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What is the best indicator of performance and efficiency of a mixer?

Q: We will be constructing a plant that will require many agitators. We have bids out to suppliers and have received responses. In addition to the cost, what would be the best indicator of performance and efficiency that could be used given the normal feedback from vendors such as installed power, weight, gearboxes, shafts, shaft speed, impellers, and materials of construction? Is there a best factor or ratio of these parameters that one could use to decide upon the best vendor? Simple relationships would be preferred that do not need such things as mixing constants and fluid densities.

A:

The easy answer to the best mixer is the one that gives the most mixing, since too much mixing is better than too little mixing.  Without any other means of assessment, the best fluid mixing is normally associated with torque (power divided by speed.)  If you want a conversion factor to get to a real value, multiply 63025 times power (in horsepower) divided by rotational speed (in rpm) to get torque (in inch-lbs).

The problem with torque, power, or any other measure of performance is that the fluid only sees what is delivered by the impellers.  It would be easy to evaluate the mixers if you only had to divide the motor horsepower by the rotational speed of the mixer.  However, many impellers do not fully (as in 85% or 90%) load the motor.  Some impellers only load the motor to 50% or less, so the ideal situation would be to calculate the power required by the impeller and then divide by the rotational speed of the mixer.

However, the simple answer is to work with the motor power and divide by the rotational speed of the mixer shaft.  If the mixer is designed properly, the gear reducer should be able to handle the power and the torque.  The shaft should be able to handle the torque and bending loads.  The weight of the shaft and impellers should not cause a critical speed problem.  The impellers should be sized to properly load the motor.  If the impellers are not sized properly, increasing or decreasing impeller size should be the easiest correction to a mixer design.

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.

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