Refrigerant, Secondary

(also secondary coolant), a liquid or gas that is used in refrigerating systems as an intermediate medium for heat transfer from the object to be cooled to the refrigerant, which boils in the evaporator of the refrigerating machine. Systems that operate with secondary refrigerants are used in cases where the direct cooling of an object by means of a boiling refrigerant is impossible, difficult, or uneconomical, for example, when there are many users or when the users are far from the machine room.

Secondary refrigerants should have a low freezing point, a low viscosity, a high specific heat, and a high thermal conductivity. They should be nontoxic, flameproof, and neutral with respect to building materials.

Solutions of salts in water, called refrigeration brines, are used as secondary refrigerants. Sodium chloride brine is employed for temperatures down to – 15°C; magnesium chloride brine, down to – 27°C; and calcium chloride brine, down to – 45°C. Antifreezes and Freons are used in low-temperature refrigerating systems. Aqueous solutions of propylene glycol are employed for temperatures down to – 47°C; aqueous solutions of ethylene glycol, down to – 60°C; Freon-30, down to – 90°C; and Freon-11, down to – 100°C.

Water is used as the secondary coolant in air-conditioning systems.