Thread Content
Vibration of heat exchangers: As the production scale increases, the size of the heat exchangers, the flow velocity of the fluids, and the span between supports all increase, sometimes exceeding allowable limits. This reduces the stiffness of the tube bundle and increases the likelihood of vibration occurring. Vibration can cause pipes to leak, wear out, become fatigued, or break, and it may also generate harsh noises; this not only reduces the lifespan of the equipment but also harms people’s health. Once a vibration-related accident occurs, it often takes a considerable amount of time to analyze and repair it. Due to the complex factors that affect vibration, the magnitude of damping is difficult to estimate accurately, and the rate of tube wear and damage is hard to determine; moreover, these phenomena cannot be described using simple mathematical formulas. As a result, it can be said that the theoretical calculation methods available to date cannot be used in practical engineering applications to analyze vibrations accurately. In the existing specifications for heat exchangers, there are also no clear provisions regarding vibration analysis methods and anti-vibration design criteria. However, practice has shown that by utilizing existing research findings to conduct necessary estimates and analyses of vibrations during the design phase, and by taking certain vibration prevention measures, most destructive vibrations can be avoided. Causes of fluid-induced vibration: The tube bundle in a heat exchanger is an elastic material; it is disturbed by the flowing fluid, causing it to deviate from its equilibrium position and resulting in vibrations in the tubes. Such vibrations are known as flow-induced vibrations. In fact, every heat exchanger experiences some degree of vibration during operation, and the source of this vibration may be caused by fluid flow on the shell side or the tube side ； Vibrations caused by fluctuations or pulsations in fluid velocity ； Mechanical vibration transmitted through pipes or supports, and so on. Sometimes there may be multiple vibration sources, one or several of which could be the main cause of the vibrations. Some vibration sources are relatively easy to predict, whereas vibrations induced by fluids are more difficult to forecast. Some experiments and operational experience indicate that the vibration of heat exchangers is mainly caused by the flow of the fluid on the shell side, while the vibration induced by the flow of the fluid on the tube side can often be ignored. Under normal circumstances, in the shell-side fluid, the amplitude of vibrations induced by longitudinal flows that move parallel to the tube axis is small, and the probability of structural damage caused by such vibrations is also much lower compared to those resulting from transverse flows. Therefore, there is greater concern about the vibration problems caused by lateral flow. The three distinct causes of flow-induced vibration that are currently recognized are: vortex shedding, turbulent flutter, and fluid-elastic rotation (also known as fluid-elastic instability). (1) Vortex shedding: When fluid flows transversely past a single cylinder, at high Reynolds numbers, the Karman vortices formed in the wake behind the cylinder cause two sets of vortices moving in opposite directions to shed periodically, resulting in a certain shedding frequency. When the fluid flows transversely through the tube bundle, Karman vortices are also generated behind the bundle. For tube bundles with a small spacing, this phenomenon occurs only in the first few rows around the periphery of the bundle, whereas in tube bundles with a large spacing, it can occur throughout the entire bundle. When vortices detach, the fluid exerts an alternating positive and negative force on the circular tube; the frequency of this force is the same as that of vortex detachment, causing the tube to vibrate perpendicular to the flow direction at the vortex detachment frequency or a frequency close to it. When the vibration frequency of the circular tube is a multiple or divisor of the vortex shedding frequency, the vortices shed uniformly at the same frequency across the entire length of the cylinder (circular tube) at the same time; the shedding frequency and the vibration frequency are in sync, and this is what is known as resonance. Vortex shedding itself can also generate a certain sound. This is because, under certain conditions, it induces a certain order of standing wave between the two walls of the gas chamber, which is perpendicular to both the tube and the flow direction, as shown in the figure below. This standing wave reflects back and forth between the walls surrounding the tube bundle, without transmitting energy outward; meanwhile, vortex shedding continuously supplies energy. When the frequency of the standing wave coincides with the frequency of vortex shedding, it induces intense acoustic standing wave vibrations in the gas chamber – gas vibration – resulting in significant noise. (2) Fluid-elastic rotation: When gas flows transversely through the tube bundle, the hydrodynamic forces resulting from the asymmetry of the fluid can cause one of the tubes in the bundle to shift instantaneously from its original