If a small particle is moving around an obstruction (a water droplet, for example) in the flow stream,it may come in contact with (be intercepted by) that object because of the particle’s physical size.This interception of particles on the collector usually occurs on the sides before reaching the top or bottom. Because the center of a particle follows the streamlines around the droplet, collusion occurs if the distance between the particle and droplet is less than the radius of the particle. Collection of particles by interception results in an increase in overall collection efficiency.
This effect is characterized by the separation number, which is the ratio of the particle diameter
to the droplet diameter (USEPA-81/10, p. 9-8) and is expressed as: