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Industrial Stacks

Dynamic behavior of stacks

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Dynamic behavior of chimney

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Large slender structures such as stacks are protruding above all structures and therefor effected by the wind.

The wind in relation to the slenderness and shape of the structure can lead to deflections of the chimney top in crosswind direction.

For slender structures such as chimney’s subjected to wind loading there are 3 main actions to consider:

  • 1. Gust winds, they displace the chimney in the same direction as the wind load.
  • 2. Vortex shedding, this phenomenon occurs when the natural frequency of the chimney corresponds with the vortices shed from opposite sides of the structure resulting in cross gas flow oscillations.
  • 3. Ring oscillations (ovalling) are a pulse oval oscillation of the cylindrical shell of the chimney.
Vortex shedding oscillations give rise to a fluctuating load perpendicular to the wind direction.
 
Large and lots of crosswind movement leads to maximum fatique stress and the start of cracks in the material is arisen.
 
Everybody knows the example of a paperclip which is bended a few times until it cracks, this is fatigue. Even the strongest material can be broken by fatigue.

Vortex shedding calculation should be applied if the slenderness ratio of the structure is greater than 30, which is the case for almost all large chimneys. The slenderness ratio is the height of the chimney divided by the diameter of the chimney. ( for e.g. 80m / 2,5 m = 32)

The wind speed at which vortex shedding can occur is called the critical wind speed, often is this at low wind speed for e.g. 5 m/s so the chimney is often dragged into vortex shedding.

Vortex shedding can be prevented or reduced with 2 complete different types of solutions, each with their own pro’s and contra’s.
 

 

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