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 or producing fluid flow in axial direction relative to a shaft rotating around its central 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 being perpendicular to the central axis. The central hub has three groups of first bolt holes arranged to form a pattern. Three equally-spaced blades extend radially outwardly from the central hub. Each blade has a root portion. The blade 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 corre ^¬ sponding 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. Each blade further comprises a straight leading edge, a trailing edge and a tip edge.

The applicant of the present application has previously designed a blade of an axial flow impeller and an axial flow impeller, disclosed in WO 2013/124539 Al, the de ^¬ sign and dimensioning of the blade having excellent characteristics in terms of flow pattern, low energy consumption, high pumping capacity, strong axial flow with a small power consumption and low shear, high pumping efficiency, scalability and low fabrication costs. However, the presented blade design is suitable for blades which are connected to the central hub by welding. Now, there has existed a need to develop a blade and an impeller which can be constructed without any welding, so that such a structure can provide for lower manufacturing costs and enhanced fatigue strength. Therefore, the attachment of the blades to the central hub by bolted joints is a desirable ap- proach. However, the bolted joints need redesigning of the central hub, the pattern of the bolted joint at ^¬ tachment and the form of the blade so that an equally good performance can be achieved compared to the impel ^¬ ler and blade design presented in WO 2013/124539 Al .

OBJECTIVE OF THE INVENTION

The objective of the invention is to provide an impel ^¬ ler having excellent performance characteristics, low fabrication costs and a long fatigue life.

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 the hub is perpendicular to the central axis. The central hub having three groups of first bolt holes arranged to form a pattern. The impeller further comprises three equally-spaced blades extending radial ^¬ ly outwardly from the central hub. Each blade has a root portion. The blade 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. Each blade comprises a straight leading edge, a trailing edge and a tip edge.

According to the invention the tip edge is straight and has a right angle with a radius extending from the cen ^¬ tral axis to the tip edge. The number of holes in each group of first and second holes is at least five, pref- erably eight. The pattern in which the first holes and second holes are arranged in each of the respective groups of holes is in a form of an ellipse having a center and a major axis which is substantially parallel to the radius and placed at a distance therefrom. The leading edge is, in the direction to rotation, behind an imaginary radial line intersecting the central axis of the shaft and the center of the ellipse. The leading edge is at an angle of 58°± 2° in relation to said ra ^¬ dial line. The trailing edge has two straight edge por- tions which are at an angle of 150°± 5° to each other. The tip of said angle is located approximately in the middle of the length of the trailing edge. The tip is rounded. The area of the blade is divided into four planar portions by three straight bends. A first bend extends along the blade in a direction which is at an angle of 16°± 2° in relation to the radius and, in the direction of rotation, forwards of the radius. The first bend divides the blade to said root portion and a first profile portion. The root portion and the first profile portion meet at the first bend such that that the first profile portion is angled at an angle of 16°± 2° downwardly from the root portion. A second bend ex ^¬ tends along the blade in a direction which is at an angle of 12°± 2° in relation to the radius and, in the direction of rotation, backwards from the radius. The second bend divides the blade further to a second pro ^¬ file portion. The first profile portion and the second profile portion meet at the second bend such that that the second profile portion is angled at an angle of 10°± 2° downwardly from the first profile portion. A third bend extends along the blade in a direction which is at an angle of 21°± 2° in relation to the radius and, in the direction of rotation, backwards from the radius. The third bend divides the blade further to a third profile portion. The second profile portion and the third profile portion meet at the third bend such that that the third profile portion is angled at an an ^¬ gle of 8°± 1° downwardly from the second profile por ^¬ tion.

The advantage of the impeller is that it is able to provide all performance benefits as the prior art im ^¬ peller disclosed in WO 2013/124539 Al with lower manu ^¬ facturing costs and higher fatigue life.

In one embodiment of the hydrofoil impeller, the cen- tral hub is in the form of a triangular plate with rounded corners. Each corner has one group of first holes .

In one embodiment of the hydrofoil impeller, the length of the blade is 0.85xR ± 0. lxR, wherein R is the length of the radius from the central axis to the tip edge.

In one embodiment of the hydrofoil impeller, the corner between the leading edge and the tip edge is rounded with a radius of 0.125xR ± 0.02xR, wherein R is the length of the radius from the central axis to the tip edge .

In one embodiment of the hydrofoil impeller, the corner between the trailing edge and the tip edge is rounded with a radius of 0.125xR ± 0.02xR, wherein R is the length of the radius from the central axis to the tip edge .

