Experimental investigation on the pull-down performance of a -80°C ultra-low temperature freezer

This paper presents an experimental investigation on the pull-down performance of a -80°C ultra-low temperature freezer. Experiments were conducted in a two-stage cascade refrigeration system with an inner recuperative heat exchanger in low temperature cycle (LTC). In addition, the pull-down performances of the freezer were discussed on different startup temperatures ranging from -20°C to 24°C when the ambient temperature was fixed at 26°C. In this paper, the cooling process was divided into early start-up phase, rapid cooling phase and stable cooling phase, and it was found that thermal parameters changed obviously with the startup temperatures, especially in the rapid cooling phase. The experimental results showed that there were maximum discharge temperatures of LTC compressor during pull-down process, and the maximum values increased with the rise of startup temperatures. When the startup temperature was -20°C, the maximum discharge temperature of LTC compressor was 8.1% (11.6°C) lower than that when the startup temperature was 24°C. As two factors affecting the discharge temperature of LTC significantly, the superheating degree was the main influencing factor in rapid cooling phase, while the decrease of the evaporation and condensation pressures had a more significant effect in the stable cooling phase. Besides, the volumetric efficiency of high temperature cycle (HTC) compressor deteriorated more obviously during the pull-down process, and had higher potential for performance improvement compared with that of LTC.