Handbook - Refrigeration

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REFRIGERATION

G u i d e B o o k 4

3

3 _E

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STRATEGYSTRATEGY S S T T R R A A T T E E G G Y Y EFFICIENCY EFFICIENCY ENERGY ENERGY EARNINGS EARNINGS

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HOW TO SAVE

ENERGY AND MONEY

IN

IN REFRIGERAREFRIGERATIONTION

This booklet is part of the 3E strategy series. It provides advice on practical This booklet is part of the 3E strategy series. It provides advice on practical ways of improving energy efficiency

ways of improving energy efficiency in industrial refrigeration applications.in industrial refrigeration applications. Prepared for the European Commission DGXVII by:

Prepared for the European Commission DGXVII by: The Energy Research Institute

The Energy Research Institute Department of Mechanical

Department of Mechanical EngineerinEngineeringg University of Cape Town

University of Cape Town Private Bag Private Bag Rondebosch 7701 Rondebosch 7701 Cape Town Cape Town South Africa South Africa www.eri.uct.ac.za www.eri.uct.ac.za

This project is funded by the European Commission and co-funded by the This project is funded by the European Commission and co-funded by the Dutch Ministry of Economics, the South African Department of Minerals Dutch Ministry of Economics, the South African Department of Minerals and Energy and Technical Services International (ESKOM), with the Chief and Energy and Technical Services International (ESKOM), with the Chief contractor being ETSU.

contractor being ETSU.

Neither the European Commission, nor any person acting on behalf of Neither the European Commission, nor any person acting on behalf of thethe commission, nor NOVEM, ETSU, ERI, nor any of the information commission, nor NOVEM, ETSU, ERI, nor any of the information sources is responsible for the use of the information contained in this sources is responsible for the use of the information contained in this publication.

publication.

The views and judgements given in this publication do not necessarily The views and judgements given in this publication do not necessarily represent the

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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

3

_E

STRATEGYSTRATEGY

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HOW TO SAVE

ENERGY AND MONEY

IN REFRIGERATION

ACKNOWLEDGEMENTS

Other titles in the 3E

Other titles in the 3E strategy series:strategy series:

HOW

HOWTO SATO SAVE ENEVE ENERGY AND MONRGY AND MONEYEY::THE 3E STRATEGYTHE 3E STRATEGY HOW

HOWTO SATO SAVE ENERGY AND MONEVE ENERGY AND MONEY IN ELECTRIY IN ELECTRICITY USECITY USE HOW

HOWTO SATO SAVE ENERGY AND MONEVE ENERGY AND MONEY IN BOILERSY IN BOILERSAND FURNACESAND FURNACES

HOW

HOWTO SATO SAVE ENERGY AND MONEVE ENERGY AND MONEY IN COMPRESSY IN COMPRESSEDEDAIR SYSTEMSAIR SYSTEMS HOW

HOWTO SATO SAVE ENERGY VE ENERGY AND MONEY IN AND MONEY IN STEAM SYSTEMSSTEAM SYSTEMS

HOW

HOWTO SATO SAVE ENEVE ENERGY AND MONERGY AND MONEY INSULAY INSULATION SYSTION SYSTEMSTEMS

Copies of these guides may be obtained from: Copies of these guides may be obtained from: The Energy Research Institute

The Energy Research Institute Department of Mechanical

Department of Mechanical EngineerinEngineeringg University of Cape Town

University of Cape Town Private Bag Private Bag Rondebosch 7701 Rondebosch 7701 Cape Town Cape Town South Africa South Africa Tel No: +27 (0) 21 650 3892 Tel No: +27 (0) 21 650 3892 Fax No: +27 (0) 21 686 4838 Fax No: +27 (0) 21 686 4838 E-mail: 3E@eng.uct.ac.za E-mail: 3E@eng.uct.ac.za Website:

Website: http://www.3e.uct.ac.zhttp://www.3e.uct.ac.zaa

The Energy Research Institute would like to

The Energy Research Institute would like to acknowledge the following for their contributionacknowledge the following for their contribution in the production of the guide:

in the production of the guide:



 Energy Technology Support Unite (ETSU), UK, for permission to Energy Technology Support Unite (ETSU), UK, for permission to use informationuse information

from the ‘’Energy Efficiency Best

from the ‘’Energy Efficiency Best Parctice’’ series of handbooks.Parctice’’ series of handbooks.



 Energy Conservation Branch, Department of Energy, Mines and Energy Conservation Branch, Department of Energy, Mines and Resources, Canada,Resources, Canada,

for permission to use

for permission to use information from the ‘’Energy Management’’ series of manuals.information from the ‘’Energy Management’’ series of manuals.



 TLV Co, Ltd, for permission to use figures from their set TLV Co, Ltd, for permission to use figures from their set of handbooks on steam.of handbooks on steam. 

 Wilma Walden for graphic design work Wilma Walden for graphic design work (walden@grm.co.za).(walden@grm.co.za). 

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QUICK 'CHECK-LIST' FOR SAVING E

NERG

Y AND

MONEY IN

REFRIGERA

TION SYSTEMS

This list is a

This list is a selected summary of energy and cost savings opportunities outline in the selected summary of energy and cost savings opportunities outline in the text. Many moretext. Many more

are detailed in the body of the

are detailed in the body of the booklet. These are intended to be a quick 'checklist'.booklet. These are intended to be a quick 'checklist'.

EQUIPMENT MAINTENANCE

EQUIPMENT MAINTENANCE (Chapter 3):(Chapter 3):



 Ensure that there is good and regular maintenance of all equipment.Ensure that there is good and regular maintenance of all equipment.



 Avoid blockage of air flow through and around heat exchanges (e.g. evaporators andAvoid blockage of air flow through and around heat exchanges (e.g. evaporators and

condensers). condensers).



 Make sure that fouling of Make sure that fouling of primary and secondary refrigeration circuits is kept to primary and secondary refrigeration circuits is kept to a minimum.a minimum.



 Maintain isolation standards where Maintain isolation standards where appropriate.appropriate.

EFFICIENT USE

EFFICIENT USE OF OF THE REFRIGERATION SYSTEMTHE REFRIGERATION SYSTEM (Chapter 5):(Chapter 5):



 Keep operating hours to a Keep operating hours to a minimum.minimum.



 Ensure that the cooling load is kept to a Ensure that the cooling load is kept to a minimum.minimum.



 Avoid operating refrigeration plant under Avoid operating refrigeration plant under part-load conditions.part-load conditions.



 Investigate the possibility of Investigate the possibility of improving control functions.improving control functions.



 Reschedule production cycles to reduce peak electrical demand.Reschedule production cycles to reduce peak electrical demand.

AL

ALTERATIONS TO TERATIONS TO THE EXISTING THE EXISTING PLANTPLANT (Chapters 3 and 5):(Chapters 3 and 5):



 Utilise waste heat where possible.Utilise waste heat where possible.



 Where appropriate, retrofit plant with more energy efficient Where appropriate, retrofit plant with more energy efficient components.components.



 Increase evaporator temperature to increase system COP.Increase evaporator temperature to increase system COP.



 Reduce condensing temperature to increase system COPReduce condensing temperature to increase system COP



 Upgrade automatic controls in refrigeration plants to provide accurate and flexible Upgrade automatic controls in refrigeration plants to provide accurate and flexible operation.operation.



 Replace high-maintenance, centrifugal compressors with compressors selected for highReplace high-maintenance, centrifugal compressors with compressors selected for high

efficiency when operating at part

efficiency when operating at part load conditions.load conditions.



 Upgrade insulation on primary and secondary refrigerant piping circuits.Upgrade insulation on primary and secondary refrigerant piping circuits.

REFRIGERANTS

REFRIGERANTS (Chapter 4):(Chapter 4):



 Review energy efficiency when replacing CFC with ozone benign refrigerants. (This might notReview energy efficiency when replacing CFC with ozone benign refrigerants. (This might not

have an energy saving effect). have an energy saving effect).

G u i d e B o o k E s s e n t i a l s

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AUDITING

AUDITING (Chapter 5)(Chapter 5)

Refrigeration efficiency is usually expressed as the

Refrigeration efficiency is usually expressed as the coefficient of performance (COP), defined as:coefficient of performance (COP), defined as:

C

COOPP ==

Once the system performance has been established it is useful to identify the contribution of each plant Once the system performance has been established it is useful to identify the contribution of each plant component to the total system

component to the total system power input. Suitable electricity submeters can be power input. Suitable electricity submeters can be installed for this purpose. Theinstalled for this purpose. The

main contributors are normally: main contributors are normally:



 compressors (typically 65%);compressors (typically 65%);



 condenser pumps (typically 5%);condenser pumps (typically 5%);



 condenser fans (typically 10%);condenser fans (typically 10%);



 evaporator pumps (typically 15%);evaporator pumps (typically 15%);



 lights (typically 5%).lights (typically 5%).

The next stage is to

The next stage is to divide the total cooling load amongst the various process requirements. This should allowdivide the total cooling load amongst the various process requirements. This should allow

the loads that significantly affect costs to be highlighted. the loads that significantly affect costs to be highlighted.

Cooling effect (kW) Cooling effect (kW) Power input to compressor (kW) Power input to compressor (kW)

3

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T a b l e o f c o n t e n t s

1.

1. INTRODUINTRODUCTIONCTION...1...1

1.1

1.1 Purpose...Purpose...1...1

2.

2. THE RTHE REFRIGEREFRIGERAATION TION PROCESSPROCESS...2...2

2.1

2.1 The The vapour vapour compressiocompression n cyclecycle...2...2

2.2.

2.2. Reverse Reverse Carnot Carnot Cycle...Cycle...4.4

2.2.1

2.2.1 Coefficient Coefficient of of Performance...Performance...4...4

2.3

2.3 Theoretical Theoretical Vapour Vapour CompressioCompression n CycleCycle...5...5

2.3.1

2.3.1 Model Model Coefficient Coefficient of of Performance...Performance...6...6

2.3.2

2.3.2 Practical Practical Considerations...Considerations...7...7

2.4

2.4 Absorption Absorption Cycle...Cycle...11...11

2.5

2.5 Special Special Refrigeration Refrigeration SystemsSystems...13...13

2.6

2.6 Variations on Variations on the the simple Carnot simple Carnot circuit...circuit...13...13

2.6.1

2.6.1 Suction/liquid Suction/liquid heat heat exchanger...exchanger...13...13

2.7

2.7 Multiple Multiple evaporator evaporator circuitscircuits...1414

2.7.1

2.7.1 Multiple Multiple compressor compressor SystemsSystems...15...15

2.7.2

2.7.2 Cascade Cascade SystemsSystems...17...17

2.7.3

2.7.3 Heat Heat Pump Pump SystemsSystems...18....18

3.

3. EQUIPMENEQUIPMENTT...20...20

3.1

3.1 CompressorsCompressors...20...20

3.1.1

3.1.1 Types Types of of compressor housingcompressor housing...20...20

3.1.2

3.1.2 Hermetic Hermetic and and semi-hermetic compressorssemi-hermetic compressors...20...20

3.1.3

3.1.3 Open Open compressorscompressors...20...20

3.1.4

3.1.4 Reciprocating Reciprocating compressorcompressors...s...21...21

3.1.5

3.1.5 Screw Screw compressors.compressors...21....21

3.1.6

3.1.6 Scroll Scroll compressorscompressors...22...22

3.1.7

3.1.7 Compressor Compressor performance performance datadata...22...22

3.1.8

3.1.8 Capacity Capacity control...control...22.22

3.2

3.2 Evaporators....Evaporators...23...23

3.2.1

3.2.1 Direct Direct expansionexpansion...23...23

3.2.2

3.2.2 Flooded...Flooded...2424

3.2.3

3.2.3 Oil Oil control control in in evaporators...evaporators...25...25

3.2.4 Energy

3.2.4 Energy efficient operation efficient operation of evaporatorsof evaporators...27...27

3.2.5

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3

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STRATEGYSTRATEGY

3.3

3.3 Expansion Expansion devices...devices...28...28

3.3.1

3.3.1 Thermostatic Thermostatic expansion expansion valvesvalves...28...28

3.3.2

3.3.2 Float Float valve valve systems...systems...30...30

3.4

3.4 Condensers...Condensers...32...32

3.4.1

3.4.1 Air-cooled Air-cooled condensers...condensers...32...32

3.4.2

3.4.2 Water-cooled Water-cooled condensers...condensers...32...32

3.4.3

3.4.3 Evaporative Evaporative condensers...condensers...33...33

3.4.4 Loss of condenser efficiency due

3.4.4 Loss of condenser efficiency due to air in to air in systemsystem...38...38

4.

4. REFRIGREFRIGERANTSERANTS...35...35

4.1

4.1 Desirable Desirable CharacteristicsCharacteristics...35...35

4.2

4.2 Common Refrigerants Common Refrigerants - - Vapour Compression Cycles...Vapour Compression Cycles...38..38

4.3

4.3 Common Refrigerants Common Refrigerants - - Absorption Cycle...Absorption Cycle...38....38

4.4

4.4 Brines Brines and and Secondary Secondary Coolants...Coolants...38...38

5.

