What is the difference in the stress on the tube sheet of fixed-tube-sheet heat exchangers and U-tube heat exchangers?

The main difference is that the heat exchange tubes are not subjected to pulling forces, and the shell is not affected by the heat exchange tubes. The heat exchange tubes also exert no thrust or supporting force on the tube sheet.

The tube bundle contracts or expands due to the temperature difference of the heat transfer fluid, which in turn subjects the tube sheet to stress; however, the tube sheet in a U-tube heat exchanger is less affected.

I agree with the opinion above: U-shaped tubes can expand and contract, so the thermal stress is lower. Also, I think that for the same type of shell, U-shaped tubes would result in fewer tube bundles compared to those with fixed tube sheets, and this surely has an impact on the stress distribution design as well.

Compared with the tube sheet of a U-tube heat exchanger, the fixed tube sheet may be subjected to thrusts or pulls generated by the thermal expansion and contraction of the heat exchange tubes, resulting in a poor stress condition; whereas the latter can basically eliminate this force, leading to a better stress condition.

The last edit to this post was made by HSLJHZ on 2016-2-17 at 09:44. Let’s start by discussing their common points: 1. Both are derived using shell theory; therefore, when calculating the internal stresses in the tube sheet, only the bending stresses caused by circumferential and radial moments are taken into account, while shear stresses are ignored. As a result, both are controlled using a stress value that is 1.5 times the allowable stress (stability criterion)； 2. The weakening effect of tube holes on the tube sheet is considered in both cases (stiffness weakening coefficient η, strength weakening coefficient μ) ； 3. The consideration of the restraining effect of surrounding supports on the tube sheet is consistent: for clamped tube sheets, shear force is considered while bending moment is not; for welded tube sheets, both shear force and bending moment are considered; and for flanged tube sheets, the influence of bolt torque must also be taken into account. Differences between the two: U-tube: (1) It does not take into account the supporting effect of the tube bundle on the tubes ； (2) The effect of temperature difference is not considered. Without considering elastic support, there is only one tube sheet, with no support at the tail, and deformation is coordinated ； Fixed tube sheet: (1) Consider the supporting role of the tube bundle on the tubes ； (2) Consider the effect of the temperature difference between the tube side and the shell side cylinder on the tube sheet ； (3) The new GB151 specifies the impact of the axial deformation of the expansion joint caused by internal pressure on the fixed-tube-sheet heat exchanger ； (4) Using a model with a very narrow pipe-free area around it, the effect of the annular pipe-free area was ignored ； (5) The model assumes complete symmetry of the tube sheets at both ends in terms of dimensions, material, constraints, and stress; therefore, it requires that the tube sheets at both ends be made of the same material and have the same dimensions (see GB151) ； (6) Consider separately the cases where the pressure in the tube/shell side acts alone or simultaneously, as well as the presence or absence of a temperature difference. Therefore, 3*2=6 operating conditions should be verified. It treats the tube sheet as an approximately axisymmetric structure, with the tube sheets at both ends having the same material and the same thickness, as well as the same boundary conditions.

So, how do U-tube heat exchangers compare with floating-head heat exchangers? Most of our company’s units are of the floating-head type, and many that could use a U-tube structure have not adopted it either. Looking forward to your reply!

In a U-tube heat exchanger, the tube sheet is subjected to bending moment but no shear force, while the fixed tube sheet experiences both shear force and bending moment; in addition, the effect of flange moments must also be taken into account. For the six combinations specified in the fixed tube sheet standards, it is not necessary to calculate them all if a qualitative analysis can be conducted.