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THERMAL DESIGN OF
WITH HIGH POUR POINT FLUID
. .
Consultant, Equipment Engineering (Heat Exchangers)Foster Wheeler, Chennai, India
ec n ca resen a on
13th
HTRI CC Meet, Chennai, India14th February 2011
2011 Foster Wheeler. All rights reserved.
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To develop Thermal Designas per HTRI program for
Air Cooled Heat Exchanger Unitfor fuel oil cooling having
high pour point, with an aim to illustrate the use of a simple
practical idea avoiding costly appurtenance.
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Fuel oil conditions
Fluidname HighSulphurFuelOil(HSFO)
Parameter Units Case1 Case2
.
(allliquid)
.
(allliquid)
Temperature in/out DegC 131/84 129/65
*
Allowable pressuredrop
Kg/cm*2
.
3.5
.
2.5
Specific heat in/out Kcal/kgC 0.47/0.42 0.46/0.42
. . .
Thermalconductivity in/out Kcal/mhrC 0.09/0.1 0.09/0.1
Density
in/out Kg/m*3 973.4
/989 904
/923
Liq.surfacetension in/out Dyne /cm 30.7/33.7 27.5/32
Foulingresistance C hrm*2/Kcal 0.001 0.001
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ourpo n eg
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Air side conditions
Forced draft fan arrangement.
Design ambient air temperature = 42 C (Summer)
Minimum air temperature = 11 C (Winter)
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Design compliance: Standards & Specifications
Design to comply with requirements of
API 661 / ISO 13706
and relevant project specifications
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Design compliance: Minimum tube wall temperature API 661
As per API 661, when fluids with high pour points are handled, theminimum tube wall temperature attained during the operation shall bemaintained at or above the specified minimum tube wall temperature.
The specified minimum tube wall temperature shall be 14 C abovethe pour point of the fluid handled. Refer Table C.2.7 Table C1 ofAPI- 661 category 5.
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HTRI Xace, program
Design modelling is done in HTRI Xace, for both the given cases atboth summer and winter ambient air conditions.
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Salient features of the modelling
Bare tubes are used instead of conventional high fin tubes, as finned
surface did not help in improving heat transfer co-efficient of highv scous u .
A combination of bare tube & finned tubes also did not provideimprovement.
Hot air recirculation is not used, as suitable temperature control isobtained with tube side fluid flow reversal (co-current flow) in winter,without enhancin the set oint of desi n ambient air tem erature.
Counter-current air flow in summer and co-current air flow in winter isused to control minimum tube wall temperature in winter within API
For proper operation in both cases, a bay is isolated from operationin one case.
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Salient features of the modelling
A distributed pass-type tube layout (two passes in a row) is used tohave even flow with proper velocity (at least 0.3 m/s) in tubes of highviscous fluid.
Tube inserts are not used since proprietary devices are not preferredby the user.
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Controlling case
The controlling Case is Case 2 with higher heat duty.
However Case 1 is found to be pressure drop controlling.
Design performed for Case 2 with suitable geometry & configurationand then it is checked with Case 1 for passing.
Final design is one that passed both the cases with same geometryand configuration with required air flow control for each case.
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Performance parameters
Parameter Units Case1 Case2
Flowrate Kg/hr Summer Winter Summer Winter
Designambient
air
Deg
C 42 11 42 11
empera ure
Flowpattern CounterCurrent
Co-
Current
Counter
Current
Co-
Current
Heatdut MM 4.138 5.572
Kcal/hr
No.ofBays(with2bundles) 4 5
Airfacevelocity m/s 0.85 0.42 2.5 0.69
Totalairflow 1000Kg/hr 955.6 467.2 3491 971.75
Mintubemetaltemp(HTRI
finalresults
line
8
DegC 59.2 65.6 48.03 46.8
Allowablemin.tubemetal
temperature
DegC 49 42
(AsperAPI661=Pourpoint+14C)
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H2O
. . . .
Tubepressuredrop Kg/Cm*2 3.0 1.16 1.495 1.511
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HTRI Minimum tube wall temperature Summer, Counter currentna es gn
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A
partial
screen
shot
of
HTRI
output
of
the
case.
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HTRI Minimum tube wall temperature Winter, Counter current
e ow e
specified
minimum
.
Thedesignisnot
acceptable
2011FosterWheeler.Allrightsreserved.
Apartial
screen
shot
of
HTRI
output
of
the
case.
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HTRI Minimum tube wall temperature Winter, Co-current
Final Results
Released to the following HTRI Member Company:
Foster Wheeler Energy Ltd
FWEL
. .
HTRI CC 13
ACHE HIGH POUR POINT. CASE 2 WINTER(CO-CURRENT)
Simulation-Horizontal air-cooled heat exchanger forced draft co-current to cross flow
1 Process Data Airside Tubeside
u name
3 Fluid condition Sens. Gas Sens. Liquid4 Total flow rate (1000-kg/hr) 971.753 198.000 *
5 Weight fraction vapor, In/Out (--) 1.000 1.000 0.000 0.000
6 Temperature, In/Out (Deg C) 11.00 34.88 129.00 65.00
, . . . .
8 Wall temperature, Min/Max (Deg C) 46.73 83.42 46.80 83.77
9 Pressure, In/Out (kgf/cm2A) 1.033 1.033 6.033 4.523
10 Pressure drop, Total/Allowed (mmH2O) (kgf/cm2) 1.508 0.000 1.511 2.500
11 Pressure Drop, A-frame reflux section (kgf/cm2)- . .
13 - In/Out (m/s) 0.31 0.31
14 Film coefficient, Bare/Extended (kcal/m2-hr-C) 28.39 28.39 32.39
15 Mole fraction inert (--)
16 Heat transfer safety factor (--) 1 1
17 Fouling resistance (m2-hr-C/kcal) 0.000000 0.001000
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ApartialscreenshotofHTRIoutputofthecase.
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Bundle configuration: Pass arrangement
Passes
1
Tubesidein
VentRows
3 3
66
4
7
5
2
3
4
510 9
7 810
SPLITBUNDLESDrain
Vent
A
7
6
8
9
12
13
11
14
15
18
11
14
15
18
16
17
TubesideoutDrain
1919 20
ViewA
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Bundle
is
split
into
two,
due
to
weight
limitations
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Other accessories for operation
A steam coil and outlet louvres are suggested so that the bundlecan be warmed for start-up and to unfreeze if there is congealingin the tubes.
Variable Frequency Drive (VFD) control of air flow for processoutlet temperature.
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Schematic arrangement of the Air Cooled Heat Exchanger
ALL DIMENSIONS IN mm
T1 V
OUTLET
LOUVRES
BUNDLES
T2
D
(COLUMN CENTRES)
HTRI Model
Apartialscreenshotof
HTRIoutput
.
(COLUMN
OneBay
neB
undle
CENTRES)O
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Conclusion
The final design is an optimised unit which avoided the need ofcostly boxed-up hot air recirculation cabins.
It is recommended that for every case of high pour point fluid
cooling, counter current / co- current design shall be examinedfirst before deciding the need of hot air recirculation.
When the pour point is high enough, and not able to obtain acontrol of minimum tube wall temperature as per API 661, hot air
recirculation must be used.
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Thank ou.Any questions?
www.fwc.com
V.DWARAKAKRISHNA
Consultant,Equipment
Engineering
(Heat
Exchangers)
FosterWheeler,Chennai,Indiadwarakakrishna_v@fwuk.fwc.com
2011 Foster Wheeler. All rights reserved.