Filter Facts: IAQ and Efficiency
Part 1: The Three Types of Air Filters
Part 2: HVAC's Impact on IAQ and Energy Efficiency
The Three Types of Air Filters
By Philip Winterland - October 2009
The air-filtration industry has seen its share of changes over the last decade. Manufacturers have created new products that not only have improved indoor air quality (IAQ) but also have reduced the cost of installing and operating air-filtration equipment in commercial and institutional facilities.
Besides technology improvements, industry organizations are adopting standards, guidelines and requirements that promote healthy IAQ. Debate continues on the impact IAQ can have on building occupants, including whether productivity and overall occupant health improves as air quality improves.
As maintenance and engineering managers continue to focus on IAQ and its impact on their facilities and occupants, manufacturers are providing them with more efficient HVAC systems and components, including air filters. These technology advances include reduced pressure drops and improved filter efficiency, as well as smaller fans and motors, due to the reduced pressure drop across the filters.
Types of Filters
When the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) introduced IAQ standards, it signaled an important change in the air-filtration industry. ASHRAE has been promoting the adoption of the minimum efficiency reporting value (MERV) system to standardize definitions of efficiencies.
MERV measures a filter's arrestance — a filter's ability to remove large airborne particles from the air — and dust-spot efficiency — its ability to remove small airborne particles — and assigns a number based on the filter's ability to remove the particles. Several types of air filters are common in commercial HVAC systems:
Fiberglass filter. This throwaway air filter is the most common type. Layered fiberglass fibers are laid over each other to form the filter media and typically are reinforced with a metal grating that supports the fiberglass to prevent failure and collapse.
Polyester and pleated filters. These filters are similar to fiberglass filters but typically have a higher resistance to airflow and a superior dust-stopping ability.
High efficiency particulate arrestance (HEPA) filters. These units filter the air passing through them at a very fine scale. The U.S. Department of Energy (DOE) and its contractors use HEPA filters that meet DOE standard STD-3020-97, Specification for HEPA Filters Used by DOE Contractors, to filter 99.97 percent of all particles 0.3 microns or larger.
Washable air filters. These products are not as common and rely on the build-up of dust along the cloth to improve the efficiency of the filter. Industrial processes involving high volumes of coarse dust are typical applications.
Because filters are fairly simple — they essentially are porous membranes that allow air to flow through them — little change has occurred to improve problems associated with air pressure drop and its impact on energy efficiency.
Comments
Curtis Bowe wrote re: The Three Types of Air Filters
on 10/9/2011 4:42:10 PM
I am a fan of the washable air filters. I'm a big fan of eliminating waste and being able to use the same air filter over and over saves waste. It doesn't matter if you need washable Honeywell or washable Aprilaire filters, washable filters are made for every unit. Help eliminate waste and go with a washable filter!
Steve Voros wrote re: The Three Types of Air Filters
on 2/24/2011 8:25:41 PM
Washable air filters are, in my view, an under used resource. They can be effective and easy to maintain providing there is regular maintenance.
Regards,
Steve
rcox3338 wrote re: The Three Types of Air Filters
on 10/29/2009 11:54:24 AM
This article brings up a good point about the importance of air filter pressure loss as it affects the energy consumed by an HVAC system. Indeed, the filtration industry has been working diligently to reduce airflow resistance through a combination of filter pleating and the use of synthetic filter media.
One point of clarification is needed, however. There is a big difference between an electrostatic air filter appliance, as described in the article, and a commercial pleated air filter using electret-treated filter media. Some room air cleaning appliances have been associated with ozone generation. Electret-treated pleated filters used in commercial HVAC units have NOT been associated with ozone generation. These disposable filters increase fine particle capture efficiency and do not require the time-consuming plate cleaning described by the author.
For most commercial and institutional buildings, one of the easiest steps to take in an effort to reduce HVAC system energy costs is to choose a filter with a lower resistance to airflow. Filters that combine an electret treatment for initial and fine particle capture efficiency with a robust underlying mechanical filtration structure for sustained particle capture efficiency will also help to improve IAQ.
