Wed, April 14th, 2021

Frequently Asked Questions

What Does Efficiency Mean?

(1) The ability of a device to remove particulate or gaseous material from an airstream by measuring the concentration of the material upstream and downstream of the device.
(2) In the ASHRAE 52.1 Standard test method, it is a measure of the ability of a filter to remove the staining portion of atmospheric dust from the test air. This is officially termed Atmospheric Dust Spot Efficiency.

For air filters, efficiency is determined by the relative size of the filter fibers, the density of the fibers in the filter, the adhesive, if applied, and the velocity at which the air is moving through the filter. Generally, large filter fibers are effective on large particles at high velocities and small filter fibers work best on small particles at low velocities. Proper filter design, including choice of fiber diameter, density of media, and the proper adhesive, are necessary for a filter to function properly. With proper design it is possible for a filter to function well beyond its nominal theoretical efficiency range. For example, a properly designed synthetic media filter, with the proper adhesive, will function as well or better in the medium efficiency range than a dry fine fibered filter.

If the filter fibers are designed to remove large particles, they will not be effective on smaller particles regardless of the depth of the filter. For this reason, properly designed filters must be used to stop fine dust particles. Also, many filter fibers will initially stop dust particles, but will release them after unless the proper adhesive is employed to permanently trap the particles.

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