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Section 4 DUST FILTERS A pneumatic system in a grain elevator exhausts to the atmosphere through an air-cleaning device that removes the collected dust for disposal. The most effective air-cleaning device now used in elevators is the fabric filter (sometimes called baghouse). A less effective device, the cyclone, is also used. (See Section 2 for a general comparison of these and two other devices, wet scrubbers and electrostatic precipitators.) The great advantage of the filter is its efficiency. The device efficiently collects large particles as well as very small (submicron) and very light particles. The cyclone, on the other hand, does not collect fine or very light particles effectively. A well operated filter collects particles at an overall efficiency of more than 99 percent. Cyclones, in contrast, are 60 to 95 percent efficient. Particles small enough to be inhaled tend to pass through a cyclone uncollected. Because such particles have become an environmental concern, the use of cyclones in the grain industry has been declining. However, the devices are often used in series with filters. This dual arrangement is covered at the end of this section; otherwise the section is devoted to fabric filtration. Fabric-Filtration Practice A typical filter is shown in Figure 4-1. The fabric is in the form of stockings (tubes or rocks) in the figure, but other designs employ fabric leafs (envelopes). As the filter operates, a mat of dust builds up on the dirty side of the fabric. It is this mat that filters dust from the entering air. Resistance to airflow increases as the mat builds up, so the accumulated dust must be removed periodically. It may be removed by shaking or vibrating the fabric or by use of air in reverse flow, reverse jet, or reverse pulse. The dust dislodged falls into the lower chamber, or hopper, and is discharged through the air lock. At intervals, all bags or stockings must be removed from the filter and cleaned thoroughly. Air-to-Cloth Ratio An important factor in the performance of a filter is the air-to-cloth ratioâthe cubic feet per minute of air being filtered per square foot of fabric in the filter. The best air-to-cloth ratio for a filter depends on factors such as the dust being handled, the design of the filter, and the filtration fabric, or medium. 29
30 Valve Body Assembly: -Plenum Chamber -Butterfly Valve Reverse Air Pressure Blower âr- Rotating (_J Chamber Drive Magnehelic Gauge ; I- .- / Dust '.' ' -â ; Laden '. Wire Sleeves with Filter Stockings Bin Level Indicator Rotary Air Lock Discharge FIGURE 4-1 Typical fabric filter.
31 Filter performance and optimum air-to-cloth ratio also vary with humidity and the salt content of the air. However, humidity and salt content are high enough to be significant only in a coastal area reaching from Texas to Maryland (Figure 4-2) . This coastal belt extends 50 to 100 miles inland. Filters are generally classified as high-ratio or low-ratio. High-ratio filters use felt media with continuous cleaning, such as reverse-pulse or reverse-flow cleaning. Air-to-cloth ratios in high-ratio filters should not exceed 9 to 1 in the coastal belt described above. Elsewhere in the United States, they should not exceed 11 to 1. Low-ratio filters use woven media cleaned by shaking, gentle reverse flow, or intermittent methods. Air-to-cloth ratios in low-ratio filters should not exceed 2.5 to 1 in the coastal band (see Figure 4-2) . They should not exceed 3 to 1 elsewhere in the United States. (See also last paragraph of this Section, under Cyclone-Filter Facilities.) Air-to-cloth ratios higher than recommended here can be used, and the filter can be made correspondingly smaller. However, higher ratios increase maintenance and operational problems and shorten bag life. They also increase pressure loss across the filter and so increase energy costs. Furthermore, ratios higher than recommended above sometimes give less effective dust control. Lower air-to-cloth ratios tend to minimize maintenance, increase bag life, reduce pressure loss and energy costs, and give generally more trouble-free operation. Caution should be used with air-to-cloth ratios in elevators handling any product other than whole grain. More conservative (lower) ratios should be considered for products such as malt, beet pulp, oil seeds, and low-density powders. Filter Collector Location Selection of a location for a filter at a grain elevator should be guided by the distinct hazard involved. The filter will contain a large amount of grain dust that can support either a fire or an explosion. Therefore, it is highly advisable that filter collectors be located so as to present the minimum exposure to elevator personnel. A bag filter collects the smallest and driest particles of dust in the elevator. This dust makes excellent fuel. Only an ignition source is needed to set it on fire. Because the fire would occur in the confined space of the filter collectors, airborne dust in that space could explode. Explosible concentrations of dust may easily be dispersed into the air by pulse-cleaning or by an explosion propagating into the filter. An explosion in a filter produces a fireball of significant size, although we have no way as yet to predict its exact size. Venting may considerably reduce damage to a filter in the event of an explosion. However, venting also will markedly increase the size of the fireball but venting must be done in such a way to minimize danger to personnel.
