2017 Tech Report Nov-Dec

system. Others that are becoming popular are: the hybrid cascade system, the


volatile brine cascade system, transcritical system, transcritical parallel compression system, and parallel compression ejector system. Some of these are now in use in small beverage cooler and grocery store applications in a neighborhood near you. As the technology matures it is likely that they will all find their way into large systems. THE CASCADE SYSTEM The cascade system has been used with refrigerants that have a low critical point—the point at the top of the Mollier Diagram, above which gas cannot be condensed—which for CO2 is 87.76ºF (31.0ºC) and 1055.3 psig (72.8 bar). Refrigerants other than CO2, such as R23 and R503—no longer available— have been used in the low stage of cascade systems to operate at temperatures down to -150ºF (-101ºC). CO2 is limited by its triple point to temperatures above -69.8ºF (-56.6ºC). The cascade system is often confused with two stage systems. Two stage systems use one refrigerant and compress the refrigerant vapor through two stages from low—evaporator—pressure,

Figure 1 Simple Cascade System

to intermediate pressure, and to high—condensing— pressure. This is done to reduce the compression ratio of the individual low stage and high stage compressors. The two- stage system achieves improved efficiency as the volumetric efficiency of the compressors increases with the lowering of the compression ratio.

Cascade systems use two systems interconnected by a heat-exchanger that is the evaporator for the high stage system, and the condenser for the low stage system. This type system uses two different refrigerants. One refrigerant that has a low critical point in the low stage system. The other is a common refrigerant,


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