Effect of Inter-stage Pressure on the Performance of a Two Stage Refrigeration Cycle Using Inter Cooler

Effect of inter-stage pressure on the performance of a two stage refrigeration cycle using inter cooler is studied based on thermodynamic modelling. Six common refrigerants are selected for the analyses. These are R134a, R22 and R143a as synthetic refrigerants and propane, carbon dioxide and nitrous oxide as natural refrigerants. The designed input range is -50oC to -30oC for evaporator temperature and 40oC to 60oC for heat rejection temperature. Three classical relations for estimating inter-stage pressure in a two stage refrigeration system are selected and the performance is evaluated against optimized inter-stage pressure corresponding to the best COP. Results show that for trans-critical cycles, the deviation of optimized inter-stage pressure with the classical ones is considerable, while for sub-critical cycles, COP is very less sensitive to inter-stage pressure. Further, comprehensive analysis of CO2 based trans-critical refrigeration system is presented for a wider range covering -50oC to 10oC evaporation temperature and 35oC to 60oC gas cooler outlet temperature. A correlation for estimating optimum inter-stage and gas cooler pressure for the same is also formulated. With gradual increase in focus towards adoption of natural refrigerants like CO2, the effect of inter stage pressure is expected to be an important consideration in warmer climatic condition.