Thermodynamics

Chapter 11 | 621WARMenvironment7Q H6CondenserExpansionAvalveCompressorHeat exchangerEvaporatorTDecrease incompressorworkQ H68 Heat53 Condenser 2378A52Expansionvalve4BQ LEvaporatorQ LCompressor14B1Q LIncrease inrefrigerationcapacitysCOLD refrigeratedspaceFIGURE 11–10A two-stage cascade refrigeration system with the same refrigerant in both stages.In the cascade system shown in the figure, the refrigerants in both cyclesare assumed to be the same. This is not necessary, however, since there is nomixing taking place in the heat exchanger. Therefore, refrigerants with moredesirable characteristics can be used in each cycle. In this case, there wouldbe a separate saturation dome for each fluid, and the T-s diagram for one ofthe cycles would be different. Also, in actual cascade refrigeration systems,the two cycles would overlap somewhat since a temperature differencebetween the two fluids is needed for any heat transfer to take place.It is evident from the T-s diagram in Fig. 11–10 that the compressor workdecreases and the amount of heat absorbed from the refrigerated spaceincreases as a result of cascading. Therefore, cascading improves the COPof a refrigeration system. Some refrigeration systems use three or fourstages of cascading.EXAMPLE 11–3A Two-Stage Cascade Refrigeration CycleConsider a two-stage cascade refrigeration system operating between the pressurelimits of 0.8 and 0.14 MPa. Each stage operates on an ideal vaporcompressionrefrigeration cycle with refrigerant-134a as the working fluid. Heatrejection from the lower cycle to the upper cycle takes place in an adiabaticcounterflow heat exchanger where both streams enter at about 0.32 MPa.