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34 / June 2020 powderbulk.com critical for proper dust collector design to prevent can velocity from leading to a potential failure mode. Your material's properties and filter media selection There are many types of filter media, and your choice will have a large impact on the cost and performance of your dust collector. In addition to application aspects such as air volume and temperature, material proper- ties can have a large effect on a filter's performance and the life of the filter elements. First, you must determine whether your dust collec- tor will use bag or cartridge filters. Bag filters or socks are the least expensive, but cartridge filters offer better filtration efficiency and can reduce the dust collector size. When making a filter media decision, however, it's important to note that certain material character- istics may dictate the answer. Pleated cartridge filters squeeze more filter area into a given volume, but some material shapes will get caught in the pleats and pulse-cleaning won't dislodge them. This most often occurs when the particle shape is flat, flakey, or jagged, so bag filters are the better choice in this scenario. Other material properties, such as adhesiveness and cohesiveness, can cause further filtration problems. These sticky materials may adhere to the filter element. This is especially true for standard bag filters made with felted fibers; materials can stick to them and cause blinding and premature failure. You may need to use a filter element with a polytetrafluoroethylene (PTFE) surface membrane to prevent the material from stick- ing. Alternatively, if your material contains oil, then the material is sure to blind the filter, and an oleophobic filter media (one that repels oil) should be considered. Still, other material properties can cause potential hazards that must be addressed. If a dust explosion is possible with your material, then the dust collector and filter elements must be designed with this in mind. Several methods for preventing and, at the very least, minimizing the effects of potential explosions are available. Many explosion protection methods are quite expensive but may be required for safety reasons and to comply with OSHA regulations and NFPA standards. You'll want to determine if these explo- sion protection measures are even necessary for your material. If so, you'll need to determine your material's explosibility factors, such as minimum ignition energy and maximum pressure generated, which can only be established through testing. Once determined, these values can be readily used by the filter supplier or sys- tem designer to choose the proper safeguards for the filter media, filter unit, and ducting. Other tips and considerations Material samples. When obtaining a material sample for evaluation, your test sample should be taken using the proper methods to get a representative sample. For example, the top of the sample pile will probably have less dust than the bottom, and the sample will need to be made up of material from both locations. The sample should represent actual material that will be filtered. In addition, if your material is friable, then the material as it enters the dust collector may have smaller particle sizes than the material as it entered the feedpoint or as received from the supplier. This is espe- cially true with a high-velocity pneumatic conveying system. Make sure your sample represents the material being handled at the subject separator. Calculations. When calculating airflow and velocity, be sure to base the figures on conditions at the sep- arator. Due to gas (air) compressibility, the air volume and velocity can be much different at the air filtration system's end than at the beginning. In other words, don't forget to include the effects of pressure or vacuum, tem- perature, and altitude in your air volume calculations. Air velocity in any separation device is important, so take care to ensure a uniform distribution of air as it enters and exits the unit. Turbulence and areas of high or low velocity will cause particle re-entrainment and can be avoided with the proper design of transitions, adjacent ducting, and elbows. PBE For further reading Find more information on this topic in articles listed under "Dust collection and dust control" and "Particle/Powder analysis" in Powder and Bulk Engineering's comprehen- sive article index in the December 2019 issue or the article archive on PBE's website, www.powderbulk.com. Todd Smith (toddsmith@k-state.edu, 316-350-5865) is the business and strategy manager for Kansas State University's Bulk Solids Innovation Center. He's spent more than 35 years in the bulk solids industry working in a variety of engineering and management positions. He has a mechanical engineering degree from KSU and an MBA from Kansas Wesleyan University. K-State Bulk Solids Innovation Center Salina, KS 785-404-4918 https://bulk-solids.k-state.edu