Agitator / Mixer Sizing [Top Entry] 2

[takiyasamsama]

Hi,

I need to know regarding sizing of mixers, specifically top entry:

1. Generally which party gives the mixer sizing? Is it vendor/supplier or engineers?
2. What are the parameters required for sizing? (e.g. viscosity, mixing action, etc.)
3. If engineers where to produce sizing, what formula or reference can be referred to (valid ref.)?

I'm a static guy and I would like to know or generally understand the concept of mixer sizing and selection of mixer. Generally when it comes to sizing, I will pass to our rotating guy and what normally they would to is rely on vendor sizing based on any relevant process parameters. Besides relying on vendor/supplier alone is there any method for us to determine the sizing of mixer?

Thank you

 

[gr2vessels]

The mixer selection is almost entirely a vendor design, supply and process guarantee. They are engineers, specialised in their field.
Usually, the calculations are related to their own proprietary equipment, development and studies. The manufacturers are locked in fierce competition and their offer is usually backed with significant liability warranties. Accordingly, your engineering book calculations might not be representative and your results might be miles out of reality.
Normally, you ask several reputed mixer manufacturers to bid for the job and the winner will provide some calculations. Unfortunately, the calculations are proprietary to the manufacturer and they will not disclose their secrets to anyone. They will provide the relevant data for you to design the vessel, foundation, piping, etc. They can also provide a complete skidded product with connection flanges and terminal JB's to your specification.
Hope this helps.
Cheers,
gr2vessels

 

[takiyasamsama]

Thanks for your response gr2vessels.

The reason for asking is that currently we have to design a vessel with a mixer which the data is for us to come out with. As for now we are suggesting the vessel to be around one-quarter of a meter with 6 inches of vessel connection. I'm not sure how things are going to turn out. In fact our process team cannot produce approximation regarding the process condition.

anyway thank you

 

[geesamand]

I think gr2vessels has the situation about right. Engineering companies sometimes have specifications and agitator designs but typically they are happy to re-use the same specs year upon year, decade upon decade. So they tend to not be even slightly optimal.

There are some published books that can give guidance but they are not the most current sources. Of them, the Handbook of Industrial Mixing is best of the bunch.

Designing your specifications to a process guarantee is very sensible, but only if you have good knowledge of your fluid rheology and can accurately monitor your inputs and outputs. Lacking those elements, a performance guarantee won't hold up. If you can provide a sample, a good agitator manufacturer can do the rheological analysis. A good agitator designer will ask a lot of question - probably enough to drive you a little nuts.

The agitator manufacturers carry the strongest and most current data. Some agitator manufacturers are really engaged in R&D, others are not so much. Take great care with designs that rely strictly on CFD - without lab work to keep the CFD on track with reality - too many agitation variables are not predictable within CFD codes. You might have a gander at recent work published in the North American Mixing Forum, World Mixing Forum, or AIChE to see which manufacturers are progressing the state of the art. In this environment it's not surprising that the design methods are protected by proprietary measures - however with mutual secrecy and NDE agreements you're not going to fly blindly.

It's true that for some mixing problems, variances in calculations and agitator designs may yield performance varying by an order of magnitude or more. So if mixing performance really does drive your profit, go find the industry leader in your estimation and work directly with them.

 

[geesamand]

Another thing to consider is loads applied to the vessel. The simplest top entry agitator design involves no bearings in the seal area or inside the tank. This can put significant bending stresses in the mixer shaft and that gets applied to the vessel nozzle. Not only must stress be acceptable, but rigidity of the neck to prevent harmful vibration. It's vitally important to take the nozzle loads from the agitator design and confirm the vessel design against it.

 

[MJCronin]

As always geesamand provides solid advice.....

The stiffness of the structure supporting the mixer is very important. At one time Chemmineer provided guidelines for the top head stiffness and recommended thickness based on the estimated mixer hp and diameter.

The nozzle that supports the mixer is typically reinforced with a generous repad and multiple gussets running between the nozzle flange and repad.

http://www.eng-tips.com/viewthread.cfm?qid=278248

Good basic discussion here also

http://www.chemicalprocessing.com/articles/2006/059/

MJCronin
Sr. Process Engineer

 

[geesamand]

The stiffness of the structure supporting the mixer is very important. At one time Chemmineer provided guidelines for the top head stiffness and recommended thickness based on the estimated mixer hp and diameter.

Which is valid for only the very basic mixing application. Popular variations in the mixer construction and configuration can vastly increase these nominal loads.

Not sure if it's valid for them to proscribe vessel parameters with any validity - but its their legal problem, not mine.