ASHRAE STANDARDS

The 2007 version of ASHRAE Standard 52.2 quantifies the fractional particle-size efficiency of filters at various particle sizes. A test indicates a filter's ability to remove airborne particles with diameters between 0.3 and 10 microns (Table 1).

A MERV is assigned to a filter depending on particle-size efficiency in three particle-size ranges: 0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns. MERVs range from 1 to 16, but high-efficiency-particulate-air (HEPA) filters, which have MERVs of 17 to 20, are tested with different standards. For general HVAC applications, a MERV of 1 is least efficient, while a MERV of 16 is most efficient. Recent studies suggest that a MERV of 8 is enough to provide good HVAC-system cleanliness and efficient operation, even though the current minimum requirement, according to ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality, is a MERV of 6.

In addition to MERVs, it is important to review fractional particle-size-efficiency curves. These curves show a filter's efficiency in capturing particles of a specific size at a particular airflow rate. Keep in mind that MERVs designate the minimum efficiency at which a filter will perform.

FILTER TECHNOLOGIES AND STYLES

There are many styles of HVAC filters on the market, but only a few types of commonly used filter media.

Some commercial facilities continue to use older flat-panel filters with filter media made of spun glass or high-loft nonwovens. These often are called “throw-away” filters. They typically capture particles through straining and impaction. These filters typically have MERVs of 5 or below and efficiency values of less than 35 percent for particles in the 3-to-10-micron range. That means more than 65 percent of particles in that range, plus virtually all smaller particles, pass through the filter without being captured.

Pleated filters commonly are made with a blend of cotton/polyester or synthetic media. Pleating of the media provides a larger filter-surface area than panel filters. Most pleated filters have MERVs of 6 to 11, with filter-media depths of 1, 2, or 4 in. These filters use a combination of straining, impaction, interception, and diffusion to capture particles. Depending on the filter, capture efficiencies for particles in the 3-to-10-micron range can be 35 to 85 percent. These filters are common in most commercial buildings.

Extended-surface filters also are made with synthetic media or fiber-glass or cellulose/glass-fiber blends. They include bag or pocket, rigid-cell, aluminum-separator, and V-bank filters. Most have filter-media depths of 12 in., although media in pocket filters can be up to 30 in. deep. Pocket filters offer an even greater filter-surface area than pleated filters to provide maximum efficiency with the lowest pressure drop and longest life. Pocket filters use a combination of straining, impaction, interception, and diffusion to capture particles and typically have MERVs of 11 to 15, capturing up to 95 percent of particles larger than 3 microns. Each filter has unique properties related to its rigidity, airflow resistance, and service life and should be compared on a case-by-case basis.

In most commercial buildings, a MERV 8 pleated filter is the recommended minimum. In some buildings, it may be appropriate to use a medium-efficiency pre-filter and a high-efficiency extended-surface filter. The major advantage of using a pre-filter is to extend the life of a high-efficiency filter. In the absence of pre-filters, high-efficiency filters may load with large particles quickly, causing airflow resistance to increase. This can increase the load on system fans and, in extreme cases, induce filter bypass because of leakage at joints in the HVAC system.

The goal of filter installation is to avoid bypass air, which remains unfiltered because it bypasses the filtration media. This unfiltered air contains particulate matter that contaminates housings, coils, fans, and ducts. Bypass occurs when filter media is not sealed properly in the filter frame, when filters are not gasketed properly in filter racks, or when air-handler doors and ducts are not sealed properly.

Gaps around high-efficiency filters or filter housings can decrease filter performance. For a 1-mm gap, bypass flows may be less than 5 percent of the total airflow; for a 10-mm gap, bypass flows can increase to 25 to 35 percent of the total airflow. Air, like any gas or liquid, will flow through the area with the least resistance. Because higher efficiency filters have a larger resistance to airflow, bypass air has a larger effect on high-performance filters.¹ Also, bypass air reduces MERVs. Mathematical models have shown that a 1-mm gap causes a MERV 15 filter to perform as a MERV 14 filter, while a 10-mm gap causes a MERV 15 filter to perform as a MERV 8 filter.¹

CONCLUSION

Effective air filtration provides the primary defense for building occupants and HVAC equipment against particulate pollutants generated within a building as well as pollutants from air drawn into a building from an HVAC system. Because indoor air can be two to five times more polluted than outdoor air, HVAC engineers would be well served to learn more about proper filter selection.

REFERENCE

1. Ward, M., & Siegel, J. (2005). Modeling filter bypass: Impact on filter efficiency. ASHRAE Transactions, 111, 1,091-1,100.

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David Matela, CAFS, is a market manager with Kimberly-Clark Filtration Products, a supplier of HVAC filter media. He has worked for the company for 11 years, during which he has been involved in consumer-product development and nonwoven-material and process development, areas in which he holds eight patents. He can be contacted at dmatela@kcc.com.

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