content transfers the heat it has gained to ambient air or (kJ/kg).water and turns back into a liquid (condenses). In
this way heat is absorbed, or removed, from a low The refrigeration cycle can be broken down into temperature source and transferred to one at a the following stages:higher temperature.
1 - 2 Low pressure liquid refrigerant in the There are a number of factors, which make the evaporator absorbs heat from its operation of the vapour compression cycle surroundings, usually air, water or some possible:
2.1 THE VAPOUR
COMPRESSION
CYCLE
other process liquid. During this process it cooling for this process is usually achieved
changes its state from a liquid to a gas, and by using air or water. A further reduction in
at the evaporator exit is sl ight ly temperature happens in the pipe work and
superheated. liquid receiver (3b - 4), so that the
refrigerant liquid is sub-cooled as it enters
2 - 3 The superheated vapour enters the the expansion device.
compressor where its pressure is raised.
There will also be a big increase in 4 - 1 The high pressure sub-cooled liquid passes
temperature, because a proportion of the through the expansion device, which both
energy put into the compression process is reduces its pressure and controls the flow
transferred to the refrigerant. into the evaporator.
3 - 4 The high pressure superheated gas passes It can be seen that the condenser has to be capable
from the compressor into the condenser. of rejecting the combined heat inputs of the
The initial part of the cooling process (3 - evaporator and the compressor; i.e. (1 - 2) + (2 - 3)
3a) desuperheats the gas before it is then has to be the same as (3 - 4). There is no heat loss or
turned back into liquid (3a - 3b). The gain through the expansion device.
2 3
2. THE REFRIGERATION
PROCESS
Figure 1: Single stage vapour compression circuit and pressure enthalpy diagram (source: ETSU)
2.2. REVERSE CARNOT
CYCLE
2.2.1 COEFFICIENT OF
PERFORMANCE
� 3 to 4 is constant entropy (ideal)
expansion from a higher to a lower
pressure through the throttling device.
The Carnot Cycle is a theoretical model From the diagram, the concept of Coefficient of representing the basic processes of a heat engine. A Performance (COP) is derived. The COP is the heat engine is a devide which produces work from ratio of the cooling or Refrigeration Effect (RE), to heat. The Reverse Carnot cycle produces a transfer the work required to produce the effect.of heat from work. From the model, the maximum
theoretical performance can be calculated,
establishing criteria to which real refrigeration
cycles can be compared.
The following processes occur in the Reverse The refrigeration effect is represented as the area Carnot Cycle (Figure 2).under the process line 4 - 1.
� 4 to 1 is the absorption of heat at the RE = T × (s - s )L 1 4evaporator, a constant temperature
boiling process at T .LWhere, RE = Refrigeration effect (kJ)� 1 to 2 is constant entropy (ideal) T = Temperature (K)Lcompression. Work input is required and s , s = Entropy [kJ/kg·K)J1 4the temperature of the refrigerant
increases.
The theoretical work input (W ) (i.e. energy � 2 to 3 is heat rejection at the condenser, a S
requirement) for the cycle is represented by the constant temperature process at T .H
area "within" the cycle line 1-2-3-4-1. Example: two refrigeration machines of similar
capacity are compared. One has a COP of 4.0 while W = (T - T ) × (s s ) kJ/kgS H L 4 1 the second a COP of 3.0 at the same operating
conditions. The first machine with the higher COP The equation for coefficient of performance (COP) is the most efficient, producing 1.33 times the is obtained by dividing the refrigeration effect (RE) refrigeration effect for the same work input of the by the theoretical work input (W ).S second machine. The figures above show the effect
of evaporator and condenser temperatures on the COP = =
COP for various types of chillers.
The coefficient of performance for this theoretical The theoretical COP can also be expressed in
system is temperature dependent and can be terms of enthalpy, where the difference in energy
reduced to:content of the refrigerant at various points of the
cycle define the cooling effect and the work input.COP (Ideal) =
Actual systems are not as efficient as the ideal or COP =
theoretical model (i.e. lower COP), but the
following general conclusion applies: The smaller
the temperature difference between the heat sink
and the heat source, (T - T ) the greater the H L
efficiency of the refrigeration (or heat pump)
system. The COP, a measure of the energy
The Carnot cycle, although a useful model to assist required to produce a given refrigeration effect, is
in the understanding of the refrigeration process, an excellent means of comparing the efficiencies of
has certain limitations. One limitation is the lack of similar equipment.
2.3 THEORETICAL VAPOUR
COMPRESSION CYCLE
4 5
Figure 2: Reverse Carnot Cycle (source: CEMET)
REWS
T x (s - )L 1
(T - ) x H
s4
s4T (s - )L 1
T L(T - )H TL
Figure 3: Effects of evaporator and condensing temperature on chiller COP. (source: CEMET)
(h - h )1 4 (h - h )2 1
2.2. REVERSE CARNOT
CYCLE
2.2.1 COEFFICIENT OF
PERFORMANCE
� 3 to 4 is constant entropy (ideal)
expansion from a higher to a lower
pressure through the throttling device.
The Carnot Cycle is a theoretical model From the diagram, the concept of Coefficient of representing the basic processes of a heat engine. A Performance (COP) is derived. The COP is the heat engine is a devide which produces work from ratio of the cooling or Refrigeration Effect (RE), to heat. The Reverse Carnot cycle produces a transfer the work required to produce the effect.of heat from work. From the model, the maximum
theoretical performance can be calculated,
establishing criteria to which real refrigeration
cycles can be compared.
The following processes occur in the Reverse The refrigeration effect is represented as the area Carnot Cycle (Figure 2).under the process line 4 - 1.
� 4 to 1 is the absorption of heat at the RE = T × (s - s )L 1 4evaporator, a constant temperature
boiling process at T .LWhere, RE = Refrigeration effect (kJ)� 1 to 2 is constant entropy (ideal) T = Temperature (K)Lcompression. Work input is required and s , s = Entropy [kJ/kg·K)J1 4the temperature of the refrigerant
increases.
The theoretical work input (W ) (i.e. energy � 2 to 3 is heat rejection at the condenser, a S
requirement) for the cycle is represented by the constant temperature process at T .H
area "within" the cycle line 1-2-3-4-1. Example: two refrigeration machines of similar
capacity are compared. One has a COP of 4.0 while W = (T - T ) × (s s ) kJ/kgS H L 4 1 the second a COP of 3.0 at the same operating
conditions. The first machine with the higher COP The equation for coefficient of performance (COP) is the most efficient, producing 1.33 times the is obtained by dividing the refrigeration effect (RE) refrigeration effect for the same work input of the by the theoretical work input (W ).S second machine. The figures above show the effect
of evaporator and condenser temperatures on the COP = =
COP for various types of chillers.
The coefficient of performance for this theoretical The theoretical COP can also be expressed in
system is temperature dependent and can be terms of enthalpy, where the difference in energy
reduced to:content of the refrigerant at various points of the
cycle define the cooling effect and the work input.COP (Ideal) =
Actual systems are not as efficient as the ideal or COP =
theoretical model (i.e. lower COP), but the
following general conclusion applies: The smaller
the temperature difference between the heat sink
and the heat source, (T - T ) the greater the H L
efficiency of the refrigeration (or heat pump)
system. The COP, a measure of the energy
The Carnot cycle, although a useful model to assist required to produce a given refrigeration effect, is
in the understanding of the refrigeration process, an excellent means of comparing the efficiencies of
has certain limitations. One limitation is the lack of similar equipment.
2.3 THEORETICAL VAPOUR
COMPRESSION CYCLE
4 5
Figure 2: Reverse Carnot Cycle (source: CEMET)
REWS
T x (s - )L 1
(T - ) x H
s4
s4T (s - )L 1
T L(T - )H TL
Figure 3: Effects of evaporator and condensing temperature on chiller COP. (source: CEMET)
(h - h )1 4 (h - h )2 1
accounting for changes of state. The figure below condenser. Step 2 2' is the initial de-superheating
shows a vapour compression cycle approximating of the hot gas at the condenser or intermediate
the effect of the cycle on the refrigerant, assuming equipment, and 2' - 3 is the condensation process.
ideal equipment, where:
� 1 - 2 Compression.
� 2 - 2' Desuperheating.
� 2' - 3 Constant Temperature
Condensation.As in the Reverse Carnot cycle, the coefficient or
� 2 - 4' Throttling.performance is:
� 4' - 1 Constant Temperature
Evaporation. COP(refrig) = refrigeration effect
Work input
Assuming that the compression process starts at
COP(refrig) = =point 1 as a saturated vapour, energy added in the
form of shaft work will raise the temperature and
pressure. Ideally, this is a constant entropy process Where h ' = h4 3
represented by a vertical line on the T-s diagram.
Departures from the ideal Carnot cycle are The net result is superheating of the vapour to
apparent.point 2. Process 2 2' 3 is heat rejection at the
2.3.1 MODEL COEFFICIENT
OF PERFORMANCE
� [h - h ](theoretical) is larger than [h - limitations such as equipment size, system pressure, 2 1 2
and design temperatures at the evaporator and h ](Carnot).1
condenser, reduce the effectiveness of actual � [h - h ](theoretical) is smaller than [h - 1 4 1
1systems. Actual COPs are 20 to 30 per cent of the h ](Carnot).4
theoretical COP based on the Carnot cycle
operating at the same conditions. Individual The net effect is a COP reduction.components, such as the compressor, may have an
effectiveness of 40 to 60 per cent of the theoretical The throttling process reduces the refrigerant
COP (Figure below). These limitations, and pressure from the condensing (high) pressure side
techniques used to reduce their input on cycle to the evaporator (low) pressure side. By definition,
efficiency, are now discussed.throttling is a constant enthalpy process. The
enthalpy at point 3 is equal to that at point 4', thus h 3
= h '. Energy is degraded in this process, therefore 4
the entropy must increase from point 3' to 4.
Operating temperatures in actual cycles are
established to suit the temperatures required at the
cold medium and the temperature acceptable for
the heat sink. The practical temperature gradient
required to transfer heat from one fluid to another
Refrigeration and heat pump cycles are more through a heat exchanger is in the range of 5 to 8ºC.
complex than the theoretical vapour compression This means that the refrigerant entering the
cycle discussed in the previous sector. Practical evaporator should be 5 to 8ºC colder than the
2.3.2.1 Heat Transfer
2.3.2 PRACTICAL
CONSIDERATIONS
6 7
Figure 4: Basic Refrigeration Cycle. (source: CEMET)
T L(T - )H T L
h - h1 4 h - h2 1
Figure 5: Effectiveness of Reciprocating compressors. (source: CEMET)
1 An example of measuring COP directly is given in Appendix 3
accounting for changes of state. The figure below condenser. Step 2 2' is the initial de-superheating
shows a vapour compression cycle approximating of the hot gas at the condenser or intermediate
the effect of the cycle on the refrigerant, assuming equipment, and 2' - 3 is the condensation process.
ideal equipment, where:
� 1 - 2 Compression.
� 2 - 2' Desuperheating.
� 2' - 3 Constant Temperature
Condensation.As in the Reverse Carnot cycle, the coefficient or
� 2 - 4' Throttling.performance is:
� 4' - 1 Constant Temperature
Evaporation. COP(refrig) = refrigeration effect
Work input
Assuming that the compression process starts at
COP(refrig) = =point 1 as a saturated vapour, energy added in the
form of shaft work will raise the temperature and
pressure. Ideally, this is a constant entropy process Where h ' = h4 3
represented by a vertical line on the T-s diagram.
Departures from the ideal Carnot cycle are The net result is superheating of the vapour to
apparent.point 2. Process 2 2' 3 is heat rejection at the
2.3.1 MODEL COEFFICIENT
OF PERFORMANCE
� [h - h ](theoretical) is larger than [h - limitations such as equipment size, system pressure, 2 1 2
and design temperatures at the evaporator and h ](Carnot).1
condenser, reduce the effectiveness of actual � [h - h ](theoretical) is smaller than [h - 1 4 1
1systems. Actual COPs are 20 to 30 per cent of the h ](Carnot).4
theoretical COP based on the Carnot cycle
operating at the same conditions. Individual The net effect is a COP reduction.components, such as the compressor, may have an
effectiveness of 40 to 60 per cent of the theoretical The throttling process reduces the refrigerant
COP (Figure below). These limitations, and pressure from the condensing (high) pressure side
techniques used to reduce their input on cycle to the evaporator (low) pressure side. By definition,
efficiency, are now discussed.throttling is a constant enthalpy process. The
enthalpy at point 3 is equal to that at point 4', thus h 3
= h '. Energy is degraded in this process, therefore 4
the entropy must increase from point 3' to 4.
Operating temperatures in actual cycles are
established to suit the temperatures required at the
cold medium and the temperature acceptable for
the heat sink. The practical temperature gradient
required to transfer heat from one fluid to another
Refrigeration and heat pump cycles are more through a heat exchanger is in the range of 5 to 8ºC.
complex than the theoretical vapour compression This means that the refrigerant entering the
cycle discussed in the previous sector. Practical evaporator should be 5 to 8ºC colder than the
2.3.2.1 Heat Transfer
2.3.2 PRACTICAL
CONSIDERATIONS
6 7
Figure 4: Basic Refrigeration Cycle. (source: CEMET)
T L(T - )H T L
h - h1 4 h - h2 1
Figure 5: Effectiveness of Reciprocating compressors. (source: CEMET)
1 An example of measuring COP directly is given in Appendix 3
desired medium temperature. The saturation When the superheating occurs at the evaporator, 0 the enthalpy of the refrigerant is raised, extracting temperature at the condenser should be 5 to 8 C
additional heat and increasing the refrigeration above the temperature of the heat rejection
effect of the evaporator. When superheating medium (Figure below).
occurs in the compressor suction piping, no useful
cooling occurs.The area enclosed by line l - 2 - 3 - 4' - l, which
describes the cycle, has increased because of the
temperature difference required to drive the The increase in refrigeration effect, caused by
transfer process. There has been an increase in the superheating in the evaporator, is usually offset by a
work required to produce the refrigeration effect decrease in refrigeration effect at the compressor.
because the temperature difference has increased, Because the volumetric flow rate of a compressor is
(T - T ). constant, the mass flow rate and refrigerating effect H L
are reduced by decreases in refrigerant density
caused by the superheating. The relative effects of
increases in enthalpy and decreases in density must
be calculated in detail. A study of the system design
may be practical only for systems over 500 kW in In the refrigerant cycle, refrigerant gas becomes capacity. There is a loss in refrigerating capacity of superheated at the evaporator and at the about one per cent for every 2.5ºC of superheat in compressor (Figure 6). During the evaporation the suction line of a reciprocating compressor. process the refrigerant is completely vaporized Insulation on suction lines will minimize the part-way through the evaporator. As the cool undesirable heat gain.refrigerant vapour continues through the
evaporator, additional heat is absorbed which
Refrigerant superheating also occurs at the superheats the vapour. Pressure losses, caused by
compressor. The refrigerant enters the compressor friction, further increase the amount of superheat.
2.3.2.2 Superheat
as a saturated vapour. Increasing the pressure will gas) leaving the compressor will reduce the
increase the temperature and cause superheat. required condenser capacity, and provide a high-
Friction, system inefficiency and the work added, grade heat source for other process use. A typical
raise the entropy and superheat above that application would be the preheating of boiler make-
occurring in the theoretical cycle. Superheat, up or process water. The total amount of heat
caused by the compression process, does not available as superheat can be difficult to predict, as
improve cycle efficiency, but results in larger the superheat fluctuates with changes in load
condensing equipment and large compressor conditions. If a use can be found for low-grade heat,
discharge piping. the total condensing load can be reclaimed. This
can result in substantial energy savings.
Desuperheating is the process of removing excess
heat from superheated refrigerant vapour, and
when accomplished by means external to the cycle,
can be beneficial to system performance.
Desuperheating the suction gas is often impractical
because of the low temperatures (less than 10 ºC)
and the small amount of available energy. Some Liquid subcooling occurs when a liquid refrigerant is
superheat is required to prevent slugs of liquid cooled at constant pressure to below the
refrigerant from reaching the compressor and condensation temperature (Figure 7). When
causing serious damage. At design conditions, subcooling occurs by a heat transfer method
superheat can account for 20 per cent of the heat external to the refrigeration cycle, the refrigerating
rejected at the condensers, and often raises effect of the system is increased because the
condensing temperatures above 45ºC. enthalpy of the subcooled liquid is less than the
enthalpy of the saturated liquid. Subcooling of the
Desuperheating the high-pressure refrigerant (hot liquid upstream of the throttling device also reduces
2.3.2.3 FLASH GAS AND
SUBCOOLING
8 9
Figure: 6: Heat exchanger limitations and the effects of superheating. (source: CEMET)
Figure 7: Effect of Subcooling (source: CEMET)
desired medium temperature. The saturation When the superheating occurs at the evaporator, 0 the enthalpy of the refrigerant is raised, extracting temperature at the condenser should be 5 to 8 C
additional heat and increasing the refrigeration above the temperature of the heat rejection
effect of the evaporator. When superheating medium (Figure below).
occurs in the compressor suction piping, no useful
cooling occurs.The area enclosed by line l - 2 - 3 - 4' - l, which
describes the cycle, has increased because of the
temperature difference required to drive the The increase in refrigeration effect, caused by
transfer process. There has been an increase in the superheating in the evaporator, is usually offset by a
work required to produce the refrigeration effect decrease in refrigeration effect at the compressor.
because the temperature difference has increased, Because the volumetric flow rate of a compressor is
(T - T ). constant, the mass flow rate and refrigerating effect H L
are reduced by decreases in refrigerant density
caused by the superheating. The relative effects of
increases in enthalpy and decreases in density must
be calculated in detail. A study of the system design
may be practical only for systems over 500 kW in In the refrigerant cycle, refrigerant gas becomes capacity. There is a loss in refrigerating capacity of superheated at the evaporator and at the about one per cent for every 2.5ºC of superheat in compressor (Figure 6). During the evaporation the suction line of a reciprocating compressor. process the refrigerant is completely vaporized Insulation on suction lines will minimize the part-way through the evaporator. As the cool undesirable heat gain.refrigerant vapour continues through the
evaporator, additional heat is absorbed which
Refrigerant superheating also occurs at the superheats the vapour. Pressure losses, caused by
compressor. The refrigerant enters the compressor friction, further increase the amount of superheat.
2.3.2.2 Superheat
as a saturated vapour. Increasing the pressure will gas) leaving the compressor will reduce the
increase the temperature and cause superheat. required condenser capacity, and provide a high-
Friction, system inefficiency and the work added, grade heat source for other process use. A typical
raise the entropy and superheat above that application would be the preheating of boiler make-
occurring in the theoretical cycle. Superheat, up or process water. The total amount of heat
caused by the compression process, does not available as superheat can be difficult to predict, as
improve cycle efficiency, but results in larger the superheat fluctuates with changes in load
condensing equipment and large compressor conditions. If a use can be found for low-grade heat,
discharge piping. the total condensing load can be reclaimed. This
can result in substantial energy savings.
Desuperheating is the process of removing excess
heat from superheated refrigerant vapour, and
when accomplished by means external to the cycle,
can be beneficial to system performance.
Desuperheating the suction gas is often impractical
because of the low temperatures (less than 10 ºC)
and the small amount of available energy. Some Liquid subcooling occurs when a liquid refrigerant is
superheat is required to prevent slugs of liquid cooled at constant pressure to below the
refrigerant from reaching the compressor and condensation temperature (Figure 7). When
causing serious damage. At design conditions, subcooling occurs by a heat transfer method
superheat can account for 20 per cent of the heat external to the refrigeration cycle, the refrigerating
rejected at the condensers, and often raises effect of the system is increased because the
condensing temperatures above 45ºC. enthalpy of the subcooled liquid is less than the
enthalpy of the saturated liquid. Subcooling of the
Desuperheating the high-pressure refrigerant (hot liquid upstream of the throttling device also reduces
2.3.2.3 FLASH GAS AND
SUBCOOLING
8 9
Figure: 6: Heat exchanger limitations and the effects of superheating. (source: CEMET)
Figure 7: Effect of Subcooling (source: CEMET)
flashing in the liquid piping. The work input is cent for an 8 cylinder unit. For centrifugal
reduced, and the refrigeration effect is increased equipment, the bypass varies with the load and
because (h h ) is less than (h h '). impeller characteristics.1 4 1 4
Subcooling refrigerant R-22 by 13ºC increases the
refrigeration effect by about 11 per cent. If
subcooling is obtained from outside the cycle, each
degree increase in subcooling will improve system When a refrigeration system operates with the capacity by approximately one per cent. Subcooling evaporator temperature close to 0ºC, or less, from within the cycle may not be as effective frosting of the evaporator coil is inevitable. because of offsetting effects in other parts of the Examples of this would be the frosting of heat cycle. pump evaporator coils during winter operation, or
freezer evaporators. Ice buildup on the coils lowers Subcooling capacity can be increased by providing the heat transfer rate, effectively reducing the additional cooling circuits in the condenser or by refrigeration effect. The suction temperature will immersing the liquid receiver in a cooling tower fall as the heat transfer rate falls, further increasing sump. Most systems provide 5 to 7ºC subcooling at the rate of ice buildup. For systems operating under the condenser to improve system efficiency. these conditions defrosting accessories are
available from the equipment manufacturer.
Defrost is performed by reversing the refrigerant
flow, so that the system operates in an air-Hot gas bypass is a method of placing an artificial conditioning mode, using the evaporator as the heat load on the refrigeration system to produce condenser to reject heat through the frosted coils. stable suction pressures and temperatures, when In a heat pump system used for heating, a back-up the refrigeration load is very low. The heat load is heating system is required to prevent chilling the produced by bypassing hot gas from the space during the defrost mode. Defrosting is a compressor discharge to the evaporator inlet or major consumer of energy. It is important that the the compressor suction. While permitting stable controls optimise the defrost cycle to avoid compressor operation at low load, hot gas bypass unnecessary defrosting while preventing unwanted wastes energy. Bypass is required to maintain ice build-up.evaporator temperature above freezing, and
prevent frosting of the coil, freezing of the chilled
water, and compressor cycling.
The total refrigeration load on a compressor with
The heat pump is a separate class of compression hot gas bypass will be equal to the actual (low) load
refrigeration equipment whose main purpose is to plus the amount of hot gas bypass. Typically, the hot
transfer heat from a low temperature heat source gas bypass on a reciprocating machine is 25 per cent
to a higher temperature heat sink for heating, rather of the nominal refrigeration capacity for a 4 cylinder
than for cooling. The coefficient of performance in unit, 33 per cent for a 6 cylinder unit and 37.5 per
2.3.2.5 EVAPORATOR FROSTING
2.3.2.4 HOT GAS BYPASS
2.3.2.6 HEAT PUMP CYCLE
the heating configuration is: The steps in an absorption refrigeration cycle are:
COP(Heat Pump) = 1. Liquid refrigerant is vaporized in the
evaporator absorbing heat from the = T H medium to be cooled
(T - T )H L 2. The suction effect necessary to draw the
vapour through the system is ac-In a heat pump system where both heating and
complished by bringing the refrigerant into cooling are required, a special four-way valve is
contact with a solvent. The solvent's affinity used to reverse the functions of the evaporator and
for the refrigerant causes the refrigerant to condenser. In this manner, the coil or exchanger is
be absorbed by the solution, reducing the used to supply heating or cooling as required.
pressure of the refrigerant vapour. The Alternatively, the piping or ductwork system
absorption process releases heat which external to the heat pump can be provided with
must be removed from this portion of the valves or dampers to reverse the primary air or fluid
cycle. The solution of refrigerant and flows, without the reversing valve. The heat pump
solvent (weak liquor) is pumped from cycle is identical to a standard refrigeration cycle on
the absorber at low pressure, to the a T-s diagram (Figure 2).
generator at a high pressure.
