Hydrocarbon Processing - February 2022 - 73

Heat Transfer
only 4% higher for the helical baffle.
The increase in overall heat transfer
coefficient (U) due to improved
heat transfer with the helical baffle
was partly compensated by the
use of a rotated square pitch with
segmental baffle.
* A marginal improvement was seen
in mean temperature difference
(MTD) with the helical baffle due
to the absence of leakage streams.
* The effective surface area was 6%
lower for the helical baffle due
to higher overall U and MTD,
described above.
* The capital cost with the helical
baffle will be much higher than
with the segmental baffle-the
reduction in cost of the proprietary
heat exchanger due to the 6%
reduction in surface area will be
more than offset by the higher cost
of the heat exchangera
design.
For the crude/VR PDT + quench exchanger-III:
*
The overall heat transfer rate is
almost same in both helical and
segmental baffle designs. Any
increase in overall U due to more
effective heat transfer with the
helical baffle was compensated by
use of a rotated square pitch with
the segmental baffle.
* The MTD is almost identical
for both designs since the impact of
the leakage stream in the segmental
baffle design case in minimal.
* The effective surface area is the
same in both helical and segmental
baffle designs due to similar U
and MTD values.
* The capital cost with the helical
baffle will be significantly higher than
the segmental baffle due to the higher
cost of the heat exchangera
design.
For the crude/VR PDT + quench exchanger-II:
*
The overall heat transfer rate is
almost the same in both helical
and segmental baffle designs. Any
increase in overall U due to more
effective heat transfer with the
helical baffle was compensated by
use of a rotated square pitch with
the segmental baffle.
* Marginal improvement in MTD was
achieved with the helical baffle due
to the absence of leakage streams.
* The effective surface area is 4%
TABLE 1. Proprietary heat exchangera
Service/construction
High viscosity fluid
on shell side
Reduced shell-side fouling
Smaller surface area
scores over conventional S&T exchanger
Remarks
Absence of baffle-to-shell bypassing and continuous drawing
of fluid from the periphery to center of bundle
Due to near plug flow condition achieved throughout
the bundle length
More heat transfer for the same allowable pressure drop
due to better mixing of flow and no unnecessary change
of direction of flow inside shell
TABLE 2. Fluid properties and performance parameters of four heat exchangers
Service
Crude/
VR PDT
TEMA type2
Heat duty, MM Kcal/hr
Shell-side fluid
Tube-side fluid
S/S viscosity (in/out), cP
S/S fouling factor, m2-hr-°C/Kcal
exchanger
AES
4.4649
VR + slop
Crude
10.5/16.5
0.002
T/S fouling factor, m2-hr-°C/Kcal 0.000614
S/S temp. (in/out), °C
T/S temp. (in/out), °C
Allowable DP (S/S), kg/cm2
251.1/227.1
201.4/212.2
4.1
greater for the segmental baffle due
to the lower MTD described above.
* The capital cost with the helical
baffle will be much higher than
with the segmental baffle since the
reduction in cost of the proprietary
heat exchanger due to the 4%
reduction in surface area will be
more than offset by the higher cost
of the heat exchanger design.
For the crude/VR PDT + quench exchanger-I:
*
Marked improvement (~ 9%)
was seen in the overall heat transfer
for the segmental baffle, due to the
change in exchanger arrangement
to four shells in series with two
tube passes rather than two shells
in series and two in parallel with
four tube passes in the case of the
helical baffle design.
* Significant MTD improvement
was seen in the segmental baffle
design, where the negative impact
of leakage streams is compensated
by improvement in F factor to the
increase in the number of shells in
series from two to four.
* The increase on overall U and
MTD resulted in a reduction in
Crude/VR
PDT + quench
exchanger-III
AES
9.1234
VR + slop +
quench
Crude
4.9/7.81
0.002
0.000819
279.5/249.4
223.5/244.7
4.9
Crude/VR
PDT + quench
exchanger-II
AES
9.4985
VR + slop +
quench
Crude
4.36/6.42
0.002
0.000819
309.8/276.3
246.4/267.8
2.8
Crude/VR
PDT + quench
exchanger-I
AES
12.4146
VR + slop +
quench
Crude
2.94/4.36
0.002
0.000819
351.5/309.8
266.7/313.2
1.5
the effective surface area by 14%
compared to the helical baffle design.
* The capital cost with the helical
baffle will be significantly higher
than with the segmental baffle due
to a 14% increase in surface area and
proprietary heat exchangera
design.
Capital and installation cost:
It is evident that the capital and
installation cost of the four services
discussed here will be higher for
the proprietary heat exchanger
compared to conventional S&T
exchangers with a segmental baffle.
Maintenance cost: The required
time to reach the design fouling
resistance value will be more with a
helical baffle compared to segmental
baffle exchangers due to higher
shell-side velocity. Additionally, the
cleaning frequency for the specified
fouling factors will be less for helical
baffle exchangers. Therefore, the
maintenance cost will be lower
for the proprietary heat exchanger
compared to conventional S&T
exchangers with a segmental baffle.
Operating cost: The shell-side
calculated pressure drop for all four
services with segmental baffles are
Hydrocarbon Processing | FEBRUARY 2022 73
Hydrocarbon Processing - February 2022

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