In recent years flow-induced vibration has joined heat transfer and pressure drop as primary concerns when designing shell-and-tube heat exchangers. Two types of flow-induced vibration occur-tube vibration and acoustic vibration. Tube vibration involves motion of the tubes and can lead to damaged tubes. Tubes vibrate at their natural frequencies as a result of the shell-side fluid flowing past them. When the flow velocity becomes sufficiently high, the tubes vibrate with enough amplitude to strike the baffles, one another, or the shell. The result is that the tubes wear so thin that they can leak or the joint fastening them to the tubesheet fails and the exchanger must be taken out of service for repair and modification. Acoustic vibration causes very loud noises, up to 150 db, but seldom any tube motion or tube damage.

Section 4.6 presents an overview of the problems, the methods for predicting their occurrence, comparisons of predictions with field experience, and some suggestions for preventing vibration problems.

There has been a trend toward larger heat exchangers with increased shell-side velocities to improve heat transfer and to reduce the possibilities of fouling. Scale-ups of exchanger designs have been made without consideration of the effects that geometry and flow conditions have in causing flow-induced vibration.

Although many heat exchangers have developed vibration problems, how to prevent them is generally understood. What is often missing is the ability to adequately model the actual geometry and flow conditions. Most vibration experiments have been conducted under controlled conditions using single tubes or ideal tube banks exposed to uniform cross flow or parallel flow. Few investigations have addressed the specific problems associated with industrial heat exchanger configurations. Application of results from ideal tests is often difficult because of differences in geometry, in the way the flow is coupled to the motion of the tubes, and in the nonuniformity of the velocities throughout the bundle. Consequently, the ability to accurately predict the intensity of flow-induced vibration or the probability of damage is less than certain. However, heat exchangers can be designed that will not develop vibration problems if construction and operation constraints are respected.

Fluid flowing across or parallel to tubes can provide the energy required to excite them into vibration. Tubes vibrate at discrete frequencies that depend primarily on their geometry, means of support, and material properties. The lowest frequency at which a tube vibrates is called its fundamental natural frequency. The intensity of vibration is evidenced by the amount of periodic movement of the tube, with the largest movement often at the midspan between adjacent supports. The extent of the peak-to-peak movement about the at-rest centerline is termed the amplitude of vibration. Energy must be fed continuously to the tubes to sustain vibration as internal and external damping dissipate energy. Prolonged tube vibration with large amplitudes leads to mechanical failure of the tubes, which then permits leakage of fluids between the tube and shell sides of the exchanger. Mechanical failure of tubes is usually the result of one of the following: