Abstract: A particle size classifier in which feed material composed of product and overs moves downwardly through a separator unit by the action of negative pressures created by a first air blower associated with the collection of product and a second air blower associated with the collection of overs. As the feed material moves downwardly, the product and the overs are separated with the product passing from within a screen unit to outside the screen unit and the screen unit preventing passage of the overs. Blowback air, applied to the screen unit from the outside cleans the cylindrical screen unit to permit continued passage of product through the screen unit.