5. ENERGY MANENERGY MANAGEMENT OPPOAGEMENT OPPORRTUNITITUNITIESES...39...39

5.1

5.1 Housekeeping Housekeeping Opportunities....Opportunities...39...39

5.1.1

5.1.1 General General maintenance...maintenance...39...39

5.1.2

5.1.2 Plant Plant operationoperation...40...40

5.1.3

5.1.3 Instrumentation....Instrumentation...40...40

5.1.4

5.1.4 Trouble Trouble shootingshooting...42...42

5.1.5

5.1.5 Housekeeping Housekeeping Worked Worked Examples...Examples...42...42

5.2

5.2 Low Low Cost Cost Opportunities...Opportunities...45.45

5.2.1

5.2.1 Low Low Cost Cost Worked Worked Examples...Examples...46...46

5.3

5.3 Retrofit Retrofit Opportunities...Opportunities...47...47

APPENDIX

APPENDIX 1: GLOSSAR1: GLOSSARY OF TERMSY OF TERMS...49...49

APPENDIX

APPENDIX 2: ENERGY2: ENERGY, , VOLUME AND MASVOLUME AND MASS CS CONVERSONVERSION ION FACTFACTORSORS...57...57

APPENDIX 3: EXAMPLE OF MEASUR

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Throughout history, humans have used various Throughout history, humans have used various forms of refrigeration. Simple cooling forms of refrigeration. Simple cooling arrangements, such as those provided by iceboxes arrangements, such as those provided by iceboxes

The following summarizes the purpose of this The following summarizes the purpose of this and root cellars, allowed long term storage of

and root cellars, allowed long term storage of

guide. guide. perishable foods. These, and other simple

perishable foods. These, and other simple techniques, though largely supplanted by techniques, though largely supplanted by



 Introduce the subject of Refrigeration andIntroduce the subject of Refrigeration and

mechanical refrigeration equipment, are still used mechanical refrigeration equipment, are still used

Heat Pumps as used in the Industrial, Heat Pumps as used in the Industrial, by campers, cottagers and people in remote or less

by campers, cottagers and people in remote or less

Commercial and Institutional Sectors. Commercial and Institutional Sectors. developed areas.

developed areas.



 Make building owners and operatorsMake building owners and operators

aware of the potential energy and cost aware of the potential energy and cost Mechanical refrigeration systems were first built

Mechanical refrigeration systems were first built inin

savings available through the savings available through the implemen- the late nineteenth century, but did not become

the late nineteenth century, but did not become

tation of Energy Management tation of Energy Management Oppor-commonplace until the 1940s. Although

commonplace until the 1940s. Although

tunities. tunities. mechanical refrigeration provides benefits such as

mechanical refrigeration provides benefits such as refrigerated storage independent of season or refrigerated storage independent of season or

·· PrProvovidide mee meththodods of s of cacalclcululatatining thg the poe potetentntiaiall

climate, and better living and working climate, and better living and working

energy and cost savings, using simple worked energy and cost savings, using simple worked environments, the energy costs related to

environments, the energy costs related to

examples. examples. operation of these systems are significant. This

operation of these systems are significant. This guide examines refrigeration and heat pump guide examines refrigeration and heat pump systems and identifies where

systems and identifies where energy consumptionenergy consumption

and costs may be reduced. and costs may be reduced.

1.1 PURPOSE

1. INTRODUCTION

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The majority of refrigeration systems are driven by

The majority of refrigeration systems are driven by  The temperature at which The temperature at which refrigerant boilsrefrigerant boils

a

a machimachine, ne, which which comprcompresses esses and and pumps pumps varies varies with with its its pressure; pressure; the the higher higher thethe

refrigerant

refrigerant vapour vapour around around a a sealed sealed circuit. circuit. Heat Heat is is pressure, pressure, the the higher higher the the boiling boiling point;point;

absorbed and rejected through heat exchangers.

absorbed and rejected through heat exchangers.  When refrigerant liquid boils, changing its When refrigerant liquid boils, changing its

These

These systems systems work work on on what what is is called called a a vapour vapour state state to to a a gas, gas, it it absorbs absorbs heat heat from from itsits

ccoommpprreessssiioon n ccyyccllee.. ssuurrrroouunnddiinnggss;;



 Refrigerant can be changed back from a Refrigerant can be changed back from a gasgas

There

There are are other other types types of of plant plant which which can can be be used used to to to to a a liquid liquid by by cooling cooling it, it, usually usually by by using using air air

obtain

obtain a a cooling cooling effect, effect, such such as as absorption absorption cycle cycle or or water.water.

systems, but these are not in common use and are systems, but these are not in common use and are only

only econoeconomicamically lly viabviable le wherwhere e there there are are large large NoteNote:: In In the the refrirefrigerageration tion indusindustry try the the termterm

ssuupppplliiees s oof f wwaasstte e hheeaatt.. eevvaappoorraattiioon n iis s uusseed d iinnsstteeaad d oof f bbooiilliinngg. . AAllssoo, , iif f a a ggaas s iiss

heated above its boiling point it is said to be heated above its boiling point it is said to be superheated and if liquid is cooled below its superheated and if liquid is cooled below its condensing temperature it is sub-cooled.

condensing temperature it is sub-cooled.

To enable the refrigerant to be condensed it has to To enable the refrigerant to be condensed it has to be compressed to a higher pressure, and it is

be compressed to a higher pressure, and it is at thisat this

point that energy has to be used to drive the point that energy has to be used to drive the machine that performs this task. The machine is machine that performs this task. The machine is Heat can only flow naturally from a hot to

Heat can only flow naturally from a hot to a colder a colder

called a compressor and it is usually driven by an called a compressor and it is usually driven by an body. In refrigeration system the opposite must

body. In refrigeration system the opposite must

electric motor. electric motor. occur. This is achieved by using a substance called a

occur. This is achieved by using a substance called a refrigerant, which absorbs heat and hence boils or refrigerant, which absorbs heat and hence boils or

The operation of a simple refrigeration system is The operation of a simple refrigeration system is evaporates at a low pressure to form a gas. This

evaporates at a low pressure to form a gas. This gasgas

shown in Figure 1. The diagram shows the shown in Figure 1. The diagram shows the is then compressed to a higher pressure, such that it

is then compressed to a higher pressure, such that it

refrigerant pressure (bars) and its heat content refrigerant pressure (bars) and its heat content transfers the heat it has gained to ambient air or

transfers the heat it has gained to ambient air or

(kJ/kg). (kJ/kg). water and turns back into a liquid (condenses). In

water and turns back into a liquid (condenses). In this way heat is absorbed, or removed, from a

this way heat is absorbed, or removed, from a lowlow

The refrigeration cycle can be broken down into The refrigeration cycle can be broken down into temperature source and transferred to one at a

temperature source and transferred to one at a

the following stages: the following stages: higher temperature.

higher temperature.

1

1 - - 22 LoLow w prpresessusure re liliququid id rerefrfrigigereranant t in in ththee

There are a number of factors, which make the There are a number of factors, which make the

evaporator absorbs heat from its evaporator absorbs heat from its operation of the vapour compression cycle

operation of the vapour compression cycle

surroundings, usually air, water or some surroundings, usually air, water or some possible:

possible:

2.1

2.1 THE

THE

V

APOUR

COMPRESSION

CYCLE

2. THE REFRIGERATION

PROCESS

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other

other process process liquid. liquid. During During this this process process it it cooling cooling for for this this process process is is usually usually achievedachieved

changes

changes its its state state from from a a liquid liquid to to a a gas, gas, and and by by using using air air or or water. water. A A further further reduction reduction inin

at

at the the evapevap oratorat or or exit is exit is sligslig hthtly ly temperatutemperature re happehappens ns in in the the pipe pipe work work andand

ssuuppeerrhheeaatteedd.. lliiqquuiid d rreecceeiivveer r ((33b b - - 44)), , sso o tthhaat t tthhee

refrigerant liquid is sub-cooled as it

refrigerant liquid is sub-cooled as it entersenters

2

2 - - 33 ThThe e susuperperheaheated ted vavapopour ur ententers ers the the the the expexpansansioion n dedevivice.ce.

compressor where its pressure is raised. compressor where its pressure is raised. The

There will alsre will also be o be a big increa big increase in ase in 4 4 - - 11 The The highigh h prepressussure re subsub-co-cooleoled d liqliquid uid papassessess

temperature,

temperature, because because a a proportion proportion of of the the through through the the expansion expansion device, device, which which bothboth

energy

energy put put into into the the compression compression process process is is reduces reduces its its pressure pressure and and controls controls the the flowflow

ttrraannssffeerrrreed d tto o tthhe e rreeffrriiggeerraanntt.. iinntto o tthhe e eevvaappoorraattoorr..

