PBS_Powder & Bulk Solids

Powder & Bulk Solids, August 2015

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www.PowderBulkSolids.com 11 A u g u s t 2 0 1 5 when space or headroom is limited, the compact cartridge collector is often the best choice. Baghouses are typically selected when venti- lating processes (especially hot applications up to 500°F), when used as a product collector, or when handling more difficult dusts (hygro- scopic, sticky, etc.). Figure 3 shows a baghouse installed on a metals processing application. Cartridge collectors are typically selected for ventilating storage silos, transfer points, and general material handling of dusts such as limestone, cement, and rock products. Because of their compact design, cartridge collectors can be mounted directly on silos, transfer points, and other indoor locations where space is limited. Figure 4 shows a cartridge collector mounted in a packaging plant. For cases where the process requirements could be addressed using either bag or car- tridge filters, the choice often comes down to capital costs. Cartridge collectors are typi- cally more economical up to 20,000 acfm, and baghouses are typically more economical at higher flow rates, although there are excep- tions. Life-cycle costs also should be consid- ered, such as how often the filters need to be changed and the cost of replacement filters. In general, replacement costs for filter bags with support cages are less than cartridges on a per unit basis without installation. Pressure drop is generally the same across the filters, so there is not much difference in operating costs, de- pending on the application. Once a technology is selected, the design process is not finished. Engineers should pay special attention to filter selection, filter clean- ing system, inlet configuration, flow distribu- tion, and control flexibility to most effectively match these factors to the intended applica- tion. The cost will obviously depend on the design and features selected. For example, a PTFE membrane filter might be selected if required to meet a lower particulate emission standard, but will increase the overall cost. In addition, corrosion-resistant materials may be required for some components of the clean- ing system; metal corrosion could eventually degrade the structural integrity of the filtration system, which could lead to undesired release to the surrounding environment. Right-Sizing for Best Performance Once the filtration design engineer has de- cided between a baghouse and a cartridge collector, he/she can turn his/her attention to sizing the system. The main sizing criteria for a baghouse system is the air-to-cloth (A/C) ratio, which is defined as the amount of air be- ing filtered relative to the amount of filtering media (see Equation 1 below). The A/C ratio is a feet per minute (fpm) ratio, and is often referred to as face velocity. A lower A/C ratio is generally associated with improved performance. Difficult process ap- plications typically have A/C rations of 3:1 to 4:1, while easier applications have A/C ratios up to 6:1. If the air filtration system is de- signed with too high of a ratio, the cost will be less, but a number of operational issues could result, such as blinded filters, decreased filter life, and unit pluggage. A second sizing factor, the interstitial "can" velocity, also should be considered for bag- house design. The can velocity is defined as the upward velocity in between the filters and will vary by manufacturer depending on the filter diameter, center-to-center filter spacing, and distance from filter to wall. A high can velocity may cause re-entrainment of the dust after cleaning. While the A/C ratio is a useful design pa- rameter for systems using cloth filters, it is not useful for pleated or cartridge filters. If it were, one could simply add more pleats to an element to increase the surface area, thereby lowering the A/C ratio and improving perfor- mance. The low air-to-cloth ratio is deceiving, however, as the tight pleat spacing renders some of the media useless for filtering. One common result is dust bridging between the tighter pleats, which makes it difficult to pulse the dust out. Certain applications, such as collecting sugar dust, need a wide pleat spacing to prevent dust buildup in between the pleats. This in turn increases the A/C ratio, but can actually improve performance. See variations of pleats shown in Figure 5. The proper way to size a cartridge collector, independent of pleat spacing, is based on cfm per element (see Equation 2 below). For common applications such as nuisance dust collection, silos, and transfer points, car- tridge type dust collectors should be sized for 300 to 400 cfm per element; for more difficult applications such as light dusts, a lower value of 200 to 300 cfm per element can be used; for simple applications such as air blast rooms, a higher value of 400 to 500 cfm per element is acceptable. Cartridge collectors typically feature a con- ventional vertical design. Horizontal cartridge collectors are sometimes specified, but it's im- portant to recognize that only one-half of the cartridge may be used for filtering. The dust piles up on the top half of the cartridge, while the lower half doesn't see the dust due to poor air flow distribution. As the dust is pulsed off Figure 3 - Baghouse installed on metals processing ap- plication Figure 4 - Cartridge collector mounted in a packaging plant. Figure 5 - Variations in pleats Air to Cloth Ratio = Filtration Area (ft 2 ) Volume (acfm) Element Volume = Number of Elements Volume (acfm) While pulse-jet filter bags and cartridge collectors are similar in terms of how they work, selection depends on a number of design parameters.

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