Air filters perform an important function in commercial and institutional facilities. Because indoor air is typically two to five times more polluted than outdoor air, air filters are needed to remove respirable particles such as microorganisms, dust and allergens from the breathing air.
Not all air filters are created equal. Indeed, there are marked differences among various types of filters and especially filter media when it comes to air cleaning and energy use. Depending on the filter media used, even two filters with the same filtration efficiency rating (MERV) can vary widely in both particle capture performance and energy consumption.
Until recently, filter media was considered a commodity and was specified and purchased solely on the purchase price. But today, the industry is realizing the advantages of HVAC air filters with mechano-electret synthetic filter media for their added value and long-term cost savings.
A Brief History of Air Filtration
Air filters were initially developed in the 1930s to protect forced-air heating system elements from dust, and thus reduce the risk of building fires. Air filters of this period consisted of loose mats of fibrous glass, animal and synthetic fibers, metal mesh and various fabrics of cellulose and other porous materials. In the 1940s and 1950s, buildings started to use air filters to protect the cooling coils of the HVAC system from dust, thus preventing fouling. Filtration efficiencies for these early filters were very low.
In the 1980s and 90s, the industry started looking at the ability of filters to not only protect HVAC equipment, but also to clean the air stream for building occupants. Advances in styles and types of filters began to emerge. And more attention began to be paid to the filter media—the material within the filter that removes particles and impurities/pollutants from the air. Today, air filtration is seen as playing a role in green building initiatives, energy conservation, sustainable building design, and occupant health and productivity.
Why Is Filter Media So Important?
Effective air filtration provides the primary defense 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.
Most of the respirable dust and particles people breathe into their lungs is approximately seven microns or smaller—a fraction of the size of a grain of sand, and lung-damaging dust can be as small as 0.5 micrometers. It is the job of the filter media to captur≠≠e these respirable particles, both to keep them from fouling the HVAC equipment and to keep them out of the breathing air.
Poor indoor air quality (IAQ) significantly influences the occurrence of communicable respiratory illnesses, allergy and asthma symptoms, and sick building symptoms. Poor IAQ is so prevalent that 50 percent of all illnesses are either caused or aggravated by poor IAQ.
Poor IAQ can also lead to absenteeism and reduced productivity among building occupants. It’s estimated that U.S. adults miss about 14.5 million work days each year because of poor IAQ. And the total cost to the U.S. economy from poor IAQ is estimated to be as high as $160 billion a year.
In addition to their role in IAQ, filters also play a major role in the energy consumed in operating the HVAC system. In fact, energy costs account for 81 percent of the total life cycle cost of a filter system.
The energy used by filters is directly proportional to the airflow resistance of the filter media. A filter media with a lower resistance to airflow requires the HVAC system motor to overcome less resistance to deliver the required air flow, thus reducing the blower or fan motor’s energy consumption.
Filter Media Technology & Filter Styles
There are many styles of HVAC air filters on the market today, though only a few types of commonly used filter media. In most commercial buildings, the minimum recommended filter is a MERV 8 pleated filter, which should be used on each return air grill for air handlers used during construction to obtain credits under the USGBC LEED-EB program. Facilities can also obtain LEED credits relating to Indoor Environmental Quality by installing MERV 13, 14 or 15 air filters prior to building occupancy.
Panel Filters. Some commercial facilities still use this pre-WWII technology in their air filtration system. Often called “throw-away” filters, these filters are typically made with fiberglass media and priced lower than filters made with other media types. The fiberglass media in panel filters provides good temperature resiliency, but low filtration efficiencies (less than 20 percent for particles of three to 10 m≠≠≠icrons). That means that more than 80 percent of these particles, plus virtually all of the smaller particles, pass through the filter without being captured.
Panel filters also may be made with high-loft polyester (PET) media, which provides for a low resistance to airflow, but also has a lower particle capture efficiency. This means that, while the filters may be good at protecting HVAC equipment from damaging particles, they may not be effective at protecting people from smaller particles that can cause IAQ problems.
Pleated Filters. Pleating of the filter media provides a larger filtering surface area within a given space than with panel filters. Most pleated filters on the market today are MERV 8-11 (though MERV 13 pleated filters have recently been introduced) with filtering media depths of two or four inches. Filtration efficiencies of pleated filters range from 35 percent to more than 85 percent for particles of three to 10 microns (MERV 6 to MERV 11). MERV 13 pleated filters provide more than 90 percent efficiencies capturing particles in the one to 10 micron range.
