Thermodynamics

288 | ThermodynamicsSurrounding mediumsuch as the kitchen airQ H800 kPa30°CCONDENSER800 kPa60°CEXPANSIONVALVECOMPRESSORW net,in120 kPa–25°C120 kPa–20°CFIGURE 6–19Basic components of a refrigerationsystem and typical operatingconditions.EVAPORATORQ LRefrigerated spaceWarm environmentat T H > T LRQ LQ HCold refrigeratedspace at T LRequiredinputW net,inDesiredoutputFIGURE 6–20The objective of a refrigerator is toremove Q L from the cooled space.The refrigerant enters the compressor as a vapor and is compressed to thecondenser pressure. It leaves the compressor at a relatively high temperatureand cools down and condenses as it flows through the coils of the condenserby rejecting heat to the surrounding medium. It then enters a capillary tubewhere its pressure and temperature drop drastically due to the throttling effect.The low-temperature refrigerant then enters the evaporator, where it evaporatesby absorbing heat from the refrigerated space. The cycle is completed asthe refrigerant leaves the evaporator and reenters the compressor.In a household refrigerator, the freezer compartment where heat is absorbedby the refrigerant serves as the evaporator, and the coils usually behind therefrigerator where heat is dissipated to the kitchen air serve as the condenser.A refrigerator is shown schematically in Fig. 6–20. Here Q L is the magnitudeof the heat removed from the refrigerated space at temperature T L , Q His the magnitude of the heat rejected to the warm environment at temperatureT H , and W net,in is the net work input to the refrigerator. As discussedbefore, Q L and Q H represent magnitudes and thus are positive quantities.Coefficient of PerformanceThe efficiency of a refrigerator is expressed in terms of the coefficient ofperformance (COP), denoted by COP R . The objective of a refrigerator is toremove heat (Q L ) from the refrigerated space. To accomplish this objective,it requires a work input of W net,in . Then the COP of a refrigerator can beexpressed asDesired outputCOP R (6–7)Required input Q LW net,inThis relation can also be expressed in rate form by replacing Q L by Q . L andW net,in by Ẇ net,in .The conservation of energy principle for a cyclic device requires thatW net,in Q H Q L 1kJ2(6–8)