GB2393220A

GB2393220A - Centrifugal fan with noise reducing plate

Info

Publication number: GB2393220A
Application number: GB0219956A
Authority: GB
Inventors: Richard Hayes Teague
Original assignee: Fans & Blowers Ltd
Current assignee: Fans & Blowers Ltd
Priority date: 2002-08-28
Filing date: 2002-08-28
Publication date: 2004-03-24
Anticipated expiration: 2022-08-28
Also published as: GB0219956D0; GB2393220B

Abstract

A centrifugal fan comprises a casing 10 with an inlet 11 and an outlet 14, an impeller 13 and a plate 22 mounted adjacent a cut-off point between the outlet 14 and the casing 10. The plate 22 extends away from the outlet 14 in a direction opposite to a direction of rotation 16 of the impeller 13. The width of the plate 22 decreases as it extends away from the outlet 14 (see figure 4) such that, in use, noise generated by the release of a pressure wave, as a blade of the impeller 13 passes the outlet 14, is reduced. The plate 22 may be fixed in position or attached to a pivot 23 such that its position may be adjusted. Adjustment of the position of the plate 22 may be automatically controlled.

Description

1 2393220

Centrifugal Ean or_Bloer The present invention relates to [ens and blowers, and in particular Lo centrifugal fans and blowers.

Centrifugal fans (also known as radial fans) are used in many situations where it is desired to increase the pressure of a flow of air or other fluid. They may be used, for example, in chemical plants, in ventilation 10 systems, for the supply of combustion air to burners for gas boosting, for landfill gas extraction, and in heating systems. They may also be used to handle the pumping of mixtures of gas with dust or other particulate materials, which mixtures can be made to flow in the manner of a 15 fluid. Centrifugal fans may also be used to provide suction. Known centrifugal fans have a casing containing a rotatable impeller. Rotation of the impeller draws in air 20 at an inlet in the casing and causes it to flow to an outlet in the casing of the fan, increasing the pressure of the air or providing suction as it does so. The impeller has a plurality of blades, and the rotation of the impeller moves air past the blades.

In one known type of centrifugal fan, the blacJes are normally held between two parallel plates of the impeller, with those plates rotating within the casing of the fan. One plate has an aperture therein through which 5 air can pass from an inlet in the casing to the space between the plates, where it is forced outwardly by the blades to the periphery of the impeller, and hence to an outlet. Such known impellers usually have blades equally spaced around the impeller, and impellers for different 10 purposes may have different blade designs. In a centrifugal fan, the surface of each blade may lie perpendicular to the direction of rotation, or may be inclined either forward or backward relative to the direction of rotation. Furthermore, the blades may be 15 flat or may be curved either in the same direction as, or against, the direction of rotation. The blades may be in the form of a sheet or may have an aerofoil cross-

section. 20 In another known type of centrifugal fan, the blades project from a hub to form a paddle-like arrangement, and the impeller does not have the parallel plates as described previously. Others have only one plate, and yet others have at least one conical plate, with the aid of 25 the cone at the axis of the impeller.

is also known to provide multiple stage fans, in which there are a plurality of impellers in series. Normally, each impeller has its own outer casing, with the casings then being secured together to form the whole fan.

A common feature of such centrifugal fans is that air is drawn into the rotating impeller at its axis and is forced outwardly to the periphery of the impeller by the blades. Having reached the periphery of the impeller, the 10 air is forced into the space between the circumference of the impeller and the casing. Thus, a pressure wave moves outwardly to the periphery of the impeller and is released into the space between the circumference of the impeller and the casing. The rotation of the impeller 15 further causes the air in this space to have a tangential component of movement so that the air flows to the outlet in the casing.

As the impeller rotates, the peripheral end of each blade 20 repeatedly passes the casing cut-off point, which is where the arrangement of the casing is such to allow for an outlet in the casing. Thus, the casing and outlet meet at the casing cut-off point. As a blade passes this cut-

off point, the space between the circumference of the 25 impeller and the casing, into which air is forced by the rotating impeller, is suddenly reduced. Thus, each time a

blade passes the casing cut-off point, there is a sudden transition from the pressure wave being released into a relatively larger space before the cut-off point to it being released into a relatively smaller space after the 5 cut-off point. It is the release of the pressure wave into this suddenly reduced space that generates a blade passing frequency (BPF) tonal noise. Thus, the action of the impeller results in the generation of sound pressure at the BPF that can be heard during use of the fan. This 10 noise is generally undesirable, whether the fan is used in an industrial or domestic setting. For example, there is generally a need to minimize noise levels within an industrial workplace such as a factory, and a desire to have minimal noise emitted by fans used in, for example, 15 offices and residential buildings.

