International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Fired Heater Design and Simulation
Mahesh N. Jethva1, C. G. Bhagchandani2
1
2
M.E. Chemical Engineering Department, L.D. College of Engineering, Ahmedabad-380 015
Associate Professor, Chemical Engineering Department, L.D. College of Engineering, Ahmedabad-380 015
Abstract- In fired heaters, heat is released by combustion of fuels
into an open space and transferred to process fluids inside tubes.
The tubes are ranged along the walls and roof of the combustion
chamber. The heat is transferred by direct radiation and
convection and also by reflection from refractory walls lining the
chamber. The design and rating of a fired heater is a moderately
complex operation. Here forced draft fired heater, which is fired
by fuel gas, has been treated. For that all required equations and
generalizations are listed from different fired heater design
methods as per requirement. A fired heater design calculations
are performed using Microsoft Excel Programming software and
the same fired heater data are used in HTRI simulation software
for simulation and comparision purpose.
(rectangular c/s) or vertical (cylindrical c/s) in shape. Same
way, a fired heater may be classified depending on location of
the burners and type of the draft.
II. Radiant Section Design
A. Radiant Heat Transfer in Radiant Section:
Applying basic radiation concepts to process-type heater
design, Lobo & Evans developed a generally applicable rating
method that is followed with various modifications, by many
heater designers. Direct radiation in the radiant section of a
direct fired heater can be described by the equation shown
below.
Keywords- Radiant heat transfer, Convective heat transfer,
Shield section, Heat balance, HTRI simulation, Comparision.
=
ℱ(
−
)
Where,
=
=
I. Introduction
A fired heater is a direct-fired heat exchanger that uses the hot
gases of combustion to raise the temperature of a feed flowing
through coils of tubes aligned throughout the heater.
Depending on the use, these are also called furnaces or
process heaters. Some heaters simply deliver the feed at a
predetermined temperature to the next stage of the reaction
process; others perform reactions on the feed while it travels
through the tubes.
Fired heaters are used throughout hydrocarbon and chemical
processing industries such as refineries, gas plants,
petrochemicals, chemicals and synthetics, olefins, ammonia
and fertilizer plants. Most of the unit operations require one or
more fired heaters as start-up heater, fired reboiler, cracking
furnace, process heater, process heater vaporizer, crude oil
heater or reformer furnace.
Heater fuels include light ends (e.g. refinery gas) from the
crude units and reformers as well as waste gases blended with
natural gas. Residual fuels such as tar, pitch, and Bunker C
(heavy oil) are also used. Combustion air flow is regulated by
positioning the stack damper. Fuel to the burners is regulated
from exit feed temperature and firing rate is determined by the
level of production desired.
A typical fired heater will have following four sections: (1)
Radiant section, (2) Shield section, (3) Convection section,
and (4) Breeching and stack. A fired heater may be a box
ISSN: 2231-5381
ℱ
=
=
=
=
=
Radiant heat transfer, Btu/hr
Stefan-Boltzmann constant,
0.173E-8 Btu/ft2-hr-R4
Relative effectiveness factor of the tube bank
Cold plane area of the tube bank, ft2
Exchange factor
Effective gas temperature in firebox, °R
Average tube wall temperature, °R
B. Heat Balance In The Radiant Section:
There are four primary sources of heat input as well as four
sources of heat output to the radiant section. We can now set
up the heat balance equation as follows:
+
+
+
+
+
+
=
Where,
=
=
=
=
=
=
=
=
heat liberated by fuel, Btu/hr (LHV)
sensible heat of combustion air, Btu/hr
sensible heat of steam used for oil atomization,
Btu/hr
sensible heat of recirculated flue gases, Btu/hr
heat absorbed by radiant tubes, Btu/hr
Radiant heat to shield tubes, Btu/hr
heat loss in firebox through furnace walls,
bridgewall, casing, etc., Btu/hr
heat of flue gases leaving the radiant section,
Btu/hr
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
C. Total Heat Transfer in Radiant Section (if Shield Section
is present):
The total heat transfer in firebox when shield section is
present will be as follows:
= ( ∝
+
)ℱ(
−
ℎ = 0.023( )
)+
.
(
)
.
(
)
.
≥15,000,
And for vapor flow with
ℎ = 0.021( )
+
.
.
.
