Additional allowances for excess air should be used above those
given by boiler manufacturers for predicted performance data. Expected
operating conditions must be explicitly defined and equipment systems must be
designed to meet all operating modes. The following design parameters can be
used as a guide:
a) Collector should be a compartmental design with sufficient
compartments so that construction and performance criteria are met with one
compartment cleaning and one off-line for maintenance.
b) Maximum gross air-to-cloth ratio to be 2.0 to 2.5 ACFM/square
feet of filter area.
c) Maximum net air-to-cloth ratios with one module off-line for
cleaning and reverse air fan cleaning should be 2.5 to 3.0 ACFM/square feet of
d) Minimum reverse air flow to be 1.50 ACFM/square feet of filter
area in a module.
e) Bags are usually of woven fabric with a maximum bag length to
diameter ratio of 38 to 1 and an adequate number of anti-collapse rings sewn
into the fabric.
f) Maximum total baghouse pressure drop from inlet flange to
outlet flange with one module cleaning to be 6 in. w.g.
g) Hoppers should be designed with adequately sloped side walls
for the fly ash characteristics to be encountered. Minimum slope of the
valley should be 55 degrees. Hoppers should have poke holes, strike plates,
hopper vibrators and high ash level indicators. Hopper capacity should be 8
to 10 hours using one-third of the hopper volume.
h) Provide access to all working and maintenance areas with a
protective penthouse to allow for bag maintenance.
i) Design inlet and outlet ductwork plenums for equal gas
distribution to all modules.
j) Flue gas temperature must be controlled at a minimum 25 degrees
F (13.9 degrees C) above the dew point and below the maximum temperature
limits of the fabric filter for the expected load range of the boiler.
k) The collector housing, hoppers, inlet and outlet plenums, and
reverse air plenum should all be insulated. A minimum gas flow should be
maintained through the reverse air fan and plenum even when not in a cleaning
cycle. This will avoid fan or duct corrosion due to the temperature of
residual flue gases dropping below their dewpoint. Provide walking surfaces
to all maintenance and access locations. Insulate probes, pressure taps, and
any other sources of heat sink that may cause localized cooling and corrosion
of the internal walls.