An air preheater (APH) is any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process.
They may be used alone or to replace a recuperative heat system or to replace a steam coil.
The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas.
As a consequence, the flue gases are also conveyed to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the conveyance system and the flue gas stack.
It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).
It is installed between the economizer and chimney.