There are four groups of variables that influence dust
collector performance. They are variables regarding:
1.
dust,
2.
fluid,
3.
operation,
4.
Dust Collector.
Variables regarding dust are size, shape, density of
dust particles, and agglomerating tendency
Variables regarding fluid are density and viscosity.
These are functions of air or gas composition, temperature, and barometric
pressure.
The effect of dust and fluid variables can be
simplified by using the terminal velocity theory. Terminal settling velocity,
commonly called terminal velocity or "settling velocity”, is the velocity
of a particle, falling in still air or another fluid, after it stops
accelerating. So it is clearly determined by dust and fluid’s properties.
Variables regarding operation are gas velocity through
the collector, or capacity in cubic feet per minute, and dust loading,
generally expressed in grains of dust per cubic foot of gas.
There are an infinite number of collector variables.
With a collector of a given design, the cumulative
effect of all these groups determines the shape of the fractional efficiency
curve.
Interstitial Velocity and Can Velocity
When sizing a pulse-jet dust collector, it is
important to consider not just air-to-cloth ratio (Filtration Velocity) but
also upward velocities, interstitial velocity and can velocity.
Upward velocity occurs when a hopper inlet is used on
a pulse-jet or a reverse-air baghouse. Dusty air is introduced into the hopper
and travels upward into the filter housing where cleaned gas passes through the
filter bags and dust is deposited on the exterior of the bags.
Can velocity is the vertical flow velocity above the
hopper level, but before reaching the bottom of the bags.
Can velocity is commonly confused with interstitial
velocity, and with good reason. These are practically the same when the filter
bags extend down to the hopper level.
Interstitial velocity is defined as the upward
velocity of air through the open area between the filter bags inside a dust
collector. Interstitial velocity changes in value from its maximum at the bottom
of the bags to zero just below the tube sheet.
The Importance of Terminal Velocity
If a particle’s falling velocity is zero at the
position of maximum interstitial velocity, then it will become suspended there.
Theoretically, we can assume the falling velocity of a
particle or an agglomerate of particles to be the sum of the terminal velocity
and the interstitial velocity. So, at this position, if a particle’s falling
velocity is zero, then the particle’s terminal velocity is equal to the
interstitial velocity.
Any particle
that has a terminal velocity bigger than the maximum interstitial velocity will
overcome the maximum buoyancy at this position and fall into the hopper, while
any particle that has a terminal velocity less than the maximum interstitial
velocity will be re-entrained by the upward flow and carried back to the bag
surface. The results can be a high pressure drop, excessive use of compressed
air, and shortened bag life.
We can also assume that a particle’s terminal velocity
is unchanged in the distance of the whole bag length while the particle or
agglomerate is falling down after being pulsed off the bag surface.
When a baghouse’s interstitial velocity is equal to some
particle’s terminal velocity, the corresponding equivalent particle size can be calculated. Any
particle that is bigger will overcome the maximum buoyancy at this position and
fall into the hopper. Any particle that is smaller than this size will be
re-entrained by the upward flow and carried back to the bag surface.
At the same time, any dust particle
carried by the upward air flow from the hopper and with terminal velocity
bigger than can velocity may not reach the bag surface and fall out into the
hopper without filtration. The critical diameter of the particle whose terminal
velocity equals to the can velocity can be calculated as well.
As long as the particle size
distribution of the dust is known, then the weight of the dust that can reach
the bag surface can be estimated. Afterwards, whether a pre-cleaner is needed ahead
of a baghouse can be decided. Note: The choice of whether a pre-cleaner is
needed is decided by how much dust will settle on the surface of the bags, not
by the inlet dust loading.
Having dead space under the bag array provides a low
can velocity, creating an internal dropout chamber that helps distribute and
minimize horizontal flows that can cause abrasion problems at the bottom of the
bags.
This is the reason why interstitial
velocity and can velocity are so important in baghouse design. They are
implicitly related to particles’ terminal velocity.
How to Obtain Terminal Velocity
Terminal velocity in air can be determined directly by
air elutriation, or similar methods, such as using the Roller Elutriator, or
the Bahco Micro Particle Classifier.
If experimental data is somehow not available, then we
have to use an empirical formula to estimate the terminal velocity of a
particle or an agglomerate of particles. When dust is pulsed off the bag
surface, they are normally agglomerates of particles.
What Airvate Can Help
If you need to
know the terminal velocity for your dust in an application, dust collection or
pneumatic conveying, please contact Airvate through email. Airvate knows how to either
estimate or precisely calculate it. After so many years in baghouse design and
calculation, Airvate has accumulated a lot of terminal velocity data for
different kinds of particles in air.
If you need transport velocity, saltation velocity, and pickup velocity in an application, dust collection or pneumatic
conveying, please contact Airvate too through email.
Field Measurement Services Airvate Can Do
If your baghouse or cartridge filter has a performance problem, Airvate can do some measurements and calculations, the result will definitely help to find the root-cause of the problem. “Measurement facilitates root-cause analysis that leads to solutions of problems,” so told us by the quality guru from the 20th century, W. Edwards Deming.
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