Filtration efficiency and aging of electret filters

S. Schumacher*, R. Jasti, A.M. Todea, C. Asbach, Institute for Energy and Environmental Technology e.V. (IUTA); A. Hellmann, Technical University Kaiserslautern, Germany

Particle filters find applications in various fields, e.g., to improve the air quality, to protect components from dust or to reduce emissions. An important aspect in the choice of an appropriate filter is to reach a high filtration efficiency combined with a low pressure drop. To achieve this, often electret filters are used. Such filters carry electric charges on the fibers, which interact either directly with charged particles by Coulomb attraction (electrophoresis) or by interaction with induced dipoles in uncharged particles (dielectrophoresis). Thus, the filtration efficiency is enhanced compared to the purely mechanical uncharged filter medium without an increase of the pressure drop. However, a known drawback of electret filters is that the efficiency can drastically degrade with time due to exposure to particles, gases or humidity.

Although there are numerous studies on electret filters, they provide a heterogeneous and sometimes even contradictory picture, which leaves several questions open. For example, little is known about the charge distribution on the fibers, the particle size dependence of the individual electrostatic precipitation mechanisms and the relative importance of different aging effects. To address these questions, the filtration efficiency of electret filters with respect to differently charged aerosols in the size range from a few 10 nm up to several micrometers was investigated. To separate the electrostatic from the mechanical contributions, the filter are also investigated after discharging with isopropyl alcohol.

To provide a test aerosol in different charge states, three methods are employed: (i) a Kr-85 bipolar neutralizer to establish a well-defined bipolar equilibrium charge distribution, (ii) a self-developed unipolar diffusion charger to provide highly negatively or positively charged particles, and (iii) a self-developed electrostatic precipitator, which lets pass only completely uncharged particles. From the dependence of the filtration efficiency on the charge state of the test aerosol, the different filtration mechanisms can be separated and conclusions on the charge state of the filter can be drawn. Furthermore, the particle-covered filter substrates are investigated by microscopic imaging techniques to analyze the charge distribution on the fibers...

Session: G11 - Short Oral + Poster Presentations II
Day: 14 March 2018
Time: 14:45 - 16:45 h

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