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Pulse Jet Pointers
Vol. 2, No. 6

The Big Bad Bag Bamboozle

 Here are some common pulse jet bag/cage sizes:
4-1/2" dia.'s
Cloth area (sf)

5-1/2" dia.'s
Cloth area (sf)

Knowing the cloth area, and selecting an air-to-cloth ratio, you can determine CFM capacity, but air-to-cloth ratio is not the full story. Two hundred feet per minute upward velocity under the bags (sometimes called "can velocity") is the upper limit, regardless of how "easy" the dust might be. In fact, fine dust (minus 44 micron/325 mesh) might not clean off the bags if the upward velocity is over 100 feet per minute. Therefore, the air-to-cloth ratio is limited if long bags are used.

A third factor comes from shaker bag experience: Aspect ratio.

Short fat bags have a lower aspect ratio than long skinny bags. The practical fact here is that a long skinny bag will have too much internal velocity to clean properly.

Finally, we come to "gross cloth." To beat the competition, some manufacturers have claimed over-diameter bags (6" diameter on a 5-1/2" cage or 5" diameter on a 4-1/2" cage). Unfortunately, the air knows how big the cage is, and the bag is going to pull down on that cage as soon as the air starts through. In fact, an overly large bag is going to suck into the spaces between cage wires and increase the aspect ratio.

Some bag makers even add in obstructed areas, such as over the bag collar and the bottom disk, to gain more "gross cloth" area. We rate our cloth area on cage size, not six to twelve percent higher based on "gross areas."

The Sneaky Scrim Scandal

Scrim is a cheesecloth-like material weighing about 1-1/2 ounces per square yard upon which felt fibers of a pulse jet bag are needled into a mat. The scrim is usually of the same material as the felt itself. Fairly recent technology has made it possible to eliminate the scrim, but the scrimless bag will not hold up to flexing and abrasion. The usual parameters, such as permeability or ounce weight will not reveal the presence or absence of the scrim. Two things will tell— examination of the cloth or premature failure of the bag. People don't do the former, so they get the latter. The savings in initial cost (about 20%) will hardly justify using this bag, especially if you consider labor and downtime.

Incidentally, if the bags are Nomex, another way to cheapen the bag is by adding glass fibers, as much as 20%, which also shortens the service life.

Our survey of ten major bag suppliers revealed a willingness to furnish scrim or scrimless material as specified, if specified, and to furnish scrimless material if not specified. Inquiries seldom specify, so we are left with the problem of either "getting out the word" or taking our lumps!

The Air-to-Cloth Ratio

Many magazine ads these days claim new Blow Pipe & Venturi designs that will "boost the air/cloth ratio." As pulse jet technology comes of age, there have been many divergent patterns of Blow Pipe and Venturi design. Please don't buy it for its pleasing appearance. The bottom line is will it clean the bags? Our design efforts have held that factor as most important. Another important feature of any system is service life. Even if all the apparatus is located on the "clean side," bags still do break, and all that fancy technology gets an abrasive blasting job! We use Schedule 40 blow pipes 1/4" thick abrasion resistant cast aluminum Venturis — both items more than twice as thick as most other manufacturers, and seven times that of some!

The Electric Fan

Over one hundred years ago, in 1885, they invented the electric fan. It was, to quote one authority of that day, "a very refreshing invention." It stood a foot high and ran on batteries. We have come a long way!

Oil Mist & Smoke Collector

Beckert & Hiester's Oil mist & smoke collector for mineral base and water soluble cutting oils from machining operations have efficiencies to 95% in the NBS test. Sizes starting at 800 CFM. No high voltage power pack or ionizing wires to fail. No arcing to cause fires. No cell plates to clean. For more information call us (toll free).

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Since 1921, Industrial Dust Control, Ventilation and Process Equipment

Beckert & Hiester, Inc.
P.O. Box 1885
Saginaw, MI  48605-1885
Local and International - 989-792-3443 or 989-793-2420 Toll Free - 800 332-4031
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