Flue Gas Air Pre Heater consist of straight tube bundles which pass through the outlet ducting of the boiler and open at each end outside of the ducting. 

Ambient air is forced by a fan through ducting at one end of the preheater tubes and at other end the heated air from inside of the tubes emerges into another set of ducting, which carries it to the boiler furnace for combustion. 

Inside the ducting, the hot furnace gases pass around the preheater tubes, transferring heat from the exhaust gas to the air inside the preheater.