Thermodynamics

Chapter 11 | 631(a) The maximum and minimum temperatures in the cycle are determinedfrom the isentropic relations of ideal gases for the compression and expansionprocesses. From Table A–17E,T 1 460 R ⎯→ h 1 109.90 Btu/lbm and P r1 0.7913P 2P r2 P r1 (4)(0.7913) 3.165 ⎯→ e h 2 163.5 Btu/lbmT 2 683 R (or 223°F)P 1T 3 540 R ⎯→ h 3 129.06 Btu/lbm and P r3 1.3860P 4P 3P r4 P r3 (0.25)(1.386) 0.3465 ⎯→Therefore, the highest and the lowest temperatures in the cycle are 223 and97°F, respectively.(b) The COP of this ideal gas refrigeration cycle isCOP R q Lw net,inq Lw comp,in W turb,oute h 4 86.7 Btu/lbmT 4 363 R (or 97°F)whereThus,q L h 1 h 4 109.9 86.7 23.2 Btu>lbmW turb,out h 3 h 4 129.06 86.7 42.36 Btu>lbmW comp,in h 2 h 1 163.5 109.9 53.6 Btu>lbmCOP R 23.253.6 42.36 2.06(c) The rate of refrigeration isQ # refrig m # 1q L 2 10.1 lbm>s2 123.2 Btu>lbm2 2.32 Btu/sDiscussion It is worth noting that an ideal vapor-compression cycle workingunder similar conditions would have a COP greater than 3.11–9 ■ ABSORPTION REFRIGERATION SYSTEMSAnother form of refrigeration that becomes economically attractive whenthere is a source of inexpensive thermal energy at a temperature of 100 to200°C is absorption refrigeration. Some examples of inexpensive thermalenergy sources include geothermal energy, solar energy, and waste heatfrom cogeneration or process steam plants, and even natural gas when it isavailable at a relatively low price.As the name implies, absorption refrigeration systems involve the absorptionof a refrigerant by a transport medium. The most widely used absorptionrefrigeration system is the ammonia–water system, where ammonia(NH 3 ) serves as the refrigerant and water (H 2 O) as the transport medium.Other absorption refrigeration systems include water–lithium bromideand water–lithium chloride systems, where water serves as the refrigerant.The latter two systems are limited to applications such as air-conditioningwhere the minimum temperature is above the freezing point of water.