The present invention relates to a hydrofoil impeller for producing fluid flow in axial direction relative to a shaft rotating around its central axis in an agitated tank .

BACKGROUND OF THE INVENTION In prior art, e.g. document JP 2005087876 discloses a hydrofoil impeller with three blades and comprising a central hub which is connected to a rotatable shaft. The central hub is in the form of a flat plate with a uniform thickness and being perpendicular to the cen- tral axis of the shaft. The central hub has three groups of four first bolt holes arranged to form a pat ^¬ tern. The number of groups of first bolt holes corre ^¬ sponds to the number of blades attached to the central hub. Thus, three blades extend radially outwardly from the central hub, each blade having a root portion. The root portion is in a form of a flat plate with a uniform thickness, said root portion has a group of four second bolt holes. The second bolt holes are arranged in a corresponding pattern in relation to the pattern of the first bolt holes so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints. A problem with the attachment of blades to the central hub of the impeller with bolted joints wherein the bolts are arranged in linear rows is that the dynamic stresses caused by fluid forces, when the impeller is in operation during agitation of the fluid, are unevenly exerted at the bolted joints and the material of the hub and blades near to the bolted joints affecting strongly on the fatigue life of the impeller. Fatigue occurs when a material is subjected to repeated loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators. Eventually a crack will reach a critical size, the crack will propagate suddenly, and the structure will fracture.

OBJECTIVE OF THE INVENTION

The objective of the invention is to alleviate the dis- advantages mentioned above.

SUMMARY OF THE INVENTION

According to an aspect, the present invention provides a hydrofoil impeller for producing fluid flow in axial direction relative to a shaft rotating around its cen ^¬ tral axis in an agitated tank. The impeller comprises a central hub which is connected to the shaft. The cen ^¬ tral hub is in the form of a flat plate with a uniform thickness and is perpendicular to the central axis. The central hub has a number of groups of first bolt holes arranged to form a pattern, the number of groups of first bolt holes corresponding to a number of blades attached to the central hub. The impeller further com ^¬ prises at least three blades extending radially out- wardly from the central hub, each blade having a root portion. The root portion is in a form of a flat plate with a uniform thickness. The root portion has a group of second bolt holes arranged in a corresponding pat ^¬ tern in relation to the pattern of the first bolt holes so that the group of second bolt holes can be aligned with the group of first bolt holes and bolts can be placed through the first and second bolt holes to form bolted joints. According to the invention the number of holes in each group of first and second holes is at least five. The pattern to which the first holes and second holes are arranged in each of the respective groups of holes is in a form of a closed curved shape.

The advantage of the invention is that the arrangement of the pattern of holes and bolts in a form of a closed curved shape tends to equalize dynamic stresses caused by fluid forces acting on the impeller, which dynamic stresses, when the impeller is in use during agitation, are exerted at the bolted joints. The fatigue life of the impeller is improved. The arrangement of the inven ^¬ tion also makes it possible to construct the impeller cost-effectively from a few parts with bolted joints without any need for welds, and also thereby improve the fatigue strength. The highest stress concentration factor of the new structure is significantly lower than old design has and new structure has only a few criti- cal stress spots.

In one embodiment of the hydrofoil impeller, the cen ^¬ tral hub comprises an upper surface and a lower surface which is parallel to the upper surface. The root por- tion of each blade abuts against the lower surface of the central hub. In one embodiment of the hydrofoil impeller, the number of the blades is three. The central hub has a form of a triangle with rounded corners. The group of first bolt holes is disposed at each corner of the central hub. The pattern in which the first holes and second holes are arranged in each of the respective groups of holes has a form of an ellipse. The central hub having a shape of a triangle has an advantage that the vertical flow of the fluid in the downwards direction near to the shaft and passing the central hub is substantially unobstructed by the central hub.

In one embodiment of the hydrofoil impeller, the number of the blades is five. The central hub has a form of a circular disc. The closed pattern in which the first holes and second holes are arranged in each of the re ^¬ spective groups of holes has a form of a circle.

In one embodiment of the hydrofoil impeller, impeller comprises five pieces of first spacer plates each of which has a pattern of bolt holes corresponding to the pattern of groups of first bolt holes and second bolt holes for each of the bolted joints. The first spacer plates are arranged against and underneath the root portion of the blade to form spacing elements for the bolted joints.

In one embodiment of the hydrofoil impeller, the impel ^¬ ler comprises five pieces of second spacer plates each of which has a pattern of bolt holes corresponding to the pattern of the bolt holes of the first spacer plate, and which second spacer plates are arranged against the upper surface of the central hub to form spacing elements for the bolted joints.

