Baghouse Related Articles
Is Your
Baghouse the Real Bottleneck?
Apr 1, 2000 12:00
PM
Andy Winston
An aggregate operation has many systems working
together. When problems arise, detailed troubleshooting usually pinpoints
several areas to address. So why is it that whenever there is an airflow
problem, a dusting problem, or an emissions problem, the baghouse alone is
blamed? Is the system's bottleneck just the
baghouse part or baghouse filter bags? In
most cases, the answer is no.
In most aggregate plants, the dust-collection
system has five major components: collection hood(s), ductwork, dust
collector (baghouse), fan, and material handling equipment. If any of these
areas is poorly designed or operating inefficiently, the entire system will
not perform properly. Instead of making major changes to one component
(often the baghouse), the solution may be to make minor changes to several
components.
Airflow through hoods It is easy to tell if a hood
is not pulling sufficient airflow-because there is dust everywhere. But, a
hood can pull too much dust-laden air (Figure 1). To design an efficient
hood, capture velocity, face velocity and duct velocity must be understood.
* Capture velocity is the speed of air at any point
in front of the hood that is necessary to overcome opposing air currents and
to capture dust-laden air by causing it to flow into the hood.
* Face velocity is the speed of dust-laden air at
the opening of the hood.
* Duct velocity is the speed of the dust-laden air
once inside the ductwork. In order to move particulate to the dust
collector, the duct velocity must be equal to or greater than the minimum
air velocity required to move particles in the air stream.
The hood must generate a flow pattern and capture
velocity sufficient to control dusting without collecting excessive dust. In
standard material handling applications such as conveyor belt transfer,
bucket elevators, air slides, vessel loading, and crushing and grinding, the
goal is to de-dust the area without moving material off of the conveying
equipment.
If the capture velocity is too high at the pick-up
point, excessive particulate will be introduced into the ductwork and
eventually to the dust collector. Inside the dust collector, the filters
will collect more dust than they were designed for, leading to high dust
loading. This leads to the baghouse operating at high differential pressure
(DP) and requiring accelerated filter cleaning. Under these conditions, the
filters wear out faster and require more cleaning energy.
One solution is to enclose the hoods as much as
possible (Figure 1). Enclosing the hood keeps out air currents surrounding
the hood, thus requiring less airflow to control the dust. This requires
minimal costs and installation time.
Another problem is found on an airslide hood when
the opening to the ductwork is too close to the airslide casing. This
creates excessive duct velocity where air entersthe ductwork. This can
result in a higher capture velocity.
The solution is to build an additional box on top
of the casing with the same height and width of the casing. The box should
have square sides and a 45degrees sloped transition into the duct. The vent
point should be in the middle of the discharge points. This additional area
will slow the capture velocity to capture just the dust, not the material.
The ideal capture velocity for material handling applications is 200 to 550
fpm.
Once the hoods are designed and installed properly,
blast gates need to be installed to achieve system balance. Due to changing
airflows through the system, adjustments are needed to ensure proper capture
velocity at each hood.
Dust highway The ductwork is the highway of the
dust-collection system. Proper duct velocity for most rock-dust applications
should be between 3,500 and 4,000 fpm. Speeds lower than 3,500 fpm could
allow particulate in the airstream to fall, causing dust buildup in the
duct. Speeds higher than 4,000 fpm could cause abrasion, especially in
ductwork branches and elbows.
The most common design problems involve installing
improperly sized ductwork. If the ductwork is too small in diameter, the
result is high duct velocities. This is usually evident if the ductwork has
numerous holes and weakened areas. If abrasion is present, a patch will soon
be worn away and the problem will still exist.
If the ductwork is too large, the duct velocities
will be too low and result in dust buildup. Dust buildup in the duct creates
a maintenance problem and reduces the cross sectional area of the duct which
affects overall flow and air velocities at the hood.
The most common mistake with ductwork is the
addition of vent points after a system has been designed and installed
(Figure 2). This is usually the hurried addition of a vent point, using
whatever size duct is available and attaching it to the existing duct in the
most convenient place.
Additional vent points installed without
considering existing vent points will result in an imbalance of the system.
Usually, this means the vent point furthest from the dust collector will not
have adequate volume to de-dust its area.
Clearing the air The baghouse is where dirty air
becomes clean air, where dust from the airstream is discharged into a hopper
or vessel. Air velocities are a critical aspect of a properly working
baghouse BPS.
