AU2001100256A4 - Impeller for centrifugal fan - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2(2)

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION INNOVATION PATENT Application Number: Lodged: Invention Title: IMPELLER FOR CENTRIFUGAL FAN The following statement is a full description of this invention, including the best method of performing it known to us IMPELLER FOR CENTRIFUGAL FAN FIELD OF THE INVENTION The present invention relates generally to fans of the type which are typically, although not exclusively, driven by an electric motor and are used in commercial, industrial or domestic applications. More particularly, the invention concerns an improved impeller for a centrifugal fan.

BACKGROUND OF THE INVENTION Fans are typically selected for a given airflow at a given pressure, called the duty point of the fan. This relationship between airflow and pressure is the major characteristic of a fan. Noise and cost are also important considerations.

There are two general types of fans which have pressure/flow characteristics suiting certain applications. Axial fans, where the fan blades extend radially from the rotation axis of the fan, generally produce higher flows and lower pressures. Centrifugal fans, on the other hand, where the blades extend parallel to the rotation axis of the fan, produce greater pressures and lower flow rates. Typical performance characteristics of axial and centrifugal fans are depicted in Figure 1.

The present applicant had a requirement for a fan with a certain performance characteristic for use in a cooking hood exhaust in fast food outlets.

Noise levels and cost are also critical in this application. Available axial fans of a suitable size and noise level did not develop enough pressure to overcome the resistance of the cooking hood filters. Available backward curved centrifugal fans with the desired flow developed too much pressure and so required a larger motor. An impeller with a performance characteristic between the two was required. This is shown in Figure 1 as the "desired characteristic".

A fan impeller, known as a "mixed flow" impeller, having a performance characteristic between that of an axial impeller fan and a centrifugal impeller fan, can have the desired performance. However, mixed flow impellers are generally more expensive than centrifugal impellers or axial impellers and the ones available to the applicant were too expensive for the application.

A fan can be "over-specified" for the application but this is not desired for other reasons such as increased motor power, physical size or noise. Reducing the number of blades can also bring the performance nearer to that required but this adjustment is coarse and, if the number of blades is reduced too far, a commercially produced impeller can lack rigidity and lose its balance too easily.

There therefore remained a need for an impeller which can produce a performance characteristic between that of an axial fan and a centrifugal fan and do this at a lower cost than known mixed flow impellers.

SUMMARY OF THE INVENTION The present invention accordingly provides an impeller for a centrifugal fan, the impeller including a plurality of blades wherein the length of each blade, measured from a leading edge to a trailing edge of the blade, varies across its width, wherein each blade is of constant width, having two parallel sides, and wherein the length of each blade is a minimum at a position intermediate its two sides.

Preferably each blade has a concave leading edge and/or trailing edge.

The leading edge and/or the trailing edge may further be V-shaped or may be Ushaped.

The present invention also provides an impeller for a centrifugal fan, the impeller including a back plate, an inlet plate parallel to the back plate and a plurality of blades having parallel sides secured between the back plate and the inlet plate, wherein the length of each blade, measured from a leading edge to a trailing edge of the blade, is a minimum at a position intermediate the back plate and inlet plate.

The concept behind the present invention is that reducing the area of the blades, without changing the intake or discharge areas, reduces the pressure development with little effect on free flow. This is embodied in the present invention by reducing the blade area while maintaining the same overall dimensions.

2a To assist the further understanding of the invention, reference is now made to the accompanying drawings which illustrate a preferred embodiment of the invention. It is to be appreciated that this embodiment is given by way of illustration only and the invention is not to be limited by this illustration.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows performance characteristics for a typical axial fan and a typical centrifugal fan, together with a desired characteristic; Figure 2 shows a schematic representation of a typical arrangement of a prior art centrifugal fan; Figure 3 shows a perspective view of a prior art impeller for a centrifugal fan; Figure 4 shows a perspective view of an impeller according to an embodiment of the present invention; Figure 5 shows a profile of a blade of the impeller shown in Figure 4; Figures 6 to 13 show alternative profiles of a blade.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring firstly to Figure 1, there is shown pressure/flow performance characteristics for a typical axial fan and for a typical centrifugal fan. It can be seen that, in general, an axial fan produces higher flow rates at lower pressures whereas a centrifugal fan produces higher pressures at lower flow rates.

In the application of a cooking hood exhaust in fast food outlets a performance characteristic between that of typical axial fans and centrifugal fans is desired. This is also shown in Figure 1 as the "Desired Characteristic".

Figure 2 shows a schematic representation of a typical centrifugal fan. The fan includes an impeller 10, mounted within a casing 12, and driven by a motor 14 about a rotation axis 16. The casing 12 includes an air inlet 18 having an inlet cone 20 to direct air into a central portion of the impeller 10. The air is then discharged from an outlet 22 of the casing 12. It should be understood that this is merely an illustrative example for the purposes of showing a prior art impeller in a typical application. In other applications the axis of rotation 16 may instead be vertical.