In one embodiment of the hydrofoil impeller, each blade comprises a leading edge and a trailing edge in the di ^¬ rection of the rotation. The attachment of the blade at the root portion provided by the closed curve shaped pattern of bolted joints provided by said first and second bolt holes and bolts is located, in the direc- tion of the rotation, in front of and near to the side of the leading edge of the blade and substantially be ^¬ hind the trailing edge of the preceding blade.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to pro ^¬ vide a further understanding of the invention and constitute a part of this specification, illustrate embod ^¬ iments of the invention and together with the description help to explain the principles of the invention. In the drawings:

Figure 1 is a schematic elevation side view of a reac ^¬ tor tank equipped with a first embodiment of the impel ^¬ ler in accordance of the invention,

Figure 2 is an axonometric view of the first embodiment of the impeller of the invention seen obliquely from above, Figure 3 is an axonometric view of the impeller of Fig ^¬ ure 2 seen obliquely from below, Figure 4 is a top view of the impeller of Figures 2 and 3, with one of the blades shown as detached,

Figure 5 is an axonometric view of an impeller in ac- cordance of a second embodiment of the invention seen obliquely from above,

Figure 6 is an axonometric view of the impeller of Fig ^¬ ure 5 seen obliquely from below,

Figure 7 is a top view of the impeller of Figures 5 and 6,

Figure 8 is a plan view of the first spacer plate,

Figure 9 is a plan view of the second spacer plate, and Figure 10 is a section X-X taken from Figure 7. DETAILED DESCRIPTION OF THE INVENTION

In Figure 1 is shown a hydrofoil impeller 1 for produc ^¬ ing fluid flow in axial direction relative to a shaft 2 rotating around its central axis x in an agitated tank 3.

Referring to Figures 2 to 7, the impeller 1 comprises a central hub 4. The central hub 4 is connected to the shaft 2. Preferably, the central hub 4 has a central hole to which the shaft 2 is attached by an interfer- ence fit. Also any other suitable connecting means may alternatively be used, including welding. However, while it is an objective to improve fatigue life, welding is preferably avoided. The central hub 4 has a form of a flat plate with a uniform thickness. The cen ^¬ tral hub 4 is perpendicular to the central axis x. The central hub 4 has a number of groups of first bolt holes 5 arranged to form a pattern. The number of groups of first bolt holes corresponds to the number of blades 6 attached to the central hub 4. In the embodi ^¬ ment shown in Figures 1 to 4, the number of blades 6 is three. In the embodiment shown in Figures 5 to 7, the number of blades 6 is five.

The blades 6 extend radially outwardly from the central hub 4. Each blade 6 has a root portion 7. The root por ^¬ tion 7 is in a form of a flat plate with a uniform thickness. In the shown two embodiments the whole blades are in a form of a flat plate with a uniform thickness. Each of the blades has three straight bends which divide the blades to four planar angled succes ^¬ sive profile portions.

The root portion 7 of the blade has a group of second bolt holes 8 which are arranged in a corresponding pat ^¬ tern in relation to the pattern of the first bolt holes 5 so that the group of second bolt holes 8 can be aligned with the group of first bolt holes 5 and bolts 9 can be placed through the first and second bolt holes to form bolted joints.

The number of holes in each group of first and second holes is at least five. In the preferred embodiments shown in Figures the number of holes in each group of first and second holes is eight. The pattern in which the first holes 5 and second holes 8 are arranged in each of the respective groups of holes is in a form of a closed curved shape. The closed curved shape can be an ellipse or a circle.

As can be seen especially in Figures 2 and 3 for the first embodiment of the impeller and in Figures 5, 6, and 10 for the second embodiment of the impeller, the central hub 4 comprises an upper surface 10 and a lower surface 11 which is parallel to the upper surface. The root portion 7 of each blade 6 is abutted against the lower surface 11 of the central hub 4.

In the embodiment shown in Figures 2 to 4, the number of the blades 6 is three. The central hub 4 has a form of a triangle with rounded corners. The group of first bolt holes 5 is disposed at each corner of the central hub 4. The pattern in which the first holes 5 and sec ^¬ ond holes 8 are arranged in each of the respective groups of holes is in a form of an ellipse.

In the embodiment shown in Figures 5 to 7, the number of the blades 6 is five. The central hub 4 is in the form of a circular disc. The pattern, in which the first holes 5 and second holes 8 are arranged in each of the respective groups of holes, has a form of a cir ^¬ cle .

Referring to Figures 5 to 10, the impeller 1 comprises five pieces of first spacer plates 12, one of which is shown separately in Figure 8. Each first spacer plate 12 has a pattern of bolt holes 13 corresponding to the pattern of groups of first bolt holes 5 and second bolt holes 8 for each of the bolted joints. As can be seen from Figures 6 and 10, the first spacer plates 12 are arranged against and underneath the root portion 7 of the blade 6 to form spacing elements for the bolted joints. The impeller 1 also comprises five pieces of second spacer plates 14, one of which is shown sepa ^¬ rately in Figure 9. Each second spacer plate 14 has a pattern of bolt holes 15 corresponding to the pattern of the bolt holes 13 of the first spacer plate 12, and which second spacer plates 14 are arranged against the upper surface 11 of the central hub 4 to form spacing elements for the bolted joints. The spacer plates 12 and 14 used together with the central hub 4 and the root portion 7 give a suitable elongation length for the bolts 9 to enhance their fatigue strength.

Referring to Figures 4 and 7, each blade 6 comprises a leading edge 15 and a trailing edge 16 in the direction of the rotation. The attachment of the blade 6 at the root portion 7 provided by the closed curve (i.e. el ^¬ lipse or circle) shaped pattern of bolted joints pro ^¬ vided by said first and second bolt holes 5, 8 and bolts 9 is located, in the direction of the rotation, in front of and near to the side of the leading edge 15 of the blade 6 and substantially behind the trailing edge 16 of the preceding blade.

While the present invention has been described in con- nection with an exemplary embodiment, and implementa ^¬ tions, the present invention is not so limited, but ra ^¬ ther covers various modifications, and equivalent ar- rangements, which fall within the purview of prospec ^¬ tive claims.