3. Heat is added to the weak liquor to drive
the refrigerant out of solution. A heat
exchanger is located between the
absorber and generator. Heat is removed from
the strong liquor (solution with high solvent The absorption refrigeration cycle is similar to the and low refrigerant concentrations) leaving vapour compression cycle, however instead of the generator, and is added to the weak using a compressor, high pressures are obtained by liquor entering the generator, reducing the cycle applying heat to a refrigerant solution. heat input.The system operates on the principle that variations 4. Further heat added to the weak liquor in in refrigerant solubility can be obtained by changing the generator drives the refrigerant out of solution temperatures and pressures. Absorption solution providing a high pressure systems in industry often use ammonia as the refrigerant vapour. The hot solvent, still refrigerant in a water solvent, whereas in containing some refrigerant (strong liquor), commercial and institutional applications water is returns to the absorber through the heat used as the refrigerant in a lithium bromide solvent. exchanger where the solvent cycle
repeats.
The basic components of an absorption system are 5. Vapour at high-pressure and temperature
the vapour absorber, solution transfer pumps, and a flows to the condenser where heat is
vapour regenerator (solvent concentrator) in rejected through a coil or heat exchanger
addition to the evaporator and condenser. during the condensation process.
2.4 ABSORPTION CYCLE
10 11
2Refrigeration effect plus work input
Net work input
2 i.e. 'Heat 'pumped' to the hot surface.
flashing in the liquid piping. The work input is cent for an 8 cylinder unit. For centrifugal
reduced, and the refrigeration effect is increased equipment, the bypass varies with the load and
because (h h ) is less than (h h '). impeller characteristics.1 4 1 4
Subcooling refrigerant R-22 by 13ºC increases the
refrigeration effect by about 11 per cent. If
subcooling is obtained from outside the cycle, each
degree increase in subcooling will improve system When a refrigeration system operates with the capacity by approximately one per cent. Subcooling evaporator temperature close to 0ºC, or less, from within the cycle may not be as effective frosting of the evaporator coil is inevitable. because of offsetting effects in other parts of the Examples of this would be the frosting of heat cycle. pump evaporator coils during winter operation, or
freezer evaporators. Ice buildup on the coils lowers Subcooling capacity can be increased by providing the heat transfer rate, effectively reducing the additional cooling circuits in the condenser or by refrigeration effect. The suction temperature will immersing the liquid receiver in a cooling tower fall as the heat transfer rate falls, further increasing sump. Most systems provide 5 to 7ºC subcooling at the rate of ice buildup. For systems operating under the condenser to improve system efficiency. these conditions defrosting accessories are
available from the equipment manufacturer.
Defrost is performed by reversing the refrigerant
flow, so that the system operates in an air-Hot gas bypass is a method of placing an artificial conditioning mode, using the evaporator as the heat load on the refrigeration system to produce condenser to reject heat through the frosted coils. stable suction pressures and temperatures, when In a heat pump system used for heating, a back-up the refrigeration load is very low. The heat load is heating system is required to prevent chilling the produced by bypassing hot gas from the space during the defrost mode. Defrosting is a compressor discharge to the evaporator inlet or major consumer of energy. It is important that the the compressor suction. While permitting stable controls optimise the defrost cycle to avoid compressor operation at low load, hot gas bypass unnecessary defrosting while preventing unwanted wastes energy. Bypass is required to maintain ice build-up.evaporator temperature above freezing, and
prevent frosting of the coil, freezing of the chilled
water, and compressor cycling.
The total refrigeration load on a compressor with
The heat pump is a separate class of compression hot gas bypass will be equal to the actual (low) load
refrigeration equipment whose main purpose is to plus the amount of hot gas bypass. Typically, the hot
transfer heat from a low temperature heat source gas bypass on a reciprocating machine is 25 per cent
to a higher temperature heat sink for heating, rather of the nominal refrigeration capacity for a 4 cylinder
than for cooling. The coefficient of performance in unit, 33 per cent for a 6 cylinder unit and 37.5 per
2.3.2.5 EVAPORATOR FROSTING
2.3.2.4 HOT GAS BYPASS
2.3.2.6 HEAT PUMP CYCLE
the heating configuration is: The steps in an absorption refrigeration cycle are:
COP(Heat Pump) = 1. Liquid refrigerant is vaporized in the
evaporator absorbing heat from the = T H medium to be cooled
(T - T )H L 2. The suction effect necessary to draw the
vapour through the system is ac-In a heat pump system where both heating and
complished by bringing the refrigerant into cooling are required, a special four-way valve is
contact with a solvent. The solvent's affinity used to reverse the functions of the evaporator and
for the refrigerant causes the refrigerant to condenser. In this manner, the coil or exchanger is
be absorbed by the solution, reducing the used to supply heating or cooling as required.
pressure of the refrigerant vapour. The Alternatively, the piping or ductwork system
absorption process releases heat which external to the heat pump can be provided with
must be removed from this portion of the valves or dampers to reverse the primary air or fluid
cycle. The solution of refrigerant and flows, without the reversing valve. The heat pump
solvent (weak liquor) is pumped from cycle is identical to a standard refrigeration cycle on
the absorber at low pressure, to the a T-s diagram (Figure 2).
generator at a high pressure.
3. Heat is added to the weak liquor to drive
the refrigerant out of solution. A heat
exchanger is located between the
absorber and generator. Heat is removed from
the strong liquor (solution with high solvent The absorption refrigeration cycle is similar to the and low refrigerant concentrations) leaving vapour compression cycle, however instead of the generator, and is added to the weak using a compressor, high pressures are obtained by liquor entering the generator, reducing the cycle applying heat to a refrigerant solution. heat input.The system operates on the principle that variations 4. Further heat added to the weak liquor in in refrigerant solubility can be obtained by changing the generator drives the refrigerant out of solution temperatures and pressures. Absorption solution providing a high pressure systems in industry often use ammonia as the refrigerant vapour. The hot solvent, still refrigerant in a water solvent, whereas in containing some refrigerant (strong liquor), commercial and institutional applications water is returns to the absorber through the heat used as the refrigerant in a lithium bromide solvent. exchanger where the solvent cycle
repeats.
The basic components of an absorption system are 5. Vapour at high-pressure and temperature
the vapour absorber, solution transfer pumps, and a flows to the condenser where heat is
vapour regenerator (solvent concentrator) in rejected through a coil or heat exchanger
addition to the evaporator and condenser. during the condensation process.
2.4 ABSORPTION CYCLE
10 11
2Refrigeration effect plus work input
Net work input
2 i.e. 'Heat 'pumped' to the hot surface.
6. The pressure of the liquid refrigerant is
reduced by passing through a throttling
device before returning to the evaporator
section. The complete cycle is shown in
Figure 8.
The generator may be equipped with a rectifier for
selective distillation of refrigerant from the solution.
This feature is common in large ammonia systems.
Performance of an absorption chiller is measured
by the COP, the ratio of actual cooling or heating
effect, to the energy used to obtain that effect. The
best ratios are less than one for cooling and 1.2 to
1.4 for a heat pump application. Compared to
compression cycles this is low, but if high-
temperature waste heat can be utilized to
regenerate the refrigerant, refrigeration can be
obtained at reasonable costs.
System performance is affected by:
The flow diagram of a two-shell lithium bromide
� Heat source temperature. chiller is shown in Figure 9. Figure 10 shows an
� Temperature of medium being cooled. alternative configuration of an absorption machine
� Temperature of the heat sink. using only a single shell. Actual installations vary
considerably in layout, number of components and Well water, or any other clean water below l5ºC,
accessories, application and refrigerant type. can be used for cooling or precooling ventilation air,
or a process.
Steam jet refrigeration systems use steam ejectors to
reduce the pressure in a tank containing the return
water from a chilled water system. Flashing a
portion of the water in the tank reduces the liquid
temperature. The chilled water is then used directly The cooling effect of an evaporator is proportional or passed through an exchanger to cool another to the length of the line between points 1 and 2 in heat transfer fluid.
2.5 SPECIAL 2.6 VARIATIONS ON THE REFRIGERATION
SIMPLE CARNOTSYSTEMSCIRCUIT
2.6.1 SUCTION/LIQUID HEAT
EXCHANGER
Figure 8: Absorption Refrigeration Cycle. (source: CEMET)
Figure 9: Diagram of a Two-Shell Lithium
Bromide Cycle Water Chiller.
(source: CEMET)
Figure 10: Single shell configuration. (source: CEMET)
12 13
6. The pressure of the liquid refrigerant is
reduced by passing through a throttling
device before returning to the evaporator
section. The complete cycle is shown in
Figure 8.
The generator may be equipped with a rectifier for
selective distillation of refrigerant from the solution.
This feature is common in large ammonia systems.
Performance of an absorption chiller is measured
by the COP, the ratio of actual cooling or heating
effect, to the energy used to obtain that effect. The
best ratios are less than one for cooling and 1.2 to
1.4 for a heat pump application. Compared to
compression cycles this is low, but if high-
temperature waste heat can be utilized to
regenerate the refrigerant, refrigeration can be
obtained at reasonable costs.
System performance is affected by:
The flow diagram of a two-shell lithium bromide
� Heat source temperature. chiller is shown in Figure 9. Figure 10 shows an
� Temperature of medium being cooled. alternative configuration of an absorption machine
� Temperature of the heat sink. using only a single shell. Actual installations vary
considerably in layout, number of components and Well water, or any other clean water below l5ºC,
accessories, application and refrigerant type. can be used for cooling or precooling ventilation air,
or a process.
Steam jet refrigeration systems use steam ejectors to
reduce the pressure in a tank containing the return
water from a chilled water system. Flashing a
portion of the water in the tank reduces the liquid
temperature. The chilled water is then used directly The cooling effect of an evaporator is proportional or passed through an exchanger to cool another to the length of the line between points 1 and 2 in heat transfer fluid.
2.5 SPECIAL 2.6 VARIATIONS ON THE REFRIGERATION
SIMPLE CARNOTSYSTEMSCIRCUIT
2.6.1 SUCTION/LIQUID HEAT
EXCHANGER
Figure 8: Absorption Refrigeration Cycle. (source: CEMET)
Figure 9: Diagram of a Two-Shell Lithium
Bromide Cycle Water Chiller.
(source: CEMET)
Figure 10: Single shell configuration. (source: CEMET)
12 13
Figure 1. Additional cooling can be obtained by temperatures . In genera l, the evaporating
increasing the amount of subcooling at the inlet to temperature below which a suction/liquid heat
the expansion device. exchanger no longer becomes viable is about 15º
C. Care must also be taken when using these heat
The temperature of the refrigerant leaving the exchangers on systems with R22 and R717
evaporator will be lower than that of the liquid (ammonia) refrigerants, where the increased
entering the expansion device. Therefore, it is suction temperature at the compressor could result
possible to reduce the liquid temperature by using a in an excessive discharge temperature.
heat exchanger between these two pipes. A
schematic layout showing how a suction/liquid heat
exchanger can be incorporated into a refrigeration
circuit is given in Figure 2.
It is often desirable to operate more than one
evaporator on the same system. This is not a
problem if all if the evaporators are working it the
same temperature, as they can simply be connected
in parallel.
If, however, one evaporator is required to work it a
lower temperature than the others, it will be
necessary to operate the compressor(s) at the
pressure required by the lower temperature. The
other evaporators will then have to be controlled at
a higher pressure by installing evaporator pressure
regulators between the exit of the evaporator and
the suction into the compressor(s). The
disadvantage of this is that operating the system at
Figure 11: The suction line heat exchanger. the lower suction pressure will reduce the
(source: ETSU) compressor's efficiency and capacity. If the main
load has the lower temperature, then the cost of
installing an additional system for the small higher It must be remembered that there will be a
temperature load would probably not be corresponding increase in the suction gas
economic, despite the increase in efficiency which temperature entering the compressor, which will
would result. If the opposite case exists, it will reduce its capacity as the gas will be less dense and,
almost certainly be better to put the small low therefore, a lower mass of refrigerant will be
temperature load on its own individual system and pumped by the compressor. Experience has shown
run the main load at a higher, and hence more that an overall improvement in the system's
energy efficient, evaporating pressure.efficiency will be gained at high evaporating
2.7 MULTIPLE
EVAPORATOR
CIRCUITS
2.7.1 MULTIPLE COMPRESSOR
SYSTEMS
2.7.1.2 INTERNALLY COMPOUNDED
COMPRESSORS
2.7.1.1 TWO STAGE SYSTEMS
2.7.1.3 EXTERNALLY COMPOUNDING
COMPRESSORS
taken to ensure the liquid does not get significantly
warmer, so that it begins to evaporate, before it
enters the expansion device.
In many systems the load is too great to be handled
practically with one compressor. In these cases
compressors are connected in parallel, which has
the added advantage that their use can be cycled in
order to adjust the capacity to suit the load.
Two stage compression can be achieved within
one, specially designed, compressor. The gas is
compressed to the intermediate pressure in the
first, low stage cylinder(s) and then compressed to
the condensing pressure in the high stage Two stage, or compound, systems are used when
cylinder(s). The intermediate condition is called the there is a large difference between the evaporating
interstage pressure, and some form of cooling is and condensing temperatures. This usually occurs
usually used to reduce the temperature of the when process or product storage conditions
refrigerant before it enters the second stage of require a low evaporating temperature, such as in
compression.freeze drying or ice cream storage.
The selection and application of such a compressor At these compression ratios two stage systems
is relatively simple; however, there are a limited have to be used because a single stage system
number of compressor variations available. would result in an unacceptably high discharge
Selecting a design that matches a specific system temperature in the compressor. In addition, in
requirement usually results in a compromise which some cases two stage compression can give more
is made at the expense of energy efficiency. The efficient compressor operation.
fixed volume ratio of the two compression stages
also means that efficiencies are lower than they There is no easy rule to determine where two stage
could be where demand varies.compression, with its additional design and
installation complexity, becomes preferable to
single stage compression. Generally, with
refrigerants like R22, two stage compression may
be used on systems using suction cooled
compressors evaporating below about - 30ºC.
In this case two stage compression is achieved by
There are two ways that two stage compression using two separate compressors - one for the low
can be achieved and the method selected will affect stage and another for the high. This more flexible
efficiency. In both cases, additional system capacity approach enables the system designer to match a
can be obtained by first passing the refrigerant used compressor combination to the load more
for interstage cooling through a liquid line accurately and select the most economical
subcooler. If this method is used, care must be interstage pressure.
14 15
Figure 1. Additional cooling can be obtained by temperatures . In genera l, the evaporating
increasing the amount of subcooling at the inlet to temperature below which a suction/liquid heat
the expansion device. exchanger no longer becomes viable is about 15º
C. Care must also be taken when using these heat
The temperature of the refrigerant leaving the exchangers on systems with R22 and R717
evaporator will be lower than that of the liquid (ammonia) refrigerants, where the increased
entering the expansion device. Therefore, it is suction temperature at the compressor could result
possible to reduce the liquid temperature by using a in an excessive discharge temperature.
heat exchanger between these two pipes. A
schematic layout showing how a suction/liquid heat
exchanger can be incorporated into a refrigeration
circuit is given in Figure 2.
It is often desirable to operate more than one
evaporator on the same system. This is not a
problem if all if the evaporators are working it the
same temperature, as they can simply be connected
in parallel.
If, however, one evaporator is required to work it a
lower temperature than the others, it will be
necessary to operate the compressor(s) at the
pressure required by the lower temperature. The
other evaporators will then have to be controlled at
a higher pressure by installing evaporator pressure
regulators between the exit of the evaporator and
the suction into the compressor(s). The
disadvantage of this is that operating the system at
Figure 11: The suction line heat exchanger. the lower suction pressure will reduce the
(source: ETSU) compressor's efficiency and capacity. If the main
load has the lower temperature, then the cost of
installing an additional system for the small higher It must be remembered that there will be a
temperature load would probably not be corresponding increase in the suction gas
economic, despite the increase in efficiency which temperature entering the compressor, which will
would result. If the opposite case exists, it will reduce its capacity as the gas will be less dense and,
almost certainly be better to put the small low therefore, a lower mass of refrigerant will be
temperature load on its own individual system and pumped by the compressor. Experience has shown
run the main load at a higher, and hence more that an overall improvement in the system's
energy efficient, evaporating pressure.efficiency will be gained at high evaporating
2.7 MULTIPLE
EVAPORATOR
CIRCUITS
2.7.1 MULTIPLE COMPRESSOR
SYSTEMS
2.7.1.2 INTERNALLY COMPOUNDED
COMPRESSORS
2.7.1.1 TWO STAGE SYSTEMS
2.7.1.3 EXTERNALLY COMPOUNDING
COMPRESSORS
taken to ensure the liquid does not get significantly
warmer, so that it begins to evaporate, before it
enters the expansion device.
In many systems the load is too great to be handled
practically with one compressor. In these cases
compressors are connected in parallel, which has
the added advantage that their use can be cycled in
order to adjust the capacity to suit the load.
Two stage compression can be achieved within
one, specially designed, compressor. The gas is
compressed to the intermediate pressure in the
first, low stage cylinder(s) and then compressed to
the condensing pressure in the high stage Two stage, or compound, systems are used when
cylinder(s). The intermediate condition is called the there is a large difference between the evaporating
interstage pressure, and some form of cooling is and condensing temperatures. This usually occurs
usually used to reduce the temperature of the when process or product storage conditions
refrigerant before it enters the second stage of require a low evaporating temperature, such as in
compression.freeze drying or ice cream storage.
The selection and application of such a compressor At these compression ratios two stage systems
is relatively simple; however, there are a limited have to be used because a single stage system
number of compressor variations available. would result in an unacceptably high discharge
Selecting a design that matches a specific system temperature in the compressor. In addition, in
requirement usually results in a compromise which some cases two stage compression can give more
is made at the expense of energy efficiency. The efficient compressor operation.
fixed volume ratio of the two compression stages
also means that efficiencies are lower than they There is no easy rule to determine where two stage
could be where demand varies.compression, with its additional design and
installation complexity, becomes preferable to
single stage compression. Generally, with
refrigerants like R22, two stage compression may
be used on systems using suction cooled
compressors evaporating below about - 30ºC.
In this case two stage compression is achieved by
There are two ways that two stage compression using two separate compressors - one for the low
can be achieved and the method selected will affect stage and another for the high. This more flexible
efficiency. In both cases, additional system capacity approach enables the system designer to match a
can be obtained by first passing the refrigerant used compressor combination to the load more
for interstage cooling through a liquid line accurately and select the most economical
subcooler. If this method is used, care must be interstage pressure.
14 15
The design and selection process is far more system where the compression work is done by
complicated than with the internally compounded either two positive displacement compressors or
variation, but the use of computer selection by two stages of a multistage centrifugal unit. The
programs make it easier and quicker. To limit the flash intercooler subcools the refrigerant liquid to
final discharge temperature interstage cooling is the evaporator by vaporizing a portion of the
used, usually by injecting a small quantity of refrigerant after the first throttling stage. The flash
refrigerant into the gas flow although other suitable gas returns at an intermediate point in the
sources of cooling could be used. compression process to improve the compression
efficiency by cooling the superheated gas (Figure
A multistage system is used when large 13).
temperature and pressure differences exist
between the evaporator and the condenser. Figure In large systems with a number of evaporators and
12 illustrates the basic arrangement for a two-stage large compression (temperature) ratios, the
number of flash intercoolers and compression below the process or product storage temperature.
stages is increased to maximize system efficiency. The condenser for this system is also the
evaporator of the high pressure system. The high
stage system transfers the heat from this condenser
evaporator to the external condenser. The low
pressure system can therefore use a refrigerant
which has a suitably low boiling point for the Cascade systems are another method of application, and its condensing pressure can be kept overcoming the problems in applications requiring at a safe level by the high stage of the cascade.low evaporating temperatures. Two separate
refrigeration circuits are used, usually with different A cascade system cannot be as efficient as a well refrigerants in each circuit.designed externally compounded system, because
The evaporator of the low pressure system is there is a loss in efficiency due to the heat transfer
2.7.2 CASCADE SYSTEMS
Figure 12: Schematic of 2-Stage Refrigeration System. (source: CEMET)
Figure 13: Diagram of a 2-Stage Vapour Compression Cycle. (source: CEMET)
Figure 14: Three stage Cascade System. (source: CEMET)
Figure 15: Two stage cascade system with booster circuit. (source: CEMET)
16 17
The design and selection process is far more system where the compression work is done by
complicated than with the internally compounded either two positive displacement compressors or
variation, but the use of computer selection by two stages of a multistage centrifugal unit. The
programs make it easier and quicker. To limit the flash intercooler subcools the refrigerant liquid to
final discharge temperature interstage cooling is the evaporator by vaporizing a portion of the
used, usually by injecting a small quantity of refrigerant after the first throttling stage. The flash
refrigerant into the gas flow although other suitable gas returns at an intermediate point in the
sources of cooling could be used. compression process to improve the compression
efficiency by cooling the superheated gas (Figure
A multistage system is used when large 13).
temperature and pressure differences exist
between the evaporator and the condenser. Figure In large systems with a number of evaporators and
12 illustrates the basic arrangement for a two-stage large compression (temperature) ratios, the
number of flash intercoolers and compression below the process or product storage temperature.
stages is increased to maximize system efficiency. The condenser for this system is also the
evaporator of the high pressure system. The high
stage system transfers the heat from this condenser
evaporator to the external condenser. The low
pressure system can therefore use a refrigerant
which has a suitably low boiling point for the Cascade systems are another method of application, and its condensing pressure can be kept overcoming the problems in applications requiring at a safe level by the high stage of the cascade.low evaporating temperatures. Two separate
refrigeration circuits are used, usually with different A cascade system cannot be as efficient as a well refrigerants in each circuit.designed externally compounded system, because
The evaporator of the low pressure system is there is a loss in efficiency due to the heat transfer
2.7.2 CASCADE SYSTEMS
Figure 12: Schematic of 2-Stage Refrigeration System. (source: CEMET)
Figure 13: Diagram of a 2-Stage Vapour Compression Cycle. (source: CEMET)
Figure 14: Three stage Cascade System. (source: CEMET)
Figure 15: Two stage cascade system with booster circuit. (source: CEMET)
16 17
coil) temperature falls. This is particularly true in air- result from lack of proper cleaning. Absorption
to-air, space-heating systems where heat output chillers face reductions in refrigerating capacity of
decreases as the outdoor temperature lowers. up to 24 per cent, with power increases of 7.5 per
cent, from poor maintenance.
Owners of refrigeration and heat pump equipment
should follow the manufacturer's service and
maintenance recommendations to maintain
maximum system efficiency over the life of the
equipment, leaking seals, poor lubrication and faulty
controls will reduce system life and performance.
A simple procedure, such as regular cleaning of the
evaporator and condenser, has a marked effect on
performance. Table 7 shows the effect of dirty heat
transfer elements on an air-cooled reciprocating
compressor system. Reductions in refrigerating
capacity up to 25 per cent, with simultaneous Figure 17: Typical schematic of water-to-air
increases in power input of up to 40 per cent, can heat pump system. (source: CEMET)
2.7.3.1 EFFECTS OF MAINTENANCE
ON SYSTEM EFFICIENCY
between the two systems. It does, however, offer In each case the first term refers to the heat source
more flexibility, as a small low temperature load for heating applications, or the heat sink for cooling.
could be interfaced with an existing h igh The second term refers to the secondary
temperature system. In many cases cascading is the refrigerant used for process or space heating and
only alternative if very low temperatures are cooling. For example:
required.
� An air-to-air heat pump (Figure 16)
Refrigerants used in each stage may be different and provides heating or cooling. In the cooling
are selected for optimum performance at the given mode, heat is removed from the air in the
evaporator and condenser temperatures. An space and discharged to the outside air. In
alternative arrangement uses a common condenser the heating mode, heat is removed from
with a booster circuit to obtain two separate the outside air and discharged to air in the
evaporator temperatures (Figure 15). space.
