Experimental Study of Electrostatic Capture Mechanisms in Commercial Electret Filters

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Abstract
Electret filters are widely used in applications requiring high-filtration efficiency and low-pressure drop. These filters rely on electrostatic particle capture mechanisms in addition to the conventional mechanical capture mechanisms. This study reports experimental data collected on the performance of three types of commercially available fibrous electret filters: corona-charged fibrillated split-fiber media (type A), triboelectrically charged mixed-fiber media (type B) and corona-charged meltblown media (type C). The filtration efficiency of these filters was measured as a function of particle size (i.e., 0.05 to 0.5 μm) and charge state (i.e., singly charged and neutral) for two face velocities. The same experiments also were performed on discharged electret filters to obtain information on the pure mechanical capture mechanisms. The magnitude of the effective surface electric field for each tested electret filter was estimated from a simplified Coulombic capture model, giving information on the ranking of the charge level for the tested electret filters. The single-fiber efficiencies for Coulombic and dielectrophoretic capture mechanisms were isolated by assuming negligible interaction among the different particle capture mechanisms. The single-fiber efficiencies were fitted by power law expressions to a combination of parameters suggested by dimensionless numbers derived from theory. The experimentally obtained power law exponents were in good agreement with those predicted by theory (Brown, 1981; Emi et al., 1987) using simplified charge configurations. No direct semi-empirical correlations in terms of dimensionless parameters could be obtained due to the lack of reliable information on the charge density of the tested electret filters.

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