Where,
=
=
Convective heat transfer to radiant tubes, Btu/hr
Convective heat transfer to shield tubes, Btu/hr
Where the Reynolds number is,
=
III. Convection Section Design
A. Overall Heat Transfer Coefficient,
:
=
Where,
Overall heat transfer coefficient, Btu/hr-ft2-F
Total outside thermal resistance, hr-ft2-F/Btu
=
=
And,
=
+
+
Where,
=
=
=
Outside thermal resistance, hr-ft2-F/Btu
Tube wall thermal resistance, hr-ft2-F/Btu
Inside thermal resistance, hr-ft2-F/Btu
And the resistances are computed as,
=
Where,
=
ℎ
ℎ
=
=
=
=
=
=
=
)(
1
+
ℎ
Heat transfer coefficient, liquid phase, Btu/hr-ft2-°F
Thermal conductivity, Btu/hr-ft-°F
Inside diameter of tube, ft
Absolute viscosity at bulk temperature, lb/ft-hr
Absolute viscosity at wall temperature, lb/ft-hr
Heat transfer coefficient, vapor phase, Btu/hr-ft2-°F
Bulk temperature of vapor, °R
Wall Temperature of vapor, °R
Mass flow of fluid, lb/hr-ft2
Heat capacity of fluid at bulk temperature, Btu/lb-°F
ℎ
)
)(
)
Effective outside heat transfer coefficient, Btu/hrft2-F
Inside film heat transfer coefficient, Btu/hr-ft2-F
Tube-wall thickness, ft
Tube wall thermal conductivity, Btu/hr-ft-F
Outside tube surface area, ft2/ft
Mean area of tube wall, ft2/ft
Inside tube surface area, ft2/ft
Inside fouling resistance, hr-ft2-F/Btu
B. Inside film heat transfer coefficient, ℎ :
The inside film coefficient needed for the thermal calculations
may be estimated by several different methods. The API
RP530, Appendix C provides the following methods,
For liquid flow with
Where,
=
ℎ
=
=
=
=
=
ℎ
=
=
=
=
×
For two-phase flow,
1
ℎ
=(
=(
And the Prandtl number is,
1
=
×
Where,
=
ℎ
=
=
=ℎ
+ℎ
Heat transfer coefficient, two-phase, Btu/hr-ft2-°F
Weight fraction of liquid
Weight fraction of vapor
C. Effective outside heat transfer coefficient ( ℎ ) for Fin
tubes:
ℎ =ℎ
Where,
=
ℎ
=
=
=
=
(
+
)
Average outside heat transfer coefficient, Btu/hrft2-F
Fin efficiency
Total outside surface area, ft2/ft
Fin outside surface area, ft2/ft
Outside tube surface area, ft2/ft
i. Average outside heat transfer coefficient, ℎ :
≥10,000,
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
ℎ =
Where,
=
ℎ
=
ℎ
=
1
= 0.35 + 0.65
1
+
(ℎ + ℎ )
ii. Outside film heat transfer coefficient, ℎ :
(
)
)
an inline pattern,
Outside heat transfer coefficient, Btu/hr-ft2-F
Outside radiation heat transfer coefficient, Btu/hrft2-F
Outside fouling resistance, hr-ft2-F/Btu
ℎ =
.
(
= 0.20 + 0.65
)
Where,
=
=
Fin height, in
Fin spacing, in
Non-equilateral & row correction,
For fin tubes arranged in,
Staggered pattern,
.
.
(
:
Where,
=
=
=
=
=
Colburn heat transfer factor
Mass velocity based on net free area, lb/hr-ft2
Heat capacity, Btu/lb-F
Gas thermal conductivity, Btu/hr-ft-F
Gas dynamic viscosity, lb/hr-ft
= 0.7 + 0.7 − 0.8
Inline pattern,
= 1.1 − 0.75 − 1.5
.
(
+ 460
)
+ 460
=
=
=
iv. Mass Velocity,
Reynolds number correction
Geometry correction
Non-equilateral & row correction
Outside diameter of fin, in
Outside diameter of tube, in
Average gas temperature, F
Average fin temperature, F
Reynolds number correction,
:
= 0.25
×
=
:
For segmented fin tubes arranged in,
a staggered pattern,
= 0.55 + 0.45
(
.
)
an inline pattern,
= 0.35 + 0.50
(
.
)
)
(
.
)
.
:
=
Where,
=
=
And,
Net Free Area,
Mass flow rate of gas, lb/hr
Net free area, ft2
:
−(
)
Where,
Reynolds number
Geometry correction,
(
Number of tube rows
Longitudinal tube pitch, in
Transverse tube pitch, in
=
.
Where,
=
(
)
.
Where,
=
=
=
=
=
=
=
.
Where,
iii. Colburn heat transfer factor, :
=
(
.
