Heat transfer represents one of the most important steps in industrial processes. The subject in it broadest sense can be divided into two categories. The first is transfer of heat at temperatures well below 0° C, such as in air liquefaction plants. Heat transfer at low temperature is normally termed refrigeration and fluids involved in such cases are therefore called refrigerants. The second category involves transfer of heat at temperatures near atmospheric up to 1000 °C and above. The most common fluids applicable in this range includes air, products of combustion (flue gas) and water. The nature of the fluids involved, in particular combustion products and steam, and the urge to conserve energy, establish a strong link between simple indirect heat transfer and energy conservation.