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Baghouse Dust Collectors for
Air Pollution
Control in the Carbon Black Industry
The production of carbon black presents a unique set of
challenges for dust collection technology. The material itself is light
in weight, with small particles ranging from 0.02 to 0.09 micron. (As
a reference, the average particle size in cigarette smoke is 0.3 micron.)
This requires a filtration system that is extremely efficient.
Because most collection occurs at temperatures in excess of 400°F
(200°C), selection of the filter material is critical. And the product
has a low pH, making equipment and bags susceptible to acidic attack.
Up until recently, the most common equipment used to collect
carbon black has been a reverse-air baghouse. Now, however, AeroPulse
pulse-jet collectors have demonstrated a number of advantages in new systems
and retrofit installations within the carbon black industry. These units
are successfully operating as both reactor and dryer baghouses.
Reverse-air vs. Pulse–jet collectors
Reverse-air collectors in high temperature applications typically use
woven fiberglass bags. Dust-laden gas enters from the inside of the bag
and clean gas exits through the bag wall. However, the woven glass bag,
by itself, is relatively inefficient as a filtration media. In practice,
the bag simply serves as a mechanical structure for the build-up of a
dust layer, or cake, which provides the actual filtration. Periodically,
the bags are cleaned by passing a reverse flow of air through them, a
process which require shutting down a portion of the baghouse. This means
that in order to maintain a continuous output from a reverse-air baghouse,
one extra compartment must be provided. Individual compartments are then
taken off-line, in rotation, for periodic cleaning. In addition, these
units cannot be over cleaned because they rely on the cake to provide
filtration. They also operate on relatively low grain loadings as well
as very low air-to-cloth ratios. When used as a reactor baghouse in a
carbon black application, these reverse-air units typically run at 1.1
to 1.3 cfm of gas to each square foot of filter.
In contrast, pulse-jet collectors use Woven bags with P.T.F.E. Membrane
where the bag itself provides filtration. These bags are supported on
metal cages and the dust-laden gas enters from the outside, with clean
gas exiting from the inside. To clean the bag, a short burst of gas is
injected into the inside of the bag. This pulse consists of about 20-25%
compressed air and 75-80% gas which has just been cleaned. The combination
of compressed air and induced gas creates a wave which mechanically shakes
the dust from the outside of the bag. In addition, the flow of air is
reversed through the bag, removing fine particles trapped between fibers,
and providing thorough cleaning. Pulse jet baghouse can operate at high
grain loadings, with air-to-cloth ratios that are typically 2 1/2
times higher than reverse-air units.
Pulse-jet advantages
Because of these fundamental differences in operating principles, pulse-jet
collectors offer several significant advantages in carbon black applications
including:
• constant, high-volume production throughout the life of the
bags.
• simpler systems, with less operating valves to maintain.
• less susceptibility to corrosion.
• smaller units that require less space to install.
Because woven bags cannot be thoroughly cleaned, the pressure drop across
the bag typically "creeps"up over the operating life of the
bag. As pressure increases, the fan is able to move less gas through the
collector and production decreases. Bags with a P.T.F.E. membrane bags
maintain a uniform pressure drop that translates into a constant production
rate.
In the typical carbon black installation, the loading in the gas stream
leaving the reactor is approximately 18 grains per cubic foot. Because
of their limited ability to cope with high grain loadings, woven filter
systems require a cyclone collector in front of the baghouse, increasing
both differential temperature and pressure. In addition, the need for
a modular design usually results in a four-compartment collector. Pulse-jet
units eliminate the need for the cyclone and its associated airlock valve,
plus the dampers required to segregate different modules. A typical four-compartment
reverse-air system has five airlocks and eight dampers. A pulse jet system
requires a single airlock. With the very corrosive environment present
in carbon black installations, this represents a significant reduction
in maintenance.
Gas velocity through woven bag collectors is significantly lower than
with membrane filters. And, the reverse-air design presents larger metal
surfaces because of its multiple compartments. These factors, plus the
temperature drop in the cyclone contribute to greater cooling of the gas
inside the collector. When this cooling is sufficient to drop the gas
below the dew point, major corrosion problems can result, particularly
in a dryer baghouse.
Because they provide higher air-to-cloth ratios and don’t require
multiple housings, pulse-jet units are more compact than comparable reverse-air
systems. This means they require less space and can be retrofitted quite
easily into existing reverse air installations.
AeroPulse advantages
With 35 years of dust collection experience behind them, AeroPulse
baghouses are designed to provide long life and reliable service
in even the most difficult applications.
Less energy for cleaning
AeroPulse baghouses use 4 1/2 inch diameter
bags mounted on 7-inch centers, as opposed to most pulse-jet designs
which use 6-inch bags on 8-inch centers. The 70 to 90 psi air and
induced gas used to clean the bags results in a pressure of 14 to
18 inches of water at the wall of the 4 1/2
inch bags. With larger bags, less cleaning energy is produced at
the bag wall. (Typically 10 x 12 inches of water). Therefore, more
compressed air is required for the same cleaning efficiency.
Better collection efficiency
The smaller AeroPulse bag design results in greater open space
between bags. Therefore, for the same gas flow through the baghouse,
the up-ward gas velocity is lower in an AeroPulse unit. This means
less dust is re-entrained during the cleaning cycle. In addition,
AeroPulse has pioneered the successful use of high-side inlet connections
which also serve to reduce upward gas velocity. The combination
of these two factors means an AeroPulse collector performs significantly
better in applications involving small, light-weight particulate
such as carbon black.
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Easier bag removal
An AeroPulse baghouse features true top bag removal. The bag assembly,
consisting of bag, cage and venturi, is smaller in diameter than the tubesheet
hole. This design eliminates two major problems frequently encountered
when trying to remove snap-in bags. Because bags can shrink tightly around
the cage, it is often necessary to cut a snap-in bag from its cage before
the cage can be removed. Or, a hole in a snap-in bag can cause the bag
to fill with dust so that it is bigger than the tubesheet opening through
which it must be removed. In either case, the AeroPulse design allows
the assembly to be removed easily. If it is necessary to change a bag,
the process is accomplished outside of the collector, then the assembly
is simply reinstalled.
Longer bag life
The design of the AeroPulse venturi incorporates lugs under the
tubesheet. These lugs serve to provide lateral stability, preventing
bags from moving in the gas stream which can cause wear. Cages are
fabricated of either 304 or 316 SS to minimize corrosion. In addition,
the method used for mounting bags to cages assures a snug fit at
the bottom of the cage despite variations in bag length. Without
this snug fit, bags wear at the bottom of the cage, as proven by
the number of bags which do fail at this point. |
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AeroPulse pulse-jet baghouses are at work right now in over fifty carbon
black installations. They can provide you with a lower installed cost
and better operating efficiencies. To find out more, contact us at the
address shown below.
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