10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Filtration Efficiency and Discharging Behavior of Electret Filters
STEFAN SCHUMACHER, Rama Rao Jasti, Maximilian Kerner, Kilian Schmidt, Albert Hellmann, Sergiy Antonyuk, Christof Asbach, Institut für Energie- und Umwelttechnik e.V. (IUTA)
Abstract Number: 660 Working Group: Aerosol Physics
Abstract Electret filters find many applications since they offer a high initial filtration efficiency combined with a low pressure drop. They carry electric charges on the fibers, which interact either directly with charged particles by Coulomb attraction (electrophoresis), or by interaction with induced dipoles in both charged and uncharged particles (dielectrophoresis). However, a known drawback is that the efficiency of electret filters can drastically degrade with time due to exposure to particles, gases, solvents, or humidity.
In this study, the efficiency of electret filters with respect to differently charged aerosols was investigated to learn more about the charge and particle size dependence of the different electrostatic deposition mechanisms. Therefore, at first a unipolar diffusion charger was developed to provide highly negatively or positively charged particles. First experiments showed that the filtration efficiency strongly depends on the charge state of the particles. Furthermore, a pronounced discrepancy between measurements conducted with sodium chloride (NaCl) and di-ethyl-hexyl-sebacat (DEHS) is found, the origin of which has been further investigated.
To separate the electrostatic from the mechanical contributions, the filters were discharged with isopropyl alcohol (IPA) using two different techniques: On the one hand, the filters were immersed in liquid IPA, as applied for example in the European standard EN 779 for general ventilation filters. On the other hand, the filters were discharged in a cabinet (TDC 584, Topas GmbH) with IPA saturated air, as proposed by the new international standard ISO 16890, which replaces the EN 779 in 2018. Up to now, the comparability of the two methods is not well known. In our study, partially strong deviations in the efficiency of the two methods were found, the origin of which needs further investigation, but might have a strong impact on the reliability of the standards. Furthermore, we found that also for the discharged filters, charged particles are more efficiently deposited than neutral particles, hinting at the importance of image charge effects that are often neglected.
The experimental investigations are supported by 3D direct numerical simulation (DNS) of the filter media using the self-developed software DNSlab. Therefore, a realistic 3D model of the fiber structure is generated and differently charged areas are randomly distributed on the fiber surfaces to compare the effect of different charge configurations. The resulting electric field is computed by explicit integration of the fiber surface charges, while the flow field is obtained by numerical solution of the Navier-Stokes equations. Afterwards the trajectories of either unipolarly or bipolarly charged particles are tracked by the Lagrangian method considering all relevant forces. The corresponding filtration efficiency curves can be directly compared to the experimental results.