Dave Dickey Forum Moderator 290 Posts
Re: What's the best way to check mixing design?13 July 2011 at 1:29pm
You could check many aspects of the mixer design -- some easy, others more difficult. You can check the power requirement, based on impeller type, impeller diameter, rotational speed, fluid density, and viscosity.
The power number is a characteristic of the impeller type and geometric dimensions. The power number for a typical 4-blade, 45-degree pitched-blade impeller is 1.37, for a 3-blade hydrofoil the power number is about 0.31, and for a 4-blade, straight-blade (radial flow) impeller use a power number of 3.96. Use one of the two attached power calculations for turbulent power, depending on the units you prefer to use. The motor should be 10% to 20% greater than the calculated impeller power. A correction may be needed if the viscosity results in a Reynolds number less than 20,000.
Torque can be calculated from the power and speed of the mixer. Impeller power and speed should be used for the torque to evaluate mixing intensity. Motor power and mixer speed should be used to evaluate the drive torque. Calculations can be made for shaft diameter and natural frequency, see Chapter 21 of the Handbook of Industrial Mixing (Paul, Atiemo-Obeng, and Kresta; John Wiley & Son, 2004.) Other aspects of the mixer selection can be investigated depending on the application with other chapters in the Handbook.
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.
We are designing a mixing process for a new installation in India. The first two mix tanks will be small volume -- 5 drum and 10 drum capacities. The question is based upon the 5 drum tank, which will be about 43 inches in diameter. A colleague has suggested that I locate the lower impeller 2 inches off the dish bottom. He suggests a radial impeller with a diameter of 4 - 6 inches. To avoid pump cavitation with the impeller in this location, he requested that I offset the mixer from the tank center by 1/2 blade diameter or 7 inches. In total he suggests three impellers. The two upper impellers would be axial type. For speed he suggests 60 RPM, 120 RPM and 240 RPM. The drivers behind these suggestions are several: 1) Produce volumes from 2 - 5 drums in the same tank. 2) Dissolve powders quickly when added early in the formula. Traditionally our tanks are designed using standard guidelines: Impeller 1/3 tank diameter spaced 1 impeller diameter off the bottom and 1 impeller diameter between each. Do his suggestions make sense? Our products are metal working fluids.
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