Pleated filters may be made with polyester or cotton/polyester media, needlefelt media, or bi-component spunbond media. One problem with needlefelt media is that while it provides good mechanical efficiency for particle capture, it is often subject to higher airflow resistance, and thus increased energy costs in operation. Bi-component spunbond filter media has durability advantages in terms of strength and tear resistance compared with fiberglass and meltblown media, and it is competitively priced against cotton/poly and needlefelt media. It also has high dust-holding capacity for longer filter life.
Pocket Filters. Pocket filters may be made with meltblown polypropylene media or air-laid fiberglass media. The thick, dense structure of meltblown polypropylene media makes it hard for dust, dirt, germs and water droplets to pass through. Air-laid fiberglass media gives pocket filters good mechanical efficiency for particle capture, but with a higher resistance to airflow than certain synthetic media of the same efficiency. Higher-efficiency pocket filters (65-95 percent efficiency) can also use synthetic meltblown filter media, which resists shedding and is stable and resilient.
Stiff Pocket Filters. Stiff pocket filters can be made with either fiberglass media or synthetic media. Bi-component synthetics may offer good dust-holding capacity, while fiberglass media often holds up in applications using UV light.
Rigid Cell Filters. These may be made with air-laid fiberglass or bi-component spunbond media, which offers high particle capture efficiency at a relatively low cost. Bi-component spunbond media also resists shedding and doesn’t absorb moisture.
Mini-Pleated Filters. Wet-laid fiberglass media provides good mechanical efficiency for particle capture, while synthetic filter media typically offers better durability – especially important in avoiding damages to very expensive filters.
Look Inside the Filter The HVAC air filter industry has made significant strides in the recent years, allowing for the production of nonwoven (synthetic) filtration media that has both a performance and cost advantage over traditional materials for HVAC filters.
One advantage of synthetic filter media can be seen in the methods through which various media types capture airborne particles. Non-synthetic filter media captures particles only through mechanical methods such as straining (particle sizes are larger than the spaces between the fibers in the media), as well as through impingement and interception (particles collide with and remain attached to a filter media fiber based on a number of forces).
Synthetic filter media, on the other hand, not only works by mechanical methods, but it also has the ability to be electrostatically charged, which increases initial filtration efficiency without affecting resistance to airflow. The filter fibers are charged, through various processes, to create a charge differential that generates the attractive force. This leads to high initial filtration efficiency. Sustained efficiency is provided through complimentary mechanical methods.
Filters that provide a good balance of mechanical and electret efficiency will almost always outperform a filter that relies solely on mechanical efficiency. The electrostatic effects in an electret-charged media are particularly useful in increasing the capture efficiency for submicron particles. Mechano-electret filters also deliver lower airflow resistance than mechanical-only filters, which translates directly to reductions in energy consumption and cost. However, the combination of different electret treatment patterns/charge distributions means that all electret filters are not created equal.
When evaluating filters that combine mechanical and electret filtration methods, it’s best to look for a depth-loading media with a density gradient structure. This combination can help to reduce airflow resistance, enhance dust loading and prevent face loading of the filter.
Filters with synthetic filter media also have energy cost savings advantages, especially when compared with filters made of fiberglass. For example, for a filter of a given configuration and efficiency, the version containing synthetic media can have a lower airflow resistance than its fiberglass counterpart, allowing buildings to maintain filtration efficiency in HVAC systems while lowering their energy costs.
Additional advantages of air filters made with synthetic media include:
Synthetic media made of polyolefin fibers is hydrophobic and will not absorb moisture that would support microbial growth;
Synthetic media has the ability to provide superior performance while using less media than other filters—a good way to reduce raw material usage and waste. Furthermore, a high-capacity pleated filter made with synthetic media can extend filter life and reduce changeouts, also reducing waste streams – a wise sustainability strategy;
Filters containing synthetic media can be made without binders, which can cause off-gassing; and
Synthetic filter media can be made of thermally bonded, continuous polyolefin fibers that resist shedding to help keep HVAC ductwork and components cleaner.
From high filtration efficiencies to reduced energy requirements, there are many reasons to select air filters with synthetic filter media that has a good balance of a robust mechanical structure and an electret charge. The HVAC system, coupled with the right filters, can heave a pronounced effect on a building’s ability to maintain a healthier, more productive work environment.