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33 A filter should be situated for ease of maintenance, as well as in the interest of safety. If the unit is installed off the ground, maintenance can be eased by use of catwalks and ladders. Instruments for checking the performance of the filter should be in the elevator's control room where they can be monitored continually. A baghouse should also be located so that it can conveniently feed the dust-storage facility, whether by gravity or mechanically or pneumatically. A final consideration is noise from the exhaust fan. However, if the filter is located remotely because of the explosion hazard, noise should not be a problem. Filter Materials and Construction Materials and type of construction for a bag filter should be selected on three general grounds; ease of assembly and installation; service life; and maintenance requirements. The optimum materials and construction may vary for particular installations. With any filter, however, attention should be paid to the following points; 1. Magnehelic gauge or manometer. 2. Gauge of housing. 3. Type of service platform and access ladder. 4. Adequate liners at wear points. 5. Life and class rating of bearings and gear reducers. 6. Wind-loading, dead-loading, and seismic requirements of structural legs. 7. Type and construction of air lockâmachined housing, close tolerance; nonmachined housing, flex tip. 8. Rating of motorâTotally enclosed, fan cooled, Class II, Group G. 9. Explosion vents and vent ratio. Housing should have high enough pressure rating to withstand rupture pressure of vents. 10. High-temperature limit switch. 11. Negative pressure rating of filter housing. Selection of Filter Media The selection of filter media should be based on; 1. Type of filter. 2. Type of dust being handled. 3. Temperature of air being handled. 4. Unusual characteristics of air and its contents; acidic, alkaline, moisture, etc. Filter media can be made of a variety of materials. Those available include cotton, wool, polyester, acrylic, nylon, Nomex, and polypropylene. Wool is used rarely because it is costly and does not retain its size and shape well in service. Acrylic fiber, like wool, is costly and is used only in special applications in the starch industry. Nylon is used occasionally in acidic conditions, but has no advantages in the grain industry. Nomex, a heat-resistant fiber, would be used only where process air was hotter than 250Â°F. Cotton, polyester, and polypropylene are the most widely used filter materials in the grain industry. Of these three, polyester is the most common.
34 Filter media are made in woven form or as felts. Woven media are relatively lightweight and are used primarily in shaker and reverse-flow filters. The material can be cotton, polyester, or polypropylene. Felt media, the more common form, are used basically in reverse-jet and pulse-jet filters. They are made by needling or shrinking a loose, continuous blanket of fibers into a dense felt. Polyester is by far the most common fiber in felts. Normal weight for polyester felt is 12 to 19 ounces per square yard. The fiber is relatively inexpensive and holds its size and shape well in service. Polyester also can be laundered or dry-cleaned and so can be renewed indefinitely. Polyester felt can be made with a slick surface (eggshell finish) on one or both sides. This type of surface provides better dust release and has been found beneficial in high-humidity areas like the Gulf Coast. A smooth finish also can be produced by singeing the fibers at the surface of the felt with a direct flame. This type of finish also improves dust release in humid areas. Polypropylene felt is favored by some over polyester because it gives better dust release. The two fibers are basically interchangeable, however. Their costs are similar, as are their retention of size and shape in service. A few new types of filter media are said to offer ultrahigh efficiencies for submicron particles. One of these media is a felt-back membrane with extremely small pores. Another is an arrangement of pleated paper. These media, along with the so-called absolute filters, would be considered for cleaning air to be returned to the inside of the building. Returning air to a building is a questionable practice, however, especially if the dust involved is flammable, explosive, or toxic. A word of caution is in order on the selection of filter media for specific installations. Generally, the medium recommended by the manufacturer of the filter will be the most suitable. Woven fabrics usually will blind (dust will plug the pores) or otherwise work inefficiently on a reverse-jet filter and should not be considered for such facilities. Also, lightweight felts (8 to 10 ounces per square yard) should be avoided. Disposal of Collected Dust Grain dust discharged from a filter remains a serious fire and explosion hazard. The collected dust should always be disposed of with these hazards in mind. The following practices are recommended. Transporting Dust Collected dust may be conveyed to storage mechanically over a short distance with one or no change in direction. Long, complicated screw or drag conveyors require their own dust-collection systems, which only add to the problem.
