A centrifugal fan including a fan hub lying generally in a plane normal to the axis of fan rotation and having a central opening for receiving a motor housing, and a plurality of fan blades of substantially identical construction spaced circumferentially evenly around the fan hub and extending away from the fan hub in a direction parallel to the axis of fan rotation. Each fan blade includes a leading edge at its innermost radial periphery, and the leading edge, at least at the axially upper one-half section thereof, is convexly curved in the shape of a quarter ellipse having a major axis, Mg, substantially parallel to the axis of fan rotation and a minor axis, mh, normal to the major axis Mg. The fan also includes a continuous shroud joined to each fan blade at an axially upper section of the radially outer portion thereof and the shroud is concave in cross-section. The fan orifice used in conjunction with the fan has an entry portion that is convex in cross-section and is in...