GB2249269A - Mixer - Google Patents

Abstract

A batch paddle mixer comprises two rotatable shafts (12, 14) each supporting a number of paddles. The surface area of the free ends of the paddles (18, 20) is reduced by providing notches (22, 30, 32) in the paddle (18, 20) so as to provide improved performance. The peripheral rotational speed may be at least 1.8m/sec. The paddles may be disposed at an angle to the shafts (12, 1.4) (Fig 1, not shown) and to the arms (16) (Fig 5, not shown). <IMAGE>

Description

MIXER This invention relates to mixers and particularly, but not exclusively to batch paddle mixers.

The shape of the lifting blades of known mixers is such as to cause segregation of differing particle sizes if peripheral blade velocity is high. As a result of this the mixing zone, velocity and efficiency of these known mixers are reduced leading to an increase in mixing time.

The present invention has been made from a consideration of this problem.

According to the present invention there is provided a mixer comprising at least two rotatable shafts, a plurality of paddles being connected to each shaft, wherein the surface area of at least a part of one of said paddles is reduced at and/or towards the edge of said paddle, said edge being remote from the shaft on which said paddle is mounted.

The mixer of the present invention can be successfully operated at speeds in excess of that previously used.

According to a second aspect of the invention there is provided a method of mixing components using a mixer comprising two shafts, a plurality of paddles being connected to each shaft, wherein the peripheral rotational speed of the mixer is at least 1.8m/sec.

Preferably the rotational speed of the mixer is at least 2m/sec.

In a preferred embodiment of the invention the mixer comprises a plurality of paddles at least one of which and preferably each comprising at least one indentation in its outside edge. The profile of the indentation is preferably such as to provide progressive dispersal and exit of the material from the paddle, for example a V-shaped notch, so as to extend and improve the mixing area. At least one and preferably both of the side edges of at least one and preferably all of the paddles may be inwardly inclined towards the central longitudinal axis of the paddle.

The paddles may be mounted on arms which extend from the shaft. The arms of the paddles may be arranged in a parallel manner. The arms of paddles provided on opposite sides of one shaft may be arranged on the same axis. Alternatively the arms of paddles provided on opposite sides of the same axis may not be arranged on the same axis. In such circumstances the paddles mounted on these axis may be mounted off centre on these arms such that both paddles rotate through the same area.

The mixer is preferably operated at a peripheral blade speed of at least 1.8m/sec and the mixer is filled with material to be mixed to a level not greater than the underside of the shafts. The material to be mixed is preferably particulate although fluids may also be mixed.

The applicants have found that the material being processed should be subjected to the most extensive interaction and the broader the zone of interaction then the more effective the performance will be.

In order that the invention may be more readily understood specific embodiments thereof will now be described by way of example only with reference to the accompanying drawings in which: Fig.1 shows the mixing chamber of one mixer in accordance with the invention; Fig. 2 is a front view of the mixing chamber of Fig. 1; Fig. 3 is a side elevation of one type of paddle shown in Figs. 1 and 2; Fig. 4 is a side elevation of the other type of paddle shown in Figs. 1 and 2; Fig. 5 is a side elevation of a modified paddle of a mixer in accordance with the present invention; and Fig. 6 shows a modified paddle arrangement.

Referring to Figs. 1 to 4 there is shown a mixing chamber 10 into which the materials to be mixed are introduced. The materials are introduced to a level below the level of two counter rotation shafts, 12,14 located in the mixing chamber 10. Sets of four arms 16 periodically extend from each shaft 12,14 at ninety degrees to one another. A paddle 18,20 is located at the free end of each arm 16, each paddle 18,20 being disposed at a 45" angle relative to the shafts 12,14.

It can be seen from Fig. 2 that the distance between the two shafts, 12,14 is such that the paddles 18,20 on both shafts 12,14 rotate through the same area A and that the paddles 18,20 from either shaft 12,14 will not come into contact with the paddles 18,20 of the other shaft 12,14.

