EP0085659A2

EP0085659A2 - A method for mixing liquids

Info

Publication number: EP0085659A2
Application number: EP83850007A
Authority: EP
Inventors: Hjalmar Fries
Original assignee: Flygt AB
Current assignee: Xylem Water Solutions AB
Priority date: 1982-02-01
Filing date: 1983-01-18
Publication date: 1983-08-10
Also published as: FI79037B; FI824463A0; JPS6230810B2; FI79037C; SE439592B; JPS58207935A; EP0085659A3; CA1222506A; FI824463L; SE8200546L

Abstract

The invention concerns a method for mixing liquids.

The mixing takes place by help of a propeller having a diameter which is at least 40% of the liquid depth of the tank and where the thrust centra of the propeller blades are displaced to a'radius of 65-75% of the total radius of the blade from the propeller centre.

Description

When mixing liquids with propeller mixers two methods have been used up to now. According to one, the propeller is arranged on a vertical or somewhat inclined shaft, while according to the other, the driving shaft is horizontal with the driving unit arranged outside the tank. In both cases it is tried to avoid circulation around the symmetry axle as this decreases the mixing effect.
In order to save energy during mixing, translation movements are preferred as turbulent energy has a worse distribution ability. The turbulent movements shall only be big enough to secure the essential mixing. A rotaing movement is tried to be obtained where the central part is involved, without unnecessarily extensive micro turbulence.
The possibility of obtaining such a flow is decreased when particles are sedimented in the liquid, as these particles are drawn towards the centre due to the so-called tea-cup effect.
By the methods used up to now, these particles cannot leave the centre part because of the bad media exchange (Fig 1).- Mixers having hotizontal shafts have therefore been directed across the tank or in an angle less than 10° from the diameter. The central dead zone and the co-rotation are then eliminated. It turns out however, that another two rotation centres are built up on each side of the flowcenter- line.
When using mixers having vertical shafts a central dead zone may be avoided if the co-rotation is checked by baffles arranged at the tanksides (fig 2). A toroidal spiral movement, controlled to a certain extent of an upwards or downwards going flow and an opposite periphery flow is then obtained,which is superposed a checked rotation movement.
This method has however several disadvantages. A long propeller part must be used, which means strong loads on motor and gear box. Further a slow rotating mixer must be used as well as expensive baffles, which often consume more energy than what is necessary for the mixing process. Another way to solve the problem to make the central part take part in the mixing process is to inject air in this part, thus obtaining, secondary flows which ensure media exchange and diminish sedimentation. This method means however often problems with vibrations in the mixer depending on the air influencing the propeller and the hydraulic connection between the geometry of the tank and the propeller. This has been solved by providing the propeller with a flow ring which stops radial flows and prevents air from streaming from the pressure side to the suctionside of the impeller.
The technique to inject air is of course only profitable if aeration is demanded during the mixing.
According to the invention it is however possible to use a free propeller without any ring, which is arranged on a horizontal shaft and having a big diameter when compared with the depth of the tank in such a way, that a co-operation between co-rotation, the main flow rotation of the propeller and the free tipturbulences brings about a media exchange at the central part, the tipturbulence then sweeping in a spiral form through the bottom layer of the tank thus preventing sedimentation. This is obtained by help of a special propeller design in combination with the tank design. The invention which brings about a possibility of an energy saving and extensive mixing, without risking propeller vibrations, is described more closely below with reference to enclosed drawings.
When regarding a wing tip (Fig 3) in a laminar liquid flow,a tip turbulence accurs caused by the over pressure on the underside and the under pressure on the upper side. This turbulence follows the liquid backwards. Such a flow can be counteracted by help of a tip plate on the wing, but such a plate is in its turn the origin of corresponding turbulence phenomena that remove the gain by eliminating the turbulence first mentioned.
When regarding a propeller having a flow ring (Fig 4), it can be seen that the liquid leaves the propeller along a spiral formed line, the rotation of which origins from the torque of the propeller shaft. If the flow ring is removed, a tip turbulence occurs. The latter is directed in the opposite direction of the main flow rotation and if it becomes too extensive, an important loss and a diffuse flow from the impeller occurs (Fig 5). The problem is stressed if the propeller diameter is big, compared with the water depth, as the circumstances differ very much between the moments when the propeller blades are vertical and horizontal resp.
According to the invention the propeller is so designed, that tip turbulence is limited and moving like a spiral formed whirl along the cylindric surface of the main flow. It is also possible to so dimension the tip turbulence, that it mainly corresponds with the layer at the tank bottom and that it sweeps in spiral form over said layer, thus preventing sedimentation and resuspending particles already sedimented.
The size of the tip turbulence by a free flowing propeller depends on the pressure difference between the two sides of the blade at the tips and of the position of the thrust centre. As the thrust is proportional to the dynamic pressure, a lifting force for a certain area is obtained which is proportional to the square radius to the centre. By conventionally designed propellers, the tips then give a very strong lifting force. By forming the tips vary ellipsoidally and displace the blade area towards the centre, a limitation of the tip turbulence is obtained. If the lifting force centre is located within 70% of the blade counted from the centre, tip turbulences are obtained which have a diameter of about 10% of the propeller diameter. This dimension corresponds quite well with the layer thickness in a tank having a depth of 1,1 - 1,2 times the propeller diameter. By such dimensioning the spiral formed circulation flow depending on the torque, will be big enough to exchange the liquid in the tank centre if the ratio tank diameter-liquid depth is within the area 1-5. By a greater ratio, the number of mixers can be increased to obtain the same spiral movement and to keep the tip turbulence effect in the bottom-layer.
In the description above, circular tanks have been referred to. The method can however also be used for oval tanks (Fig 6). In such a tank, where there is a rotating flow by the ends and a laminar flow there between, a special advantage is obtained by a free flow propeller because of the overflow of tip turbulence which occur at the partition wall (Fig 6). Immediately behind the linking by the partition wall, a death zone.normally occurs where particles sediment. If the propeller then origins distinct tip turbulences, these will sweep regularly with the rotation of the main flow into this zone and resuspend the material. Like in circular tanks, it is important that the tip turbulence is small when compared with the total flow diameter, as otherwise the tip turbulence will diffund into the main flow and stop acting at the tank bottom.
The spiral formed toroid movement as an energy saving mixing method, may be improved in various ways. The machine may be arranged to oscillate in the horizontal plane obtaining that the spiral movement switches its rotation centre and the tip turbulences sweep More congruently over the tank bottom. More energy may then be saved as the effect necessary for the mixing,decreases.
Another way to decrease the energy demand and the variations in the propeller load further, is to gradually set the angle of incidence for the blade depending on if the blade is directed towards the centre or the periphery of the tank. There is namely always a risk that the propeller blade will be over loaded at its centre where the flow speed is lower than at the periphery. In combination with an oscillating movement, this setting gives possibilities to exact adjust the flow for the mixing intensity demanded. There is of course an advantage but not a necessity, that the whole unit is submersible.
In the description above a mixer having a propeller with two blades has been referred to. The theory is however valid for any suitable number of blades, but the effect is not so marked by bigger numbers.

