Theory of size classification of airborne dust clouds by elutriation

Abstract

Particle size classification systems are considered of the type in which an airborne dust cloud is drawn at a low velocity (so that inertial forces are negligible) through a vertical or horizontal duct (elutriator) in which the larger particles are removed by gravitational settlement. It is shown that within such a system the concentration of particles of given falling speed f remains constant along any (particle) flow tube; tubes coming from the open cloud contain particles at the original cloud concentration, whereas those originating from the underside of an impervious surface are empty. It follows that if particles are drawn upwards on to a horizontal inverted filter immersed in the cloud, there being a uniform vertical air velocity v immediately beneath the filter surface, then the number of particles collected will be a fraction 1 - (f/v) of those originally contained in the sampled air. The addition of a vertical elutriator tube beneath the filter has no effect on the selectivity, in particular the velocity profile within the tube is of no consequence and it is not possible to obtain a sharper cut than that indicated by the above formula. If the elutriator is fitted with a convergent upper section before the filter, the sharpness of the classification will be diminished.