Analytical model for the initial efficiency of compressed nonwoven electret media for air filtration

One of the main design goals for filter media is to reach a high filtration efficiency and dust holding capacity while the pressure loss is low. During manufacturing, such media are exposed to compression over the entire surface of the media by transportation via conveyor belt rolls. Further processing steps such as pleating brings in additional local compressed filter regions. It is obvious that the (local) compaction of the nonwoven material changes both the flow resistance and the aerosol capturing. Although experimental measurement series are very helpful for the consideration of these effects, they are associated with substantial efforts in time and cost. The present work considers the influence of compression on the filter performance of air filter media and proposes a model-based prediction of pressure drop and filter efficiency. The application of the method requires only measurements for the uncompressed filter media. The prediction of the pressure drop uses the Darcy law and the efficiency is based on single fiber efficiency (SFE) models for both mechanical capturing and electret media. The presented approach uses a collection of these models paired with correction factors to find an optimal setup for matching the experimental data in the uncompressed state. For variations of compression level as well as media face velocities, the pressure drop, and fractional efficiencies can be predicted without changing the model setup or needing additional experimental data. In the case of electret media, this means that the proposed method can be applied if the charge density of the fibers does not change significantly under compression. The model is validated by comparing the predictions for different compression levels and media face velocities with corresponding experimental data obtained for different nonwoven materials. The proposed analytical model allows for a fast and accurate evaluation of the filter medium’s flow resistance and filter efficiency depending on the level of compression.