Hydrocarbon Processing - August 2021 - 73

Heat Transfer
° Estimated based on firebox dimension as per internal
design practices
* FT factor = 0.744
° Maximum tube skin temperature, °R = 1,751
° Mean tube skin temperature, °R = 1,670
° Average flue gas temperature, °R= 1,938
° Use Eq. 2 specified above.
The maximum/peak local heat flux (shown in FIGS. 4 and 5)
= 1.2 × 1.4 × 0.744 × 7,800 = 9,750 Btu/hr/ft2
.
However, if in the above case a conventional value of FT
factor
= 1 is considered, then the calculated peak flux would increase
to 1.2 × 1.4 × 1 × 7,800 = 13,100 Btu/hr/ft2
when compared to an FT
tional approach (FT
. Comparing
the peak flux values for both options (TABLE 1), it is evident
that the FT
factor of 0.744 lowers the peak flux by almost 25.6%
factor of 1. Further, with the convenfactor
of 1), the peak flux will lead to significantly
higher TMT, resulting in high tube thickness. The furnace
designer will be forced to consider a lower average radiant
heat flux to maintain the peak heat flux constant to keep TMT
within range for the selected tube thickness.
For this condition (FT
be designed for 5,800 Btu/hr/ft2
heater has an inlet and outlet temperature of 330°C (626°F)
and 349°C (660°F), respectively. The heater is designed for
an average allowable radiant heat flux of 7,800 Btu/hr/ft2
for
single-fired arrangement. The tube size is 8.625-in. OD and
tubes are placed at a 2D center-center spacing. The tubes are
placed vertically along the walls of a vertical cylindrical furnace.
The weld-weld tube length is 60 ft. The outlet temperature
is considered as the bulk fluid temperature for calculation.
The peak flux for this heater is estimated as:
* FC
factor = 1.8
° Tubes are placed in 2D spacing in a
single-fired arrangement
° Tube OD/tube spacing = 16/8.625 = 1.9
° Refer API 530 Figure B.1 to calculate FC
* FL factor = 1.6
° Estimated based on firebox dimension
as per internal design practices
factor of 1), the heater is estimated to
average radiant heat flux. This
would lead to almost a 34% extra heat transfer area compared to
the previous option. This increase in area can result in increasing
the heater cost by ~10%, limiting heater turndown, and occupying
a larger plot space. Therefore, the careful selection of
FT
factor is emphasized, as it is critical in optimizing the size and
cost of a heater system for such services.
Case study 2. This case considers a product fractionator reboiler
heater (FIGS. 6 and 7) in a hydroprocessing unit. The
15,950
14,950
13,950
12,950
11,950
10,950
9,950
8,950
From radiant inlet tube to outlet tube
FIG. 4. Hydrogen heater: Peak radiant heat flux profile.
1,300
1,250
1,200
1,150
1,100
1,050
1,000
From radiant inlet tube to outlet tube
FIG. 5. Hydrogen heater: Radiant box TMT profile.
Process parameter
Fc
FL
Peak heat flux, Btu/hr/ft2
Max. tube metal temp.
(outlet tube),°F
Extra heat transfer area
compared to Option 1, %
24,000
23,000
22,000
21,000
20,000
From radiant inlet tube to outlet tube
FIG. 6. PFR heater: Peak radiant heat flux profile.
1,000
950
850
900
800
750
700
From radiant inlet tube to outlet tube
FIG. 7. PFR heater: Radiant box TMT profile.
Hydrocarbon Processing | AUGUST 2021 73
TABLE 1. Comparison of radiant peak flux in a hydrogen heater
FT = 0.744 (Option 1) FT
Average radiant heat flux,
Btu/hr/ft2
7,800
1.2
1.4
9,750
1,275
-
1.2
1.4
9,744
1,275
34.5%
= 1 (Option 2)
5,800
as 1.8
Tube metal temperature, °F
Peak radiant heat flux, Btu/hr-ft2
Tube metal temperature, °F
Peak radiant heat flux, Btu/hr-ft2
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