Hanspeter Baumgartner

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.

High-speed cinematographic recording and numerical simulation of particles depositing on electret fibres

Abstract A method is described for visualization and recording, as well as for numerical simulation of particle motion during the deposition onto single electrically charged filter fibres (electret fibres). For this a microscope and a high-speed cine camera are used to record the trajectories of particles in the process of flowing around, and being deposited on, a fibre (two-dimensionally in a plane perpendicular to the fibre axis). The particle trajectories are then modeled, based on the experimental data and suitable mathematical models for the fluid flow and the electric fields of the fibre charges. As the fibre charge density, in general, is not known, it is used as a fitting parameter in the model calculation. Using the example of split-fibres with a rectangular cross-section and a bipolar electric charge it is shown that, using models known from the literature, very good agreement can be achieved between experiment and simulation. With this electret fibre type, the trajectory simulations, assuming an ideal bipolar charge configuration, gave effective charge densities between roughly 100 and 500 μAs m−2, corresponding to total surface charge densities between 230 and 1150 μAs m−2.

The collection performance of deep-bed electret filters

Deep-bed electret filters are used to separate fine particles (diameters < 10μm) from gas flows. Compared with conventional deep-bed filters having similiar pressure losses, they operate with substantially higher collection efficiencies. In this paper, two experimental methods allowing the charge characteristics of single electret fibres to be determined, are presented and discussed. Furthermore, the results of the investigated fractional efficiency measurements are given for experiments conducted on two different filter materials, for a particle size range of 10 nm to 10 μm, and at a filter face velocity of 10 cm/s.