Multistage Refrigeration

the process of heat transfer from a lower temperature level to a higher level (that is, cooling) that is accomplished in refrigeration equipment by means of several closed refrigeration cycles operating in series. This is the method used for deep freezing where the cooling agent in the low-temperature cycle condenses as a result of evaporation of the cooling agent in the next cooling cycle, with a higher temperature. As a rule no more than four cycles are used, because the construction of the equipment would become too complicated. The refrigeration cycles used may be the same, or they may have various thermodynamic heat transfer principles and cooling agents.

During the late 1800’s the Swiss physicist R. Pictet used multistage refrigeration to liquefy air. His equipment had three refrigeration cycles. In the first high-temperature two-stage cycle the working substance was methyl chloride (CH₃C1), in the middle cycle it was ethylene (C₂H₄), and in the third it was oxygen (O₂). The method was subsequently improved and was used to produce liquid hydrogen and helium.

Multistage refrigeration is usually used to reach temperatures as low as — 110°C in experimental chambers and for technological purposes in chemistry, medicine, and biology.

Multistage refrigeration with two vapor-compression cycles is most common. The cooling agent usually used in the high-temperature cycle is Freon 22 (CHC1F₂) and in the low-temperature cycle it is Freon 13 (CF₃C1). To produce temperatures down to — 90°C, the low-temperature cycle has a single stage using Freon 13; for temperatures below — 90°C, it has two stages. Heat is transferred from the low-temperature cycle to the high-temperature cycle in a heat exchanger (evaporator-condenser) as a result of condensation of the low-temperature cooling agent and boiling of the high-temperature cooling agent. The method can be improved by the use of more efficient cooling agents, better compressor designs, and more efficient heat exchangers.