35 Dust is best moved longer distances pneumatically. Metallic, electrically conductive pipe should be used. Long-sweep elbows and other features in accord with good engineering practice should be employed. Nonconductive plastic pipe should be avoided because of the risk that static electricity will create hot spots, which are a potential ignition source. A pneumatic transport system must supply enough air to fluidize the dust and transport it to storage. Dust Storage Dust should be stored in a noncombustible tank not housed in a building. The tank should be located so as to minimize the length of charging and discharging conveyors. The hopper should have a minimum angle of 60 degrees, and one side of the discharge opening should be flush with the side wall. If the dust is to be stored for a considerable time, the bottom of the tank may require some sort of mechanical agitation for loadout to preclude bridging or other condition problems. The tank should only be large enough to hold 10 days' production of dust or 150 percent of the capacity of the vehicle being loaded, whichever is greater. Returning Dust to Grain Stream Where a long, complicated dust-transport system would be required, it may be desirable instead to return the collected dust to the grain stream. However, the dust should not be recirculated through the grain-handling and dust-collection systems. Such recirculation can be avoided by the following restrictions; 1. Dust should not be returned to bucket elevators. 2. Dust should always be returned to the grain downstream from the point where it was first collected. 3. Dust should not be returned to spaces around machinery where it would again be drawn into the dust-collection system. 4. Dust returned to conveyors or spouts must be returned so that it is beneath the grain stream. Disposal Alternatives Alternative methods of dust disposal are slurrying in water and agglomerization or palletization. Particularly at elevators associated with processing plants, water containing slurried dust can be employed in the process. Grain dust can be agglomerated or pelletized to yield a product that can be handled safely. The pelletizing process has inherent fire problems and must be separated from the grain-handling operation by a fire wall. Magnets should be installed ahead of the pellet mill to scavenge tramp metal. Fire monitors also must be installed.
36 Cyclone-Filter Facilities Cyclones are commonly used in series with bag filters in grain elevators. The exhaust fan must have additional horsepower to handle the pressure drops across both cyclone and filter. The advantage is that cyclones tend to collect a higher percentage of larger particles, which can be returned to the grain stream. The smaller particles tend to pass through the cyclone to the filter, where they are collected for disposal in some other way. However, dust collected by the cyclone may still contain a good deal of fine, dry material. Therefore, this dust should be returned to the grain with the same precautions that apply to dust from the filter. Use of a cyclone ahead of a bag filter does not as a rule affect the optimum air-to-cloth ratio of the filter. The dust loading on the filter is lower with the cyclone than without it. On the other hand, the filter is less efficient. The reason is that the larger particles captured by the cyclone would otherwise contribute to the collection efficiency of the mat of dust that builds up on the filter fabric. These factorsâlower dust loading and lower efficiency--tend to balance each other. Thus, although adding a cyclone reduces the dust loading on the filter, it does not permit the air-to-cloth ratio to be increased.