Thermodynamics

628 | ThermodynamicsThis and other refrigeration cycles used for the liquefaction of gases canalso be used for the solidification of gases.11–8 ■ GAS REFRIGERATION CYCLESAs explained in Sec. 11–2, the Carnot cycle (the standard of comparison forpower cycles) and the reversed Carnot cycle (the standard of comparisonfor refrigeration cycles) are identical, except that the reversed Carnot cycleoperates in the reverse direction. This suggests that the power cycles discussedin earlier chapters can be used as refrigeration cycles by simplyreversing them. In fact, the vapor-compression refrigeration cycle is essentiallya modified Rankine cycle operating in reverse. Another example is thereversed Stirling cycle, which is the cycle on which Stirling refrigeratorsoperate. In this section, we discuss the reversed Brayton cycle, better knownas the gas refrigeration cycle.Consider the gas refrigeration cycle shown in Fig. 11–16. The surroundingsare at T 0 , and the refrigerated space is to be maintained at T L . The gasis compressed during process 1-2. The high-pressure, high-temperature gasat state 2 is then cooled at constant pressure to T 0 by rejecting heat to thesurroundings. This is followed by an expansion process in a turbine, duringwhich the gas temperature drops to T 4 . (Can we achieve the cooling effectby using a throttling valve instead of a turbine?) Finally, the cool gasabsorbs heat from the refrigerated space until its temperature rises to T 1 .WARMenvironmentQ HTHeatexchanger3 2Q H24TurbineW net,in3Compressor14HeatexchangerQ L1Q LsCOLDrefrigerated spaceFIGURE 11–16Simple gas refrigeration cycle.