Shell and tube heat exchangers are widely used in industrial CO2 extraction machines. With the expansion of the production scale of CO2 extraction, the size of the heat exchanger, the flow rate of the fluid, and the span of the support have all increased, even exceeding the allowable limit, thereby reducing the rigidity of the tube bundle and increasing the possibility of vibration.
Heat Exchanger Vibration
Vibration can make pipes leak, wear, fatigue, break, and even be accompanied by harsh noises, 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. Due to the intricate factors affecting vibration, the size of the damping effect is difficult to estimate accurately, and the speed of tube 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 to analyze vibration in engineering practice.
In the existing specifications for heat exchangers, there is also a lack of clear regulations on vibration analysis methods and anti-vibration design criteria. However, the practice has proved that if the necessary estimation and analysis of vibration can be carried out by using the existing research results in the design, and some anti-vibration measures are taken, then some destructive vibrations can be mostly avoided.
Three 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. This vibration is called flow-induced vibration. In fact, each heat exchanger has more or less vibration during operation. 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 pipeline or bracket Propagated dynamic mechanical vibrations, etc. Sometimes there may be many sources of vibration, 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 fluid flow on the shell side, and the vibration caused by the fluid flow on the tube side can often be ignored. In general, in the shell-side fluid, the amplitude of the vibration excited by the longitudinal flow flowing parallel to the tube axis is small, and the probability of structural damage caused by the vibration is also much smaller than that of the lateral flow. Therefore, people are more concerned about the vibration caused by lateral flow.
Three different causes of fluid-induced vibration are currently recognized: eddy shedding, turbulent buffeting, and fluid-elastic rotation (or fluid-elastic instability).
Eddy current 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 columns of vortices in opposite directions to shed alternately and alternately. A certain shedding frequency is produced. When the fluid flows laterally through the tube bundle, the Karman vortex will also be generated after the tube bundle, and for the small-pitch tube bundle, this phenomenon only occurs in the first few rows of the outer tube bundle, and for the large-pitch tube bundle, it can occur in the entire tube bundle. When the eddy current sheds, the fluid exerts a positive and negative alternating force on the circular tube. The frequency of this force is the same as the eddy current shedding frequency so that the circular tube will vibrate perpendicular to the flow direction at the vortex shedding frequency or a frequency similar to it. If the vibration frequency of the circular tube is equal to the multiple or submultiple of the eddy current shedding frequency, the vortex will shed uniformly 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 called resonance.
Eddy current shedding itself can also produce a certain sound. This is because, under certain conditions, it will arouse a certain order standing wave between the two walls of the air chamber, which is perpendicular to both the pipe and the flow direction, as shown in the figure below. This standing wave is reflected back and forth between the walls where the tube bundle is located, without propagating energy outward, while the eddy current shedding continuously inputs energy. When the standing wave frequency and the eddy current shedding frequency are coupled, a strong acoustic standing wave in the air chamber will be induced. Vibration – gas vibration, resulting in a lot of noise.
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 have 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 vibrated. 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. Unlike the former, eddy current shedding is an unstable phenomenon that occurs behind the tube and causes the tube to vibrate. It is a hydromechanical phenomenon that does not depend on the movement of the tube at all, and the elastic rotation of the fluid does not depend on any instability. The phenomenon is caused by the interaction of the flow fields of adjacent tubes.
Turbulent flowing fluids have random fluctuation components in all directions over a wide frequency range. When the fluid flows downstream or laterally around the outside of the tube, these turbulent components transmit energy to the tube, resulting in random vibration of the tube. The vibration of the tube caused by the turbulent flow generated by the shell-side fluid flowing through the tube bundle is the most common form of vibration. When the dominant frequency of this turbulent flow is in sync with the natural frequency of the tube, a typical resonance occurs. If the shell-side fluid is a gas, at a certain velocity, the dominant frequency of turbulent buffeting may also produce acoustic resonances.