3

3 - - 44 The The high high prespressure sure supesuperheatrheated ed gas gas passpasses es It It can can be be seen seen that that the the condcondenser enser has has to to be be capacapableble

from

from the the compressor compressor into into the the condenser. condenser. of of rejecting rejecting the the combined combined heat heat inputs inputs of of thethe

The

The initial part initial part of the of the cooling process cooling process (3 - (3 - evaporator and evaporator and the the compressor; compressor; i.e. i.e. (1 (1 - - 2) 2) + + (2 (2 - - 3)3)

3a) des

3a) desuperheats the uperheats the gas befogas before it re it is then is then has has to to be be the the same same as as (3 (3 - - 4). 4). There There is is no no heat heat loss loss or or

turned

turned back back into into liquid liquid (3a (3a - - 3b). 3b). The The gain gain through through the the expansion expansion device.device.

Figure 1: Single stage vapour

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2.2.

2.2. REVERSE

REVERSE

CARNOT

CYCLE

2.2.1

2.2.1 COEFFICIENT

COEFFICIENT

OF

PERFORMANCE



 3 to 4 is constant entropy (ideal)3 to 4 is constant entropy (ideal)

expansion from a higher to a lower expansion from a higher to a lower pressure through the throttling device. pressure through the throttling device. The Carnot Cycle is a theoretical model

The Carnot Cycle is a theoretical model _{From the diagram, the concept of Coefficient of}_{From the diagram, the concept of Coefficient of}

representing the basic processes of a heat engine.

representing the basic processes of a heat engine. AA _{Performance (COP) is derived. The COP is the}_{Performance (COP) is derived. The COP is the}

heat engine is a

heat engine is a devide which produces work fromdevide which produces work from _{ratio of the cooling or Refrigeration Effect (RE), to}_{ratio of the cooling or Refrigeration Effect (RE), to}

heat. The Reverse Carnot cycle produces a

heat. The Reverse Carnot cycle produces a transfer transfer _{the work required to produce the effect.}_{the work required to produce the effect.}

of heat from work. From the

of heat from work. From the model, the maximummodel, the maximum

theoretical performance can be calculated, theoretical performance can be calculated,

establishing criteria to which real refrigeration establishing criteria to which real refrigeration cycles can be compared.

cycles can be compared.

The following processes occur in the Reverse The following processes occur in the Reverse

The refrigeration effect is represented as the area The refrigeration effect is represented as the area Carnot Cycle (Figure 2).

Carnot Cycle (Figure 2).

under the process line 4 - 1. under the process line 4 - 1.



 4 to 1 is the absorption of heat at the4 to 1 is the absorption of heat at the

R

RE E = = T T × LL× ((s s - 11- ss ))44

evaporator, a constant temperature evaporator, a constant temperature boil

boiling proing process at Tcess at T ..LL

Where,

Where, RE RE = = Refrigeration Refrigeration effect effect (kJ)(kJ)



 1 to 2 is constant entropy (ideal)1 to 2 is constant entropy (ideal)

T

T = LL = TempTemperatuerature re (K)(K)

compression. Work input is required and compression. Work input is required and

ss , , s 11 s = 44 = EntEntropropy y [kJ[kJ/kg/kg·K·K)J)J

the temperature of the refrigerant the temperature of the refrigerant

increases. increases.

The theoretical

The theoretical work input (work input (WW ) (i.e. energy ) (i.e. energy



 2 to 3 is 2 to 3 is heat rejection at the condenser, aheat rejection at the condenser, a SS

requirement) for the cycle is represented by the requirement) for the cycle is represented by the cons

constant temtant temperatperature proure process at Tcess at T ..HH

Figure 2: Reverse Carnot Cycle (source: CEMET) Figure 2: Reverse Carnot Cycle (source: CEMET)

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aarreea a ""wwiitthhiinn" " tthhe e ccyycclle e lliinne e 11--22--33--44--11.. EExxaammppllee: : ttwwo o rreeffrriiggeerraattiioon n mmaacchhiinnees s oof f ssiimmiillaar r capacity are compared. One has a COP of 4.0 while capacity are compared. One has a COP of 4.0 while W

W = SS= ((T T - HH- TT ) LL) × × ((s s ss ) 44 11) kkJJ//kkgg the second a COP of 3.0 at the same operating the second a COP of 3.0 at the same operating

conditions. The first machine with the

conditions. The first machine with the higher COPhigher COP

The equation for coefficient of

The equation for coefficient of performance (COP)performance (COP) _{is the most efficient, producing 1.33 times the}_{is the most efficient, producing 1.33 times the}

is obtained by dividing the refrigeration effect (RE)

is obtained by dividing the refrigeration effect (RE) _{refrigeration effect for the same work input of}_{refrigeration effect for the same work input of the}_the

by the theo

by the theoretical woretical work input rk input (W(W ).SS). second machine. The figures above show the effectsecond machine. The figures above show the effect

of evaporator and condenser temperatures on the of evaporator and condenser temperatures on the COP

COP = = ==

COP for various types of

COP for various types of chillers.chillers.

The coefficient of performance for this theoretical The coefficient of performance for this theoretical

The theoretical COP can also be expressed in The theoretical COP can also be expressed in system is temperature dependent and can be

system is temperature dependent and can be

terms of enthalpy, where the difference in energy terms of enthalpy, where the difference in energy reduced to:

reduced to:

content of the refrigerant at various points of the content of the refrigerant at various points of the cycle define the cooling effect and the work input. cycle define the cooling effect and the work input. COP (Ideal) =

COP (Ideal) =

Actual systems are not as efficient as the ideal or

Actual systems are not as efficient as the ideal or COP =COP =

theoretical model (i.e. lower COP), but the theoretical model (i.e. lower COP), but the following general conclusion applies: The smaller following general conclusion applies: The smaller the temperature difference between the heat sink the temperature difference between the heat sink

and th

and the heae heat sout source, (rce, (T T - THH - T ) the gLL) the greater reater thethe

efficiency of the refrigeration (or heat pump) efficiency of the refrigeration (or heat pump) system. The COP, a measure of the energy system. The COP, a measure of the energy

The Carnot cycle, although a useful

The Carnot cycle, although a useful model to assistmodel to assist

required to produce a given refrigeration effect, is required to produce a given refrigeration effect, is

in the understanding of the refrigeration process, in the understanding of the refrigeration process, an excellent means of comparing the efficiencies of

an excellent means of comparing the efficiencies of

has certain limitations. One limitation is the

has certain limitations. One limitation is the lack of lack of

similar equipment. similar equipment.

