The cascade refrigeration system is a freezing system that uses two kinds of refrigerants having different boiling points, which run through their own independent freezing cycle and are joined by a heat exchanger. This system is employed to obtain temperatures of -50 to -80°C or ultra-low temperatures lower than them. At such ultra-low temperatures, a common single-refrigerant two-stage compression system limits the low-temperature characteristics of the refrigerant to a considerably poor level, making the system significantly inefficient. The efficiency is improved by combining two kinds of refrigerants having different temperature characteristics. This system has the following advantages compared to a two-stage compression system:
•Energy is saved because the system allows the use of refrigerants that have suitable temperature characteristics for each of the higher-temperature side and the lower-temperature side.
•It allows stable ultra-low-temperature operation.
•The running cost is inexpensive.