Fired Heater Design and Simulation

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, =

+

−

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( (

.

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

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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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