Abstract: An impeller assembly, comprising a generally planar disc oriented in a first plane and having a circular outer edge; and a plurality of blades mounted to the disc and extending beyond the edge of the disc, with the blades in cross-section having a shape that is substantially symmetrical along a center line and having a first segment and a second segment disposed on opposite sides of the center line, with one segment being symmetrical with at least a portion of the other segment, and with the blades mounted so that the respective center line of each blade is at an angle relative to the first plane in which the disc lies. In another variation, the blades are mounted so the center line of each blade is offset from the plane in which the disc lies.