fan shall be sized for an output volume and static pressure rating
sufficient for pressure losses, excess air requirements at the burner or
grate, leakages, temperature, and elevation corrections for a dirty boiler
with worst ambient conditions, all at full combustion to meet net rated
output at normal firing condition. In addition, fan sizing shall include
minimum margins of 10 percent volume and 21 percent static pressure, plus
margins of 5 degrees C 10 degrees F for forced-draft fans and 22 degrees C
40 degrees F for induced-draft fans. Induced-draft fans shall be designed
for handling hot flue gas at the maximum outlet temperature adjusted for
surface fouling. [Induced-draft fans shall be provided with outlet
dampers.] Noise levels for fans shall not exceed 85 decibels at 914 mm 3
foot station. Fan bearings shall be [air cooled] [or] [water-cooled], and
backward curved fan blade type with bearings not requiring water cooling
may be of the self-aligning antifriction type. [Scroll sheets and rotor
blades shall have liners.]
2.9.1
Draft Fan Control
**************************************************************************
NOTE: Variable speed control, inlet vane control
and inlet damper control are, in descending order of
efficiency, capable of control draft fan conditions.
The choice is based on economics. However, in
erosive services, inlet vane control is not
desirable.
**************************************************************************
Forced-draft centrifugal fans shall have [inlet vane controls] [variable
speed control] where indicated. Induced-draft centrifugal fans shall have
[inlet vane control] [inlet damper control] [variable speed control].
[Axial propeller fans shall have variable propeller pitch control.] Inlet
vanes or dampers shall be suitable for use with combustion control
equipment.
2.9.2
Draft Fan Drives
**************************************************************************
NOTE: Where motor starters for mechanical equipment
are provided in motor control centers, delete the
description of motor starters.
**************************************************************************
Fan shall be driven by an electric motor. Electric motor shall be
[drip-proof] [totally enclosed nonventilated] [totally enclosed fan-cooled]
[totally enclosed fan-cooled, suitable for installation in a Class II,
Division 1, Group F, hazardous location conforming to NFPA 70]. [Motor
starter shall be magnetic [across-the-line] [reduced voltage start] type
with [general-purpose] [weather-resistant] [water-tight] [dust-tight]
[explosion-proof] enclosure and shall be furnished with four auxiliary
interlock contacts.]
2.10
AIR DUCTS
Air ducts connecting the forced-draft fan units with the stoker plenum
chamber shall be designed to convey air with a minimum of pressure loss due
to friction. Ductwork shall be galvanized sheet metal conforming to ASTM A
653/A 653M. Ducts shall be straight and smooth on the inside with laps
made in direction of air flow. Ducts shall be externally braced and shall
be so installed and anchored as to be completely free from vibration.
SECTION 15555A
Page 51
Previous Page
