Thermodynamics

All the processes described are internally reversible, and the cycle executedis the ideal gas refrigeration cycle. In actual gas refrigeration cycles,the compression and expansion processes deviate from the isentropic ones,and T 3 is higher than T 0 unless the heat exchanger is infinitely large.On a T-s diagram, the area under process curve 4-1 represents the heatremoved from the refrigerated space, and the enclosed area 1-2-3-4-1 representsthe net work input. The ratio of these areas is the COP for the cycle,which may be expressed aswhereCOP R q Lw net,inq L h 1 h 4w turb,out h 3 h 4w comp,in h 2 h 1The gas refrigeration cycle deviates from the reversed Carnot cyclebecause the heat transfer processes are not isothermal. In fact, the gas temperaturevaries considerably during heat transfer processes. Consequently, thegas refrigeration cycles have lower COPs relative to the vapor-compressionrefrigeration cycles or the reversed Carnot cycle. This is also evident fromthe T-s diagram in Fig. 11–17. The reversed Carnot cycle consumes a fractionof the net work (rectangular area 1A3B) but produces a greater amountof refrigeration (triangular area under B1).Despite their relatively low COPs, the gas refrigeration cycles have twodesirable characteristics: They involve simple, lighter components, whichmake them suitable for aircraft cooling, and they can incorporate regeneration,which makes them suitable for liquefaction of gases and cryogenicapplications. An open-cycle aircraft cooling system is shown in Fig. 11–18.Atmospheric air is compressed by a compressor, cooled by the surroundingair, and expanded in a turbine. The cool air leaving the turbine is thendirectly routed to the cabin.Chapter 11 | 629T2q LGas(11–11)w comp,in w refrigerationturb,outcycle3B4A1ReversedCarnotcycleFIGURE 11–17A reserved Carnot cycle producesmore refrigeration (area under B1)with less work input (area 1A3B).sQHeatexchanger32W net,inTurbineCompressor4Cool airout1Warm airinFIGURE 11–18An open-cycle aircraft cooling system.