Fired Heater Design and Simulation

Transcription

1 Fired Heater Design and Simulation Mahesh N. Jethva 1, C. G. Bhagchandani 2 1 M.E. Chemical Engineering Department, L.D. College of Engineering, Ahmedabad Associate Professor, Chemical Engineering Department, L.D. College of Engineering, Ahmedabad 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. Keywords- Radiant heat transfer, Convective heat transfer, Shield section, Heat balance, HTRI simulation, Comparision. 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 (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. Q = σαa F(T T Q = Radiant heat transfer, Btu/hr σ = Stefan-Boltzmann constant, 0.173E-8 Btu/ft 2 -hr-r 4 α = Relative effectiveness factor of the tube bank A = Cold plane area of the tube bank, ft 2 F = Exchange factor T = Effective gas temperature in firebox, R T = 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: Q + Q + Q + Q = Q + Q + Q + Q Q = heat liberated by fuel, Btu/hr (LHV Q = sensible heat of combustion air, Btu/hr Q = sensible heat of steam used for oil atomization, Btu/hr Q = sensible heat of recirculated flue gases, Btu/hr Q = heat absorbed by radiant tubes, Btu/hr Q = Radiant heat to shield tubes, Btu/hr Q = heat loss in firebox through furnace walls, bridgewall, casing, etc., Btu/hr Q = heat of flue gases leaving the radiant section, Btu/hr ISSN: Page 159

2 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: Q = σ( A + σ αa F(T T + Q + Q Q = Convective heat transfer to radiant tubes, Btu/hr Q = Convective heat transfer to shield tubes, Btu/hr III. Convection Section Design A. Overall Heat Transfer Coefficient, U : U = 1 R U = Overall heat transfer coefficient, Btu/hr-ft 2 -F R = Total outside thermal resistance, hr-ft 2 -F/Btu And, R = R + R + R R = Outside thermal resistance, hr-ft 2 -F/Btu R = Tube wall thermal resistance, hr-ft 2 -F/Btu R = Inside thermal resistance, hr-ft 2 -F/Btu And the resistances are computed as, R = 1 h R = ( t k ( A A R = ( 1 + R h ( A A h = Effective outside heat transfer coefficient, Btu/hrft -F h = Inside film heat transfer coefficient, Btu/hr-ft 2 -F t = Tube-wall thickness, ft k = Tube wall thermal conductivity, Btu/hr-ft-F A = Outside tube surface area, ft 2 /ft A = Mean area of tube wall, ft 2 /ft A = Inside tube surface area, ft 2 /ft R = Inside fouling resistance, hr-ft 2 -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, h = 0.023( k d R. P. ( μ μ. And for vapor flow with R 15,000, h = 0.021( k d R. P. ( T T. Where the Reynolds number is, And the Prandtl number is, R = d G μ P = C μ k h = Heat transfer coefficient, liquid phase, Btu/hr-ft 2 - F k = Thermal conductivity, Btu/hr-ft- F d = Inside diameter of tube, ft μ = Absolute viscosity at bulk temperature, lb/ft-hr μ = Absolute viscosity at wall temperature, lb/ft-hr h = Heat transfer coefficient, vapor phase, Btu/hr-ft 2 - F T = Bulk temperature of vapor, R T = Wall Temperature of vapor, R G = Mass flow of fluid, lb/hr-ft 2 C = Heat capacity of fluid at bulk temperature, Btu/lb- F For two-phase flow, h = h W + h W h = Heat transfer coefficient, two-phase, Btu/hr-ft 2 - F W = Weight fraction of liquid W = Weight fraction of vapor C. Effective outside heat transfer coefficient (h for Fin tubes: (EA + A h = h A h = Average outside heat transfer coefficient, Btu/hrft -F E = Fin efficiency A = Total outside surface area, ft 2 /ft A = Fin outside surface area, ft 2 /ft A = Outside tube surface area, ft 2 /ft i. Average outside heat transfer coefficient, h : For liquid flow with R 10,000, ISSN: Page 160

