I came across Ergun's equation on another post here, not sure if that would be entirely ideal. My task for my project is to create a device that sucks in polluted air such as smog and use a HEPA filter to filter out the polluted matter. I want to find the pressure drop through a clean filter and also as time goes on with a dirty filter. This pressure drop value is needed to justify whether or not a second fan is needed after the filter to provide continuous air flow.