To make the filter bags perform to the optimum at the vessel, you need to know how to choose the right filter bags for the filtration system.
Polypropylene, Polyester, Nomex and Nylon are the four major synthetic fiber-made filter media used for making filter bags, which are able to be made of needle-punched filter felts and woven filter meshes or filter fabrics.
Due to its chemical resistance, water solvents and reasonable prices, polypropylene filter felt-made filter bags are most commonly used for industrial filtration. There are two major felts filter bags, one is self-supported and scrim-supported, the former is the felt without any scrim, but thanks to the production and punching technology, felt can achieve strength without scrim at lower price, the latter one is the felt made with a scrim (usually it is HDPE filter mesh) “sandwiched” in the center of felt to increase the felt strength.
To ensure synthetic fibers are not migrated or dropped from the felt into clean or filtered fluid/liquid, needle-punched filter felts should be “glazed” or “singed”.
Some people prefer a sewn bag seam for the perceived assurance of the strength of a mechanically secure, sewn seam. Obviously if the seam fails and the bag breaks, dirt will enter the downstream flow. One area of concern with a sewn seam is the small holes made by the needle that sews the bag. These holes, if larger than the micron rating of the bag, can allow unwanted material to by-pass the filter bag. This can be a greater problem if the bag is not properly sewn. A good quality bag will be made from media that is silicone free and will be sewn with silicone free thread so that the filtrate is not contaminated.
Welded construction filter bags have the fabric fused together to form the bag and secure the sealing ring. Because there are no sewn holes there is no particle by-pass through the seam. Sometimes a customer may worry that a welded seam would not be as strong as a sewn one, although this is no longer the case because of modern fabric welding technology.
Another construction consideration is the type of sealing ring. This is a ring at the opening of the bag that helps seal it to the housing. It can be made of plastic or metal. The ring material is important not only for chemical compatibility, but also for disposal purposes. If the bag is going to be disposed of by incineration, it’s important that the ring be plastic, and not metal. Again, don’t forget to consider the ring material of the bag when looking at chemical compatibility of the application.
Filter bags are generally available in micron ratings from 1/2 to 1200. It’s important to know that almost all bags have what’s called a “nominal” rating. This means that the bag will allow some percentage of particles larger than its micron rating to by-pass the bag. In other words, a nominally rated 25 micron bag will allow some larger than 25 micron particles to enter the downstream flow. This usually is not a problem since, in many applications, the proper size bag is determined through testing different micron ratings to determine the one that will retain the required size particles. Absolute rated bags are available that will stop 99.9% of the rated size particles but they can cost up to 30 times what a nominal rated bag does.