The design of electrostatically augmented moving bed granular gas filters

Abstract

Granular gas bed filters have been used in industry for a considerable period and mathematical descriptions of dust capture have allowed rigorous design of static beds. Provision for bed movement and electrostatic augmentation, which allows much thinner continuous beds to be used, requires adaptation of design methods for these phenomena. Design methods that allow for this are developed for a cross-flow bed with vertical bed movement and a number of granule and dust types. Direct current charging is applied to the bed itself and to the particles before they enter the bed. In the case of electrostatic augmentation, it is shown that simple models of spherical particles describe the mechanism adequately. The advantages of pre-charging dust particles before they enter the bed are indicated by calculation and proved experimentally. Parameters to describe the enhancement of filtration efficiency by the collected dust are obtained experimentally. It is shown that the factors controlling re-entrainment vary with particle size. For the dust particles less than 1,5 micrometers in size, re-entrainment is linked closely to the electrostatic capture mechanism which is dominant in that size range. For particles approaching 10 micrometer, re-entrainment can be neglected as the impaction efficiency, which is dominant for particles of this size and larger, approaches unity. A complex situation exists between these particle sizes as the magnitude and predominance of capture mechanisms in this region are determined by a number of operational parameters. It did not prove possible to develop predictive equations for re-entrainment efficiency using the results of this study. A number of heuristics are however developed that allow rational design by the use of the empirical parameters found, and that will be valid for the range of parameters used in this work.