3. Gas liquefaction

Numerous methods can be used to produce cold at very low temperatures, but only those commonly used for gas liquefaction are discussed below. In most cases, the choice of refrigeration production method depends on the intended use of the cold produced. Consequently, for very low temperatures, refrigeration machines whose sole purpose is to produce cold are rarely found on the market. The only notable exception is the Philips machine used to liquefy nitrogen or to produce cold for hydrogen or helium liquefaction, but its size is limited (a few dozen kilowatts of mechanical power).