麦格劳-教育或展示需thermo 8e chap lecturesi 8e.pptx

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麦格劳-教育或展示需thermo 8e chap lecturesi 8e.pptx

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CHAPTER11

REFRIGERATIONCYCLESCopyright?2015TheMcGraw-HillEducation.Permissionrequiredforreproductionordisplay.Thermodynamics:AnEngineeringApproach8thEditioninSIUnits

YunusA.?engel,MichaelA.BolesMcGraw-Hill,2015

2ObjectivesIntroducetheconceptsofrefrigeratorsandheatpumpsandthemeasureoftheirperformance.Analyzetheidealpressionrefrigerationcycle.Analyzetheactualpressionrefrigerationcycle.Reviewthefactorsinvolvedinselectingtherightrefrigerantforanapplication.Discusstheoperationofrefrigerationandheatpumpsystems.Evaluatetheperformanceofinnovativepressionrefrigerationsystems.Analyzegasrefrigerationsystems.Introducetheconceptsofabsorption-refrigerationsystems.

3REFRIGERATORSANDHEATPUMPSThetransferofheatfromalow-temperatureregiontoahigh-temperatureonerequiresspecialdevicescalledrefrigerators.Anotherdevicethattransfersheatfromalow-temperaturemediumtoahigh-temperatureoneistheheatpump.Refrigeratorsandheatpumpsareessentiallythesamedevices;theydifferintheirobjectivesonly.forfixedvaluesofQLandQH

4THEREVERSEDCARNOTCYCLEBothCOPsincreaseasthedifferencebetweenthetwotemperaturesdecreases,thatis,asTLrisesorTHfalls.ThereversedCarnotcycleisthemostefficientrefrig.cycleoperatingbetweenTLandTH.Itisnotasuitablemodelforrefrigerationcyclessinceprocesses2-3and4-1arenotpracticalbecauseProcess2-3involvesthecompressionofaliquid–vapormixture,whichrequiresacompressorthatwillhandletwophases,andprocess4-1involvestheexpansionofhigh-moisture-contentrefrigerantinaturbine.

5THEIDEALVAPOR-COMPRESSIONREFRIGERATIONCYCLEThepressionrefrigerationcycleistheidealmodelforrefrigerationsystems.UnlikethereversedCarnotcycle,therefrigerantisvaporizedcompletelybeforeitiscompressedandtheturbineisreplacedwithathrottlingdevice.SchematicandT-sdiagramfortheideal