International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Fired Heater Design
Mahesh N. Jethva1, C. G. Bhagchandani2
1
M.E., 2Associate 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.
II.
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.
Where,
= 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
Keywords- Convective heat transfer, Heat balance,
Radiant heat transfer, Shield section.
I.
R ADIANT SECTION DESIGN
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 (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.
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
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:
(
61
)
(
)
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Where,
= Convective heat transfer to radiant tubes,
Btu/hr
= Convective heat transfer to shield tubes,
Btu/hr
III.
Where the Reynolds number is,
And the Prandtl number is,
CONVECTION SECTION DESIGN
A. Overall Heat Transfer Coefficient,
:
Where,
= Heat transfer coefficient, liquid phase, Btu/hrft2-°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/hrft2-°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
For two-phase flow,
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,
= Heat transfer coefficient, two-phase, Btu/hr-ft2°F
= Weight fraction of liquid
= Weight fraction of vapor
( )
Where,
= Effective outside heat transfer coefficient,
Btu/hr-ft2-F
= Inside film heat transfer coefficient, Btu/hr-ft2F
= 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
C. Effective outside heat transfer coefficient (
Fin tubes:
Where,
= Average outside heat transfer coefficient,
Btu/hr-ft2-F
= Fin efficiency
= Total outside surface area, ft2/ft
= Fin outside surface area, ft2/ft
= Outside tube surface area, ft2/ft
B. Inside film heat transfer coefficient, :
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
≥10,000,
And for vapor flow with
for
i.
Average outside heat transfer coefficient,
:
Where,
= Outside heat transfer coefficient, Btu/hr-ft2-F
= Outside radiation heat transfer coefficient,
Btu/hr-ft2-F
= Outside fouling resistance, hr-ft2-F/Btu
≥15,000,
62
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
ii.
Outside film heat transfer coefficient,
Inline pattern,
:
(
Where,
=
=
=
=
=
iii.
)
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
iv.
Number of tube rows
Longitudinal tube pitch, in
Transverse tube pitch, in
Mass Velocity,
:
Colburn heat transfer factor, :
Where,
(
)
=
=
Where,
=
=
=
=
=
=
=
And,
Net Free Area,
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,
Mass flow rate of gas, lb/hr
Net free area, ft2
:
Where,
:
=
=
=
=
Cross sectional area of box, ft2
Fin tube cross sectional area/ft, ft2/ft
Effective tube length, ft
Number tubes wide
=
=
=
=
=
Fin height, ft
Outside diameter of tube, ft
Transverse tube pitch, ft
fin thickness, ft
number of fins, fins/ft
Where,
=
Reynolds number
=
Geometry correction, :
For segmented fin tubes arranged in, a staggered
pattern,
v.
an inline pattern,
Surface Area Calculations:
For the prime tube,
And for solid fins,
For solid fin tubes arranged in, a staggered pattern,
(
(
)
)
And for segmented fins,
an inline pattern,
(
)
((
)(
)
(
Where,
=
=
Fin height, in
Fin spacing, in
Where,
=
=
=
=
=
Non-equilateral & row correction, :
For fin tubes arranged in, Staggered pattern,
(
)
63
Outside diameter of tube, ft
number of fins, fins/ft
fin thickness, ft
Fin height, ft
Width of fin segment, ft
))
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Table I
Radiant Section Design
And then,
vi.
PROPERTY
Tube
Fin Efficiency, :
For segmented fins,
And for solid fins,
(
)
Where,
And,
Where,
Combustion
Firebox
Process fluid
For segmented fins,
(
)
Flue gas
And for solid fins,
vii.
α (Radiant)
α (Shield)
Acp (Radiant)
Acp (Shield)
αAcp (Radiant)
αAcp (Shield)
(αAcp)r+(αAcp)s
AR/
((αAcp)r+(αAcp)s)
Fin Tip Temperature, :
The average fin tip temperature is calculated as
follows,
(
)
Where,
=
=
IV.
Gas Temperature, F
Tube Wall Temperature, F
Partial pressure
Mean beam length
P*l
Emissivity
Exchange factor
Radiantion
Heat Transfer
EXCEL P ROGRAMMING
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.
64
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
Btu/hr
MM Kcal/hr
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
3.37*10^7
8.488
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3, March 2013)
Table II
Convection Section Design
PROPERTY
Fin
Tube
Process Fluid
Flue Gas
Inside Film HT co-
DETAIL
Height, in (lf)
Thickness, in (tf)
No of fins, fins/ft
(nf)
Ther. Cond., Btu/hrft-⁰F (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/lb-⁰F (cp,l)
Sp. Heat (Vap),
Btu/lb-⁰F (cp,v)
Viscosity (Liq),
lb/hr-ft (µl)
Viscosity (Vap),
lb/hr-ft (µv)
Mass flow rate,
lb/hr
Wt fraction (Liq)
(Wl)
Wt fraction (Vap)
(Wv)
Fouling factor,hrft2-⁰F/Btu (Rfi)
Inlet temp, ⁰F (t1)
Outlet temp, ⁰F (t2)
Mass flow rate,
lb/hr (Wg)
Ther. Cond., Btu/hrft-⁰F (kg)
Sp. Heat, Btu/lb-⁰F
(cp,g)
Viscosity, lb/hr-ft
(µg)
hi, Btu/hr-ft2-⁰F
efficient
Mass Velocity of
Flue Gas
Colburn HT Factor
Outside Film HT
co-efficient
Average Outside HT
co-efficient
Fin Efficiency
Effective Outside
HT co-efficient
Overall HT coefficient
LMTD
HT Area
Convection Heat
Transfer
AMOUNT
1
0.05118
60
21.292
8.626
0.5
5
4
18.307
16
959
12.83
Gn, lb/hr-ft2
1017.79
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
Table III
Heat Balance
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)
609.8
621.1
0.04939
0.11995
0.694
0.8985
0.31448
0.0508
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
1054905.3
V.
0.7
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.
0.3
0.00391
1472
788
42620.545
REFERENCES
[1]
[2]
[3]
0.0353
[4]
0.3087
[5]
0.0883
461.16
65
Process Heat Transfer by Donald Q. Kern,
http://www.heatexchangerdesign.com,
API 560, Fired Heaters for General Refinery Service, 4 th edition,
August 2007,
Chemical Process Equipment: Selection and Design by Stanley
M. Walas,
Paper by Asutosh Garg, Optimized Fired Heater saves Money.