制冷技术英文版Ch5090531.docx

制冷技术英文版Ch5090531.docx

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制冷技术英文版Ch5090531

Chapter5.VaporandGasRefrigerationCycles

5-1）MechanicalPoweredVaporCompressionRefrigerationCycle

5-2）HeatPoweredVaporCompressionRefrigerationCycle

—VaporAbsorptionRefrigeration

5-3）HeatPoweredvaporCompressionRefrigerationCycle

—VaporAdsorptionRefrigeration

5-4）HeatPoweredvaporCompressionRefrigerationCycle

—VaporJetRefrigeration

5-5）RefrigerationCyclebyGasCompressionandAdiabatic

Expansion

5-1）MechanicalPoweredVaporCompressionRefrigerationCycle

（1）Introductionofmechanicalpoweredvaporcompressionrefrigerationcycle

Fig.2-1,TheModelforAnalysisofRefrigerationCycle

Themechanicalvaporcompressionrefrigerationisthemostcommonrefrigerationcycle.Itsadvantages,incomparisonwithothertypesofrefrigerationsystems,includethecompactofthesystem;highcoefficientofperformance（COP）;beingreliable,safeandflexibleinoperation;relativelysimpleinmaintenance;andlowinitialcosts.

（2）Basicvaporcompressionrefrigerationcycle

举个例子

HeretaketheRefrigerantR134aasanexampletoshowhowtocalculatethecycle’scoolingcapacityandCOPbyassumingthattherefrigerantleavestheevaporatoratthetemperatureof-20°Canditiscondensedat40°C.

Forthecaseofevaporatingtemperature

andcondensingtemperature

thethermalpropertiesofR134acanbefoundfromthediagramortableoftherefrigerantR134aasbelow:

Evaporatingpressure

Condensingpressure

ThespecificenthalpiesofR134aatthesestatesare:

（Itisanisentropicprocessfrompoint1topoint2.）

Fig.5-1,Schematicandalogp-hdiagramsforthebasicvaporcompressioncycle

Theprocess1-2isareversible,adiabatic（isentropic）compression

（5-1）

Theprocess2-3isanisobaricheatrejectionprocess

（5-2）

Theprocess3-4isanirreversiblethrottlingprocess,

Theprocess4-1isanisobaricconstantpressureheatadmissionprocess

（5-3）

Thecoefficientofperformanceofthecyclecanbecalculatedas:

（5-4）

IfthemassflowrateoftherefrigerantR134athroughthiscycleism=0.1kg/s,thentherefrigerationcapacity,thecondensingloadandtheworkofcompressioncanbegottenas:

5-2）HeatOperatedVaporCompressionRefrigerationCycle

（1）

—VaporAbsorptionRefrigeration

（还是蒸汽压缩式制冷，降温方法一样（p.53），区别只在于压缩方式）

有热能可以利用的场合

Thereisabundantthermalenergyappearedindifferentformsintheworld,suchassolarthermal,geothermal,variouswastedheatsandbiomassenergyetc.Theseenergiescanbeusedtodriverefrigerationandair-conditioningsystems.

3种热驱动的蒸汽压缩式制冷

Therearethreekindsofvaporcompressionrefrigerationcyclesthatcanbedrivenbythermalenergy.Theyare:

1，theabsorptionrefrigerationcycle,2，theadsorptionrefrigerationcycleand3，vaporjetrefrigerationcycle.Thesecyclessharesimilartechnologiesthatareusedinthevaporcompressionrefrigerationcycle,i.e.,throttlingevaporatingandcondensing.buttheyaredrivenbythermalenergy.Theserefrigerationcycleswilldiscussedinthischapter.

（1）Principlesofabsorptionrefrigeration

Fig.5-2,essentialcomponentsofthevaporabsorptioncycle

Themechanicalcompressorisreplacedbyathermalcompressorwhichconsistsofabsorber,solutionpump,generator（orboiler）andliquidvalve.Thisgroupofcomponents‘sucks’vaporfromtheevaporator,anddelivershighpressurevaportothecondenser,justasthemechanicalcompressordoesbutthevaporisactuallyabsorbedbyaliquidabsorbent.Aquaammoniaandaqualithiumbromidesolutionsarecommonlyusedinvaporabsorptionrefrigerationsystems.

氨水吸收的蒸汽压缩式制冷系统

Theabsorptionofammoniabywaterisanexothermicprocess.Thestrongsolutionformedintheabsorberispumpedtothegeneratorathigherpressure.Inthegenerator,thestrongsolutionisboiledbyheating,andthevaporgivenoffisrectifiedtonearlypureammoniaanddeliveredtothecondenser.Thereisaheatexchangerinterposedbetweenthegeneratorandabsorber.Thehotweaksolutionfromthegeneratortransferstheheattothestrongsolutionfromtheabsorber.Tomaintainthedifferenceinpressuresbetweenthegeneratorandabsorber,avalveisinstalledinthepipe[4,5]

TherefrigerantsandabsorbentinH2O-LiBrsystemandNH3–H2Osystem

Absorptioncycle

refrigerant

Absorbent

H2O–LiBrsystem

H2O

LiBrsolution

NH3–H2Osystem

NH3

H2O

溴化锂水吸收的蒸汽压缩式制冷系统

Inlithiumbromide-waterabsorptionrefrigerationsystems,wateristherefrigerantandlithiumbromideistheabsorbent.Thisexplainsthatthelithiumbromideabsorptionsystemisstrictlylimitedtoevaporationtemperaturesabove0ºC;andtheammoniaabsorptionsystemismainlyusedforlowtemperaturesbelow0ºC.Waterasasolventinammoniaabsorptionsystemispresentinthevaporsorectificationisrequiredtoremoveit,whereasLiBr（ahygroscopicsalt）isalmostnon-volatileattheoperatingconditionssorectificationisnotnecessary

（2）Compositionofmixtures

Calculationofabsorptionrefrigeratorsrequiressomeknowledgeofthethermodynamicsofsolutions（溶液热力学）andofhowtheirpropertiesdependonthecomposition.

