Hydrocarbon Processing - May 2021 - 67

Heat Transfer
non-tie rod end tubesheet pair is
also prepared.
* The tube-to-tubesheet joint on the
shell side is tested for shell-side
hydrotest pressure. Any leakage
must be detected with a drop
in pressure because-with the
presence of the shroud shell-there
is no visibility in the space between
the pair of tubesheets.
The manufacturer can alter the intermediate fabrication and testing sequences
based on shop facilities, experience and
individual technical capability.

These heat exchangers are also required
in polysilicon manufacturing plants, as
well as solar power plants. Distilled hexane reboilers, hexane heaters and titanium chloride heaters in the petrochemical
industry are further examples.
This paper has presented reasons for
selection of such types of heat exchangers, various types of stresses in tubes,
tubesheets and shroud shell, fabrication,
assembly sequences and testing methodologies. Either of the two proposed

assembly sequence methods can be adopted by a manufacturer.
LITERATURE CITED
Standards of the Tubular Exchanger Manufacturers
Association (TEMA), 9th Ed., New York, January
2007.
2
Perry's Chemical Engineers Handbook, 7th Ed.,
Mc-Graw Hill Publications, 1997.
3
A Working Guide to Shell and Tube Heat Exchangers,
Stanley Yokell, Mc-Graw Hill, 1990.
1

PURUSHOTTAM M. MISAL works for Fluor in
New Delhi, India. The author can be reached at
Purushottam.m.misal@fluor.com.

Drawbacks of double-tubesheet
heat exchangers. Although the total

surface area of the exchanger is more, the
effective surface area is reduced significantly because the effective tube length is
measured between the inside faces of the
shell-side tubesheet. The tube length surface area in the shroud area is not considered as a heat transfer area. This increases
the required tube length and, in turn, the
overall length of the exchanger, further
increasing the cost of the heat exchanger.
The addition of two more tubesheets
in double-tubesheet construction increases the cost further.
As discussed in previous sections, many
criticalities and difficulties are involved in
tubesheet/baffle/support plate drilling
and machining, especially to achieve tube
hole concentricity and tubesheet surface
parallelism. Additionally, challenges exist
in the correct sequence of assembly, making it difficult to produce quality product.
Maintenance of these heat exchangers
can be very difficult, particularly tube removal and replacement since the tube has
been fixed with tubesheets in four places. A
better option is to plug the damaged tubes.
The arrangement is only possible in
a fixed tubesheet, a U tube and outsidepacked floating head. During the operation, the vents/drains of the shroud shell
must be regularly monitored to avoid mixing of hazardous or carcinogenic fluid into
the atmosphere.
Takeaway. Well-planned fabrication and
assembly sequences can be useful while
manufacturing double-tubesheet heat exchangers. These heat exchangers are also
required in the pharmaceutical industry for sanitary applications and are designed to meet that industry's high-quality requirements and hygienic standards.

FIG. 6. (D-F) A fabrication and assembly sequence (A-F) has been presented for a fixed
double-tubesheet heat exchanger.
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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