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Refrigerants

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All refrigerants are liquefied by the removal of heat energy. However, the cryogenic refrigerants such as helium, hydrogen, nitrogen, and others, are referred to as permanent gasses.  There is a temperature above which a gas cannot be liquefied regardless of the amount of pressure that is applied. That temperature is called the critical temperature. For ordinary refrigerants the critical temperature are above 93.33  ℃, with the exception of carbon dioxide. By contrast, the critical temperatures  of the common cryogenic refrigerants are: nitrogen -147 ℃; hydrogen -236.8 ℃; and helium -268℃, within less than -12.2 C degrees of absolute zero. This means that each gas must be cooled below its critical temperature before it can be liquefied. The critical pressure is the pressure needed to liquefy the gas at the critical temperature. The refrigeration equipment necessary to keep these refrigerants at cryogenic temperatures is more complex than that needed for common refrigeran