Hydrocarbon Processing - April 2022 - 37

Heat Transfer
K. DARU, Air Products and Chemicals Inc.,
Houston Texas; A. PANCHAL, Independent
Consultant, Mumbai, India; P. GALA, Reliance
Industries Ltd., Mumbai, India
Uncommon lessons: Shell-and-tube
heat exchangers-Part 1
Shell-and-tube heat exchangers
(STHEs) are the most common heat
transfer process equipment in all types
of industrial plants. Due to the variety of
available construction options, an STHE
can be suitably designed for most processes
and design conditions and can be constructed
from different materials or combination
of materials. STHEs are typically
manufactured in accordance with international
or national codes or standards,
with the American Society of Mechanical
Engineers (ASME) Code1
being the most
popular, along with associated Tubular
Exchangers Manufacturers Association
(TEMA)2
tute (API) Standard 6603
and American Petroleum Instirequirements.
According
to the ASME Code, an
STHE is a multi-chamber vessel, with two
chambers for most constructions that are
commonly addressed as shell-side and
tube-side or channel-side. In an STHE,
the tubesheet, tubes, floating head (if
applicable) and internal expansion bellow
(if applicable) are the common elements.
Depending upon process needs,
the STHE may have a variety of external
and internal components, such as cylinders,
concentric and/or eccentric cones,
dished heads, thick or thin expansion
bellows, tubes (straight, u-tube, low fin,
twisted, etc.), girth flanges, tubesheets,
bolted covers, nozzles, multi-purpose
nozzles, baffles, tie-rods, impingement
rods, sealing strips, sliding strips, bundle
pulling eye bolts, saddle support, lug support
and skirt support (for large and long
exchangers). FIG. 1 illustrates a floating
head type heat exchanger and its major
parts/components.
Apart from designing components to
comply with applicable pressure and coincident
temperature conditions and evaluating
thicknesses or stresses in different
components, there are specific requirements
in the ASME Code, TEMA Standard
and the API Standard 660 that are often
not complied with in design and early
fabrication stages. Based on the authors'
opinion, failure to identify these during
design reviews leads to costs that must be
corrected at later stages in fabrication.
This article focuses on specific types of
lessons and design limitations that none
of the commercial design programs (software,
Excel spreadsheets, etc.) will identify
with their " help " menus or warnings.
Merely checking drawings with respect
to design calculations is insufficient to
ensure thorough compliance and reliable
product design that will operate (thermally
and mechanically) as intended.
The authors' purpose is to shed light on
those uncommon but valuable lessons that
are not discussed in reference manuals or
design guides. These lessons will be helpful
in engineering and design activities.
Channel
cover
Channel
Girth flange
Nozzle
Shell
Maximum allowable working pressure
(MAWP). The authors encountered
shell-side MAWP calculation for an exchanger
with a thick (flanged and flued)
expansion bellow. This expansion bellow
was designed (separately) using finite
element analysis (FEA) with shell-side
design pressure; therefore, the MAWP
for the expansion bellow is the shell-side
design pressure. Surprisingly, a higher
shell-side MAWP obtained from a design
report generated via a computer program
was indicated on the drawings and accordingly
was carried over in the ASME
forms and equipment nameplate. The
higher MAWP was based on the design
of other parts of the shell-side chamber,
excluding the expansion bellow.
Note: When a part of a pressure
chamber (in this example, the bellow)
is designed using an FEA program, the
pressure used to design the part becomes
the MAWP of that part, unless a higher
MAWP is used in the FEA. This should
not be missed when evaluating the governFloating
head
cover
Saddle
Floating head
Tubesheet
FIG. 1. Floating head heat exchanger.
Hydrocarbon Processing | APRIL 2022 37
Hydrocarbon Processing - April 2022

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