An effective air filtration strategy provides a prime defence for building occupants and HVAC equipment against pollutants generated within a building as well as pollutants from air drawn into a building from the HVAC system.
Indoor air pollution is a problem in commercial and institutional buildings. In fact, indoor air is typically two to five times more polluted than outside air. That is one reason why 50 percent of all illnesses are thought to be either caused by or aggravated by poor indoor air quality (IAQ). Poor IAQ is more than just a nuisance and a health hazard. In the U.S. alone, it costs the nation's economy upwards of $160 billion each year in medical costs and reduced productivity.
Most of the respirable dust and particles people breathe into their lungs is approximately 5 microns or smaller – a fraction of the size of a grain of sand. Not all air filter media is capable of efficiently capturing such small particles. That is why taking a closer look at filter media is so important. Choosing the right filter media and overall air filter design can provide higher efficiency removal of respirable particles associated with poor IAQ.
In the past, filter media was considered a commodity and was specified and purchased solely on the purchase price. Today, facility professionals are increasingly aware of issues associated with poor IAQ. In their efforts to improve IAQ for building occupants, they are realising the added value and benefits of HVAC air filters with filter media utilising a combination of a robust mechanical structure and an electret charge.
How air filter media works
Air filters capture particulates on the filter media, which is the material within the filter that removes particles and impurities/pollutants from the air. This capture requires two things to happen. The first is that the particle will collide with or be removed by the ‘fibres’ that make up the filter media. The second is the probability that the particle, once contacting the filter, will continue to adhere to the media fibre.
Air filter media is available in two types: mechanical-only and electro-mechanical. Mechanical-only air media relies solely on the four particle capture processes explained below. Electro-mechanical air filter media relies on these same processes, along with one major enhancement: an electret charge.
According to the NAFA Guide to Air Filtration (Fourth Edition, 2007), there are four primary methods of mechanical particle capture.
Impingement
Impingement is the mechanism by which large, high-density particles are captured. As air flows through a filter, it must bend or change direction many times to flow around the filter fibres. Because of their inertia, larger particles resist change in direction and attempt to continue on in their original direction. Thus, they collide with, and adhere to the fibres.
Interception
Interception occurs when a particle follows the air stream, but still comes into contact with the fibre as it passes around it. If the forces of attraction between the fibre and the particle are greater than the force of the airflow to dislodge it, the particle will stick to the fibre. Interception is enhanced when the size of the fibre is closest to the size of the particle.
Diffusion
Diffusion explains the capture of very small particles at lower air velocities. As the contaminated air passes through the filter media, minute particles will tend to move from areas of higher concentration and will take an erratic path described as Brownian Motion. This erratic path increases the probability that particles will come in contact with fibres and will stay attached to them. Diffusion works best with fine filter fibres and very low air velocities.
Straining
Straining occurs when the smallest dimension of a particle is greater than the distance between adjoining filter media fibres.
Enhancing the mechanical structure with electret treatments
Particle capture in electro-mechanical air filter media can be enhanced by adding an additional process that harnesses one of nature's most powerful forces: electrostatic charge. Electro-mechanical air filter media attracts particles that have a natural charge or that pick up a natural charge as they pass through the air.
Electrostatically charged filter media was introduced in the 1970s and began to see commercial adoption in the 1990s. While electrostatically modified media includes a broad class of materials, such as fibrillated electret-charged films and triboelectrically charged needled felts, today, most electro-mechanical air filter media is filament-based, using one of a number of nonwoven forming techniques and synthetic fibre types, including meltblown polyolefins and spunbond polyolefins, to name a few. Corona charging is generally 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 filter media with a robust underlying mechanical structure.
First, a well designed electret-treated 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.
Second, the electrostatic effects created in an electret-charged media are particularly useful in increasing the capture efficiency for submicron particles. This is because, 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.
Third, studies have shown that filtration efficiency of electret-treated media is unaffected by relative humidity and by long-term warehousing at high temperatures (54.5°C/130°F), meaning the electret effect is very resistant to age-related decay under conditions relevant to its storage and use.
Fourth, electro-mechanical media almost always delivers lower airflow resistance in the same filter construction as a mechanical-only filter. This translates into reductions in energy consumption and costs. Mechanical-only filters, on the other hand, 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 – see Figure 1. Lower airflow resistance and reduced energy consumption also means electro-mechanical media filters can help reduce greenhouse gas generation – a wise sustainability strategy. Sustainability is also achieved when one considers that filters using only mechanical filtration methods typically require additional mass of filter media to attain the same filtration level as an electro-mechanical filter. In many cases, this additional mass consumes more non-renewable natural resources.
It is important to remember that 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 electro-mechanical media filters are not created equally.
How to select the right air filter
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.
That's why it's important to look for an electrets-treated media filter that has depth-loading media with a gradient density structure in which the media's fibres are more loosely packed on the upstream side and more densely packed on the downstream side. Figure 2 illustrates this. This structure helps to reduce airflow resistance, enhance dust loading and prevent face loading of the filter.
ASHRAE has an important HVAC industry standard that addresses filter efficiency: ASHRAE 52.2-2007. The ASHRAE 52.2 Standard measures the fractional particle size efficiency (PSE) of an HVAC filter. This indicates the filter's ability to remove particles of differing sizes between 0.3 and 10 micrometers 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. Recent studies suggest that an appropriate minimum efficiency for office buildings is MERV 7 to 11 or better to provide good HVAC system cleanliness and efficient operation.
When evaluating filter performance under ASHRAE 52.2, it is important to examine the filter's efficiency in all particle size ranges: E1 (very fine particles in the 0.3 to 1.0 micrometer range), E2 (fine particles in the 1.0 to 3.0 micrometer range), and E3 (coarse particles in the 3.0 to 10.0 micrometer range). The E1, E2, and E3 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 and helping building occupants avoid illness due to poor IAQ. Unfortunately, many pleated filters today have very low E1 and E2 efficiencies.
Conclusion
A nonwoven filter media that uses a combination of mechanical structure and electret treatment provides a means of achieving high initial efficiency and sustained high efficiency in HVAC air filters.
From high filtration efficiencies to reduced energy requirements, there are many reasons to select air filters utilising media that has a good balance of a robust mechanical structure and an electret treatment.
Resources
NAFA Guide to Air Filtration. Fourth Edition, 2007. Published by the National Air Filtration Association.