2.3

2.3 THEORETI

THEORETI

CAL

V

APOUR

COMPRESSION CYCLE

RE RE W W SS T T LL x x ((s - s - 11 )) ( (T - T - HH ) ) xx ss44 ss44 T TLL ((s s 11 - - )) T T LL ( (T T HH - - T T LL))

Figure 3: Effects of evapo

Figure 3: Effects of evaporator anrator and condend condensing tempsing temperaturerature on chiller COe on chiller COPP. . (sour(source: ce: CEMET)CEMET)

(

(h h - 11 - hh44))

(

(14)

accounting

accounting for for changes changes of of state. state. The The figure figure below below condenser. condenser. Step Step 2 2 2' 2' is is the the initial initial de-superheatingde-superheating

shows

shows a a vapour vapour compression compression cycle cycle approximating approximating of of the the hot hot gas gas at at the the condenser condenser or or intermediateintermediate

the

the effect effect of of the the cycle cycle on on the the refrigerant, refrigerant, assuming assuming equipment, equipment, and and 2' 2' - - 3 3 is is the the condensation condensation process.process.

ideal equipment, where: ideal equipment, where:



 1 - 1 - 2 2 Compression.Compression.



 2 - 2' Desuperheating.2 - 2' Desuperheating.



 2' - 3 2' - 3 Constant TemperatureConstant Temperature

Condensation.

Condensation. _{As in the Reverse Carnot cycle, the coefficient or}_{As in the Reverse Carnot cycle, the coefficient or}



 2 - 4' Throttling.2 - 4' Throttling.

performance is: performance is:



 4' - 1 4' - 1 Constant TemperatureConstant Temperature

Evaporation.

Evaporation. _{COP(refrig) = refrigeration effect}_{COP(refrig) = refrigeration effect}

Work input Work input Assuming that the compression process starts at

Assuming that the compression process starts at

COP(refrig)

COP(refrig) = = ==

point 1 as a saturated vapour, energy added in the point 1 as a saturated vapour, energy added in the form of shaft work will raise the temperature and form of shaft work will raise the temperature and pressure. Ideally, this is a

pressure. Ideally, this is a constant entropy processconstant entropy process Where h Where h₄₄'' = = hh₃₃

represented by a vertical line on the T-s diagram. represented by a vertical line on the T-s diagram.

Departures from the ideal Carnot cycle are Departures from the ideal Carnot cycle are The net result is superheating of the vapour to

The net result is superheating of the vapour to

apparent. apparent. point

point 2. 2. Process Process 2 2 2' 2' 3 3 is is heat heat rejection rejection at at thethe

2.3.1

2.3.1 MODEL

MODEL

COEFFICIENT

OF PERFORMANCE

Figure 4: Basic Refrigeration Cycle. (source: CEMET) Figure 4: Basic Refrigeration Cycle. (source: CEMET)

T T LL ( (T T HH - - T T LL )) h h 11 - h- h44 h h 22 - h- h11

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

 [h [h - h₂₂ - h ](the₁₁](theoretioretical) cal) is is larglarger ter than han [h [h - ₂₂ - limitatiolimitations ns such such as as equipequipment ment size, size, systesystem m prespressure,sure,

and design temperatures at the evaporator and and design temperatures at the evaporator and h

h ](C11](Carnarnot)ot)..

condenser, reduce the effectiveness of actual condenser, reduce the effectiveness of actual



 [h - h[h ₁₁ - h ](t₄₄](theoheoretreticaical) is sl) is smalmaller tler than [han [h h --₁₁

1 1

systems. Actual

systems. Actual COPs are 20 to 30 per COPs are 20 to 30 per cent of thecent of the

h

h ](C44](Carnarnot)ot)..

theoretical COP based on the Carnot cycle theoretical COP based on the Carnot cycle operating at the same conditions. Individual operating at the same conditions. Individual The net effect is

The net effect is a COP reduction.a COP reduction.

components, such as the compressor, may have an components, such as the compressor, may have an effectiveness of 40 to 60 per cent of the theoretical effectiveness of 40 to 60 per cent of the theoretical The throttling process reduces the refrigerant

The throttling process reduces the refrigerant

COP (Figure below). These limitations, and COP (Figure below). These limitations, and pressure from the condensing (high) pressure side

pressure from the condensing (high) pressure side

techniques used to reduce their input on cycle techniques used to reduce their input on cycle to the evaporator (low) pressure side. By definition,

to the evaporator (low) pressure side. By definition,

efficiency, are now discussed. efficiency, are now discussed. throttling is a constant enthalpy process. The

throttling is a constant enthalpy process. The enthalpy at point 3 is equal to that at point 4', thus h

enthalpy at point 3 is equal to that at point 4', thus h33

= h

= h '. Energy is degrad44'. Energy is degraded in this process, thered in this process, thereforeefore

the entropy must increase from point 3' to 4. the entropy must increase from point 3' to 4.

Operating temperatures in actual cycles are Operating temperatures in actual cycles are established to suit the temperatures required at the established to suit the temperatures required at the cold medium and the temperature acceptable for cold medium and the temperature acceptable for the heat sink. The practical temperature gradient the heat sink. The practical temperature gradient required to transfer heat from one fluid to another required to transfer heat from one fluid to another Refrigeration and heat pump cycles are more

Refrigeration and heat pump cycles are more _{through a heat exchanger is in the range of 5}_{through a heat exchanger is in the range of 5 to 8ºC.}_{to 8ºC.}

complex than the

complex than the theoretical vapour compressiontheoretical vapour compression _{This means that the refrigerant entering the}_{This means that the refrigerant entering the}

cycle discussed in the previous sector. Practical

cycle discussed in the previous sector. Practical _{evaporator should be 5 to 8ºC colder than the}_{evaporator should be 5 to 8ºC colder than the}

2.3.2.1 Heat Transfer

2.3.2 PRACTICAL

CONSIDERATIONS

Figure 5: Effectiveness of Reciprocating compressors. (source: CEMET) Figure 5: Effectiveness of Reciprocating compressors. (source: CEMET)

1

An example of

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desired medium temperature.

desired medium temperature. The saturation The saturation When When the the superheating superheating occurs occurs at at the the evaporator,evaporator,

0 0

the enthalpy of the refrigerant is raised, extracting the enthalpy of the refrigerant is raised, extracting temperature at the

temperature at the condenser shocondenser should be 5 to uld be 5 to 88 CC

additional heat and increasing the refrigeration additional heat and increasing the refrigeration above the temperature of the heat rejection

above the temperature of the heat rejection

effect of the evaporator. When superheating effect of the evaporator. When superheating medium (Figure below).

medium (Figure below).

occurs in the

occurs in the compressor suction piping, no usefulcompressor suction piping, no useful

cooling occurs. cooling occurs. The area enclosed by line l - 2 - 3 - 4' - l, which

