EXPERIMENTAL-STUDY OF ELECTROSTATIC CAPTURE MECHANISMS IN COMMERCIAL ELECTRET FILTERS

Electret filters are widely used in applications requiring high-filtra tion efficiency and low-pressure drop. These filters rely on electrost atic particle capture mechanisms in addition to the conventional mecha nical capture mechanisms. This study reports experimental data collect ed 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 ch arged meltblown media (type C). The filtration efficiency of these fil ters was measured as a function of particle size (i.e., 0.05 to 0.5 mu m) and charge state (i.e., singly charged and neutral) for two face v elocities. The same experiments also were performed on discharged elec tret filters to obtain information on the pure mechanical capture mech anisms. The magnitude of the effective surface electric field for each tested electret filter was estimated from a simplified Coulombic capt ure model, giving information on the ranking of the charge level for t he tested electret filters. The single-fiber efficiencies for Coulombi c and dielectrophoretic capture mechanisms were isolated by assuming n egligible 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 derive d 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-empiri cal correlations in terms of dimensionless parameters could be obtaine d due to the lack of reliable information on the charge density of the tested electret filters. (C) 1998 American Association for Aerosol Re search.