Hydrofoil impeller

1. A hydrofoil impeller for producing fluid flow in an axial direction relative to a shaft rotating around its central axis in an agitated tank, the impeller comprising:

a central hub which is connected to the shaft, the central hub being in the form of a flat plate with a uniform thickness and being perpendicular to the central axis (x), the central hub having 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; and

at least three blades extending radially outwardly from the central hub, each blade having a root portion, the root portion being in a form of a flat plate with a uniform thickness, the root portion having a group of second bolt holes 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, wherein the number of holes in each group of first and second holes is at least five, wherein 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 a closed curved shape,

wherein each blade comprises a leading edge and a trailing edge in the direction of the rotation, and the attachment of the blade at the root portion provided by the closed curve shaped pattern of bolted joints provided by the first and second bolt holes and bolts is located, in the direction of rotation, in front of a midpoint of the blade defined between the leading edge and trailing edge of the blade.

2. The hydrofoil impeller according to claim 1 , wherein the central hub comprises an upper surface and a lower surface which is parallel to the upper surface, and the root portion of each blade abuts against the lower surface of the central hub.

3. The hydrofoil impeller according to claim 1 , wherein the number of the blades is three, the central hub is in the form of a triangle with rounded corners, the group of first bolt holes is disposed at each corner of the central hub, and the pattern in which the first holes and second holes are arranged in each of the respective groups of holes is in the form of an ellipse.

4. The hydrofoil impeller according to claim 1 , wherein the number of the blades is five, the central hub is in the form of a circular disc, and the pattern in which the first holes and the second holes are arranged in each of the respective groups of holes is in a form of a circle.

5. The hydrofoil impeller according to claim 4 , wherein the 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, and which first spacer plates are arranged against and underneath the root portion of the blade to form spacing elements for the bolted joints.

6. The hydrofoil impeller according to claim 5 , wherein the impeller comprises five pieces of second spacer plates each of which has a pattern of bolt holes corresponding to the pattern of the bold 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.