THIS INVENTION relates to a fluid displacement machine, an accessory therefor and a method of guiding intake fluid onto an impeller of the fluid displacement machine.

According to a first aspect of the invention, there is provided an accessory for use with a fluid displacement machine having a casing, an impeller, an impeller drive shaft and a fluid intake area, the accessory comprising a secondary impeller mountable for free rotation in the proximity of the fluid intake area.

The accessory may include a first free rotating mounting device for mounting on the drive shaft.

In this specification and in the appended claims, unless the context clearly indicates otherwise, the term"free rotating mounting device"includes a device for mounting an apparatus to a rotatable shaft in such a way that the apparatus is rotatable independently with reference to the rotatable shaft.

Furthermore, the term"impeller"includes any apparatus which drives, forces or displaces a fluid and inclues, but is not limited to an impeller for a pump, fan and the like, a propeller, a runner and a rotor.

The free rotating mounting device preferably includes a bearing arrangement for mounting to the drive shaft.

In a preferred form of the invention, the secondary impeller features a substantially conical body having a leading edge and a base with a plurality of vanes mounted on the body.

The vanes may extend from the leading edge to the base of the body describing an arcuate path.

In a preferred form of the invention the vanes include leading edges which are orientated transverse with reference to the rotation axis of the secondary impeller.

In one form of the invention, the secondary impeller inclues a second free rotating mounting device for mounting on the casing of a fluid displacement machine.

The second free rotating mounting device is preferably mountable to the leading edge of the impeller body.

The first free rotating mounting device may be mountable to the base of the body of the secondary impeller.

In an alternative form of the invention, the secondary impeller comprises a central hub and a plurality of vanes extending radially from the hub.

The first free rotating mounting device may be mounted to the hub of the secondary impeller.

In a preferred form of the invention, the secondary impeller is mountable in the proximity of the fluid inlet area of the fluid displacement machine.

With the above arrangement, rotation of the fluid displacement machine's impeller (the primary impeller), causes fluid to enter the machine via the fluid inlet area which fluid in turn causes the secondary impeller to rotate.

The vanes of the secondary impeller may be configured to cause the secondary impeller to rotate in the same direction as the impeller of the fluid displacement machine in use.

Alternatively, the vanes of the secondary impeller may be configured to cause the secondary impeller to rotate in the opposite direction to the impeller of the fluid displacement machine in use.

The fluid displacement machine may be any one of an axial flow fan, a centrifugal fan, a pump, a compressor and a turbine.

According to a second aspect of the invention, a fluid displacement machine which inclues a casing, an impeller, a drive shaft for the impeller and a fluid intake area is provided, the machine including a secondary impeller as described above which is mounted for free rotation in the proximity of the fluid intake area.

According to a third aspect of the invention, a method of guiding intake fluid onto an impeller of a fluid displacement machine including a casing, an impeller, a drive shaft for the impeller and a fluid intake area comprises the step of mounting a free rotating secondary impeller as described above in association with the impeller of the machine This invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is an isometric view of one embodiment of a secondary impeller according to the invention; Figure 2 is an isometric view of the impeller of figure 1 mounted in association with the impeller of a fluid displacement machine; Figure 3 is a front view of the impeller of figure 1;

Figure 4 is a sectioned side view of the impeller of figures 1 and 2 mounted in association with a fluid displacement machine; and Figure 5 is a sectioned side view of a second embodiment of a secondary impeller according to the invention, mounted in association with a fluid displacement machine.

Referring now in particular to figures 1,2 and 3, a secondary impeller 10 according to the invention is shown to comprise a conical body 12 having a leading edge 14 and a circular, flanged base 16.

Four vanes 18 are mounted to the body 12 and describe arcuate paths from the leading edge 14 of the body to the base 16 as shown. Each vane 18 features a leading edge 18.1 which extends transversely relative to the rotational axis 20 of the impeller 10.

It will be appreciated that the number of vanes and the vane shapes will affect the performance of the impeller.

Base 16 features mounting holes 22 for mounting a free rotating mounting device to, as will be described hereunder.

In figure 4, a secondary impeller 10 according to the invention is shown to be mounted to the drive shaft 24 of a fluid displacement machine 26 (in

this embodiment, a centrifugal fan). Machine 26 include an impeller 28 (the primary impeller) and a casing 30 including a fluid inlet area 32. The secondary impeller is shown to be mounted in the fluid inlet area 32 of the machine 26.

Secondary impeller 10 is mounted to shaft 24 by way of a free rotating mounting device 34. Device 34 comprises a bearing assembly 36 which is mounted in a free rotating fashion to shaft 24 and a mount plate 38 which is mounted to the flanged base 16 of the secondary impeller body 12 by way of bolts 40.

In use, the primary impeller 28 rotates and draws fluid, in this embodiment, air, into the fluid inlet area 32 in direction A. The air causes the secondary impeller 10 to rotate in the same direction as the primary impeller 28.

Rotation of the secondary impeller 10 causes the intake air to be guided radially in direction B onto the primary impeller 28.

In figure 5, an alternative embodiment of a secondary impeller 42 according to the invention is shown to comprise a central hub 44 which is mounted by way of a free rotating mounting device 46 to the drive shaft 48 of a fluid displacement machine (in this embodiment, an axial flow fan). The machine inclues an impeller 47 (the primary impeller).

Four vanes 50 extend radially from the hub 44 as shown in the drawing.

Device 46 comprises a bearing assembly 52 and allows the secondary impeller 42 to be mounted to shaft 48 in a free rotating fashion. Secondary impeller 42 is mounted in a fluid inlet area 54 of the fluid displacement machine as shown.

In use, rotation of the primary impeller 47 causes fluid, in this embodiment, air, to be drawn into the fluid inlet area 54 in direction A. The secondary impeller 42 guides the inlet air in direction B onto the primary impeller.

Applicant has found that the operation of a secondary impeller as described with reference to figures 1,2 and 3, when mounted to a centrifugal fan, has improved the efficiency of the fan significantly in terms of increased volume displacement and pressure. Tests indicated an increase in efficiency of a centrifugal air fan from 68% standard, i. e. not fitted with the secondary impeller as described to 85% when fitted with the secondary impeller as described.