A regenerative heat exchanger, or more commonly a regenerator, is a type of heat exchanger where the flow through the heat exchanger is cyclical and periodically changes direction. It is similar to a countercurrent heat exchanger. However, a regenerator mixes the two fluid flows while a countercurrent exchanger maintains them separated. The temperature profile remains at a nearly constant temperature, and this includes the fluid entering and exiting each end.In regenerative heat exchangers, the fluid on either side of the heat exchanger is nearly always the same fluid. The fluid is cycled through the heat exchanger, often reaching high temperatures. The fluid may go through an external processing step, and then it is flowed back through the heat exchanger in the opposite direction for further processing. Usually the application will use this process cyclically or repetitively. Thus, in regenerative heat exchangers, a fluid incoming to a process is heated using the energy contained in the fluid exiting this process.