EFFECT OF LOADING WITH AIM OIL AEROSOL ON THE COLLECTION EFFICIENCY OF AN ELECTRET FILTER

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Abstract
Loading of an electret filter changes the distribution of electrical field in the filter from its preloading condition, and, therefore, affects the filtration efficiency of the filter. Liquid droplets collected on electret filters cause degradation of the electrostatic enhancement of filtration efficiency because of charge neutralization and the formation of a dielectric coating over die charged fibers. In this study, calculations were made for the penetration of aerosol particles through a spun-type electret filter as a function of the particulate loading. An assumption was made that each charge collected neutralized one charge of opposite polarity on the fibers of the filter. It was also assumed that the electrostatic charges present on the particles followed the Boltzmann equilibrium charge distribution. The decrease in fiber charge and resulting increase in penetration were calculated as a function of time and of total particulate loading on the filter. The calculated penetrations were compared with experimental measurements of loading on a spun fiber electret filter challenged with monodisperse liquid droplets of bis-Ethylhexyl Sebacate with equilibrium charge distribution and with zero charge. The rate at which the penetration increased was found to be the same for particles with zero charge as for particles with equilibrium charge distribution. For 1 um diameter droplets with equilibrium charge the theory predicted complete discharge of the filter at a loading of around 200 g/m2. Experimentally, only about 0.3 g/m2 was required. This difference indicates the presence of additional mechanisms for the discharge of the fibers.