Abstract: A skimmer is secured to the top of a cyclone separator. The skimmer includes an outer cylinder forming an extension of the conventional air exhaust duct of the separator and an inner concentric cylinder terminating within the lower end of the outer cylinder. A specially constructed spiral directing wall means is secured within the space and directs the fine dust laden peripherial layer of the exhaust air through the outer peripheral portion of the exhaust duct for selective discharge into and from a discharge opening and duct means. A door is provided for the opening, and has a central pivot means to simultaneously open and close the reverse sides of the door. The cylinders define an annular space aligned with the peripherial layer of the upwardly moving air from the cyclone separator. The peripherial air layers move into and through the annular space into the discharge duct or back into the center of the center cylinder while the central relative clean air passes through the center cylinder for discharge.