Transmission of thermal energy from one location to another by means of a temperature gradient existing between the two locations. It may take place by conduction, convection, or radiation. Heat transfer is involved in many types of industrial operations, including distillation, evaporation, canning of foods, baking, curing, etc. In some cases, a heat exchanger is utilized. Fluids of high heat capacity are widely used to remove unwanted heat or to transfer it from one place to another within a system; examples are air, water, ethylene glycol.See Coolant.