GB2142985A - Impeller for a double inlet centrifugal fan - Google Patents

Abstract

The impeller includes a centre plate 30, a plurality of securing blocks 40 welded to the periphery of the centre plate, and blades 33 welded to the blocks so as to extend on both sides of the centre plate. Wear protection members 35 may be screwed to the outer surface of blocks 40. <IMAGE>

Description

SPECIFICATION Impeller for a centrifugal fan This invention relates to an impeller for a double inlet centrifugal fan.

Double inlet centrifugal fans are used for pumping large quantities of gas, for instance gas from induced draught boilers. Such a fan has an axial inlet on each side of the impeller. Usually the impeller shaft passes right through the fan to a bearing at each end so that both inlets are annular and surround the shaft.

The impeller normally comprises a centre plate perpendicular to the fan axis with blades welded to it and extending from its opposite sides. The blades are angled to impart energy to the gas. The ends of the blades remote from the centre plate are usually connected to one another by an annular side plate through the aperture in which the incoming air passes.

The fans are designed and used with a view to the stress that can be tolerated by the blades, by the welds by which they are attached to the centre plate, and by the centre plate. The blades which are located on the two sides of the centre plate can, in the circumferential direction, either be in staggered relation to one another or in register. In the former case, where the blades on each side are intermediate the blades on the other side, the load of the blades on the centre plate is more distributed, and the plate can flex slightly which means that there is less stress in the welds by which the blades are held on the centre plate. In the latter case, where the blades on the sides are in line, the stress and distortion in the centre plate are less but the stresses in the welds by which the blades are attached are correspondingly increased.

Double inlet centrifugal fans are frequently employed for moving gases which contain dust which erodes those parts of the impeller which it contacts.

For this reason it is well known to provide replaceable liners on selected parts of the impeller. In particular, liners are applied to the blade surfaces and also to the surfaces of the centre plate which face the inlets. As can be imagined, particularly when aerofoil shaped as opposed to flat blades are employed, the provision of the liners can be expensive and difficult to achieve, particularly as a number of different pieces may have to be employed. Gaps between the liner pieces through which dust can penetrate almost inevitably arise. One known way to protect the centre plate is simply, when possible, to remove it, that is to cut out parts which are radially outward of the blades.This, however, is a less than fully satisfactory solution, particularly when aerofoil blades are employed, due to the fact that further difficulty is then experienced with attaching the blades to the centre plate.

We have now found that at least some of the abovementioned difficulties can be reduced by attaching the blades to the centre plate of an impeller for a double inlet centrifugal fan using securing blocks, the blocks being welded to the centre plate and the blades being welded to the blocks.

According to the present invention, therefore, there is provided an impeller for a double inlet centrifugal fan including a centre plate, a plurality of securing blocks welded to the centre plate, and blades welded to the blocks so that blades extend on both sides of the centre plate.

The securing blocks can be more strong and robust than the centre plate and thus provide a more secure means of attaching the blades. Also, more advantageous and stronger welds can be achieved between the blocks and each of the centre plates and blades than can normaliy be made between the centre plate and blades directly. It is also preferable and important that the block can extend axially of the impeller beyond the width of the centre plate. This means that the welds holding the blades are shifted towards the outer ends of the blades from the centre plate, which much reduces the stress on the welds for given blade loading. As the blocks are stronger than the blades an overall strength increase results.

Normally, the free ends of the blades on each side of the centre plate are connected by sideplates to which they are welded. Preferably two blades are attached to each securing block, one extending to each side of the centre plate so that the arrangement is the in-line arrangement referred to above.

It is expected that the invention will usually find application in cases where the centre plate is cut away between blades to minimise erosion. Particularly, though not necessarily in combination with this feature the blocks are preferably welded to the periphery by the centre plate.

Additional strength can be given to the impeller by reason of the nature of the securing blocks, which are intended to be strong and robust, and particularly by the preferred feature that the blocks extend beyond, and so generally speaking are wider than, the centre plate. The welding points by which the blades are attached are thus moved axially away from the plane of symmetry of the impeller. The further the weld points are so moved the less the stress which the welds have to withstand for a given blade shape. This effect is particularly significant for intial displacements of the weld point away from the centre line and the rate of improvement diminishes such that at about one third of the span of the blades no further advantage in relieving weld stress can be obtained.A reduction of up to 40% in blade weld stress can, however, be obtained relative to presently known constructions. Other advantages are that full penetration welds can be made between the securing blocks and the blades. Further, the stresses on the centre plate are reduced due to the welding of the blades being to the more robust securing blocks, while with the in-line arrangement, the stresses applied to the plate are symmetrical. The welds of the securing blocks to the centre plate can be butt or double fillet welds of high strength.

Mention was made above of the problems which currently exist in securing replaceable liners to the blades and centre plate of an impeller for a double inlet fan. An advantage for the present invention is that the securing blocks are very appropriate for use as anchoring points for protective liners for the blades. Such liners can be provided at least to cover the central sections of the blades and sometimes to extend completely across the blades. The fixing to the securing blocks, for instance using screws, is more secure and easy than fixing to the blades as has been done before.

The securing blocks have to be of sufficient strength and thickness to allow secure anchoring for the blades. They will then also be adequate mounts for the protective liners. The blocks can be shaped by forging or machining.

The securing blocks can take various configurations. They may, for instance, be rectangular or I-shaped in cross-section when viewed in planes including the impeller axis. Both constructions offer the possibility of improving the weld connection with aerofoil blades. The blocks are preferably solid.

In a preferred form an I-shaped section securing block has a radially inward extension which can be butt welded to the centre plate, thus providing a still stronger stress resistant attachment. In sections perpendicular to the impeller axis the securing blocks will usually match the blade profile.

