How to Select A Filter Bag

Filter Bag Basics


For your Hayward plastic bag filter to perform the way you expect it to, you have to select the right filter bag. After all, it's the bag that does the actual filtering, not the vessel itself.


Sizes

Hayward has two sizes of plastic filter vessels: a single length and a double length. The single length housing takes a bag size called by the industry as either P1, N1, or #11. The double length vessel takes a P2, N2, or #12 bag. Maximum flow rates are 50 gpm for a #11 bag and 110 gpm for a #12 bag.


Fabrics

The four most common fabrics used in the construction of industrial filter bags are polypropylene, polyester, Nylon and Nomex, although others are available for special applications. Bags can be made of both felt and mesh material.

Polypropylene felt bags are probably the most common type in use. They provide good to excellent chemical resistance to mineral acids, organic acids, alkalis, organic solvents, and micro organisms at temperatures up to 200F, at a very reasonable cost. Felt bag fabrics are available in what's called self-supported or scrim supported material. Scrim supported means that the fabric is made with a woven support material "sandwiched" between the felt material for additional strength. However, current technology now produces self-supported material that is just about as strong as scrim support and at a lower cost.

One thing to make sure of with a felt bag, no matter what material it is constructed of, is that its outside surface is singed or glazed. This is necessary to prevent fabric fibers from the bag getting into the clean, filtered process media. After selecting the fabric material of the filter bag, the next thing to look at is how the bag is constructed. Filter bags are made with sewn construction or welded construction.


Construction

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.


Micron Ratings

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.