IE41451B1 - Surface aerator and a rotor therefor - Google Patents

Abstract

1470926 Aerating liquids MACHINEFABRIEK W HUBERT & CO BV 13 Aug 1975 [29 Aug 1974] 33772/75 Heading B1C A rotatable surface aerator has a boss 1, 2 carrying a plurality of radial blades 3 interconnected by a ring 7, the blades extending at 10 beyond the ring and having lower edges which extend perpendicularly to the rotational axis at 5 and thereafter upwardly, and upper edges which extend outwardly and upwardly the adjacent blade portion 9 being bent in the intended direction of rotation. Preferably the angle of the portion 9 is 30‹ to 60‹ to the plane of the blade, and a part of the lower edge of the blade may be bent in the same sense. As shown the ring 7 is upwardly convex but it may be planar or frusto-conical, or diminished to a slip.

Description

This invention relates to rotors for surface aerators, and to surface aerators having such rotors. In use the rotor is generally mounted on a spindle which is vertical. The aerators of the invention are particularly applicable to oxidation plants for sewage treatment, such as oxidation ditches or canals and oxidation tanks. f In aeration tan'ks and oxidation ditches, besides the f so-called brush aerators, there have been used aerators having bladed rotors. While rotating, the blades are in contact with the surface of the liquid to agitate it and to introduce air into the liquid.

Because of the oxygen demand of aerobic bacteria, the quantity of oxygen that can be introduced to the water by an aerator is a significant factor. Such an aerator is normally driven by means of an electric motor and a reduction gear. In relation to operating costs, the amount of oxygen that is brought into the water per unit of power is also a significant factor.

As the rate of solution of atmospheric oxygen into water is reduced if the water already contains a certain concentration of oxygen, it is also important with an aerator that the water circulation is as -2414S1 great as possible in order that as much oxygen-poor water as possible can come into contact with atmospheric oxygen.

It is a disadvantage of aerators known and in use that if they are used in an oxidation ditch of the carrousel type (i.e. built in a simple circuit shape) which is to operate according to the so-called Pasveer conditions the flow speed obtained in some locations is insufficient to keep the sludge in the circuit in suspension. Because these known aerator types sometimes have an insufficiently large propulsion capacity, costly provisions must often be made in a carrousel circuit to bring the said flow speed up to the required 1evel.

Such provisions involve among other things circular aeration zones and legs of the circuit which are shallower than the aeration zones.

The fundamental parameters determining the rate of oxygen transfer are the saturation deficit, the specific interface between water and air, the rate of molecular diffusion, and the rate of interface renewal. In practice, the rate of oxygenation is mainly controlled by the magnitude of the interfacial area produced by aeration. The main factors with respect to air diffusion are bubble size, rate of air flow and diffuser submergence. -341481 The principal parameters in mechanical areation are size (diameter) of the aerator (rotor or cone), its peripheral speed and its depth of submergence.

Also the shape of the tank and the pattern of flow induced by aeration influence the oxygenation capacity and the amount of oxygen transferred per kWh of energy.

According to the preseht invention, there is provided a rotor for a surface aerator having a boss, a plurality of blades connected to the boss, and a ring spaced from the boss and interconnecting the blades, the blades extending outwardly beyond the ring and having first edges which are lowermost in the operative orientation of the rotor and which outwardly from the boss extend perpendicularly to the rotational axis and there15 after continuously upwardly, and second edges which are uppermost in the said operative orientation and which extend outwardly and upwardly, the upper portions of the blades adjacent said second edges being turned forwardly in one direction of rotation. This direction of turn of the upper portions of the blades is in the intended preferential direction of rotation in use.

One embodiment of the aerator rotor according to the invention will now be described by way of example with reference to the accompanying drawings,in which:-441451 Figure 1 is a plan view of the aerator rotor; Figure 2 is a side view of a blade part of the boss of the aerator rotor of Figure 1; and Figure 3 is a cross-section on the line 111 - 111 in Figure 2.

The aerator rotor shown in the drawing has a boss 1 which in use is mounted on a spindle of a surface aerator and is rotated about a vertical axis. At its lower end the boss 1 is provided with a streamlined portion 2 and carries a number of radially extending blades 3 which are mounted around it, these being in the present embodiment eight blades 3. The blades 3 have a portion 4 near the boss 1 where the lower edge 5 of the blade is horizontal. Further from the boss, the blade has a continuously upwardly and outwardly sloping lower edge 6.

A generally frustro-conical ring 7, whose width can vary according to the particular case and which in fact has a wing-form in cross-section i.e. is slightly convex upwardly, interconnects the blades 3 in the manner shown in Figure 1.

The ring 7 is spaced from the boss 1, so that an annular opening 8 is left between the boss 1 and the inner circumference of the ring 7. s The vertically arranged blades 3 are at portions adjacent the upper edge which slopes continuously upwardly in a straight line, turned over in this case at an angle of 45° in the preferential direction P of rotation (i.e. the intended direction in which the agitator is to be rotated). -541451 The angle at which the portions adjacent the upper edge are inclined is preferably in the range 30° to 60° to the horizontal. Each blade 3 extends at 10 beyond the outer circumference of the ring 7, together with a turned over portion 11.

