TM 5-815-1/AFR 19-6
bags longer than 10 to 12 feet should be provided with
are required to indicate whether necessary dilution air-
walkways at the upper and lower bag attachment
dampers or pre-cooling sprays are operating correctly.
levels.
A well-instrumented fabric filter system protects the
b. Hopper and disposal equipment. The dust-collec-
investment and decreases chances of malfunctions. It
tion hopper of a baghouse can be constructed of the
also enables the operating user to diagnose and correct
same material as the external housing. In small light
minor problems without outside aid.
c. Gas preconditioning. Cooling the inlet gas to a
duty, hoppers 16 gage metal is typical. However, metal
wall thicknesses should be increased for larger
fabric filter reduce the gas volume which then reduces
baghouses and hopper dust weight. The walls of the
required cloth area; extends fabric life by lowering the
hopper must be insulated and should have heaters if
filtering temperature; and permits less expensive and
condensation might occur. The hopper sides should be
durable materials to be used. Gas cooling is mandatory
sloped a minimum of 57 degrees to allow dust to flow
when the effluent temperature is greater than the max-
freely. To prevent bridging of certain dusts, a greater
imum operating temperature of available fabrics. Three
hopper angle is needed, but continuous removal of the
practical methods of gas cooling are radiation con-
dust will also alleviate bridging. If dust bridging is a
vection cooling, evaporation, and dilution.
significant problem, vibrators or rappers may be
(1) Radiation convection cooling enables fluctua-
installed on the outside of the hopper. The rapping
tions in temperature, pressure, or flow to be
mechanism can be electrically or pneumatically oper-
dampened. Cooling is achieved by passing the
ated and the size of the hopper must be sufficient to
gas through a duct or heat-transfer device and
hold the collected dust until it is removed. Overfilled
there is no increase in gas filtering volume.
hoppers may cause an increased dust load on the filter
However, ducting costs, space requirements,
cloths and result in increased pressure drop across the
and dust sedimentation are problems with this
collector assembly. Storage hoppers in baghouses
method.
which are under positive or negative pressure warrant
(2) Evaporative cooling is achieved by injecting
the use of an air-lock valve for discharging dust. Since
water into the gas stream ahead of the
this will prevent re-entrainment of dust or dust blow-
filtering system. This effectively reduces gas
out. A rotary air valve is best suited for this purpose.
temperatures and allows close control of
c. For low solids flow, a manual device such as a
filtering temperatures. However, evaporation
slide gate, trip gate, or trickle valve may be used,
may account for partial dust removal and
however, sliding gates can only be operated when the
incomplete evaporation may cause wetting
compartment is shut down. For multicompartmented
and chemical attack of the filter media. A
units, screw conveyors, air slides, belt conveyors or
visible stack plume may occur if gas
bucket conveying systems are practical. When a screw
temperatures are reduced near to or below the
conveyor or rotary valve is used, a rapper can be
dew point.
operated by a cam from the same motor.
(3) Dilution cooling is achieved by mixing the gas
steam with outside air. This method is
9-5.
Auxiliary equipment and control
inexpensive but increases filtered gas volume
systems
requiring an increase in baghouse size. It is
possible the outside air which is added may
also require conditioning to control dust and
ric filter system depends upon continuous control of
moisture content from ambient conditions.
gas temperature, system pressure drop, fabric pressure,
gas volume, humidity, condensation, and dust levels in
9-6.
Energy requirements.
hoppers. Continuous measurements of fabric pressure
The primary energy requirement of baghouses is the
drop, regardless of the collector size, should be pro-
power necessary to move gas through the filter. Resis-
vided. Pressure gages are usually provided by the filter
tance to gas flow arises from the pressure drop across
manufacturer. With high and with variable dust load-
the filter media and flow losses resulting from friction
ings, correct fabric pressure drop is critical for proper
and turbulent effects. In small or moderately sized
operation and maintenance. Simple draft gages may be
baghouses, energy required to drive the cleaning mech-
used for measuring fabric pressure drop, and they will
anism and dust disposal equipment is small, and may
also give the static pressures at various points within
be considered negligible when compared with primary
the system. Observation of key pressures within small
fan energy. If heating of reverse air is needed this will
systems, permits manual adjustment of gas flows and
require additional energy.
actuation of the cleaning mechanisms.
9-7.
Application
b. The number and degree of sophistication of pres-
sure-sensing devices is relative to the size and cost of
a. Incinerators. Baghouses have not been widely
the fabric filter system. High temperature filtration will
used with incinerators for the following reasons:
require that the gas temperature not exceed the
(1) Maximum operating temperatures for fabric
tolerance limits of the fabric and temperature displays
filters have typically been in the range of 450
9-10
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