The present invention aims to address the problem of centrifugal fans or blowers emitting an undesirable noise during use. Generally, the invention lies in providing a 20 means for progressively restricting the release of a pressure wave as a blade of a fan or blower passes adjacent to the casing outlet before passing the casing cut-off point. This then acts as a noise reduction device such that the fan is quieter during use than such a fan 25 or blower without the device.

The said means reduces the sound pressure generated at the BPF as a blade passes the fan or blower outlet, at the Casio- cut-off point, thus reducing the sound pressure heard.

The sound pressure value of the BPF tonal noise, and hence the sound pressure heard, is directly influenced b the way in which a pressure wave is allowed to be released into the reduced space between the circumference 10 of the impeller and the casing as a blade passes the fan or blower casing cut-off point. The way in which a pressure wave is released is in part determined by the relationship between the impeller and the casing. The position of an impeller blade with respect to the casing 15 depends on the blade design. For example, for centrifugal fans in which the surface of a blade is perpendicular to the sides of the casing, all of the energy contained within a pressure wave is released at once. Thus, there is a sudden transition from all of the energy of a 20 pressure wave being released into the relatively larger space between the circumference of the impeller and the casing before the cut-off point to it being released into the relatively smaller space after the cut-off point.

This sudden restriction in the release of energy causes 25 the BPF noise as the blade passes the cut-off point. The

present invention thus eliminates the sudden energy release into the reduced space and makes it progressive.

Preferably, the means for progressively restricting the 5 release of a pressure wave is a body, such as a plate, mounted in the casing adjacent the outlet and extending in the casing away from the outlet in a direction opposite to the direction of rotation of the blade(s).

The surface of the body decreases in width as it extends 10 away from the outlet.

Accordingly, the present invention may provide a centrifugal fan or blower having a casing with an inlet and an outlet, a rotatable impeller having one or more 15 blades within the casing for forcing fluid from the inlet to the outlet, and a body mounted in the casing adjacent the outlet which body extends in the casing away from the outlet in a direction opposite to the direction of rotation of the blade(s) and the surface of which 20 decreases in width as it extends away from the outlet in a direction opposite to the direction of rotation of the blade(s). Preferably, the body extends from the casing cut-off 25 point, at which point the outlet meets the casing, such that it projects into the space between the circumference

of the impeller and the casing into which a pressure wave is released before a blade of the impeller passes the casing cut-off point. The body partially blocks said space in a way that progressively increases towarcis the 5 cut-off point, so the release of a pressure wave past the body is progressively reduced as a blade moves towards the casing cut-off point. However it is not necessary that the body terminates at the casing cut-off point. It is possible for the body to have a part extending in the 10 outlet and a part extending in the casing, such that parts of the body lie on each side of the cut-off point.

Nor is it necessary that the body and the casing cut-off point make contact. The body may be spaced from the casing cut-off point provided the body is in the claims 15 and extends away from the outlet.

Generally, the effect of the body is to partially extend the casing beyond the cut-off point in a direction opposite to the direction of rotation of the blades(s), 20 but which extension progressively reduces. Preferably, the body extends essentially along the projected path of the casing from the casing cut-off point. The size and shape of the body is such that it partially blocks the space between the circumference of the impeller and the 25 casing into which a pressure wave is released before a blade passes the casing cut-off point. Preferably, the

shape of the body is such that the degree of blockage caused to the pressure wave by the body is at a minimum furthest away from the cut-off point and gradually increases towards the cut-off point. Thus, the body 5 ensures that not all of a pressure wave is released at once into the reduced space between the circumference of the impeller and the casing as a blade passes the cut-off point by progressively restricting the release of the wave before the blade passes the cut-off point. The 10 resulting progressive restriction of the release of the energy contained within the pressure wave consequently reduces the sound pressure generated and the noise heard.

