Page 245 - Chemical Process Equipment - Selection and Design

3.11. FIRED HEATERS 215

.I& Equations and Other Relations for Fired Heater Design
4. Radiant zone heat transfer

J
4000
-""I\ \ loo I J '
2. Radiant zone heat balance

0, is the enthalpy absorbed in the radiant zone, Qa is the enthalpy of the entering air, a,
that of the entering fuel, Q, is the enthalpy loss to the surroundings, Qg is the enthalpy of
the gas leaving the radiant zone; Qa and Q,are neglected if there is no preheat, and
Q,/Qn is about 0.02-0.03; Q,, is the total enthalpy released in the furnace
3. Enthalpy Qs, of the stack gas, given by the overall heat balance
Qs/Q,,=l +(l/Q")(Q~+a,-Q,-Q,-Q,,,,,,,i,,)
4. Enthalpy Qg, of the flue gas as a function of temperature, "F

Qg/Qn = [a+ b(T/l000-0.1)~(T/1000-0.1)
z= fraction excess air
a = 0.22048 - 0.350272 + 0.92344~'
b= 0.016086 + 0.293932 - 0.481392'
5. Absorptivity, a, of the tube surface with a single row of tubes
LY = 1 - 10.0277 + 0.0927(~ - l)l(x - 1)
x= (center-to-center spacing)/(outside tube diameter)

6. Partial pressure of CO, + H,O
P= 0.288 - 0.229~ + 0.090~'
x=fraction excess air
7. Mean beam lengths L of radiant chambers
Dimensional Ratioa
Rectangular Furnaces Mean Length L (ft)
1. 1-1-1 to 1-1-3 2/3 vfurnace volume, (R3)
1-2-1 to 1-2-4
2. 1-1-4 to 1-1-m 1.0 x smallest dimension
3. 1-2-5 to 1-2-8 1.3 x smallest dimension
4. 1-3-3 to I-m-m 1.8 x smallest dimension
Cylindrical Furnaces
5. dxd 2/3 diameter
6. dx2dto dxsd 1 x diameter
a Length, width, height in any order.
8. Emissivity @ of the gas (see also Fig. 8.20).
@ = a + b(fL) + c(fL)'
PL = product of the partial pressure (6) and the mean beam length (7)
z= (T, +460)/1000
a=0.47916-0.19847~ +0.0225692'
b= 0.047029 + 0.06992 - 0.015282'
c= 0.000803 - 0.007262 + 0.001597~~

9. Exchange factor F
F= a + bdp + c@'
@ = gas emissivity, (8)
z= A, J@AR
a=0.00064+0.0591z+0.00101z2
b = 7.0256 + 0.49082 - 0.0582'
c= -0,144 - 0.5522 + 0.0402'