Selection of suitable natural refrigerants pairs for cascade refrigeration system

摘要

This study presents thermodynamic analyses and optimization of a two-stage cascade system employing a choice of eight natural refrigerants leading to the most suitable refrigerants pair for wide ranges of operating temperatures; the optimal pair was based on best system performance (cooling coefficient of performance) as well as best volumetric cooling capacity (compressor compactness). Out of the 56 possible combinations of high temperature and low temperature fluids with eight natural refrigerants - namely, ammonia, carbon dioxide, propane, propylene, n-butane, isobutane, ethane, and ethylene - the primary selection has been done based on the criteria of normal boiling point and critical point of both low temperature and high temperature fluids following the guideline of the operating temperatures of a refrigerant to be within its critical point and normal boiling point. All the selections have been made based on an optimum intermediate temperature leading to maximum cooling coefficient of performance for an individual pair. Effect of compressor isentropic efficiency on optimization and selection has been studied as well. Two selection charts along with tables have been developed, one for the highest coefficient of performance and another for the highest volumetric capacity; these design tools are expected to offer useful guidelines to select the best refrigerant pair for specific ranges of condenser and evaporator temperatures.