Heat Exchanger
File: PVE-3520
Last Updated: Sept. 29, 2009
DV
Commentary:
Why use FEA on Heat Exchangers?
ASME UHX rules cover the design of tubesheets, tubes and the shell next to the tubesheet. But the rules are limited to designs with uniform hole patterns that cover the complete tubesheet. What if the hole pattern is not uniform, or in the case of this sample, the holes are not a uniform size?
 A heat exchanger with non-uniform hole sizes |
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Burst testing is an economical way to validate inexpensive products. However it may be unreasonable to use destructive testing to validate costly or large items such as a heat exchanger; in this case finite element analysis (FEA) is the most logical approach.
Using FEA to Replace ASME Code Rules
The ASME code rules must be used if they are applicable. In this case the standard code rules can not be applied so FEA is allowed. The UHX rules account for three stresses in the design of an exchanger:
- the tubesheet stress caused by pressure and thermal loading
- the shell stress next to the tubesheet caused by tubesheet rotation
- the tube compression or tension loading caused by tubesheet displacement and thermal expansion.
Per UHX rules, these stresses are analyzed for the following load cases in fixed tube exchangers:
- Tube side pressure only
- Shell side pressure only
- Tube + shell side pressure
- Thermal loads only
- Tube side pressure + thermal loads
- Shell side pressure + thermal loads
- Tube side pressure + shell side pressure + thermal loads
For a finite element analysis to replace the UHX rules for a fixed tubesheet exchanger the three stresses need to be studied for the seven load cases.
FEA Results
See sample 6 for an in depth discussion of the FEA method applied to a simple pressurized object. Here the same methods are used with a more complicated model.
The illustration below shows the mesh used for this sample. A solid mesh has been applied to the tubesheet and adjacent shell and a shell mesh applied to the tubes. The mesh is reduced in size at locations of interest such as the tubesheet, the tubesheet to tube junction, and the adjacent shell.
 Close up of the mesh used in the sample stud. A fine mesh is used for the tubesheet and the shell next to the tubesheet. The tubes and outer shell are made of shell elements. |
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 Load case 2 - shell side pressure only. |
The sample FEA report walks through all seven load cases and checks all three stresses for each case. Each stress is compared to the ASME allowable stress to determine pass/fail for each load case.
The reported results can also be further used to provide design optimization and cycle life results.
 Deformation plot with the displacements magnified 100x. The rotation of the tubesheet with the adjacent shell bending it causes can be seen. |
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 Deformation plot with the displacements magnified 100x. The rotation of the tubesheet with the adjacent shell bending it causes can be seen. |
Summary:
FEA can be used to address ASME code rules where calculations cannot be applied. It is an excellent, and in some cases the only option to validate a design. It can be cost effective, reduce lead time and expedite registration.
We have successfully used this FEA method to provide reports justifying heat exchanger designs reviewed by Authorized Inspectors and review engineers.
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