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.

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CALCULATIONS:

Impeller Power Calculation

Impeller Power Calculation_SI

Reynolds Number Evaluation with Definition

Reynolds Number Evaluation with Metric Conversion

Torque Evaluation SI with Definition

Torque Evaluation with Definition