Abstract: In accordance with the present invention, there is provided a simple solution to retain the vortex in the confined position, within the fan, thereby permitting higher operational fan speeds a lower energy consumption. By using the false-bottom, which eliminates at least one vertex of the rear wall of the inlet chamber, an aerodynamic and structural asymmetry is introduced at the bottom of the inlet box, near the impeller inlet, that is sufficient to retain the impeller vortex totally within the impeller and inlet cone, and prevent its expansion into the inlet box proper, while still permitting the pre-spin effects of the parallel bladed inlet dampers at reduced load conditions.