Experimental assessment of phase separator pressure control in a R1150/R600a autocascade system for ultra-low temperatures

This work presents an experimental evaluation of an autocascade (AC) refrigeration system operating at ultra-low temperatures (-80°C) with a natural refrigerant mixture of R1150/R600a. A micrometric valve was introduced at the condenser outlet to create an intermediate pressure level and modifying the fractionation conditions at the phase separator. The aim was to enrich the vapor stream in ethylene, enhancing cooling capacity and coefficient of performance (COP). The conducted experimental campaign included optimization of capillary tube lengths and micrometric valve opening degrees. With the optimal conditions, results show that the introduction of the micrometric valve significantly altered circulating compositions, increasing the ethylene proportion in the vapor line by 13%. The cooling capacity rose to 442.2 W, representing a 49.6% increase compared to the baseline cycle. Furthermore, the COP increased by 25.9%, reaching 0.425, an efficiency comparable to that of two-stage cascade systems. These findings provide the first experimental evidence of performance gains from introducing an intermediate pressure level in AC cycles and demonstrate the feasibility of this modification for improving the efficiency of ultra-low temperature applications while maintaining the simplicity of single-compressor operation.