3 1 h = 1 (h + h + R h = Outside heat transfer coefficient, Btu/hr-ft 2 -F h = Outside radiation heat transfer coefficient, Btu/hrft -F R = Outside fouling resistance, hr-ft 2 -F/Btu ii. Outside film heat transfer coefficient, h : h = jg c ( k. c μ j = Colburn heat transfer factor G = Mass velocity based on net free area, lb/hr-ft 2 c = Heat capacity, Btu/lb-F k = Gas thermal conductivity, Btu/hr-ft-F μ = Gas dynamic viscosity, lb/hr-ft iii. Colburn heat transfer factor, j: j = C C C d. ( T d T C = Reynolds number correction C = Geometry correction C = Non-equilateral & row correction d = Outside diameter of fin, in d = Outside diameter of tube, in T = Average gas temperature, F T = Average fin temperature, F Reynolds number correction, C :. C = 0.25R R = Reynolds number = d G μ Geometry correction, C : For segmented fin tubes arranged in, a staggered pattern, an inline pattern, For solid fin tubes arranged in, a staggered pattern, C = e (. C = e (. an inline pattern, C = e (. C = e (. l = Fin height, in s = Fin spacing, in Non-equilateral & row correction, C : For fin tubes arranged in, Staggered pattern, C = e (. e (. Inline pattern, C = e (. e (. iv. Mass Velocity, G : And, Net Free Area, A : N = Number of tube rows P = Longitudinal tube pitch, in P = Transverse tube pitch, in G = W A W = Mass flow rate of gas, lb/hr A = Net free area, ft 2 A = A (A L N A = Cross sectional area of box, ft 2 A = Fin tube cross sectional area/ft, ft 2 /ft L = Effective tube length, ft N = Number tubes wide A = N L P A = d + 2l t n l = Fin height, ft d = Outside diameter of tube, ft P = Transverse tube pitch, ft t = fin thickness, ft n = number of fins, fins/ft v. Surface Area Calculations: For the prime tube, A = πd (1 n t And for solid fins, ISSN: Page 161

4 A = πd (1 n t + πn (2l d + l + t d + 2l And for segmented fins, T = Gas Temperature, F T = Tube Wall Temperature, F A = πd 1 n t + 0.4πn (d (d l 0.4 w + t + w t +πn w d = Outside diameter of tube, ft n = number of fins, fins/ft t = fin thickness, ft l = Fin height, ft w =? w = Width of fin segment, ft And then, vi. Fin Efficiency, E: For segmented fins, And for solid fins, And, For segmented fins, And for solid fins, A = A A E = x( x E = y(0.45 ln d d (y y = x( x x = tanh (mb mb B = l + ( t 2 m = ( h t + w. 6k t w m = ( h 6k t. vii. Fin Tip Temperature, T : The average fin tip temperature is calculated as follows, T = T + T T ( 1 ( e. + e. 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. Table 1 Radiant Section Design PROPERTY DETAIL AMOUNT Tube OD, in (d o thickness, in (t w No of tubes (N t 40 (Radiant No of tubes (N t 12 (Shield Effective length, ft (L e (Radiant Effective length, ft (L e (Shield Tube spacing, in (CC 16 (Radiant No of tubes per row 4 (N t/r (Shield Transverse pitch, in 16 (P t (Shield Combustion Fraction excess air 0.15 Firebox Diameter, ft (D Process fluid Mean wall temperature, (T t, ⁰R Flue gas Flue gas temperature (T g, ⁰R α (Radiant ( α (Shield Assumed (- 1 A cp (Radiant ft A cp (Shield ft αa cp (Radiant ft αa cp (Shield ft (αa cp r +(αa cp s ft A R / A T, ft ((αa cp r +(αa cp s Area of Shield Section, ft 2 (A s A R, ft A R / 0.17 ((αa cp r +(αa cp s Partial pressure atm (P Mean beam length ft P*l atm-ft Emissivity E Exchange factor F ISSN: Page 162

