COLLECTION MECHANISMS OF ELECTRET FILTER

Abstract

Collection efficiency of a single electret fiber, which carries permanent positive and negative charges, was studied. Theoretically, it was obtained by solving the equation of particle motion taking account of the induced and Coulombic forces simultaneously. When either induced or Coulombic forces dominates particle collection, the collection efficiency was found to be proportional to 2/5 power of induced force parameter K_In, and 3/4 power of Coulombic force parameter K_C, respectively. However, when both forces are effective simultaneously, the efficiency was not expressed by the simple superposition of both effects because of negative interaction between both forces. Experimentally, collection efficiency of a single electret fiber was measured by using monodisperse sodium chloride particles ranging from 0.01 to 0.4 μm in diameter for filtration velocities from 5 to 200 cm/s, under different charging state of particles, i.e., uncharged, singly or doubly charged and charged in Boltzmann equilibrium. It was found that the experimental efficiency was markedly influenced by the small change in the charging state of particles, and that both Coulombic and induced forces affect the collection of charged particles simultaneously. The dependency of experimental collection efficiency on dimensionless parameters K_In and K_C coincided with the theoretical calculation. Based on the theoretical and experimental results, the following semi-empirical expression for the single fiber collection efficiency, which is applicable to particles in any charging state, was presented. $