Deep-Bed Electret Filters: The Determination of Single Fiber Charge and Collection Efficiency

Deep-bed electret filters are used to separate small solid or liquid particles from gas flows. Compared with conventional deep-bed filters of similiar pressure loss, they operate with substantially higher collection efficiencies. During the course of filtration, however, the electrical charges of the deposited particles neutralize the electret fibers, temporarily decreasing the collection efficiency. In this paper, experimentally determined fractional separation functions of different types of electret filters are presented for the particle size range from 10 nm to 10 , um at a filter face velocity of 10 cm/s. The initial efficiencies are compared to that of a conventional (glass fiber) filter and also to those of the electret filters in discharged state. The results of long-term filtration experiments show that a complex time-dependent behavior exists for different filter materials. Furthermore, two experimental methods are presented and discussed which determine the charge characteristics of single electret fibers.