In typical automotive filtration applications, laminar flow occurs, so you can consider the application of log-normal pore distribution technology. A more important structure is that loose paper not only has a larger number of pores than compact paper, but also finer. The hole size has a wide range, so it is impossible to use conventional papermaking methods to produce a paper sheet with a large number of uniform pores.
In the case of organic liquid with a certain pressure head as the background, let air pass through the paper in a boosted state, and then analyze the air flow to obtain the distribution of pores. Unfortunately, it is not yet possible to establish a correlation between the measured pore distribution and the filter paper’s particle removal efficiency. The main reason lies in the complexity of the mechanism by which particles are removed when the liquid flows through the filter medium. A simple filtration process, in fact, it is very dangerous to infer anything about filtration performance based on the pore size value.
Therefore, the current design of automotive filter paper is mainly based on accumulated experience and knowledge plus new ideas provided by paper structure analysis, rather than based on the unquestionable reality of filtering behavior.
The filter paper has a positive asymmetry that is easy to see. The upper surface is looser and the permeability is high, and the lower surface gradually becomes tighter and the permeability is low. The filter performance of the filter paper is obviously determined by which surface of the filter paper is used as the upstream side during filtration. Therefore, the filter paper manufacturer should mark the lower surface of the filter paper (usually a color line) to make it easy for the filter manufacturer to recognize and use the upper surface as the upper side.
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Link to this article：Discussion on Asymmetry of Filter Paper in Automobile
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