Heat exchanger (HE) is a device built for efficient heat transfer from one fluid to another, whether the fluids are separated by a solid wall so that they never mix, or the fluids are directly contacted. In the process industries the transfer of heat between two fluids is generally done in heat exchangers. The most common type is one in which the hot and the cold fluid do not come into direct contact with each other but are separated by a tube wall or a flat or curved surface. The transfer of heat is accomplished from the hot fluid to the wall or tube surface by convection, through the tube wall or plate by conduction and then by convection to the cold fluid.