Air filtration by moving filaments

It is shown that droplets may be removed from an air stream by impaction on a number of filaments mounted radially on a central disk which is rotated in a plane at right angles to the direction of flow. Published theoretical and experimental results indicate that particles will be impacted on the filaments when the rate of rotation of the disk is sufficient for the Stokes number of the smallest particle which is to be removed to be equal to at least 12. Experiments are described with a filter composed of 390 filaments, 10 cm long and 460 μm thick, mounted on a disk 56 cm in diameter. When the disk was rotated at 1900 rev min-1, just sufficient to ensure that the Stokes number for 4 μm particles of unit-density material was 12 and that the air flow of 2500 ft3 min-1 was fully swept, the measured filtration efficiency was equivalent to 90 ± 5% for 4 μm droplets of unit-density material and progressively more for the larger sizes. Filtration efficiencies were determined by the use of sodium-24 as a radioactive tracer in conjunction with cascade impactors. Filtration devices of the type described have the advantages that they are compact, self-cleaning and offer little resistance to large rates of air flow.