Agitator Torque Calculation

[Nickf_Zar]

TL;DR Summary

Calculating the torque required for an agitator/scraper.

Hi All,

I must calculate the torque and for an agitator motor. An anchor type agitator/scraper, removing crust off the edge of a tank.

My instinct is the max or startup torque will be = (Moment of Inertia) x (Angular Acceleration) + (Drag force of the submerged impeller)* Max radius + (Frictional forces on the edge of the tank)* Max Radius

Tmax = ( I * α ) + [ ( Cd * A * ρ * V² * ½ ) + (µ * F) ] * D/2

• The motor has a soft start but is operated at constant RPM.
• The idea is to get a worst-case scenario to design from.
• Please suggest if you think this problem can be solved with a better solution.

I have very little experience with this type of stuff and would greatly the feedback.

My Questions to the community

• Is this analysis correct?
• What would be an acceptable coefficient of drag to use for a plate with cut-outs in it without doing an experiment
• What would be an acceptable coefficient of friction to use, for the solidified sealing compound attached to the walls?

Thank you

 

[jrmichler]

I must calculate the torque and for an agitator motor. An anchor type agitator/scraper, removing crust off the edge of a tank.

If it's scraping crust, then it's a slow speed machine, and you can ignore the acceleration torque. Scraping crust is not a friction problem, it's a "cutting through the crust" problem. There are no published numbers for the force to do this, so you need to run some experiments. One possible experiment is to pull a hand scraper with a spring scale to directly measure the scraping force. Do your best to measure the worst case force, then add a substantial safety factor.

 

[Lnewqban]

How the material could solidify if the agitator is constantly working?
Or it is the case that the agitator is only activated after the solidification occurs?