The effect electrical mobility of aerosol particles on air filtration

Effects of particle charge on the penetration of aerosols through electrets and fiberglass filter media were studied to establish (1) relationships between penetration and non-dimensional electrostatic filtration parameter, (2) relative influence of Coulombic and dielectrophoretic forces on the penetration of submicron particles, and (3) loading effect causing degradation of filter performance as a function of particle loading.;Experiments were carried out using monopolar or "spun" space charge electrets, dipolar "Elitolon" electrets and fiberglass filter mats that do not carry significant electrostatic charges. These filters were tested at two filtration velocities: (a) 5 cm/s and (b) 15 cm/s against aerosols containing either solid particles of polystyrene latex spheres (PLS) or liquid droplets of bis-ethyl hexyl sebacate (BES) of known aerodynamic size and electrostatic charge distribution. The charge distributions were controlled so that test aerosols contained either unipolarly charged (positive or negative) or bipolarly charged (both positive and negative) particles. The experimental data were correlated with respect to electrical mobility or nondimensional electrostatic filtration parameters calculated from the sum of Coulombic and dielectrophoretic forces. Experimental data on electrets showed exponential decay of penetration with increasing values of electrostatic parameters. The penetration decreased by three orders of magnitude when this parameter increased from 0 to +0.5. Similar increase in electrostatic parameter was not possible for uncharged fiberglass filters. The penetration decreased only by a factor of two when particle charge level increased from zero to approximately 30 electrons/particles.;Experimental data on electrets showed that Coulombic forces are dominant in aerosol filtration for submicron particles. However, at low charge levels, the dielectrophoretic filtration mechanism becomes more effective than Coulombic forces as the particle size increased.;There was continued decrease in penetration as the electrets were loaded with liquid droplets. This adverse effect continued regardless whether the droplets were at Boltzmann charge equilibrium or were unipolarly charged. For solid particles having equilibrium charge, there was no significant adverse effect due to particle loading.