Thermodynamics

Chapter 11 | 623 10.05 kg>s231270.92 251.882 kJ>kg4 10.039 kg>s231255.93 239.162 kJ>kg4 1.61 kW(c) The COP of a refrigeration system is the ratio of the refrigeration rate tothe net power input:COP R Q# LW # 7.18 kWnet,in1.61 kW 4.46Discussion This problem was worked out in Example 11–1 for a single-stagerefrigeration system. Notice that the COP of the refrigeration systemincreases from 3.97 to 4.46 as a result of cascading. The COP of the systemcan be increased even more by increasing the number of cascade stages.Multistage Compression Refrigeration SystemsWhen the fluid used throughout the cascade refrigeration system is the same,the heat exchanger between the stages can be replaced by a mixing chamber(called a flash chamber) since it has better heat transfer characteristics. Suchsystems are called multistage compression refrigeration systems. A twostagecompression refrigeration system is shown in Fig. 11–12.WARMenvironmentExpansionvalveExpansionvalve6Flashchamber78Q H5Condenser4T4High-pressurecompressor5 293796 32Low-pressure1compressor81EvaporatorQ LsCOLDrefrigerated spaceFIGURE 11–12A two-stage compression refrigeration system with a flash chamber.