The progress of cryogenic engineering and the role of refrigerators are described. Small refrigerators, mostly for electronics application, are coming to be standardized, while large refrigerators have a trend to become multipurpose. For future applications of cryogenic equipments and devices, the development and cost of refrigeration by refrigerators will become the most important factor.
Working principle and theoretical thermodynamic efficiency of refrigeration cycles with regenerator which have been developed for practical use or have a prospective application are described. Six refrigeration cycles, namely Kirk cycle, Vuilleumier cycle, Gifford-MacMahon cycle, Higa cycle, Pulse tube refrigerator and Vortex tube are included. The last one is not practically used as a refrigeration cycle at present, but it would be worth while to describe it as one of the possible lines of consideration for future refrigeration cycles. Most of the cycles can be extended to a cascade cycle of two or three stages to realize lower refrigeration temperature, but it is not described here to avoid complexity.