Multi-Parameter Optimization of a Low-Temperature Auto-Cascade Refrigerating Machine Thermodynamic Efficiency

The object of this study is a low-temperature refrigerating machine operating on the auto-cascade cycle and capable of producing cold at two temperature levels simultaneously in a wide range of temperatures down to -195oC. A multi-component zeotropic mixture of refrigerants is used as the working fluid. A hermetic lubricated compressor, one phase separator, a triple-section internal recuperative heat exchanger and two evaporators make the main components of the refrigerator. Sophisticated correlations between the operating parameters of the refrigerator were revealed when analyzing the basic principles of the machine operation. A comprehensive procedure of the auto-cascade cycle thermodynamic calculations considering peculiarities of the working fluid flow transformations through the loop was developed and reported. Building upon this procedure, an algorithm and computer program of full-scale numerical calculations to determine optimal values of such parameters as the working mixture component concentrations, operating temperatures and pressures considering the machine thermodynamic efficiency as a criterion function was implemented. To demonstrate the program operability and potentials the design calculations of the refrigerator operating at the temperature level of -150 oC were conducted and reported. The analysis of the results of calculations confirmed validity and particular relevance of the proposed approach implying the full-scale optimization of the system operating parameters.