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Increase mixing efficiency by putting baffles in the mixing tank

March 24, 2014

Putting baffles in the mixing tank but the solid raw materials can probably stick into the edges of the baffles. The best way to increase the mixing efficiency of a mixer that handles 500 – 2000 centipoise liquid suspension, use 3 or 4 baffles in the way, also consider the mixing blades. All vertical blades provide perfect dispersion but poor to mediocre mixing.

To increase mixing you have to create a pumping effect which can be done by angling of blades. Decreasing the blade angle from 90′ produces a vertical flow along with its radial flow. Considering that your blades have a diameter of approx 0,65-0,7D (hydraulic half diameter of the wessel) the vertical flow go down to wessel bottom than rises from the area between blades and wessel wall.

As this happen, radial flows from the blades cut this rising vertical flow continuously, providing a perfect mixing. You can adjust the angle of blades to create the desired mixing profile, higher angles giving a dispersion-dominat mixing and lower angles (down to 45′) giving a pumping-dominant mixing. The tangent value of the blade angle gives the ratio of radial flow/vertical flow, so represent the dispersive characteristic of you blade system. For example a blade angle of 63′, tangent value=2, produces 2 parts of radial flow and 1 part of vertical flow.

Assuming that you have an elliptic bottom dish, just place the blades starting from the bottom of cylinder, giving a 0,35D center-to-center spacing. After first blade rotate the second by 60′ clockwise (same rotation direction with the shaft!) and same for the third also. In doing so you have a 3-element distributed propeller which is perfect for all kinds of viscous liquids and if you have enough power it can vortex a 20.000 cPs liquid.

If your wessel too large, increase the number of blades in the same manner. For a perfect functioning mixer, it is better to have two sets of 3-element groups aligned in continuous manner. To help this system, it may be advisable to install a bottom scraper (dish-blade spacing at about 5-10mm), all vertical and at 0,5D diameter. This will be especially useful if you are about to dissolve high density salts at relatively low speeds.

For the rotational speed, think your needs. As an idea, if you choose the blade diameter 0,67D, the blade width 0,067D, the blade angle 63′-56′ and max tip speed of the blades 6m/s you can vortex a liquid of 5.000 to 20.000 cPs depending on the flow characteristics (consider kinematic viscosity rather than dynamic viscosity) at full speed. Power requirement may be in the range of 1,5 to 3 kw/m3 roughly.

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