� An air-to-water system extracts heat from
ambient or exhaust air to heat or preheat
water used for space or process heating.
A heat pump is a device used to transfer heat from a
lower temperature to a higher temperature, for � A water-to-air system (Figure 17) provides
heating the warmer area or process. In many heating and cooling of air with water as the
installations, reversible heat pumps are used, which heat sink or source.
heat or cool the process, or space.
� A water-to-water system extracts heat from
A four-way reversing valve is used to reverse the a water source while simultaneously
refrigerant flow, to permit the use of the coils or rejecting heat to a water heat sink, to either
exchangers in either the condenser or evaporator heat or cool a space or process.
mode. With a fixed refrigerant circuit and no
reversing valve, the secondary refrigerant flows can � Earth-to-air and earth to water systems have
be reversed through appropriate external valve or limited use. Practical application is limited
damper arrangements. to space heating where the total heating or
cooling effect is small, and the ground coil
Various heat source and heat sink arrangements are size is equally small.
possible, depending on heating and cooling
requirements. The COP for heat pump systems varies from 2 to 3
for small air-to-air space heating systems, to 5 or 6
� Air-to-air. for large systems that operate across small
� Air-to-water. temperature differences.
� Water-to-air.
Most heat pump systems are provided with a � Water-to-water.
backup heat source to offset reductions in heat � Earth-to-air.
output as the evaporator (heat source: outdoor � Earth-to-water.
2.7.3 HEAT PUMP SYSTEMS
Figure 16: Typical schematic of an air-to-air heat pump system. (source: CEMET)
18 19
coil) temperature falls. This is particularly true in air- result from lack of proper cleaning. Absorption
to-air, space-heating systems where heat output chillers face reductions in refrigerating capacity of
decreases as the outdoor temperature lowers. up to 24 per cent, with power increases of 7.5 per
cent, from poor maintenance.
Owners of refrigeration and heat pump equipment
should follow the manufacturer's service and
maintenance recommendations to maintain
maximum system efficiency over the life of the
equipment, leaking seals, poor lubrication and faulty
controls will reduce system life and performance.
A simple procedure, such as regular cleaning of the
evaporator and condenser, has a marked effect on
performance. Table 7 shows the effect of dirty heat
transfer elements on an air-cooled reciprocating
compressor system. Reductions in refrigerating
capacity up to 25 per cent, with simultaneous Figure 17: Typical schematic of water-to-air
increases in power input of up to 40 per cent, can heat pump system. (source: CEMET)
2.7.3.1 EFFECTS OF MAINTENANCE
ON SYSTEM EFFICIENCY
between the two systems. It does, however, offer In each case the first term refers to the heat source
more flexibility, as a small low temperature load for heating applications, or the heat sink for cooling.
could be interfaced with an existing h igh The second term refers to the secondary
temperature system. In many cases cascading is the refrigerant used for process or space heating and
only alternative if very low temperatures are cooling. For example:
required.
� An air-to-air heat pump (Figure 16)
Refrigerants used in each stage may be different and provides heating or cooling. In the cooling
are selected for optimum performance at the given mode, heat is removed from the air in the
evaporator and condenser temperatures. An space and discharged to the outside air. In
alternative arrangement uses a common condenser the heating mode, heat is removed from
with a booster circuit to obtain two separate the outside air and discharged to air in the
evaporator temperatures (Figure 15). space.
� An air-to-water system extracts heat from
ambient or exhaust air to heat or preheat
water used for space or process heating.
A heat pump is a device used to transfer heat from a
lower temperature to a higher temperature, for � A water-to-air system (Figure 17) provides
heating the warmer area or process. In many heating and cooling of air with water as the
installations, reversible heat pumps are used, which heat sink or source.
heat or cool the process, or space.
� A water-to-water system extracts heat from
A four-way reversing valve is used to reverse the a water source while simultaneously
refrigerant flow, to permit the use of the coils or rejecting heat to a water heat sink, to either
exchangers in either the condenser or evaporator heat or cool a space or process.
mode. With a fixed refrigerant circuit and no
reversing valve, the secondary refrigerant flows can � Earth-to-air and earth to water systems have
be reversed through appropriate external valve or limited use. Practical application is limited
damper arrangements. to space heating where the total heating or
cooling effect is small, and the ground coil
Various heat source and heat sink arrangements are size is equally small.
possible, depending on heating and cooling
requirements. The COP for heat pump systems varies from 2 to 3
for small air-to-air space heating systems, to 5 or 6
� Air-to-air. for large systems that operate across small
� Air-to-water. temperature differences.
� Water-to-air.
Most heat pump systems are provided with a � Water-to-water.
backup heat source to offset reductions in heat � Earth-to-air.
output as the evaporator (heat source: outdoor � Earth-to-water.
2.7.3 HEAT PUMP SYSTEMS
Figure 16: Typical schematic of an air-to-air heat pump system. (source: CEMET)
18 19
The following major components are required in is contained in a common gas-tight housing.
vapour compression refrigeration systems. Hermetic compressors are built into a welded shell,
and there is no access to the internal parts for � Refrigerant compressors. servicing or repair. Semi-hermetic compressors are � Evaporators. assembled with removable covers, usually sealed by � Throttling devices. gaskets, enabling a limited amount of access for on-� Condensers. site maintenance.� Heat rejection equipment.
Both types of compressor are designed and built
with specially selected motors. The motor's size
and type is matched to the motion work of the
compressor for specific applications and The purpose of the compressor in a refrigeration refrigerants. To obtain the maximum efficiency the system is to draw the low pressure refrigerant gas compressor must be closely matched to the system from the evaporator and compress it to a higher duty.pressure. This enables the gas to be condensed
back into liquid by some convenient low cost Hermetic compressors and larger semi-hermetic source of cooling, such as air or water. compressors are usually suction-cooled, the
refrigerant passing over the motor windings before
entering the compressor cylinders. This helps to
cool the motor windings, but reduces the capacity
of the compressor. Externally cooled types, where
the gas passes directly into the cylinders, are usually Most compressors are driven by an electric motor, about 8% more efficient than the equivalent sometimes built into a common casing. Other suction-cooled models but are only available up to compressors have an external drive, the shaft a motor size of about 5 kW.passing through a rotating gas seal where it exits
from the pressurised casing.
This type of compressor has an external drive shaft
allowing a suitably sized motor to be selected and
connected to it, either with a direct coupling or via These compressors have the motor directly belts. It is important to size the motor accurately in attached to the main shaft, and the whole assembly relation to the compressor's duty. Running motors
3.1 COMPRESSORS
3.1.1 TYPES OF COMPRESSOR
HOUSING
3.1.3 OPEN COMPRESSORS
3.1.2 HERMETIC AND SEMI-
HERMETIC COMPRESSORS
at below their design duty reduces their power � improved flow through valves:
factor and their efficiency. o less restricted gas flow path,
o reduced pressure drop;
When comparing the input power requirements of
open and semi-hermetic compressors, the motor's � minimised heat transfer from discharge to
efficiency and losses due to the drive have to be suction gas.
taken into account for open drive machines.
Such modifications can improve efficiency by up to Where extended operation of the plant is 20%, although in many cases the capital cost of the envisaged it could prove viable to invest in an compressor will be higher because of the increased energy efficient (high efficiency) motor. At present complexity of manufacturing.the cost will be higher than a standard motor but
this could change as the price differential between It is critical to the reliability of reciprocating standard and high efficiency motors is decreasing. compressors that liquid refrigerant or large The payback time, derived by a simple cost analysis, quantities of oil are not injected into the cylinders, will usually be less than two years given the long as this will cause mechanical failure in the running hours and may show a better return on compressor.investment.
Screw compressors are available for duties from
about 50 kW up to thousands of kilowatts and are Reciprocating compressors are the most common
generally used on medium to high temperature types of compressor and are available for a wide
applications. The geometry of the compressor range of applications.
determines its optimum pressure ratio. Operation
away from this ratio will significantly reduce its The design of a compressor is optimised for
efficiency. For this reason manufacturers usually operation within a designated application envelope
produce a range of machines with different with specified refrigerants. Operating a compressor
operating characteristics.at high temperature conditions with valves
designed for low temperature operation could A large quantity of oil is injected into screw result in losses of up to 10% in the extraction rate. compressors to seal the running clearances With many compressors it could also result in the between the rotors and the casing. The oil has to be motor being overloaded and tripping its protection removed from the refrigerant in a suitable sized device.separator. A significant amount of the heat of
compression is absorbed by the oil, which must be Compressors have been developed with improved
removed by an oil cooler. It is preferable to cool the efficiencies. The main areas of improvement are:
oil by using a supply of air or water. Using a supply of
refrigerant for cooling can reduce the system � clearance volume reduction;
3.1.5 SCREW COMPRESSORS3.1.4 RECIPROCATING
COMPRESSORS
3. EQUIPMENT
20 21
The following major components are required in is contained in a common gas-tight housing.
vapour compression refrigeration systems. Hermetic compressors are built into a welded shell,
and there is no access to the internal parts for � Refrigerant compressors. servicing or repair. Semi-hermetic compressors are � Evaporators. assembled with removable covers, usually sealed by � Throttling devices. gaskets, enabling a limited amount of access for on-� Condensers. site maintenance.� Heat rejection equipment.
Both types of compressor are designed and built
with specially selected motors. The motor's size
and type is matched to the motion work of the
compressor for specific applications and The purpose of the compressor in a refrigeration refrigerants. To obtain the maximum efficiency the system is to draw the low pressure refrigerant gas compressor must be closely matched to the system from the evaporator and compress it to a higher duty.pressure. This enables the gas to be condensed
back into liquid by some convenient low cost Hermetic compressors and larger semi-hermetic source of cooling, such as air or water. compressors are usually suction-cooled, the
refrigerant passing over the motor windings before
entering the compressor cylinders. This helps to
cool the motor windings, but reduces the capacity
of the compressor. Externally cooled types, where
the gas passes directly into the cylinders, are usually Most compressors are driven by an electric motor, about 8% more efficient than the equivalent sometimes built into a common casing. Other suction-cooled models but are only available up to compressors have an external drive, the shaft a motor size of about 5 kW.passing through a rotating gas seal where it exits
from the pressurised casing.
This type of compressor has an external drive shaft
allowing a suitably sized motor to be selected and
connected to it, either with a direct coupling or via These compressors have the motor directly belts. It is important to size the motor accurately in attached to the main shaft, and the whole assembly relation to the compressor's duty. Running motors
3.1 COMPRESSORS
3.1.1 TYPES OF COMPRESSOR
HOUSING
3.1.3 OPEN COMPRESSORS
3.1.2 HERMETIC AND SEMI-
HERMETIC COMPRESSORS
at below their design duty reduces their power � improved flow through valves:
factor and their efficiency. o less restricted gas flow path,
o reduced pressure drop;
When comparing the input power requirements of
open and semi-hermetic compressors, the motor's � minimised heat transfer from discharge to
efficiency and losses due to the drive have to be suction gas.
taken into account for open drive machines.
Such modifications can improve efficiency by up to Where extended operation of the plant is 20%, although in many cases the capital cost of the envisaged it could prove viable to invest in an compressor will be higher because of the increased energy efficient (high efficiency) motor. At present complexity of manufacturing.the cost will be higher than a standard motor but
this could change as the price differential between It is critical to the reliability of reciprocating standard and high efficiency motors is decreasing. compressors that liquid refrigerant or large The payback time, derived by a simple cost analysis, quantities of oil are not injected into the cylinders, will usually be less than two years given the long as this will cause mechanical failure in the running hours and may show a better return on compressor.investment.
Screw compressors are available for duties from
about 50 kW up to thousands of kilowatts and are Reciprocating compressors are the most common
generally used on medium to high temperature types of compressor and are available for a wide
applications. The geometry of the compressor range of applications.
determines its optimum pressure ratio. Operation
away from this ratio will significantly reduce its The design of a compressor is optimised for
efficiency. For this reason manufacturers usually operation within a designated application envelope
produce a range of machines with different with specified refrigerants. Operating a compressor
operating characteristics.at high temperature conditions with valves
designed for low temperature operation could A large quantity of oil is injected into screw result in losses of up to 10% in the extraction rate. compressors to seal the running clearances With many compressors it could also result in the between the rotors and the casing. The oil has to be motor being overloaded and tripping its protection removed from the refrigerant in a suitable sized device.separator. A significant amount of the heat of
compression is absorbed by the oil, which must be Compressors have been developed with improved
removed by an oil cooler. It is preferable to cool the efficiencies. The main areas of improvement are:
oil by using a supply of air or water. Using a supply of
refrigerant for cooling can reduce the system � clearance volume reduction;
3.1.5 SCREW COMPRESSORS3.1.4 RECIPROCATING
COMPRESSORS
3. EQUIPMENT
20 21
capacity by up to 10%, with a corresponding loss of compressors that the correct running speed of the
efficiency. compressor has been used. With semi-hermetic
compressors this speed is fixed by the design of the
built-in motor.
The scroll type of rotary compressor has been the
subject of extensive development in recent years,
as improved machining techniques have made its To maintain the maximum system efficiency in
production viable. systems with widely varying loads, it is important to
be able to vary the duty of the compressor. In a Scroll compressors are being increasingly applied to multi-compressor system this can either be medium and small air-conditioning applications achieved by switching a number of compressors off because of their quiet, low vibration operation and or by reducing their individual pumping capacities. good efficiency. Their efficiency advantage over The best way to save energy is always to switch off reciprocating compressors at lower compression any unnecessary machines.ratios makes them ideal for high temperature
refrigeration applications, such as beer cellar and
milk tank cooling.
Scroll compressors are also being developed for There are a number of methods used to reduce the lower temperature applications. capacity of compressors:
� blocked suction gas,
� suction valve lifting;
� discharge gas recirculation
The extraction rate and power input of a
compressor depend principally on the evaporating
and condensing temperatures. Compressor
performance is usually presented in graphical (Fig)
or in tabular format.
These data are presented at specific rating
conditions, and corrections have to be made to
take into account actual site operating conditions
for:
� suction gas temperature;
� liquid subcooling.
Figure 18: Typical compressor performance
Care must be taken with data for open data. (source: ETSU)
3.1.6 SCROLL COMPRESSORS
3.1.8 CAPACITY CONTROL
3.1.8.1 RECIPROCATING COMPRESSORS
3.1.7 COMPRESSOR PERFORMANCE
DATA
When selecting a compressor, it is important to about 50% capacity, but below this it falls off very
check the manufacturer's data to ensure that the quickly.
model chosen is of an energy efficient design. The
reduction in input power should match, as closely as
possible, the reduction in refrigeration duty.
It is also worthwhile checking whether There are two principal types of evaporator:
supplementary compressor cooling is required � direct expansion (sometimes called "dry
while capacity control is in operation, as this will expansion" or DX);
need additional energy.� flooded.
The number of stages of capacity reduction that can
be obtained will depend on the design of the
compressor, and is usually a function of the number
of cylinders. On suction cooled compressors the These are commonly used to cool either air or a minimum capacity is often limited by the loss of liquid. The expansion device used with this type of cooling of the motor. evaporator is an expansion valve.
A direct expansion evaporator used for cooling air
is shown in Figure 19. There are many different
designs available using plain or finned tube, both
with and without forced circulation of air or some The capacity of a large screw compressor can be
process fluid. Certain tube designs incorporate varied from 100% down to 10% by using a slide
internal devices to maximise heat exchange and vane. The part load efficiency is acceptable down to
3.2 EVAPORATORS
3.2.1 DIRECT EXPANSION
3.1.8.2 SCREW COMPRESSORS
Figure 19: Liquid distribution on a direct expansion circuit. (source: ETSU)
22 23
capacity by up to 10%, with a corresponding loss of compressors that the correct running speed of the
efficiency. compressor has been used. With semi-hermetic
compressors this speed is fixed by the design of the
built-in motor.
The scroll type of rotary compressor has been the
subject of extensive development in recent years,
as improved machining techniques have made its To maintain the maximum system efficiency in
production viable. systems with widely varying loads, it is important to
be able to vary the duty of the compressor. In a Scroll compressors are being increasingly applied to multi-compressor system this can either be medium and small air-conditioning applications achieved by switching a number of compressors off because of their quiet, low vibration operation and or by reducing their individual pumping capacities. good efficiency. Their efficiency advantage over The best way to save energy is always to switch off reciprocating compressors at lower compression any unnecessary machines.ratios makes them ideal for high temperature
refrigeration applications, such as beer cellar and
milk tank cooling.
Scroll compressors are also being developed for There are a number of methods used to reduce the lower temperature applications. capacity of compressors:
� blocked suction gas,
� suction valve lifting;
� discharge gas recirculation
The extraction rate and power input of a
compressor depend principally on the evaporating
and condensing temperatures. Compressor
performance is usually presented in graphical (Fig)
or in tabular format.
These data are presented at specific rating
conditions, and corrections have to be made to
take into account actual site operating conditions
for:
� suction gas temperature;
� liquid subcooling.
Figure 18: Typical compressor performance
Care must be taken with data for open data. (source: ETSU)
3.1.6 SCROLL COMPRESSORS
3.1.8 CAPACITY CONTROL
3.1.8.1 RECIPROCATING COMPRESSORS
3.1.7 COMPRESSOR PERFORMANCE
DATA
When selecting a compressor, it is important to about 50% capacity, but below this it falls off very
check the manufacturer's data to ensure that the quickly.
model chosen is of an energy efficient design. The
reduction in input power should match, as closely as
possible, the reduction in refrigeration duty.
It is also worthwhile checking whether There are two principal types of evaporator:
supplementary compressor cooling is required � direct expansion (sometimes called "dry
while capacity control is in operation, as this will expansion" or DX);
need additional energy.� flooded.
The number of stages of capacity reduction that can
be obtained will depend on the design of the
compressor, and is usually a function of the number
of cylinders. On suction cooled compressors the These are commonly used to cool either air or a minimum capacity is often limited by the loss of liquid. The expansion device used with this type of cooling of the motor. evaporator is an expansion valve.
A direct expansion evaporator used for cooling air
is shown in Figure 19. There are many different
designs available using plain or finned tube, both
with and without forced circulation of air or some The capacity of a large screw compressor can be
process fluid. Certain tube designs incorporate varied from 100% down to 10% by using a slide
internal devices to maximise heat exchange and vane. The part load efficiency is acceptable down to
3.2 EVAPORATORS
3.2.1 DIRECT EXPANSION
3.1.8.2 SCREW COMPRESSORS
Figure 19: Liquid distribution on a direct expansion circuit. (source: ETSU)
22 23
thus efficiency, by causing turbulence to keep the evaporated before reaching the outlet.
liquid in full contact with the tube wall. By monitoring the flow of refrigerant, the expansion
device maintains a superheat of about 5ºC at the
outlet of the evaporator. This ensures that the duty
is as high as is practically possible while still
protecting the compressor from liquid refrigerant
returning down the suction line. This feature is A typical evaporator will have a number of parallel important for the reliability of reciprocating circuits designed to:machines, but less so for rotary compressors.
� maximise heat transfer;
� ensure good oil return;
� minimise pressure drop.
A distributor is used to ensure refrigerant flows
evenly between the different parallel circuits.The efficiency of an evaporator can be affected by
an uneven distribution of refrigerant, and hence To enhance the heat transfer in air-cooled designs,
cooling, between the different circuits.the surface of the refrigerant-carrying tubes is
usually extended by using external fins. To This can occur if the distributor is incorrectly
maximise their surface the fins are spaced as closely positioned - it should always be vertical so that
together as possible without restricting the air flow. there is an even feed through each outlet - or if one
On low temperature systems, where ice can form distributor line becomes damaged.
on the fin surfaces, a wider spacing has to be used to
ensure adequate air flow when ice build-up occurs.It is impossible for each circuit to be totally filled
with saturated refrigerant, as there must be In the past few years compact plate heat
sufficient superheat to enable the expansion device exchangers have become increasingly popular for
to control the flow of refrigerant. This means that direct expansion cooling of liquids. Due to their
the heat transfer efficiency will be reduced at the design they have a very good heat transfer capability
end of each circuit where superheated gas is and hence high efficiency. Some larger designs can
present. Oil logging can also reduce the efficiency be disassembled for cleaning, whereas the smaller
of an evaporator - more information on this subject type are brazed together as a sealed assembly. They
is given in Section 4.3.can be used with all halocarbon refrigerants, but
because of the materials used for construction they
arc not suitable for ammonia.
There are two types of flooded evaporator:
� shell and tube;Saturated refrigerant is fed through a distributor
� plate type.into the expansion tubes where it is totally
3.2.1.1 DESIGN FEATURES
3.2.1.3 OPERATIONAL
PROBLEMS
3.2.2 FLOODED
3.2.1.2 OPERATING FEATURES
3.2.2.1 SHELL AND TUBE
3.2.2.2 PLATE TYPE
3.2.3 OIL CONTROL IN
EVAPORATORS
4.3 for more information. Fouling on the
external surfaces of the tubes, i.e. the
process fluid side, can be difficult to rectify. These are commonly used in larger applications for
This will also reduce heat transfer.cooling liquids. There are a number of different
designs but they all have the same basic
� Due to the internal volume of the shell, characteristics.
large quantities of refrigerant are required
with the corresponding cost and Design and operating features
environmental or safety issues if a leak
� In a shell and tube evaporator, the fluid to should occur.
be cooled is passed through the tubes with
the evaporating refrigerant boiling off into
gas within the body of the shell.
� The refrigerant level in the shell is Recently, the use of plate heat exchangers as maintained so that the top tube is always flooded evaporators in recirculation systems has covered with liquid. In this way the most become more common. They offer the following efficient heat exchange, liquid to liquid, is advantages over the shell and tube type:achieved over the whole of the cooling
interface. To ensure optimum efficiency, � higher heat transfer coefficients;the liquid level is usually maintained by � a smaller temperature difference between using a low pressure float valve. The the refrigerant and the cooled liquid, operation of this type of device is resu l t ing in h i gher evapora t ing explained in Section 7. Alternatively, an temperatures and therefore improved expansion device and level sensor can be system efficiency;used. � more compact units requiring less plant
room space;� The space in the upper part of the shell � smaller refrigerant charges;
allows any droplets of liquid to be � the ability to clean non-brazed assemblies, separated from the gas returning to the thus maintaining a good heat transfer compressor. This separation is sometimes capability.achieved in a different vessel called a surge
drum.
Operational problems
� Flooded shell and tube evaporators are
In order to maintain the optimum system efficiency usually large and relatively expensive.
it is important that oil is not allowed to collect in the
evaporator, coating the tubes and thereby reducing � Accumulation of oil can reduce the heat
their capability to transfer heat. Different actions transfer and hence efficiency - see Section
24 25
thus efficiency, by causing turbulence to keep the evaporated before reaching the outlet.
liquid in full contact with the tube wall. By monitoring the flow of refrigerant, the expansion
device maintains a superheat of about 5ºC at the
outlet of the evaporator. This ensures that the duty
is as high as is practically possible while still
protecting the compressor from liquid refrigerant
returning down the suction line. This feature is A typical evaporator will have a number of parallel important for the reliability of reciprocating circuits designed to:machines, but less so for rotary compressors.
� maximise heat transfer;
� ensure good oil return;
� minimise pressure drop.
