Advanced two-stage cascade configurations for energy-efficient -80 °C refrigeration

Experimental study on a -86 °C cascade refrigeration unit with environmental-friendly refrigerants R290-R170

Ultralow-temperature refrigeration faces significant issues linked to the security of the cold chain for the production, storage, transportation, and distribution of COVID-19 vaccines. The use of environmentally friendly refrigerants in cascade refrigeration systems (CRS) to provide low-temperature range is motivated by the high demand for ultralow-temperature refrigeration units. In the current study, a CRS is built to generate a low temperature of -86 °C for the storage of COVID-19 vaccines. In the CRS, the natural refrigerant combination R290-R170 is used as high-temperature and low-temperature fluids. The pull-down performance of the -86 °C freezer is explored experimentally, and the stable operating performance is determined at two different dry bulb and wet bulb temperatures. Various status monitors are set up to analyze the CRS's operation features, and several temperature monitors are put in the freezer to analyze temperature variations. The power consumption of the CRS is examined and evaluated. Finally, several key findings are summarized. The current work is the first to involve experimental measurements on -86 °C temperature generated by a CRS, which can substantially enhance experiment data in ultralow-temperature refrigeration and contribute to a more in-depth understanding of the operation performance of a -86 °C ultralow-temperature freezer.

Operation performance of an ultralow-temperature cascade refrigeration freezer with environmentally friendly refrigerants R290-R170

In the present study, the operation performance of an ultralow-temperature cascade refrigeration freezer is experimentally researched. The natural refrigerants R290-R170 are adopted as high-temperature and low-temperature fluids. The experimental test is conducted in a type laboratory with a dry bulb temperature of 32.0 °C and a wet bulb temperature of 26.5 °C. Different state monitors are set to display the system operation performance, and several temperature monitors are arranged to study the pull-down performance and temperature variations in the freezer. Based on the established experimental rig, three freezing temperatures, including - 40 °C, - 80 °C, and - 86 °C, are measured and compared. The results show that it takes about 240 min for the freezer to be pulled down to - 80 °C. During the pull-down period, different monitors all experience rapid temperature drop, and the power consumption reduces from 1461.4 W to 997.5 W. Once the target temperature is achieved, the freezer comes into periodic start-stop operation. With the set temperature ranging from - 40 °C to - 86 °C, the inlet temperature of two compressors gradually decreases, while the discharge temperature has an increase trend. The cooling effect of the pre-cooled condenser reduces with the freezing temperature, while the long connection pipe has opposite variation profile. Moreover, it is observed that for different freezing temperatures, most of the space in the freezer can be cooled down to the target temperature. It is confirmed that the present ultralow-temperature freezer can be used for the storage and transportation of COVID-19 vaccines. However, it is also found that the cascade refrigeration system is not suitable for high freezing temperature, due to high power consumption and extensive start-stop switch of refrigeration system.