Some Concepts regarding furnace design

1. Chemical Engineering Plant Design
CHE 441
Dr. Asim Khan
Assistant Professor
Email: alaeeqkhan@ciitlahore.edu.pk

2. Design of Furnaces

3. Recommended books
1. “Process Heat Transfer” by Donald Q. Kern,
McGraw-Hill edition 1997.
2. “Chemical Engineering” by Coulson & Ricardsons,
6th Edition, Elsevier.
3. “Plant Design & Economics for Chemical
Engineers”by Max S. Peters & Klaud D.
TimmerhausMcGraw Hills International Editions
4. “Unit Operations of chemical Engineering” by
MeCabe Warren L., Smith Julian C., Harriott peter 7th
Ed., 2005, McGraw Hill Inc.
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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
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Shield tubes
(combination
of
convection &
radiation
- Flue gas re-circulation
- Pre-heated oil
Before passing in
radiant section,
heated and then
maintained at high
temp in convection.

6. Heat transfer in Furnaces
 Radiation: Major mechanism (50-70%)
 Stefan-Boltzman Equation:
𝑞 𝑟 = 𝜎 𝑇4
 Heat transfer between combustion gases and tube walls:
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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)
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10. Design methods for Furnaces
 Lobo and Evan’s method
 Involves the use of overall exchange factor and Stefan-
Boltzmann 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
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11. Lobo and Evan’s method
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Taking into account the heat transferred by convection:

12. Total heat balance
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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.
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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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Acp = No. of tubes * exposed length * center to center spacing

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