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Topic: Is there a guide to finding the volume that would be mixed per second?

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We have a tank with 2.5m diameter and 1.5m height. We have an impeller with diameter 2.35m (four blades with inclination of 30 degrees to the horizontal) speed of rotation is 50 rpm. We have 6 ton of micronized powder to be mixed (average density is 1.2ton/m3). We have used a 22kw electric motor with and a planetary gearbox to transmit the necessary power. Could you please guide me in finding the volume that would be mixed per second?
  • Avatar Dave Dickey Forum Moderator 285 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Based on this description, only a portion of the powder will be mixed.  The flow properties of a "micronized powder" may be free-flowing, cohesive, or a compacted solid.  Even if the powder is suspended in a liquid, movement or mixing will be minimal.  The impeller is 94% of the tank diameter, which may allow for rotation of the powder in the region of the impeller, but the design does not provide for any vertical motion in the tank.  Whether the 30 degree angle is oriented so the motion should be upward or downward, the small gap between the impeller tip and the tank wall is insufficient for any returning or recirculating motion.
     
    The rotation of the impeller might move about 0.5 meter of vertical height in the powder (3 tons), but leave the other 0.5 meter (3 tons) undisturbed.  Even the the 3 tons of powder that could be in motion may not mix well, because rotational motion does not create much radial mixing.  This design and application is not a realistic mixing process.  The 2.5 cubic meters volume that might be mixed could take 2 minutes or much longer.  The other 2.5 cubic meters could take hours or even days to mix.
     
    Whether the motor power is sufficient or excessive depends on the flow characteristics of the powder.

    The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.

  • Avatar amid karbasi Community Member 4 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Dear Dave Dickey Thank you very much for your reply. I would like to give you some more info may be you would have a better picture of my situation. 1. The materials in question are 63micron feldspar and 63micron dolomite and 63micron calcium carbonate and 63micron silica (1% is humidity of all the materials). 2. The Flow properties of the mix is very similar to that of portland cement. 3. The bulk density of the mix is 0.9ton/m3 4. The impeller is connected to a 1.5 m long shaft with 10*10cm cross section that comes from above the tank, the width of the blade is 22cm and its length is 112.5cm( measured from the shaft . 5. After we put 6 ton of the material into the tank the motor does not start there seems to be too much torque involved for the motor to start the batch, so we put in 3ton first and then start the impeller and while the impeller is running we add the remaining 3ton. 6. While the 6ton is being rotated by the impeller we added some coloured material in to the batch to see its movement. It was pulled to the middle of the tank and submerged to the bottom while rotating and after a few minutes it emerged from the sides of the tank( but very much diluted). 7. For this reason I believe the material does get mixed eventually 8. I would like to know your oppinion about this. 9. Do you think I should reduce the impeller size and increase the rpm instead?( in order to gie room for the radial movement of the material?) I thank you in advance for your kind help.
  • Avatar Dave Dickey Forum Moderator 285 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Your comments add insights and details to your original blending question, but all seem to be consistent with my original understanding of the problem.
        1.    Uniform size particles usually are an advantage for both blending and flow.
        2.    Flow characteristics should be good, but the powder must dilate (expand) for flow and blending to occur.
        3.    The bulk density sounds correct for the materials.
        4.    Your shaft size should be good for the motor power and rotational speed.  Your blade width is less than I expected, but adequate.
        5.    Starting a rotating impeller in a blender with a full load of powder is always difficult to impossible.  Loading while the blender is running is usually required.
        6.    The flow pattern you describe indicates that the blade angle is pushing powder toward the bottom of the tank.  Pushing powder toward the bottom causes the powder to compact and become more difficult to move.  You might try reversing the direction of rotation to see if pushing upward gives better motion.  Pushing upward may make starting or restarting with a full batch possible.
        7.    From your description, the material is blending, but uniformity is still questionable.  You need to look for unmixed material.  With the mixer stopped and the power completely disconnected, blended material should be carefully removed (shoveled out).  Check to see if your added color is uniform in the locations least likely to mix well.  Check on the surface, near the walls of the tank and near the mixer shaft.  Check around the bottom of the tank where the bottom meets the side wall.  Check in the bottom center of the tank.  Check around the impeller where the blades attach to the shaft.  These locations are the places where blending is most likely to be poor.  If they are blended to your satisfaction, then the blending is done.
        8.    For vertical circulation, the impeller should be no larger than 1.75 m.  The speed may need to be increased, but try the current speed first.  50 rpm sounds like a reasonable speed for blending.

  • Avatar amid karbasi Community Member 4 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Dear Dave thanks for your reply; 1. I followed your instructions in checking the blending taking samples from different parts of the tank, they seemed ok. 2. The direction of rotation is correct and the material is not being pushed down, the reason that the material is pulled down is simply because of the vortex that has been created due to rotation of the material. 3. could you please tell me why you think the blade diameter should be 1.75m? (in other words how did you calculate this figure?) would you agree that we should have a minimum tangentional speed in order to create a flow? as you can see my rotational speed is 50rpm which transform to 6.149m/s tangentional speed. what i am trying to say is that if i reduce the diameter to 1.75 the in order to maintain the tangentional speed i need to increase the rotational speed from 50rpm to 67rpm I thank you for your patience and time. Best regards Karbasi
  • Avatar Dave Dickey Forum Moderator 285 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    1.    Sampling is about the only way to decide if you have uniform blending.
    2.    Powder material should not be pushed against a mixer surface.  Good to know the direction or rotation is correct.
    3.    The reason for the 1.75 m diameter blade is because it is about 70% of the diameter of the vessel.  You want to have at lease as much cross-sectional area for motion in the opposite direction of the flow created by the impeller.  70% is about the square root of 1/2, or 1/2 the area.  With better circulation, the additional rotational speed may not be necessary for good motion.  The tangential speed of the impeller will determine the particle size of agglomerates so increasing the rotational speed may be necessary.

  • Avatar amid karbasi Community Member 4 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Dear Dave I am going to follow your suggestion in reducing the length of the blade from 2.35 to 1.75m As the gear box that i am using cant be changed ( so no increase of rpm) I thought of increasing the width of the blade to 36cm (instead) in order to keep the flow and hopefully the torque constant. I would like to know your opinion on this? Many thanks for your help
  • Avatar amid karbasi Community Member 4 Posts

    Re: Is there a guide to finding the volume that would be mixed per second?

    Dear Dave I am going to follow your suggestion in reducing the length of the blade from 2.35 to 1.75m As the gear box that i am using cant be changed ( so no increase of rpm) I thought of increasing the width of the blade to 36cm (instead) in order to keep the flow and hopefully the torque constant. I would like to know your opinion on this? Many thanks for your help