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Multiple impellers in one shaft mixing design question

Multiple impellers in one shaft mixing design question

(OP)
Dear Professional members:

I have to bother yours a silly agitation question for me to survive in the career.

My boss asked me to design a modified pitched-blade type impeller (design for 60~90 rpm high speed

revolution speed? and created mostly in axial flow) on the center sweep shaft, and anchor impeller on

the bottom (30~50 rpm low speed revolution speed ?).

Since this reactor is operated in the laminar flow ( the process is 2nd stage of the PET process),

and the viscosity range from 1000cp to 60,000 cp). And for this wide range of viscosity process, I

wonder the impeller power and pumping in laminar flow are proportional to the cube of impeller diameter

or 5th power of impeller diameter?

And, is this multiple impeller in single shaft design work or mutli-shaft impeller to be work better

(ex. The figure in attached post)?

I appreciated if you could give me any instruction or some hint.

Thank you very much.

RE: Multiple impellers in one shaft mixing design question

silebi66
I would look at www.mixers.com Ross does a lot of multi shaft mixer designs. I am not sure that they are the correct person for your industry but you might be able to learn some things from browsing their site.

If you are going to run multiple shaft RPM's you either need multiple shafts or a shaft in a shaft which is pretty complicated to seal.
I would assume it is going to look a lot like the picture you attached. If you are really designing it from scratch then you are going to need some design equations.


Regards
StoneCold

RE: Multiple impellers in one shaft mixing design question

Ross and Myers are two companies that make that style mixer.

Here are a few comments on your attached drawing.
Putting paddles on the anchor blade (named so because it is shaped like a boat anchor) is pointless. The anchor blade turns the mass in the mixer at the same RPM so there is no shear at the paddles and all they do is add extra surface area to clean.
A bottom bearing is a maintenance headache. An anchor blade is self centering.
I've never seen any advantage to three shafts. Two will be enough for almost anything. The anchor blade takes care of wall wiping for good heat transfer and distributive mixing. The high shear blade takes care of most of the dispersive mixing. Propeller blades on the second shaft can improve top to bottom flow but are usually not needed.
The speed ratio between the two shafts is usually 5 to 20, not 2.

Double planetary mixers good option for high viscosity mixing.

RE: Multiple impellers in one shaft mixing design question

I realize this is an old thread, but maybe this can still help.

For high viscosity mixes, you do need an agitator design that engages a large portion of the tank volume and leave no dead zones. This means multiple impellers and large D/T. In the attached illustration the impellers appear to fill the volume, but rotate that section 90° and there are large dead zones between the anchor and center impellers. The contribution to mixing coming from the center impellers will be extremely limited because they are turning at the same speed as the anchor. Assuming power draw follows diameter^5, it's clear that the two central impellers will do very little. To make the central impellers significant, they'd need to rotate at a higher speed than the anchor which adds huge mechanical complexity and cost. So the dead zone now increases to the entire center of the vessel.

I don't believe your mixing problem requires multiple agitators in the same reactor. In most reactors, one well-designed agitator can mix the most challenging mixing condition and sufficiently overkill for the others. In some cases you might need to have a two-stage reactor so that high-shear mixing and high-viscosity blending can be managed in the best possible way.

I can't help but mention that with high-viscosity and non-Newtonian products, pitched-blade turbines and scraper impellers are far from efficient and can produce poor quality product. These are among the most challenging mixing applications and the list of agitator suppliers who can speak about this with true R&D-based knowledge is extremely short. I have seen order-of-magnitude improvements in this kind of reactor based on a change in agitator design. A brick-and-mortar mixing laboratory with test vessels of varying scale and measuring equipment is mandatory to unlocking the potential solutions. So if performance of this reactor is of high impact to the overall plant performance, go find that mixing expert and consult with them.

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