Forced Draft Fan or FD Fan

              Forced Draft fan or FD fan is another most important part of the boiler. The PA fan will carry the powdered coal from mills to the boiler while the ID fan will remove the gases after the combustion of the coal. The FD is also called as secondary air system which is used to provide sufficient amount of oxygen to the boiler so that combustion of coal will go smoothly. If sufficient amount of air is not supplied by the FD fan or excess amount of air is supplied then the combustion of coal will not be proper and the unburned coal will settle on the walls of the boiler. The continuous settlement of the coal powder on the walls of the boiler will lead to the formation of clinkers. 

            Clinkers are formed due to piles of unburned material in the boiler, improper burning of coal is not the only reason for formation of clinker, it may also be due to the entry of foreign particles into the boiler. All the particles other than coal are termed as foreign particles of boiler. Due to the formation of the clinkers, the ash which is resulted due to the burning of coal will settle on the clinker and starts to build into the boiler and occupy the space of the fireball. (Fireball is the area of the flame present in the boiler and if looked through the peep hole it will resemble as ball hence termed as fireball) . As the area of the fireball is occupied by the clinker and if it goes unnoticed it will result in the termination of the fire which means system is tripped.

            Operators must ensure proper working of the FD Fan and the ID fan to make sure the boiler is maintained at the proper negative furnace pressure The load of the FD fan should be reduced if any of the ID fans goes out of service( In general two ID and two FD fans are used now a days) , the negative pressure created by the ID fan is reduced, the boiler will move into the positive furnace pressure zone which results in the fire coming out of the boiler thereby reducing the lifetime and efficiency of the boiler. 

            If any of the FD fan goes off then operators should reduce the load on the ID fan as the FD fan provides air into the system, the amount of air removed by the ID fan will be more, which results in the more negative pressure in the boiler, if goes unnoticed, the negative pressure will result in the flame loss leading to unit trip. This the importance of the FD fan and its relation with ID Fan.