Preferably, for centrifugal fans having the surface of a 15 blade perpendicular to the sides of the fan casing, for example, radial or backward inclined blades, the body is a plate having a triangular shape with the apex of the triangle pointing away from the casing cut-off point. A plate of this shape allows the release of the energy 20 contained within the pressure wave to be reduced linearly. The plate may be a 45 triangle or a triangle having different angles. The apex of the triangle may be centrally positioned in the space between the sides of the casing or offset to either side. Alternatively, 25 plates having a different orientation and shape may be equally suitable. For example, a plate may have a curved

edge furthest from the casing cut-off point, which may be either concave or convex with respect to the direction of rotation of the blade(s). Further possibilities include a plate with the edge furthest from the chasing cut-off 5 point having a triangular shape cut out from it, wherein the apex of the triangular cut-out is centrally positioned in the space between the sides of the casing and points towards the casing cut- off point. Preferably, the maximum width of the plate, nearest the casing cut 10 off point, is equal to the inside width of the casing.

Alternatively, the maximum width of the plate may be equal to, or less than, the width of the impeller at its outer circumference.

15 Furthermore, the noise reduction device may be used with any type of centrifugal fan irrespective of the blade design of the impeller. However, the shape of the plate may vary depending on the blade design and the resulting differing relationship between a blade and the casing.

20 Additionally, the device may be used with fans of varying sizes and proportions. As before, the shape and size of the plate may vary accordingly. For example, with a centrifugal fan employing a wide casing and wide impeller with radial blades, the plate may similarly be triangular 25 but scaled up in size. The maximum width of the plate may be equal to the width of the impeller.

Preferably, the plate is fixed in position. Preferably, the plate is fixed by welding to the sides of the casing.

5 Alternatively, there may be provided a pivot at the edge of the plate having the greater width such that the position of the plate is adjustable. There may additionally be a level indicator fixed to the pivot and running parallel to the plate such that the level of the 10 plate may be adjusted to find the optimum position for noise reduction before locking the plate in position. For such an adjustable plate, a stop block may also be provided to prevent the edge of the plate opposite the pivot from being adjusted such that it would contact the 15 impeller. A stop block may be welded onto the casing.

Furthermore, the position of the plate may be automatically adjustable with respect to the speed of rotation of the impeller. Thus, there may be provided a means for automatic control of the plate position 20 determined by the speed of the fan. Such control may be provided by an electric or pneumatic actuator linked to the fan speed controller.

An embodiment of the present invention will now be 25 described in more detail, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a view of a centrifugal fan of the present invention; Fig. 2 shows a cross-sectional view of the fan of Fig. 1, incorporating a noise reduction device of the 5 present invention; Fig. 3 shows an enlarged cross-sectional view of the noise reduction device of Fig. 2; and Fig. 4 shows a top view of the fan and noise reduction device of Fig. 2.

Referring first to Fig. 1, a centrifugal fan has a generally circular casing 10 with an inlet 11 at its axis. The casing 10 may have a mounting plate 12 to enable the fan to be positioned in a vertical 15 configuration. Within the casing 10 is an impeller 13, which can be seen in Fig. 1 through the inlet 11, with the impeller 13 rotating to draw air into the inlet 11, and out through an outlet 14. The impeller is driven by a motor 15. It should be noted that the fan of Fig. 1 is 20 'left- handed' in that the outlet 14 points to the left relative to the inlet 13. The present invention applies also to 'right-handed' fans and the outlet 14 may be in any angular portion around the fan relative to the mounting plate.

In its external appearance, Lhe fan of Fig. l is conventional. The present invention is primarily concerned with the incorporation of a noise reduction device to the fan. An embodiment of the invention is 5 shown in more detail in Fig. 2.

Fig. 2 shows the arrangement of the fan casing 10 with respect to the impeller 13, and a noise reduction device 21 of the present invention. The blades of the impeller 10 13 are of a radial design and hence are perpendicular to the sides of the fan casing 10. The space between the circumference of the impeller 13 and the casing 10 is tapered. The space is widest adjacent to the outlet. From there, the space progressively narrows moving around the 15 circumference of the impeller 13 in a direction opposite to that of the direction of rotation 16 such that the space becomes narrowest at the fan casing cut-off point 20 immediately before it opens out at the widest part adjacent to the outlet. During use, air drawn into the 20 inlet 11 of the fan as the impeller 13 rotates is forced outwardly to the periphery of the impeller 13 by the blades. Having reached the periphery of the impeller 13 the air is forced into the space between the circumference of the impeller 13 and the fan casing 10.

