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Yes. Look at the two filter bags shown. Both have the same diameter, but the one on the right is longer. Let's assume that each application is using the same cloth ratio or cloth velocity. Dust laden air enters the bottom of each filter tube and if the bag on the right is 25% longer than the one on the left, it will have 25% greater cloth area. Therefore, the speed at which the air enters the filter bag will be 25% greater. This means that much more dust particles will be carried into the bag, requiring more frequent regeneration, as well as being susceptible to plugging. As a 'rule of thumb', the bag length should not exceed 12 times its diameter.
A further consideration in filter bag design is the method employed to keep it taught. If bags were always installed by people familiar with dust collectors, any system would suffice. However, most bag replacement is usually carried out by plant personnel and the ease with which this can be done correctly becomes important. APSCO baghouses employ a spring in tension, holding the bag to the shaker grid. This ensures that the bag will always be kept taught regardless of the installation procedure, humidity, temperature, shrinkage of filter tubes, or normal variation in bag length.
In recent years most dust collector manufacturers have used a snap ring collar arrangement to fasten the filter bag to the baffle plate. This is simple and quick, but it does put the filter bag into a "capture market" category. Filter bags are now universally manufactured by three or four "outside" vendors for dust collector manufacturers.
As dust is collected by the collector, the particles build up on the wall of the fabric. This dust buildup forms a filter aid, increasing the efficiency of the filtering system. As this happens, however, the pressure required to move the air through the filtering media (cloth and filter aid) increases until the velocity in the ductwork drops below conveying values. Before this happens a regenerative cycle must be employed to remove the filter aid and start the cycle all over again. Three common methods are employed:
a. Shaking the filter bag.
b. Low pressure reverse airflow. Modular systems employ this method, with one module reversing the air flow while the other modules continue their normal operation. c. High pressure reverse airflow. These units are commonly referred as "pulse jet" collectors.
Methods (b.) and (c.) above, allow the systems to operate on a continuous basis. Method (a.) above requires the system to be taken out of service for regeneration. It is the least expensive, and is commonly used on small to medium sized woodwork applications. Here regeneration is usually automatic, with an electrical interlock on the start/stop switch. Every time the system is stopped, a timer activates the shaker mechanism and while the bags are shaking, the system cannot be put back into operation. You can shake your filtertubes either slowly with large shaking movements, or quickly with smaller vibrations. 'Me second system (used on APSCO units) is less stressful, thus prolonging the life of filter tubes. The shaking cycle is usually performed four times a day, at coffee breaks, at lunch, and at close of the day's activities. We do not recommend a manual filter shaker.
As often but no more than necessary. That sounds like a silly answer, but in fact the required time between filter tube replacements varies with each application. The best answer is to follow this guide: Keep one extra set of filter tubes on hand. Then once a year program a replacement day (preferably on a warm summer Saturday) and replace all the filter tubes. Now the used ones can be cleaned in lukewarm water or drycleaned by a commercial cleaner. Inspect and replace or repair any damaged filter tube and store them in a dry location for the next summer's replacement day. With the spring tension system (see Question 8 above) slight shrinkage will not preclude reusing the filter tubes. This process will result in the most economical and troublefree operation of your system.
If you are using a filter-fabric type of dust collector, the answer generally yes. However, if the dust collected is carcinogenic, you cannot. Certain woods such as mahogany fall into this category. Check with your local authorities if you have any questions. In this case a simple indoor/outdoor splitter damper in the return air ductwork allows you to exhaust or return the air at your option. Returning the air can save a great deal of your heating costs.
Many fine dusts including wood dusts are highly explosive. APSCO, at the request of the Underwriters insurance company designed an explosion vent/access door. This access door is installed on all APSCO dust collectors. These doors are sized to conform to a maximum 30/1 volume/area ratio.
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