It is well known that filter media are exposed to compression during the manufacturing process. The transportation via conveyor belt rolls compacts the medium over the entire surface, changing the filter performance, i.e. efficiency, pressure drop and dust holding capacity. Further processing steps such as pleating compress the filter media locally at the tops and bottoms. The nonuniform permeability leads to a reorientation of the flow field so that the filter surface is not utilized optimally. Experimental measurement series are very helpful for the design and optimization of the manufacturing process and the nonwoven fabric. Nonetheless, they are associated with large efforts in time and cost.

This work provides an overview of the influence of compression on the filter performance of flat filter media. The pressure drop across the media as well as the fractional efficiency are predicted by models. The approach is based solely on measured material characteristics of the uncompressed nonwoven and requires no additional experimental effort for the compressed material (see Figure 1).

The data basis for the analysis of pressure drop is provided by five nonwovens, which differ by their fields of application and media structure to ensure a broad validity of the presented model. SEM images as well as measurements of the thickness, the area weight and the correlation between pressure and velocity characterize the uncompressed media. The prediction of the pressure drop for multiple compression levels is based on the Darcy-Forchheimer law [1] whereby relevant model parameters (permeability, inertial loss coefficient) are scaled with suitable models [2] referring to the change of material thickness and the corresponding reduction of porosity.

Based on one of the five filter media, which is an electret filter, the experiments are extended to include the fractional efficiency. Single fiber efficiency (SFE) models [3] are a reasonable choice for modeling porous media that consist of fibers. The mechanisms of diffusion, direct interception, inertial impaction, electrophoresis and dielectrophoresis are distinguished. Numerous models are available in the literature (see e.g. [4]), although the accuracy of the models can vary depending on the filter medium or moreover the manufacturing process. A suitable combination of these models paired with weighting factors describes the initial efficiency and can be extended by considering only the changes of material thickness and porosity.

The proposed methods allow for ...