An ultralow-temperature cascade refrigeration unit with natural refrigerant pair R290-R170: Performance evaluation under different ambient and freezing temperatures

The global demand for ultralow-temperature (ULT) refrigeration units gets greatly promoted, for storage, transportation, and distribution of COVID-19 vaccines. In this work, a ULT freezer is developed with a cascade refrigeration system (CRS) utilizing environmentally friendly refrigerants R290 and R170. The performance of the ULT freezer is experimentally evaluated under different ambient temperatures Tamb and freezing temperatures Tfreezing. The result shows that once the refrigeration system starts, the freezer enters a pull-down period and then reaches stable or periodic on-off operation. The monitored temperatures present drastic variations in the low-temperature cycle (LTC) and show a relatively stable start-up process in the high-temperature cycle (HTC). The monitored temperature rises when Tamb increases from 16 degrees C to 32 degrees C. The increasing Tamb brings about a larger temperature drop crossing the pre-cooled condenser, cascade heat exchanger, and hightemperature condenser, and a smaller temperature reduction through the anti-condensation. As Tfreezing decreases from -60 degrees C to -86 degrees C, the suction/discharge gas temperatures increase in the low-temperature compressor, while the other monitored temperatures reduce. The largest temperature non-uniformity in the freezer is 9.19 degrees C, and the lowest wall temperature can reach -90.52 degrees C. With Tamb ranging from 16 degrees C to 32 degrees C, the power consumption of the freezer increases from 896 W to 912 W. When Tfreezing varies from -60 degrees C to -86 degrees C, the CRS's consumed power reduces from 804 W to 904 W. The present freezer can easily obtain low temperatures e.g., -81 degrees C, and reach a lower temperature, such as -86 degrees C with proper improvements to reduce cold loss.