Fluid impeller

A mixed flow fan comprising:

• one or more fluid impelling blades;
• a hub adjacent to an edge of each of said one or more fluid impelling blades, at least one blade facing surface of said hub being formed substantially as a segment of a sphere of radius r1 about a center of curvature;
• a shroud adjacent to an outer edge of each of said one or more fluid impelling blades, at least one blade facing surface of the shroud being formed substantially as a segment of a sphere of radius r2 about the center of the curvature;
• wherein
• each of said one or more fluid impelling blades has a hub abutting edge curved substantially to fit against a sphere of radius r1 and a shroud abutting edge curved substantially to fit against a sphere of radius r2 so that each of said one or more fluid impelling blades is attachable to said hub and said shroud at various angles about an axis passing through the center of curvature, each angle of said various angles in use achieving a different performance and at each angle of said various angles said hub abutting and said shroud abutting edges of one or more fluid impelling blades remain substantially abutting said hub and said shroud respectively,
• said hub, said shroud and said blades forming a single rotatable assembly,
• each of said one or more fluid impelling blades is pivotable about a respective mounting point an said shroud,
• each of said one or more fluid impelling blades is pivotable about a respective mounting point on said hub, and
• the mounting points on said shroud and said hub lie substantially on an axis passing through the center of curvature of said hub and said shroud.