Hydrocarbon Processing - February 2022 - 74

Heat Transfer
TABLE 3. Size and performance data for both helical and single segmental baffle designs
Parameters
Baffle type
Shell ID, mm
Tube length, mm
No. of shells, S / P
No. of passes
Tube OD, mm
Tube layout, degrees
Baffle pitch, mm1
Baffle cut, %
Surface area, m2
MTD, °C
Calculated DP (S/S), kg/cm2
Avg. velocity, m/sec
Heat transfer co-efficient
(S/S), m2
-hr-°C/Kcal
Overall U, m2-hr-°C/Kcal
Thermal resistance:
shell, % of total
Thermal resistance:
fouling, % of total
174.7
33.6
48.5
1 Baffle pitch assumed for proprietary heat exchangera
the same-if not lower in some
cases-than those with helical
baffles. Therefore, the operating
cost (which is related to the
pressure drop) of these exchangers
with a segmental baffle will be
same, if not lower, compared to
those with a helical baffle.
Total lifecycle cost: As noted
here, all components of the
total lifecycle cost-excluding
maintenance cost-are lower for
the conventional S&T exchanger
with segmental baffle option
compared to the proprietary heat
exchanger option for the four
services considered here. Since
a lower maintenance cost will be
more than compensated by the
higher capital, installation and
operating costs of the proprietary
heat exchanger, compared to a
conventional S&T exchanger,
the proprietary heat exchangera
option will be unviable from a total
lifecycle cost perspective.
Total lifecycle cost
optimization: Since the average
shell-side velocity with a helical
baffle is about 1.5-2 times the
74 FEBRUARY 2022 | HydrocarbonProcessing.com
167.5
37.5
46.7
171.3
27.2
53.3
176.8
27.4
53.9
179.4
29.5
56.3
2 Baffle cut is not applicable for proprietary heat exchangera
velocity with a segmental baffle,
the proprietary heat exchanger will
have a lower propensity for shellside
fouling. With a fouling factor
much lower than 0.002 Kcal/m2
-
hr-°C, which is more appropriate
for a segmental baffled exchanger,
the size of the proprietary heat
exchanger can be considered. Since
the overall size-and, therefore, the
capital and installation costs-are
dependent on additional surface
area required to overcome fouling
(all four sizes are controlled by
fouling, as explained above), a lower
fouling factor would have resulted
in a much lower surface area for
the proprietary heat exchanger.
This would have resulted in a lower
capital cost, making the proprietary
heat exchangera
option viable in
terms of the overall lifecycle cost.
Takeaway. While selecting the proprietary
heat exchanger, the inherent advantage
of higher shell-side velocity with
a helical baffle must be considered while
specifying the shell-side fouling factor. A
conservative approach of using the same
fouling factor as a conventional segmental
baffled exchanger can make the proprietary
heat exchanger option unviable.
While selecting an alternate option of
a conventional S&T exchanger with a segmental
baffle, careful shell arrangement selection
is vital for multiple shell exchangers,
tube and baffle geometries to optimize
the use of allowable pressure drop and
make the exchanger more compact.
The total lifecycle cost, rather than
only CAPEX, must be considered by
owner-operators and contractors during
front-end engineering and design
(FEED) when selecting the best heat exchanger
option for a particular service.
NOTES
a Lummus Technology's HELIXCHANGER™
b HTRI Xist
RITABRATA PRAMANIK has more than 35 yr of
experience in the design and engineering of heat
transfer equipment in the oil and gas, petrochemical,
LNG, fertilizer, chemical and power sectors. He was
a Fluor global Subject Matter Expert (SME) in heat
exchanger thermal design and retired as a Design
Director from the Fluor New Delhi office.
N. R. SRINATH has more than 14 yr of experience
in the design and engineering of heat transfer
equipment in the energy and chemical sectors.
He began his career with Foster Wheeler India in
2007 and has worked at Fluor New Delhi since 2013.
176.3
32.9
56.7
157.1
32.6
49.4
171.7
28.9
52.4
Crude/VR PDT
exchanger
Helical
1,250
6,000
2 × 1
2
25
90
205
Note 2
808.6
31.6
2.2
1.5
519
Single segment
1,180
7,000
2 × 1
2
25
45
250
25.6
858.5
31.4
1.8
0.74
447
Crude/VR PDT +
quench exchanger-III
Helical
1,320
6,000
4 × 1
2
25
90
260
Note 2
1,775.6
30
4.9
1.8
643
Single segment
1,280
6,500
4 × 1
2
25
45
275
25
1,773.4
29.9
4.7
0.97
645
Crude/VR PDT +
quench xchanger-II
Helical
1,480
8,000
2 × 1
4
25
90
255
Note 2
1,521.6
34.8
2.8
1.5
626
Single segment
1,480
8,500
2 × 1
4
25
45
300
22.5
1,578.7
34.5
1.6
0.63
535
Crude/VR PDT +
quench exchanger-I
Helical
1,400
8,000
2 × 2
4
25
90
205
Note 2
2,763.6
28.6
1.5
0.87
497
Single segment
1,380
7,315
4 × 1
2
25
45
446
25.4
2,433
29.8
1.5
0.51
593.5
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