What is the difference between primary refrigerant fluids and heat transfer fluids?

Primary fluids are always used in compression refrigeration cycles that include a compression, condensation, expansion and evaporation sequence; therefore, they are direct expansion fluids because they directly evaporate in the utility that requires refrigeration.

Heat exchange fluids exchange heat with primary fluids to transfer cold into large circuits, thus optimising costs and safety because they are not hazardous fluids. This remarkably reduces levels of circulating primary fluids and the probability of accidental losses.

The primary refrigerants used belong to the family of halogenated hydrocarbons (halocarbons). They are halogenated hydrocarbon molecules in which some hydrogen atoms are replaced by halogen atoms (chlorine and fluorine). Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) belong to this family. Certain hydrocarbons (e.g. methane, propane, butane, etc), anhydrous ammonia and carbon dioxide are used as primary refrigerants, besides the halogenated hydrocarbon family.