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Topic: Can you calculate the power required by an anchor impeller to mix a fluid of known viscosity and density?

anonymous
Is it possible to calculate the power (in HP) of a motor required to mix product of viscosity X if you know the dimensions of the vessel and assume some standard design of mixing anchor? I want to be able to specify the power of the mixing motor to mix product with a specified viscosity in a vessel of known dimensions.
  • Avatar Dave Dickey Forum Moderator 279 Posts

    Re: Can you calculate the power required by an anchor impeller to mix a fluid of known viscosity and density?

    Last edited: 5 September 2012 at 12:25pm

    It is possible to calculate the power required by an anchor impeller to mix a fluid of known viscosity and density.  The motor power is at least 110% of the calculated impeller power.  The motor power may be 125% to 200% of the impeller power, depending on the accuracy and certainty of the fluid viscosity.  The power calculation is probably easier than the explanation.
     
    The impeller power is calculated by the Power Calculation - Viscous.  The viscosity and rotational speed come from the fluid properties and mixer operation, the viscous power number and impeller diameter will be explained further.  The viscous power number is calculated first from just the geometry according to Anchor - Power Number Viscous.  The tank diameter is the stated starting point for the calculation question.  A typical anchor impeller has a diameter 90% to 95% of the tank diameter.  The blade width is typically about 10% of the impeller diameter.  The number of arms is almost always two.  The height of the vertical blade may be about the same as the impeller diameter, but the power will depend on the liquid level, if it is less than the height of the anchor blade.  The determination of these values depends on the mixer design and application.
     
    However, this viscous power number value previously calculated only applies for a Reynolds number less than 15.  So the Reynolds number must be calculated according to Reynolds Number Evaluation w Definition.  With the Reynolds number a correction factor, Correction Factor - Viscous Power, can be calculated depending on the range for the Reynolds number.  That correction factor can be multiplied times the Anchor - Power Number Viscous previously calculated.  The corrected viscous power number can be used along with the other dimensions and properties in the Power Calculation introduced at the beginning of the explanation.
     
    As a final comment - an anchor impeller is a poor selection as an impeller type because is provides almost no vertical, top to bottom, motion.  It only provides motion at the vessel wall and usually has a long blend time with poor mixing results.  Better impellers, such as a helical ribbon impeller, are recommended for viscous mixing applications.  In some cases, a combination of an anchor and other impellers on separate shafts will provide adequate mixing.

    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 RAGHU RAMAYYA KARUTURI Community Member 1 Post

    Re: Can you calculate the power required by an anchor impeller to mix a fluid of known viscosity and density?

    Last edited: 6 September 2012 at 1:35pm
    How to calculate power of motor with 4 Nos of baffles(width is 10% of vessel diameter) and anchor sweep is 70% of vessel diameter?
  • Avatar Dave Dickey Forum Moderator 279 Posts

    Re: Can you calculate the power required by an anchor impeller to mix a fluid of known viscosity and density?

    Last edited: 8 September 2012 at 12:00am
    You ask a good question about an anchor impeller 70% of the tank diameter with four baffles. Now let me ask a question Why would you do that? I have seen many different mixer deisgns, but I know of no testing or measurements on the type of system you describe. Without physical measurements on a similar system, especially an unusual one, power requirements are extremely difficult to predict, even with computer models. An anchor impeller should be used near the vessel wall, since keeping the fluid moving at the wall is about the only thing an anchor does well. I presume from this question, such a vessel and mixer exist and you are trying to decide how to use it or if it will work at your current viscosity. The system you describe may have all of the bad features of an unbaffled tank because of the whole vertical effect of an anchor impeller, while lacking the the positive features of an anchor to create motion at the tank wall. Typical impellers used with baffles have radial or axial flow blades, because the flow needs to have a vertical circulation pattern to be effective. Some vertical motion is needed for mixing, other wise an addition at the surface will remain near the surface for a long time. An anchor impeller offers almost no vertical motion, only rotational motion. Solid body rotation of the fluid, typical for an anchor impeller, does little mixing. With more details about the anchor blade width and height it might be possible to use data from tested impeller designs to estimate power requriements for some viscosities. However, even with the power information, I doubt that the impeller and baffle system you describe would blend liquids or do almost any other mixing function.
  • Avatar Manoj Tadwalkar Community Member 1 Post

    Re: Can you calculate the power required by an anchor impeller to mix a fluid of known viscosity and density?

    Last edited: 30 September 2012 at 10:42pm
    Thank you for clarifying the method of calculation of the power consumption of an anchor agitator. I need your help for estimating the power consumption of a waste recovery mixer. How to calculate the power consumption of the agitator system given in the enclosed drawing. The W/D ratio is 1/10.
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