Heat transfer is an interaction that occurs between materials by reason of the temperature difference between them. It is the most common type of interaction in heat exchange equipment Heat transfer between the streams is usually affected indirectly: the streams are separated by a solid material, such as a metal tube wall or plate, or even a plastic membrane, and the heat passes
from the first fluid through the solid material to the second fluid. The consequences of the heat transfer are often local increases in the temperature of the cooler fluid and decreases in that of the warmer fluid; they may also entail the change of phase of one or both fluids.
Heat transfer can also take place when the fluids are in direct contact, for example, when one fluid is warm water and the other is cool air. Direct-contact heat transfer is very common when the cooling water of a steam power station is to be cooled in its turn. The relevant heat exchange device is then usually called a cooling tower.