An air preheater (APH) is an equipment designed to heat air before another process with the primary objective of increasing the thermal efficiency of the process.

The purpose of the air preheater is to recover the heat from the flue gas thereby increasing the thermal efficiency of the equipment by reducing the useful heat lost in the flue gas.  As a consequence, the flue gases are also sent to the flue gas stack at a lower temperature, allowing simplified design of the ducting and the flue gas stack.  It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

This air preheat system preheats the combustion air to increase overall efficiency by means of an air heater which utilises the hot flue gases. The cool flue gases are then passed through an induced draft fan an exhausted to a stack and the combustion air is supplied via a forced draft fan.

These units can be of either recuperative or regeneration type.

APH systems become more profitable with increasing fuel costs, with increasing process inlet temperature (i.e. higher stack flue-gas temperature), and with increasing fired duty.