Hydrocarbon Processing - August 2021 - 74

Heat Transfer
* FT factor = 0.99
° Maximum tube skin temperature, °R = 1,231
° Mean tube skin temperature, °R = 1,218
° Average flue gas temperature, °R = 1,860
° Use Eq. 2 specified above.
The maximum/peak local heat flux = 1.8 × 1.6 × 0.99 ×
7,800 = 22,240 Btu/hr/ft2
(TABLE 2).
It is evident from the above case study that there is insignificant
impact on the FT factor (0.99 vs.1) and peak heat
flux (22,240 Btu/hr/ft2
vs. 22,464 Btu/hr/ft2) and subsequent
TMT calculation. This will be true for heaters where
the process fluid temperature variation from the radiant inlet
to outlet is small. In such cases, an FT
able assumption.
factor of 1 is a reasonTABLE
2. Comparison of radiant peak fl ux in a product
fractionator reboiler
1
Process parameter
Avg. radiant heat fl ux, Btu/hr/ft2
FC
FL
FT
Peak heat fl ux, Btu/hr/ft2
Max. tube metal temp.
(outlet tube), °F
FT = 0.99
7,800
1.8
1.6
0.99
22,240
840
FT
= 1
2
7,800
1.8
1.6
1
22,464
841
3
4
TABLE 3 shows a comparison of the impact of the FT
the two aforementioned heaters.
factor in
Takeaway. This work has demonstrated the criticality of
the FT
factor in fired heater design with case studies covering
two extreme cases. It is worth noting that the overall process
temperature difference in Case 1 was 240°C while it was only
19°C in Case 2. It is evident that the larger the difference in
inlet and outlet temperatures across the heater, the lower the
FT
factor and the higher the consequent impact on peak heat
factor based on the service and
flux calculation in the radiant section. It is for this reason that
a vigilant selection of the FT
operating conditions will encompass a significant reduction in
heat transfer area and better operational flexibility, resulting in
a more competitive design.
LITERATURE CITED
American Petroleum Institute (API) 530, " Calculation of heater-tube thickness in
petroleum refineries, " 7th Ed., April 2015.
Barletta, T., " Why vacuum unit fired heaters coke, " Process Consulting Services
Inc., Houston, Texas, October 2004.
Niccum, G. and S. White, " Realities of heat flux in fired heaters, " Process
Consulting Services, Houston, Texas, November 2020.
Bernhagen, P., " Coker heater design: The heart of the coking process, " Fired heater
division, Foster Wheeler USA Corp., June 2019.
KAPIL BATRA works as Deputy Manager in the heat transfer
department at Engineers India Ltd. in New Delhi. Mr. Batra
has a progressive work experience of 8 yr in fired heaters and
combustion systems. He is a chemical engineering graduate
from the Laxminarayan Institute of Technology (LIT)-Nagpur.
NEW VERSION
InstruCalc
CONTROL VALVES * FLOW ELEMENTS * RELIEF DEVICES * PROCESS DATA
InstruCalc 9.0 calculates the size of control valves, fl ow
elements and relief devices and calculates fl uid properties,
pipe pressure loss and liquid waterhammer fl ow. Easy to
use and accurate, it is the only sizing program you need,
enabling you to: size more than 50 diff erent instruments;
calculate process data at fl ow conditions for 54 fl uids,
in either mixtures or single components, and 66 gases;
and calculate the orifi ce size, fl owrate or diff erential
range, which enables the user to select the fl owrate with
optimum accuracy.
Updates include Engineering Standard
Upgrades and Operational Improvements
in InstruCalc Version 9.0
Please contact J'Nette Davis-Nichols
for more information at
Jnette.Davis-Nichols@GulfEnergyInfo.com
NAVNEET AGARWAL works as Deputy General Manager in the
heat transfer department at Engineers India Ltd. in New Delhi.
He has more than 22 yr of experience in the fields of heat and
mass transfer equipment. His work focuses primarily on fired
heater design, revamp and troubleshooting for the hydrocarbon
industry and manages a team of engineers working on fired
heaters and combustion systems. Mr. Agarwal earned a B.Tech
degree in chemical engineering from the Indian Institute of Technology, BHU.
TABLE 3. Comparison of process parameters in a hydrogen
heater and product fractionator reboiler
Process parameter
Radiant section inlet temperature, °C
Radiant section outlet temperature, °C
Maximum tube skin temperature, °R
Mean tube skin temperature, °R
Average fl ue gas temperature, °R
FT
FC
FL, calculated based on
heater dimensions
Average radiant heat fl ux, Btu/hr/ft2
Radiant peak heat fl ux, Btu/hr/ft2
Diff erential peak radiant heat fl ux
with FT
impact, Btu/hr/ft2
Decrease in peak heat fl ux due to
FT
factor at radiant outlet, %
74 AUGUST 2021 | HydrocarbonProcessing.com
25.6
Negligible
Hydrogen
heater
326
566
1,751
1,670
1,938
0.744
1.2
(double-fi red)
1.4
7,800
9,750
3,355
Fractionator
reboiler
330
349
1,231
1,218
1,860
0.99
1.8
(single-fi red)
1.6
7,800
22,240
Negligible
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