Thermodynamics

630 | ThermodynamicsCOLDrefrigerated space5 4WARMenvironmentTurbineHeatexchanger6RegeneratorQ3Heatexchanger2Compressor1T45Q H36Q L21W net,insFIGURE 11–19Gas refrigeration cycle with regeneration.T, °FThe regenerative gas cycle is shown in Fig. 11–19. Regenerative coolingis achieved by inserting a counter-flow heat exchanger into the cycle. Withoutregeneration, the lowest turbine inlet temperature is T 0 , the temperatureof the surroundings or any other cooling medium. With regeneration, thehigh-pressure gas is further cooled to T 4 before expanding in the turbine.Lowering the turbine inlet temperature automatically lowers the turbine exittemperature, which is the minimum temperature in the cycle. Extremely lowtemperatures can be achieved by repeating this process.T max800T min·Q H3421·Q LEXAMPLE 11–5The Simple Ideal Gas Refrigeration CycleAn ideal gas refrigeration cycle using air as the working medium is to maintaina refrigerated space at 0°F while rejecting heat to the surrounding medium at80°F. The pressure ratio of the compressor is 4. Determine (a) the maximumand minimum temperatures in the cycle, (b) the coefficient of performance,and (c) the rate of refrigeration for a mass flow rate of 0.1 lbm/s.FIGURE 11–20T-s diagram of the ideal-gasrefrigeration cycle described inExample 11–5.sSolution An ideal gas refrigeration cycle using air as the working fluid isconsidered. The maximum and minimum temperatures, the COP, and therate of refrigeration are to be determined.Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas withvariable specific heats. 3 Kinetic and potential energy changes are negligible.Analysis The T-s diagram of the gas refrigeration cycle is shown inFig. 11–20. We note that this is an ideal gas-compression refrigerationcycle, and thus, both the compressor and the turbine are isentropic, and theair is cooled to the environment temperature before it enters the turbine.