A distributor is used to ensure refrigerant flows
evenly between the different parallel circuits.The efficiency of an evaporator can be affected by
an uneven distribution of refrigerant, and hence To enhance the heat transfer in air-cooled designs,
cooling, between the different circuits.the surface of the refrigerant-carrying tubes is
usually extended by using external fins. To This can occur if the distributor is incorrectly
maximise their surface the fins are spaced as closely positioned - it should always be vertical so that
together as possible without restricting the air flow. there is an even feed through each outlet - or if one
On low temperature systems, where ice can form distributor line becomes damaged.
on the fin surfaces, a wider spacing has to be used to
ensure adequate air flow when ice build-up occurs.It is impossible for each circuit to be totally filled
with saturated refrigerant, as there must be In the past few years compact plate heat
sufficient superheat to enable the expansion device exchangers have become increasingly popular for
to control the flow of refrigerant. This means that direct expansion cooling of liquids. Due to their
the heat transfer efficiency will be reduced at the design they have a very good heat transfer capability
end of each circuit where superheated gas is and hence high efficiency. Some larger designs can
present. Oil logging can also reduce the efficiency be disassembled for cleaning, whereas the smaller
of an evaporator - more information on this subject type are brazed together as a sealed assembly. They
is given in Section 4.3.can be used with all halocarbon refrigerants, but
because of the materials used for construction they
arc not suitable for ammonia.
There are two types of flooded evaporator:
� shell and tube;Saturated refrigerant is fed through a distributor
� plate type.into the expansion tubes where it is totally
3.2.1.1 DESIGN FEATURES
3.2.1.3 OPERATIONAL
PROBLEMS
3.2.2 FLOODED
3.2.1.2 OPERATING FEATURES
3.2.2.1 SHELL AND TUBE
3.2.2.2 PLATE TYPE
3.2.3 OIL CONTROL IN
EVAPORATORS
4.3 for more information. Fouling on the
external surfaces of the tubes, i.e. the
process fluid side, can be difficult to rectify. These are commonly used in larger applications for
This will also reduce heat transfer.cooling liquids. There are a number of different
designs but they all have the same basic
� Due to the internal volume of the shell, characteristics.
large quantities of refrigerant are required
with the corresponding cost and Design and operating features
environmental or safety issues if a leak
� In a shell and tube evaporator, the fluid to should occur.
be cooled is passed through the tubes with
the evaporating refrigerant boiling off into
gas within the body of the shell.
� The refrigerant level in the shell is Recently, the use of plate heat exchangers as maintained so that the top tube is always flooded evaporators in recirculation systems has covered with liquid. In this way the most become more common. They offer the following efficient heat exchange, liquid to liquid, is advantages over the shell and tube type:achieved over the whole of the cooling
interface. To ensure optimum efficiency, � higher heat transfer coefficients;the liquid level is usually maintained by � a smaller temperature difference between using a low pressure float valve. The the refrigerant and the cooled liquid, operation of this type of device is resu l t ing in h i gher evapora t ing explained in Section 7. Alternatively, an temperatures and therefore improved expansion device and level sensor can be system efficiency;used. � more compact units requiring less plant
room space;� The space in the upper part of the shell � smaller refrigerant charges;
allows any droplets of liquid to be � the ability to clean non-brazed assemblies, separated from the gas returning to the thus maintaining a good heat transfer compressor. This separation is sometimes capability.achieved in a different vessel called a surge
drum.
Operational problems
� Flooded shell and tube evaporators are
In order to maintain the optimum system efficiency usually large and relatively expensive.
it is important that oil is not allowed to collect in the
evaporator, coating the tubes and thereby reducing � Accumulation of oil can reduce the heat
their capability to transfer heat. Different actions transfer and hence efficiency - see Section
24 25
are required to control oil, depending on the type system duty between a number of smaller
of evaporator and refrigerant. evaporators, isolating some as the load diminishes.
Ammonia systems
The main rule with this type of evaporator, whether
� Oil is almost totally insoluble in ammonia it is being used with halocarbons or ammonia, is to
and will separate out, collecting in the maintain an adequate refrigerant velocity to carry
bottom of the evaporator and must be the oil through the tube assembly.
periodically drained, either manually or
automatically. This is not a hazardous Problems can occur if the evaporator has to
operation providing proper safety operate over a wide range of loads, as the flow
precautions are taken. A careful log must might not be sufficient at the lowest duty to achieve
be kept recording any oil added to or the minimum required velocity. Under these
removed from the system.conditions it may be necessary to split the total
3.2.3.1 DIRECT EXPANSION 3.2.3.2 FLOODED EVAPORATORS
EVAPORATORS
� Any control connections made to the the evaporator.
lower part of the evaporator's shell must The size of evaporator should be decided at the be above the highest possible oil level. Oil design stage by evaluating the additional evaporator is very viscous at low temperatures and can capital cost and the resulting lower running costs, cause a restriction in small bore pipes.and comparing the simple paybacks obtained by
Halocarbon systems each option.
� Some refrigerants, for example R11 and The heat transfer will be influenced by factors such
Rl2, are completely miscible with oil under as:
all operating conditions and no special � oil logging;
action is required to prevent oil logging.� fouling and corrosion of heat transfer
surfaces;� Other refrigerants, for example R22 and
� incorrect control of the refrigerant flow or R502, are miscible at high temperatures
level in the evaporator;but, at low temperatures, an oil rich layer
· frost build up.will form on the top of the liquid
refrigerant. By carefully positioning tapping
points ill the evaporator's shell, this oil rich
mixture can be removed from the
evaporator and transferred into a rectifier.
As noted before, allowance must be made in the fin The rectifier is then heated to boil the
spacing to allow for ice build-up on evaporators majority of the refrigerant out of the oil
operating with refrigerant temperatures below before it is returned to the compressor.
0ºC. To maintain an adequate air flow through the The most energy efficient method of
fin block it has to be defrosted periodically, supplying this heat is to use the warm
requiring the use of heat.refrigerant in the liquid line which incurs no
additional energy costs, and has the further
Energy efficient defrosting depends on the advantage of increasing the liquid
following factors:subcooling. A typical oil rectification
arrangement is shown in Figure 20.
� initiating a defrost operation only when it
becomes necessary through loss of
performance;
� using the most efficient method of applying
the necessary heat;
� ensuring that the defrost heat is evenly
The efficiency of a refrigeration system is increased distributed over the whole of the fin block;
when the evaporating temperature increases. This � ·stopping the defrost cycle as soon as the
can be achieved by: fin block is totally clear of ice;
� ·maximising the size of the evaporator; � ·minimising the amount of defrost heat
� maintaining the peak heat transfer rate of absorbed by the process fluid or product.
3.2.5 DEFROSTING
3.2.4 ENERGY EFFICIENT
OPERATION OF
EVAPORATORS
Figure 20:Typical oil rectification system diagram. (source: CEMET)
26 27
are required to control oil, depending on the type system duty between a number of smaller
of evaporator and refrigerant. evaporators, isolating some as the load diminishes.
Ammonia systems
The main rule with this type of evaporator, whether
� Oil is almost totally insoluble in ammonia it is being used with halocarbons or ammonia, is to
and will separate out, collecting in the maintain an adequate refrigerant velocity to carry
bottom of the evaporator and must be the oil through the tube assembly.
periodically drained, either manually or
automatically. This is not a hazardous Problems can occur if the evaporator has to
operation providing proper safety operate over a wide range of loads, as the flow
precautions are taken. A careful log must might not be sufficient at the lowest duty to achieve
be kept recording any oil added to or the minimum required velocity. Under these
removed from the system.conditions it may be necessary to split the total
3.2.3.1 DIRECT EXPANSION 3.2.3.2 FLOODED EVAPORATORS
EVAPORATORS
� Any control connections made to the the evaporator.
lower part of the evaporator's shell must The size of evaporator should be decided at the be above the highest possible oil level. Oil design stage by evaluating the additional evaporator is very viscous at low temperatures and can capital cost and the resulting lower running costs, cause a restriction in small bore pipes.and comparing the simple paybacks obtained by
Halocarbon systems each option.
� Some refrigerants, for example R11 and The heat transfer will be influenced by factors such
Rl2, are completely miscible with oil under as:
all operating conditions and no special � oil logging;
action is required to prevent oil logging.� fouling and corrosion of heat transfer
surfaces;� Other refrigerants, for example R22 and
� incorrect control of the refrigerant flow or R502, are miscible at high temperatures
level in the evaporator;but, at low temperatures, an oil rich layer
· frost build up.will form on the top of the liquid
refrigerant. By carefully positioning tapping
points ill the evaporator's shell, this oil rich
mixture can be removed from the
evaporator and transferred into a rectifier.
As noted before, allowance must be made in the fin The rectifier is then heated to boil the
spacing to allow for ice build-up on evaporators majority of the refrigerant out of the oil
operating with refrigerant temperatures below before it is returned to the compressor.
0ºC. To maintain an adequate air flow through the The most energy efficient method of
fin block it has to be defrosted periodically, supplying this heat is to use the warm
requiring the use of heat.refrigerant in the liquid line which incurs no
additional energy costs, and has the further
Energy efficient defrosting depends on the advantage of increasing the liquid
following factors:subcooling. A typical oil rectification
arrangement is shown in Figure 20.
� initiating a defrost operation only when it
becomes necessary through loss of
performance;
� using the most efficient method of applying
the necessary heat;
� ensuring that the defrost heat is evenly
The efficiency of a refrigeration system is increased distributed over the whole of the fin block;
when the evaporating temperature increases. This � ·stopping the defrost cycle as soon as the
can be achieved by: fin block is totally clear of ice;
� ·maximising the size of the evaporator; � ·minimising the amount of defrost heat
� maintaining the peak heat transfer rate of absorbed by the process fluid or product.
3.2.5 DEFROSTING
3.2.4 ENERGY EFFICIENT
OPERATION OF
EVAPORATORS
Figure 20:Typical oil rectification system diagram. (source: CEMET)
26 27
3.3 EXPANSION DEVICES
3.3.1 THERMOSTATIC
EXPANSION VALVES
� thermostatic expansion valve;
� high pressure float valve;
� low pressure float valve.The purpose of an expansion valve is to:
Capillary tubes (which just drop the refrigerant � reduce the pressure of the liquid pressure but cannot regulate flow) are used in refrigerant from the condensing pressure domestic type systems. These are factory to the evaporating pressure;assembled and cannot be adjusted.� modulate the flow of liquid refrigerant into
the evaporator
Correct selection and installation of expansion
valves is very important, because their incorrect
operation will reduce the efficiency and reliability of
a system.Thermostatic expansion valves are used on most
commercial installations. A typical example shown There are three types of expansion valve widely in Figure 21. The refrigerant pressure is dropped used in commercial and industrial refrigeration:
through an orifice, and the flow of refrigerant is across them varies widely, for example if the
regulated by a needle valve and diaphragm condensing pressure 'floats' with ambient. To cope
arrangement. The diaphragm is moved by the with such conditions other valves are now available.
pressure inside the controlling phial, which senses
the temperature of the refrigerant leaving the
evaporator which should be approximately 5º
C higher than the evaporating temperature, to
ensure there is no liquid refrigerant present which Balanced port valves are very similar in design and
could damage the compressor. This temperature operation to the conventional thermostatic valve
difference is the superheat setting of the valve and apart from a special internal balanced port design.
can he set by adjusting the valve. Correct setting is This allows the valve to control inlet pressure
vital to the efficient and reliable operation of the accurately over a much wider range. These valves
refrigeration system. cost approximately 20% more than a conventional
valve, but are currently available only in a limited
If the load on the evaporator changes, then the range of sizes.
temperature of the refrigerant leaving the
evaporator will also change. The controlling phial
will sense this and automatically adjust the
refrigerant flow to accommodate the load change.
A major disadvantage of thermostatic valves is that Electronic expansion valves work in a similar way to
they cannot work well if the pressure difference thermostatic valves, except that the temperature is
3.3.1.1 BALANCED PORT VALVES
3.3.1.2 ELECTRONIC EXPANSION
VALVE
Table1: Types of Liquid Coolers
Type of cooler Usual Refrigerant Feed Usual Range of Commonly Used with
Device Capacity (kW) Refrigerant Numbers
Flooded shell-and-bare-tube Low pressure float 175-1750 717 (Ammonia)
Flooded shell-and-finned-tube Low pressure float
High pressure float, fixed orifice(s), weir(s) 175-35 000 11, 12, 22, 113
114, 134a, 500, 502
Spray-type-shell-and-tube Low pressure float
High pressure float 350-1750 11, 12, 13B1, 22,
113, 114, 134a
Direct-expansion shell-and-tube Thermal expansion valve 17.5-1250 12, 22, 134a, 500, 502, 717
Flooded Baudelot cooler Low pressure float 35-350 717
Direct-expansion Baudelot cooler Thermal expansion valve 17.5-85 12, 22, 134a, 717
Flooded double-pipe cooler Low pressure float 35-85 717
Direct-expansion double-pipe cooler Thermal expansion valve 17.5-85 12, 22, 134a, 717
Shell-and-coil cooler Thermal expansion valve 7-35 12, 22, 134a, 717
Flooded tank-and-agitator Low pressure float 175-700 717
Figure 21: Thermostatic Expansion Valve. (source: CEMET)
28 29
3.3 EXPANSION DEVICES
3.3.1 THERMOSTATIC
EXPANSION VALVES
� thermostatic expansion valve;
� high pressure float valve;
� low pressure float valve.The purpose of an expansion valve is to:
Capillary tubes (which just drop the refrigerant � reduce the pressure of the liquid pressure but cannot regulate flow) are used in refrigerant from the condensing pressure domestic type systems. These are factory to the evaporating pressure;assembled and cannot be adjusted.� modulate the flow of liquid refrigerant into
the evaporator
Correct selection and installation of expansion
valves is very important, because their incorrect
operation will reduce the efficiency and reliability of
a system.Thermostatic expansion valves are used on most
commercial installations. A typical example shown There are three types of expansion valve widely in Figure 21. The refrigerant pressure is dropped used in commercial and industrial refrigeration:
through an orifice, and the flow of refrigerant is across them varies widely, for example if the
regulated by a needle valve and diaphragm condensing pressure 'floats' with ambient. To cope
arrangement. The diaphragm is moved by the with such conditions other valves are now available.
pressure inside the controlling phial, which senses
the temperature of the refrigerant leaving the
evaporator which should be approximately 5º
C higher than the evaporating temperature, to
ensure there is no liquid refrigerant present which Balanced port valves are very similar in design and
could damage the compressor. This temperature operation to the conventional thermostatic valve
difference is the superheat setting of the valve and apart from a special internal balanced port design.
can he set by adjusting the valve. Correct setting is This allows the valve to control inlet pressure
vital to the efficient and reliable operation of the accurately over a much wider range. These valves
refrigeration system. cost approximately 20% more than a conventional
valve, but are currently available only in a limited
If the load on the evaporator changes, then the range of sizes.
temperature of the refrigerant leaving the
evaporator will also change. The controlling phial
will sense this and automatically adjust the
refrigerant flow to accommodate the load change.
A major disadvantage of thermostatic valves is that Electronic expansion valves work in a similar way to
they cannot work well if the pressure difference thermostatic valves, except that the temperature is
3.3.1.1 BALANCED PORT VALVES
3.3.1.2 ELECTRONIC EXPANSION
VALVE
Table1: Types of Liquid Coolers
Type of cooler Usual Refrigerant Feed Usual Range of Commonly Used with
Device Capacity (kW) Refrigerant Numbers
Flooded shell-and-bare-tube Low pressure float 175-1750 717 (Ammonia)
Flooded shell-and-finned-tube Low pressure float
High pressure float, fixed orifice(s), weir(s) 175-35 000 11, 12, 22, 113
114, 134a, 500, 502
Spray-type-shell-and-tube Low pressure float
High pressure float 350-1750 11, 12, 13B1, 22,
113, 114, 134a
Direct-expansion shell-and-tube Thermal expansion valve 17.5-1250 12, 22, 134a, 500, 502, 717
Flooded Baudelot cooler Low pressure float 35-350 717
Direct-expansion Baudelot cooler Thermal expansion valve 17.5-85 12, 22, 134a, 717
Flooded double-pipe cooler Low pressure float 35-85 717
Direct-expansion double-pipe cooler Thermal expansion valve 17.5-85 12, 22, 134a, 717
Shell-and-coil cooler Thermal expansion valve 7-35 12, 22, 134a, 717
Flooded tank-and-agitator Low pressure float 175-700 717
Figure 21: Thermostatic Expansion Valve. (source: CEMET)
28 29
sensed electronically and this signal opens and the high (receiver) pressure or the low
closes the orifice via a small electrical motor. The (evaporator) pressure of the system.
valve can therefore operate with a wider difference
in pressure across it. A further advantage is that they
can be easily integrated into an electronic or
microprocessor control system. Figure 22 shows an
electronic expansion valve with a direct expansion
air cooler.A typical HP float valve is shown in Figure 23. This
type of valve is used to maintain a liquid level in the Electronic valves are much more expensive than
receiver and operates at receiver pressure.conventional thermostatic valves, and will give a
payback of less than a year only on systems with a The receiver pressure controls the pilot line
capacity greater than 100kw.pressure, and as this pressure varies the expansion
valve opens and closes to supply liquid refrigerant
from the receiver to the evaporator.
An HP float valve is used in large industrial systems
A float valve system uses a float chamber with a with single evaporators. As it provides no control of
separate modulating expansion valve, connected by the level of refrigerant in the evaporator, the
a pilot line. The float chamber can either operate at amount of refrigerant in the system must be
3.3.2.1 HIGH PRESSURE (HP)
FLOAT VALVE
3.3.2 FLOAT VALVE SYSTEMS
correct, i.e. the system is said to be critically evaporator and operates at evaporator pressure.
charged. To ensure correct operation , the This liquid level affects the pressure in the pilot line,
evaporator must be fitted with a level gauge which and as the pressure varies the expansion valve
is checked regularly. modulates the supply of liquid from the receiver to
the evaporator.
LP float valves are used on systems which have
more than one evaporator connected to one
compressor or to several compressors in parallel.
A typical LP float system is shown in Figure 24. An
It is important that the expansion valve is fitted at a LP float valve is used to maintain a liquid level in the
3.3.2.2 LOW PRESSURE (LP)
FLOAT VALVE
Figure 22: Electronic expansion valve on direct expansion air cooler. (source: ETSU)
Figure 23: High pressure float valve. (source: ETSU)
Figure 24: Low pressure float expansion system. (source: ETSU)
30 31
sensed electronically and this signal opens and the high (receiver) pressure or the low
closes the orifice via a small electrical motor. The (evaporator) pressure of the system.
valve can therefore operate with a wider difference
in pressure across it. A further advantage is that they
can be easily integrated into an electronic or
microprocessor control system. Figure 22 shows an
electronic expansion valve with a direct expansion
air cooler.A typical HP float valve is shown in Figure 23. This
type of valve is used to maintain a liquid level in the Electronic valves are much more expensive than
receiver and operates at receiver pressure.conventional thermostatic valves, and will give a
payback of less than a year only on systems with a The receiver pressure controls the pilot line
capacity greater than 100kw.pressure, and as this pressure varies the expansion
valve opens and closes to supply liquid refrigerant
from the receiver to the evaporator.
An HP float valve is used in large industrial systems
A float valve system uses a float chamber with a with single evaporators. As it provides no control of
separate modulating expansion valve, connected by the level of refrigerant in the evaporator, the
a pilot line. The float chamber can either operate at amount of refrigerant in the system must be
3.3.2.1 HIGH PRESSURE (HP)
FLOAT VALVE
3.3.2 FLOAT VALVE SYSTEMS
correct, i.e. the system is said to be critically evaporator and operates at evaporator pressure.
charged. To ensure correct operation , the This liquid level affects the pressure in the pilot line,
evaporator must be fitted with a level gauge which and as the pressure varies the expansion valve
is checked regularly. modulates the supply of liquid from the receiver to
the evaporator.
LP float valves are used on systems which have
more than one evaporator connected to one
compressor or to several compressors in parallel.
A typical LP float system is shown in Figure 24. An
It is important that the expansion valve is fitted at a LP float valve is used to maintain a liquid level in the
3.3.2.2 LOW PRESSURE (LP)
FLOAT VALVE
Figure 22: Electronic expansion valve on direct expansion air cooler. (source: ETSU)
Figure 23: High pressure float valve. (source: ETSU)
Figure 24: Low pressure float expansion system. (source: ETSU)
30 31
level below the liquid surface in the receiver, in are used, such as electronic expansion valves.
order to prevent refrigerant gas going through the
valve and hence reducing efficiency. A level gauge
must be fitted to the receiver so that the liquid level
can be checked to ensure adequate performance is In an air-cooled condenser the refrigerant maintained. condenses inside tubes over which air is forced by
fans. To improve the heat transfer, the tube surface
is usually extended using corrugated metal fins.
A well designed plant should operate with a
condensing temperature no higher than l4ºC above There are three types of condenser in widespread
the ambient temperature. With larger condensers use:
it is common practice to control the head pressure
by switching off or slowing down fans, although this � air-cooled (using ambient air);
is inefficient.� water-cooled (using mains, river or cooling
tower water);If air-cooled condensers are being used in a
� evaporative cooled (using ambient air and corrosive atmosphere (for example, near the sea or
recirculated water).in polluted air) then a suitable tube/fin material
combination or a coating should be used.The two latter types take advantage of the lower
wet bulb ambient temperature and the greater heat Air-cooled condensers are susceptible to blockage
transfer affect of water, and therefore operate with by air borne debris such as dust, feathers, packaging,
lower condensing temperatures. When comparing and so on. They must be regularly cleaned (but not
different condenser types the power requirements with refrigerant) to prevent a build up of
of associated fans, pumps and heaters should be contamination, as this will reduce the air flow and
taken into account. In general, systems under 100 hence increase the condensing pressure.
kW capacity use air-cooled condensers unless
there is a space or noise restriction.
For a given capacity, a larger condenser will result in
a lower condensing temperature and hence better
efficiency. Problems can be caused on installations
Water-cooled condensers are of the shell and tube which use thermostatic expansion valves if the
type. The cooling water flows in tubes inside the condensing (head) pressure varies widely. Such
shell, and refrigerant inside the shell condenses on valves are unable to control refrigerant flow reliably
the outside of the cold tubes. Heat transfer is under such conditions, and reduced efficiency and
improved as the water velocity is increased. An reliability will result. Some form of head pressure
efficient system will work with a temperature rise of control may be used to raise the head pressure
5ºC for the water passing through the condenser, artificially, although this is inefficient and is not
and a difference of 5ºC between the condensing necessary if more sophisticated expansion devices
3.4.1 AIR-COOLED CONDENSERS
3.4 CONDENSERS
3.4.2 WATER-COOLED
CONDENSERS
temperature and that of the water leaving the likely to cause a problem, cleanable condensers
condenser. On very small commercial installations should be used.
mains water is often used directly, although this is
becoming less common on new installations due to
water metering.
On larger installations the water will be cooled in a
cooling tower, where the cooling effect is achieved In an evaporative condenser, refrigerant is
by evaporating some of the cooling water into the condensed in tubes which are wetted and over
air. Blockages in the air or water side will significantly which air is forced. The water used to wet the
reduce the efficiency of the cooling tower. Such outside of the tubes is recirculated, although a
blockages are common and are normally caused by certain amount of make up water will be needed.
hard water deposits or algae growth. Water should Evaporative condensers should operate with similar
be treated to prevent these and also to prevent temperatures to the water-cooled conden-
bacteria growth. The cooling tower should cool the ser/cooling tower combination above. The water
water to within 13 - l8ºC of the wet bulb ambient used will require treatment, as described for water-
temperature (which can be up to 10ºC lower than cooled condensers above.
the dry bulb temperature).
The water side of the condenser is also liable to
blockage caused by hard water deposits. If this is
Air and other non-condensable gases in a
refrigeration system will increase the condensing
temperature and hence reduce efficiency. For
example, in a medium temperature ammonia
system working with a condenser which contains
15% air, the running costs will increase by 12%.
Air can remain in a system after installation or
service, if the system has been inadequately
evacuated prior to charging with refrigerant. While
running, air can be drawn into a system operating at
a suction condition lower than atmospheric
pressure, if there is a leak on the low side of the
system.
