performance of the embodiments will be optimized where the curvature the fan assembly 11 comprises a fan rotor 12 having a plurality of vanes 13 the rotor 12 being adapted to be rotated by an electric motor not shown the fan motor is supported within a housing 14 having an inlet 16 and an outlet 17 the housing 14 has a whirl shaped form at least on the internal surfaces which resembles the shape of shellfish of the genus trochus this shape corresponds generally to the streamlines of a vortex in the drawings it is to be appreciated that the form indicated on the external surfaces is intended to correspond with the form of the internal surface although in a real fan the form of the external surface is not of importance to the performance of the fan as such and may be quite different from the internal surfaces indeed the housing might be constructed with an internal shroud which comprises a separate component from the external surface of the housing and it is to be appreciated that where such a design is undertaken it is the internal surfaces of the separate shroud which must conform to the principles as described herein in the first embodiment the housing is formed in two portions 18 and 19 the first of these comprises an inlet portion 18 which includes the inlet 16 and also provides mounting means not shown to support the fan motor to which the fan rotor 12 is attached the inlet portion 18 also acts as a shroud around outer extents of the.