In a Steam Boiler operation assembly, Industrial Fans and Blowers i.e. Forced Draft fans are used to draw air into combustion chamber, where it mixes with the fuel being supplied. FD fan is used to regulate appropriate amount of air to fuel ratio in combustion chamber to maximize the fuel efficiency.
Induced Draft fans are used in industrial steam boilers to draw flue gases from the combustion chamber and through the rest of the boiler assembly (Air Pre-heater, Dust Collectors, ESPs etc.) to the stack. Induced Draft fan is always assembled before chimney.