Indoor air pollution is a problem in many public buildings. In fact, 50 percent of all illnesses
are thought to be either caused or aggravated by poor indoor air quality (IAQ), a problem that
costs the United States upwards of $160 billion each year in medical costs and reduced
productivity. One solution to the problem is an effective air filtration strategy, which serves as
a prime defense for building occupants against indoor airborne pollutants.
Most of the particles people breathe into their lungs are approximately 3 microns or smaller
— a fraction of the size of a grain of sand. It is the job of the air filter media to capture
these respirable particles. However, not all air filter media is capable of efficiently capturing
the tiniest particles that can cause health problems.
In the past, filter media was seen as a commodity and was specified solely on price. Today,
to improve IAQ, the industry is realizing the added value and benefits of heating, ventilation and
air conditioning (HVAC) air filters made with synthetic, nonwoven filter media that balances a
robust mechanical structure with an electret charge.
Filters, such as the array shown here, that provide a good balance of a robust mechanical
structure and an electret treatment will almost always outperform a filter media that relies solely
on mechanical efficiency. Photographs courtesy of Kimberly-Clark Professional Filtration.
Air Filtration Methods
For air filters to capture particles on the filter media, the particle must collide with or
be removed by the filter media fibers and must continue to adhere to the media fibers.
According to the National Air Filtration Association’s NAFA Guide to Air Filtration (Fourth
Edition, 2007), there are four primary methods of mechanical particle capture:
Impingement: As air flows through a filter, it changes direction to flow around the filter
fibers. Because of their inertia, larger particles resist change in direction and attempt to
continue in their original direction, thus colliding with and adhering to the fibers.
Interception: A particle follows the airstream and contacts the fiber as it passes around it.
If the forces of attraction between the fiber and the particle are greater than the force of the
airflow to dislodge it, the particle will stick to the fiber. Interception is enhanced when the
size of the fiber is closest to the size of the particle.
Diffusion: As air passes through the filter media, minute particles move from areas of higher
concentration and take an erratic path called Brownian motion, thus increasing the probability that
particles will contact the fibers and stay attached to them. Diffusion works best with fine filter
fibers and very low air velocities.
Straining: Straining occurs when the smallest dimension of a particle is greater than the
distance between adjoining filter media fibers.
Nonsynthetic, mechanical-only air filter media relies solely on the four particle-capture
processes explained above. Synthetic, electro-mechanical air filter media relies on these same
processes, along with one major enhancement: an electret charge.
Figure 1: In synthetic filter media with gradient density structure, fibers are more
loosely packed on the upstream side and densely packed on the downstream side.
Electret Treatments Enhance Mechanical Structure
Particle capture in synthetic air filter media can be enhanced by adding an electrostatic
charge. In fact, filters that provide a good balance of a robust mechanical structure and an
electret treatment will almost always outperform a filter media that relies solely on mechanical
efficiency.
While electrostatically modified media includes a broad class of materials, such as
fibrillated electret-charged films and triboelectrically charged needled felts, most
electro-mechanical air filter media is filament-based, using one of a number of nonwoven forming
techniques and synthetic fiber types, including meltblown polyolefins and spunbond polyolefins.
Corona charging is considered to be the best method for large-scale electret treatment of
electro-mechanical air filter media.
There are several benefits of imparting an electret treatment to synthetic filter media with
a robust underlying mechanical structure:
Electret-treated synthetic media can be manufactured to provide high initial and high
sustained efficiency over the filter lifecycle. Filters that are designed to provide only
mechanical efficiency begin their life at their lowest particle removal efficiency and rely on the
building of the dust cake in the filter to increase efficiency.
The electrostatic effects created in an electret-charged synthetic media are particularly
useful in increasing the capture efficiency for submicron particles. While submicron particles are
much smaller than the void spaces present in most commercial electret media, the electrostatic
forces within the media structure allow those particles to be removed with high efficiency.
The filtration efficiency of electret-treated synthetic media is unaffected by relative
humidity and by long-term warehousing at high temperatures (130°F), meaning the electret effect
resists age-related decay during storage and use.
Electro-mechanical synthetic media almost always delivers lower airflow resistance in the
same filter construction as a mechanical-only filter, especially those made of fiberglass. This
characteristic translates into reduced energy consumption and costs. Mechanical-only filters tend
to create significant drag or resistance because their filtration mechanisms cause disruption of
the particles in the airstream.
The reduced airflow resistance and energy consumption also means electro-mechanical media
filters can help reduce greenhouse gas generation.
Additional Advantages Of Synthetic Media
Synthetic media made of polyolefin fibers is hydrophobic and will not absorb moisture that
would support microbial growth.
Certain synthetic media can provide superior performance while using less media than other
filters. Furthermore, a high-capacity pleated filter made with synthetic media can have an extended
life, reducing changeouts.
Filters containing synthetic media can be made without binders, which can cause off-gassing.
Synthetic filter media can be made of thermally bonded, continuous polyolefin fibers that
resist shedding to help keep HVAC ductwork and components cleaner.
Evaluating Air Filters
When evaluating synthetic mechano-electret air filters, look for those that have
depth-loading media with a gradient density structure in which the media’s fibers are more loosely
packed on the upstream side and more densely packed on the downstream side
(See Figure 1). This structure helps to reduce airflow resistance, enhance dust loading
and prevent face loading of the filter. Prevention of face loading is important during filter
removal. Reducing the dust cake located on the surface of the filter media lowers the probability
that the dust and contaminants will be dislodged during filter removal. This can save maintenance
costs by reducing the time required to complete a filter changeout and associated cleanup.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 52.2
Standard measures an HVAC filter’s fractional particle size efficiency (PSE), which indicates the
filter’s ability to remove particles ranging between 0.3 and 10 microns in diameter. A MERV, or
Minimum Efficiency Reporting Value, is assigned to a filter based on a minimum PSE. A MERV 1 is
least efficient, while a MERV 16 is most efficient. When evaluating filter performance under ASHRAE
52.2, examine the filter’s efficiency in all particle size ranges: E1 (very fine, 0.3 to 1.0
microns); E2 (fine, 1.0 to 3.0 microns); and E3 (coarse, 3.0 to 10.0 microns). These three
efficiencies represent the true measure of filter performance and give users a more complete
picture of the filter’s particle capture performance. High E1 and E2 efficiencies are critical for
providing for good IAQ.
Tony Fedel, P.E., is associate marketing director, Kimberly-Clark Professional Filtration,
United States.
April/May/June 2013