Air Discharge Stack - Design 1

[DaveNZ]

Hi All Just interested to get some pointers from anyone who has designed an air discharge stack before. Say from a factory or industrial plant. In particular I am interested in stack height to material thickness proportions, helical spoilers, foundation constraints, intermediate support/ties, and anything else that may be of use. Am looking at a stack about 15 to 20m in height (thats 50 to 65 ft for US cousins out there). Also have seen some pictures of "bad" blower hook ups, but little to indicate what the best configuration is. Any help here? Stack is likely to be less than 1m (3'4&quot in diameter. Thanks to anyone who can help.

 

[meca]

DaveNZ,

I can't answer all of your questions, but I can offer some feedback on the structural design. In order to do some rough calculations, here are some assumptions I made:

Mat'l: A-36 Carbon Steel
Corrosion Allow: 1.5 mm
Height: 20 m
Dia: 1 m
Support: Fixed at base and free standing
Wind: ASCE 7-98, 100 mph, Exp. C, Cat. III

Based upon these assumptions, the stack can be 6 mm thick. Some local reinforcement may be required at the blower inlet. The stack would have a critical wind speed of 12.4 m/s, which could cause large deflections up to 0.7 m. To minimize these deflections you could use a vibration damper.

Regards,

Chris

http://www.mecaconsulting.com

PS I can email the analysis output to you if you email me direct at chris@mecaconsulting.com

 

[jheidt2543]

6 mm? Please take another look at this!

 

[meca]

1/4 inch wall or 6 mm is very common on small stacks such as this one. If you expect large corrosion, then perhaps a heavier wall is required.

Please elaborate on why you feel 6 mm is inadequate.

Regards,

Chris

 

[jheidt2543]

Chris,

I have to admit that I didn't run the calculations and may have stuck my slide rule in my mouth. However, it seemed to me that a 3' diameter, 1/4" thick steel "pipe" 65' long standing upright in the air and subject to 2'-4" (.7 m) deflection was iffy (.7/20 = .0035 which = 1/28 when I'm use to seeing structural deflections limited to 1/180, 1/240, 1/360). Sometimes intuition is not all that great, I'll run the numbers.

 

[meca]

I may not have been clear on my original post. The deflection of 0.7 m is due to wind induced vibration (vortex shedding), and is totally unacceptable. That is why I suggested that a vibration damper be used, which would bring the deflection down to an acceptable level.

For the along wind loads, the deflection that I calculated is only about 0.085 m (3.3&quot. This is assuming that my wind load assumptions are realistic for this location.

Thanks for helping me clarify this point.

Thanks,

Chris

http://www.mecaconsulting.com

 

[ricellis]

I am designing a similar chimney.
The 0.7m. due to vortex shedding occurs perpendicular to the wind direction, correct? Will vibration occur in the direction of the wind? I can't find anything in wind loading codes for this effect

Thanks,

Richard

 

[jhardy1]

ricellis,

Yes, the vibrations at the critical wind speed occur across the wind direction (cross-wind response), and can arise at a critical velocity a lot less than your maximum design velocity. (Critical velocity is a function primarily of the diameter of the stack and its natural frequency.)

Vibration in the along-wind direction will also occur, and is best quantified by conducting a dynamic wind response analysis (Gust Factor method). Most wind codes have some sort of dynamic model which should be used for the along-wind design of wind-sensitive structures, such as tall chimneys.

You can get some really good documents on chimney design from CICIND - the International Committee on Industrial Chimneys -

http://www.cicind.org

- however, you will need to pay to download most of their documents, such as their steel chimney design code and commentaries.

Hope this helps.