A heat exchanger is a device that transfers heat from one fluid to another or between a fluid and the environment. In the open type, or direct contact type of heat exchanger, there is no intervening surface between the fluids. In the closed type or indirect contact heat exchangers, our definition pertains to a device that is employed in the transfer of heat between two fluids or a surface and a fluid. Shah (1981) and Mayinger (1988) have categorized heat exchangers according to the following properties:

• Transfer processes • Surface compactness • Number of fluids • Flow arrangement • Construction • Number of fluid passes • Heat transfer mechanisms • Type of surface

Recuperators (or closed-type heat exchangers) are heat exchangers in which heat transfer occurs between two fluid streams at different temperatures that are separated by a thin solid wall (a parting sheet or tube wall). Heat is transferred by convection from the hotter fluid to the wall surface and by convection from the wall surface to the cooler fluid. Figure 25.1 shows four examples of the recuperator that are surface-type exchangers. The recuperator is the only type of heat exchanger that we will consider in detail.