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10 Questions about Filtration

Tue, 01/17/2006 - 7:27am
Close-up of wet-laid media
By Richard Smolen and Viola A. Bielobradek Tempted to try a different filter to improve your dust collector’s performance? How do you choose when there are so many options? Start by reviewing the answers to 10 frequently asked questions about mechanical filtration. The array of filter elements on the market can generate anxiety when it’s time to specify the best media for specific applications. The following report offers answers to some of the most frequently asked questions about mechanical filtration elements. It focuses on pleated cartridge filters and pleated bags and examines the three most popular styles of media available today. These are wet-laid, spun-bonded, and needled felt media. 1. What media type is least expensive and why are there such significant differences in price? The wet-laid category is the least expensive. The difference in pricing is dictated by the cost of production as well as varying performance characteristics for each group of media. The least expensive category is created in a wet process. Cellulose fiber slurry is mixed with a chemical resin that holds the fibers together and protects them against moisture. The cellulose fiber can be a hardwood, softwood, or even grass. The mixture is pressed flat, cured in high-temperature ovens, and dried. Because of their excellent efficiency characteristics, wet-laid media types are good for a wide array of applications and are suitable for operating temperatures up to 200F. The 100 percent cellulose wet-laid media category, though, is not recommended for high-grain loading or high-moisture applications. However, it is important to note that there are cellulose blends available, also created in the wet process, that exhibit excellent moisture tolerance. 2. What can spun-bonded media offer? Spun-bonded media, also known as non-woven media, can easily handle higher grain loading. This fine denier polyester media can also be treated to repel moisture and oil contamination. This style is seen as moderately expensive but offers high levels of efficiency. It’s produced when dried polyester chips are put into a hopper and extruded by a melt-spinning machine while molten. The continuous filaments are spun, drawn, and opened by a spinning cylinder, dispersed on a net conveyor, and laminated uniformly. The filament webs are then thermal bonded with an embossed roller in order to form a sheet. Spun-bonded media types are recommended for operating temperatures up to 275F. It is worthwhile to note that there are various treatments and finishes available to handle application specific needs. One example is the ePTFE membrane. It can be laminated onto spun-bonded media to provide extraordinary release characteristics for product reclaim applications.
Inside look at needled felt media
3. Which media type is most widely used in the chemical processing industry? Since the chemical processing industry is a diverse industry, media selection really is application specific. Having said that, we’ve observed that most of the industry deals with abrasion and higher operating temperature challenges quite frequently. Out of the three media classes, the needled felt type would be most appropriate for those extreme applications. This category is also moderately expensive. It is produced when short felt fibers are pressed together and mechanically fixed with a needle punch machine. The style offers varying levels of efficiency based upon weight and fiber composition. In some cases, a limited amount of media area per cartridge requires dust cake buildup for maximum efficiency. It is recommended for applications with operating temperatures up to 500F. 4. How important is choosing the right media for a given application? Crucial. The wrong media type may cause all types of problems from media blinding to premature filter failure or even to collector fire. Also, the right media choice may significantly improve dust collecting operation. It is absolutely imperative, however, that all the application aspects be considered when selecting the media and the filters especially in the chemical processing industry. 5. What are some of the signals that would indicate there might exist a media change/upgrade opportunity? Filter upgrades should be made to improve specific performance criteria. For instance, reasons to upgrade include if your system is operating with higher-than-standard pressure drop or if filters are retaining too much product. Upgrading filters in these two cases will likely mean switching from one pleated style to another. You also may decide to upgrade filters because there has been a change in your process that requires a new approach to dust collection or because regulations demand lower plant emissions and better plant air quality. Or, you may need to achieve a longer filter service life to reduce production interruptions caused by filter leakage or insufficient efficiency and to lower maintenance costs. 6. What can a typical upgrade yield? Changing from wet-laid or needled felt to spun-bonded media can improve strength, dust release, abrasion resistance, and efficiency. Although the filter price increases, a higher cost is justified by an extended service life and improved performance. Upgraded filters also bring about better filtration efficiencies with lower emissions, improved product reclamation, lower compressed air usage, lower fan amps due to lower pressure drop, and a lower maintenance cost with less frequent change-outs. It may also be the answer to premature filter failure. 7. What causes filters to fail prematurely? Thermal exposure, chemical attack, abrasion, cleaning insufficiency, and moisture saturation are factors that shorten filter service life. If you are practicing preventive maintenance, filters should be analyzed annually. If filters are failing prematurely, they should be tested. Specific laboratory testing can determine what is causing these failures.
Details of spun-bonded media with ePTFE membrane
8. What are examples of laboratory tests and their possible results? One test is the Frazier Permeability test. A low number scored on this test may indicate decreased efficiency and increased pressure drop. The test records the volume of air, in CFM, that flows through one square foot at 0.5 in. W.G. pressure drop. Another test is the Wet and Dry Mullen Burst test. Low readings indicate deterioration of used media. The test involves a rubber bladder that is pressurized under clamped sections of media. When both the wet and dry media burst, the pressure is recorded in PSI. A very low reading indicates that the media may not hold up under repeated pulsing for an extended period of time. And finally, a microscopic examination of the media can show pore blockage, clean-side contamination, and particulate penetration, allowing the lab technician to assess the potential recovery of porosity and the remaining service life of a given filter or pleated bag. 9. What specialized finishes are available? Fire-retardant, ePTFE membrane, carbon impregnation, metallized, or hydro and olieophobic finishes can be used to address very specific application needs. 10. How should these finishes be used? A fire-retardant finish, which is not fireproof, can be sprayed onto wet-laid or spun-bonded media to retard combustion. An ePTFE membrane can be laminated onto spun-bonded and needled felt media to capture fine particulate. It yields improved efficiency and cake release. Carbon impregnation is a process applied to wet-laid media in the slurry stage and results in excellent static dissipation properties. A metallized finish can be applied to spun-bonded media to produce improved cake release and excellent static dissipation properties. And finally, hydro or olieophobic finishes can be applied to spun-bonded media for excellent moisture and mild oil-mist tolerance, efficiency, and strength. About the Authors: Richard Smolen is the filtration design engineer at TDC Filter/Midwesco, 1331 S. 55th Ct., Cicero, IL 60804. He has a bachelor’s degree in mechanical engineering and experience in laboratory work and testing. Viola A. Bielobradek is the marketing manager at TDC Filter/Midwesco and has a bachelor’s degree in commerce. Questions about this article can be addressed to the authors at smolenr@tdcfilter.com and bielobradekv@tdcfilter.com or by calling 800-424-1910. Additional information is available at www.tdcfilter.com and www.midwescofilter.
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