Case 1:

User is designing the Heat exchanger NFU type and getting higher stresses at shell/channel joint.

We recommend to understand why the stresses are higher and how to reduce it.
Go through the Tubesheet design section from report below, You can see the formula of membrane stress and bending stress highlighted in the yellow and allowable criteria highlighted in green.

Here, We can increase the allowable stresses by selecting higher grade material.
Also stresses can be reduced by increasing the minimal thickness of the shell or by reducing the length of the tubes. 

Another option is to changing the heat exchanger type from NFU to BFU.
It is completely a designer's responsibility to choose the feasible workaround to reduce the stresses. 

Case 2:

Designing a double fixed tubesheet heat exchanger is challenging and complex since various design cases need to be considered while determining the tubesheet thickness including differential thermal expansion between the shell and tube sides.

 

Shown below is a simple NEN type double fixed tubesheet exchanger modeled using AutoPIPE Vessel and as one can see there are several errors reported:

The errors reported are as follows:

25: insufficient strength in shell-channel joint

27: longitudinal membrane strength in tubes exceeded

28: longitudinal membrane stress in shell exceeded

633: tube-to-Tubesheet joint strength not adequate

Also the thickness of the tubesheet is excessively high at 278 mm.

There may be several ways to resolve each of the above errors individually and onus lies on the designer what method he may want to choose. However, one of the most commonly used methods for resolution of these types of tubesheet related errors in double fixed tubesheet exchangers is by including an expansion joint, since most of the time the tubesheet related errors in double fixed tubesheet may be due to stress arising out of differential thermal expansion between the tubeside and the shell side.

The expansion joint can be added by clicking on the icon and entering the spring rate in the dialog box that appears as shown below, there are also options for flanged and bellows expansion joint:

NOTE - Determining expansion joint spring rate is the complete responsibility of the designer based on the expected mean metal temperatures of the shell side and tube side during various operating scenarios - the spring rate entered (200 daN/mm) is just for example :

Note that the location of the expansion joint can be entered in the dialog box, however it can be moved in the 2D sketcher directly by selecting the component, right click and selecting the option "Move".

The strength calculations are performed once again and as shown below there are no errors this time and the expansion joint is shown clearly. The design is complete. Observe that the thickness of the tubesheet is also significantly reduced (24 mm) thereby leading to an optimized design:

In the report, the calculation of the tubesheet shows clearly the inclusion of the expansion joint spring constant (200 daN/mm or 2,000 kg/mm):