Expert ForumsMixing

Topic: How can we achieve an optimum mixing system?

How can we achieve an optimum mixing system? Currently, I have various sizes of mixer, one of which has a diameter of 103 cm and height of 109 cm. All were used for mixing paints with density 1,300 kg/m3 and viscosity 8-9 poise. 1. What is the suitable blade type for mixing such high density and viscosity? How about 4 blades (composed of T-shaped fixed crosscurrent on the top of another T shaped)? 2. And If d = stirrer diameter D = mixer diameter what is the optimum d/D? 3. What is the optimum rpm to provide good mixing in this case? How to define good mixing? Is there any quantitative measurement (reynolds number range or power number range) which represent good mixing? 4. How do we define suitable power of stirrer motor?
  • Avatar Dave Dickey Forum Moderator 290 Posts

    Re: How can we achieve an optimum mixing system?

    Last edited: 12 September 2012 at 10:48am

    1.    The impeller description is not entirely clear, but it sounds as if two flat blades are mounted near the bottom the shaft and then two more flat blades are mounted to the shaft about a meter above the bottom blades.  For typical blades 103 cm in diameter, the blade width would be about 20 cm.  That design would be suitable for mixing the paint.
    2.    The impeller (stirrer? [d]) diameter should be about 40% of the vessel (mixer? [D]) diameter:  d/D = 0.4.  So a 103 cm diameter impeller size could be used in a 260 cm diameter vessel.
    3.    The optimum rpm for good mixing with the impellers and vessel previously described would be about 45 rpm.  The best quantitative description for this application is in terms of torque per volume at about 0.06 N m / m3.  Reynolds number and power number do not describe good mixing.  Reynolds only describes whether flow is turbulent, transitional, or laminar, which are ranges of conditions, not mixing intensities.  For the impeller diameter, fluid properties and rotational speed in this application, the Reynolds number is about 1,000.  Power number applies to an impeller geometry (two sets of two flat blades mounted to a shaft) and describes the relationship between impeller diameter, rotational speed, and fluid properties (fluid properties as a value of Reynolds number).  The power number for the described impeller is about 4.66.  A high power number might describe an impeller with wide blades and a low power number an impeller with narrow blades, but not whether the mixing is good or not.
    4.    The suitable power for the motor can come from either using the torque per volume value given in 3. at (0.06 N m / m3), or from the fluid properties in combination with the impeller diameter and rotational speed.

    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 dinesh khatri Community Member 1 Post

    Re: How can we achieve an optimum mixing system?

    Last edited: 30 September 2012 at 10:42pm
    i want to mix solid carbonyl iron particle with grease whose density is approx 5 times of water after mixing.Which motor(H.P.) and stirrer use to proper mixing of these? vessel size:3 litre in which approx 2 litre mixture is be prepaired