Investigation of Precooling-Related Losses in Small-Scale Helium Liquefaction Based on a Two-Stage Gifford-McMahon Cryocooler Working Near 4 K

Due to the extremely high ratio of sensible heat to latent heat for helium (>70), a sufficient precooling process is critical to reducing the consumption of limited cooling capacity at 4 K by helium’s sensible heat and improving the helium liquefaction rate. However, because of the difficulties in cryogenic measurement, practical helium precooling process lacks sufficient experimental date to guide its optimization. In this study, an analysis method for precooling-related losses is proposed and an experimental platform is established based on a two-stage 4 K GM cryocooler. The extraction characteristics of excess cooling and its impact on the cooling capacity at 4 K are measured, and the distribution of losses during the precooling process is quantitatively analyzed. Results indicate that the cooling capacity loss caused by the superheat of helium is the main loss affecting the liquefaction rate. This loss is governed by the minimum extractable temperature of excess cooling, which is determined by cylinder-end temperature of the cryocooler. Effective use of the excess cooling at the end of cylinder for precooling helium can reduce the helium temperature from above 10 K to 8.2 K and the corresponding liquefaction rate loss from 63% to 47.8%, representing an effective approach to improve precooling efficiency and increase the helium liquefaction rate.