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Mechanical Cleaning Dust Collectors

Shakers & Mechanical Cleaning Dust Collector

Mechanical cleaning dust collectors are used in processes where non-cleanable filters become an expensive option due to large dust content. It consists of bags with an inlet for dusty air stream at the bottom. The bags are suspended from a rope for improved agitation and fixed at the bottom. Dust filled air enters the bag, passes through the filter and collects on the inside walls of the bag. As dust collects inside the bag, the pressure drop rises (initial pressure drop could be at 0.1 to 0.2 inches w.c.). When pressure drop reaches between 2 to3.5 inches of w.c., filter media is cleaned by pulling up and down on the pipe (by shaking). After cleaning, pressure drop readjusts to 0.5 – 1.0 inch w.c.

The fan is on the dusty side and its paddle wheel moves the air and receives incoming dust. Used in manual paper trim operations, these shakers used sateen weave cloth as filter material because of the tight weave and flexibility.
 

While selecting mechanical cleaning shaker collectors the basic criteria to be considered are:

• filtering velocity
• dust load
• residual pressure drop

Since time is consumed while cleaning, the inlet load into the dust collector must take into account (a) the time required for cleaning and (b) intervals of time between cleanings. For example, inlet load capacity could be 1000 CFM where cleaning is done twice in an eight hourly shift. But where cleaning is done only once in an eight hourly shift a capacity of 500 CFM would suffice. It is important to remember that the residual pressure drop is related to dust holding capacity of filter media. Dust holding capacity of filter media is related to dust characteristics.

Envelope Filter Bags

The collector bags are sewn into envelopes in this type. The envelopes are provided with spacers made of wire inserts or open foam between them to keep the filter media from collapsing. Envelope filter bags save space. Cleaning can be done by manual shaking or automatically by motor power. Springs are attached to the closed end of the envelope bags to keep the bags taut. This arrangement prevents filter media from stretching after cleaning cycles. Spring tension must be checked and readjusted at appropriate intervals

Envelope filter bags are effective at transfer points on belt conveyors, screening, clamping stations, grinding and abrasive blast cabinets where dust is mechanically generated. However their efficiency suffers in processes such as dryers and furnaces where dust particles get stuck in the filter media. This drawback is because the cleaning mechanism in these processes is not good enough to sustain filter element life and low residual pressure drops.

Tubular Shakers

Tubular shakers are tube shaped dust collectors that effectively handle the problem of dust particles that get stuck in the filter media. In these type of shaker collectors, an inlet opening is provided at the bottom of the bag. Dust filled air enters through the inlet and dust particles collect on the inside of the bags. The cloth on the bags is kept taut by the use of a tension device. Cleaning process is a variation of the shaker mechanism.

The high cleaning efficiency of this type of shaker collectors is due to the high ratio between the bag opening diameter (3-12 inches) to length of the bag (20-35 inches). This provides tubular shakers with better cleaning than other shaker collector types.

These shakers however require continuous cleaning, which stops the industrial process. To counter this drawback, a continuous cleaning compartmental collector system was designed with separate modules inside the collector with separate ‘dampers’. This facilitates cleaning of individual modules by closing a damper while directing the flow into other compartments, thereby keeping the process continuous. Individual compartments can be thus cleaned ‘off line’ by stopping fan flow into that compartment. Compartments can range from 2 to 20.

Continuous cleaning shakers hold an advantage in low volume applications and in environments where compressed air is not available for filter cleaning requirements. They however suffer from the drawbacks of the ‘off line’ cleaning process and high maintenance since they have large number of internal moving parts. The low air to cloth ratio makes these collectors larger in size and they are more expensive than other collectors.