Figure 3 shows a typical prior art centrifugal impeller as may be used in the fan of Figure 2. The impeller 10 includes a back plate 30 having a drive flange 32 to allow the impeller 10 to be secured to a shaft of a drive motor (14 in Figure 2) or other suitable drive means. The impeller also includes an inlet plate 34 which is curved to improve airflow into the impeller 10. Between the back plate 30 and inlet plate 34 are rectangular blades 36. The blades 36 are secured to the back plate 30 and inlet plate 34. In the embodiments shown, rivets are used to secure the blades but any other suitable fixing may be employed, for example welding, bolts, or slots and lugs.

When viewed from the top in Figure 3, the direction of rotation of the impeller 10 is clockwise. Thus, each blade 36 has a leading edge 38 and a trailing edge 40. The leading edge 38 is radially the innermost edge of the blade 36 whereas the trailing edge 40 aligns with the diameter of the back plate 30 and inlet plate 34.

For ease of description of the prior art impeller 10, and of the impeller of the present invention, the length of the blade 36 is defined herein as the distance from the leading edge 38 to the trailing edge 40. The width of the blade 36 is defined herein as the distance between the two parallel sides engaging the back plate 30 and inlet plate 34.

Referring now to Figure 4, there is shown an impeller 40 according to an embodiment of the present invention. Similar to the prior art impeller 10 of Figure 3, the impeller 40 of the present invention includes a back plate 50 having a drive flange 52 and an inlet plate 54. Mounted between the back plate 50 and inlet plate 54 is a plurality of blades 56, each having a leading edge 58 and trailing edge 60. In the embodiment shown, the leading edge 58 is concave, such that the length of the blade 56 is a minimum at a point intermediate its two sides engaging the back plate 50 and inlet plate 54. Once again, the blades 56 are fixed to the back plate 50 and inlet plate 54.

It will be appreciated that the depth of the concave cutout in the leading edge 58 can be varied in order to achieve a desired performance characteristic.

In the embodiment shown, the cutout is generally V-shaped but it could instead be made more of a U-shape so as to form a more gradual curve between the two sides. Also, it should be appreciated that the cutout could instead have been made in the trailing edge 60 of the blade 56, rather than in the leading edge 58.

It will also be appreciated that various materials may be used to construct the impeller of the invention. The impeller shown in the drawings is made of galvanised steel but aluminium, plastic or composite fibre materials could alternatively be used.

Figure 5 shows a plan view of a single blade 56 of the impeller 40 shown in Figure 4. Once again, the blade 56 includes a leading edge 58 and a trailing edge 60. In an assembled impeller, side 62 of the blade 56 is fixed to the inlet plate 54 and side 64 is riveted to the back plate Further alternative blade shapes are shown in Figures 6 to 13. Clearly, different profiles may be employed by scalloping out the leading and/or trailing edges of a standard blade. Each profile will produce a different performance characteristic and small changes can be made to the profile in order to fine tune the impeller for a particularly desired characteristic.

The performance of a fan can thus be finely controlled by the sizing and position of the scallop. The blades can be produced on numerically controlled machinery and the scalloped blades can be made at virtually the same cost as a standard blade. By keeping the envelope dimensions of the blade the same, the inlet and back plates of the impeller can be the same as a standard prior art impeller. The impeller can also be assembled on the same machinery using the same techniques.

The impeller of the present invention can also be used as a direct replacement for standard impellers, requiring no modification to the fan housing.

This enables the same basic fan to be tuned to suit the application at hand.

By maintaining the same number of blades as a standard impeller and keeping the attachment to the inlet and back plates the same the rigidity of the impeller is not compromised. By way of comparison, to simply reduce the number of blades or the length of each blade would reduce the rigidity of the impeller.

It will also be seen that the impeller 40 shown in Figure 4 includes six blades. The number of blades may also be varied in order to further tune the performance characteristic for a particular application.

6 Although a preferred embodiment of the invention is described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.

Claims (4)

1. An impeller for a centrifugal fan, the impeller including a plurality of blades wherein the length of each blade, measured from a leading edge to a trailing edge of the blade, varies across its width, wherein each blade is of constant width, having two parallel sides, and wherein the length of each blade is a minimum at a position intermediate its two sides.

2. An impeller according to claim 1 wherein each blade has a concave leading edge and/or trailing edge.

3. An impeller according to claim 1 or claim 2 wherein each blade has a V- shaped leading edge and/or trailing edge.

4. An impeller for a centrifugal fan, the impeller including a back plate, an inlet plate parallel to the back plate and a plurality of blades having parallel sides secured between the back plate and the inlet plate, wherein the length of each blade, measured from a leading edge to a trailing edge of the blade, is a minimum at a position intermediate the back plate and inlet plate. An impeller according to claim 4 wherein each blade has a V-shaped leading edge and/or trailing edge. DATED this 14th day of January 2002 FANTECH PTY LTD WATERMARK PATENT TRADEMARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA UIP1009AUOO RLT:SLB:JL