The Difference Between Gas and Liquid Filtration

It is tempting to think of filtration performance as being totally determined by the media, but in reality filtration performance is also determined by the type of filtration being carried out.  For example, the exact same media may perform very differently when used as a liquid filter than when used as a gas filter.  Let me give a very concrete example.

A HEPA filter is a particular grade of air filter media for clean room applications.  It is usually made of wet laid glass microfibers, and by definition it is at least 99.97% efficient against an aerosol of 0.3 µm DOP droplets at a face velocity of 5.33 cm/s. This is a pretty absolute air filter, and one must use some kind of discrete particle counting to actually measure the particle penetration.

It is also possible to try to use the exact same media to filter a dilute suspension of 0.3 µm acrylic latex particles in water, at roughly the same face velocity.  Such a suspension is slightly turbid, and one can measure the concentration of particles using a simple turbidity media.  If one carries out this experiment, they discover that the media is 0% efficient as a liquid filter for removal of these 0.3 µm particles.

How do we explain this dramatic difference in performance?  In gaseous aerosol filtration the basic rule is “if you touch, you stick”.  If an aerosol particle contacts a fiber in the media, it will adhere and wet the fiber.  So efficiency is determined by the statistics of impaction, interception, and diffusion.  The typical microglass media uses very fine fibers, so the efficiency is very high.

For liquid filtration there are other forces that come into play, such as much higher viscous forces trying to drag the particle off the fiber, and extremely high electrostatic ionic forces, which very often repel the ionically charged particle from the ionically charged fiber.  Thus, liquid filtration efficiency is more often controlled by the mean pore size defined by the array of fibers in the media.  In a microglass HEPA  media the typical mean pore size is in the neighborhood of 5 µm, which is much too large to have any efficacy at all against a 0.3 µm particle.

By Norm Lifshutz

Norm Lifshutz is the current Treasuer of the AFS.

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