TM 5-810-15
burners and any other resistance between the fan
entering gas being handled as a safety factor. ID fan
and the furnace at the air flow rate required for
type will be selected based on grain loading
proper combustion. The volume of the air to be
according to table 7-9.
handled is dependent on the air pressure
(elevation), moisture content if moisture exceeds 1
Table 7-9. ID Fan Type.
or 2 percent by weight, temperature and excess air
required. Factors of safety to be added to the air
Grains/actual cubic feet
ID fan type
flow requirements to obtain test blockrating are 20
0.1 to 0.2
Airfoil
percent excess volume and 32 percent excess pres-
>0.2
Radial tipped, backward in-
sure for coal fired boilers. Add 25 degrees F to
clined, or radial bladed
temperature of the air being handled as a safety
factor.
ID fan control type will be selected based on
(1) FD fan will be airfoil type to provide
temperature and grain loading according to table
lower power consumption. Airfoil fans will have
7-10.
inlet vane controls to provide low part load power
consumption.
Table 7-10. ID Fan Control Type.
(2) FD fan design will include the following
features. Shafts will be designed to have critical
Grains/actual
speeds not less than 1.4 times the operating speed.
Temperature
cubic foot
Type ID fan control
Bearings will be antifriction type with L10 life of
100,000 hours. Wheel stresses should not exceed
#500 F
#0.2
inlet vane
50 to 60 percent of yield strength while using finite
>500 F
>0.2
inlet box dampers
difference methods and 75 percent of yield strength
at operating temperature while using finite element
ID Fan design will include the following features.
methods. Stress rupture should be considered for
Shafts will be designed to have critical speeds not
elevated temperature. Variable speed fan fatigue
less than 1.4 to 1.5 times the operating speed.
life should be evaluated to avoid premature failure
Bearings shall be antifriction type with L10 life of
due to low cycle fatigue. An impact response test
100,000 hours. Partial liners will be included for
should be performed to avoid high cycle fatigue
airfoil or backward curved fans. Partial or full
due to resonance. Resonance speed of fan support
width liners will be included for other blade types
system should not be less than 1.2 times operating
based on dust loading and velocities. Housings will
speed.
be provided with liners or replaceable heavier scroll
c. Induced draft fans. ID fans can operate under
for fans with severe dust loading. Wheel stresses
erosive conditions even though these fans are
should not exceed 50 to 60 percent of yield
located downstream of the particulate (fly ash)
strength while using finite difference methods and
collection equipment. Erosion is controlled by us-
75 percent of yield strength at operating tempera-
ing abrasion resistant material and limiting top
ture while using finite element methods. Stress
speed. The ID fans move the gas from the furnace,
rupture should be considered for elevated tempera-
through the superheater if required, boiler bank,
ture. Variable speed fan fatigue life should be
economizer, ductwork, scrubbers, baghouse and
evaluated to avoid premature failure due to low
stack. Corrosion must be considered if tempera-
cycle fatigue. An impact response test should be
tures of flue gas are within 30 degrees F of the dew
performed to avoid high cycle fatigue due to
point. The type of fan is usually straight radial with
resonance. Resonance speed of fan support system
shrouds (modified radial) or radial tip design
should not be less than 1.25 times operating speed.
(forward curved, backware inclined) with wearing
d. Fan control methods.
strips when dust burden is high. Maximum speed
(1) Dampers are used on the fan discharge at
should be 1200 revolutions per minute (rpm). Even
either the stoker plenum, the boiler outlet or in
when flue gases are normally cleaned through a
ducting and function to raise system resistance,
baghouse before they reach the fan dirty gases can
thus raising operating points higher on the fan
be bypassed around the baghouse and impinge on
curves and altering fan output. Input power can
the fan blades or wheel. Therefore, the fan must be
decrease somewhat on decreased volume output if
constructed to resist fly ash and dust buildup and to
fan efficiency increases. Dampers are closed on
give better wear resistance. Factors of safety to be
startup of the boiler to reduce the starting load on
added to the air flow requirements to obtain test
the motor.
block rating are 20 percent excess volume and 32
(2) Variable inlet vanes are used to change
percent excess pressure for coal fired boilers. Add
characteristic curves of FD fans. Vanes impart a
a minimum of 25 degrees F to the temperature of
prespin to the gas and by the alteration of the pitch
7-12
Previous Page
