An air pre heater (APH) is a general term to describe 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. The purpose of the air pre heater 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 sent to the flue gas stack (or chimney) 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