1. Product families

Since the aim with these fluids is to produce cold at generally low temperatures by vaporizing the liquid, the triple points of these fluids must be at even lower temperatures. Their vapour pressures must therefore be fairly high or, if preferred, their boiling temperatures must be low, which implies the choice of simple molecules. We can thus use :

• simple elements ;

• pure compounds (inorganic or organic);

• azeotropic mixtures, whose behavior is comparable to that of pure bodies, or zeotropic mixtures, whose behavior is more complex.

The simplest elements (hydrogen, helium, nitrogen, oxygen, etc.) enable the low temperatures used in cryogenic techniques to be reached. These cryogens are not generally used in compression cycles as described...