Chimney Design

[neilmark]

Currently involved with circular steel chimney design and possibly require to design an oval shape chimney. Does anybody have any useful information regarding how or if vortex shedding effects these shapes?

Using CICIND we design circular chimney shells for axial and bending stress. We are thinking for an oval chimney say 25m high we will need to calculate section properties such as a buckling moment capacity etc...

Has anybody had experience with Oval Chimneys??

Regards

 

[SrVaro]

In the past I have used the Structural Engineering Handbook (by Gaylord, Gaylord, and Stallmeyer) Chapter 32 for the design of stacks (chimneys). Within it there are several checks required including ovaling.

 

[neilmark]

Hi, thanks for the response and book reference, but I think you may have misread the question.

We have been asked to look at designing an oval shaped chimney (not check a circular chimney for ovaling).

Regards

 

[Chris1968]

I've been designing steel chimneys for a number of years now and have always done my best to guide the client away from eliptical chimneys due to a lack of information on the subject, it's not always possible though. I have not designed an eliptical stack to CICIND as yet only to the now withdrawn BS4076.
The ratio of length to width of the cross section will in part guide you to appropriate force coeficients along with the Reynolds Number maybe 1.2 or higher dependent on the ratio and substantially less at 90 degrees but is not likely to be the case of prime consideration.

(Introduction To Fluid Mechanics CH9 - Drag & Lift...CD = 0.2 to 1.2 but read further)
(Withdrawn CP3 CH5 PT2 1972 - Table 14... Cf (same as CD)=0.2 to 1.7)

Depending on where you are geographically I'd be checking through the Eurocodes or ASME standards for a more upto date value.
Preliminary steps for me anyway would be to work out my minimum windshield dimensions for the height,
calculate section properties in X & Y,
consider what an equivalent circular section would be to these properties in each direction,
run them with CD=1.7 for the broad face equivalent and using the load from the actual broad face and 0.2 to 0.7 the other,
consider the output (take special notice of the critical wind speed for oscilation if you can get this above 1.25*Vcr,n in both directions things are looking promising but require further verification,
carry to more detailed assesment now you have something looking promising and model for FEA with the apertures/reinforcing note any change in frequency and it's effect on the critical wind speeds for each direction and some CFD.
Validating the stresses to a given code is always the stumbling block if you can get the critical wind speed high enough though you are only woried about along wind loads,the eliptial chimneys I've done in the past have always been over designed for this reason.
If your FEA package handles fatigue you may want to run this also.
I think you really need to try and stick with all Eurocode if you can detect the relevant information or all ASME, mixing and matching is not good.
If not the above with a good measure of common sense, I wish I could be more helpful and best of luck with the project.

Chris

 

[Chris1968]

Should have stated more clearly above that the prelim runs are for assesing the plate stresses and you need to be sure the mass/m is the same as the elipse.....this is a rough way of obtaining a sensible start size only.