Dear Sirs,

For the design analysis of a high and slender vent gas scrubber, I've chosen the vortex shedding analysis to investigate a possible limited fatigue life. The vortex shedding analysis for tall vertical vessels is based on the non-dimensional Strouhal number, S = f.D/v [Bednar, p. 111].

Since I've run into design problems with this specific scrubber, I've performed the analysis in both English and metric units to check the consistency. More specific for this scrubber, the PVElite 4.2 output calculated a very limited service life, due to too high alternating stresses. I discovered that the calculation of the critical wind speed (based on the eigen frequencey f, weigthed diameter D, and Strouhal S =0.2) and the velocity pressure Qh remain both unchanged, i.e. both outcome are indentical (Vh = 117.85 m/s and 117.85 miles/hr, Qh = 41.88 N/m2 and 41.88 psf).

Therefore I've the following questions:
- What is the meaning of the constant 0.6818 in the PVElite output ?
- How is the weigthed diameter Davg of a vessel determined, used in the Strouhal equation ?
- If vortex shedding showed out to be critical for a vessel design (limited service life due to too high alternating stresses) what design variables do I have ?
- Do you know the excistence of a recommended engineering guideline for vortex breakers (spiralized rafter or beams on the OD of tall vertical columns and stacks ?

Kind regards,

Dear Sir,

The design of tall thin cylindrical stuctures can be quite tricky. It is empirically difficult to estimate whether or not a tall tower will resonate. This is of course due to the fact that typically there are platforms, ladders, piping and other equipment that may disrupt eddy currents that may force a damped oscillation of the structure. To my knowledge, there are no highly detailed design guidelines for the typical process tower vortex shedding calculations. In essence computer software must make the assumption that the shell is bare and smooth, much like a plain vent stack.

The analysis in PVElite is based on the National Building Code of Canada and papers written by Kanti Mahajan and Ed Zorilla. You might also check the design guideline STS-92 published by ASME. This is for stack design.

The 0.6818 is 3600/5280, the number of seconds in 1 hour divided by the number of feet in a mile. The result of this calculation is always meant to be in miles per hour. In metric if you just multiply the numbers the answer does not work out. The same is true for Qh. I will make this more clear in the near future, displaying answers in both English and Metric Units. Doing this however does not change the calculated loadings on the vessel (the final results).

The average diameter is obtained by multiplying the element diameter times their respective lengths and dividing the sum of the lengths over the upper 1/3 of the vessel.

If resonance does really prove to be a problem, it may be necessary to alter the stiffness of the tower which will change the fundamental frequency of vibration or adding strakes or similar devices to break up the wind flow around the column. Unfortunately, the latter has the unwanted effect of creating higher loads (shear and moment) to the entire tower, right down to the foundation.

I hope this helps,