How Do Media Air Filters Actually Work?

People and equipment work better in clean air; this is intuitively obvious.  People come equipped with an air cleaning system called the mucociliary escalator.  This biological system traps dust, pathogens, and pollutants in the sticky mucus secreted by the sinuses.  The mucus trickles down the throat and into the trachea and bronchia, and is moved up and out of the lung air passages by the coordinated wave-like motion of the tiny hairs lining the upper portion of the airways.  We then either cough it up or swallow it, leaving the clean air to enter our lungs.  One of the driving factors behind air cleaning is to reduce or eliminate the load on this biological air cleaning system by using some means of mechanical filtration, such as media air filters, to clean the air before we inhale it.

Common Air Cleaning Technologies

In order to achieve clean air in the workplace or at home, there are a number of commercially available air cleaning technologies on the market.  These include:

• Electrostatic Precipitators (electronic air cleaners)
• Wet collectors (scrubbers)
• Dry centrifugal collectors (think cyclonic inertial impaction of wood dust)
• Fabric collectors (media filtration)

Mechanisms of Media Air Filters

This paper  by Air Quality Engineering focuses on media filtration and addresses the question: “How do media filters actually work?”  As it turns out, there are five different mechanisms employed by media filters, and they are outlined below. Starting from coarse particle size to finer particles they are:

Straining

This is the most obvious of the media air filter mechanisms wherein the particles being removed from the air are larger than the openings or spaces between the filter fibers.   This mechanism misses most of the airborne particles because the spaces between filter fibers are much larger than most particulate air contaminants.  However, straining is effective in removing lint, hair, and coarse wood dust.

Impingement

Media air filters are a maze of fibers with intentionally circuitous paths through the filter. As the air passes through the maze of filter fibers, it changes direction many times, working its way through the filter.  As the air changes direction, some of the airborne particles do not change direction with it due to the mass and inertia of the particles.  Although small enough to pass through the space between the fibers, these particles don’t change direction fast enough to avoid slamming into the filter fibers, and they “hit and stick,” removing them from the air stream.

Interception

The first two mechanisms noted are pretty easy to visualize, but we need to  turn up the magnification on our mental microscope to understand interception.  This is actually a special case of impingement, but what separates it from impingement is the very small particle size collected via this mechanism.  These very small particles can easily follow the path of air flow as it passes through the mass of filter fibers.  However, due to their microscopic size, they will adhere to filter fibers that they come in contact with as the air passes through the filter.  They do not stick due to inertial impaction.  Rather, they adhere due to the inherent adhesive forces between the particle and the fiber.  These adhesive forces work at the atomic level between neutral atoms or molecules and are called “Van Der Walls Forces” after the scientist who first described them.

Diffusion

This media air filter mechanism affects the smallest particles.  These particles are so small that their direction and velocity are driven by random motion as if they were gases rather than particles.  They do not follow the path of the moving air but follow a kind of bizarre meandering motion through the filter until they bump into a fiber.  They then adhere to the fiber via Van Der Walls as described above.  Most extremely efficient filters employ this mechanism.

How Filter Cake Improves Efficiency

One aspect of media filtration that is not widely acknowledged is that after a media air filter has been in service, we start filtering with an additional material consisting of the collected particulate matter.  This collected particulate matter is called “cake’ and represents another “layer” of filtration.  Strangely, this means that a filter that has been in service will actually be more efficient than a new filter.

When Filters Need to Be Replaced

Eventually, the filter will become so “caked up” that it’s easier for the air to go around the filter through leaks where the filter mates with the rest of the air cleaning system. In order to avoid this overload condition, Air Quality Engineering, Inc. includes static pressure gauges with many of its media air cleaners so that the operator will be aware when it’s time to change out the filter.