The first problem area in a baghouse is inlet
design. The secret to a good inlet design is to slow air speed from 3,500 to
4,000 fpm down to 2,800 fpm or slower. A good baffle system helps reduce
speed. A baffle turns the air, making it go to the filters in a straight,
even pattern. The baffle also knocks some of the heavier dust into the
hopper before it even gets to the filters. If air speed is not reduced
enough, the dust-laden air will blast into the filters, causing premature
filter failure and emissions.
In most cases, it is best to slow air into the
baghouse as much as possible, without causing a reduction in the duct
velocity (resulting in dust buildup in the ducts) or the capture velocity
(resulting in dusting at the pick-up point).
Baghouse sizing is critical to a properly operating
system. The air-to-cloth (A/C) ratio is the relationship between the amount
of dusty air per minute flowing through the baghouse and the total amount of
filter media in the baghouse. These are general guidelines and will be
different if a system has high grain loading, frequent temperature
fluctuations, high moisture content or unique dust composition.
Can velocity is important for pulse-jet baghouse
sizing. Can velocity is the speed of dust-laden air as it moves between the
filters. Because a pulse-jet collector cleans the filters on-line (airflow
is not stopped to clean the filters), a high can velocity (above 300 to 350
fpm) will not allow proper cleaning.
Each time a filter is pulsed with air, the dustcake
is momentarily blown off the filter surface and falls down due to gravity.
If the can velocity is too high, the dust is not allowed to fall off the
filtersand into the hopper. It can also cause filter blinding as the small
dust particles that are blown off during cleaning are blown back onto the
filter and eventually forced into the interstices of the filter media.
High can velocity is caused when the filters are
too close together or when air volume is too high within the baghouse. An
effective solution is to use pleated filter elements. Pleated elements offer
more media area per filter than a standard filter bag, which allows an
operator to install fewer filters (spaced farther apart) or to install
filters of staggered length. Either option can reduce problems resulting
from can velocity.
One area often ignored on a baghouse is the
cleaning system controls. In almost every application, filter cleaning based
on DP is preferred over cleaning based on time intervals. This ensures that
the filters are only being cleaned when necessary, thus prolonging filter
life. Changing the cleaning control to be based on DP is simple and
inexpensive.
BHA recommends starting to clean at 4.5 to 5 in.
w.c. and stopping at 4 to 4.5 in. w.c. Cleaning only when necessary achieves
more consistent DP and airflows. With high-efficiency filters such as ePTFE
membrane filters or spun-bonded pleated filters, lower DP levels than this
can be maintained.
The right fan The fan is the engine that supplies
energy to the dust-collection system. An improperly sized fan can hinder the
performance of other components (hoods, ductwork, baghouse). The fan must be
sized to handle a calculated volume of air at a calculated system pressure.
If the fan volume is undersized, it cannot pull the desired volume. If the
fan pulls too much air, it puts unnecessary stress on the baghouse, which
can lead to all types of maintenance problems.
cyclone dust collector
industrial dust collector
jet dust collector
torit dust collector
pulse jet dust collector
used dust collector
If the fan is not sized for the correct system
pressure, it will not pull the correct volume of air. This usually occurs
when existing fans are used on a new application with only the volume
capacity taken into consideration.
Just speeding up a fan will not always result in
more air volume. System pressure must be calculated and matched to a fan
curve.
Dumping the dust At this point in the
dust-collection process, the dust has been captured (hood), transported
(ductwork), filtered and collected (baghouse). Now the unwanted dust must be
conveyed away from the baghouse. To do this properly, screw conveyers, air
slides, and air locks must be sized correctly to evacuate the hopper(s).
Once dust has been collected, it should be taken
completely away from the dust-collection system. If hoppers are allowed to
stay full of dust, commonly, the air stream coming into the baghouse lifts
the dust and takes it back to the filters.
This creates a recirculating dust load in the
baghouse which can quickly reduce the efficiency and life of the filters.
Air locks at the bottom of the hopper must create a good seal. If not, dust
will be reintrained into the inlet gas stream. It acts like a hood, picking
up whatever dust it can.
This makes the baghouse work harder and
deteriorates performance. Air locks must be installed and used as they were
designed. In practical terms, do not take rotors out of rotary feeders or
wire double flap valves open.
If air locks cause maintenance headaches, install
ones that are sized properly. Air locks are reasonable in price and, if
properly sized, can significantly reduce maintenance time.
When experiencing airflow or dusting problems,
check all the components of the system first. Don't settle for a quick fix
in one area. Usually, a quick fix just moves the problem to another area or
puts unwanted stress on the whole system.
Understanding and using system guidelines on all
components, and properly fixing the real problem areas, is the best way to
create and maintain a reliable dust-collection system and to reduce
maintenance costs.
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