59.What are the Secondary Air Dampers and how they differ between wall fired and corner fired boilers?

The damper controlling the combustion air to fuel at the burner is known as Secondary air. The air which is used to transport fuel to the burner is Primary air. Normally secondary air will be admitted very close to the fuel. In a wall fired boiler the secondary air will be spinned around the fuel spary. If the secondary air axial velocity component is greater than flame propagation velocity, then the flame will become unstable. Hence the excess air will be supplied at the outer circle. This air is known as Tertiary air. In a corner fired boiler, the secondary air is known as Fuel air, the damper is called FAD. The air will be admitted axially at the bottom and top of the fuel nozzle and will rotate at the peripheral of the fire ball. The excess air will be admitted at he next  level of each FAD outlet and called Auxiliary air and damper is called AAD.