Study of the migration behavior of mixed refrigerants and energy efficiency during actuators action process in auto-cascade refrigeration systems

The auto-cascade refrigeration cycle (ARC) using mixed refrigerants is widely used to produce temperatures below -40 degrees C. Due to the vapor-liquid separation at the condenser outlet, the ARC system contains three distinct streams: the total stream, the vapor stream and the liquid stream. The three streams exhibit diverse variations in mass flow rates and composition circulation concentrations. The composition distribution and migration behavior in any stream undergoes complex variations with the actuation operation. Existing research only obtained the composition concentration shift relationship or inferred composition migration behavior from the pressure and temperature variations. The underlying causes of changes in thermodynamic parameters have not been revealed. This article experimentally investigates composition migration behavior during the actuator regulation in the ARC system using R600a/R170. From the perspective of composition migration behavior, this study elucidates the variations of thermodynamic parameters during actuators' operation. The results show that the compressor speed variations lead to an increase in the circulating flow rate. The composition circulating concentration exhibits non-monotonic variation. The increased fan speed accelerates the condensation of mixed refrigerants. The circulating concentration of R170 decreases from 50.4% to 48.38%. The expansion valve before the evaporator regulates the compositions and flow rate, while the other valve primarily manages the total flow rate. The optimal energy efficiency corresponds to a compressor speed of 3000r, fan speed of 1980r and valve openings of 30% and 50%. Studying composition migration behavior provides actuators with optimization regulation strategies for ARC systems.