Filter efficiency & pre-treatment (Why multiple pass efficency goes up)

As a filter goes into use, it has pores that let the fluid through, and retain the particles. As the filter begins to remove particles, some pores may plug, however, large particles will lodge on the media, partially blocking pores, which reduces the sizes of particles that can now pass through that pore.

This is the principle behind diatomaceous aquarium filters, and the pre treatment of large industrial filters before the new filter media is put on line.



Pre-coatingTo ensure the filter bags have a long usage life they are commonly coated with a filter enhancer (pre-coat). The use of chemically inert limestone (calcium carbonate) is most common as it maximises efficiency of dust collection (including fly ash) via formation of what is called a dustcake or coating on the surface of the filter media.

More from the filter experts at K&N:



The dirt particles collected on the surface of a K&N element have little effect on air flow during much of its service life because there are no small holes to clog. Particles are stopped by layers of crisscrossed cotton fibers and held in suspension by the oil. As the filter begins to collect debris, an additional form of filter action begins to take place because air must first pass through the dirt particles trapped on the surface.  That means a K&N air filter continues to exhibit high air flow throughout the life of the filter while it is accumulating dirt.  At the same time, the air flow for an average paper air filter can decrease dramatically as the paper element gets dirty.  So as dirt accumulates, the performance advantages of a K&N air filter can increase!

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