(also heat-transfer medium or coolant), a mobile medium used for transferring heat from a hotter object to a cooler object. Heat-transfer fluids are used for cooling, drying, and heat treatment in heat-supply, heating, and ventilation systems and in industrial heat devices. The most common mediums are flue (stack) gases, water, steam, liquid metals (potassium, sodium, and mercury), Freons, and aerosols of bulk materials.
In the process of transferring heat, heat-transfer fluids may change their state of aggregation, as in the case of boiling liquids and condensing vapors, or they may remain in the same state, as with nonboiling liquids, superheated vapors, and noncondensing gases. In the first instance, the temperature of the heat-transfer fluid is unchanged, since only the heat of transformation is transferred; in the second, the temperature of the fluid increases or decreases. There are special requirements for the heat-transfer fluids (coolants) in nuclear reactors.