PREDICTION OF HEPA FILTER COLLECTION EFFICIENCY WITH A BIMODAL FIBER SIZE DISTRIBUTION

Estimation of particle penetration through HEPA (High Efficiency Parti culate Air) filters with the conventional filtration theory requires a ccurate measurements of average fiber size and variance of fiber diame ters. However, it is not an easy task to obtain these properties becau se fiber size distribution varies to a large extent from point to poin t in a HEPA filter. In the present work, measurements of representativ e fiber size distribution from scanning electron micrographs of HEPA f ilter cross-section are performed and the particle penetrations are pr edicted with the measured fiber size distribution. They are compared w ith those from the conventional method (measurement from filter surfac es) and experimental data. As a result, even for filters with inhomoge neity factor delta(p)<1 by the conventional method, the present method gave delta(p)>1, suggesting that delta(p)<1 is caused by the failure in measurement for representative fiber size distribution. However, th e improvement in prediction with more reliable filter properties is no t satisfactory because the conventional filtration theory cannot accou nt for the contribution of fine fibers properly. Introduction of bimod al fiber size distribution together with the measurement of fiber size distribution in a cross-section of a filter is found to give comparab le prediction results with the conventional method without resorting t he inhomogeneity factor of filter packing.