Efficiency can decline as liquid accumulates on a fibrous filter. The amount of liquid retained affects efficiency, and can be influenced by drainage, re-entrainment, and evaporation. In this study, equations were developed to describe non-stationary filtration and drainage for 0.88 cm thick wetted fibrous filters as a function of saturation ratio, a measure of liquid retention. Filtration efficiency can be predicted by modifying existing single-fiber efficiency theory to account for increases in interstitial velocity and removal of some fibers from effective collection due to the presence of liquid. An empirical expression was developed from experimental data to predict drainage rates from saturation ratios. During experiments, no re-entrainment was observed under conditions comparable to those found in industry.
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Acknowledgements2The work was made possible in part by a gift from Ford Motor Company and the United Auto Workers to support research in air engineering at the University of North Carolina at Chapel Hill and by EPA STAR Graduate Fellowship dU914812-01. The authors wish to thank Evanite Fiber Corporation for supplying glass "bers used in this study.