Induced vibration of any system involves the coupling of some exciting forces with an elastic structure. In the case of flow-induced vibration, the exciting forces result from the flow of the shellside fluid and the elastic structure in the bundle of tubes. The exciting forces fluctuate at characteristic frequencies which increase continuously with increasing flow rate. The tubes vibrate only at unique responding frequencies called their natural frequencies. Coupling occurs when the exciting frequencies match the responding frequencies and tube vibration results.
The natural frequency of tubes depends primarily on their geometry and material of construction. The intensity of vibration is evidenced by the amount of periodic movement; the extent of this peak-to-peak movement about the at-rest centerline is termed the amplitude of vibration. Energy must be available to excite the tubes into vibration. The energy of vibration is dissipated by internal and external damping. The exciting force could be the result of:
- fluid dynamic mechanisms as a result of flow parallel to or across the tubes
- pulsations of a compressor or pump
- mechanical vibrations transmitted through a structure.
Unless amplified by resonant phenomena, the flow forces normally enountered in equipment are not sufficient to cause damage. Resonance, which can increase the tube deflection by orders of magnitude, occurs when the frequency of a cyclic exciting force coincides with the natural frequency of the tube
In order to predict the occurrence of flow-induced vibration, the phenomena that produces the exciting forces and the dynamic response by the tubes must be understood. The determination of tube natural frequencies is relatively straight-forward. However, determination of the exciting forces created by the shellside fluid flow is extremely more difficult. The shellside flow in a heat exchanger follows a complex flow path. It is subjected to changes of direction, acceleration, and deceleration. At times, the flow is either perpendicular to the tubes (crossflow), axially along the tubes (parallel flow), or at any angle in between. Flow phenomena in crossflow include vortex shedding, turbulent buffeting, and fluid-elastic whirling, The flow phenomena found in parallel flow includesaxial-flow eddy formation.
