INFLUENCE OF INTERNAL STRUCTURE OF FIBROUS FILTER ON COLLECTION PERFORMANCE AT LOW PECLET NUMBER

The fiber size of HEPA (High efficiency particulate air) filters has b een drastically decreased in the pursuit of higher collection performa nce, i.e., high collection efficiency and low pressure drop. The reduc tion of fiber size as well as the importance of collection of ultrafin e particles has raised various problems in particle collection at extr emely low Peclet numbers, Pe. The present work focuses on clarifying t he filtration problems at low Pe by studying the collection characteri stics of ideal model filters. The main findings of the present work ar e: (1) the log-penetration equation is not valid at low Pe because sin gle fiber efficiency in the vicinity of the filter surface is higher t han the inside; (2) filter with a lower packing density has a higher c ollection performance for Pe<1; (3) the single fiber efficiency of an inclined fiber can be predicted with the prediction equation for perpe ndicular fibers by defining Pe based on the airflow velocity component normal to the fiber axis; and (4) the effect of inclined fibers on th e particle penetration is not significant so the assumption of perpend icular fiber alignment to the main airflow can be introduced without l oss in accuracy.