Effect of Charging State of Particles on Electret Filtration

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Abstract
Collection performance of an electret filter with rectangular fibers was studied experimentally for cases in which electrostatic effect and Brownian diffusion are predominant by using particles from 0.02 to 0.4 μm in diameter and at different charging states. A single fiber collection efficiency ηED was found to be expressible as a function of dimensionless parameters of Peclet number Pe, and Coulombic and induced force parameters, K c and K In as, here, A, B, C, and D are the numerical constants depending upon the charging density of electret fiber. Indices of each dimensionless parameter determined through the experiment coincided with the previous theory. A maximum penetration of particles appeared in the transition region of predominant collection mechanisms, i.e., between Brownian diffusion and induced force effect, ranging smaller than 0.1 μm in diameter for uncharged particles, and between Coulombic and induced force effects, ranging larger than 0.1 μm in diameter, for charged particles. Semiempirical expressions for a single electret fiber collection efficiency and a most penetrating particle size, applicable to particles in any charging state, were obtained taking account of Brownian diffusion, and induced and Coulombic force effects simultaneously. Undulation of the penetration observed in the filtration of particles in charge equilibrium was explained by using the semiempirical expression for a single fiber efficiency and charge distribution on a particle.

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