What is exactly Draught System in Boiler ????

Draught System in Boiler:

The draught system is basically used in boilers due to the reasons ---

1. To supply to the furnace the required quantity of air for complete combustion

2. To remove the gaseous products of combustion from the furnace and throw these through chimney

FD Fan: The FD fan or blower is installed near or at the base of the boiler to force the air through the cool bed and other passages through the furnace; it creates a positive pressure in the furnace and overcomes the resistance in the air pre-heater and grate.

ID Fan: The ID Fan is located at or near the base of the chimney and it provides negative pressure in the furnace and flues. It overcomes draught losses through boiler, economizer, and air pre-heater and the connecting flues. Its function is similar to that of the natural draught.

     There is a minimum amount of air, which is required by the fuel to burn completely. But always air in excess is used because whole of the air supplied for combustion purpose does not come in contact with the fuel completely and as such a portion of the fuel may be left unburnt. 

But if a large quantity of excess air is used, it exercises a cooling effect on combustion products. Hence a proper control of the right amount of excess air has to be maintained for optimum combustion efficiency. This can be ensured when the smoke from the chimney becomes just visible.