=
=
=
=
Cross sectional area of box, ft2
Fin tube cross sectional area/ft, ft2/ft
Effective tube length, ft
Number tubes wide
=
=
+2
=
=
=
=
=
Fin height, ft
Outside diameter of tube, ft
Transverse tube pitch, ft
fin thickness, ft
number of fins, fins/ft
)
v. Surface Area Calculations:
For the prime tube,
For solid fin tubes arranged in,
a staggered pattern,
=
(1 −
)
And for solid fins,
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Where,
=
(1 −
)+
(2
+
+
+2
=
=
)
Gas Temperature, F
Tube Wall Temperature, F
And for segmented fins,
=
(
+
1−
+ 0.2)
(
+ 0.4
2 − 0.4
+ 0.2)
+
IV. Excel Programming
+
Design of different sections of fired heater has been
performed using Microsoft Excel Programming. For the
calculation purpose, different calculation methods and
equations are used in the programming.
Where,
=
=
=
=
=
=
Outside diameter of tube, ft
number of fins, fins/ft
fin thickness, ft
Fin height, ft
?
Width of fin segment, ft
Table 1 Radiant Section Design
PROPERTY
Tube
And then,
=
−
vi. Fin Efficiency, :
For segmented fins,
= (0.9 + 0.1 )
And for solid fins,
( − 1) + 1)
= (0.45 ln
Where,
Combustion
Firebox
Process fluid
= (0.7 + 0.3 )
And,
tanh(
=
)
Flue gas
Where,
=
α (Radiant)
α (Shield)
Acp (Radiant)
Acp (Shield)
αAcp (Radiant)
αAcp (Shield)
(αAcp)r+(αAcp)s
AR/
((αAcp)r+(αAcp)s)
+( )
2
For segmented fins,
ℎ
=(
+
)
6
.
And for solid fins,
=(
6
ℎ
)
.
vii. Fin Tip Temperature, :
The average fin tip temperature is calculated as follows,
=
+
−
ISSN: 2231-5381
(
(
.
1
+
2
.
)
)
Partial pressure
Mean beam length
P*l
Emissivity
Exchange factor
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DETAIL
OD, in (do)
thickness, in (tw)
No of tubes (Nt)
(Radiant)
No of tubes (Nt)
(Shield)
Effective length, ft
(Le) (Radiant)
Effective length, ft
(Le) (Shield)
Tube spacing, in (CC)
(Radiant)
No of tubes per row
(Nt/r) (Shield)
Transverse pitch, in
(Pt) (Shield)
Fraction excess air
Diameter, ft (D)
Mean wall
temperature, (Tt), ⁰R
Flue gas temperature
(Tg), ⁰R
(-)
Assumed (-)
ft2
ft2
ft2
ft2
ft2
AT, ft2
Area of Shield
Section, ft2 (As)
AR, ft2
AR/
((αAcp)r+(αAcp)s)
atm (P)
ft
atm-ft
E
F
AMOUNT
8.626
0.05118
40
12
35.07
18.31
16
4
16
0.15
19.98
1097.95
2077.1
0.9086
1
1870.52
97.64
1699.51
97.64
1797.15
2103.81
97.64
306.66
0.17
0.256
13.32
3.406
0.5087
0.5129
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
Radiantion
Heat Transfer
Btu/hr
MM Kcal/hr
3.37*10^7
8.488
Table 2 Convection Section Design
PROPERTY
Fin
Tube
Process Fluid
Flue Gas
Inside Film HT coefficient
Mass Velocity of Flue
ISSN: 2231-5381
DETAIL
Height, in (lf)
Thickness, in (tf)
No of fins, fins/ft (nf)
Ther. Cond., Btu/hr-ft⁰F (kf)
OD, in (do)
Thickness, in (tw)
No of rows (Nr)
No of tubes per row
(Nw)
Effective tube length,
ft (Le)
Pitch, in (Pt)
Wall temp, ⁰F (Tw)
Wall Ther. Cond.,
Btu/hr-ft-⁰F (kw)
Inlet temp, ⁰F (t1)
Outlet temp, ⁰F (t2)
Ther. Cond., (Liq),
Btu/hr-ft-⁰F (kl)
Ther. Cond. (Vap),
Btu/hr-ft-⁰F (kv)
Sp. Heat (Liq), Btu/lb⁰F (cp,l)
Sp. Heat (Vap),
Btu/lb-⁰F (cp,v)
Viscosity (Liq), lb/hrft (µl)
Viscosity (Vap), lb/hrft (µv)
Mass flow rate, lb/hr
Wt fraction (Liq) (Wl)
Wt fraction (Vap)
(Wv)
Fouling factor,hr-ft2⁰F/Btu (Rfi)
Inlet temp, ⁰F (t1)
Outlet temp, ⁰F (t2)
Mass flow rate, lb/hr
(Wg)
Ther. Cond., Btu/hr-ft⁰F (kg)
Sp. Heat, Btu/lb-⁰F
(cp,g)
Viscosity, lb/hr-ft (µg)
hi, Btu/hr-ft2-⁰F
AMOUNT
1
0.05118
60
21.292
Gn, lb/hr-ft2
1017.79
8.626
0.5
5
4
Gas
Colburn HT Factor
Outside Film HT coefficient
Average Outside HT
co-efficient
Fin Efficiency
Effective Outside HT
co-efficient
Overall HT coefficient
LMTD
HT Area
Convection Heat
