Theory of refrigeration machines - Mechanical compression machine. Fluids

To absorb heat from a medium to be cooled (refrigeration machines) or to heat a medium with free heat whose temperature has been raised (heat pumps), the various thermodynamic systems use refrigerants, substances that undergo temperature and pressure variations or phase transitions during the thermodynamic (or thermochemical in some cases) transformations of the systems that use them. These refrigerants are :

• or solids which, depending on their type, undergo mechanical, electronic or magnetic variations accompanied by thermal phenomena. The heat they absorb or release requires the use of heat- or cold-transfer fluids. Systems that use these fluids are still in their infancy;

• or fluids, which is the most common case, and the systems that use them are widespread. These refrigerants are easy to move around in systems, which is a major advantage. They can undergo thermodynamic transformations, sometimes of considerable magnitude, without changing their physical state. This is the case, for example, with – refrigerants (or "cryogens") used in refrigeration or cryogenic cycles to produce very low temperatures – which remain permanently gaseous. The compression systems we are concerned with here all use refrigerants that change physical state during cycles, thus benefiting from the high enthalpies of liquid ↔ vapour transformation.