In one embodiment of the hydrofoil impeller, the thick- ness of the blade is 0,02xR ± O.OlxR wherein R is the length of the radius from the central axis to the tip edge .

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 shows a plan view of the impeller of Figures 1 to 3 seen from above,

Figure 5 is a schematic plan view showing one blade of the impeller of Figure 4,

Figure 6 is a section VI-VI from Figure 5,

Figure 7 is a view VII-VII from Figure 5, and Figure 8 is a vie VIII-VII 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 4, 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 to avoid any connecting by welding. The central hub 4 is in the form of a flat plate with a uni- form thickness. The central hub 4 is perpendicular to the central axis x. The central hub 4 has three groups of first bolt holes 5 which are arranged to form an el ^¬ liptical pattern. Three equally-spaced blades 6 extend radially outwardly from the central hub 4. Each blade 6 has a root portion 7. The blade 6 is in a form of a flat plate with a uniform thickness. The root portion 7 has a group of second bolt holes 8 arranged in a corre ^¬ sponding elliptical in relation to the elliptical pat ^¬ tern of the first bolt holes 5. The number of holes in each group of first and second holes 5, 8 is eight. 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. Each blade 6 comprises a straight lead- ing edge 10, a trailing edge 11 and a tip edge 12.

As can be seen from Figures 2 to 4, the central hub 4 is in the form of a triangular plate having rounded corners. One group of first holes 5 is arranged at each corner of the central hub 4. The triangular form of the central hub 4 is advantageous because it allows axial flow near to the shaft 2. Referring to Figure 4, the tip edge 12 of the blade 6 is straight. The tip edge 12 is at right angle in rela ^¬ tion to a radius r which extends from the central axis x to the tip edge 12.

The elliptical pattern of the bolt holes 5 an 8 and bolts 9 has a center 13. The major axis 14 of the el ^¬ lipse is substantially parallel to the radius r. The major axis 14 of the ellipse is placed at a distance d from the radius r.

The leading edge 10 of the blade 6 is, in the direction to rotation, behind an imaginary radial line T intersecting the central axis x of the shaft 2 and the cen- ter 13 of the ellipse. The leading edge 10 is at an an ^¬ gle a of 58°± 2° in relation to the radial line. The leading edge 10 is also at an angle δ of 18 °± 2° in relation to the radius r which extends from the central axis x to the tip edge 12.

The trailing edge 11 of the blade 6 has two straight edge portions 15 and 16 which are at an angle β of 150°± 5° to each other. The tip of said angle β is lo ^¬ cated approximately in the middle of the length of the trailing edge 11. The tip of the angle is rounded.

Reference is made to Figures 4 to 8.

The area of the blade 6 is divided into four planar portions 7, 17, 18, 19 by three straight bends 20, 21, 22.

A first bend 20 extends along the blade 6 in a direc ^¬ tion which is at an angle γ of 16°± 2° in relation to the radius r and, in the direction of rotation, for- wards of the radius r. The first bend 20 divides the blade 6 to the root portion 7 and a first profile por ^¬ tion 17. The root portion 7 and the first profile por- tion 17 meet at the first bend 20 such that that the first profile portion 17 is angled at an angle of 16°± 2° downwardly from the root portion 7 A second bend 21 extends along the blade 6 in a direc ^¬ tion which is at an angle o of 12°± 2° in relation to the radius r and, in the direction of rotation, backwards from the radius r. The second bend 21 divides the blade 6 further to a second profile portion 18. The first profile portion 17 and the second profile portion 18 meet at the second bend 21 such that that the second profile portion is angled at an angle of 10°± 2° down ^¬ wardly from the first profile portion 17. A third bend 22 extends along the blade 6 in a direc ^¬ tion which is at an angle Θ of 21°± 2° in relation to the radius r and, in the direction of rotation, backwards from the radius r. The third bend 22 divides the blade 6 further to a third profile portion 19. The sec- ond profile portion 18 and the third profile portion 19 meet at the third bend 22 such that that the third pro ^¬ file portion 19 is angled at an angle of 8°± 1° down ^¬ wardly from the second profile portion 18. Referring to Figure 4, the length L of the blade 6 is 0.85xR ± O.lxR. The corner between the leading edge 10 and the tip edge 12 is rounded with a radius of 0.125xR ± 0.02xR. The corner between the trailing edge 11 and the tip edge 12 is rounded with a radius of 0.125xR ± 0.02xR. Referring to Figure 6, the thickness s of the plate material blade 6 is 0,02xR ± O.OlxR. R is the length of the radius r from the central axis to the tip edge 12. 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.