Induced Draft Fans

MIL-HDBK-1003/6

efficiency, controllability and vibration free operation throughout the entire

operating range. Backward inclined or backward curved single thickness blade

centrifugal fans or variable pitch vane axial fans, are preferred. Inlets for

the fans should have silencers with screens to attenuate entrance noises and

prevent objects from entering the system.

The air flow rate shall be sufficient to provide the air required

for proper combustion plus air leakage. The static pressure of the FD fan

shall be calculated for the pressure drop through the inlet air duct, air

heater, air metering device, dampers or vanes, air ducts, fuel bed, burners

and any other resistance between the fan and the furnace. If there is not an

induced draft fan, the forced draft fan must also provide sufficient static

pressure for the furnace, stack effect and other losses. The volume of air to

be provided is dependent on air pressure (elevation), air temperature and

fuel. To assure that the fans will not limit a boiler's performance, margins

of safety are added into the calculations for net fan requirement. The

margins of safety are:

a)

Coal fired:

20 percent excess volume, 32 percent excess

pressure

b) Oil and gas fired:

10 to 15 percent excess volume, 20 to 25

percent excess pressure

c)

25 degrees F (13.9 degrees C) increase in temperature at the

fan inlet.

Induced Draft Fans. Induced draft (ID) fans handle high

8.3.5

temperature corrosive gas which may contain erosive ash. Erosion is

controlled by using abrasion resistant material and limiting top speed. For

coal firing, ID fans may be required to operate under variable temperatures

due to load, bypass of baghouse and scubber. Temperatures can vary from 250

to 450 degrees F (121 to 232.3 degrees C) depending on operation. The

bearings are usually water cooled and have radiation shields on the shaft

between rotor and bearings to avoid overheating. The ID fans move the air

from the furnace, through the heat recovery equipment, ductwork, pollution

control equipment, and stack. Corrosion must be considered if temperatures of

flue gas are within 30 degrees F (16.7 degrees C) of dewpoint. ID fans are

designed with approximately the same factor of safety as FD fans.

Fan Selection. The method of establishing characteristics for FD

8.3.6

and ID fans in a balanced draft system is described in para. 8.3.6.1 and

8.3.6.2.

Forced Draft Fan. (Refer to "Steam" p. 17-10.)

8.3.6.1

EQUATION:

Wa = Tac + Ea + La = Tnr

(17)

Tnr + Wsm = Wbr

150