25 The rotation of the impeller 13 further causes the air in the space to flow to the outlet 14 in the casing 10. As

the impeller 13 rotates, the peripheral end of each blade repeatedly passes the tan casing cut-off point 20. Each time the end of a blade passes from the widest to the narrowest space between the end of the blade and the 5 casing to aL the fan casing cut-off point 20, the pressure wave released into the suddenly reduced space results in the generation of noise. The noise reduction device 21 comprises a plate 22 projecting out from the fan casing cut-off point 20. The plate 22 extends 10 essentially along the projected path of the fan casing from the fan casing cut-off point 20. In doing so, the plate 22 partially blocks the space between the circumference of the impeller and the casing into which a pressure wave is released before a blade passes the cut 15 off point 20. Thus, the release of a pressure wave past the plate 22 is progressively restricted. This removes the sudden transition from the pressure wave being released into a relatively larger to a relatively smaller space and consequently reduces the noise emitted by the 20 fan. The plate 22 is fixed to a pivot 23, comprising a threaded spindle, along its wider edge adjacent to the cut-off point 20. Additionally, a level indicator 24 is fixed to the pivot 23 such that it is parallel to the plate 22. A stop block 25 prevents the edge of the plate 25 22 opposite the pivot 23 from being positioned such that

it would contact the impeller 13. The noise reduction device 21 is shown enlarged in Fig. 3.

In an alternative embodiment (not shown), the plate 22 is 5 fixed in position, eliminating the pivot 23, level indicator 24 and stop block 25.

Fig. 4 shows a top view of the fan incorporating the noise reduction device 21 of Fig. 2. The plate 22 is 10 triangular with the apex of the triangle furthest away from the pivot 23. One end of the threaded spindle of the pivot 23 protrudes through and beyond a side of the fan casing 10. The level indicator 24 is fixed to the protruding end of the spindle such that it provides a 15 visual indicator outside the fan casing 10 of the level of the plate 22 inside the fan casing 10. There is provided a nut 26 on the spindle outside the fan casing 10 that when positioned flush to the casing locks the pivot 23, and hence the plate 22, in position.

Moreover, although Fig.4 illustrates an embodiment of the noise reduction device having a triangular plate, further embodiments may provide plates having different shapes and orientations depending on the design of the blades of 25 the impeller.

r It has previously been mentioned that fans may have multiple impellers in series, each with their own casing.

lhe noise reduction device of the present invention may be used in any or all stages of such a fan.

Claims

Claims:

1. A centrifugal fan or blower having: (a) a casing with an inlet and an outlet formation) 5 (b) a rotatable impeller having one or more blades within the casing for forcing fluid from the inlet to the outlet formation, and (c) a body mounted in the casing; wherein at least a part of the body extends in a space 10 between the circumference of the blade(s) and the casing, adjacent a point at which the outlet formation meets the casing, said at least part of a body extending in a direction opposite to the direction of rotation of the blades(s), and said at least part of the body decreasing 15 in width as it extends away from the outlet formation in a direction opposite to the direction of rotation of the blade(s).

2. The centrifugal fan or blower according to claim 1, 20 wherein the body is mounted at a said point at which the outlet formation meets the casing.

3. The centrifugal fan or blower according to claims 1 or 2, wherein the body is a plate.

4. The centrifugal fan or blower according to claim 3, wherein the plate is triangular shaped with the apex of the triangle pointing into the casing, away from the outlet formation.

5. The centrifugal fan or blower according to claim 3, wherein the plate has a triangular cut-out, with the apex of the cut-out positioned between the sides of the casing and pointing towards the outlet formation, the width of 10 the plate being defined by the material of the plate excluding the cut-out.

6. The centrifugal fan or blower according to claim 3, 4 or 5, wherein the plate is fixed in position.

7. The centrifugal fan or blower according to any one of claim 3, 4 or 5, wherein the plate is pivoted.

8. The centrifugal fan or blower according to claim 7, 20 wherein the pivot has a level indicator fixed thereto.

9. The centrifugal fan or blower according to claim 7 or 8, further including a stop block to prevent contact between the plate and the impeller.

10. The centrifugal fan or blower according to claim 7, 8 or 9, further including a means for automatic control of the plate position determined by a speed of the impeller.

11. The centrifugal fan or blower substantially as described herein as an embodiment of the invention with reference to and illustrated in the accompanying drawings.