With the expansion of the production scale, the size of the heat exchanger, the flow rate of the fluid, and the span of the support all increase, even exceeding the allowable limit, thereby reducing the rigidity of the tube bundle and increasing the possibility of vibration.
Vibration can cause pipe leakage, wear, fatigue, fracture, and even harsh noise, which not only reduces the life of the equipment, but also damages people's health. Once the vibration causes an accident, it often takes a long time to analyze and repair it. Due to the complex factors affecting vibration, the size of the damping effect is difficult to accurately estimate and the speed of pipe wear and damage is difficult to determine, and they cannot be described by simple mathematical formulas. It can be said that the theoretical calculation methods so far cannot be used. Accurately analyze vibration in engineering practice. In the existing specifications related to heat exchangers, there is still a lack of clear regulations on vibration analysis methods and anti-vibration design criteria. However, practice has proved that if the existing research results can be used in the design to make necessary estimation and analysis of vibration, and some anti-vibration measures are taken, then most of the destructive vibration can be avoided.
1) Causes of fluid-induced vibration
The tube bundle of the heat exchanger is an elastic body, which is disturbed by the flowing fluid and leaves its equilibrium position, and the tubes vibrate, which is called flow-induced vibration. In fact, every heat exchanger has more or less vibration when it is working. The vibration source may be the vibration caused by the fluid flow on the shell side or the tube side; the vibration caused by the fluctuation or pulsation of the fluid velocity; through the pipe or support Propagated dynamic mechanical vibrations, etc. Sometimes there may be more vibration sources, and one or several of them may be the main source of vibration. Some vibration sources are relatively easy to predict, while fluid-induced vibrations are more difficult to predict.
Some experiments and operating experience show that the vibration of the heat exchanger is mainly caused by the flow of the fluid on the shell side, and the vibration caused by the fluid flow on the tube side can often be ignored. Generally, in the shell-side fluid, the amplitude of the vibration excited by the longitudinal flow parallel to the pipe axis direction is small, and the probability of structural damage caused by the vibration is much smaller than that of the transverse flow. Therefore, people are more concerned about the vibration caused by cross flow.
At present, it has been recognized that there are three different causes of fluid-induced vibration: eddy current shedding, turbulent buffeting and fluid elastic rotation (or fluid elastic instability).
(1) Vortex shedding
When the fluid flows laterally through a single cylinder, at a large Reynolds number, the Karman vortex (or Karman vortex street) formed in the wake behind the tube causes two rows of vortices in opposite directions to shed periodically and alternately. There is a certain shedding frequency. When the fluid flows laterally through the tube bundle, Karman vortices will also be generated behind the tube bundle, and for small-pitch tube bundles, this phenomenon only occurs in the first few rows around the tube bundle, and for large-pitch tube bundles, it can occur in the entire tube bundle. When the vortex is shedding, the fluid exerts an alternating positive and negative force on the tube. The frequency of this force is the same as the vortex shedding frequency, which will make the tube vibrate perpendicular to the flow direction at the vortex shedding frequency or a frequency close to it. If the vibration frequency of the circular tube is equal to the multiple or submultiple of the vortex shedding frequency, the vortex will shed evenly at the same frequency along the full span of the cylinder (circular tube) at the same time, and the shedding frequency and the vibration frequency are synchronized, which is the so-called resonance.
The eddy current shedding itself can also produce a certain sound. This is because under certain conditions, it will excite a standing wave of a certain order between the two walls of the air chamber that is perpendicular to the pipe and the flow direction, as shown in the figure below. This standing wave reflects back and forth between the walls where the tube bundle is located, and does not transmit energy outward, while the vortex shedding continuously inputs energy. When the standing wave frequency and the vortex shedding frequency are coupled, a strong air chamber acoustic standing wave will be induced Vibration - gas vibration, making a lot of noise.
