INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Reactor Agitator number of blades vs number of baffles

Reactor Agitator number of blades vs number of baffles

(OP)
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?

RE: Reactor Agitator number of blades vs number of baffles

Rubbish!

Good luck,
Latexman

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

RE: Reactor Agitator number of blades vs number of baffles

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

RE: Reactor Agitator number of blades vs number of baffles

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

RE: Reactor Agitator number of blades vs number of baffles

(OP)
Thanks for the opinions. Rubbish it is then!

RE: Reactor Agitator number of blades vs number of baffles

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.

RE: Reactor Agitator number of blades vs number of baffles

(OP)
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.

RE: Reactor Agitator number of blades vs number of baffles

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.

RE: Reactor Agitator number of blades vs number of baffles

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.

RE: Reactor Agitator number of blades vs number of baffles

(OP)
@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?!" :)

RE: Reactor Agitator number of blades vs number of baffles

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

RE: Reactor Agitator number of blades vs number of baffles

(OP)
Thanks Ed! I did not know that.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close