A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method

This paper focuses on cascading an ideal vapor compression cycle and determ ining the optimal intermediate temperatures based on the entropy generation minimization method. Only superheating and throttle losses of the cycle ar e considered since they are inherent to the ideal vapor compression refrige ration cycle. The second law equations have been developed in terms of spec ific heats and temperature ratios with the intent of reducing involved prop erty modeling. Also the entropy generation was expressed in terms of a sing le independent variable and minimized to develop an advanced rule for selec ting optimum intermediate temperatures. Results for a cascade system operat ing between reduced temperatures of 0.684 and 0.981 with R-134a as the work ing fluid are presented. The approximate method presented here predicted th e optimum intermediate reduced temperature for a two-stage system to be 0.8 8, a difference of 2% from the optimum. The method presented was a much bet ter predictor of the optimum temperature than the geometric mean method whi ch was 0.82, a difference of 5% from the optimum. The entropy generation di stribution of the optimum solution was investigated. For a two-stage system , the lower stage and higher stage entropy generation was 44% and 56%, resp ectively. In comparison to the single stage, the two-stage reduced losses b y 78%. (C) 2000 Elsevier Science Ltd and IIR. All rights reserved.