Forced Draft Fans

What is Forced Draft Fan?

Forced Draft Fan is a type of a fan supplying pressurized air to a system. In the case of a Steam Boiler Assembly, this fan is of great importance. As we know, for a Steam Boiler to generate steam, combustion must occur, be it any fuel (wood/coal/rice husk/pet coke/LDO/Furnace oil) and for combustion to exist one of its three main requirements, is the air. And this requirement is compensated by a Forced Draft Fan.

This fan supplies air to the Air-Preheater, where it captures the heat from the flue gases coming from the Boiler Outlet.

Introduction to FD fans

Most of the time, use  the forced draft fans in boilers. Install it  in front of the boiler air preheater. It sucks the air  from the atmosphere.  And then sends the air to the air preheater and heated to the requested temperature. Use part of it as the secondary air of the boiler and directly send to the furnace of the boiler through the burner.  But the other part enters the pulverized coal preparation system as a desiccant. Or as primary air, convey coal powder to the furnace through the burner (depleted air powder feeding system), or as tertiary air directly into the furnace through the burner (hot air powder feeding system)

What is the difference between ID fan and FD fan?
Induced Draft:- A fan is used to extract the air from boiler post combustion to exhaust through the chimney. This is called ID fan.  1. It located in between the Boiler/ESP and the chimney/stack. Suction from ESP / Boiler and discharge to the stack. 2. Used to exhaust gases to atmosphere through chimney. 3. In balanced draft used to maintain slight negative pressure (-10 mm Hg typically) with respect to atmosphere to prevent pressurization of boiler(reduces the risk of explosion.). 4. Size is largest as it exhaust all air (secondary air and primary air) and is first to start as is necessary for safe boiler operation to prevent pressurization of boiler.

Forced draft:- A fan is used to force air into the boiler to provide more air for combustion. This is called FD fan, the location near mills and discharges directly to wind box of boiler for uniform air distribution. 1. Suction from atmosphere and discharge to the wind box of the boiler. 2. Used to provide secondary air that is the main air supply for fuel combustion. 3. It pressurizes boiler so is started only after the ID fan is started. Is much smaller than ID fan.