Reactor Agitator number of blades vs number of baffles

[plantprowler]

I came across a new factoid today that I had never read of before, so thought I'd get opinions on here:

We have a reactor with a four bladed agitator and the vessel has 4 full height baffles (8% width). Someone mentioned that it is better to go with 3 baffles in this case i.e. not have the same number of baffles and agitator blades. He wasn't able to articulate his reasoning to my satisfaction but I've read of other situations (e.g. gears) where designers try to avoid teeth-ratios that are integral.

With 4 blades + 4 baffles it might be plausible that the blades pass all baffles at the same instant?

Just wondering if there is any opinions on this matter. For sure, I cannot remember reading in any authoritative reference that the number of blades and baffles cannot be the same.

Thoughts?

 

[Latexman]

Rubbish!

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[StoneCold]

I have to go with Latex man on this. If the material was extremely thick and the agitator to baffle clearance was small you might have a specialized case. But that would probably be a design mistake from the beginning and not a blade to baffle ratio issue.

Regards
StoneCold

 

[EdStainless]

I'll third the rubbish vote, it would only matter if the design was bad. If the fit was so tight (that a lot of force was generated at passing) and the mixer was not stiff enough, then it would matter, but it would be the least of your issues.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[plantprowler]

Thanks for the opinions. Rubbish it is then!

 

[geesamand]

4 bladed impellers are used with 4 baffle tanks all day, every day without hesitation and I recommend it when normal agitator sizing is present. But that doesn't make the notion rubbish, entirely.

When the agitator blades and baffles are separated by a reasonable distance, it's a non-issue. If the agitator blades get close, it can become a problem. Not everyone realizes that an overhung agitator design (no bearings / bushings in the tank) can deflect several inches in normal circumstances. So with normal hydraulic instability, the movement of the agitator shaft can produce proximity issues at the bottom impeller. Many agitator impeller designs use relatively compact hub ears and in the presence of even moderate unstable loads, will fatigue through the blade. With multi-impeller systems the agitator should have each impeller rotated a bit from the next. This will help mute any blade-pass response.

I suppose in the end I agree with the other 3 - this effect only seems to show when the overall agitator design is poor. But if it happens to be the first point of failure, it still matters.

 

[plantprowler]

Thanks @geesmand. Actually with the reactor in question the issue was solved by adding a bottom support bush. So you may be right about this.

But if it is a deflection / proximity to baffle issue then why is 4/4 worse than (say) 3/4.

 

[Compositepro]

It is probably most important to keep the forces on the agitator shaft balanced and symmetrical due to flexibility of the long shaft. This would call for a 4/4 arrangement and not 4/3.

 

[geesamand]

I'm glad you challenged that idea.

Offhand the only boogeyman I can come up with is interaction with a structural harmonic, which is more likely to excite when the 4 combined pulses at 4x shaft speed. This is more likely to excite the system than smaller pulses at 12x shaft speed. It doesn't happen often, but it can/does happen.

Something else may come to me on my commute or in the shower - I will drop in if something else comes up.

 

[plantprowler]

@geesmand

Thanks. Do post the results of your shower-commute meditations. I'm like one of those pesky kids that cannot stop asking "But why?!"

 

[EdStainless]

In high speed centrifugal pumps where vanes and blades are kept close you always use different counts (7 and 8 for example) to keep the blade pass harmonics low. A mixer shouldn't have such issues unless the shaft is too long/thin.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[plantprowler]

Thanks Ed! I did not know that.