Optimization of compressors displacement ratio in R134a/R23 cascade refrigeration system

A dimensionless parameter, the compressor displacement ratio, is proposed to investigate the performance of the R134a/R23 cascade system and optimize the match of compressors of the system. The parameter is defined as the ratio of the compressor inlet volume flow rates between the low temperature cycle and the high temperature cycle. A numerical model is established to analyze the effect of the displacement ratio on the COP and the intermediate temperature. The relationship between the optimal displacement ratio and the operating parameters is discussed under various conditions when the evaporating temperature, condensing temperature and cascade temperature difference are ranged within -75~-55 ℃, 20~50 ℃ and 5~13 ℃, respectively. The results show that the intermediate temperature increases with the displacement ratio, while the COP rises first and then drops slowly. The variation of the operating temperature exerts a stronger impact on the COP than the displacement ratio. The optimal displacement ratio increases with the condensing temperature and the evaporation temperature, and decreases when the cascade temperature difference rises. The correlations of the optimal displacement ratio are obtained by fitting data of the working condition parameters and the optimum compressor displacement ratio.