It is possible to check for air and other non-
condensable gases when the system is not working
3.4.3 EVAPORATIVE
CONDENSERS
3.4.4 LOSS OF CONDENSER
EFFICIENCY DUE
TO AIR IN SYSTEM
Figure 25: Draw-through-type of evaporative
condenser (source: CEMET)
32 33
level below the liquid surface in the receiver, in are used, such as electronic expansion valves.
order to prevent refrigerant gas going through the
valve and hence reducing efficiency. A level gauge
must be fitted to the receiver so that the liquid level
can be checked to ensure adequate performance is In an air-cooled condenser the refrigerant maintained. condenses inside tubes over which air is forced by
fans. To improve the heat transfer, the tube surface
is usually extended using corrugated metal fins.
A well designed plant should operate with a
condensing temperature no higher than l4ºC above There are three types of condenser in widespread
the ambient temperature. With larger condensers use:
it is common practice to control the head pressure
by switching off or slowing down fans, although this � air-cooled (using ambient air);
is inefficient.� water-cooled (using mains, river or cooling
tower water);If air-cooled condensers are being used in a
� evaporative cooled (using ambient air and corrosive atmosphere (for example, near the sea or
recirculated water).in polluted air) then a suitable tube/fin material
combination or a coating should be used.The two latter types take advantage of the lower
wet bulb ambient temperature and the greater heat Air-cooled condensers are susceptible to blockage
transfer affect of water, and therefore operate with by air borne debris such as dust, feathers, packaging,
lower condensing temperatures. When comparing and so on. They must be regularly cleaned (but not
different condenser types the power requirements with refrigerant) to prevent a build up of
of associated fans, pumps and heaters should be contamination, as this will reduce the air flow and
taken into account. In general, systems under 100 hence increase the condensing pressure.
kW capacity use air-cooled condensers unless
there is a space or noise restriction.
For a given capacity, a larger condenser will result in
a lower condensing temperature and hence better
efficiency. Problems can be caused on installations
Water-cooled condensers are of the shell and tube which use thermostatic expansion valves if the
type. The cooling water flows in tubes inside the condensing (head) pressure varies widely. Such
shell, and refrigerant inside the shell condenses on valves are unable to control refrigerant flow reliably
the outside of the cold tubes. Heat transfer is under such conditions, and reduced efficiency and
improved as the water velocity is increased. An reliability will result. Some form of head pressure
efficient system will work with a temperature rise of control may be used to raise the head pressure
5ºC for the water passing through the condenser, artificially, although this is inefficient and is not
and a difference of 5ºC between the condensing necessary if more sophisticated expansion devices
3.4.1 AIR-COOLED CONDENSERS
3.4 CONDENSERS
3.4.2 WATER-COOLED
CONDENSERS
temperature and that of the water leaving the likely to cause a problem, cleanable condensers
condenser. On very small commercial installations should be used.
mains water is often used directly, although this is
becoming less common on new installations due to
water metering.
On larger installations the water will be cooled in a
cooling tower, where the cooling effect is achieved In an evaporative condenser, refrigerant is
by evaporating some of the cooling water into the condensed in tubes which are wetted and over
air. Blockages in the air or water side will significantly which air is forced. The water used to wet the
reduce the efficiency of the cooling tower. Such outside of the tubes is recirculated, although a
blockages are common and are normally caused by certain amount of make up water will be needed.
hard water deposits or algae growth. Water should Evaporative condensers should operate with similar
be treated to prevent these and also to prevent temperatures to the water-cooled conden-
bacteria growth. The cooling tower should cool the ser/cooling tower combination above. The water
water to within 13 - l8ºC of the wet bulb ambient used will require treatment, as described for water-
temperature (which can be up to 10ºC lower than cooled condensers above.
the dry bulb temperature).
The water side of the condenser is also liable to
blockage caused by hard water deposits. If this is
Air and other non-condensable gases in a
refrigeration system will increase the condensing
temperature and hence reduce efficiency. For
example, in a medium temperature ammonia
system working with a condenser which contains
15% air, the running costs will increase by 12%.
Air can remain in a system after installation or
service, if the system has been inadequately
evacuated prior to charging with refrigerant. While
running, air can be drawn into a system operating at
a suction condition lower than atmospheric
pressure, if there is a leak on the low side of the
system.
It is possible to check for air and other non-
condensable gases when the system is not working
3.4.3 EVAPORATIVE
CONDENSERS
3.4.4 LOSS OF CONDENSER
EFFICIENCY DUE
TO AIR IN SYSTEM
Figure 25: Draw-through-type of evaporative
condenser (source: CEMET)
32 33
and the temperatures have had a chance to Any air should be safely purged from the system by
stabilise. If there is no air present, then the a skilled refrigeration technician, with minimum
temperature in the condenser should be equivalent refrigerant emission to the atmosphere.
to the temperature of the ambient air or of the
water flowing through a water-cooled condenser.
If air is in the system the temperature will be higher. 4.1 DESIRABLE
CHARACTERISTICS
� Large heat of vaporization to minimize
equipment size and refrigerant quantity.
� Low specific volume in the vapour phase
to minimize compressor size. This aspect is Refrigerants for Industrial, Commercial and critical for reciprocating and screw type Institutional refrigeration and heat pump systems compressors.are selected to provide the best refrigeration effect � Low liquid phase specific heat to minimize at a reasonable cost. The following characteristics the heat transfer required when are desirable. subcooling the l iquid below the
condensing temperature.� Non-flammable to reduce the fire hazard. � Low saturation pressure required at � Non-toxic to reduce potential health desired condensing temperatures to
hazards. eliminate requirement for heavy duty or
4. REFRIGERANTS
Table 2: Physical Properties of some Refrigerants
Refrigerant Chem. Molec. Boiling Freez. Critical Critical Critical
Formula Mass Point Point Temp Pressure Volume
(NBP), ºC ºC kPa L/kg
°C
Helium He 4.0026 -268.9 None -267.9 228.8 14.43
Hydrogen H 2.0159 -252.8 -259.2 -239.9 1315 33.212
Air 28.97 -194.3 --- -140.7 3772 3.048
Oxygen O 31.9988 -182.9 -218.8 -118.4 5077 2.3412
Methane CH 16.04 -161.5 -182.2 -82.5 4638 6.1814
Tetrafluoro-methane CF 88.01 -127.9 -184.9 -45.7 3741 1.5984
Ethylene C H 28.05 -103.7 -169 9.3 5114 4.372 4
Trifluoromethane CHF 70.02 -82.1 -155 25.6 4833 1.9423
Chlorotrifluoro-methane CClF 104.47 -81.4 -181 28.8 3865 1.7293
aCarbon Dioxide CO 44.01 -78.4 -56.6 31.1 7372 2.1352
Propylene C H 42.09 -47.7 -185 91.8 4618 4.4953 6
Propane C H 44.10 -42.07 -187.7 96.8 4254 4.5453 8
Chlorodifluoro-methane CHClF 86.48 -40.76 -160 96.0 4974 1.9042
Chloropenta-fluoroethane CClF CF 154.48 -39.1 -106 79.9 3153 1.6292 3
34 35
and the temperatures have had a chance to Any air should be safely purged from the system by
stabilise. If there is no air present, then the a skilled refrigeration technician, with minimum
temperature in the condenser should be equivalent refrigerant emission to the atmosphere.
to the temperature of the ambient air or of the
water flowing through a water-cooled condenser.
If air is in the system the temperature will be higher. 4.1 DESIRABLE
CHARACTERISTICS
� Large heat of vaporization to minimize
equipment size and refrigerant quantity.
� Low specific volume in the vapour phase
to minimize compressor size. This aspect is Refrigerants for Industrial, Commercial and critical for reciprocating and screw type Institutional refrigeration and heat pump systems compressors.are selected to provide the best refrigeration effect � Low liquid phase specific heat to minimize at a reasonable cost. The following characteristics the heat transfer required when are desirable. subcooling the l iquid below the
condensing temperature.� Non-flammable to reduce the fire hazard. � Low saturation pressure required at � Non-toxic to reduce potential health desired condensing temperatures to
hazards. eliminate requirement for heavy duty or
4. REFRIGERANTS
Table 2: Physical Properties of some Refrigerants
Refrigerant Chem. Molec. Boiling Freez. Critical Critical Critical
Formula Mass Point Point Temp Pressure Volume
(NBP), ºC ºC kPa L/kg
°C
Helium He 4.0026 -268.9 None -267.9 228.8 14.43
Hydrogen H 2.0159 -252.8 -259.2 -239.9 1315 33.212
Air 28.97 -194.3 --- -140.7 3772 3.048
Oxygen O 31.9988 -182.9 -218.8 -118.4 5077 2.3412
Methane CH 16.04 -161.5 -182.2 -82.5 4638 6.1814
Tetrafluoro-methane CF 88.01 -127.9 -184.9 -45.7 3741 1.5984
Ethylene C H 28.05 -103.7 -169 9.3 5114 4.372 4
Trifluoromethane CHF 70.02 -82.1 -155 25.6 4833 1.9423
Chlorotrifluoro-methane CClF 104.47 -81.4 -181 28.8 3865 1.7293
aCarbon Dioxide CO 44.01 -78.4 -56.6 31.1 7372 2.1352
Propylene C H 42.09 -47.7 -185 91.8 4618 4.4953 6
Propane C H 44.10 -42.07 -187.7 96.8 4254 4.5453 8
Chlorodifluoro-methane CHClF 86.48 -40.76 -160 96.0 4974 1.9042
Chloropenta-fluoroethane CClF CF 154.48 -39.1 -106 79.9 3153 1.6292 3
34 35
high pressure equipment. R11, R12, R502 and R22) are being phased out by
� Low pressure portion of the cycle should international agreement.
be above atmospheric pressure to prevent
inward leakage of air and water vapour into The Montreal Protocol on substances suspected of
the refrigerant piping. attacking ozone was first agreed in 1988, and has
� High heat transfer coefficients. now been signed by over 90 countries. HCFCs such
as R22, which have much lower ozone depletion
Physical properties of various common refrigerants potentials than CFCs, are termed transition
are listed in Table 2 substances and cannot be considered long term
refrigerants. New HCFCs are being developed
The relative safety and hazard level of various which, together with R22, are being used today to
refrigerants have been compiled and classified replace CFCs in many applications.
under ANSI Code B9.l l97l and by Underwriter's
laboratories. Table 2 provides a listing of these New refrigerants which do not attack ozone are
properties for various refrigerants. also being developed. R134a, the first of these to
become commercially available, has been
Many refrigerants widely used today belong to the developed to replace R12 on mobile air-
family of chemica ls called CFCs (ch loro- conditioning and small refrigeration applications. It
fluorocarbons) which are suspected of breaking is not a 'drop in' replacement for R12, although it
down ozone in the upper atmosphere. This operates with very similar temperatures and
environmental concern is causing major changes in pressures. It is not miscible with the mineral oils
refrigerant development and use. CFC and HCFC currently used with CFCs and HCFCs, so new
(hydrochlorofluorocarbon) type refrigerants (e.g. synthetic oils have been developed. Systems
running with R12 can be retrofilled with Rl34a if the synthetic oils. Very few single substances are totally
oil is also changed, providing that the components suitable as refrigerants, and therefore blends of new
in the system can be used with the new refrigerant. and existing substances are being developed.
Successful retrofills have been carried out, with Blends have already been developed based on
minimum disruption to the cooling application. HCFCs and are currently being used as transition
substances. Care must be taken, however, to
Further ozone benign refrigerants are being ensure that the blend remains consis tent
developed which will also need to use the new throughout a plants lifetime.
Ammonia NH 17.03 -33.3 -77.7 133.0 11417 4.2453
Dichlorodi-fluoromethane CCl F 120.93 -29.79 -158 112.0 4113 1.7922 2
Difluoroethane CH CHF 66.05 -25.0 -117 113.5 4492 2.7413 2
Sulphur Dioxide SO 64.07 -10.0 -75.5 157.5 7875 1.9102
Chlorodifluoro-ethane CH CClF 100.5 -9.8 -131 137.1 4120 2.2973 2
Methyl Amine CH NH 31.06 -6.7 -92.5 156.9 74553 2
Octafluorocyclo-butane C F 200.04 -5.8 -41.4 115.3 2781 1.6114 8
Butane C H 58.13 -0.5 -138.5 152.0 3794 4.3834 10
Dichlorofluoro-methane CHCl F 102.92 8.8 -135 178.5 5168 1.9172
Ethyl AmineC H NH 45.08 16.6 -80.6 183.0 56192 5 2
Trichlorofluoro-methane CCI F 137.38 23.82 -111 198.0 4406 1.8043
Ethyl Ether C H O 74.12 34.6 -116.3 194.0 3603 3.7904 10
Dichlorohexa-fluoropropane C Cl F 220.93 35.69 -125.4 180.0 2753 1.7423 2 6
Trichloroethylene CHCl=CCl 131.39 87.2 -73 271.1 50162
Water H O 18.02 100 0 374.2 22103 3.1282
Table 3: Non-CFC refrigerants
1 2 3 4Ref. ODP GWP Availability BP at Efficiency Application
1Bar (ºC)
R22 0.05 0.34 Now -40.8 Better than R12; R12, R502 replacement
same as R502
MP39 0.03 0.22 Now -28.9 Similar to R12 Medium temp retail food
MP66 0.035 0.024 Now -30.7 Similar to R12 R12 replacement
HP81 0.03 0.52 Now -47.4 Same to slightly Medium temp retail food,
better than R12 ice machines, vending
R134a 0 0.34 Now -26.1 Same to slightly Medium and high temp
worse than R12 food retail
69S 0.04 4.0 Now approx. -50.0 Same to slightly Low temp close coupled
better than R502 systems
69L 0.028 4.09 Now -50.6 Same to slightly Low temp remote systems
better than R502
HP80 0.02 0.63 Now -49.0 Slightly worse Low temp retail food,
than R502 transport
FX10 0.023 0.76 Now -49.7 Slightly better Low temp
than R502
HP62 0 0.94 1993/4 -46.5 Similar to R502 Low temp
FX40 0 0.88 1993 for trials -55.0 Slightly worse Low temp
than R502
KLEA60 0 0.35 Now -38.0 to 45.0 Similar to R502 Low temp
Notes:
1. The ODP (ozone depletion potential) is relative to R11 for which OP=1.
2. The GWP (direct greenhouse warming potential) is relative to R11 for which GWP=1.
3. BP = boiling point
4 The efficiency is based on limited test data (not theoretical calculations) in the case of newly developed refrigerants and is therefore
an indication of expected efficiency in actual installations. Much of this data is provisional - the actual effect on efficiency of any new
refrigerant should be checked at the operating conditions of the system.
36 37
high pressure equipment. R11, R12, R502 and R22) are being phased out by
� Low pressure portion of the cycle should international agreement.
be above atmospheric pressure to prevent
inward leakage of air and water vapour into The Montreal Protocol on substances suspected of
the refrigerant piping. attacking ozone was first agreed in 1988, and has
� High heat transfer coefficients. now been signed by over 90 countries. HCFCs such
as R22, which have much lower ozone depletion
Physical properties of various common refrigerants potentials than CFCs, are termed transition
are listed in Table 2 substances and cannot be considered long term
refrigerants. New HCFCs are being developed
The relative safety and hazard level of various which, together with R22, are being used today to
refrigerants have been compiled and classified replace CFCs in many applications.
under ANSI Code B9.l l97l and by Underwriter's
laboratories. Table 2 provides a listing of these New refrigerants which do not attack ozone are
properties for various refrigerants. also being developed. R134a, the first of these to
become commercially available, has been
Many refrigerants widely used today belong to the developed to replace R12 on mobile air-
family of chemica ls called CFCs (ch loro- conditioning and small refrigeration applications. It
fluorocarbons) which are suspected of breaking is not a 'drop in' replacement for R12, although it
down ozone in the upper atmosphere. This operates with very similar temperatures and
environmental concern is causing major changes in pressures. It is not miscible with the mineral oils
refrigerant development and use. CFC and HCFC currently used with CFCs and HCFCs, so new
(hydrochlorofluorocarbon) type refrigerants (e.g. synthetic oils have been developed. Systems
running with R12 can be retrofilled with Rl34a if the synthetic oils. Very few single substances are totally
oil is also changed, providing that the components suitable as refrigerants, and therefore blends of new
in the system can be used with the new refrigerant. and existing substances are being developed.
Successful retrofills have been carried out, with Blends have already been developed based on
minimum disruption to the cooling application. HCFCs and are currently being used as transition
substances. Care must be taken, however, to
Further ozone benign refrigerants are being ensure that the blend remains consis tent
developed which will also need to use the new throughout a plants lifetime.
Ammonia NH 17.03 -33.3 -77.7 133.0 11417 4.2453
Dichlorodi-fluoromethane CCl F 120.93 -29.79 -158 112.0 4113 1.7922 2
Difluoroethane CH CHF 66.05 -25.0 -117 113.5 4492 2.7413 2
Sulphur Dioxide SO 64.07 -10.0 -75.5 157.5 7875 1.9102
Chlorodifluoro-ethane CH CClF 100.5 -9.8 -131 137.1 4120 2.2973 2
Methyl Amine CH NH 31.06 -6.7 -92.5 156.9 74553 2
Octafluorocyclo-butane C F 200.04 -5.8 -41.4 115.3 2781 1.6114 8
Butane C H 58.13 -0.5 -138.5 152.0 3794 4.3834 10
Dichlorofluoro-methane CHCl F 102.92 8.8 -135 178.5 5168 1.9172
Ethyl AmineC H NH 45.08 16.6 -80.6 183.0 56192 5 2
Trichlorofluoro-methane CCI F 137.38 23.82 -111 198.0 4406 1.8043
Ethyl Ether C H O 74.12 34.6 -116.3 194.0 3603 3.7904 10
Dichlorohexa-fluoropropane C Cl F 220.93 35.69 -125.4 180.0 2753 1.7423 2 6
Trichloroethylene CHCl=CCl 131.39 87.2 -73 271.1 50162
Water H O 18.02 100 0 374.2 22103 3.1282
Table 3: Non-CFC refrigerants
1 2 3 4Ref. ODP GWP Availability BP at Efficiency Application
1Bar (ºC)
R22 0.05 0.34 Now -40.8 Better than R12; R12, R502 replacement
same as R502
MP39 0.03 0.22 Now -28.9 Similar to R12 Medium temp retail food
MP66 0.035 0.024 Now -30.7 Similar to R12 R12 replacement
HP81 0.03 0.52 Now -47.4 Same to slightly Medium temp retail food,
better than R12 ice machines, vending
R134a 0 0.34 Now -26.1 Same to slightly Medium and high temp
worse than R12 food retail
69S 0.04 4.0 Now approx. -50.0 Same to slightly Low temp close coupled
better than R502 systems
69L 0.028 4.09 Now -50.6 Same to slightly Low temp remote systems
better than R502
HP80 0.02 0.63 Now -49.0 Slightly worse Low temp retail food,
than R502 transport
FX10 0.023 0.76 Now -49.7 Slightly better Low temp
than R502
HP62 0 0.94 1993/4 -46.5 Similar to R502 Low temp
FX40 0 0.88 1993 for trials -55.0 Slightly worse Low temp
than R502
KLEA60 0 0.35 Now -38.0 to 45.0 Similar to R502 Low temp
Notes:
1. The ODP (ozone depletion potential) is relative to R11 for which OP=1.
2. The GWP (direct greenhouse warming potential) is relative to R11 for which GWP=1.
3. BP = boiling point
4 The efficiency is based on limited test data (not theoretical calculations) in the case of newly developed refrigerants and is therefore
an indication of expected efficiency in actual installations. Much of this data is provisional - the actual effect on efficiency of any new
refrigerant should be checked at the operating conditions of the system.
36 37
'Energy Management Opportunities' is a term that surfaces reduces the heat transfer
represents the ways that energy can be used wisely to efficiency, requiring higher temperature
reduce operating costs. A number of Energy differences to maintain the heat transfer
Management Opportunities, subdiv ided into rate. An increase in temperature difference
Housekeeping, Low Cost, and Retrofit categories are reduces the COP.
outlined in this section with worked examples or
written text to illustrate the potential energy savings. � Repair suction and liquid line insulation to This is not a complete listing of the opportunities reduce superheating of suction gas and loss available for refrigeration and heat pump systems. of subcooling. Refrigerant lines gain heat However, it is intended to provide ideas for when they are located in spaces that are management, operating, and maintenance personnel not air-conditioned, increasing the system to allow them to identify other opportunities that are load without producing useful cooling.applicable to a particular facility. Other guides in this � Calibrate controls and check operation on series should be considered. a regular basis to ensure that the
refrigeration and heat pump systems
operate efficiently.
� Maintain specified refrigerant charge in
refrigeration and heat pump equipment.
Insufficient refrigerant reduces system
performance and capacity. Reduced mass
flow rates of refrigerant causes excessive
superheating of the refrigerant at the
evaporator which reduces the efficiency of
the compressor, and increases the
condensing temperatures.Implemented housekeeping opportunities are Energy � Provide unrestricted air movement around Management actions that are done on a regular basis condensing units and cooling towers to and never less than once a year. The following are eliminate short circuiting or the airstreams typical Energy Management Opportunities in this which causes a higher condensing category: temperature and pressure.
� Minimize the simultaneous operation of � Keep heat transfer surfaces of evaporators heating and cooling systems. Strategically
and condensers clean, through regular located thermometers will help identify inspection and cleaning. Fouling of the this problem.
5.1 HOUSEKEEPING
OPPORTUNITIES
5.1.1 GENERAL
MAINTENANCE
Table 3 gives information on non-CFC refrigerants Water, is the most common refrigerant, and is used
that are available now and on those that will be in combination with lithium bromide as the
available in the near future. absorbent.
Secondary refrigerants, brines and heat transfer
fluids find common use in refrigeration applications. Freons: R22, used primarily in air conditioning; R-12,
These liquids are cooled or heated by the primary used primarily in medium- and high-temperature
refrigerant and transfer heat energy without a refrigeration (R-134a is now used as a replacement
change of state. Their use is common where:for R-12); R-502, used primarily in low-temperature
refrigeration. R-500 can still be found in older � Large refrigerant quantities would equipment. otherwise be required.
� Toxicity or flammability of the refrigerant is Ammonia, refrigerant R-717, one of the earliest
a concern.refrigerants, is now limited to industrial applications
� Central refrigeration is used to produce because of its high toxicity. High cycle efficiency,
cooling for a number of remote locations. low specific volume, high latent heat and low cost
Many examples exist where brines and led to its popularity, particularly in ice rink facilities
secondary coolants are used.and other applications where large temperature
� Chilled water or glycol-water solutions for differences were required.
air-conditioning and process cooling.
� Calcium chloride or sodium chloride in Carbon dioxide is a non-toxic, non-flammable,
solution with water for ice production in odourless, colourless, and inert gas. Because of high
skating rink applications.operating pressures and high horsepower
� Propylene glycol and water solutions for requirements its use as a refrigerant is limited to
use in food and potable water refrigeration specific industrial applications.
systems.
� Hydrocarbon refrigerants in the liquid
phase for extremely low temperature
applications.
Selection of the brine type and concentration is
made on the basis of freezing point, crystallization
Ammonia is a refrigerant used with water as the temperature, specific volume, viscosity, specific
absorbent (solvent). Use of ammonia is declining heat and boiling point. Toxicity, flammability and
with the introduction of refrigerants that have low corrosion characteristics are secondary factors, but
toxicity and operate at lower system pressures. must be considered in the overall analysis.
