In the previous article (Three Causes Of Vibration And Noise In Shell And Tube Heat Exchangers), we described: Shell and tube heat exchangers are widely used in industrial carbon dioxide 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 increased. Even the allowable limits are exceeded, thereby reducing the stiffness of the tube bundle and increasing the possibility of vibration. Today we will talk about how to stop it from happening, and four measures.
The vibration caused by the shell and tube heat exchanger during the operation of the CO2 extraction process of the CO2 extraction machine is very harmful, so it should be considered in the design to minimize the possibility of fluid-induced vibration. Eliminating all the possibilities of exciting vibration in the heat exchanger tube bundle is the most fundamental way to prevent vibration. Therefore, the prediction or verification of the vibration of the shell and tube heat exchanger should be an important part of ensuring the safe operation of the heat exchanger. well done.
But vibration does not necessarily cause mechanical damage, and many heat exchangers vibrate without incident. Of course, this does not mean that vibration can be turned a blind eye. When there is a possibility of vibration as a result of the prediction, the following four anti-vibration and vibration reduction measures can be taken.
Four anti-vibration and vibration reduction measures
Decrease the velocity on the shell side
If the shell side flow rate remains unchanged, the pipe spacing can be increased. This approach works well when there are pressure drop constraints in the design, but increases the shell diameter or increases the tube length.
If the original single inlet and outlet at both ends of the shell (the fluid bypasses the baffles and flows through the shell once) are 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 from the shell. Outflow at one end, as shown in the figure below, can greatly reduce the cross-flow velocity.
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 there is no pipe arrangement at the notch of the bow-shaped baffle, the spans that were 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 view) can also be added between the two baffles, which has no effect on the pressure drop but has a certain benefit for 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 large.
Raise the sound frequency
Insert the damping plate into the shell so that its width direction is parallel to the transverse flow direction and its length direction is parallel to the tube axis, which can increase the frequency of acoustic vibration and make it inconsistent with the frequency of eddy current shedding and turbulent buffeting. The position of the vibration-damping plate should be on the antinode of the standing wave waveform of acoustic vibration.
Increase the thickness of the baffle or intermediate support plate
Structurally, increasing the thickness of the baffle plate or the intermediate 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. Machining chamfers on both sides of the baffle tube hole have a certain effect on reducing vibration damage.
In addition to the structural attention to avoid vibration, it is also necessary to pay attention to some factors that affect the heat exchanger in operation, such as: do not allow the shell radial flow rate to exceed the limit allowed by the vibration analysis, even for a short period of time. It is detrimental to the service life of the heat exchanger.