Problems encountered in the calculation of expansion joints for fixed-tube-sheet heat exchangers
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When designing a conventional fixed-tube-sheet heat exchanger, the design pressure on the tube side is 6.2 Mpa at 180°C, while the design pressure on the shell side is 2.0 Mpa at 210°C. The thickness of the tube sheet proved to be unreasonablely large; therefore, the use of expansion joints was considered. But why did the inclusion of expansion joints not have any effect, and in fact made the tube sheet even thinner than if no such joints had been used? Without overtime, the calculated thickness of the tube sheet is 120 mm, while it exceeds 120 mm when expansion joints are used. What is the reason for this? Thank you all for your advice!
Search the forum; it seems this topic was discussed before as well. It’s probably related to the stiffness of the expansion joints. I didn’t quite understand it, so I hope someone can explain it in more detail.
By adding expansion joints, the axial stiffness of the shell side **decreases**, which in turn **reduces** the stress on the tube sheet. So shouldn’t the tube sheet become thicker instead? This is just my personal opinion; it may not be correct.
This post was last edited by jia717 on 2009-9-2 at 21:42. When the temperature difference between the tube side and the shell side of a fixed-tube-sheet heat exchanger is too large, significant stresses are generated in the shell side and the heat exchange tubes. To reduce such stresses, expansion joints are generally installed.
What was said upstairs is correct, but the problem is that I added expansion joints, and the calculations for those joints have been approved; yet the thickness of the tube sheet resulting from these calculations is greater than that of the tube sheet without expansion joints. Please explain why this is the case.
Ugh, why do the answers to the questions keep going in circles? I personally think that the structure of the expansion joint you designed is unreasonable; as a result, its SPRING RATE is too high. In other words, a large force is required for this expansion joint to stretch or shrink by 1 mm, and the stress generated on the cylinder by this force clearly exceeds the stress that would occur if there were no expansion joint at all... Therefore, please adjust parameters such as the height, wave width, and bending radius of the expansion joint yourself, so as to minimize its elastic ratio – that should do it.
What was said upstairs makes a lot of sense; it might be possible to adjust the geometric dimensions of the expansion joint and give it a try – perhaps the dimensions of the expansion joint aren’t quite optimal
Thank you for the advice from the two people above... I checked the results and even wrote a program in Excel to run some calculations; I found that the main problem lies in the fact that, under the hazardous operating conditions where only the tube bank pressure Pt is considered, the radial stress σr on the tube sheet exceeds the allowable value. Without using expansion joints, with a tube sheet thickness of 130 mm as calculated, σr equals 194.2 Mpa. With expansion joints in place, σr rises to 211.2 Mpa, which is above the allowable stress level. Moreover, on the tube sheet design page, I tried entering various values for the stiffness of the expansion joint, and the same result was obtained each time; I’m not quite sure why. Actually, after comparing with other stress data, it can be seen that the stress does decrease significantly after the installation of expansion joints; however, σr increased and exceeded the allowable stress range: (
In fixed-tube-sheet heat exchangers, during operation, a temperature difference exists between the tube bundle and the shell, which leads to different rates of thermal expansion in the tube side and the shell side. As a result, temperature differential stresses are generated between the tube bundle and the shell. The magnitude of these stress forces is determined by the temperature difference between them. Sometimes, these stresses can be strong enough to cause the tube bundle to bend, thereby damaging it. Since thermal stress is caused by structural inconsistencies rather than by the requirements of operating conditions, efforts should be made to reduce its impact on the structure during heat exchanger design. The most common method for eliminating or reducing thermal stress in fixed-tube-sheet heat exchangers is to install expansion joints; however, sometimes, when the temperature difference in the production process is small, meaning the thermal stress is not significant, it is not necessary to install expansion joints. Taking both economic efficiency and safety into account, a decision can be made in heat exchanger design regarding whether it is necessary to install expansion joints. Hehe, this is a passage from my graduation project; I was lazy and just copied and pasted it. It states that the thickness of the tube sheet has nothing to do with expansion joints. The calculation method for the tube sheet thickness is described in detail in the design manuals, but here I’m providing a simpler approach – one that remains reliable (safe and secure, with a slightly larger thickness). 1) Calculate the tube sheet thickness based on its bending strength. 2) Calculate it based on its shear strength. 3) The minimum thickness of the tube sheet (without including any additional thickness allowance): 3/4d
Following the previous reply: an omission was made; taking the maximum value of the three gives the desired result