Thermodynamics

Chapter 11 | 625T4h 4 = 274.48 kJ/kgh 5 = 95.4752h 2 = 255.93h 7 = 55.16 h 6 = 95.47 976 h 3 = 251.88 3h 9 = 255.108h 8 = 55.16h 1 = 239.161sFIGURE 11–13T-s diagram of the two-stagecompression refrigeration cycledescribed in Example 11–4.andw in w comp I,in w comp II,in 11 x 6 21h 2 h 1 2 112 1h 4 h 9 2The enthalpy at state 9 is determined from an energy balance on the mixingchamber,E # out E # in112h 9 x 6 h 3 11 x 6 2h 2h 9 10.20492 1251.882 11 0.204921255.932 255.10 kJ>kgAlso, s 9 0.9416 kJ/kg · K. Thus the enthalpy at state 4 (0.8 MPa, s 4 s 9 ) is h 4 274.48 kJ/kg. Substituting,w in 11 0.2049231255.93 239.162 kJ>kg4 1274.48 255.102 kJ>kg 32.71 kJ/kg(c) The coefficient of performance isCOP R q L 146.3 kJ>kgw in 32.71 kJ>kg 4.47Discussion This problem was worked out in Example 11–1 for a single-stagerefrigeration system (COP 3.97) and in Example 11–3 for a two-stage cascaderefrigeration system (COP 4.46). Notice that the COP of the refrigerationsystem increased considerably relative to the single-stage compressionbut did not change much relative to the two-stage cascade compression.Multipurpose Refrigeration Systemswith a Single CompressorSome applications require refrigeration at more than one temperature. Thiscould be accomplished by using a separate throttling valve and a separatecompressor for each evaporator operating at different temperatures. However,such a system is bulky and probably uneconomical. A more practical and