This is basically A heat transfer equipment to heat air prior to entering in the furnace with a primary object to increase thermal efficiency of hot flue gases are passed through the tubes and air outside the tubes. The heat of the boiler flue gas is recovered in APH to increase the air temperature & the thermal efficiency is increased. Another advantage is that the flue gas is sent to chimney stack at lower temperature allowing simplified design of ducting & flue gas stack. It also allow controls the temperature of flue gas leaving the stack to meet emissions regulation / pollution control.