Performance of Electrically Augmented Fibrous Filters, Measured with Monodisperse Aerosols

Abstract

A study of the performance of filters in an external electric field has been carried out in controlled conditions using monodisperse aerosols with a charge distribution close to a Boltzmann. The effects of electric field strength, particle diameter, fiber diameter, and filtration velocity have been measured. The results are in basic agreement with existing theory for the capture of particles by polarization forces and indicate that this process is dominant, although other filtration processes are also significant, particularly when the electric field is low. Reasonable correlation between single fiber efficiency and the dimensionless parameter for capture by polarization forces is observed. The agreement is improved by the inclusion of the hydrodynamic factor into the calculation, but this does not remove all discrepancy. If filters are exposed to an electric field for a protracted period, a small persistent residual polarization of the fibers is observed. Loading the filter with aerosol in the laboratory has little effect on its performance, though eventually the aerosol captured in the vicinity of the live electrode causes shorting out.

Effects of an external electric field on the collection efficiency of air filters: Filtration mechanisms with an external e-field

“…Kao et al (1987) proposed a theoretical model to predict the collection efficiency of a fibrous filter under a strong electric field and compared the calculation results with their experimental results. Thorpe and Brown (2003) investigated the effects of electric field strength, size of particles and filter fiber, and filtration velocity on collection efficiency. In their study, monodisperse particles with Boltzmann charge distributions were used as test particles.…”

Aerosol Filtration by Fibrous Filters: A Statistical Mechanics Approach

“…where h 1,2 is the Hamaker constant and D the distance between the particle(s)/surface. An approximate expression for the Hamaker constant of two bodies (1 and 2) interacting across a medium 3, none of them being a conductor, is (4) where h is the Planck's constant, v e is the main electronic adsorption frequency in the UV (assumed to be the same for the three bodies and typically around 3 × 10 15 s -1 ), n i the refractive index of phase i, ε i the static dielectric constant of phase i, k the Boltzmann constant and T the absolute temperature.…”

“…Adhesive forces involved in filtration process are also studied. These forces include van der Waals forces [2,3] and electrostatic forces [4,5] between particle and fibrous filters. 1 However, the arrangement formats of fibers in a filter will always have great impact on the aerosol filtration since fibers interact during filtration process.…”