furnace design

Some Concepts regarding furnace design

1. Chemical Engineering Plant Design CHE 441 Dr. Asim Khan Assistant Professor Email: [email protected]

2. Design of Furnaces

3. Recommended books “Process Heat Transfer” by Donald Q. Kern, McGraw-Hill edition 1997. “Chemical Engineering” by Coulson & Ricardsons , 6 th Edition, Elsevier. “Plant Design & Economics for Chemical Engineers”by Max S. Peters & Klaud D. TimmerhausMcGraw Hills International Editions “Unit Operations of chemical Engineering” by MeCabe Warren L., Smith Julian C., Harriott peter 7 th Ed., 2005, McGraw Hill Inc. 3 Recommended Books

4. Fired Heaters Fired directly by fuel (Oil, Gas, Coke) Application based on requirements (High flow rates, high temperatures) Capacity: 3 – 10 MW High thermal efficiency (Scarcity of fuel) Applications Process feed stream heaters Refineries (Atmospheric and vacuum distillation, thermal cracking, high temperature gas processing) Steam boilers Direct fired reactors Reformers 4

5. Basic Construction of Furnaces 5 Shield tubes (combination of convection & radiation Flue gas re-circulation Pre-heated oil Before passing in radiant section, h eated and then m aintained at high t emp in convection.

6. Heat transfer in Furnaces Radiation: Major mechanism (50-70%) Stefan- Boltzman Equation: Heat transfer between combustion gases and tube walls:   6

7. Emissivity of combustion gases Diatomic gases having low emissivites H2O, CO2, SO2 having good emissivities Total radiation of combustion products: Temperature of gas No. of radiating molecules (volume of gas and conc.) Emissivity: (P * L) Product of partial pressure of gas and mean beam length (Average depth of the blanket of flue gas in all directions) 7

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10. Design methods for Furnaces Lobo and Evan’s method Involves the use of overall exchange factor and Stefan- B oltzmann equation Application in refineries Wilson, Lobo and Hottel method Used when accuracy is not desired Preliminary estimate method Orrok -Hudson method Used for estimation of changes in the firing rate and air/fuel ratio Wohlenburg method Only used for coal-fired furnaces 10

11. Lobo and Evan’s method 11 Taking into account the heat transferred by convection:

12. Total heat balance 12 Efficiency of furnace:

13. Example of Lobo & Evan Method A furnace is to be designed for a total duty of 50,000,000 Btu/hr. The overall efficiency is to be 75%. Oil fuel with a lower heating value of 17,130 Btu/ lb is to be fired with 25% excess air (corresponding to 17.44 lb air/ lb fuel), and the air pre-heated to 400 deg F. Steam for atomizing the fuel is 0.3 lb / lb of oil. The furnace tubes are to be 5 in. OD on 8 ½ in centers, in a single-row arrangement. The exposed tube length is to be 38’6’’. The average tube temperature in the radiant section is estimated to be 800 deg F. Design the radiant section of the furnace having a radiation section average flux of 12,000 Btu/hr-ft2. 13

14. 14 Parameters Specifications Total duty 50,000,000 Btu/ hr Efficiency of furnace 75 % Heating value of fuel 17, 130 Btu/ lb Excess air (Pre-heated to 400 deg F) 25 % (17.44 lb air/ lb of fuel) Steam for atomization 0.3 lb / lb of fuel Furnace tubes Do Centers arrangement Exposed Length 5 inch 8.5 in Single row 38’6” Average tube temp. in the radiant section 800 deg F Average flux in the radiant section 12,000 Btu/hr-ft2

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16. 16 A cp = No. of tubes * exposed length * center to center spacing

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