The area enclosed by line l - 2 - 3 - 4' - l, which describes the cycle, has increased because of the describes the cycle, has increased because of the temperature difference required to drive the

temperature difference required to drive the The increase in refrigeration effect, caused by The increase in refrigeration effect, caused by

transfer process. There has been an increase in the

transfer process. There has been an increase in the superheating in the evaporator, is usually offset by superheating in the evaporator, is usually offset by aa

work required to produce the refrigeration effect

work required to produce the refrigeration effect decrease in refrigeration effect at the compressor.decrease in refrigeration effect at the compressor.

because the temperature difference has

because the temperature difference has increased,increased, Because the volumetric flow rate of Because the volumetric flow rate of a compressor isa compressor is

((T T - HH- TT ))..LL constant, the mass flow rate and refrigerating effectconstant, the mass flow rate and refrigerating effect

are reduced by decreases in refrigerant density are reduced by decreases in refrigerant density caused by the superheating. The relative effects of caused by the superheating. The relative effects of increases in enthalpy and decreases in density must increases in enthalpy and decreases in density must be calculated in detail. A study of the system design be calculated in detail. A study of the system design may be practical only for systems over 500 kW in may be practical only for systems over 500 kW in In the refrigerant cycle, refrigerant gas becomes

In the refrigerant cycle, refrigerant gas becomes

capacity. There is a loss

capacity. There is a loss in refrigerating capacity of in refrigerating capacity of

superheated at the evaporator and at the superheated at the evaporator and at the

about one per cent for every 2.5ºC of

about one per cent for every 2.5ºC of superheat insuperheat in

compressor (Figure 6). During the evaporation compressor (Figure 6). During the evaporation

the suction line of a reciprocating compressor. the suction line of a reciprocating compressor. process the refrigerant is completely vaporized

process the refrigerant is completely vaporized

Insulation on suction lines will minimize the Insulation on suction lines will minimize the part-way through the evaporator. As the cool

part-way through the evaporator. As the cool

undesirable heat gain. undesirable heat gain. refrigerant vapour continues through the

refrigerant vapour continues through the evaporator, additional heat is absorbed which evaporator, additional heat is absorbed which

Refrigerant superheating also occurs at the Refrigerant superheating also occurs at the superheats the vapour. Pressure losses, caused by

superheats the vapour. Pressure losses, caused by

compressor. The refrigerant enters the compressor compressor. The refrigerant enters the compressor friction, further increase the amount of superheat.

friction, further increase the amount of superheat.

2.3.2.2 Superheat

Figure: 6: Heat exchanger limitations and the effects of superheating. (source: CEMET) Figure: 6: Heat exchanger limitations and the effects of superheating. (source: CEMET)

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as

as a a saturated saturated vapour. vapour. Increasing Increasing the the pressure pressure will will gas) leaving the gas) leaving the compressor will reduce thecompressor will reduce the

increase

increase the the temperature temperature and and cause cause superheat. superheat. required required condenser condenser capacity, capacity, and and provide provide a a high-

high-Friction,

Friction, system system inefficiency inefficiency and and the the work work added, added, grade heat grade heat source source for for other other process process use. use. A A typicaltypical

raise

raise the the entropy entropy and and superheat superheat above above that that application application would would be be the the preheating preheating of of boiler boiler make-

make-occurring in the theoretical cycle. Sup

occurring in the theoretical cycle. Superheat, erheat, up up or or process process water. water. The The total total amount amount of of heatheat

caused by the compression pro

caused by the compression process, does not cess, does not available available as as superheat superheat can can be be difficult difficult to to predict, predict, asas

improve

improve cycle cycle efficiency, efficiency, but but results results in in larger larger the the superheat superheat fluctuates fluctuates with with changes changes in in loadload

condensing

condensing equipment equipment and and large large compressor compressor conditions. conditions. If If a a use use can can be be found found for for low-grade low-grade heat,heat,

d

diisscchhaarrgge e ppiippiinngg.. tthhe e ttoottaal l ccoonnddeennssiinng g llooaad d ccaan n bbe e rreeccllaaiimmeedd. . TThhiiss

can result in

can result in substantial energy savings.substantial energy savings.

Desuperheating

Desuperheating is the process of removing excessis the process of removing excess

heat from superheated refrigerant vapour, and heat from superheated refrigerant vapour, and when accomplished by means

when accomplished by means external to the cycle,external to the cycle,

can be beneficial to system performance. can be beneficial to system performance. Desuperheating the suction gas is often

Desuperheating the suction gas is often impracticalimpractical

because of the low temperatures (less than 10 ºC) because of the low temperatures (less than 10 ºC) and the small amount of available energy. Some

and the small amount of available energy. Some Liquid subcooling occurs when a liquid refrigerant isLiquid subcooling occurs when a liquid refrigerant is

superheat

superheat is is required required to to prevent prevent slugs slugs of of liquid liquid cooled at constant pressure to below thecooled at constant pressure to below the

refrigerant

refrigerant from from reaching reaching the the compressor compressor and and condensation condensation temperature temperature (Figure (Figure 7). 7). WhenWhen

causing

causing serious serious damage. damage. At At design design conditions, conditions, subcooling subcooling occurs occurs by by a a heat heat transfer transfer methodmethod

superheat can account for 20 per cent of the

superheat can account for 20 per cent of the heatheat externalexternal to the refrigeration cycle, the refrigeratingto the refrigeration cycle, the refrigerating

rejected

rejected at at the the condensers, condensers, and and often often raises raises effect effect of of the the system system is is increased increased because because thethe

ccoonnddeennssiinng g tteemmppeerraattuurrees s aabboovve e 4455ººCC.. eenntthhaallppy oy of tf thhe se suubbccoooolleed ld liiqquuiid id is ls leesss ts thhaan tn thhee

enthalpy of the saturated

enthalpy of the saturated liquid. Subcooling of theliquid. Subcooling of the

Desuperheating

Desuperheating the the high-pressure high-pressure refrigerant refrigerant (hot (hot liquid liquid upstream upstream of of the the throttling throttling device device also also reducesreduces

2.3.2.3 FLASH GAS AND

SUBCOOLING

Figure 7: Effect of Subcooling (source: CEMET) Figure 7: Effect of Subcooling (source: CEMET)

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flashing

flashing in in the the liquid liquid piping. piping. The The work work input input is is cent for cent for an 8 an 8 cylinder unit. cylinder unit. For centrifugFor centrifugalal

reduced,

reduced, and and the the refrigeration refrigeration effect effect is is increased increased equipment, equipment, the the bypass bypass varies varies with with the the load load andand

b

beeccaauusse e ((h h h ) 11 h ) iis 44 s lleesss s tthhaan n ((h h h '11 h '))..44 iimmppeelllleer r cchhaarraacctteerriissttiiccss..