(2) Fluid elastic rotation
When the gas flows laterally through the tube bundle, the fluid force generated by the asymmetry of the fluid can cause a tube in the tube bundle to undergo an instantaneous displacement from its original position, so the flow field alternates, destroying the adjacent The tubes are balanced so that they are also displaced and in 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 cause damage. Such vibrations are called hydroelastic vibrations. Different from the former, vortex shedding is an unstable phenomenon that occurs behind the tube and causes the tube to vibrate. It is a hydrodynamic phenomenon completely independent of the tube motion, and the fluid elastic rotation does not depend on any instability. phenomenon, but due to the interaction of the flow fields of adjacent pipes.
(3) Turbulent buffeting
Fluid flowing in turbulence has random fluctuation components in a wide frequency range in all directions. When the fluid flows downstream or laterally around the pipe, these turbulent components transmit energy to the pipe, resulting in random vibration of the pipe. The vibration of the tube caused by the turbulence generated by the shell-side fluid flowing through the tube bundle is the most common form of vibration. When the main frequency of the turbulent fluctuation is in sync with the natural frequency of the tube, typical resonance occurs. If the shell side fluid is gas, at a certain speed, the main frequency of turbulent buffeting may also produce acoustic resonance.
The research in the above three aspects shows that the vibration of the tube bundle is closely related to the natural frequency of the tube and the acoustic vibration frequency of the air chamber.
2) Prediction and prevention of vibration
The harm caused by vibration is very great, so the possibility of fluid-induced vibration should be taken into account in the design to a minimum. The most fundamental way to prevent vibration is to eliminate all the possibility of exciting vibration in the tube bundle of the heat exchanger. Therefore, the prediction or verification of the vibration of the shell-and-tube heat exchanger should be taken as an important part of ensuring the safe operation of the heat exchanger. do well.
However, vibration does not necessarily cause mechanical damage. Many heat exchangers vibrate without accidents. Of course, this does not mean that you can turn a blind eye to vibration. When the prediction result is likely to vibrate, the following anti-vibration and vibration reduction measures can be taken.
(1) Reduce the flow velocity on the shell side. If the flow rate on the shell side remains unchanged, the pipe distance can be increased. This approach is acceptable when there are pressure drop limitations in the design, but increases the shell diameter or length of tubing.
If the original single inlet and outlet located at both ends of the shell (the fluid bypasses the baffle and flows through the shell at one time) is changed to a split flow heat exchanger with the inlet in the middle and the outlet at both ends, the fluid is divided into two halves and flows from the shell to either side. Outflow at one end, as shown in the figure below, can greatly reduce the cross-flow velocity.
(2) Increase the natural frequency of the tube. The natural frequency of the pipe is inversely proportional to the square of the support span, so reducing the support span of the pipe is the most effective way to increase the natural frequency of the pipe.
If no tubes are arranged at the gaps of the bow-shaped baffles, those spans originally supported by every other baffle can be shortened and the natural frequency can be increased. This method is said to be the most effective solution to the vibration problem, and its structure is shown in the figure below. If necessary, an intermediate support plate (a support plate cut off at both ends, as shown in the elevation) can be added between the two baffles, which has no effect on the pressure drop but has some benefits on heat transfer . The natural frequency can also be increased by changing the pipe material or increasing the thickness of the pipe wall, but the effect is not very great.
(3) Increase the frequency of acoustic vibration. The vibration damping plate is inserted in the shell so that its width direction is parallel to the cross flow direction and its length direction is parallel to the pipe axis, which can increase the frequency of acoustic vibration and make it inconsistent with the frequency of eddy current shedding and turbulent chattering. The position of the damping plate should be on the antinode of the acoustic vibration standing wave waveform.
(4) In terms of structure, increasing the thickness of the baffle or the middle support plate can reduce the shearing effect on the pipe and increase the damping of the system when the gap between the holes is constant. Processing chamfers on both sides of the baffle tube hole has a certain effect on reducing vibration damage.
In addition to paying attention to avoiding vibration from the structure, some factors that affect the heat exchanger in operation must be paid attention to. Detrimental to the service life of the heat exchanger.
February 16, 2023
