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Since fluid in the shell is at a different temperature
than the fluid in the tubes, there are corresponding differences in
expansion of shell and tubes. Some provision must be made in design to
compensate for this difference in thermal expansion.
U-Tube Design
In the U -tube exchanger, the shape of the tubes takes care of
differential expansion. As the name implies, the tubes have a
hairpin shape, with both ends of the tube fastened to one tubesheet.
This U bend design allows each tube to expand and contract
independently.
Floating Head Designs
This type meets the expansion problem by having one stationary
tubesheet, and one free to move - float - back and forth as the
tubes expand and contract under the influence of temperature changes.
Fixed Tubesheet Design
The fixed tubesheet heat exchanger employs straight tubes secured
at both ends into tubesheets, which are welded to the shell. When a
thermal expansion problem exists, an expansion joint is incorporated
in the shell. This permits the shell to expand and contract.
When the possibility of intermixing of the shell side
and tube side fluids cannot be tolerated, double tubesheet
construction will offer positive assurance against one fluid leaking
into the other at a tube to tubesheet joint. A second tubesheet is
installed with a small space between tubesheets - usually open to the
atmosphere. |