The invention extends to a centrifugal fan of the double inlet type provided with such an impeller.

The invention will be more clearly understood from the following description which is given by way of example only with reference to the accompanying drawings in which: Figures 1 and 2 are partial perspective views of known impellers for double inlet centrifugal fans; Figure 3 is a partial perspective view of an impeller according to the invention; Figures 4A and 4B are sectional views showing use of a first form of securing block for attaching blades to the centre plate in the present invention, Figure 4A being in a plane perpendicular to the axis of the impeller and Figure 4B in a plane including that axis; Figures 5A and 5B are similar sectional views showing the use of a second form of securing block; and Figures 6A and 6B are similar sectional views showing the use of a third form of securing block.

In the prior art arrangement shown in Figure 1, a centre plate 10 of an impeller for a double inlet centrifugal fan has blades 11 welded to it and extending from both sides. These blades are staggered in the circumferential direction that is to say the blades on one side have their fixing points intermediate those of the blades on the other side.

Attached to the outer ends of the blades 11 are respective side plates 12 which are secured by welding.

Figure 2 shows an alternative prior art embodiment again having a centre plate 10 with blades 11 welded thereto and extending from each side and having on their outer ends side plates 12. In this case, however, the blades 11 are in an in-line arrangement, that is to say viewed axially they are aligned with each other in the circumferential direction of the centre-plate.

Figure 3 shows part of an impeller according to the present invention, this having a centre plate 30 supported on a mounting 31 by conical supports 32.

The centre plate is cut away so as to have a stepped periphery offering a plurality of faces inclined to the radial direction. The purpose of this stepped shape is to reduce the parts of the centre plate most liable to dust erosion in use. On each face a securing block which is not visible in Figure 3 is welded. To each side of the securing blocks are welded blades 33, in the in-line arrangement, and at the outer ends of the blades side plates 34 are again provided. While the nature and shape of the securing blocks will be described below, attention is directed in Figure 3 to a wear protection liner 35 fixed by screws to the securing block and extending over part of the width of each blade. Such liners can be provided across the entire width of the blades if desired.

Figures 4A and 4B show a first embodiment of securing block 40 used for attaching two aerofoil type blades to the centre plate 30, each blade having upper and lower sides meeting at leading and trailing edges. In this embodiment the securing block is solid and of rectangular cross-section in a section including the impeller axis. It is attached by two fillet welds 41 to the centre plate. The upper and lower sides of the blades 33 are welded with fillet welds 42 to inner and outer edges of the securing block 40, which in section perpendicular to the axis of the impeller has the same profile as the blades. It can be seen from Figures 4A and 4B that the liner 35 is screwed directly onto the securing block 40 allowing a particularly effective attachment, and that it extends along the circumferential length of the outer face of the blades.

In the embodiment of Figure 5 the only difference is that the securing block 50 has an I-shape in a cross-section including the impeller axis. The upper and lower limbs 51,52 to which the blades 33 are attached have greater thickness than the blades themselves. Thus, not only is the stress in the blade welds less than in the prior art, but further strength increase arises from the thicker and more robust limbs of the block.

The further alternative embodiment of Figures 6A and 6B has the difference of an extension 61 to the securing block 60, this extending radially inwards to be butt welded at 62 to the centre plate 30. This offers better fatigue resistance and is also easier to check by means of radiography than is a double fillet weld such as is employed in the other embodiments.

In all embodiments it will have been noted that the weld connection between the blades and the securing block is axially displaced towards the end of the blade. This means that the stress which the weld has to support is reduced and that the point of highest stress is now in the block itself. The block, being unitary and large, is well able to resist it, and offers increased robustness in the area of attachment of the blades relative to constructions where the blades are attached directly to the centre plate. A result, of course, is that higher stress loads can be contemplated so that greater impeller speeds and fan capacity become available.

Claims (15)

1. An impeller for a double inlet centrifugal fan including a centre plate, a plurality of securing blocks welded to the centre plate, and blades welded to the blocks so that blades extend on both sides of the centre plate.

2. An impeller according to claim 1 wherein the blocks extend, in the axial direction of the impeller, beyond the centre plate so that the welds between the blades and the securing blocks are located axially outwardly of the centre plate.

3. An impeller according to claim 1 or 2 wherein two blades are welded to each securing block, the blades extending on opposite sides of the centre plate.

4. An impeller according to claim 1,2or3 wherein the centre plate is cut away between the blades to minimise erosion.

5. An impeller according to any preceding claim wherein the blocks are welded to the periphery of the centre plate.

6. An impeller according to any preceding claim wherein the securing blocks are solid.

7. An impeller according to any preceding claim wherein the securing blocks are rectangular in sections including the impeller axis.

8. An impeller according to any one of claims 1 to 6 wherein the securing blocks are I-shaped in sections including the impeller axis.

9. An impeller according to claim 8 wherein the blocks are secured to the centre plate by double fillet welds.

10. An impeller according to claim 8 wherein the I-shaped securing blocks each have an extension directed radially inwardly towards the centre plate.

11. An impeller according to claim 9 wherein the extension is butt welded to the centre plate.

12. An impeller according to any preceding claim wherein the blades are attached to the securing blocks by full penetration welds.

13. An impeller according to any preceding claim having lining plates covering at least the part of each blade adjacent the centre of the impeller, the plates being secured to the securing blocks.

14. An impeller for a centrifugal fan of double inlet type substantially as hereinbefore described with reference to and as illustrated in Figures 3 and one of Figures 4 to 6 of the accompanying drawings.

15. A centrifugal fan of the double inlet type including an impeller according to any preceding claim.