The effect of loading dust type on the filtration efficiency of electrostatically-charged filters.

Ventilation filters composed of electrostatically-charged fibers, also referred to as electret filters,are know to have the potential to decrease in filtration efficiency with use. However, little datahave been available on whether such decreases are seen in actual applications. ASHRAE(American Society of Heating and Refrigerating and Air-conditioning Engineers) is developinga new test method (draft ASHRAE 52.2P Method of Testing General Ventilation Air-cleaningDevices for Removal Efficiency by Particle Size) that includes procedures intended to reveal ifa filters efficiency decreases with use. The work described in this paper had three purposes: 1)conduct tests to quantify the changes in filtration efficiency that electret filters undergo withactual use, 2) assess the adequacy of the draft ASHRAE 52.2P methodology to reveal thesechanges, and 3) if needed, develop a laboratory test method that more closely reveals thesechanges than is achieved with the draft ASHRAE 52.2P methodology.Three types of electrostatically-charged filters were evaluated: a rigid cell filter charged via anelectrodynamics spinning process, a pleated panel filter charged via a corona charging process, anda residential filter charged via a split-fiber process. The filtration efficiency measurementscovered the 0.3-10 micrometer diameter size range. Exposures consisted of outdoor ambientair, in-home air, ASHRAE dust, ASHRAE dust without carbon black, and a submicrometer saltaerosol.Results show that all the ambient and in-home exposed filters had substantial decreases infiltration efficiency. Laboratory tests using the draft ASHRAE 52.2P procedures did notreproduce these results well, often showing either little change or increases with loading ratherthan decreases. The submicrometer salt aerosol came closest to duplicating the outdoor and inhomeaerosol exposure results, although the magnitude of the efficiency decrease wasunderestimated in some cases.