4.2 COMMON 4.4 BRINES AND
REFRIGERANTS - SECONDARY
VAPOUR COMPRESSION COOLANTS
CYCLES
4.3 COMMON
REFRIGERANTS -
ABSORPTION CYCLE
38 39
5. ENERGY MANAGEMENT
OPPORTUNITIES
'Energy Management Opportunities' is a term that surfaces reduces the heat transfer
represents the ways that energy can be used wisely to efficiency, requiring higher temperature
reduce operating costs. A number of Energy differences to maintain the heat transfer
Management Opportunities, subdiv ided into rate. An increase in temperature difference
Housekeeping, Low Cost, and Retrofit categories are reduces the COP.
outlined in this section with worked examples or
written text to illustrate the potential energy savings. � Repair suction and liquid line insulation to This is not a complete listing of the opportunities reduce superheating of suction gas and loss available for refrigeration and heat pump systems. of subcooling. Refrigerant lines gain heat However, it is intended to provide ideas for when they are located in spaces that are management, operating, and maintenance personnel not air-conditioned, increasing the system to allow them to identify other opportunities that are load without producing useful cooling.applicable to a particular facility. Other guides in this � Calibrate controls and check operation on series should be considered. a regular basis to ensure that the
refrigeration and heat pump systems
operate efficiently.
� Maintain specified refrigerant charge in
refrigeration and heat pump equipment.
Insufficient refrigerant reduces system
performance and capacity. Reduced mass
flow rates of refrigerant causes excessive
superheating of the refrigerant at the
evaporator which reduces the efficiency of
the compressor, and increases the
condensing temperatures.Implemented housekeeping opportunities are Energy � Provide unrestricted air movement around Management actions that are done on a regular basis condensing units and cooling towers to and never less than once a year. The following are eliminate short circuiting or the airstreams typical Energy Management Opportunities in this which causes a higher condensing category: temperature and pressure.
� Minimize the simultaneous operation of � Keep heat transfer surfaces of evaporators heating and cooling systems. Strategically
and condensers clean, through regular located thermometers will help identify inspection and cleaning. Fouling of the this problem.
5.1 HOUSEKEEPING
OPPORTUNITIES
5.1.1 GENERAL
MAINTENANCE
Table 3 gives information on non-CFC refrigerants Water, is the most common refrigerant, and is used
that are available now and on those that will be in combination with lithium bromide as the
available in the near future. absorbent.
Secondary refrigerants, brines and heat transfer
fluids find common use in refrigeration applications. Freons: R22, used primarily in air conditioning; R-12,
These liquids are cooled or heated by the primary used primarily in medium- and high-temperature
refrigerant and transfer heat energy without a refrigeration (R-134a is now used as a replacement
change of state. Their use is common where:for R-12); R-502, used primarily in low-temperature
refrigeration. R-500 can still be found in older � Large refrigerant quantities would equipment. otherwise be required.
� Toxicity or flammability of the refrigerant is Ammonia, refrigerant R-717, one of the earliest
a concern.refrigerants, is now limited to industrial applications
� Central refrigeration is used to produce because of its high toxicity. High cycle efficiency,
cooling for a number of remote locations. low specific volume, high latent heat and low cost
Many examples exist where brines and led to its popularity, particularly in ice rink facilities
secondary coolants are used.and other applications where large temperature
� Chilled water or glycol-water solutions for differences were required.
air-conditioning and process cooling.
� Calcium chloride or sodium chloride in Carbon dioxide is a non-toxic, non-flammable,
solution with water for ice production in odourless, colourless, and inert gas. Because of high
skating rink applications.operating pressures and high horsepower
� Propylene glycol and water solutions for requirements its use as a refrigerant is limited to
use in food and potable water refrigeration specific industrial applications.
systems.
� Hydrocarbon refrigerants in the liquid
phase for extremely low temperature
applications.
Selection of the brine type and concentration is
made on the basis of freezing point, crystallization
Ammonia is a refrigerant used with water as the temperature, specific volume, viscosity, specific
absorbent (solvent). Use of ammonia is declining heat and boiling point. Toxicity, flammability and
with the introduction of refrigerants that have low corrosion characteristics are secondary factors, but
toxicity and operate at lower system pressures. must be considered in the overall analysis.
4.2 COMMON 4.4 BRINES AND
REFRIGERANTS - SECONDARY
VAPOUR COMPRESSION COOLANTS
CYCLES
4.3 COMMON
REFRIGERANTS -
ABSORPTION CYCLE
38 39
5. ENERGY MANAGEMENT
OPPORTUNITIES
5.1.2 PLANT OPERATION
5.1.3 INSTRUMENTATION
instrumentation should be considered to
measure/monitor:
Plant performance will be maintained if the system � pressures;is monitored and appropriate remedial action taken � temperatures;when necessary. Adequate instrumentation is � current and/or power.necessary to enable a plant to be easily monitored.
The use of computers to analyse data will help to Figure 26 shows where such measurements should highlight areas which should be investigated before be taken on a water chilling system.problems develop.
Many compressors can he used on part capacity,
and the number of cylinders operating on a
reciprocating compressor can be indicated by the
signal to the solenoid valves which unload cylinders. There should be sufficient instrumentation on a
On centrifugal or screw compressors an analogue plant to enable the performance to be assessed and
indication of the control signal can be useful.faults diagnosed. With smaller commercial systems
pressure gauges, thermometers and amp probes of Level gauges should be fitted to all vessels that
the type carried by service engineers are likely to be contain liquid refrigerant, i.e. liquid receivers, shell
sufficient. With larger installations permanent and tube evaporators and condensers, and
40 41
Figure 26 Simple direct expansion water chilling system
(source: ETSU)
Tab
le 4
: An
exam
ple
of
a lo
g s
heet.
COM
PRESS
OR
CONDENSE
REVAPORATOR
Su
ctio
n te
mpe
ratu
reDi
scha
rge
tem
pera
ture
Cond
ensi
ng l
iqui
dW
ater
Evap
orat
ing
Reco
mm
ende
dHo
urs
Amps
tem
p
valu
eRu
nSa
tura
ted
Ac
tual
Satu
rate
dAc
tual
Com
p.Oi
l Di
ff.Sa
tura
ted
Liqu
idIn
let
Exit
Flow
Inle
tEx
it
P1
T1
P2
T2Lo
adin
gpr
ess
P3lin
ete
mp.
Tem
p.Ra
tete
mp
tem
p.
1 to
5
P1+
3ºC
to%
>3b
arP2
-2ºC
T4
T5F1
P4P5
P1
+7º
Cto
7/
10ºC
2/5º
C55
0
P2-5
ºCl/
min
Date
Tim
e
18.7
.92
1400
2326
92.5
3.0
6.
130
.057
.210
04.
230
.028
.08.
43.
654
70.
50
NB:
Tem
pera
ture
s ta
ken
from
pre
ssur
e ga
uges
(P)
ref
er t
o sa
tura
ted
tem
pera
ture
s fr
om d
ual
scal
e ga
uges
5.1.2 PLANT OPERATION
5.1.3 INSTRUMENTATION
instrumentation should be considered to
measure/monitor:
Plant performance will be maintained if the system � pressures;is monitored and appropriate remedial action taken � temperatures;when necessary. Adequate instrumentation is � current and/or power.necessary to enable a plant to be easily monitored.
The use of computers to analyse data will help to Figure 26 shows where such measurements should highlight areas which should be investigated before be taken on a water chilling system.problems develop.
Many compressors can he used on part capacity,
and the number of cylinders operating on a
reciprocating compressor can be indicated by the
signal to the solenoid valves which unload cylinders. There should be sufficient instrumentation on a
On centrifugal or screw compressors an analogue plant to enable the performance to be assessed and
indication of the control signal can be useful.faults diagnosed. With smaller commercial systems
pressure gauges, thermometers and amp probes of Level gauges should be fitted to all vessels that
the type carried by service engineers are likely to be contain liquid refrigerant, i.e. liquid receivers, shell
sufficient. With larger installations permanent and tube evaporators and condensers, and
40 41
Figure 26 Simple direct expansion water chilling system
(source: ETSU)
Tab
le 4
: An
exam
ple
of
a lo
g s
heet.
COM
PRESS
OR
CONDENSE
REVAPORATOR
Su
ctio
n te
mpe
ratu
reDi
scha
rge
tem
pera
ture
Cond
ensi
ng l
iqui
dW
ater
Evap
orat
ing
Reco
mm
ende
dHo
urs
Amps
tem
p
valu
eRu
nSa
tura
ted
Ac
tual
Satu
rate
dAc
tual
Com
p.Oi
l Di
ff.Sa
tura
ted
Liqu
idIn
let
Exit
Flow
Inle
tEx
it
P1
T1
P2
T2Lo
adin
gpr
ess
P3lin
ete
mp.
Tem
p.Ra
tete
mp
tem
p.
1 to
5
P1+
3ºC
to%
>3b
arP2
-2ºC
T4
T5F1
P4P5
P1
+7º
Cto
7/
10ºC
2/5º
C55
0
P2-5
ºCl/
min
Date
Tim
e
18.7
.92
1400
2326
92.5
3.0
6.
130
.057
.210
04.
230
.028
.08.
43.
654
70.
50
NB:
Tem
pera
ture
s ta
ken
from
pre
ssur
e ga
uges
(P)
ref
er t
o sa
tura
ted
tem
pera
ture
s fr
om d
ual
scal
e ga
uges
interstage vessels on two stage systems. The
normal refrigerant level, and the acceptable
maximum and minimum levels should be marked
oil the gauge. Worked examples are used to illustrate potential
energy and cost savings. The examples are
considered typical of conditions found in
refrigeration and heat pump systems.
The instrumentation fitted to a system enables on-
site plant operators or off-site contractors to
monitor performance and detect faults before they
cause major decline in efficiency.
Over time the performance of a 175 kW
refrigeration system, with an air-cooled, packaged � Log Sheetscondensing unit, deteriorated. Investigation
revealed that the space where the condensing unit Plant log sheets should be kept, containing was located had been converted to a storage area information on normal operation as well as with stacked materials. Air flow to the condenser recording day to day operation. These logs was blocked, causing short circuiting of the cooling allow performance to be assessed air stream.providing that:
On a day when the ambient temperature was - data is measured and recorded
35ºC, the air entering the condenser was 46.1ºC. accurately
The actual refrigerating load was 120 kW. - information is correctly analysed
Manufacturer's data for 120 kW cooling indicates - problems found are followed by
that the compressor power is 42.3 kW at 35ºappropriate action arid recorded.
C, and 49.76 kW at 46.1ºC. The system operates
2000 hours per year at the elevated temperature. Table 4 shows an example log sheet for the plain
Removal of the stored materials from the shown in Figure 26. The data recorded on a log
condenser vicinity would prevent short circuiting sheet for a specific plant will depend on the
and lower the air temperature entering the characteristics of that plant.
condenser to the ambient temperature. Electricity
cost is 0.10R/kWh
Compressor energy required at 46.1ºC
= 2000 x 49.76
= 99 520 kWhFrom monitoring the refrigeration system, several
irregularities can be linked directly to savings Compressor energy required at 35ºC
potential. Below table 5 gives a list of such potential = 2000 x 42.3
symptoms. = 84 600 kWh
5.1.5 HOUSEKEEPING
WORKED EXAMPLES
5.1.3.1 PLANT MONITORING
5.1.5.1 REDUCE CONDENSING
TEMPERATURE
5.1.4 TROUBLE SHOOTING
42 43
Table 5: Common faults on refrigeration systems
Major Other Fault Solution Operational symptom symptoms cost penalty
Low cooling duty Bubbles in liquid System undercharged LP Add refrigerant to Up to 25% or more reductioncompared with line and low or zero float or TEV system correct level in duty and COPcompressor curves subcooling from
condenser
On HP float systems: HP float valve stuck open, Determine why bypass Up to 50% reduction in bypassed, gas passing valve was opened duty and COP
initially. Correct fault and close bypass valve
High actual compressor Broken or obstructed Repair valve and identify Loss of duty in proportion discharge temperature reciprocating and rectify cause of to cylinders affectedand low compressor compressor suction blockage or obstructionabsorbed power valve
High actual compressor Broken or obstructed Repair valve and identify Loss of duty and COP discharge temperature reciprocating and rectify cause of in proportion to cylinders
compressor discharge breakage or obstruction affectedvalve
Poor evaporator Low evaporating pressure Fouling of air/water side Clean evaporator and Up to 15% loss of COP, effectiveness high water/air side of evaporator locate and cure source 25% loss of cooling duty
pressure drop of fouling
Low evaporating pressure Blocked suction strainer Clean suction strainer. Up to 30% reduction in COPhigh apparent superheat Identify and rectify source
of blockage
Loss of oil from Oil accumulation in Remove excess oil, install Up to 25% reduction in COPcompressor flooded evaporator effective oil drain or crankcase rectification system
Loss of oil from Poor oil return from Re-design suction side Up to 25% reduction in compressor expansion valve system pipework duty and COPcrankcase
In all systems, possible Obstruction in liquid line Locate and clear Up to 15% loss in COP, subcooling, high obstruction. Identify 25% loss of cooling duty
high liquid line cause and rectifysuction superheat
Poor condenser High condensing Very high overcharge of Remove excess charge Up to 10% loss of duty, effectiveness temperature, high LP float or TEV system 15% reduction in COP
liquid subcooling
High condensing, high Air or non-condensable Purge non-condensable Up to 10% loss in COPliquid subcooling gas in system gas in system
High water/air side Fouling of air-water side Clean condenser and Up to 25% loss in COP, 10% pressure drop of condenser locate and cure source loss in duty
of fouling
Low suction superheat LP float and TEV: possible Incorrect or faulty expansi Identify and rectify fault Up to 15% reduction in duty.
low compressor discharge on device control Potential compressor failure
temperature due to liquid carry over
High suction superheat HP float: low liquid level in System undercharged Add refrigerant to correct Up to 10% loss of duty.
evaporator level 7½% reduction in COP
interstage vessels on two stage systems. The
normal refrigerant level, and the acceptable
maximum and minimum levels should be marked
oil the gauge. Worked examples are used to illustrate potential
energy and cost savings. The examples are
considered typical of conditions found in
refrigeration and heat pump systems.
The instrumentation fitted to a system enables on-
site plant operators or off-site contractors to
monitor performance and detect faults before they
cause major decline in efficiency.
Over time the performance of a 175 kW
refrigeration system, with an air-cooled, packaged � Log Sheetscondensing unit, deteriorated. Investigation
revealed that the space where the condensing unit Plant log sheets should be kept, containing was located had been converted to a storage area information on normal operation as well as with stacked materials. Air flow to the condenser recording day to day operation. These logs was blocked, causing short circuiting of the cooling allow performance to be assessed air stream.providing that:
On a day when the ambient temperature was - data is measured and recorded
35ºC, the air entering the condenser was 46.1ºC. accurately
The actual refrigerating load was 120 kW. - information is correctly analysed
Manufacturer's data for 120 kW cooling indicates - problems found are followed by
that the compressor power is 42.3 kW at 35ºappropriate action arid recorded.
C, and 49.76 kW at 46.1ºC. The system operates
2000 hours per year at the elevated temperature. Table 4 shows an example log sheet for the plain
Removal of the stored materials from the shown in Figure 26. The data recorded on a log
condenser vicinity would prevent short circuiting sheet for a specific plant will depend on the
and lower the air temperature entering the characteristics of that plant.
condenser to the ambient temperature. Electricity
cost is 0.10R/kWh
Compressor energy required at 46.1ºC
= 2000 x 49.76
= 99 520 kWhFrom monitoring the refrigeration system, several
irregularities can be linked directly to savings Compressor energy required at 35ºC
potential. Below table 5 gives a list of such potential = 2000 x 42.3
symptoms. = 84 600 kWh
5.1.5 HOUSEKEEPING
WORKED EXAMPLES
5.1.3.1 PLANT MONITORING
5.1.5.1 REDUCE CONDENSING
TEMPERATURE
5.1.4 TROUBLE SHOOTING
42 43
Table 5: Common faults on refrigeration systems
Major Other Fault Solution Operational symptom symptoms cost penalty
Low cooling duty Bubbles in liquid System undercharged LP Add refrigerant to Up to 25% or more reductioncompared with line and low or zero float or TEV system correct level in duty and COPcompressor curves subcooling from
condenser
On HP float systems: HP float valve stuck open, Determine why bypass Up to 50% reduction in bypassed, gas passing valve was opened duty and COP
initially. Correct fault and close bypass valve
High actual compressor Broken or obstructed Repair valve and identify Loss of duty in proportion discharge temperature reciprocating and rectify cause of to cylinders affectedand low compressor compressor suction blockage or obstructionabsorbed power valve
High actual compressor Broken or obstructed Repair valve and identify Loss of duty and COP discharge temperature reciprocating and rectify cause of in proportion to cylinders
compressor discharge breakage or obstruction affectedvalve
Poor evaporator Low evaporating pressure Fouling of air/water side Clean evaporator and Up to 15% loss of COP, effectiveness high water/air side of evaporator locate and cure source 25% loss of cooling duty
pressure drop of fouling
Low evaporating pressure Blocked suction strainer Clean suction strainer. Up to 30% reduction in COPhigh apparent superheat Identify and rectify source
of blockage
Loss of oil from Oil accumulation in Remove excess oil, install Up to 25% reduction in COPcompressor flooded evaporator effective oil drain or crankcase rectification system
Loss of oil from Poor oil return from Re-design suction side Up to 25% reduction in compressor expansion valve system pipework duty and COPcrankcase
In all systems, possible Obstruction in liquid line Locate and clear Up to 15% loss in COP, subcooling, high obstruction. Identify 25% loss of cooling duty
high liquid line cause and rectifysuction superheat
Poor condenser High condensing Very high overcharge of Remove excess charge Up to 10% loss of duty, effectiveness temperature, high LP float or TEV system 15% reduction in COP
liquid subcooling
High condensing, high Air or non-condensable Purge non-condensable Up to 10% loss in COPliquid subcooling gas in system gas in system
High water/air side Fouling of air-water side Clean condenser and Up to 25% loss in COP, 10% pressure drop of condenser locate and cure source loss in duty
of fouling
Low suction superheat LP float and TEV: possible Incorrect or faulty expansi Identify and rectify fault Up to 15% reduction in duty.
low compressor discharge on device control Potential compressor failure
temperature due to liquid carry over
High suction superheat HP float: low liquid level in System undercharged Add refrigerant to correct Up to 10% loss of duty.
evaporator level 7½% reduction in COP
Energy Saved = 99 520 - 84 600 "Clean" refrigerant condensing temperature:
= 14 920 kWh 40.6ºC = 313.6 K
Rand savings = 14920 kWh x R0.10/kwh "Dirty" COP = 0.25* x
= R1492/yr
=0.25 x = 1.55
"Clean" COP = 0.25* x
= 0.25 x = 2.10An 880 kW centrifugal chiller with a forced draft
cooling tower is used to produce chilled water for *COP actual values estimated as
air conditioning. On a walk-through audit it was .25 x COP (theoretical)
noticed that algae was growing on the wetted
surfaces of the cooling tower. Water blowdown to
control mineral deposits and chemical feed was Change in COP = x 100
performed by leaving the blowdown valve open.
Chemical testing and treatment was neglected. = 35% (improvement)
During a plant shutdown, the heat exchanger
surfaces of the evaporator and condenser were Power required for 880 kW cooling:
examined and found to be fouled. A contractor was
hired to clean the equipment at a cost of R1,700 for "Dirty" = 568 kW
each heat exchanger and Rl,400 for the cooling
tower, for a total of R4.800. Electricity cost is
0.10/kWh."Clean" = 419 kW
Performance of the system was evaluated, before
and after the cleaning, using manufacturer's data The system operates at full load for an estimated and estimated COP values.900 hours per year. Savings because of cleaning are:
"Dirty" refrigerant suction temperature: Savings = (568 - 419) kW x1.7ºC = 274.7 K
900 hr x R0. 10/kWh
= R13410/yr"Dirty" refrigerant condensing temperature:
46.lºC = 319.1 K Simple payback = (Investment/Savings)
= 0.36 years "Clean" refrigerant suction temperature: (4 months)7.2ºC = 280.2 K
5.1.5.2 CLEAN EVAPORATORS
AND CONDENSERS
5.2 LOW COST
OPPORTUNITIES
performance of the refrigeration system
wil l offset the increased power
requirement of the cooling tower fan and
make-up water costs.
Implemented low cost opportunities are Energy Provide an automatic water treatment
Management actions that are done once and for system to add chemicals, and control
which the cost in not considered great. The following blowdown, to match the water losses of
are typical Energy Management Opportunities in cooling tower and evaporative condenser
this category. systems. Proper water treatment will
maximize heat transfer effectiveness, and
� Increase evaporator temperature to keep condensing temperatures low.
increase system COP. Benefits include reduced quantities of
Reset the temperature of the chilled water, make-up and blowdown water, and lower
glycol solution or air as a function of the operating and maintenance costs.
cooling required, to allow the evaporator � Reschedule production cycles to reduce
temperature to rise at part loads. For peak electrical demand and make more
example, the setting of the air temperature efficient use of available cooling or heating
leaving the evaporator of an air- energy. Rescheduling may permit
conditioning system can be based on the shutdown of some compressors in
latent load requirement. As the latent load multiunit systems while running others at
falls, less dehumidification is required, and optimum load and peak efficiency.
the controls adjust the evaporator Operation at higher efficiency may delay
temperature upwards. purchase of additional equipment when
Relocate the outdoor coil of an air-to-air heat total load increases
pump to a clean exhaust airstream. A � Upgrade automat ic contro l s in
building's ventilation exhaust is warmer refrigeration plants to provide accurate
than the outside ambient air during most of and flexible operation. Solid state digital
the heating season. control can optimize equipment and
� Reduce condensing temperature to system operat ion to meet load
increase system COP requirements with minimum power
Relocate air cooled condensers and heat consumption, and/or shed load to reduce
pump outdoor coils to clean exhaust short term electrical peaks.
airstreams. Generally the building's � Replace high-maintenance, centrifugal
ventilation exhaust is cooler than the compressors with compressors selected
outside ambient air when cooling is for high efficiency when operating at part
required. load conditions.
Reduce condenser water temperature by � Upgrade insulation on primary and
resetting cooling tower temperature secondary refrigerant piping circuits.
controls. Detailed analysis is required to � Provide multispeed fan motors on cooling
d e t e r m i n e w h e t h e r i n c r e a s e d towers, evaporative coolers and air cooled
44 45
TL
(T - T )H L
TL
(T - T )H L
274.7319.1 - 274.7
280.2313.6 - 280.2
(2.10 - 1.55)1.55
8801.55
8802.10
Energy Saved = 99 520 - 84 600 "Clean" refrigerant condensing temperature:
= 14 920 kWh 40.6ºC = 313.6 K
Rand savings = 14920 kWh x R0.10/kwh "Dirty" COP = 0.25* x
= R1492/yr
=0.25 x = 1.55
"Clean" COP = 0.25* x
= 0.25 x = 2.10An 880 kW centrifugal chiller with a forced draft
cooling tower is used to produce chilled water for *COP actual values estimated as
air conditioning. On a walk-through audit it was .25 x COP (theoretical)
noticed that algae was growing on the wetted
surfaces of the cooling tower. Water blowdown to
control mineral deposits and chemical feed was Change in COP = x 100
performed by leaving the blowdown valve open.
Chemical testing and treatment was neglected. = 35% (improvement)
During a plant shutdown, the heat exchanger
surfaces of the evaporator and condenser were Power required for 880 kW cooling:
examined and found to be fouled. A contractor was
hired to clean the equipment at a cost of R1,700 for "Dirty" = 568 kW
each heat exchanger and Rl,400 for the cooling
tower, for a total of R4.800. Electricity cost is
0.10/kWh."Clean" = 419 kW
Performance of the system was evaluated, before
and after the cleaning, using manufacturer's data The system operates at full load for an estimated and estimated COP values.900 hours per year. Savings because of cleaning are:
"Dirty" refrigerant suction temperature: Savings = (568 - 419) kW x1.7ºC = 274.7 K
900 hr x R0. 10/kWh
= R13410/yr"Dirty" refrigerant condensing temperature:
46.lºC = 319.1 K Simple payback = (Investment/Savings)
= 0.36 years "Clean" refrigerant suction temperature: (4 months)7.2ºC = 280.2 K
5.1.5.2 CLEAN EVAPORATORS
AND CONDENSERS
5.2 LOW COST
OPPORTUNITIES
performance of the refrigeration system
wil l offset the increased power
requirement of the cooling tower fan and
make-up water costs.