Claims

1 A method for mixing a fluid in circular or oval tanks by help of a propeller mixer having a relatively big diameter with regard to the depth of the tank, characterized in, that the liquid is rotated around the centre axis of the tank and simultanously being turned around a ring formed axis in a plane perpendicular to the centre axis in such a way that the liquid flows upwards in the centre of the tank and outwards at the surface, inclined downwards at the periphery of the tank and inwards at the bottom, that is a toroid formed spiral movement, having an upward centre flow filling the tank, or alternatively a counter directed flow, that is a toroid formed spiral movement, having a downward centre flow.

2 A method for mixing a fluid according to claim 1,
characterized in, that the propeller of the mixer is arranged with an essenitally horizontal axis, at a distance from the centre of the tank, mainly perpendicular to the radius of the tank and having the same rotation direction as the direction of the intended liquid turn and that the propeller has a diameter which is about 60-80% of the liquid depth of the tank to obtain a sufficient torque on the liquid rotating around the centre axis.

3 A method for mixing a fluid according to claims 1 or 2,
characterized in, that the propeller is arranged near. the bottom of the tank so that the tip vortexes of the propeller blades go with the bottom and interfere with its interface layer. -

4 A method for mixing a fluid according to claim 1,
characterized in, that the mixer is brought to oscillate in the horizontal plane in such a way that the rotation centre of the toroid formed spiral movement is displaced.