Two types of paddle 18,20 as illustrated in Figs.

3 and 4 are located on each shaft 12,14. Both types of paddle 18,20 have the same maximum width 'w' and the same maximum length 'l'.

Referring to the paddle illustrated in Fig. 3 a V-shaped notch 22 is provided in the outer edge 24 of the paddle 18. Similarly the corners of the paddle 18 have been removed so as to provide inclined edges 26.

These edges are inwardly inclined from the maximum width 'w' towards the longitudinal axis of the paddle 18. The depth of the notch 22 and inclined edges 26 is a fixed ratio to the outside edge 24 of the paddle 18.

For example, if the outside edge 24 of the paddle 18 is taken from the centre line t of the shaft equals one unit then the radius to the root 28 of the V-shaped notch 22 and inclined edges 26 from t equals 0.75.

Referring to the paddle of Fig. 4 two notches 30,32 are provided in the outside edge of the paddle.

The depth of these notches 30,32 is equal to that of the notch 22.

It can be seen that in the paddle area B from the root 28 of the notch 22 to the outside edge 24 of the paddle 18 the active paddle width reduces to half the width 'w' thus encouraging ranged dispersal and exit of material from the paddles and reducing segregation and degredation of the material at high peripheral velocity. The paddle profile encourages the dispersal and exit of larger particles, that is those most susceptible to centrifugal forces, from the blade over a broader route than was previously possible with known paddles. The ranged dispersal and exit of particles enables higher peripheral velocities than would be the case with square paddles. Also reduced paddle surfaces area at the outer perimeter of the paddle reduces power consumption.

This paddle design of the present invention therefore allows a number of advantages compared to the prior art.

Referring to Fig. 5 there is shown a modified paddle 18. The paddle 18 is inclined through substantially 50 relative to the arm 16. The edge of the paddle 18 is seen to be inclined away from the direction of rotation 'r'. This leads to decreased power consumption and may change the mixing characteristics of the mixer. Any number of the paddles 18,20 may be reclined in this manner.

Referring to Fig. 6 there is shown a part of the paddle arrangement of a further mixer in accordance with the invention. As before the paddles 22 are arranged in pairs mounted on opposite sides of the shaft 24. The paddles 22 are arranged along the same axis 'A'. However, the arms 26 on which the paddles 22 are mounted are not arranged along this axis A. One of the arms 26a is arranged above the axis such that the free end of that arm is fixed to the upper portion of the paddle 22a and the opposite arm 22b is arranged below the axis A such that the free end of that arm is secured to the lower portion of the paddle 22b. All of the other paddles and arms of the mixer are similarly arranged.

It is to be understood that the above described embodiments are by way of illustration only. Many modifications and variations are possible.

For example the paddle need not necessarily comprise V-shaped notches. Indentations of a different shape for example U-shaped indentations may be used.

Claims (7)

1. A mixer comprising at least two rotatable shafts, a plurality of paddles being connected to each shaft, wherein the surface area of at least a part of one of said paddles is reduced at and/or towards the edge of said paddle, said edge being remote from the shaft on which said paddle is mounted.

2. A mixer as claimed in claim 1, wherein at least one indentation is provided in at least one of said paddle.

3. A mixer as claimed in claim 1 or claim 2, wherein at least oneside edge of at least one of said paddles is inwardly inclined towards the central axis of that paddle.

4. A mixer as claimed in any preceding claim, operative to have a peripheral rotational speed of at least 1.8m/sec.

5. A method of mixing components using the mixer as claimed in any of claims 1 to 4.

6. A method of mixing components using a mixer comprising two shafts, a plurality of paddles being connected to each shaft, wherein the peripheral rotational speed of the mixer is at least 1.8m/sec.

7. A mixer as substantially described herein with reference to and illustrated by Figs. 1 to 5 and Fig.