Subcooling refrigerant R-22 by 13ºC increases the Subcooling refrigerant R-22 by 13ºC increases the refrigeration effect by about 11 per cent. If refrigeration effect by about 11 per cent. If subcooling is obtained from

subcooling is obtained from outsideoutsidethe cycle, eachthe cycle, each

degree increase in subcooling will improve system

degree increase in subcooling will improve system _{When a refrigeration system operates with the}_{When a refrigeration system operates with the}

capacity by approximately one per cent. Subcooling

capacity by approximately one per cent. Subcooling _{evaporator temperature close to 0ºC, or less,}_{evaporator temperature close to 0ºC, or less,}

from

from withinwithin the cycle may not be as effectivethe cycle may not be as effective _{frosting of the evaporator coil is inevitable.}_{frosting of the evaporator coil is inevitable.}

because of offsetting effects in other parts of the

because of offsetting effects in other parts of the _{Examples of this would be the frosting of heat}_{Examples of this would be the frosting of heat}

cycle.

cycle. _{pump evaporator coils during winter operation, or}_{pump evaporator coils during winter operation, or}

freezer evaporators. Ice buildup on the

freezer evaporators. Ice buildup on the coils lowerscoils lowers

Subcooling capacity can be increased by providing

Subcooling capacity can be increased by providing _{the heat transfer rate, effectively reducing the}_{the heat transfer rate, effectively reducing the}

additional cooling circuits in the condenser or by

additional cooling circuits in the condenser or by _{refrigeration effect. The suction temperature will}_{refrigeration effect. The suction temperature will}

immersing the liquid receiver in a cooling tower

immersing the liquid receiver in a cooling tower _{fall as the heat transfer rate}_{fall as the heat transfer rate falls, further increasing}_{falls, further increasing}

sump. Most systems provide 5 to 7ºC subcooling at

sump. Most systems provide 5 to 7ºC subcooling at _{the rate of ice buildup. For systems operating under}_{the rate of ice buildup. For systems operating under}

the condenser to improve system efficiency.

the condenser to improve system efficiency. _{these conditions defrosting accessories are}_{these conditions defrosting accessories are}

available from the equipment manufacturer. available from the equipment manufacturer.

Defrost is performed by reversing the refrigerant Defrost is performed by reversing the refrigerant flow, so that the system operates in an flow, so that the system operates in an air-Hot gas bypass

Hot gas bypass is a method of placing an artificialis a method of placing an artificial _{conditioning mode, using the evaporator as the}_{conditioning mode, using the evaporator as the}

heat load on the refrigeration system to produce

heat load on the refrigeration system to produce _{condenser to reject heat through t}_{condenser to reject heat through the frosted coils.}_{he frosted coils.}

stable suction pressures and temperatures, when

stable suction pressures and temperatures, when _{In a heat pump system used for}_{In a heat pump system used for heating, a back-up}_{heating, a back-up}

the refrigeration load is very low. The heat load is

the refrigeration load is very low. The heat load is _{heating system is required to prevent chilling the}_{heating system is required to prevent chilling the}

produced by bypassing hot gas from the

produced by bypassing hot gas from the _{space during the defrost mode. Defrosting is a}_{space during the defrost mode. Defrosting is a}

compressor discharge to the evaporator inlet or

compressor discharge to the evaporator inlet or _{major consumer of energy. It is important that the}_{major consumer of energy. It is important that the}

the compressor suction. While permitting stable

the compressor suction. While permitting stable _{controls optimise the defrost cycle to avoid}_{controls optimise the defrost cycle to avoid}

compressor operation at low load, hot gas

compressor operation at low load, hot gas bypassbypass _{unnecessary defrosting while preventing unwanted}_{unnecessary defrosting while preventing unwanted}

wastes energy. Bypass is required to maintain

wastes energy. Bypass is required to maintain _{ice build-up.}_{ice build-up.}

evaporator temperature above freezing, and evaporator temperature above freezing, and prevent frosting of the coil, freezing of the chilled prevent frosting of the coil, freezing of the chilled water, and compressor cycling.

water, and compressor cycling. The total refrigeration load on

The total refrigeration load on a compressor witha compressor with

The heat pump is a

The heat pump is a separate class of compressionseparate class of compression

hot gas bypass will be equal to the actual (low) load hot gas bypass will be equal to the actual (low) load

refrigeration equipment whose main purpose is to refrigeration equipment whose main purpose is to plus the amount of hot gas bypass. Typically, the hot

plus the amount of hot gas bypass. Typically, the hot

transfer heat from a

transfer heat from a low temperature heat sourcelow temperature heat source

gas bypass on a reciprocating machine is 25 per cent gas bypass on a reciprocating machine is 25 per cent

to a higher temperature heat sink for heating, rather to a higher temperature heat sink for heating, rather of the nominal refrigeration capacity for a 4 cylinder

of the nominal refrigeration capacity for a 4 cylinder

than for cooling. The coefficient of

than for cooling. The coefficient of performance inperformance in

unit, 33 per cent for a 6

unit, 33 per cent for a 6 cylinder unit and 37.5 per 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

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tthhe e hheeaattiinng g ccoonnffiigguurraattiioon n iiss:: TThhe e sstteepps s iin n aan n aabbssoorrppttiioon n rreeffrriiggeerraattiioon n ccyycclle e aarree:: COP(Heat

COP(Heat Pump) Pump) == 11.. LLiiqquuiid d rreeffrriiggeerraannt t iis s vvaappoorriizzeed d iin n tthhee

evaporator absorbing heat from the evaporator absorbing heat from the =

= TTHH medium to be cooledmedium to be cooled

((TT - H H - T )T )LL 22.. TThhe se suuccttiioon en effffeecct nt neecceessssaarry ty to o ddrraaw w tthhee

vapour through the system is vapour through the system is ac-In a heat pump system where both heating and

In a heat pump system where both heating and

complished by bringing the refrigerant into complished by bringing the refrigerant into cooling are required, a special four-way valve is

cooling are required, a special four-way valve is _{contact with a solvent. The solvent's}_{contact with a solvent. The solvent's affinity}_affinity

used to reverse the functions of

used to reverse the functions of the evaporator andthe evaporator and _{for the refrigerant causes the refrigerant to}_{for the refrigerant causes the refrigerant to}

condenser. In this manner, the coil or exchanger is condenser. In this manner, the coil or exchanger is

be absorbed by the solution, reducing the be absorbed by the solution, reducing the used to supply heating or cooling as required.

used to supply heating or cooling as required.

pressure of the refrigerant vapour. The pressure of the refrigerant vapour. The Alternatively, the piping or ductwork system

Alternatively, the piping or ductwork system

absorption process

absorption process releasesreleases heat whichheat which

external to the heat pump

external to the heat pump can be provided withcan be provided with

must be removed from this portion of the must be removed from this portion of the valves or dampers to reverse the primary air or fluid

valves or dampers to reverse the primary air or fluid

cycle. The solution of refrigerant and cycle. The solution of refrigerant and flows, without the reversing valve. The heat pump

flows, without the reversing valve. The heat pump _solv_solvent_{ent (wea}_(weak_{k liquo}_liquor)_{r) is}_{is pu}_{pu mp}_{mp ed}_{ed fr}_{fr om}_om

cycle is identical to a standard refrigeration cycle on

cycle is identical to a standard refrigeration cycle on _{the absorber at low pressure, to the}_{the absorber at low pressure, to the}

a T-s diagram (Figure 2). a T-s diagram (Figure 2).

generator at a high

generator at a high pressure.pressure.

