High phase-separation efficiency auto-cascade system working with a blend of carbon dioxide for low-temperature isothermal refrigeration

Abstract

A high phase-separation efficiency auto-cascade refrigeration (ACR) system working with a zeotropic mixture of CO2 is proposed in this study. The selection of a high boiling-point component is analysed. A novel zeotropic mixture consisting of a high CO2 (R744) mass fraction together with isobutane (R600a) as a carrier fluid is proposed. An experimental setup was designed, built, and verified to study the ability of the ACR to provide isothermal refrigeration at temperatures lower than the R744 triple-point temperature. The setup employs a recuperative heat exchanger (RHX) for the maximum phase-separation efficiency. The effect of the refrigerant mass charge on the ACR operation was examined. The temperature distribution across the internal heat exchangers (IHXs) was studied as well. The results revealed that the system was very sensitive to the refrigerant mass charge. With a low mass charge, the ACR did not operate correctly owing to the pinch points occurring between the hot and cold ends of the IHXs. High mass charge resulted in increased power consumption and a decreased coefficient of performance (COP). Moreover, tests were conducted to examine the ability of the ACR system, working with a binary mixture of R744/R600a, to provide isothermal refrigeration with different evaporator heat loads. The ACR system was compared in terms of the evaporation pressure and mass flow to a cascade system working with trifluoromethane (R23) in the low stage. The ACR system can be a promising alternative to cascade systems working with R23. The mixture of R744/R600a presented a significant decrease in the global warming potential (GWP) compared with R23. Near-isothermal evaporation was possible with this mixture over a range of vapour quality values. The application range of CO2 was extended to temperatures lower than the CO2 triple-point temperature, using isobutane as a solvent for dry ice. The enthalpy–mass fraction diagrams incorporating a solid fraction area were constructed for the graphical representation of the ACR system.