Air Filtration by Reverse Flow Dust Collectors

Oleg Systems Co. is a designer and engineering company for industrial dust collectors and dust collection systems. Dust collecting equipment is available in numerous designs utilizing a number of principles and featuring wide variation in effectiveness, initial cost, operating and maintenance expense, space, arrangements and material of construction.

The dust collectors are used extensively in industry for a wide range of applications. They require more space than most other types of air cleaning devices, necessiating outdoor installation in most cases. One of the most widely used type of dust collector is Fabric Dust Collector.

Fabric arresters are high efficiency, medium cost collectors. The effectiveness of passing air or gas through a fabric at low velocity has been recofnized and used for many years in air cleaning devices. Fabric is arranged in envelope or tubular (stocking) shapes. While removal appears to take place by a staining action of the media, in reality dust collection is obtained by building up a mat of the material on the dirty side of the media. This mat provides the actual filtering or straining bed. By means of this bed a high degree of removal is obtained even on sub-micron size particles.

As dust is collected on the fabric, resistance to air flow increases. Periodically the fabric must be reconditioned by shaking, vibrating, reverse jet or reverse-flow collapse which agitates suffitiently to remove the bulk of adhering material allowing it to drop into the dust hopper. In most collectors air flow must be stopped during reconditioning, otherwise released material will be re-entrained and redeposited on the fabric. Sufficient dust must adhere to the fabric to maintain the dust mat needed for maximum efficiency; therfore, after reconditioning the pressure drop will be considerably higher than loss through new fabric. Periodical cleaning of filter media is one of the most important tasks in the dust collector design. Reverse flow cleaning is generally choosen when the volumetric flow of gases is very large.

This method of cleaning inherently requires a compartment design beacause the reverse flow needed to collapse bags entrains dust that must be returned to on-line compartments of the fabric filrh compartment is equipped with one main shut-off valve and one reverse gas valve, whether the system is blown-through or drawn-through. A secondary blower and duct system is required to reverse the gas flow in the compartment to bcleaned. When a compartment is isolated for cleaning, the reverse gas circuit incrases the volumetric flow and dust loading through the collector's active compartments.

The fabric medium is reconditioned by reversing the direction of flow through the bags, which partially collapse. After cleaning, the reproduction of gas is dislodged dust to fall into the hopper.

Reverse flow cleaning reduces the number of moving parts in the fabric filter system - a maintenance advantage, especially when large volumetric flows are cleaned. However, the cleaning or reconditioning is less vigorous than other methods, and the risidual drag of the reconditioned fabric is higher. Reverse flow cleaning of dust collectors is particularly suited for fabrics like glass cloth, that require gentle cleaning. Reverse flow dust collecting bags are usually 8 to 12 inches in diameter and 22 to 33 feet long and are generally operated at low velocities in the 2 to 4 fpm range. As a consequence, reverse air dust collectros tend to be substantially larger than pulse jet dust collector designs of similar capacity.

For ambien air applications, a wooven cotton of polypropylene fabric is the usual selection for reverse flow cleaning. For higher temperatures, wooven polyester, glass fiber, or trademarked fabrics are often used. In recent years glass cloth has come into wide use because of silicone treatment of the glass fibers and employment of reverse flow techniques for cleaning. Silicone treatment provides a lubricant between fibers and increases fabric life.

Reverse flow dust collectors are limited to air conditions dry enough to prevent condensation or free moisture deposition on the fabric. With hygroscoping materials, there will be a bonding tendency between particles and fabric under high relative humidity even though no condensation takes place. Maximum recommended temperature for cotton fabric is 180 F, for wool 200 F. Higher temperatures can be handled by using synthetic materials including nylon or orlon fabrics as well as glass cloth and, occasionally, fine metallic mesh. The synthetic fabrics mau be used up to 300 F while glass cloth is acceptable to 550 F.

Rate of flow through the media varies with dust collector type, application and dust concentration. Ratings are usually selected so pressure drop will not exceed 5" WG. The smaller the particle, the more rapid the resistance rise for a given loading. For the same air flow rate and dust loading, resistance rise increases directly in proportion to time.

For additional information please refer to http://www.olegsystems.com/dustcollector/Index.html.

Oleg Chetchel
Process Air Systems Designer and Engineer
Oleg Systems Co.
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