New Engineering Insights of Pleated Cartridge Dust Collectors

Cartridge collectors have been available for over 35 years and have revolutionized the continuous cleaning pulse jet collector technology. They have reduced the emissions coming through dust collectors from typical processing operations like material handling, weld fume venting or an abrasive blast operation.

Pleated cartridge dust collectors reduced emissions coming through the collector outlets from 20-30 milligrams per cubic meter to 0.030-0.040 milligrams per cubic meter with inlet dust loadings from 900 to 1200 milligrams per cubic meter.

To recognize the new insights, it is useful to understand how this breakthrough was achieved.

Background

Pulse jet cleaning collectors with envelop or cylindrical bags, designed prior to 1978 were similar in that they used so called venturies in the bag cages.  The venturi squeezed the reverse flow cleaning jets to accelerate the cleaning jet entering the filter elements of felted media. Referring to figure 1 below, the result was that this high velocity jet ejected the dust from the filter cake on the media and propelled it towards adjoining rows of bags, which were in the filtering mode. These velocities ranged from 28,000 to 40,000 feet/ minute. As long as the pressure drop is below 2”WC across the media the collector runs with very high efficiencies, similar to those for a cartridge or mechanical shaker collector. This low pressure drop usually can be achieved with low density dusts such as fine paper dust. For other dusts, the pressure drop will rise to 4-6”WC which indicates a velocity at which it can travel. At  6”WC this velocity can be traveling up to 28,000 feet per minute with 50 lb/ cu.ft. dust density.

Figure 2 shows the action on a pleated cartridge. The air and dust is ejected from the media cake surface perpendicular to the surface. The dust is directed toward a surface that is in the cleaning mode so the dust will not penetrate through the cake. This is the reason why a pleated filter element can reduce the dust penetration by over 98% compared to a standard felted pleated bag. Figure 3 illustrates the effect of the pleated element when the filter is not designed to clean effectively. The dust collects in the valley of the pleats. When the reverse jet is activated the cleaning air takes the path of least resistance. The portion of the pleat below the bridge is not cleaned and the dust remains on the cartridge.

It has been believed erroneously for over 25 years that operating a pulse jet cartridge filter at a low filtering speed increases the collection efficiency of the filter element. There is an element of truth in this belief. The truth is only the filter area above the pleat is able to be cleaned. If a pleated filter element bridges 80% of the depth of the pleat, it is actually operating at a filtering velocity 500 % higher than the engineered filtering velocity. Not only is the filter media under the bridge not cleaned it raises the operating weight radically. Many pulsed collectors have filter operating weight of 65-80 lbs higher than the virgin filter. The operating weight for a cartridge filter with 360 square feet and a 1/64 inch thick filter cake, with density of 50 pounds per cu.ft, is 25 pounds. In a typical collector rated at 7000 ACFM with 20 cartridges, and ten valves, the excess weight of the dust is 800 pounds or so. Replacing the cartridges exposes personnel to health hazards during handling.

The correct insight is that the pleat spacing should be wide enough so that all of the media could be cleaned by the reverse air cleaning jet. Extensive lab and field tests have uncovered that 220 square feet of media can be cleaned by a 1-inch diaphragm valve at two inch pressure drop and a permeability of the media of 20 CFM per sq.ft. This means that for the tandem cartridge set, described above, where the cartridge set has 360 sq. ft. with 16 pleats per inch, only 75 percent of the media can be cleaned.

Figure 4 illustrates the results of applying this design into a typical application over time. These depictions were drawn from photos taken a week apart through an access door. The white portions of the sketches are the cleanable media.

For best operation these cartridges should have 25 % of the pleat spacing or 3.5 pleats per inch. Figure 5 shows cartridge designs with good pleat spacing of 1/4 and 1/2 inch.

Media Selection
There are two types of media that are usually applied to pleated cartridge filters.  These are cellulose based media, often reinforced with synthetic threads, and spun bond polyester media.

The cellulose media usually have sufficient stiffness to keep their shape without pinching in the tops or in the valleys of the pleat so that filtered air can flow unimpeded into the cartridge during flow reversals. They are usually banded or wrapped with an outer expanded metal or perforated core to prevent the pleats from inverting during the changes from filtering flow to cleaning flow. One limitation of cellulose based media is that humidity affects the dimensional stability of the media. As the humidity changes the length of the pleat changes enough that the pleats get curved and sometimes even crease. These are stress points that can cause premature failure from the cycling of the cleaning system. While cellulose media can be laundered, each laundering cycle changes the permeability of the media. It is only effective to launder these cartridges twice before 60% of the permeability is lost and the cartridge reaches the end of its useful life.

The first spun bond medias, as applied to pleated cartridges, were very flexible. When first started and operated at pressure drops below 1”WC, they were very effective even at filtering velocities of 12 to 14 feet per minute. Unfortunately, as the pressure rose, the tops of the pleats collapsed and had the same effect as bridging except in reverse. The tops of the pleats were rendered useless. The spun bond media could be laundered indefinitely with no loss of permeability. Recently a new spun bond media has been available. This new spun bond is constructed of a spun bond which is stiff and does not deflect. This allows the collector to operate at a low pressure drop with unlimited cartridge life and can be returned to “like new” condition by cleaning off-line. Spun Bond cartridges can also be laundered an indefinite number of times making them permanent filter elements.

Pressure drop                         Compressed air usage at 85 psig
(across the cartridges)             (needed for cleaning)           

0.90 inches w. c                        0.4 SCFM per 1000 CFM of filtered air
1.5 inches w. c.                         0.45 SCFM per 1000 CFM of filtered air
2.5 inches w. c.                         0.90 SCFM per 1000 CFM of filtered air
3.5 inches w. c.                         1.20 SCFM per 1000 CFM of filtered air

Conclusion
The designer can select and specify pleated cartridge elements that are smaller in size and more efficient by applying the engineering insights listed above to the collectors he operates. Even existing collectors can be modified by changing the cleaning systems and installing the optimum configuration of pleated cartridge design.

For more on ... Advanced Technology Dust Collectors