Cascade Refrigeration

significantly large temperature and pressure difference, one vapor compression refrigeration cycles become impractical. One of the solutions for such cases is to perform cooling in two or more stages (i.e., two or more cycles), who work in the series. These refrigeration cycles are called cascade refrigeration cycles. Thus, cascade systems used in order to obtain high temperature differences between the heat source and heat sink and are applied at temperatures from -70C -100C. Application of the three-stage compression system for boiling temperatures below -70C is limited, due to difficulties with the refrigerant temperature of freezing. The irrelevance of a threestage vapor compression systems can be avoided by applying a cascading steam-compression refrigeration machines.

Cascade refrigeration systems commonly used in the liquefaction of natural gas and some other gases. In large industrial cascade refrigerating system is shown in Fig. The most important advantage of these cascading system is that refrigerants can be selected with the appropriate properties, avoiding the larger components of the system. In these systems with several evaporators can be used in any stage of compression. Refrigerants used at each stage may be different and selected for optimum performance at a given temperature of the evaporator and condenser.

The lowest temperature that can practically be achieved in single-stage refrigeration systems is about -40 to -50˚F. A single-stage system is limited by the compression ratio of the compressor and the ambient temperature in which it must condense the refrigerant. Temperatures from -50 down to -120˚ or lower can only be achieved economically by using cascade refrigeration systems.

In a typical cascade system, a standard refrigeration system is used on the “high side” to create a cold temperature in the cascade condenser. The “low side” system is able to condense at -20 to -30˚ and evaporate as low as -120˚ with the available refrigerants before they go into vacuum. Larger systems tend to have some kind of oil separator and oil management system to keep the oil in the compressor. Some systems also employ an expansion tank to keep the refrigerant from generating extreme pressures at room temperature when the system is off.

Traditional high side refrigerants are CFC-12, HCFC-22, and CFC-502. Alternative high side refrigerants are HFC-134a, HFC-404A, and HFC-507.

Traditional low side refrigerants are CFC-13 and CFC-503. Alternative low side refrigerants are HFC-23 and HFC-508B.