Re: oil bath air filters

Posted by Nolan on July 11, 2002 at 04:52:28 from (209.48.190.220):

In Reply to: oil bath air filters posted by mike on July 10, 2002 at 00:01:07:

I'd like to offer a little clarification on the subject of air filtration (it's part of my job, advanced air filtration).

Claims about capture efficiency (99.9% for example) are meaningless without knowing the size of the particles captured. Windowscreens have a 100% capture efficiency if we are talking about particles the size of grasshoppers. Chicken wire is a fine filter, when filtering out chickens. So don't get too worked up over filter efficiency claims without knowing what size particle the claim is being made about.

Having a filter that has 80% capture efficiency of sub micron particles is wonderfull, and pointless. A sub micron sized particle is down at the size of many of the gas molecules in the air. A particle this size can't hurt your engine because it's too small to be captured between the piston and the wall. Catching it is pointless. Understand the significance of the size of the particles being captured, and not captured. A HEPA filter is overkill for an engine for example.

If you notice, most ag equipment paper filters are not the dry type used on cars, but are an oil impregnated resin. Air going through them has to contort around the fibers as it passes through the media. This design works on the principle that the particles, being heavier then air, will not make the turn, and will hit the filter fibers. Since the fibers are sticky with oil and resin, the particles get stuck on them. This is the same concept used by K&N with their oiled gauze, and others with the oiled foam filters.

Quick jump to the old fashioned oil bath filters (which is a confusing name, since the air is never bathed in oil). In these filters the oil is drawn up into a wire mesh, coating it. As air passes through this mesh, it has to make turns around the wires. The particles don't make the turn as well, hit the oil coated mesh, and get stuck. Sounds remarkably like the above "paper" filter, doesn't it? That's because it to is the exact same design and principle, just not as fine a mesh and much less convenient.

Dry paper filters work on the principle that the holes through the paper are too small for the dirt to get through. Essentially, they are a window screen type of filter. The particles don't fit through the mesh, so they build up on the surface. This type of design becomes clogged very quickly a a result.

So in quick summary, any of these types can be good, and can be bad. An oil bath filter with no mesh is virtually useless. So to is a paper or resin filter with too large openings in its fabric.

Follow Ups:

• Re: Re: oil bath air filters Buckwheat 11:24:03 07/11/02 (0)

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