The most challenging applications for filtration often involve either very small particles or huge volumes of particles – sometimes both. The filtration of buffing and lapping compound residues from solution is a good example of such an application. There are a number of “specialty” filters available which may answer the needs of the more challenging applications. The following are descriptions of a few of the alternatives.
The difficulty in challenging applications is that although nearly any filter media will work for a while, its life may only be minutes before it is totally clogged in some of these applications. The trick, then, is to find a way to extend the life of the filter media in some way or otherwise minimize the need for its frequent and costly maintenance or replacement.
In previous blogs we discussed “depth” filters which utilize a media designed to progressively remove smaller and smaller particles thereby delaying the ultimate clogging of the filter. In another approach, the filter media is “pleated” to provide more filter surface and enhance the ability of the media to collect retained particles. Bag filters are great for being able to collect and hold large volumes of particles but may have limitations when extremely small particles must be removed due to the fact that the available filtration surface is limited by the configuration.
One simple way to deal with high volume particle collection is to use a filter which is “self-cleaning.” The simplest self-cleaning filters simply provide a series of valves which periodically reverse the flow of liquid through the filter thereby forcing collected particles either to a collection tank or to drain. “Back-flushing” as this is called, is effective with some types of filter media but is seldom 100% effective in removing collected particles from the filter media. At some point, the filter media needs to be replaced as it becomes progressively more clogged with retained particles.
An “indexing media filter” uses a sheet type media supplied on a roll. The media is suspended over a receiving tank. Liquid to be filtered is delivered to the top side of the media and flows through the media either by gravity or, in some cases, with a vacuum assist. As flow through the media slows, a series of powered rollers moves fresh media in to replace the spent media over the receiving tank thereby maintaining flow through the filter.
A very specialized self-cleaning filter uses a series of grooved discs which, when held in compression act as a filter. Liquid permeates between the compressed discs from the outer diameter to the inner diameter. The grooves are designed in such a way that they actually comprise a “depth” filter, narrowing in the direction of the flow of liquid. Periodically, the pressure on the discs is released and a reverse flow of water forces trapped particles to be flushed to a holding tank for disposal or drain.
Precoat filters are used in some applications requiring the removal of very small particles. The filter media is sized to capture and hold a somewhat larger particle than those to be removed. The media is then intentionally coated with a secondary media, often diatomaceous earth or refined sand, which, pushed against the supporting media acts as the actual filter. As the coating traps particles in its narrow passages, flow diminishes. Periodically, the coating is washed off and replaced to renew the filter almost indefinitely. Although this is a labor intensive process, the cost operating this type of filter may be far less than that for other options.
Filtration technology is always advancing as manufacturers find new and better ways to do things. A full understanding of your filtration need and a little time spent with the filter manufacturers and suppliers may be well worth the effort.
– FJF –