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May 2015: Air Filtration 101

Air Filtration 101: How Do Air Filters Really Work?

By: Robert Martin, CAFS, Associate Category Manager, Kimberly-Clark Professional Filtration  |   05.07.15

Air filters have an effect on two important components of building operations — indoor air quality (IAQ) and energy consumption. Some air filter specifiers and purchasers may think they need to make a tradeoff: select a filter with excellent particle capture efficiency and accept extra energy consumption, or select a filter with less energy consumption, but also poorer filtration efficiency. This couldn’t be farther from the truth. 

In fact, filter media technology exists that combines excellent capture of the submicron particles that can cause health problems with low airflow resistance, for reduced energy consumption. The key enabler of this technology is synthetic, nonwoven filter media that combines a robust mechanical structure and an electret charge. 

4 Methods of Mechanical Particle Capture

To appreciate the difference that electret-treated filter media can make, it’s important to understand how filters work. 

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. There are four primary methods of mechanical particle capture that commercial filters employ: 

  1. 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.
  2. Interception. A particle follows the air stream and contacts the fiber as it passes around the fiber. 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.
  3. Diffusion. As air passes through the filter media, minute particles move from areas of higher concentration and take an erratic path called Brownian Motion. This increases 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.
  4. Straining. This occurs when the smallest dimension of a particle is greater than the distance between adjoining filter media fibers.

1 Way to Enhance Particle Capture

Non-synthetic, mechanical-only air filter media relies solely on the four particle capture processes explained above. Nonwoven, synthetic air filter media can take advantage of these same processes along with the power of an electret charge. 

Most electret-charged 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 synthetic air filter media. This creates an electro-mechanical structure that attracts particles with a natural charge (and those that pick up a natural charge as they pass through the air), similar to magnetic attraction. 

Electret treatments are an enhancement of an underlying mechanical structure. The combination of different electret treatment patterns/charge distributions and different mechanical structures means that all electret-charged media filters are created equally. Keep in mind that it is impossible to isolate the structural and physical properties of an electret-charged filter media from the charge distribution without impacting other filtration mechanisms and/or other filtration properties. 

2 Filter Performance Factors That Improve with Electret-Treated Media

Imparting an electret charge to synthetic, nonwoven filter media that has a robust underlying mechanical structure offers two important benefits. 

Benefit #1: The electrostatic effects created in an electret-charged synthetic media are particularly useful in increasing the capture efficiency for fine and submicron particles. These fine particles are designated as E1 and E2 particles under the ASHRAE 52.2 Test Standard, which is used to evaluate the filtration efficiency of filters and assigns a Minimum Efficiency Reporting Value, or MERV, to each filter based on its ability to filter particles of differing sizes. 

While submicron particles are much smaller than the void spaces present in most commercial electret-charged media, the electrostatic forces within the media structure allow those particles to be removed with high efficiency. In fact, recent testing showed MERV 8 filters using electret-treated media performing on average 20 percentage points higher in E1 and E2 efficiency than mechanical filters on the market today. 

Mechanical-only media, on the other hand, must use extremely fine fibers or dense structures to achieve high efficiency with submicron particles, and this can create airflow resistance in the filter. 

Benefit #2: Electret-charged synthetic media typically delivers lower airflow resistance in the same filter construction as a mechanical-only filter. This 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 air stream. The more resistance there is, the more energy is needed to push the air through the filters. 

Conclusion

An electret-treated media filter with depth loading, synthetic, nonwoven media and 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, will help to reduce airflow resistance, enhance dust loading and prevent face loading of the filter.

 

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A further suggestion is that attention be paid to documenting the amount of ventilation air actually delivered to the building occupants, as VAV boxes serving conference rooms are typically causing ventilation deficiencies.
By: David Bearg