Impeller and fan incorporating same

A fan comprising:

• a housing having an inlet and an outlet and a fluid flow path between the inlet and the outlet;
• an impeller having an axis about which the impeller is rotatable in a working direction of rotation (A), and a plurality of aerofoil blades arranged in a circular array lying spaced from and arranged from the axis, with each blade nominally at equal distance from adjacent blades, the inwardly axis-facing surface of each blade defining a longer fluid flow path across the blade than the opposite outwardly facing surface of the blade, said blades being arranged at an angle to the axis so that the impeller has a generally frusto-conical shape with a larger diameter inlet end and a smaller diameter outlet end, the inlet fluid flow into the impeller being via the larger diameter end, and with each blade having an angle of attack (θ) from 0° up to a positive angle of attack less than that at which the blade will induce turbulent fluid flow when the impeller is rotated in a fluid at a working speed in the working direction of rotation, said positive angle of attack not exceeding substantially 22°, whereby rotation of the impeller in the working direction of rotation (A) induces an inlet fluid flow generally axially towards the impeller and an outlet fluid flow away from the impeller in directions generally inclined at an angle which lies between substantially radial and substantially axial directions relative to the axis, said impeller being mounted in the flow path within the housing, and
• a drive means enabling the impeller to be rotated in its working direction of rotation (A) to cause fluid flow between the inlet to the outlet of the housing.