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When you go into the average super market to buy your groceries, you may pick from various display cases maintaining temperatures of from minus 10 degrees to plus 40 degrees Fahrenheit, because of the product they are preserving. In today’s markets, there may be as many as thirty to forty cabinets, all being cooled by as few as six refrigeration systems on the roof. These are single stage refrigerant cycles, as explained in a previous article posted earlier. With a single stage system, minus 40 degrees is about as low as you can achieve.
Cryogenics is the scientific study of what may happen to certain substances well below those limits. Examples are the preservation of fertilized embryos, or even full human remains. This study developed the need for refrigeration capacities achieving these ultra-low temperatures, and led to the development of two stage or tandem refrigeration systems, often called Cascade Systems. Now engineers are working to bring this to the commercial refrigeration field.
In such systems, the condensing unit (heat disbursing side) becomes the evaporator unit (heat accumulating side) of the second unit. You may see the principle illustrated below. This probably represents the systems which will be available commercially in the near future.