>If you strap a HEPA to an axial fan and seal it, the fan runs effectively "empty", i.e. it displaces only little air which most likely even doesn't pass the fi

>If you strap a HEPA to an axial fan and seal it, the fan runs effectively "empty", i.e. it displaces only little air which most likely even doesn't pass the filter This. I tried a 3M MERV 12 filter taped to the intake of a cheap $12 box fan and the airflow through the fan went to effectively zero. The fan actually was pulling more air from the front of the fan around the sides of the fanbox than through the filter. Increasing the fan speed just increased this backflow. You are spot on with a radial fan being what you want here, and I notice that basically every commercial air purifier uses such a fan.