reheaters

Reheaters are components used to raise the temperature of a working fluid after a cooling or expansion step within a system. The term is most often encountered in two contexts: steam power plants operating a reheated Rankine cycle and large heating, ventilating, and air conditioning (HVAC) systems that reheat conditioned air after cooling.

In steam power plants, a reheater is a heat exchanger located in the steam path between turbine stages. After partial expansion in the high-pressure turbine, steam is routed to the reheater where it is heated again by boiler heat to a higher temperature before entering the next turbine stage. Reheating reduces the moisture content of the steam at the final stages, enabling more efficient expansion, increasing turbine efficiency, and decreasing blade erosion. Reheater design can be integrated into the boiler or placed as a separate section of the steam generator. Overall, reheating can improve thermal efficiency for a given boiler output, though it adds heat transfer surface and fuel use.

In HVAC, a reheater is a heat exchanger that raises the temperature of air after it has been cooled by a cooling coil. Reheaters are common in variable-air-volume systems and large spaces that require precise humidity and temperature control. They typically use hot water, steam, or electric resistance heat. The goal is to maintain comfortable supply air while avoiding overcooling and controlling humidity, often in systems that recover energy from the cooling process.

See also: reheat cycle, steam turbine, air handling unit.