4. Cascade refrigeration (or heat pump) cycles with separate refrigerants

4.1 Description of cascade cycles

Cascade refrigeration cycles can be used to move heat over a wide temperature range, particularly to produce cold at low temperatures. Unlike multi-stage cycles, which use a single refrigerant, cascade cycles use several different refrigerants, each running in individual compression refrigeration machines that communicate with each other via heat exchangers; each of these constitutes a cell of the cascade machine. These cells, which use a single refrigerant, can be single-stage or multi-stage. The lower the final temperature to be reached, the greater the number of cells required. Heat is transferred from one cell to the next in order of increasing temperature, by thermal coupling between the cells. In practice, this coupling takes place between a...