Thermodynamics

Chapter 11 | 611WARMenvironmentT3Q HCondenser2Saturatedliquid2Q HExpansionvalve CompressorW in3W in41EvaporatorQ L4'4Q LSaturated vapor1COLD refrigeratedspacesFIGURE 11–3Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle.this isentropic compression process to well above the temperature of the surroundingmedium. The refrigerant then enters the condenser as superheatedvapor at state 2 and leaves as saturated liquid at state 3 as a result of heatrejection to the surroundings. The temperature of the refrigerant at this stateis still above the temperature of the surroundings.The saturated liquid refrigerant at state 3 is throttled to the evaporatorpressure by passing it through an expansion valve or capillary tube. Thetemperature of the refrigerant drops below the temperature of the refrigeratedspace during this process. The refrigerant enters the evaporator at state4 as a low-quality saturated mixture, and it completely evaporates byabsorbing heat from the refrigerated space. The refrigerant leaves the evaporatoras saturated vapor and reenters the compressor, completing the cycle.In a household refrigerator, the tubes in the freezer compartment whereheat is absorbed by the refrigerant serves as the evaporator. The coils behindthe refrigerator, where heat is dissipated to the kitchen air, serve as the condenser(Fig. 11–4).Remember that the area under the process curve on a T-s diagram representsthe heat transfer for internally reversible processes. The area under theprocess curve 4-1 represents the heat absorbed by the refrigerant in the evaporator,and the area under the process curve 2-3 represents the heat rejected inthe condenser. A rule of thumb is that the COP improves by 2 to 4 percent foreach °C the evaporating temperature is raised or the condensing temperatureis lowered.Freezercompartment–18°C3°CQ LKitchen air25°CEvaporatorcoilsCapillarytubeQ HCondensercoilsCompressorFIGURE 11–4An ordinary household refrigerator.