3

3.. HHeeaat t iis s aaddddeed d tto o tthhe e wweeaak k lliiqquuoor r to to ddrriivvee

the refrigerant out of solution. A heat the refrigerant out of solution. A heat exchanger is located between the exchanger is located between the absorber and generator. Heat is removed from absorber and generator. Heat is removed from the strong liquor (solution with high solvent the strong liquor (solution with high solvent The absorption refrigeration cycle is similar to

The absorption refrigeration cycle is similar to thethe _{and low refrigerant concentrations) leaving}_{and low refrigerant concentrations) leaving}

vapour compression cycle, however instead of

vapour compression cycle, however instead of _{the generator, and is added to the weak}_{the generator, and is added to the weak}

using a compressor, high pressures are obtained by

using a compressor, high pressures are obtained by _{liquor entering the generator, reducing the cycle}_{liquor entering the generator, reducing the cycle}

applying heat to a

applying heat to a refrigerant solution.refrigerant solution. _{heat input.}_{heat input.}

The system operates on the principle that variations

The system operates on the principle that variations ₄_4.. _FFu_urrtth_he_er_{r h}_he_{eaat a}_{t ad}_dd_de_ed_{d to}_{to tth}_he_{e w}_we_eaak_{k lliiq}_qu_uo_or_{r iin}_n

in refrigerant solubility can be

in refrigerant solubility can be obtained by changingobtained by changing _{the generator drives the refrigerant out of}_{the generator drives the refrigerant out of}

solution temperatures and pressures. Absorption

solution temperatures and pressures. Absorption _{solution providing a high pressure}_{solution providing a high pressure}

systems in industry often use ammonia as the

systems in industry often use ammonia as the _{refrigerant vapour. The hot solvent, still}_{refrigerant vapour. The hot solvent, still}

refrigerant in a water solvent, whereas in

refrigerant in a water solvent, whereas in _{containing some refrigerant (strong liquor),}_{containing some refrigerant (strong liquor),}

commercial and institutional applications water is

commercial and institutional applications water is _{returns to the absorber through the heat}_{returns to the absorber through the heat}

used as the refrigerant in a lithium bromide solvent.

used as the refrigerant in a lithium bromide solvent. _{exchanger where the solvent cycle}_{exchanger where the solvent cycle}

repeats. repeats. The basic components of an absorption system are

The basic components of an absorption system are ₅_5.. _V_Vaap_po_ou_{ur a}_{r at h}_{t hiiggh}_h--p_prre_essssu_{urre a}_{e an}_{nd t}_{d te}_em_mp_pe_erraattu_urre_e

the vapour absorber, solution transfer pumps, and a

the vapour absorber, solution transfer pumps, and a _{flows to the condenser where heat is}_{flows to the condenser where heat is}

vapour regenerator (solvent concentrator) in

vapour regenerator (solvent concentrator) in _{rejected through a coil or heat exchanger}_{rejected through a coil or heat exchanger}

addition to the evaporator and condenser.

addition to the evaporator and condenser. _{during the condensation process.}_{during the condensation process.}

2.4

2.4 ABSORPTION

ABSORPTION

CYCLE

22

Refrigeration effect plus work input Refrigeration effect plus work input

Net work input Net work input

2

i.e. 'Heat 'pumped' to the hot surface.

(20)

6

6.. TThhe pe prreessssuurre oe of tf thhe le liiqquuiid rd reeffrriiggeerraannt it iss

reduced by passing through a throttling reduced by passing through a throttling device before returning to the

device before returning to the evaporator evaporator

section. The complete cycle is shown in section. The complete cycle is shown in Figure 8.

Figure 8.

The generator may be equipped with a

The generator may be equipped with a rectifier rectifier for for

selective distillation of refrigerant from the solution. selective distillation of refrigerant from the solution. This feature is common in

This feature is common in large ammonia systems.large ammonia systems.

Performance of an absorption chiller is measured Performance of an absorption chiller is measured by the COP, the ratio of actual cooling or heating by the COP, the ratio of actual cooling or heating effect, to the energy used to obtain that effect.

effect, to the energy used to obtain that effect. TheThe

best ratios are less than one for cooling and 1.2 t

best ratios are less than one for cooling and 1.2 t oo

1.4 for a heat pump application. Compared to 1.4 for a heat pump application. Compared to compression cycles this is low, but if compression cycles this is low, but if high- temperature waste heat can be utilized to temperature waste heat can be utilized to regenerate the refrigerant, refrigeration can be regenerate the refrigerant, refrigeration can be obtained at reasonable costs.

obtained at reasonable costs.

System performance is affected by: System performance is affected by:

The flow diagram of a two-shell lithium bromide The flow diagram of a two-shell lithium bromide



 Heat source temperature.Heat source temperature. chiller is shown in Figure 9. Figure 10 shows anchiller is shown in Figure 9. Figure 10 shows an



 Temperature of medium being cooled.Temperature of medium being cooled. alternative configuration of an absorption machinealternative configuration of an absorption machine



 Temperature of the heat sink.Temperature of the heat sink. using only a single shell. Actual installations vary using only a single shell. Actual installations vary

Figure 8: Absorption Refrigeration Cycle. (source: CEMET) Figure 8: Absorption Refrigeration Cycle. (source: CEMET)

Figure 9: Diagram of a Two-Shell Lithium Figure 9: Diagram of a Two-Shell Lithium

Bromide Cycle Water Chiller. Bromide Cycle Water Chiller.

(source: CEMET) (source: CEMET)

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considerably in layout, number of components and

considerably in layout, number of components and Well water Well water , or any other clean water below l5ºC,, or any other clean water below l5ºC,

aacccceessssoorriieess, a, apppplliiccaatitioon an annd rd reeffrriiggeerraannt tt tyyppee.. ccaan bn be ue usseed fd foor cr coooolliinng og or pr prreeccoooolliinng vg veennttiillaattiioon an aiirr,,

or a process. or a process.

Steam jet

Steam jet refrigerationrefrigerationsystems use steam ejectors tosystems use steam ejectors to

reduce the pressure in a tank containing the return reduce the pressure in a tank containing the return water from a chilled water system. Flashing a water from a chilled water system. Flashing a portion of the water in the tank reduces the liquid portion of the water in the tank reduces the liquid temperature. The chilled water is then used directly temperature. The chilled water is then used directly

The cooling effect of

The cooling effect of an evaporator is proportionalan evaporator is proportional

or passed through an exchanger to cool another or passed through an exchanger to cool another

to the length of the line between points 1 and 2 in to the length of the line between points 1 and 2 in heat transfer fluid.

heat transfer fluid.

2.5 SPECIAL

2.6

2.6 V

V

ARI

A

TIO

NS ON

THE

REFRIGERATION

SIMPLE CARNOT

SYSTEMS

CIRCUIT

2.6.1

2.6.1 SUCTION/LIQUID

SUCTION/LIQUID

HEA

T

EXCHANGER

Figure 10: Single shell configuration. (source: CEMET) Figure 10: Single shell configuration. (source: CEMET)