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6 Criteria to Select an Overhead Stirrer

Published in Dissolving, Emulsifying Homogenizing Mixing Suspending

An online search produces an overwhelming array of options.  Below are 6 key criteria to consider when seeking the best overhead stirrer that fits your needs.

  1. Stirring Speed

Given in revolutions per minute (rpm), this will vary by model.  Most laboratory stirrers provide a speed range, such as 12-1800 rpm or 40-6000 rpm, but there are some single speed options.  Check which speeds are needed for your process and purpose of stirring.  A digital display of speed is an asset if accuracy and repeatability of the speed are needed.

  1. Volume

The stirrer’s volume capacity should meet or exceed your maximum batch size.  Volumes can be 2 L, 25 L, 200 L and anywhere in between.  Maximum volumes typically assume water viscosity unless specified by the manufacturer.

  1. Torque

Torque is the rotational force used to turn the impeller, commonly reported in Newton centimetres (N∙cm) or inch ounces (in-oz).  The higher the torque the better the stirrer is able to sustain impeller rotation in higher viscosity mixtures and when using larger diameter impellers.  A small increase in an impeller diameter (and speed) will cause a great increase in the power required for mixing, as shown in a simplified version of the power equation:

Power = rpm3 x Diameter5

The extra power needed for any increase in diameter and speed will impact your torque needs.  Remember to take this into account.

  1. Viscosity

This refers to dynamic viscosity, which is the fluid’s “thickness” when stirring.  It is an important factor for stirrers mixing batches with viscosities greater than water, or viscosities that change during the mixing.  Higher viscosity mixing will require greater torque from the stirrer motor, and you want to ensure your stirrer can maintain its rpm under changing viscosity conditions.  Be sure the overhead stirrer is able to handle the maximum viscosity of your batches.  Viscosity is reported in centipoise (cP or cps) or millipascal seconds (mPa∙s).  If you do not know the exact viscosity of your products, online viscosity tables of fluids can help provide an estimate.

  1. Options

There are numerous features which may be important to your facility and application.  Stirrer chucks offer a range of diameters so ensure your impeller shafts will fit into the chuck.  Stirrers with through shafts make adjusting the impeller to an optimal height in the vessel easy, and even provide an easy way to move the impeller up out of the way for quick changes to mixing vessels.  Reversible models offer stirring in both clockwise (standard) and counter clockwise directions.  Other features to consider are digital torque and speed display, zero torque, timer, a light, programmable mixing including automatic shutdown, data logging, and external control by a computer.

  1. Operation and Safety

Stirrer controls should be clear and intuitive to operate, especially with multiple users.  Is the stirrer designed for continuous use?   Is the stirrer maintenance free, or does it require scheduling for service?  Some facilities require verification of the speed and torque readings by calibration.  Stirrers with overload or overheat protection automatically shut down to protect the electronics if mixing conditions are outside of operational range.  Is there guarding available to shield from spinning chuck and/or shaft?  Review the safety standards that have been certified for the stirrer, such as CSA, UL, CE, FCC.  Be sure to check what is required for stirring equipment in your facility.


Determining these key criteria will narrow your options to a manageable number, to help select the overhead stirrer that is best suited to your needs.  For specific details you can further inquire with your distributor or the stirrer manufacturer.  Caframo’s team has thorough product knowledge and comprehensive experience guiding product selection for a wide range of stirring applications.  Contact us to make a selection with confidence.

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Raymond J and Raymond A.  (2015)  Propeller Turbine Mixer Design Calculator.  Retrieved April 2017, from http://www.ajdesigner.com/phpmixing/reynolds_number_mixing.php.