制冷技术英文版Ch5090531.docx
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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,
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