Hydrocarbon Processing - February 2022 - 72

Heat Transfer
R. PRAMANIK and N. R. SRINATH,
Fluor, New Delhi, India
Case study: Challenges in the selection
of a helical baffled exchanger
In a greenfield project, a proprietary
heat exchangera
has been found to be
preferable to conventional shell-and-tube
(S&T) exchangers to optimize the total
lifecycle cost (i.e., capital, operating, installation
and maintenance costs). In a
crude preheat train, where the operating
fluids are highly fouling and viscous, the
unique geometry of the heat exchanger
offers lower operating and maintenance
costs, outweighing the
higher
capital
cost and optimizing the total lifecycle
costs of these exchangers. This article
demonstrates that in a typical crude preheat
train, a detailed comparative study
between conventional S&T exchangers
and the proprietary heat exchangera
is
required before finalizing the type of exchangers.
In this study, the sizes and performances
of four exchangers in a crude
preheat train were compared between
conventional S&T exchangers with segmental
baffles and the proprietary heat
exchangera
for the same set of fluid properties
and performance parameters.
Comparison. In the proprietary heat exchangera
,
quadrant-shaped baffle plates
are positioned on the shell side at an angle
to the tube axis, creating a helical flow pattern,
as shown in FIG. 1.
The shell-side flow pattern is the main
difference between a conventional S&T
heat exchanger and the proprietary heat
exchangera
. In a conventional S&T heat
exchanger, a continuous change in flow
direction is created by the baffles (resulting
in ineffective use of pressure drop)
and steady leakage of shell-side fluid exists
through the baffle and shell clearance (resulting
in wastage in effective heat transfer
surface). The very nature of the proprietary
heat exchangera
creates a swirl
to the shell-side
baffle construction
fluid
(thereby effectively utilizing the pressure
drop in heat transfer) and continuously
brings the fluid from the bundle periphery
to the inside of the bundle (thereby effectively
reducing the leakage stream).
Notable differences between segmenFIG.
1. Typical proprietary heat exchanger.
FIG. 2. Helical baffle flow regions.
72 FEBRUARY 2022 | HydrocarbonProcessing.com
tal and helical baffles include:
1. While there are five flow paths in
conventional S&T exchangers,
there are three flow paths in the
proprietary heat exchanger with
helical baffle, as shown in FIG. 2.
The flow paths are continuous
and in parallel, unlike the alternate
pattern of window and crossflow
in the segmental baffles. Although
the bulk of the flow (analogous to
B-stream in a conventional S&T
heat exchanger) follows the main
helical path around the baffles,
the outer helical flow (analogous
to C-stream) and core flow are
also important contributors to
overall heat transfer.
2. The segmental baffle requires the
correct selection of baffle cut and
spacing to achieve a proportionate
velocity ratio between the window
and cross flow, to minimize loss of
energy to changes in velocities in
the window and crossflow regions.
The helical baffle requires an ideal
helical flow pattern to attain similar
temperature profiles for fluid in
each of the three flow paths.
3. The flow in a helical baffle
will always have a longitudinal
component.
4. There is no distinction between 45°
and 90° layouts, or between 30° and
60° layouts for the helical baffle.
exchangera
Ways in which the proprietary heat
surpasses conventional S&T
exchangers are detailed in TABLE 1.
CASE STUDY
Four heat exchangers with fluid properties
and performance parameters (listed
in TABLE 2) in a crude preheat train were
sized as conventional S&T exchangers
with segmental baffles, as well as the proprietary
heat exchanger using an incremental
software programb
. It was determined
that, for the given fluid properties
and performance parameters, conventional
S&T exchangers with segmental
baffles would have performed equally well
without impacting the total lifecycle cost
of these exchangers.
Results,
discussion and lifecycle
costs. For the crude/vacuum residue
product (VR PDT) exchanger:
* The overall heat transfer rate was
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