An air preheater is a general term to describe any device designed to heat air before another process (for example, combustion

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 in a boiler

Boiler

A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative

Recuperator

A recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat...

 heat system or to replace a steam coil.

In particular, this article describes the combustion air preheaters used in large boilers found in thermal power station

Thermal power station

A thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser and recycled to where it was heated; this...

s producing electric power

Electric power

Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...

 from e.g.