The distance from the lower edge 6 to the underside of the ring 7 gradually decreases outwardly.

As can be seen from Figures 2 and 3 each blade 3 has a so-called under blade and a so-called upper blade, divided by the ring 7. Because of the particular shape of the aerator rotor illustrated, the optimal flow of water along the several parts thereof is obtained, so that flow losses during the flow of water along the aerator rotor are, because its shape is adapted to the requirements of hydraulics, at a minimum. The portion 4 of the blade propels the water upwardly through the central opening 8 by the creation of an overpressure. The water lifted through this central opening 8 is, because of the arrangement of the vertical upper blade and of guide blade 9 placed at 45° to the vertical upper blade, as a consequence of the rotation of the aerator, carried off over the ring 7 and is slung away by the protruding parts 10 and 11 of the blade. During this -641451 flow over the ring 7 the water is in contact with the surrounding air so that additional aeration takes place. It has appeared that the presence of the central opening 8 increases the introduction of oxygen.

The water that is propelled outwardly towards the outer rim of the aerator over and under the ring 7 by the rotation of the aerator, is slung away in the form of an umbrella by the arrangenent of the vertical end 10 and of the end portion 11 of the upper blade which is turned over at 45°, the function of the end portion 11 being to turn the water film flowing along the blade more forwardly. Here again a guidance, favourable in hydraulic aspects, takes place with only slight losses of flow.

As the ring 7 does not extend to the ends of the blades of the rotor, the parts 10 and 11 of the blade 3 located beyond the ring 7 operate highly effectively, and energy losses are kept low. The efficiency of oxygen introduction is high because the water can freely move upwardly over the parts 10 and 11.

At the same time the blade area can be large so that the propulsive action of the aerator rotor in the basin in which the aerator is placed is good. -741451 Vertical blades in existing aerators are often arranged in a trailing manner, i.e. rearwardly directed with respect to the radial directions, in conformity with the blade positioning often used in a centrifugal pump. In pump technology it is known that with a trailing positioning of the blades pump flow is increased, whereas with a leading positioning higher pressure can be obtained. The aerator rotor according to the invention, however, preferably has blades that are substantially radially placed. It has appeared that this radial positioning is favourable for the aerator performance.

Many alternatives are possible within the scope of the invention. For instance a portion adjacent the lower edge of the blade 3 may also be turned over at an angle, in the same sense as the turned over portion adjacent the upper edge. Also, the width of the ring 7 can be reduced. The ring 7 may in suitable cases be diminished to a small constructional strip, and may be planar or non-planar e.g. frustocOnical or, as described above, of wing-form. Preferably the blades 3 are vertical, except for the said forwardly turned portion or portions. The forwardly turned -841451 portion may, as shown in Fig. 3, be welded onto the vertical portion and is preferably plana'.

The illustrated aerator rotor makes it possible to provide an aerator having a highly constant efficiency of oxygen introduction over the total immersion range and having a good control range. Furthermore, when it is used in an oxidation trough of the carrousel type, the aerator can have a satisfactory propulsion capacity and energy losses are lew so that the proportion of the total energy provided which is available for propulsion and oxygen introduction is high. Finally, the rotor in operation has a low axial upthrust so that axial reaction forces are reduced and the bearings of a driving reduction gear may be lighter.

Claims (9)

1. Rotor for a surface aerator having a boss, a plurality of blades connected to the boss, and a ring spaced from the boss and interconnecting the blades, the blades extending outwardly beyond the ring and having first edges which are lowermost in the operative orientation of the rotor and which outwardly from the boss extend perpendicularly to the rotational axis and thereafter continuously upwardly, and second edges which are uppermost in the said operative orientation and which extend outwardly and upwardly, the upper portions of the blades adjacent said second edges being turned forwardly in one direction of rotation.

2. Aerator rotor according to claim 1 wherein the said upper portions of the blades are inclined at an angle of 30 to 60° to the said one direction of rotation.

3. Aerator rotor according to claim 2 wherein the said upper portion of the blade is inclined at 45° to the said one direction of rotation. -1041451

4. Aerator according to any one of the claims 1 to 3 wherein the ring is non-planar.

5. Aerator rotor according to any one of the preceding claims wherein the blades are substantially radial.

6. Aerator rotor according to any one of the preceding claims wherein the said second edges of the blades slope continuously upwardly from the boss in a straight line.

7. Aerator rotor according to any one of the preceding claims wherein the lower portions of the blades adjacent said first edges are turned forwardly in the same direction of rotation as the said upper portions.

8. Rotor for a surface aerator, substantially as herein described with reference to and as shown in the accompanying drawings.

9. A surface aerator having a rotor according to any one of the preceding claims.