Tube Vibration

This section discusses tube vibration mechanisms, tube failure locations, and design criteria to prevent failures. Damaging heat exchanger tube vibration mechanisms are vortex shedding and fluid elastic whirling. Acoustic resonance can occur in exchangers but does not damage tubes and is not covered here.

Vortex shedding downstream of a single tube in cross flow is illustrated in Figure 200-7, and is a “snap shot” of a vortex forming near the tube and three other vortices that were shed earlier in sequence.

fig 1 5 - Tube Vibration

Vortices are alternately shed on each side of the tube and exert an alternating pressure (force) in the direction perpendicular to the crossflow direction. This alternating pressure makes the upstream flow first favor one side of the tube and then the other.

The vortex shedding phenomenon for a single tube occurs in heat exchanger bundles at peripheral tubes and along unblocked pass partition lanes in the vicinity of inlet and outlet nozzles, but does not penetrate far into the bundle.

The location of the next row of tubes affects the frequency of formation of vortices on the first row. Interaction between vortices from adjacent tubes usually degenerates the vortices to harmless random noise after about the third row into the bundle. That is why only peripheral tubes are of concern.

Vortex shedding frequency depends on tube layout angle. The distinction between 30 degrees and 60 degrees, and between 45 degrees and 90 degrees, however, is academic because of the extreme divergence or convergence of nozzle flows. The 60-degree layout angle governs for all triangular layouts; the 45-degree layout angle governs for all square layouts.

Damaging tube vibration occurs when the vortex shedding frequency matches one of the tube natural frequencies. The maximum allowable unsupported tube span is set so that the highest anticipated crossflow velocity will not excite the first mode natural frequency of the tube. Higher mode resonant vibration occurs at higher velocities.

Figure 1 in Standard Drawing GC-E1048 defines maximum unsupported tube spans for inlet and outlet regions of shell and tube exchangers. Vibration control involves adding partial support near nozzles as needed. These support plates do not affect thermal or hydraulic performance.

Fluid elastic whirling may occur in the first two rows beyond the baffle cut in the interior of the bundle only. These are the spans labeled “L4″ in Figure 3 in Standard Drawing GC-E1048. Clusters of at least three tubes in at least two different rows vibrate in harmony, as illustrated in Figure 200-8.

fig 1 6 - Tube Vibration

Figure 2 in Standard Drawing GC-E1048 defines maximum unsupported tube spans for interior tubes. The velocities that initiate fluid elastic whirling are greater than economic velocities and therefore rarely affect exchanger design. Problems occur when abnormally high velocities and/or abnormally long unsupported spans are used.

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