Smoke stack design 2

[PoorRod]

Just thinking about a subject that I've been wondering about for some time. At times when I'm driving past a refinery or a heating plant, I've noticed that some of the smoke stacks have a helical fluting or strakes along the exterior of the stack, usually near the top. Does anyone know the purpose of this? I have read a paper that suggests that this is for modifying the airflow around the stack to control the dynamic forces of the wind, but that doesn't explain why one stack has the helical strakes and one right next to it may not, as shown in the following photo:

http://www.ucar.edu/communications/quarterly/summer98/smoke.stack.gif

I was thinking that there might be a thermodynamic reason for this.

It's nothing that I'm working on, but it is something that I think about and say "hmm, I should know that..."

Thanks,
Poor Rod

 

[saxon]

PoorRod, It may be a case of different process streams, different manufacturers- different styles of stack design. From the looks of the one, it was built in sections of thin wall, hence the flange rings for connecting and the subsequent stiffener bands or cable to strengthen the tin walled pipe.

saxon

 

[taylorg]

PoorRod,
These strips can be used to give the exhausting medium extra bouyancy or lift by channeling the wind (picture what happens when the passing wind is directed up these strips!). Hot gases generally rise without any problem, but other types may tend to linger around the stack, or worse, may drop to ground level and deposit any nasties they contain.
Hence some have them, some don't! You'll see them mainly in built up areas and on stacks that aren't very high.

Cheers
GT

 

[GeorgeFletcher]

The photograph shows that the chimney with the strakes is free standing whilst the others are guyed.
The strakes are fitted to shed turbulence caused by other obstructions but they are only required on free standing stacks.
The jury is out on how effective they are.

 

[dooron]

I have designed both types of chimneys. As you can understand, having strakes is more expensive, then not having them, so you can start design without strakes. (Usually top 1/3 of chimney). You know the wind velocity, you know the chimney wall thickness, and you know the operating temperatures, etc. You work out the chimneys natural frequency. Struhl No. Von Karmen eddy shedding, etc.(All in the Codes) If the chimney is likey to start vibrating like mad with the wind, you have a number of options, increase the wall thickness, add guy wires or ADD STRAKES., ofcourse adding strakes increases air resistance which means thicker wall thickness (sometimes).-Thats why strakes are used.

 

[209larry]

PoorRod,

It looks like the first chimney is seamless pipe while the other one is a spiral weld.

209larry

 

[PoorRod]

I want to thank everyone for their answers.

That photo was just one that I found on the internet. The chimneys that originally sparked my interest are along a route I bike ride along every weekend. If I remember to take a digital camera one day, I'll try to take a photo and post it though now I understand the purpose of the strakes.

Rod

 

[mickeymouse]

Vibration dampening. Refer to API 560 (fired heaters for general refinery service) section 9.5 on design of self supporting stacks. Simply put, first and second critical wind velocities.

 

[rocco]

Maybe I can explain a little:
* The stack installation is devided by 4:
1. Self supported
2. Guy wire
3. Derrick structure
4. Combination guy wire and derrick structure.
Usually the self supported stack (with variety in diameter) install with wind braker, but not with another.

 

[Davidorias]

One interesting problem with strakes, in outside insulated stacks, is inside corrosion exactly in the weld. The reason is that the temperature falls until condensating temperature of the gas, because the strake acts as a refrigeration fin. Sulfuric acid (from sulfur of smoke)then acts in these points, generating corrosion.
You can also consult API 573 (Inspection of fired boilers and heaters).

 

[25362]

In addition to what was was said, in general, wind-caused vibration frequencies are inverse to the diameter of the stack, the frequencies for "ovalling" or "breathing" by wind are inverse to the diameter squared.

Experts in stack construction recommend that self-supporting steel stacks be made less sensitive to wind-induced vibration, by reducing the diameter towards the top in at least two steps. In this way the stack becomes more elastic and the excitations have different frequencies.