Implemented low cost opportunities are Energy Provide an automatic water treatment
Management actions that are done once and for system to add chemicals, and control
which the cost in not considered great. The following blowdown, to match the water losses of
are typical Energy Management Opportunities in cooling tower and evaporative condenser
this category. systems. Proper water treatment will
maximize heat transfer effectiveness, and
� Increase evaporator temperature to keep condensing temperatures low.
increase system COP. Benefits include reduced quantities of
Reset the temperature of the chilled water, make-up and blowdown water, and lower
glycol solution or air as a function of the operating and maintenance costs.
cooling required, to allow the evaporator � Reschedule production cycles to reduce
temperature to rise at part loads. For peak electrical demand and make more
example, the setting of the air temperature efficient use of available cooling or heating
leaving the evaporator of an air- energy. Rescheduling may permit
conditioning system can be based on the shutdown of some compressors in
latent load requirement. As the latent load multiunit systems while running others at
falls, less dehumidification is required, and optimum load and peak efficiency.
the controls adjust the evaporator Operation at higher efficiency may delay
temperature upwards. purchase of additional equipment when
Relocate the outdoor coil of an air-to-air heat total load increases
pump to a clean exhaust airstream. A � Upgrade automat ic contro l s in
building's ventilation exhaust is warmer refrigeration plants to provide accurate
than the outside ambient air during most of and flexible operation. Solid state digital
the heating season. control can optimize equipment and
� Reduce condensing temperature to system operat ion to meet load
increase system COP requirements with minimum power
Relocate air cooled condensers and heat consumption, and/or shed load to reduce
pump outdoor coils to clean exhaust short term electrical peaks.
airstreams. Generally the building's � Replace high-maintenance, centrifugal
ventilation exhaust is cooler than the compressors with compressors selected
outside ambient air when cooling is for high efficiency when operating at part
required. load conditions.
Reduce condenser water temperature by � Upgrade insulation on primary and
resetting cooling tower temperature secondary refrigerant piping circuits.
controls. Detailed analysis is required to � Provide multispeed fan motors on cooling
d e t e r m i n e w h e t h e r i n c r e a s e d towers, evaporative coolers and air cooled
44 45
TL
(T - T )H L
TL
(T - T )H L
274.7319.1 - 274.7
280.2313.6 - 280.2
(2.10 - 1.55)1.55
8801.55
8802.10
condensers. Normally, equipment is
selected to match the rarely attained peak
design condition. Lower outdoor wet and
dry bulb temperatures, and lower indoor Worked examples are used to illustrate potential loads, predominate. Reducing condenser cost savings. The examples are considered typical air flow to match the capacity requirement of the conditions found in building refrigeration and reduces the fan power. heat pump systems
� Evaporative coolers and condensers
operated in winter may provide adequate
capacity when operated with dry coils.
Maintenance, water and electrical costs
can be reduced. Heat tracing and pan
heaters can be turned off. The detrimental Maintain maximum heat transfer rates by
effect of icing on equipment and buildings minimizing fouling. Consider the condenser water
is eliminated. Note that the reduced system in Housekeeping Worked Example 2.
power requirements for fan and circulating Assume that half the change in performance was
pumps in cooling towers and evaporative because of condenser cleaning.
coolers may be offset by a COP decrease
caused by higher condenser temperatures. Reduced electrical costs = R 3 353 / 2
Detailed analysis is required. R 1 676
� Consider a new heat pump system instead
of a new air conditioning system, if winter An automatic water treatment system was
heating is required. The higher equipment provided for the cooling tower, to optimize water
cost will be offset by reduced heating costs make-up and blowdown, and automatically feed
during the winter season. chemicals to control fouling. Capital cost was
R3,000. Annual chemical costs are estimated at � Provide lockable covers on automatic
R800. Note that the system must be cleaned before controls and thermostats, to prevent
automatic water treatment is initiated.unauthorised tampering or adjustment.
� Use clean process cooling water that Simple payback = R 3 000 /1 676
normally goes to drain for evaporative =1.8 yrs
condenser or cooling tower make-up
water. While not conserving energy, this At the end of the first year, the cost of cleaning the will reduce operating costs. exchangers, the cooling tower, and providing
� Re-evaluate the use of hot gas bypass condenser water treatment is negligible. See
when a refrigeration unit works at part- Housekeeping Worked Example 2.
load for any significant period. It may be Other costs are reduced. Annual cleaning of possible to eliminate the bypass feature exchangers is eliminated and controlled blowdown and cycle or turn off the refrigeration reduces make-up water requirements.system.
5.2.1 LOW COST WORKED
EXAMPLES
5.2.1.1 WATER TREATMENT FOR
CONDENSER WATER
5.2.1.2 HEAT PUMP VERSUS 5.2.1.3. HOT GAS
ELECTRIC HEAT BYPASS
5.3 RETROFIT
OPPORTUNITIES
A small office addition is planned for an industrial A small manufacturing plant has a 90 kW capacity facility in Cape Town. An economical means of refrigeration plant operating at a COP of 3. The heating and cooling the addition is desirable. The compressor has six cylinders and operates at full-plant rejects waste heat in the form of warm water. load 24 hours per day, 5 days per week and 50 Loads for the proposed building, including weeks per year. During weekends the refrigeration ventilation, are 35.17 kW cooling, and 29.31 kW load is less than 10 per cent of full-load, and the unit heating. A rooftop packaged air conditioning uses hot gas bypass to avoid low suction pressures system with electric heating is proposed. The and evaporator frosting. It is proposed to eliminate estimated annual heating cost for the all-electric hot gas bypass and cycle the unit on and off to meet system is R2 45l. the low loads. Controls will be modified to
eliminate hot gas bypass and install anti-short cycle A water-to-air heat pump was considered as an
timers at a cost of R1400. The hot gas bypass alternative to the basic, air-conditioning with
imposes a cooling load of about 33 per cent on the electric heat, rooftop package initially proposed.
unit at a cost of R1188 per year. In addition to the The heat pump was selected to meet the design
cost of providing the 9 kW cooling load, by heating and cooling loads, with electric duct heaters
eliminating hot gas bypass, this R1188 can be saved.for 100 per cent backup. The COP for heating at
the given water condition was 2.25 and similar to Simple payback = R1 400/R1 188
the air-conditioner performance in the summer. = 1.2 years
The source of warm water was available 85 per
cent of the time during the heating season, and
cooling water was available throughout the cooling
season.
Annual heat pump energy costs
= (0.85xR2451)/2.25 Implemented retrofit opportunities are defined as +(0.15xR2451)energy management actions that are done once, and = 1 294for which the cost is significant. Many of the
opportunities in this category will require detailed Annual savings = R2 451- R1 294
analysis by specialists and cannot be examined in = R1 157
detail in this guide. The following are typical Energy
The extra cost for a heat pump package over Management Opportunities in the Retrofit
standard air conditioning with electric heat is category.
estimated at R 3 000
� Absorption equipment can provide low
cost cooling if dependable, high grade Simple payback = R3 000/R1 157
waste heat is available.= 2.6 years
46 47
condensers. Normally, equipment is
selected to match the rarely attained peak
design condition. Lower outdoor wet and
dry bulb temperatures, and lower indoor Worked examples are used to illustrate potential loads, predominate. Reducing condenser cost savings. The examples are considered typical air flow to match the capacity requirement of the conditions found in building refrigeration and reduces the fan power. heat pump systems
� Evaporative coolers and condensers
operated in winter may provide adequate
capacity when operated with dry coils.
Maintenance, water and electrical costs
can be reduced. Heat tracing and pan
heaters can be turned off. The detrimental Maintain maximum heat transfer rates by
effect of icing on equipment and buildings minimizing fouling. Consider the condenser water
is eliminated. Note that the reduced system in Housekeeping Worked Example 2.
power requirements for fan and circulating Assume that half the change in performance was
pumps in cooling towers and evaporative because of condenser cleaning.
coolers may be offset by a COP decrease
caused by higher condenser temperatures. Reduced electrical costs = R 3 353 / 2
Detailed analysis is required. R 1 676
� Consider a new heat pump system instead
of a new air conditioning system, if winter An automatic water treatment system was
heating is required. The higher equipment provided for the cooling tower, to optimize water
cost will be offset by reduced heating costs make-up and blowdown, and automatically feed
during the winter season. chemicals to control fouling. Capital cost was
R3,000. Annual chemical costs are estimated at � Provide lockable covers on automatic
R800. Note that the system must be cleaned before controls and thermostats, to prevent
automatic water treatment is initiated.unauthorised tampering or adjustment.
� Use clean process cooling water that Simple payback = R 3 000 /1 676
normally goes to drain for evaporative =1.8 yrs
condenser or cooling tower make-up
water. While not conserving energy, this At the end of the first year, the cost of cleaning the will reduce operating costs. exchangers, the cooling tower, and providing
� Re-evaluate the use of hot gas bypass condenser water treatment is negligible. See
when a refrigeration unit works at part- Housekeeping Worked Example 2.
load for any significant period. It may be Other costs are reduced. Annual cleaning of possible to eliminate the bypass feature exchangers is eliminated and controlled blowdown and cycle or turn off the refrigeration reduces make-up water requirements.system.
5.2.1 LOW COST WORKED
EXAMPLES
5.2.1.1 WATER TREATMENT FOR
CONDENSER WATER
5.2.1.2 HEAT PUMP VERSUS 5.2.1.3. HOT GAS
ELECTRIC HEAT BYPASS
5.3 RETROFIT
OPPORTUNITIES
A small office addition is planned for an industrial A small manufacturing plant has a 90 kW capacity facility in Cape Town. An economical means of refrigeration plant operating at a COP of 3. The heating and cooling the addition is desirable. The compressor has six cylinders and operates at full-plant rejects waste heat in the form of warm water. load 24 hours per day, 5 days per week and 50 Loads for the proposed building, including weeks per year. During weekends the refrigeration ventilation, are 35.17 kW cooling, and 29.31 kW load is less than 10 per cent of full-load, and the unit heating. A rooftop packaged air conditioning uses hot gas bypass to avoid low suction pressures system with electric heating is proposed. The and evaporator frosting. It is proposed to eliminate estimated annual heating cost for the all-electric hot gas bypass and cycle the unit on and off to meet system is R2 45l. the low loads. Controls will be modified to
eliminate hot gas bypass and install anti-short cycle A water-to-air heat pump was considered as an
timers at a cost of R1400. The hot gas bypass alternative to the basic, air-conditioning with
imposes a cooling load of about 33 per cent on the electric heat, rooftop package initially proposed.
unit at a cost of R1188 per year. In addition to the The heat pump was selected to meet the design
cost of providing the 9 kW cooling load, by heating and cooling loads, with electric duct heaters
eliminating hot gas bypass, this R1188 can be saved.for 100 per cent backup. The COP for heating at
the given water condition was 2.25 and similar to Simple payback = R1 400/R1 188
the air-conditioner performance in the summer. = 1.2 years
The source of warm water was available 85 per
cent of the time during the heating season, and
cooling water was available throughout the cooling
season.
Annual heat pump energy costs
= (0.85xR2451)/2.25 Implemented retrofit opportunities are defined as +(0.15xR2451)energy management actions that are done once, and = 1 294for which the cost is significant. Many of the
opportunities in this category will require detailed Annual savings = R2 451- R1 294
analysis by specialists and cannot be examined in = R1 157
detail in this guide. The following are typical Energy
The extra cost for a heat pump package over Management Opportunities in the Retrofit
standard air conditioning with electric heat is category.
estimated at R 3 000
� Absorption equipment can provide low
cost cooling if dependable, high grade Simple payback = R3 000/R1 157
waste heat is available.= 2.6 years
46 47
� Use a heat pump to upgrade the low superheat can be used where lower
temperature waste heat to a temperature temperature latent heat cannot. Care must
suitable for building heating. be taken in the design of the refrigerant
� Provide a thermal storage system to piping system to ensure proper return of
reduce compressor cycling, and allow liquid refr igerant and oil from the
continuous operation at full-load and desuperheater.
higher efficiency. � Use well, river or lake water as a lower
� Provide decentralized systems to match temperature cooling medium to reduce
loads with specialized requirements. For condensing temperatures. If an air-cooled
example, if a large system operates at a low condenser requires major repair or
evaporator temperature when only a small rep lacement , cons ider us ing an
portion of the load requires low evaporative condenser. Improved
temperature, provide a smal l, low performance and reduced energy cost
temperature system to serve the special because of the higher COP may justify the
area. Operate the large system at a higher added expenditure.
evaporator temperature to improve COP. � Use mechanical refrigeration equipment in
Co ns id er "p ig gy ba ck in g" th e lo w facilities, such as indoor swimming pools
temperature system onto the higher where high ventilation rates are required
tempera ture sy s tem to reduce for humidity control. Winter heating costs
temperature differences and increase for the ventilation air can be reduced by
COP. reducing the ventilation rate. The total heat
� Reclaim rejected condenser heat for space of rejection can be used to preheat the
hea ting, proces s hea ting or water ventilation supply air and preheat the
preheating. In addition to reclaiming the make-up water for the pool. Energy savings
otherwise wasted heat, the system COP result.
may be increased when a lower
temperature condensing medium is Calculations for 'retrofit' savings are site specific and
avai lable. For example, preheating in many cases involve detailed analysis. This booklet
domestic water will reduce the energy serves as a guide for the possible avenues to
required for water heating and reduce the investigate and gives a feel for energy efficiency
conden sin g temperatu re. The col d earning opportunities in refrigeration and cooling.
incoming water supply can often reduce
the condensing water temperature by 5 to
10ºC, thereby increasing the system COP.
� Desuperheat the refrigerant vapour (hot
gas) leaving the compressor. The
superheat can be recovered for process or
make-up water preheating. Because the
temperature of the hot gas is higher than
the condensing temperature, the
Glossary of terms used in commercial refrigeration
(Words in italics are other terms explained within the glossary.)
Air cooled condenser: A condenser cooled by natural or forced flow of air.
Ambient temperature: The prevailing temperature of the atmosphere surrounding the component
under consideration.
Atmospheric pressure: The pressure exerted by the column of air in the atmosphere above the
reference point.
Balanced port valve: An expansion valve which gives good system stability despite widely varying
operating conditions.
Boiling point: The temperature at which evaporation of liquid takes place at a specific
pressure.
Capacity control: Variation in the quantity of refrigerant circulated in order to vary the
refrigeration capacity.
Cascade system: A refrigeration system composed of more than one circuit where the
evaporation process of the higher temperature circuit cools the condenser of
the lower temperature circuit.
CFC: Chlorofluorocarbon a derivative of a hydrocarbon containing chlorine.
Changes of State: When sufficient heat is added or removed, most substances undergo a
change of state. The temperature remains constant until the change of state is
complete. Change of state can be from solid to liquid, liquid to vapour or vice
versa. Typical examples are ice melting and water boiling.
Condense: The process of changing a vapour into a liquid by the extraction of heat.
48 49
APPENDIX 1:
GLOSSARY OF TERMS
� Use a heat pump to upgrade the low superheat can be used where lower
temperature waste heat to a temperature temperature latent heat cannot. Care must
suitable for building heating. be taken in the design of the refrigerant
� Provide a thermal storage system to piping system to ensure proper return of
reduce compressor cycling, and allow liquid refr igerant and oil from the
continuous operation at full-load and desuperheater.
higher efficiency. � Use well, river or lake water as a lower
� Provide decentralized systems to match temperature cooling medium to reduce
loads with specialized requirements. For condensing temperatures. If an air-cooled
example, if a large system operates at a low condenser requires major repair or
evaporator temperature when only a small rep lacement , cons ider us ing an
portion of the load requires low evaporative condenser. Improved
temperature, provide a smal l, low performance and reduced energy cost
temperature system to serve the special because of the higher COP may justify the
area. Operate the large system at a higher added expenditure.
evaporator temperature to improve COP. � Use mechanical refrigeration equipment in
Co ns id er "p ig gy ba ck in g" th e lo w facilities, such as indoor swimming pools
temperature system onto the higher where high ventilation rates are required
tempera ture sy s tem to reduce for humidity control. Winter heating costs
temperature differences and increase for the ventilation air can be reduced by
COP. reducing the ventilation rate. The total heat
� Reclaim rejected condenser heat for space of rejection can be used to preheat the
hea ting, proces s hea ting or water ventilation supply air and preheat the
preheating. In addition to reclaiming the make-up water for the pool. Energy savings
otherwise wasted heat, the system COP result.
may be increased when a lower
temperature condensing medium is Calculations for 'retrofit' savings are site specific and
avai lable. For example, preheating in many cases involve detailed analysis. This booklet
domestic water will reduce the energy serves as a guide for the possible avenues to
required for water heating and reduce the investigate and gives a feel for energy efficiency
conden sin g temperatu re. The col d earning opportunities in refrigeration and cooling.
incoming water supply can often reduce
the condensing water temperature by 5 to
10ºC, thereby increasing the system COP.
� Desuperheat the refrigerant vapour (hot
gas) leaving the compressor. The
superheat can be recovered for process or
make-up water preheating. Because the
temperature of the hot gas is higher than
the condensing temperature, the
Glossary of terms used in commercial refrigeration
(Words in italics are other terms explained within the glossary.)
Air cooled condenser: A condenser cooled by natural or forced flow of air.
Ambient temperature: The prevailing temperature of the atmosphere surrounding the component
under consideration.
Atmospheric pressure: The pressure exerted by the column of air in the atmosphere above the
reference point.
Balanced port valve: An expansion valve which gives good system stability despite widely varying
operating conditions.
Boiling point: The temperature at which evaporation of liquid takes place at a specific
pressure.
Capacity control: Variation in the quantity of refrigerant circulated in order to vary the
refrigeration capacity.
Cascade system: A refrigeration system composed of more than one circuit where the
evaporation process of the higher temperature circuit cools the condenser of
the lower temperature circuit.
CFC: Chlorofluorocarbon a derivative of a hydrocarbon containing chlorine.
Changes of State: When sufficient heat is added or removed, most substances undergo a
change of state. The temperature remains constant until the change of state is
complete. Change of state can be from solid to liquid, liquid to vapour or vice
versa. Typical examples are ice melting and water boiling.
Condense: The process of changing a vapour into a liquid by the extraction of heat.
48 49
APPENDIX 1:
GLOSSARY OF TERMS
Condenser: A heat exchanger in which a vapour is liquefied by the removal of heat.
Coefficient of performance: (For a refrigerator:)The ratio of the refrigeration capacity to the power
absorbed by the compressor.(For a heat pump:) The total heat delivered to
the power absorbed by the compressor.
Compression ratio: The ration of the absolute pressures before and after compression.
Compressor: A machine for mechanically increasing the pressure of a gas.
Condensing pressure: The pressure at which a vapour changes into a liquid at a specific temperature.
Condensing temperature: The temperature of a fluid at which condensation occurs when at a known
pressure.
Condensing unit: A collection of components usually consisting of a compressor, condenser and
receiver assembled onto a common base frame.
Cycle: A cycle is a series of processes where the end point conditions or properties
of the substance are identical to the initial conditions. In refrigeration, the
processes required to produce a cooling effect are arranged to operate in a
cyclic manner so that the refrigerant can be reused.
Defrost on demand: An automatic defrost system which is initiated by an unacceptable build up of
ice and terminated when the coil has cleared.
Defrost: Elimination of an ice deposit from the surface of an evaporator.
Density of Saturated Liquid: The density of liquid at saturation temperature and pressure is expressed in 3kg/m . The specific volume of the refrigerant liquid can be calculated by taking
the inverse of the density.
Specific Volume =
Desuperheat: Removal of part or all of the superheat in a gas.
Discharge: The output side of the compressor.
Discharge temperature: The temperature of the compressed fluid discharged from the compressor.
Discharge pressure: The pressure of the compressed fluid discharged from the compressor.
Energy in Liquids and Vapours: When a liquid is heated, the temperature of the liquid rises to the boiling point.
This is the highest temperature to which the liquid can rise at the measured
pressure. The heat absorbed by the liquid in raising the temperature to the
boiling point is called sensible heat. The heat required to convert the liquid to
vapour at the same temperature and pressure is called latent heat.
Electronic expansion valve: An electro-mechanical expansion valve controlled by a microprocessor which
has sensors attached to the evaporator and adjacent pipe work.
Enthalpy (h): The total energy contained in a refrigerant is called the enthalpy. Most
refrigerant tables assume, for convenience of calculations, that the saturated
liquid at 40ºC has zero energy.
Enthalpy of liquid (h ) is the amount of energy contained in one kilogram of thef
liquid at a particular temperature, and is expressed in kJ/kg.
Enthalpy of vapour (h ) is the total energy contained in dry saturated vapour at g
a particular temperature and saturation pressure, and is expressed in kJ/kg.
Latent heat of vaporization (h ) is the amount of energy required to evaporate fg
one kilogram of liquid at a given temperature and pressure and is the
difference between the enthalpy of the liquid and the vapour. It is expressed
in kJ/kg.
The enthalpy equation is: h = h - hfg g f
Enthalpy of a mixture is a value necessary in the calculation of most practical
applications because a refrigerant usually contains a mixture of both vapour
and liquid. If the quality of the vapour is "x" then:
h = h + x(h - h )f g f
Where, h = Enthalpy of "wet" vapour (kJ/kg)
h = Enthalpy of the liquid (kJ/kg)f
h = Enthalpy of the vapour (kJ/kg)g
x = Quality of the vapour (decimal fraction)
Entropy (s): Entropy can be described as a measure of the molecular disorder of a
substance, and is used to describe the refrigeration cycle.
Entropy of saturated liquid (s ) at a given temperature and pressure condition is f
expressed in kJ/(kg·K).
50 51
1Density
Condenser: A heat exchanger in which a vapour is liquefied by the removal of heat.
Coefficient of performance: (For a refrigerator:)The ratio of the refrigeration capacity to the power
absorbed by the compressor.(For a heat pump:) The total heat delivered to
the power absorbed by the compressor.
Compression ratio: The ration of the absolute pressures before and after compression.
Compressor: A machine for mechanically increasing the pressure of a gas.
Condensing pressure: The pressure at which a vapour changes into a liquid at a specific temperature.
Condensing temperature: The temperature of a fluid at which condensation occurs when at a known
pressure.
Condensing unit: A collection of components usually consisting of a compressor, condenser and
receiver assembled onto a common base frame.
Cycle: A cycle is a series of processes where the end point conditions or properties
of the substance are identical to the initial conditions. In refrigeration, the
processes required to produce a cooling effect are arranged to operate in a
cyclic manner so that the refrigerant can be reused.
Defrost on demand: An automatic defrost system which is initiated by an unacceptable build up of
ice and terminated when the coil has cleared.
Defrost: Elimination of an ice deposit from the surface of an evaporator.
Density of Saturated Liquid: The density of liquid at saturation temperature and pressure is expressed in 3kg/m . The specific volume of the refrigerant liquid can be calculated by taking
the inverse of the density.
Specific Volume =
Desuperheat: Removal of part or all of the superheat in a gas.
Discharge: The output side of the compressor.
Discharge temperature: The temperature of the compressed fluid discharged from the compressor.
Discharge pressure: The pressure of the compressed fluid discharged from the compressor.
Energy in Liquids and Vapours: When a liquid is heated, the temperature of the liquid rises to the boiling point.
This is the highest temperature to which the liquid can rise at the measured
pressure. The heat absorbed by the liquid in raising the temperature to the
boiling point is called sensible heat. The heat required to convert the liquid to
vapour at the same temperature and pressure is called latent heat.
Electronic expansion valve: An electro-mechanical expansion valve controlled by a microprocessor which
has sensors attached to the evaporator and adjacent pipe work.
