Theoretical performance study on R600/R170 auto-cascade refrigeration cycle with mechanical pre-cooling

A mechanical pre-cooling auto-cascade refrigeration (PACR) system was proposed, R600a single-stage vapor compression system was used to provide pre-cooling for R600/R170 auto-cascade refrigeration (ACR) system. Performance of PACR system under different operating conditions was discussed. The result showed that maintaining a constant outlet temperature of 30°C for the condenser and −45°C for the evaporator in a PACR system resulted in an optimal intermediate temperature that maximized coefficient of cooling performance (COP). At this time, the optimal intermediate temperature of R600/R170 was 0°C, the COP maximum value ( COP max ) was 0.693 and exergy efficiency ( η Ex ) was 20.9%. And as the evaporator outlet temperature decreases to −70°C, the optimal intermediate temperature decreases to −10°C. The cooling capacity ( Q e ) of the low-temperature grade circulating refrigerant in the PACR system using R600/R170 at different evaporation temperatures was increased by 86.3%–108% compared to that of R134a/R23. COP max was increased by 7.1% compared to that of R134a/R23 at the evaporator outlet temperature of −60°C.