FD fan controls are done according to the boiler.Depending on the fuel rate of a particular boiler, the air requirement and hence the draft to be developed by the FD fan is decided. In all thermal power plants, with a number of boilers in operation, a master controller will decide the load sharing among boilers and hence the fuel rate.A simplified schematic diagram of Combustion air Draft Control is shown above.The fan damper is manipulated by the furnace pressure controller.

FD fan damper is generally manipulated by the airflow controller.

However, too low pressure must also be avoided to reduce air leakage into the furnace and to prevent furnace implosion in extreme cases.

The balanced draft furnace operates at negative pressures to avoid leakage of flue gas into the surroundings.

To control furnace pressure, it is required to maintain a balance between the flow in and flow out of the furnace.

Furnaces equipped with both FD Fan and ID fan are known as a Balanced Draft system.

An induced draft furnace is operated slightly below atmospheric pressure.Control is accomplished by regulating the fan speed or damper operation.An Induced Draft fan makes high stacks unnecessary.This Fan receives flue gas through the furnace and the combustion air into the furnace.This Induced Draft fan at the boiler end takes its suction from the boiler flue gas stream and discharges this flue gas to the chimney is called an induced draft fan.This type of furnace is operated slightly above atmospheric pressure.This control is accomplished by regulating the fan speed and the position of the corresponding damper.A forced draft furnace uses an FD fan or blower to force combustion air through the system by sucking air from the atmosphere.Natural draft is used to supplement the mechanical draft produced by fans.The draft that will be produced by a stack is a function of the height of the stack and the flue gas temperature.The gasses in the stack will rise, creating a vacuum (suction) that will draw the combustion gasses or flue gas out of the furnace.Gasses inside the chimney are less dense than outside the chimney.A natural draft furnace uses the stack (chimney) effect.Natural draft operates below atmospheric pressure.Furnaces are classified by the method for moving air and other gasses through the system.These Fans and chimneys produce pressure in the positive direction.Finally, when the chimney is not in a position to develop enough natural draft to accept flue gas, the induced draft (ID) fan is added to serve the purpose.After the combustion, the flue gasses travel toward the atmosphere through the economizer, air preheater, and chimney which are normally pressure-reducing components.In addition solid, liquid, and gaseous fuel adds extra pressure inside the combustion chamber.Normally in a boiler, a forced draft (FD) fan draws air from the atmosphere and gives a huge boost to the draft.Also, this should be as close to the atmosphere to minimize the ingestion of excess air drawn through openings.Generally, this draft is maintained slightly negative to prevent the combustion products, and ash from being discharged from the furnace into surrounding areas through inspection ports, doors, feeders, etc.This pressure difference is known as draft whether measured above or below atmospheric pressure.This requires a pressure difference to accelerate the gasses to their final velocity plus friction losses.The combustion of fuel requires oxygen called air, to move the required air through the fuel bed and to produce a flow of the gaseous products of combustion out from the furnace, to the chimney through the superheater, economizer, and air preheater.Furnace pressure is commonly known as draft or draft pressure or sometimes furnace draft.For the flow of Air and Fuel into the furnace and the flow of Flue gas from the furnace to the chimney. The driving force for this flow is the differential pressure between these gasses inside and outside the furnace.Considering Air and Flue gas Circuit, here the Air and Fuel such as coal or Bagasse are mixed and ignited in the combustion chamber at the furnace.

Samson boiler draft control