Transfer
j
hc, Btu/hr-ft2-⁰F
0.00543
2.0291
ho, Btu/hr-ft2-⁰F
2.599
E
he, Btu/hr-ft2-⁰F
0.9838
2.5595
Uo, Btu/hr-ft2-⁰F
1.9348
⁰F
ft2
Btu/hr
MM Kcal/hr
430.28
10102.93
8.4*10^6
2.119
18.307
Table 3 Heat Balance
16
959
12.83
609.8
621.1
0.04939
0.11995
0.694
0.8985
0.31448
PROPERTY
Assumed amount of
Radiant HT
Assumed amount of
Convection HT
Thermal Efficiency
Total Heat Input (Qfuel)
Total Heat Transferred
(Qht)
Radiant HT (Qr)
Convection HT (Qc)
Heat Loss (Qloss)
Heat out from HT area to
stack (Qstack)
%
DETAIL
AMOUNT
80
%
20
% (given)
MM Kcal/hr
MM Kcal/hr
(given)
MM Kcal/hr
MM Kcal/hr
MM Kcal/hr (2.5%
of Qfuel)
MM Kcal/hr
(=Qfuel-Qht-Qloss)
90.7
11.70
10.61
8.488
2.122
0.2924
0.7955
0.0508
V. HTRI Introduction
1054905.3
0.7
0.3
0.00391
1472
788
42620.545
HTRI Xchanger Suite® 6.0 combines in a single graphical user
environment the design, rating, and simulation of fired heaters
(Xfh®). Xfh simulates the behavior of fired heaters. The
program calculates the performance of the radiant section for
cylindrical and box (cabin) heaters and the convection section
of fired heater. It also designs process heater tubes using API
530 and performs combustion calculations. Xfh contains
different calculation modules to simulate the different parts of
a fired heater. One can run these modules separately or in
combination to model part or all of a fired heater.
0.0353
0.3087
VI. Comparision of given/calculated data and
simulated data
0.0883
461.16
The following table of comparision between given or
calculated data or results and simulated results proves that the
prepared design module is trustable tool for fired heater
design.
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International Journal of Engineering Trends and Technology- Volume4Issue2- 2013
fired heater design and simulation has been performed in
satisfactory way.
Table 4 Comparision of given/calculated data and
simulated data
PROPERTY
DETAIL
CAL.
SIMU.
DATA
DATA
Overall Performance
Heat duty
MM
10.61
10.73
kcal/hr
Efficiency (LHV)
%
90.7
85
Heat release (Total)
MM
11.69
12.63
kcal/hr
Fuel LHV
kcal/kg
13260
13278.1
Process fluid temp at C
327.28
327.88
crossover
Process fluid temp at C
346
346.64
heater outlet
Radiant Section
Fuel gas temp out
C
800
858.71
Average flux rate
kcal/hr-m2 29000
25611.2
Duty
MM
8.488
7.987
kcal/hr
Surface area
m2
166.96
311.87
Pressure drop
kgf/cm2
1.292
1.75
Convection Section
Fuel gas temp out
C
420
381.56
Outside
film kcal/hr12.5
17.63
coefficient
m2-C
Inside film coefficient kcal/hr2251.58
1854.99
m2-C
Overall
HT kcal/hr9.45
12.7
coefficient (U)
m2-C
Convection duty
MM
2.122
2.7424
kcal/hr
Surface area
m2
938.59
985.27
EMTD
C
221.27
220.4
Draft at bridgewall
mm H2O
2.3043
2.54
Pressure drop
kgf/cm2
0.58
0.547
Burners
Fuel rate
kg/hr
882.35
855.2
VII.
References
[1]
[2]
[3]
[4]
[5]
Process Heat Transfer by Donald Q. Kern,
http://www.heatexchangerdesign.com,
API 560, Fired Heaters for General Refinery Service, 4th edition, August
2007,
HTRI Xchanger Suite 6.0 software,
HTRI Manual and Help file
Conclusion
Using Microsoft Excel Programming software, a design
module has been prepared which can be used for different data
values and gives satisfactory results. In present case, the
design module gives required radiant heat transfer and
convective heat transfer in the fired heater. The specified fired
heater is also simulated in HTRI heat exchanger suite 6.0
using the same fired heater data which are used in MS Excel
design module. The table of comparision illustrates that the
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