Enthalpy (h): The total energy contained in a refrigerant is called the enthalpy. Most
refrigerant tables assume, for convenience of calculations, that the saturated
liquid at 40ºC has zero energy.
Enthalpy of liquid (h ) is the amount of energy contained in one kilogram of thef
liquid at a particular temperature, and is expressed in kJ/kg.
Enthalpy of vapour (h ) is the total energy contained in dry saturated vapour at g
a particular temperature and saturation pressure, and is expressed in kJ/kg.
Latent heat of vaporization (h ) is the amount of energy required to evaporate fg
one kilogram of liquid at a given temperature and pressure and is the
difference between the enthalpy of the liquid and the vapour. It is expressed
in kJ/kg.
The enthalpy equation is: h = h - hfg g f
Enthalpy of a mixture is a value necessary in the calculation of most practical
applications because a refrigerant usually contains a mixture of both vapour
and liquid. If the quality of the vapour is "x" then:
h = h + x(h - h )f g f
Where, h = Enthalpy of "wet" vapour (kJ/kg)
h = Enthalpy of the liquid (kJ/kg)f
h = Enthalpy of the vapour (kJ/kg)g
x = Quality of the vapour (decimal fraction)
Entropy (s): Entropy can be described as a measure of the molecular disorder of a
substance, and is used to describe the refrigeration cycle.
Entropy of saturated liquid (s ) at a given temperature and pressure condition is f
expressed in kJ/(kg·K).
50 51
1Density
Entropy of saturated vapour (s ) at a given temperature and pressure conditiong
s expressed in kJ/(kg·K).
Entropy of vaporization (s ), is the difference in entropy between the saturated fg
liquid and the saturated vapour.
Evaporation and Condensation: Unlike freezing and melting, evaporation and condensation can take place at
almost any temperature and pressure combination. Evaporation is the
gaseous escape of molecules from the surface of a liquid and is accomplished
by the absorption of a considerable quantity of heat without any change in
temperature. The vapour that leaves the surface of a boiling liquid is called
saturated vapour. The quantity of heat required to make the change of state is
called the latent heat of vaporization. Condensation occurs when the gaseous
molecules return to the liquid state.
Liquids including refrigerants, evaporate at all temperatures with increased
rates of evaporation taking place at higher temperatures. The evaporated
gases exert a pressure called the vapour pressure. As the temperature of the
liquid rises, there is a greater loss of the liquid from the surface which increases
the vapour pressure. Boiling occurs when the vapour pressure reaches the
pressure of the surrounding space. During boiling, vapour is generated at a
pressure equal to the gas pressure on the surface. If the pressure on the
surface is increased, boiling takes place at a higher temperature and the boiling
point is said to increase. Similarly, a reduction in the pressure will lower the
boiling point.
Evaporating temperature: The temperature at which a fluid vaporises within an evaporator at a specific
pressure.
Evaporating pressure: The pressure at which a fluid vaporises within an evaporator at a specific
temperature.
Evaporator: A heat exchanger in which a liquid is vaporised to produce refrigeration.
Externally cooled: A compressor which is cooled by air or water passing over the outside of its
housing.
Extraction rate: The quantity of heat which a refrigeration plant is capable of extracting under
specified conditions of time and temperature.
Fin block: A group of tubes which have been expanded into fins to form a heat
exchanger.
HCFC: Hydrochlorofluorocarbon.
Heat exchanger: A device designed to transfer heat between two physically separated fluids.
Heat recovery: The reclaim of heat from a refrigeration system for use in a heating process.
Heat Transfer: Heat energy can flow only from a higher to a lower temperature level unless
energy is added to reverse the process. Heat transfer will occur when a
temperature difference exists within a medium or between different media.
Higher heat transfer rates occur at higher temperature differences.
Hermetic compressor: A compressor directly coupled to an electric motor and contained within a
gas-tight welded casing.
High pressure switch: A switch designed to stop the compressor motor should the discharge
pressure reach a predetermined maximum valve.
Hot gas bypass: A system whereby some or all of the discharge refrigerant is passed directly
back into the compressor suction.
Immiscible: A condition where oil and refrigerant are incapable of being mixed.
Latent Heat of Fusion: For most pure substances there is a specific melting/freezing temperature
relatively independent of the pressure. For example, ice begins to melt at 0ºC.
The amount of heat necessary to melt one kilogram of ice at 0ºC to one
kilogram of water at 0ºC is called the latent heat of fusion of water and
equals 334.92 kJ/kg. The removal of the same amount of heat from one
kilogram of water at 0ºC will change it back to ice.
Liquid refrigerant injection: Introduction of liquid refrigerant into high temperature refrigerant gas to cool it.
Montreal Protocol: International legislation to phase out production of CFCs and other
substances suspected of depleting ozone.
Oil separator: A device for separating oil from refrigerant vapour.
Open compressor: A compressor driven by an external power unit, requiring a shaft seal.
tOperating conditions: The conditionsunder which a refrigeration system works, including the
evaporating pressure and condensing pressure.
Ozone depletion potential: The potential of a substance to destroy stratospheric ozone.
52 53
Entropy of saturated vapour (s ) at a given temperature and pressure conditiong
s expressed in kJ/(kg·K).
Entropy of vaporization (s ), is the difference in entropy between the saturated fg
liquid and the saturated vapour.
Evaporation and Condensation: Unlike freezing and melting, evaporation and condensation can take place at
almost any temperature and pressure combination. Evaporation is the
gaseous escape of molecules from the surface of a liquid and is accomplished
by the absorption of a considerable quantity of heat without any change in
temperature. The vapour that leaves the surface of a boiling liquid is called
saturated vapour. The quantity of heat required to make the change of state is
called the latent heat of vaporization. Condensation occurs when the gaseous
molecules return to the liquid state.
Liquids including refrigerants, evaporate at all temperatures with increased
rates of evaporation taking place at higher temperatures. The evaporated
gases exert a pressure called the vapour pressure. As the temperature of the
liquid rises, there is a greater loss of the liquid from the surface which increases
the vapour pressure. Boiling occurs when the vapour pressure reaches the
pressure of the surrounding space. During boiling, vapour is generated at a
pressure equal to the gas pressure on the surface. If the pressure on the
surface is increased, boiling takes place at a higher temperature and the boiling
point is said to increase. Similarly, a reduction in the pressure will lower the
boiling point.
Evaporating temperature: The temperature at which a fluid vaporises within an evaporator at a specific
pressure.
Evaporating pressure: The pressure at which a fluid vaporises within an evaporator at a specific
temperature.
Evaporator: A heat exchanger in which a liquid is vaporised to produce refrigeration.
Externally cooled: A compressor which is cooled by air or water passing over the outside of its
housing.
Extraction rate: The quantity of heat which a refrigeration plant is capable of extracting under
specified conditions of time and temperature.
Fin block: A group of tubes which have been expanded into fins to form a heat
exchanger.
HCFC: Hydrochlorofluorocarbon.
Heat exchanger: A device designed to transfer heat between two physically separated fluids.
Heat recovery: The reclaim of heat from a refrigeration system for use in a heating process.
Heat Transfer: Heat energy can flow only from a higher to a lower temperature level unless
energy is added to reverse the process. Heat transfer will occur when a
temperature difference exists within a medium or between different media.
Higher heat transfer rates occur at higher temperature differences.
Hermetic compressor: A compressor directly coupled to an electric motor and contained within a
gas-tight welded casing.
High pressure switch: A switch designed to stop the compressor motor should the discharge
pressure reach a predetermined maximum valve.
Hot gas bypass: A system whereby some or all of the discharge refrigerant is passed directly
back into the compressor suction.
Immiscible: A condition where oil and refrigerant are incapable of being mixed.
Latent Heat of Fusion: For most pure substances there is a specific melting/freezing temperature
relatively independent of the pressure. For example, ice begins to melt at 0ºC.
The amount of heat necessary to melt one kilogram of ice at 0ºC to one
kilogram of water at 0ºC is called the latent heat of fusion of water and
equals 334.92 kJ/kg. The removal of the same amount of heat from one
kilogram of water at 0ºC will change it back to ice.
Liquid refrigerant injection: Introduction of liquid refrigerant into high temperature refrigerant gas to cool it.
Montreal Protocol: International legislation to phase out production of CFCs and other
substances suspected of depleting ozone.
Oil separator: A device for separating oil from refrigerant vapour.
Open compressor: A compressor driven by an external power unit, requiring a shaft seal.
tOperating conditions: The conditionsunder which a refrigeration system works, including the
evaporating pressure and condensing pressure.
Ozone depletion potential: The potential of a substance to destroy stratospheric ozone.
52 53
Performance data: The extraction rate and power input of a refrigeration system.
Plant room: A secure room where most of the high pressure components of a
refrigeration system are located along with the electrical panel.
Pressure: Pressure is the force exerted on a surface, per unit area, and is expressed in
kilopascals (kPa), megapascals (MPa), bar and pounds per square inch (psig).
Process: A process is a physical or chemical change in the properties of matter, or the
conversion of energy from one form to another. In refrigeration, a process is
generally defined by the condition (or properties) of the refrigerant at the
beginning and end of the process.
Quality of Vapour: Theoretically, when vapour leaves the surface of a liquid, it is pure and
saturated at the particular temperature and pressure. In actual practice, tiny
liquid droplets escape with the vapour. When a mixture of liquid and vapour
exists, the ratio of the mass of the liquid to the total mass of the liquid and
vapour mixture is called the 'quality' and is expressed as a percentage or
decimal fraction.
Receiver: A vessel permanently installed in the refrigeration system between the
condenser and the expansion valve to provide a reservoir of liquid refrigerant.
Reciprocating: A positive displacement compressor with piston(s) moving linearly and
alternately in opposite directions in the cylinder(s).
Refrigerant: The working fluid in a refrigeration system, which absorbs heat at a low
temperature (by evaporation) and rejects heat at a high temperature (by
condensation).
Refrigeration capacity: The quantity of heat which a refrigeration plant is capable of extracting under
specified conditions of time and temperature.
Refrigerant Tables: Common properties of refrigerants are tabulated for both liquid and vapour
phases, and at different temperature pressure conditions.
Rotary: A compressor in which the rotation of the component varies the volume of
the compression chamber.
Saturation: A condition at which liquid and vapour may exist when in contact with each
other.
Saturation Pressure: Saturation pressure is normally the second column in a refrigerant table and is
expressed as MPa (absolute). To obtain gauge pressure subtract 0.101325
MPa (101.325 kpa) from the absolute pressure.
Saturation Temperature: Saturation temperature, normally the first column in a refrigerant table, and
given in K, is the temperature at which boiling will occur to produce vapour at
the given saturation pressure.
Semi-hermetic compressor: A compressor directly coupled to an electric motor and contained within a
gas-tight bolted housing.
Shut-off valve: A valve used to isolate particular items of equipment.
Sight glass: A device which allows a visual inspection of a liquid within a pressurised
container.
Specific Volume of Saturated Vapour: The specific volume of saturated vapour is the volume occupied by one
kilogram of dry saturated gas at the corresponding saturation temperature 3
and pressure, and is expressed in m /kg. Density of the vapour can be
calculated by taking the inverse of the specific volume.
Density =
Subcooled liquid: A liquid whose temperature is lower than the condensing temperature at its
given pressure.
Suction (return) temperature: The temperature at which refrigerant gas enters the compressor.
Suction cooled: A compressor in which the motor is cooled by refrigerant gas passing over the
motor windings.
Superheat: The quantity of heat added to dry saturated vapour to raise it from it
saturation temperature to a higher temperature.
Temperature: Temperature is an indication of the heat energy stored in a substance. If the
temperature of a substance was decreased to 273ºC or 0 K (Kelvin), known
as absolute zero, the substance contains no heat energy and all molecular
movement stops.
Temperature difference: The difference in temperature between two substances, surfaces or
54 55
1Specific Volume
Performance data: The extraction rate and power input of a refrigeration system.
Plant room: A secure room where most of the high pressure components of a
refrigeration system are located along with the electrical panel.
Pressure: Pressure is the force exerted on a surface, per unit area, and is expressed in
kilopascals (kPa), megapascals (MPa), bar and pounds per square inch (psig).
Process: A process is a physical or chemical change in the properties of matter, or the
conversion of energy from one form to another. In refrigeration, a process is
generally defined by the condition (or properties) of the refrigerant at the
beginning and end of the process.
Quality of Vapour: Theoretically, when vapour leaves the surface of a liquid, it is pure and
saturated at the particular temperature and pressure. In actual practice, tiny
liquid droplets escape with the vapour. When a mixture of liquid and vapour
exists, the ratio of the mass of the liquid to the total mass of the liquid and
vapour mixture is called the 'quality' and is expressed as a percentage or
decimal fraction.
Receiver: A vessel permanently installed in the refrigeration system between the
condenser and the expansion valve to provide a reservoir of liquid refrigerant.
Reciprocating: A positive displacement compressor with piston(s) moving linearly and
alternately in opposite directions in the cylinder(s).
Refrigerant: The working fluid in a refrigeration system, which absorbs heat at a low
temperature (by evaporation) and rejects heat at a high temperature (by
condensation).
Refrigeration capacity: The quantity of heat which a refrigeration plant is capable of extracting under
specified conditions of time and temperature.
Refrigerant Tables: Common properties of refrigerants are tabulated for both liquid and vapour
phases, and at different temperature pressure conditions.
Rotary: A compressor in which the rotation of the component varies the volume of
the compression chamber.
Saturation: A condition at which liquid and vapour may exist when in contact with each
other.
Saturation Pressure: Saturation pressure is normally the second column in a refrigerant table and is
expressed as MPa (absolute). To obtain gauge pressure subtract 0.101325
MPa (101.325 kpa) from the absolute pressure.
Saturation Temperature: Saturation temperature, normally the first column in a refrigerant table, and
given in K, is the temperature at which boiling will occur to produce vapour at
the given saturation pressure.
Semi-hermetic compressor: A compressor directly coupled to an electric motor and contained within a
gas-tight bolted housing.
Shut-off valve: A valve used to isolate particular items of equipment.
Sight glass: A device which allows a visual inspection of a liquid within a pressurised
container.
Specific Volume of Saturated Vapour: The specific volume of saturated vapour is the volume occupied by one
kilogram of dry saturated gas at the corresponding saturation temperature 3
and pressure, and is expressed in m /kg. Density of the vapour can be
calculated by taking the inverse of the specific volume.
Density =
Subcooled liquid: A liquid whose temperature is lower than the condensing temperature at its
given pressure.
Suction (return) temperature: The temperature at which refrigerant gas enters the compressor.
Suction cooled: A compressor in which the motor is cooled by refrigerant gas passing over the
motor windings.
Superheat: The quantity of heat added to dry saturated vapour to raise it from it
saturation temperature to a higher temperature.
Temperature: Temperature is an indication of the heat energy stored in a substance. If the
temperature of a substance was decreased to 273ºC or 0 K (Kelvin), known
as absolute zero, the substance contains no heat energy and all molecular
movement stops.
Temperature difference: The difference in temperature between two substances, surfaces or
54 55
1Specific Volume
environments involving transfer of heat.
Thermostat: An automatic switch which is responsive to temperature.
Thermostatic expansion valve: A valve which automatically regulates the flow of liquid refrigerant into the
evaporator to maintain within close limits the degree of superheat of the
vapour leaving the evaporator.
Water-cooled condenser: A condenser cooled by the circulation of water through it.
Work: Work is the energy which is transferred by a difference in pressure or force of
any kind. Work is subdivided into shaft work and flow work.
Shaft work is mechanical energy used to drive a mechanism such as a pump,
condenser or turbine. Flow work is the energy transferred into a system by fluid
flowing into, or out of, the system. Both forms of work are expressed in
kilojoules, or on a mass basis, kJ/kg.
Since South Africa mainly uses metric units, these are the first choice in this guide. However, Imperial units are
often given as well. The units used are given in the table below:
Table A1: Unit Conversions
Metric Imperial Conversion
Pressure absolute bar psi 1 barg = 14.7 psig
Pressure gauge barg psig 1 bar = 14.7 psi
Flow, volumetric l/sec cfm 1 l/s = 2 cfm (approx)
Power kW hp 1 kW = 1.34 hp
Energy kWh Btu 1 kWh = 3412.4 Btu
Specific energy J/l
Abbreviations:
psi: pounds per square inch kW: kilowatt
psig: pounds per square inch gauge hp: horsepower
l/sec: litres per second kWh: kilowatt-hour
cfm: cubic feet per minute Btu: British thermal units
J/l: Joules/litre
Pressure absolute = pressure gauge + 1 bar
1 bar = 100 kPa
Standard atmospheric pressure = 1.013 bar
Example of measuring the COP of a refrigeration system directly:
56 57
APPENDIX 2:
ENERGY, VOLUME AND MASS
CONVERSION FACTORS
environments involving transfer of heat.
Thermostat: An automatic switch which is responsive to temperature.
Thermostatic expansion valve: A valve which automatically regulates the flow of liquid refrigerant into the
evaporator to maintain within close limits the degree of superheat of the
vapour leaving the evaporator.
Water-cooled condenser: A condenser cooled by the circulation of water through it.
Work: Work is the energy which is transferred by a difference in pressure or force of
any kind. Work is subdivided into shaft work and flow work.
Shaft work is mechanical energy used to drive a mechanism such as a pump,
condenser or turbine. Flow work is the energy transferred into a system by fluid
flowing into, or out of, the system. Both forms of work are expressed in
kilojoules, or on a mass basis, kJ/kg.
Since South Africa mainly uses metric units, these are the first choice in this guide. However, Imperial units are
often given as well. The units used are given in the table below:
Table A1: Unit Conversions
Metric Imperial Conversion
Pressure absolute bar psi 1 barg = 14.7 psig
Pressure gauge barg psig 1 bar = 14.7 psi
Flow, volumetric l/sec cfm 1 l/s = 2 cfm (approx)
Power kW hp 1 kW = 1.34 hp
Energy kWh Btu 1 kWh = 3412.4 Btu
Specific energy J/l
Abbreviations:
psi: pounds per square inch kW: kilowatt
psig: pounds per square inch gauge hp: horsepower
l/sec: litres per second kWh: kilowatt-hour
cfm: cubic feet per minute Btu: British thermal units
J/l: Joules/litre
Pressure absolute = pressure gauge + 1 bar
1 bar = 100 kPa
Standard atmospheric pressure = 1.013 bar
Example of measuring the COP of a refrigeration system directly:
56 57
APPENDIX 2:
ENERGY, VOLUME AND MASS
CONVERSION FACTORS
COP is defined as the refrigeration affect (i.e. heat taken up in the evaporator) divided by the work (from the
compressor) supplied to the system. Supposing we have an 880 kW centrifugal refrigeration system. The liquid
refrigerant (134a see relevant Pressure-Enthalpy diagram) condenses at 1Mpa (P3 from section 5.1.2.1) this
corresponds to just over 40ºC. The refrigerant is then expanded (at constant Enthalpy) to a pressure of 0.22
MPa (from the PE diagram this corresponds to 10ºC) and a vapour fraction of about 35%. The outlet
temperature and pressure (T6 and P6) are measured as 0ºC and 0.2 MPa. The temperature of the brine being 3cooled is (T5 and T6) from 1 8'C at the inlet to 2ºC coming out. The flowrate of the brine is 0.0367 m /s (Fl).
Calculate the COP of the system and the flowrate of the refrigerant.
Cooling effect:
Water flow = 36.67 kg/s
Temp change for water = 18ºC-2ºC = 16ºC
Heat capacity (Cp) of water = 4.2 kJ/kg.ºC
(Cp is the amount of heat (in joules) that is given up (when the substance is cooled) or taken up (when the
substance is heated), for a change in temperature of a degree C or
K.Cp is generally given - as above - per kilogram of substance.)
Thus the cooling effect = 4.2 * 16 * 36.67 = 2464 kJ/s
COP = cooling effect/ = 2462/880 = 2.8(note: kJ/s = kW)compressor work
Refrigerant flow:
Refrigerant enthalpies
After expansion valve = 188 kJ/kg (0.22 MPa & -10ºC)
After the evaporator = 400 kJ/kg (0.2 MPa & 0ºC)
Enthalpy difference = 400 - 188 = 212 kJ/kg
Assuming heat losses between the expansion valve and suction side of the compressor are negligible, Refrigerant
flow required 2464/212 = 11.6 kg/s
Note: given the flow of refrigerant, the actual COP may be estimated directly from the refrigerant enthalpy
change (from the P-E graphs) over the evaporator, and the power drawn from the compressor.
SOURCES OF FURTHER INFORMATION
COP
APPENDIX 3: EXAMPLE OF
MEASURING COP DIRECTLY.
58 59
SOURCES OF
FURTHER
INFORMATION
For the latest news in energy efficiency technology:
“Energy Management News” is a free newsletter issued by the ERI, which
contains information on the latest developments in energy efficiency in
Southern Africa and details of forthcoming energy efficiency events.
Copies can be obtained from:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
Tel No: +27 (0) 21 650 3892
Fax No: +27 (0) 21 686 4838
E-mail: [email protected]
www.eri.uct.ac.za
COP is defined as the refrigeration affect (i.e. heat taken up in the evaporator) divided by the work (from the
compressor) supplied to the system. Supposing we have an 880 kW centrifugal refrigeration system. The liquid
refrigerant (134a see relevant Pressure-Enthalpy diagram) condenses at 1Mpa (P3 from section 5.1.2.1) this
corresponds to just over 40ºC. The refrigerant is then expanded (at constant Enthalpy) to a pressure of 0.22
MPa (from the PE diagram this corresponds to 10ºC) and a vapour fraction of about 35%. The outlet
temperature and pressure (T6 and P6) are measured as 0ºC and 0.2 MPa. The temperature of the brine being 3cooled is (T5 and T6) from 1 8'C at the inlet to 2ºC coming out. The flowrate of the brine is 0.0367 m /s (Fl).
Calculate the COP of the system and the flowrate of the refrigerant.
Cooling effect:
Water flow = 36.67 kg/s
Temp change for water = 18ºC-2ºC = 16ºC
Heat capacity (Cp) of water = 4.2 kJ/kg.ºC
(Cp is the amount of heat (in joules) that is given up (when the substance is cooled) or taken up (when the
substance is heated), for a change in temperature of a degree C or
K.Cp is generally given - as above - per kilogram of substance.)
Thus the cooling effect = 4.2 * 16 * 36.67 = 2464 kJ/s
COP = cooling effect/ = 2462/880 = 2.8(note: kJ/s = kW)compressor work
Refrigerant flow:
Refrigerant enthalpies
After expansion valve = 188 kJ/kg (0.22 MPa & -10ºC)
After the evaporator = 400 kJ/kg (0.2 MPa & 0ºC)
Enthalpy difference = 400 - 188 = 212 kJ/kg
Assuming heat losses between the expansion valve and suction side of the compressor are negligible, Refrigerant
flow required 2464/212 = 11.6 kg/s
Note: given the flow of refrigerant, the actual COP may be estimated directly from the refrigerant enthalpy
change (from the P-E graphs) over the evaporator, and the power drawn from the compressor.
SOURCES OF FURTHER INFORMATION
COP
APPENDIX 3: EXAMPLE OF
MEASURING COP DIRECTLY.
58 59
SOURCES OF
FURTHER
INFORMATION
For the latest news in energy efficiency technology:
“Energy Management News” is a free newsletter issued by the ERI, which
contains information on the latest developments in energy efficiency in
Southern Africa and details of forthcoming energy efficiency events.
Copies can be obtained from:
The Energy Research Institute
Department of Mechanical Engineering
University of Cape Town
Private Bag
Rondebosch 7701
Cape Town
South Africa
Tel No: +27 (0) 21 650 3892
Fax No: +27 (0) 21 686 4838
E-mail: [email protected]
www.eri.uct.ac.za