BCR - Collecting Dust
Effective dust control in brick grinding rooms is important for maintaining improved plant housekeeping, providing a good employee operating environment, and achieving state and federal emission standards. That’s why development of specifications for dust control was an important design goal when Acme Brick started planning a new grinding system at its Elgin, Texas, brick plant.
Designing a dust collection system for brick plant material handling operations requires consideration of four key elements: containment, capture, conveying and collection. As evidenced from Acme Brick’s new plant, careful attention to these elements, and to their relation to overall plant operating conditions, such as maintenance and physical location of equipment, are essential for achieving superior dust control.
Acme’s New Grinding PlantAcme Brick’s Elgin plant, located 30 miles east of Austin, Texas, operates two brick manufacturing lines: a stiff extrusion line, which produces architectural brick; and a molded brick line, an automated process that makes an antique style, hand-molded brick. Various mix ratios of buff, elstan and red clays are used to produce the desired brick colors. The clays and additional mix components, including sand, calcine and grog, are processed through the grinding room. This process is illustrated in the grinding room flow diagram on p. 6.
Materials are loaded into feed hoppers in the clay storage building and conveyed at a rate over 100 tons per hour to a double-deck scalping screen located in the grinding room. Materials for the molded brick plant are blended ahead of the grinding plant, then processed through the grinding circuit and stored in the 250-ton bulk materials bin. Materials for the architectural plant are individually run through the grinding plant and are stored in seven 100-ton silos.
The scalping screen has a 6-mesh bottom screen and coarse top screen. Overruns from both decks go directly to a 250-hp, single-rotor, reversible impactor mill for grinding. Material passing the bottom screen is conveyed to either a silo or the bulk bin. The grinding loop is designed to handle up to 400 tons per hour of recalculating load and consists of three 5 x 10 foot, single-deck, 6-mesh finishing screens. Material passing the 6 mesh screens goes to storage, while material larger than 6 mesh is returned to the impactor. These screens are electrically heated with DC current and equipped with rubber ball trays to aid in the reduction of blinding. Additionally, the recirculation loop features a metal detector located at the infeed to the impactor to protect it from damage.
Variable-speed drives on the feeders below the silos permit proportional feeding of individual ingredients according to the blend recipe. Mix additives are also fed into the blend prior to a ribbon mixer. All plant operations are PLC-controlled using an HMI (human machine interface) package programmed by Basic Group of Siler City, N.C. The automated blending process allows the plant to achieve better product mixing and consistency in color of the final fired product.
Dust Collection SystemA key part of the new plant is its dust collection system. Dust collection pickup points are located at every transition inside the grinding building, at the feed and discharge of the impactor, and at the feed and discharge ends of the shaker screens. Dust captured at each pickup point is conveyed through ductwork to the dust collector. Air for the dust collection system is delivered by a 125-hp backward inclined centrifugal fan rated for 32,500 cfm airflow at 13.5 in. water gauge static operating pressure. Additional fan and collector capacity is available to provide for planned future expansion.
Acme Brick developed the plant design and provided a bid package with elevations, plan view drawings and a specification package. This bid package was sent out to plant builders for bidding. Basic Group was selected to build the plant because of its performance in similar projects and its willingness to customize the plant as needed to satisfy Acme Brick’s expectations. Acme Brick and Basic Group then consulted in selecting the process ventilation contractor, Kirk & Blum of Greensboro, N.C.
The dust collection system specification was developed as a cooperative process with input from Acme Brick, Basic Group and Kirk & Blum. It was important that the design and installation incorporate good dust collection practices, and that the operating system performance be achieved and maintained.
Effective Dust Control Design PracticesContainment. Containment means enclosing the source emissions wherever practical to minimize the amount of capture air needed for control. Effective containment design at Acme Brick included specifying proper sized enclosures at the conveyor transitions, and sealing the impactor and screens. Yet with all these seals and enclosures, a key design consideration was ease of access to the machinery for maintenance and repair.
Capture. Dust control process ventilation is based on containing each dust generation source and using exhaust air to capture and transport the contained dust into ductwork. Principle guidelines for this are described in Industrial Ventilation, A Manual of Recommended Practice. (This book can be obtained from the American Conference of Governmental Industrial Hygienists, 1330 Kemper Meadow Dr., Suite 600, Cincinnati, OH 45240-1634.) Each source of dust generation is analyzed to determine the appropriate capture air volume. Included in this analysis are the containment design, operating considerations and maintenance considerations.
Allowance must be made for conveyor belt speed (to provide for overcoming induced air volume), material flow rate (to provide for displaced air volume), and sufficient in-draft velocity (to contain the dust within the enclosure). Capture volume is calculated as the sum of induced volume, displaced volume and in-draft volume. In-draft volume is determined by multiplying open area at the enclosure by the selected capture velocity. Another important design consideration is to maintain a maximum of 200 fpm velocity at the entry face of the duct inlet transition to prevent scalping unnecessary product into the ductwork.
You can’t cheat on the design by under sizing the capture volume and expect good system performance. If 1200 cfm is needed at a dust source, 800 cfm just won’t do the job. It is better to oversize and have reserve capacity than to try and just get by. Some additional need always seems to come along that robs air from the original use. Then 800 cfm becomes 600 cfm, and nothing works well.
,Conveying. Ductwork design specification is the third key element in dust collection system performance. This area includes determining conveying velocity, ductwork material and gauge, and ductwork design principles. For example, Acme specified 4000 fpm material conveying velocity to prevent material buildup in the ductwork. Maintaining velocity means that more attention must be given to managing abrasive wear, and the Elgin plant required the use of flatback elbows and heavier gauge ductwork than might typically have been used in the past. Proper sizing of ductwork and transitions for maintaining balanced conveying velocity throughout the system were also taken into consideration.
Collection. Captured dust is removed by a filter cartridge style dust collector. Acme specified a cartridge collector rather than a baghouse based on prior operating experience in its facilities. Cartridge collectors have the advantage of lower operating differential pressure drop, lower maintenance expense and a smaller physical size. An air-to-filter ratio of 2.0:1 or lower was specified for this equipment to ensure effective cleaning of the filters. Continuous on-line cleaning is accomplished using controlled pulses of compressed air regulated by an automatic timer. The dust collector inlet header is designed to absorb inlet abrasive force ahead of the collector through the use of an abrasive relieving inlet transition. Proper design of the outlet header to the fan is needed to maintain full fan performance.
David Colborne, Kirk & Blum project manager, developed the final ductwork design and calculation of the fan static pressure requirement. Proper fan selection based on the sum of hood entry loss, duct loss, and dust collector differential pressure is crucial for maintaining the desired dust collection performance and continuous system operation.
Operating ResultsCalcine is the dustiest product to go through the grinding room, and with the aid of a little water spray even it can be processed with almost no visible emissions. According to Dennis Andrews, Basic Group project manager, “We’ve had visitors come through the plant who could not believe we were running because the plant dust system is working so effectively.” That really is the best testimonial—a clean operating environment and minimal operating service needs.
Acme has been pleased with the equipment suppliers and the start-up process—especially since it did not have to take the plant off line once it was put in operation. Teamwork was probably a major factor in the new plant’s success. According to Dennis Andrews, Basic Group was able to accomplish a better design because “we had good specifications from Acme Brick on what they wanted.”