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Nonwovens / Technical Textiles

The Filter Factor

Synthetic nonwoven media provides for a balanced approach to air filtration, and mechano-electret filtration technology is key to optimum efficiency throughout filter life.

Tony Fedel, P.E.

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.

  Filtration1
Figure 1: 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.

  FiltrationSEM
Figure 2: 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.

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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 2). 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, Roswell, Ga.

May/June 2013