Electrostatic air filters may not protect your building occupants or production processes.
Air filters used in HVAC systems typically use filters that use mechanical particle capture efficiency or filters that incorporate an induced electrostatic charge to enhance the filters particle capture efficiency. Mechanical principles include straining, inertia, interception and diffusions all being related to the filter fiber density, fiber size and airborne particle size relationship to the fiber. Generally, the smaller the fiber size the higher the efficiency of the filter. Electrostatic air filters use a larger fiber size that is charged during manufacture to enhance its filtration efficiency. Filter manufacture costs are directly related to fiber size and the larger fibers of electrostatic filters sometimes gives them a price per unit advantage. The advantage comes at the sacrifice of a decreasing filter efficiency over time.
The American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) recognizes this problem in the forward of their air filter testing Standard 52.2; Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size. The direct statement is “Some fibrous media air filters have electrostatic charges that may be either natural or imposed upon the media during manufacturing. Such filters may demonstrate high efficiency when clean and a drop in efficiency during their actual use cycle.”
Air filters should always be selected based upon the size of the contaminants of concern. As an example, authorities have noted that 0.3 micron to 0.7 micron is a critical range for considering lung damaging particles and is directly related to the human lung pore opening size of 0.5 micron. These particles can enter the pore and create health problems. Particles less than 0.3 micron in size are not considered a concern as they aerodynamically are not respirable.
Authorities state, when considering protecting a buildings’ occupants from particulate contaminants of concern, a filter with a minimum efficiency reporting value (MERV) of 13, or 14, filters should be the basis of design based upon their respective 50-80% efficiency in the 0.3 micron range. As an example, MERV 14 air filters are required in critical care areas of hospitals to remove particles that could exasperate the health of individuals that already have compromised immune-systems. These filters also protect visitors and employees. The MERV 14 efficiency is based upon health-based research dating back to the 1950’s relating to specific size airborne contaminants.
Most users expect that a filter with an efficiency of MERV 14 upon installation, will have the same efficiency 3, 6 or 12 months down the road. Because electrostatic air filters can lose efficiency over time based upon the principle of particle capture used, a MERV 14 may end up as a MERV 11 or a MERV 13 may become a MERV 8. Some filters drop in efficiency in a period of weeks.
To ensure the protection of your facility ASHRAE developed an optional test wherein the manufacturer can provide not only the air filters’ MERV but also its MERV-A. The additional testing step is designed to demonstrate how an air filter will perform over time. Will it maintain its efficiency protecting the environment or will it lose efficiency over time at the sacrifice of building air quality?
Camfil publishes multiple technical materials relating to MERV, MERV-A and considerations filter purchasers should consider before installing filters in there systems. The safe route is to always ensure that the filters that you select have matching MERV and MERV-A values. Contact your local Camfil Distributor or Representative for additional information.
