Power consumption agitator with four impellers 2

[hhhansen]

Dear All
My wife is studying Chemical engineering and has been assigned an exercise covering agitators with multiple impellers.
She has calculated the power consumption for one impeller and now wants to know what would be the consumption for four impellers all vertical connected to a single shaft and running same speed. The viscosity of liquid in the batch is
500 mPa s.
I would be tempting just multiplying the single impeller power by four - but is this the right way to solve this ?.

 

[Karlando]

Hello!
I don't think that it goes just multiplying. When there is level enough in the tank that first impellers move the slurry, second and third impellers don't increase the power consumption a lot. I have at the moment 47 m3 slurry in the tank and there are two impellers in touch with impellers. Viscosity is about 50 mPa s, and the agitator takes 7,3 A. In the other tank is 27 m3 slurry and the level is little bit more than no. 1 impellers height (or level). Tank, agitator and impellers are similar and power consumption is about 7,2 A. We have three impellers at the both tanks and when they are full, the current is 7,3-7,5 A.
Our viscosity is so low comparing yours, so I think that the higher visco affects quite a lot to power consumption.
What kind of calculation your wife uses? I have tried to find a program from net, but no succes so far

 

[Latexman]

There are some correlations that show how some geometric factors (the distance off the bottom, impeller spacing, degree of baffling, and liquid coverage) affect power draw. See Oldshue's Fluid Mixing Technology for use of the proximity factor. It's in chanpter 2 in the edition I have.

However in practice on small, off-the-shelf agitators and small to medium custom designs, I've seen these factors ignored for simplicity. The finite number of standard motor sizes probably makes doing this correction a mute point anyway. On large expensive units, these factors are usually considered.

If this is an undergraduate course, multipling the power draw of one impeller by four to get the power draw of a four impeller system is probably okay, but if it's a graduate level course it may not be okay.



Good luck,
Latexman

 

[Latexman]

Now that I'm home and refreshing my recollection, Oldshue states that each identical multiple impeller typically pulls 75-95% of the power draw of an identical single impeller.

Good luck,
Latexman