The relationship between the stirring current and viscosity ?

[silebi66]

Dear Masters :

Could you please kindly help me any hint how to get the relationship between the stirring current and viscosity, by experiment or simulation ?

For example, in the PET production process, the stirring speed, temperature, liquid level, viscosity, flow resistance, heat transfer coefficient, vacuum will
make the electrical current change.

As the stirring speed increase, so the electrical current increase,too . However, the temperature in the vessel increase, the viscosity decrease, and the agitation power decrease, and the electrical current decrease also.

I wonder is any general equation found in the paper about the viscosity and electrical current for each impeller type so that we can make a simple calculation
current vs viscosity in the spreadsheet.

Please give me any advice.

Thank you so much.

 

[Compositepro]

Electric motors have something called power factor, which changes with load so current is not a very good indicator of load. This company makes power meters that are much more accurate for measuring motor load.

http://loadcontrols.com/wp-content/uploads/2015/02/Sensing-Viscosity.pdf

 

[LittleInch]

As you say there are many things which can change so no general equation is there.

I would recommend some testing and measurement of all the items to generate your graphs

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[silebi66]

Dear Masters :

If I know the AC motor's operating current, say 13A, can I say the motor power is 13A/1.3 = 10Hp ?

Refer from the graph attached, AC drive has the lowest power capability at low speed and the highest minimum speed, the DC deliverers the constant torque ?
Please give me some advice.
Thank you

 

[moltenmetal]

If the motor is on a VFD, power measurement is often already available via the VFD. If the application is fixed speed, motor power measurement will give you better information than motor current alone.

You can only correlate motor power with viscosity IF all the other parameters (speed, level, gassing, temperature etc.) are constant or separately correlated so you can remove their effects from the measurement. And you must correlate the motor power and observed viscosity by experimental measurement- there are too many variables for the predictive correlations to give you sufficient accuracy without confirmatory measurements. Sure, the people who sell agitators can get you close enough based on your service conditions and geomtetry to size a motor and a gearbox, but that's not quite the same thing as being able to infer viscosity from motor power. Remember that the motor is attached to a gearbox which will generate an amount of heat from friction which will vary with applied torque and speed, and the motor has an efficiency which varies with power and speed too.

If you want a good approximation of the sizing equations for agitators, Chemineer did a good set of articles in Chemical Engineering Magazine in the late 1980s as well as having a couple really good publications on the subject. There's also plenty of information available in the Handbook of Industrial Mixing

I can tell you from experience that if your system is too small, the frictional component of the drive system will be so large that it will make the relationship between motor power and viscosity very weak indeed- so weak that you may not even be able to infer how many agitator impellers are immersed (i.e. the very coarse level in the vessel) by watching the motor power.