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Filter
Bag Basics
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| For
your Hayward plastic bag filter to perform the way you expect it to you
have to select the right filter bag. After it's the bag that does
the actual filtering, not the vessel itself. |
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Sizes |
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| Hayward
has two sizes of plastic filter vessels, a single length and a double length.
Single length housing take 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. |
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Fabrics |
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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 come 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 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. |
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Construction |
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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. Also 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 now 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 not longer the case because
of modern fabric welding technology. |
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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. Also don't forget to consider the ring material of the bag when
looking at chemical compatibility of the application. |
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Micron
Ratings |
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bags are generally available in micron rating 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 time what a nominal rated bag does. |
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Back
to Hayward Back to PEP
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Copyright
HAYWARD Industrial Products, Inc. 1999
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Filter
Bag Basics
