Irreversibility Analysis of a Vapor Compression Cascade Refrigeration Cycle

Abstract

Hydrocarbon based energy sources such as coal, oil and natural gas have been diminishing in an increasing speed. Instead of finding alternative energy sources, we have to use the available sources more effectively. By means of the irreversibility analysis, we can determine the factors or conditions that cause the inefficiencies in any energy system. In this study, irreversibility analysis of a compression cascade refrigeration cycle that consists of a high and low temperature cycles is presented. In the high temperature cycle, the refrigerants from different classes, namely R12 (CFC), R22 (HCFC), R134a (HFC) and R404a (Azeotropic) are selected as working fluids. In the low temperature cycle, R13 is only used as a working fluid. Irreversibility analysis of refrigerant pairs, namely R12-R13, R22-R13, R134a-R13, and R404a-R13 are carried out in a compression cascade refrigeration cycle by a computer code developed. The effects of evaporator temperature, condenser temperature, and the temperature difference between the saturation temperatures of the lower and higher temperature cycles in the heat exchanger (ΔT) and the polytropic efficiency on irreversibility of the system are investigated. The irreversibility of the cascade refrigeration cycle decreases as the evaporator temperature and polytropic efficiency increase for all of the refrigerant couples considered while the irreversibility increases with the increasing values of the condenser temperature and ΔT. In the whole ranges of evaporator temperature (−65°C / −45°C), condenser temperature (30–50°C), ΔT (2–16K) and polytropic efficiency (%50/%100), the refrigerant pair R12-R13 has the lowest values of irreversibilities while the pair R404a-R13 has the highest ones. At the lower condenser temperature (<30°C) and higher polytropic efficiencies (85%–95%), the refrigerant couples except for R404a-R13 have approximately the same values of irreversibility.