Chemical Engineering February 2017 - 39

Front stationary tubesheet end.
One end of the tube bundle needs
to have a stationary tubesheet. The
tubesheet provides separation between
the shellside and tubeside
fluids. The tubesheet on the stationary
end is fixed and does not move
with thermal differential expansion. It
is often gasketed between flanges. At
least one nozzle is located in the attaching
channel. The ability to open
and close the channel with more
or less ease is part of the choice of
channel design.
TEMA rear-head type. In services
with
high
temperatures and long
tubes, differential thermal expansion
between the tubes and shell may
require a method to provide for their
differential growth. This can directly
impact the type of rear-head configuration.
If the fluid is clean and the
tube bundle does not need to be removed
to be cleaned, the differential
expansion may be accommodated
with an expansion joint. Within that
decision is the choice of which type
of expansion joint should be used.
Such expansion joints can be of the
bellows type or dished heads. Both
options are addressed in TEMA.
If the service of the exchanger
requires occasional cleaning of the
shell side, a removable design would
be chosen. The design pressure,
fluid service and gasket requirements
may dictate one design compared to
another. For instance, a " W " head is
not used with a four-tube-pass design
due to the need for a pass partition
in the return head in which the
tubesheet needs to move to account
for differential thermal expansion.
" W " heads are also not suitable for
higher design pressures.
U-tube exchangers may not be
appropriate for services in which the
tubes need to be cleaned. Fouling
and debris can accumulate in the
U-bends, resulting in tubes being
plugged and prematurely failing. In
clean tube-side flow service, the Utubes
can be the most cost-effective
approach, since it only requires one
tubesheet and only an inlet-outlet
channel. This is especially important
in high-pressure applications.
This return end also requires an even
number of tube passes. The choice
of the rear head is a function of the
cleanliness of the tube-side fluid. If
there is a significant temperature difference
between the shell-side fluid
and tube-side fluid or from startup,
a floating head may be desirable. If
the shell side is clean and there is
no requirement that the bundle be
removeable, a fixed tubesheet design
(L, M or N) with a shell expansion
joint may be more cost-effective
and require less maintenance. If the
shell fluid is not clean, and the bundle
needs to be cleanable, a pull-through
head (T) or a split-ring-type return
channel (S) may be the best choice.
Shell type. Determining the shell
type is a factor largely decided by
the thermal design engineer. The
way the shell flow transits over and
through the bundle can vary with the
shell type and the bundle baffling.
The installation of longitudinal baffles
can also have a dramatic effect on
the amount that the temperature
profile is corrected for the extent of
co-current flow.
An E shell is the most common. It is
used where the difference in the outlet
temperatures is zero or positive. It
is also most common for single-pass
counterflow design units when there
is a temperature cross of the outlet
temperatures. In cases where the E
shell would be too long, in an attempt
to make a substantial temperature
cross, an F shell may be used. The
longitudinal baffle can be sealed to
the shell ID with " lamiflex " style seals
to minimize leakage and thermal leakage.
The F shell is basically an E shell
folded in half.
Kettle units are used when vaporizing
most of the shell-side flow.
The large portion of the shell allows
for the escape of the vapor and disengagement
of the liquid from the
flooded bundle. There are other reboiler
styles, such as the thermosiphon
style, which may be horizontal
or vertical. There are some styles
that are inserted into process columns
to provide partial vaporization.
Usually, a small shell section and the
stationary end are provided with the
" stabbed-in " bundle.
The G, H, J and X are used for special
applications and as a method for
keeping velocities low. The X shell is
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39
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Chemical Engineering February 2017

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