A heat exchanger can be defined as any device that transfers heat from one fluid toanother or from or to a fluid and the environment. Whereas in direct contact heat exchangers, there is no intervening surface between fluids, in indirect contact heat exchangers, the customary definition pertains to a device that is employed in the transfer of heat between two fluids or between a surface and a fluid. Heat exchangers may be classified (Shah, 1981, or Mayinger, 1988) according to (1) transfer processes,(2) number offluids, (3) construction, (4) heat transfer mechanisms, (5) surface compactness, (6) flow arrangement, (7) number offluid passes, and (8) type ofsurf ace.
Recuperators are direct-transfer heat exchangers in which heat transfer occurs between two fluid streams at different temperature levels in a space that is separated by a thin solid wall (a parting sheet or tube wall). Heat is transferred by convection from the hot (hotter) fluid to the wall surface and by convection from the wall surface to the cold (cooler) fluid. The recuperator is a surface heat exchanger.
Regenerators are heat exchangers in which a hot fluid and a cold fluid flow alternately through the same surface at prescribed time intervals. The surface of the regenerator receives heat by convection from the hot fluid and then releases it by convection to the cold fluid. The process is transient; that is, the temperature of the surface (and of the fluids themselves) varies with time during the heating and cooling of the common surface. The regenerator is a also surface heat exchanger.
