Agitators - Constant or Variable Torque

[7carisfast]

I was always under the impression that agitators were to be categorized as constant torque applications. However, per this website, agitators tend to have this speed-torque curve which indicates that they are variable torque. Can anyone elaborate on this?

http://cipco.apogee.net/mnd/meavovr.asp

 

[jraef]

Not all agitators are the same. It depends on the fluid viscosity, paddle design, angle of operation etc. etc.

This is a good paper describing agitator applications. Although it doesn't specifically address your question, you can get from it the idea that there are a lot of variables involved. Bottom line, that article you referenced said "many" agitators and mixers, but by all means not all of them.

http://www.reliance.com/pdf/drives/whitepapers/D7747.pdf

 

[ozmosis]

A lot of agitator applications require high starting torques if a VFD is fitted. This is to overcome the changing viscosity of what they are agitating as often the mix will change as time and agitation goes on. All is fine until the agitator stops and is required to start again as suddenly you find the 'flour and water' has turned to dough.
If you go for the classic "variable torque" designed VFD, you will often find the ability to start a high load has been sacrificed for overall higher power capability but lower overload.

That's a great document by Reliance btw.

 

[DickDV]

An agitator, as it is built, is essentially a variable torque machine. However, because of the way it can be used in a batch process, it can better be sized as a constant torque application.

That is simply due to the ability of the operator or process setup people to load to the maximum over a broad spectrum of process materials usually---thus, the agitator assembly will see full capacity regardless of the speed it is to be operated at.

That pretty-well defines a constant torque application when I am asked to specify equipment.

Just an aside on this subject--- batch process agitators make excellent "trick" questions on drive application examinations just for the above reasons!

 

[aolalde]

If the viscosity of the material is kept constant, agitators and mixers are "Variable Torque".

If the viscosity changes a specific analysis is required to define the motor torque requirements.