Compositionofamixtureisexpressedasthemassfraction

ofoneofthecomponents.Forexample,inH2O–LiBrsolutionitcontainsmass

ofLiBrand

ofH2O,themassfractionofLiBrisdefinedas:

（5-5）

（3）VaporpressureofLiBr-watersolution

Thevaporpressureofaqualithiumbromidesolutionisdeterminedbyitstemperatureandmassfraction.TheirrelationshipisshowninFig.5-3.Theabscissaistemperatureinlinearscale;theordinateontheleft-handisvaporpressureinlogarithmicscale;theordinateontheright-handistemperatureinlinearscale,showsthesaturationtemperatureofpurewaterwhichhasthesamevaporpressureasaBrLisolutionatthetemperaturegivenbytheabscissa.Thelineofpurewaterisalsoshowninthefigure,whichiscorrespondingtoasolutionof

allthepointsonthelineofpurewaterhavethesamevaluesoftemperaturebothontheabscissaandontheordinateontheright-hand.

Fig.5-3,thevaporpressureofsolutionsofLiBrinwater[6]

InFig.5-3,theaccuratevalueofvaporpressurecanbefoundfromTable5-2fromthesaturatedtemperatureofpurewaterontheordinateontheright-hand.

Forexample,ifasolutionofLiBr-H2Omassfraction

=0.578isat40℃,fromtheleft-handscalethevaporpressuremaybeestimatedbetween8mbarand9mbar.Fromtheright-handscale,thetemperaturereadingofpurewaterforthesamevaporpressureisverycloseto5℃.FromthetableofpurewaterasshowninTab.5-1.,thecorrespondingvaporpressurefor5°Cis8.72mbar.

Tab.5-1,thesaturatedvaporpressuretableofpurewater[7]

Temperature

Saturated

Pressure

Temperature

Saturated

Pressure

Temperature

Saturated

Pressure

Temperature

Saturated

Pressure

0.01

6.106

1

6.571

11

13.127

21

24.877

31

44.959

2

7.060

12

14.026

22

26.448

32

47.585

3

7.580

13

14.978

23

28.104

33

50.343

4

8.135

14

15.987

24

29.851

34

53.239

5

8.725

15

17.055

25

31.692

35

56.278

6

9.353

16

18.184

26

33.631

36

59.466

7

10.020

17

19.380

27

35.673

37

62.810

8

10.728

18

20.643

28

37.822

38

66.315

9

11.481

19

21.978

29

40.083

39

69.987

10

12.280

20

23.388

30

42.460

40

73.835

50

123.499

（4）BasicLithiumbromide-waterabsorptionrefrigerationsystem

ThediagramshowninFig.5-4isabasiclithiumbromidevaporabsorptionrefrigerationsystem.AbasicH2O–LiBrabsorptionrefrigerationsystemconsistsof8maincomponents.Apartfromtheevaporator,thecondenserandtheexpansionvalvewhicharefoundinamechanicalpoweredvaporcompressionrefrigerator,otherfivecomponents,namely,apump,andabsorber,agenerator,aheatexchangerandavalvefulfillthefunctionof“thermalcompressor”:

Fig.5-4,aschemeofabasicabsorptionrefrigerationsystem

Somemanufacturersconstructtheabsorptionrefrigerationsystemsbyplacingthefourmajorcomponents（generator,absorber,condenserandevaporator）inasingleshelldividedintohigherandlowerpressureregionsasshowninFig.5-5.

Fig.5-5,asingle-effectlithiumbromide-waterabsorptionrefrigerationsystem[8]

（5）Analysisforabasicabsorptionrefrigerationsystem

a）Circulationfactor循环倍率

Animportantquantityinthecalculationofanabsorptionsystemisthemassflowrateofthestrongsolutionwhichisneededtoabsorbunitmassflowrateofvaporfromtheevaporator.Thisquantityiscalledthecirculationfactorλ.

Hence:

（5-10）

Forexample,if

and

thecirculationfactorλis7.05,

b）Enthalpyofliquidandvapor

Thefigureisbasedontheenthalpiesofliquidwaterandsolidanhydrouslithiumbromideeachbeingzeroat0℃.

Fig.5-6,specificenthalpyofsolutionsofLiBrinwater

c）Steady-flowanalysis

Assumealithiumbromidesystemoperatingatthefollowingconditions:

Evaporation:

5℃（pe=8.725mbar）,Condensation:

50℃（pc=123.45mbar）,Generator:

110℃,Absorption:

40℃.

Assumingequilibriumstatesleavingthegeneratorandtheevaporator,nopressuredrops,andcompleteheatexchange,i.e.thestrongsolutionleavestheexchangerat40℃.

Themassfractionsofthestrongandweaksolutionsaredeterminedas:

Thecirculationfactorλ=7.05byEq.5-10.

FortherefrigerationcycleshowninFig.5-4,intheprocessesfrompoint1topoint4,theworkingsubstanceispurewateroritsvapor,thereforethedataofenthalpiesofsuperheatedvaporandsaturatedvaporandliquidcanbefoundfromthesteamtables,

;

;

Intheprocessesfrompoint5topoint10,theworkingsubstanceisLiBr-H2Osolution,thereforetheenthalpiescanbefoundfromFig.5-6,