Natural Frequency Design Calculations

[wchowe]

"I am a design engineer working on a multidisciplened project. I am trying to design a welded sheetmetal box that will contain a heat exchanger. The box is attached to a turbine that is producing a certain frequency. The turbine is used to create a gas flow through the heat exchanger. What formulas are there to predict the natural frequency of the sheetmetal box (if I coin stiffinening ribs or panels into the sheetmetal)? I can calculate the natural frequency of the flat panels but I have found no help in any further calculations or design guides.
Any help would be appreciated.
Thank you,
Bill";

 

[tomirvine]

First, you need to thoroughly characterize all of the forcing frequencies produced by the turbine and the fluid flow.

The frequencies at the steady-state turbine speed are usually of greatest interest, but the start-up and shutdown frequency ramps should also be considered.

The box natural frequencies should well separated from the forcing frequencies, in order to avoid resonance.

Hand calculations are useful as a first approximation.

You will probably need to make a finite element model of the box system, however, in order analyze the box natural frequencies and mode shapes more accurately.

A good, practical reference is:

V. Adams and A. Askenazi, Building Better Products with Finite Element Analysis, OnWord Press, Santa Fe, N.M., 1999.

Available from:

http://www.amazon.com/exec/obidos/ASIN/156690160X/vibrationdata-20/002-5686648-3842444

Sincerely, Tom Irvine
Email: tomirvine@aol.com