One micron lime dust particles fall like feathers through the air at about 1.5 feet per second. Heavier and larger particles fall faster, smooth lead cannon balls terminating at around 200 feet per second.
An "off-line cleaning" type collector — theoretically —could be restarted after just five or ten seconds, depending on the depth of the hopper. We should allow for cross drafts caused by various leaks, and Brownian motion, so a minute is usually fair, and some engineers will shut down for five to be certain— except for those who collect lead cannon balls.
For continuously cleaned Pulse Jets equipped with the traditional inlet into the hopper area, the dust must fall against a rising air stream. A high inlet may be used, leaving the hopper fairly quiescent and sending the dust-laden air stream into the bag area. Sometimes this is not a good idea with abrasive dust. Many times, such as with bin mount collectors, it is just not possible to have a high inlet. Therefore, we must consider "can velocity."
Most bag collector inquiries occur because there is a difficult dust floating around the work area, causing problems for people and equipment. Floating dust means submicron or at least single digit micron fractions. An upward velocity, such as 1.5 feet per second, is often specified, not allowing for the area taken by the bags. The usual terminology is "1.5 feet per second under the bag area." As an example, a 10,000 CFM dust collector would therefore have a housing about ten-and-a-half feet square or, if cylindrical, slightly less than twelve feet in diameter,
For usual bag-packing ratios of about 35 percent (5-3/4" diameter on
8" centers or 4-3/4" diameter on 7" centers), the bags and cages need only
be about five feet high, to establish a conservative 5:1 air to cloth
ratio. Yet we see dust collectors being quoted - and alas, purchased -
with bags and cages up to fourteen feet long. Obviously, even at
ridiculously low air/cloth ratios, the can velocity is going to be too
high and the dust will not come down until the collector is shut off
(unless you have smooth lead cannon balls).
Thereupon entering the "solutions" of off-line cleaning, with dampers that leak, stick and get out of synchronization; or high inlet configurations that usually signal a short and tumultuous bag life. The smart money goes with the short bags. The yield is long bag life, lower pressure drop and higher efficiency for the lowest dollar expenditure over the life of the dust collector. Then too, ask the maintenance crew -- which bags would they rather change, the long ones or the short ones?
In 1273 the then King of England passed the very first air pollution law — as far as we know — and this concerned the emission of smoke from the burning of sea coal. In 1307 a blacksmith was convicted of violating this law. He was hanged forthwith.
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& Hiester, Inc.
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Saginaw, MI 48605-1885
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