Pleated Bags, cause problems

Several suppliers are promoting pleated bags to replace existing cylindrical bags in baghouse dust collectors. The effects of these changes are often startling.

First, we discuss the “good news”, about the effects of these changes : 

1. Collection efficiency increases which enables the users to decrease penetration of dust to the levels well documented on pleated cartridge units. The potential is outlets less than 8 x 10-5 grains per cubic foot with grain loadings below 15 grains per cubic feet. At 50 grains in pneumatic conveying applications, the outlet could still be under l0 x 10-5 grains per cubic foot. To achieve these numbers pressure drop must be kept below 6 inches water column. 
2. Bag lives can double or triple compared to cylindrical bags. 

Next we discuss the “bad news”, on the effects of these changes as they were observed:

A) The pleats on the bags were often bridged to 80-90 % of the pleat depth.

B) Users changed the pressure drop reading gauges from the 0 to 6 inch WC. range to 0 to 15 inch WC range. The reason was that the 0-6 inch gauges were pegging and pressure drops were running consistently over 12 inches.

C) The compressed air consumption which is a function of the pressure drop across the filter elements was two to four times higher than with the old cylindrical filter elements.

Conclusions:

i. The filter life is improved over the old cylindrical filter elements which were replaced.

ii. On many applications such as “bin vents” on silos venting pneumatic conveying these pressure drops will open the silo relief vents.

iii. These “bad news” effects are due to one important fact relating to application of pleated filter elements. They will capture very fine dust particles, which were formerly leaking through the filters, because of the jet velocity of the reverse air jet. With pleated filter elements these particles are collected. The collection of the finer particles is the reason for the lowered dust penetration through these elements. These finer fractions agglomerate poorly and require very low upward “can velocities” to fall into the hopper after each jet cleaning cycle. If the upward “can velocity” is too high the fine dust remains as a very low permeability filter cake. This increases the pressure drop and cleaning frequency dramatically. Once a certain pressure drop is reached, which is related to the physical properties of the dust, the dust is driven into the filter media until even “off line” cleaning will no longer restore the permeability of the media to a normal expected pressure drop. This drastically reduces filter element life.

iv. In general, the cleaning system of dust collector is not designed to work with pleated bags.

v. We have developed techniques to improve the operation of most collectors using these new filter elements. These involve lowering the “can velocities” in collectors using pleated bags. We will cover these techniques in a subsequent “service report” and “engineering bulletin”.

Please submit the particulars on your dust collector pleated filter element or cylindrical bags, to our technical support team for recommendations.

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For more ...  Retrofitting Existing Baghouses or Retrofit Cartridge Dust Collectors