position. This leads to fluctuations in the flow field, disrupting the equilibrium of the adjacent tubes and causing them to shift as well and enter a state of vibration. If there is not enough damping to dissipate its energy, the amplitude will continue to increase until the tubes collide with each other and get damaged; such vibrations are known as fluid-elastic vibrations. Unlike the former, vortex shedding is an unstable phenomenon that occurs behind the tube and causes it to vibrate; it is a hydrodynamic phenomenon that depends entirely on the flow around the tube. Fluid elastic rotation, on the other hand, is not determined by any unstable phenomena but results from the interaction of the flow fields of adjacent tubes. (3) Turbulent flutter: The fluid in turbulent flow exhibits random fluctuation components in various directions over a wide frequency range. As the fluid flows along or across the outside of the tubes, these turbulent components transfer energy to the tubes, resulting in their random vibration. This type of vibration caused by turbulence generated by the fluid flowing around the tube bundle is the most common form of vibration. When the dominant frequency of these turbulent fluctuations coincides with the natural frequency of the tubes, typical resonance occurs. If the shell-side fluid is a gas, at a certain velocity, the dominant frequency of turbulent flutter may also experience acoustic resonance. The studies in the above three aspects show that the vibration of the tube bundle is closely related to both the natural frequency of the tubes and the acoustic vibration frequency of the gas chamber. Prediction and prevention of vibration The hazards caused by vibration are significant; therefore, it is necessary to consider minimizing the possibility of fluid-induced vibration during the design phase. Eliminating all possibilities that could cause excitation in the heat exchanger tube bundle is the most fundamental way to prevent vibration; therefore, predicting or verifying vibrations in shell-and-tube heat exchangers should be carried out as an important part of ensuring their safe operation. However, vibration does not necessarily cause mechanical damage; many heat exchangers experience vibration without any accidents occurring. Of course, this does not mean that vibrations can be ignored. When vibration is likely to occur in the prediction results, the following vibration prevention and reduction measures can be taken. (1) Reduce the flow velocity on the shell side. If the shell-side flow rate remains constant, the tube spacing can be increased. This method is feasible when there are pressure drop constraints in the design, but it increases the shell diameter or the length of the tubes. By changing the original single inlet and outlet located at both ends of the shell – where the fluid flows around the baffle once before exiting the shell – to a split-type heat exchanger with the inlet in the middle and outlets at both ends, so that the fluid is divided into two streams exiting from either end of the shell, as shown in the figure below, the cross-flow velocity can be significantly reduced. (2) Increase the natural frequency of the pipe. The natural frequency of a pipe is inversely proportional to the square of its support span; therefore, reducing the support span of the pipe is the most effective way to increase its natural frequency. If no tubes are arranged at the notches of the arc-shaped baffle plates, the span distances that previously had support only every other baffle plate can be shortened, thereby increasing the natural frequency. It is said that this method can most effectively solve vibration problems, and its structure is shown in the figure below. If necessary, an intermediate support plate can also be added between the two baffle plates (a support plate with cut ends, as shown in the elevation view); it has no effect on pressure drop but offers some benefits for heat transfer. Increasing the natural frequency can also be achieved by changing the pipe material or increasing the wall thickness, but the effect is not significant. (3) Increase the acoustic vibration frequency. By inserting a vibration-damping plate into the shell, with its width direction parallel to the transverse flow direction and its length direction parallel to the tube axis, the acoustic vibration frequency can be increased so that it does not match the frequencies of vortex shedding and turbulent flutter. The position of the vibration damping plate should be at the antinode of the acoustic vibration standing wave pattern. (4) Structurally, increasing the thickness of the baffle plates or intermediate support plates – when the gap between the holes remains constant, this reduces the shear force on the tubes and increases the damping of the system. Creating chamfers on both sides of the tube holes in the baffle tubes helps to reduce the damage caused by vibrations. In addition to structurally avoiding vibrations, attention must also be paid to certain factors that affect operating heat exchangers; for example, the shell flow velocity should not exceed the limits permitted by vibration analysis, as even short-term overspeeding is detrimental to the service life of the heat exchanger.