Industrial Mixing Basics: Mixing Impeller Power

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Industrial Mixer Basics Blog Feature Image - Mixing Impeller Power

The third blog in our series on mixing impellers focuses on mixing impeller power.  As discussed in part one of this blog series, characteristics common to all mixing impellers include:

  • Flow Pattern (part two in series)
  • Impeller Power
  • Fluid Pumping Rate
  • Fluid Velocity
  • Fluid Shear

This blog discusses the mechanical power associated with a mixing impeller in a stirred tank.

The power associated with an impeller is related to four primary factors:

  1. Impeller geometry
  2. Impeller diameter
  3. Impeller rotational speed
  4. Fluid properties of the material being mixed

The relationship of these components can be expressed in the following equation:

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Power = Np * n3 * d5 * sg * K

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Np: impeller power number (geometry factor)

n: impeller rotational speed

d: impeller diameter

sg: specific gravity of fluid

K: Reynolds Number (viscosity) correction factor

 

The value of the dimensionless power number (Np) depends on several factors including impeller design, number of impellers and location within a tank, tank baffling and fluid viscosity.  Typically the power number is shown for mixing in fully turbulent flow.  A correction factor can then be applied to take all of these factors into account as illustrated above.

bar graph showing Typical power number values for common mixing impellers

The value of the dimensionless power number (Np) depends on the impeller design as well as several other factors including the number of impellers, their location within a tank, tank baffling and fluid viscosity.

 

Changes in both rotational speed and impeller diameter have an exponential effect on impeller power.  Accordingly, small changes in impeller speed and diameter can have a significant effect on mechanical power as illustrated below.   This graph plots mechanical power (as an index) verses rotational speed.

 

Graph of Mixing Impeller Rotational Speed vs Impeller Power

Small changes in impeller speed and diameter can have a significant effect on mechanical power.

 

Although understanding the factors that determine impeller mechanical power is certainly important, this is one of many factors consider in agitator design.

It is also worth noting that agitator HP by itself is not always a good measure for comparing industrial mixers.  The overall design is more dependent on torque.  The following example compares two mixers that both use a 5HP motor.

 

Agitator                 HP                      Operating Speed             Agitator Shaft Torque

Portable                   5                              1750 RPM                                180 in-lbs

Fixed Mount            5                              350 RPM                                  900 in-lbs

 

The Fixed Mount agitator above produces 5 times the torque of the Portable.  Accordingly, this agitator will have much larger components such as gearbox and shafting, and will provide a greater degree of agitation.

For More Information on Mixing Impeller Power

For more information about mixing impeller power, email applications@proquipinc.com or call us at 330-468-1850.