5 Radiantion Heat Transfer Btu/hr 3.37*10^7 MM Kcal/hr Table 2 Convection Section Design PROPERTY DETAIL AMOUNT Fin Height, in (l f 1 Thickness, in (t f No of fins, fins/ft (n f 60 Ther. Cond., Btu/hr-ft ⁰F (k f Tube OD, in (d o Thickness, in (t w 0.5 No of rows (N r 5 No of tubes per row 4 (N w Effective tube length, ft (L e Pitch, in (P t 16 Wall temp, ⁰F (T w 959 Wall Ther. Cond., Btu/hr-ft-⁰F (k w Process Fluid Inlet temp, ⁰F (t Outlet temp, ⁰F (t Ther. Cond., (Liq, Btu/hr-ft-⁰F (k l Ther. Cond. (Vap, Btu/hr-ft-⁰F (k v Sp. Heat (Liq, Btu/lb ⁰F (c p,l Sp. Heat (Vap, Btu/lb-⁰F (c p,v Viscosity (Liq, lb/hrft (µ l Viscosity (Vap, lb/hrft (µ v Mass flow rate, lb/hr Wt fraction (Liq (W l 0.7 Wt fraction (Vap 0.3 (W v Fouling factor,hr-ft ⁰F/Btu (R fi Flue Gas Inlet temp, ⁰F (t Outlet temp, ⁰F (t Mass flow rate, lb/hr (W g Ther. Cond., Btu/hr-ft ⁰F (k g Sp. Heat, Btu/lb-⁰F (c p,g Viscosity, lb/hr-ft (µ g Inside Film HT coefficient h i, Btu/hr-ft 2 -⁰F Mass Velocity of Flue G n, lb/hr-ft Gas Colburn HT Factor j Outside Film HT coefficient h c, Btu/hr-ft 2 -⁰F Average Outside HT h o, Btu/hr-ft 2 -⁰F co-efficient Fin Efficiency E Effective Outside HT h e, Btu/hr-ft 2 -⁰F co-efficient Overall HT coefficient Uo, Btu/hr-ft 2 -⁰F LMTD ⁰F HT Area ft Convection Heat Btu/hr 8.4*10^6 Transfer MM Kcal/hr Table 3 Heat Balance PROPERTY DETAIL AMOUNT Assumed amount of % 80 Radiant HT Assumed amount of % 20 Convection HT Thermal Efficiency % (given 90.7 Total Heat Input (Q fuel MM Kcal/hr Total Heat Transferred MM Kcal/hr (Q ht (given Radiant HT (Q r MM Kcal/hr Convection HT (Q c MM Kcal/hr Heat Loss (Q loss MM Kcal/hr (2.5% of Q fuel Heat out from HT area to stack (Q stack MM Kcal/hr (=Q fuel -Q ht -Q loss V. HTRI Introduction 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. VI. Comparision of given/calculated data and simulated data 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. ISSN: Page 163

6 fired heater design and simulation has been performed in satisfactory way. Table 4 Comparision of given/calculated data and simulated data PROPERTY DETAIL CAL. DATA SIMU. DATA Overall Performance Heat duty MM Efficiency (LHV % Heat release (Total MM Fuel LHV kcal/kg Process fluid temp at C crossover Process fluid temp at heater outlet C Radiant Section Fuel gas temp out C Average flux rate -m Duty MM Surface area m Pressure drop kgf/cm Convection Section Fuel gas temp out C Outside film m coefficient 2 -C Inside film coefficient m C Overall HT m coefficient (U 2 -C Convection duty MM Surface area m EMTD C Draft at bridgewall mm H 2 O Pressure drop kgf/cm Burners Fuel rate kg/hr References [1] Process Heat Transfer by Donald Q. Kern, [2] [3] API 560, Fired Heaters for General Refinery Service, 4 th edition, August 2007, [4] HTRI Xchanger Suite 6.0 software, [5] HTRI Manual and Help file VII. 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 ISSN: Page 164