Important Factors in Spark Arrester Selection
(1) Pressure drop across QUENCHER
style of unit is a function of the Reynolds number which is
proportional to the density for air. This means that a unit can be sized
smaller if operating at a higher temperature. For instance a suppressor
operating at 440 degrees F is 2/3 the size of the typical unit applied
at 70 degrees F and the pressure drop will be designed the same. This
lowers the cost of the suppressor. The density is also affected by the
water vapor in the gas stream. It has little effect at temperatures
below 125 degrees F but can be a major factor when operating at higher
temperatures.
(2) If the gas steam has dust that might drop out in the duct at the velocities in the blender style or QUENCHER suppressor, a booster must be provided to periodically remove this accumulation. If this unit is not kept clean it might pose a threat by putting an extra load on the duct-work. Without an Automatic Booster System, the suppressor might require periodic manual cleaning.
(3)
The booster design is also temperature sensitive and must be altered to
accommodate changing gas steam conditions. Most suppliers do not have
the capability to modify these booster designs.
There are several approaches to the issue of extinguishing sparks in a gas stream.
Cyclone Dust Collectors
Contrary
to common belief Cyclones are not an effective spark arrestor. For a
spark arrestor/cooler to work, there must be turbulence to be effective.
If you have turbulence in a cyclone pressure drop is very high. They
are designed to avoid turbulence. Many bag house fires occur in systems
with cyclone pre-cleaners. Amazingly the inlet baffles on the bag-house
are more effective as spark arrestors, however they are not foolproof.
Static Baffle-Box Spark Arrestor
Many
dust collector suppliers offer this type. It consists of air entering
at one end of a baffle box running over a baffle plate which drops out
the sparks and much of the dust collected.
The air exits at the other end, and then travels to the dust collector.
The big drawback is that a hopper and flexible or solid hose connection
to a collection barrel is required. Also, these devices do not
eliminate all of the sparks. There is not enough turbulence generated
to ensure hundred percent spark arrestance. Sparks may ignite the
contents of the collection bin
Mesh Filters
This
is a common stop-gap measure where the filter is placed at the exhaust
duct of hoods or installed in the duct-work. When clean, the mesh filter
will stop at best 80% of sparks. These filters do not produce enough
pressure drop to be fully effective. It only takes one spark to ignite
dust in the duct or set a dust collector on fire. The only thing these
filters do is clog up and add to your maintenance.
Blender Type Air Mixers
A
number of these air blender/mixers have been applied successfully as
spark coolers and suppressors. Over the last 5-6 years standard air
mixers have been adapted and applied between the spark generating
process and dust collector. They were applied in processes where fires
in the dust collectors had previously occurred. One supplier hired a
consultant to develop a market for these air blender/mixers as a spark
arrestor/cooler. This blender design was an outgrowth of mixing two gas
streams of different temperatures to insure a uniform temperature after
the static mixer. It was deduced that the gas stream produced turbulent
flow as it passed through the blades and this was the reason it could be
adapted to spark cooling. However, these are air mixers first and spark
arrestors second. There are performance limitations because not enough
turbulence is imparted to the spark ember.
Improved Spark Arrestors
QAM developed the QUENCHER,
which is a variation of the blender/mixer design. Employing a 60 year
old spin vane mist eliminator technology developed by Sly Manufacturing
in the early 1960’s, led QAM to vary the blade designs to have the most
effective performance, inducing maximum turbulence to the gas stream,
and lowering the cost. Maximum turbulence is the key to spark
arrestance. After several tests it was found that the air blender/mixer
design did not impart enough turbulence and some sparks got through,
especially at low gas stream velocities. Eventually, there was a
specific design which imparted the most effective swirling and
turbulence thereby extinguishing the sparks quickly and most
effectively. In fact, during testing of the QUENCHER,
the arrestor cell would light up as a ball of fire, however, one inch
past the cell nothing was left in the gas stream. These designs were
incorporated into the QUENCHER.
QAM has developed special application data in which the blade angles
are adjusted to produce minimum pressure drop for different temperatures
and gas densities. To our knowledge, no one else accounts for the gas
density effects on spark arrestors. In truth, due to the advanced
design, even applying the incorrect parameters to a QUENCHER
may not result in a failure to put out sparks. Since the pressure drop
across the blender and mixer are a function of the velocity through the
device, the development of a pneumatically operated booster was
introduced to prevent dust dropout accumulating in the static
blender/mixer. It also blows out accumulations on the blades.
Read more: Quencher Spark Arrestor
Liquid Spray Systems.
For
many years these systems were the only available systems to prevent
fires caused by sparks. The system consists of electronic detectors that
detect sparks and react to their presence. When a spark is detected
liquid sprays are actuated and water sprayed into the duct. The sprays
actually cool the gas stream below the dew point. However, in dust
collection systems, the water then wets the filter bags or cartridges.
This prevents fires but the gas flow is interrupted and the bags must be
either replaced or dried out before the process can resume. The
detector sensitivity can be lowered to prevent excessive actuations,
but, this reduces the reliability of the systems. The detector missing a
spark is an ever present danger and a fire may occur. Bag or cartridge
replacement is definitely required.
Static Blade Spark Suppressor (Tri Pass)
These
were developed in Japan to replace multiple cyclones in Coal fired
boilers. They found that the multiple cyclones did not stop sparks from
entering the dust collectors. The first ones were installed in the early
70’s. They ran at 1.5 inches of pressure drop and were fabricated from
structure angles to resist the wear of the abrasive ashes in the coal
that they fired. There are several of these applications installed in
the USA designed by one of our colleagues.
We
trust that the above information will enable you to evaluate and select
the most suitable method and supplier for your application. Buying our QUENCHER/BOOSTER combination will give you a risk free unit, fine tuned for each application.
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