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.
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,
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:
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
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
10
11. Lobo and Evan’s method
11
Taking into account the heat transferred by convection:
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