Page 247 - Chemical Process Equipment - Selection and Design

8.11. FIRED HEATERS 217

TABLE 8.ljl-(continued)
19. The partial pressure P of CO, + H,O is given in terms of the excess air by Eq. 16)
20. The product PL is found with the results of Steps 18 and 19
21. The mean tube wall temperature < in the radiant zone is given in terms of the inlet and
outlet process stream temperatures by
T, = 100 + 0.5(T, + T.)
22. The temperature Tg of the gas leaving the radiant zone is found by combining the
equations of the radiant zone heat transfer [Eq. (I)] and the radiant zone heat balance [Eq.
(211. With the approximation usually satisfactory, the equality is

The solution of this equation involves other functions of Tg, namely, the emissivity $J by
Eq. (8), the exchange factor F by Eq. (9) and the exit enthalpy ratio Qg/Qn by Eq. (4)
23. The four relations cited in Step 22 are solved simultaneously by trial to find the
temperature of the gas. Usually it is in the range 1500-1800°F. The Newton-Raphson
method is used in the program of Table 3.13. Alternately, the result can be obtained by
interpolation of a series of hand calculations
24. After Tg has been found, calculate the heat absorbed Q, by Eq. (1)
25. Find the heat flux

QIA= QF,lka,imt
and compare with value specified in Step 3. If there is too much disagreement, repeat the
calculations with an adjusted radiant surface area
26. By heat balance over the convection zone, find the inlet and outlet temperatures of the
process stream
27. The enthalpy of the flue gas is given as a function of temperature by Eq. (4). The
temperature of the inlet to the convection zone was found in Step 23. The enthalpy of the
stack gas is given by the heat balance [Eq. (3)], where all the terms on the right-hand side
are known. Q/Qn is given as a function of the stack temperature T, by Eq. (4). That
temperature is found from this equation by trial
28. The average temperature of the gas film in the convection zone is given in terms of the
inlet and outlet temperatures of the process stream and the flue gas approximately by

The flow is countercurrent
29. Choose the spacing of the convection tubes so that the mass velocity is
G = 0.3-0.4 Ib/(sec)isqft free cross section). Usually this spacing is the same as that of
the shield tubes, but the value of G will not be the same if the tubes are finned
30. The overall heat transfer coefficient is found with Eq. (IO)
31. The convection tube surface area is found by
A, = Q,/U, (LMTD)
and the total length of bare of finned tubes, as desired, by dividing A, by the effective
area per foot
32. Procedures for finding the pressure drop on the flue gas side, the draft requirements and
other aspects of stack desiqn are presented briefly by Wimpress.
[Based partly on the graphs of Wimpress, Hydrocarbon Process. 42(10), 115-126 (1963)l.

EXAMPLE 8.15 and conversion is shown in Figure 8.22. In this case, the substantial
Design of a Fired Heater differences in heat flux have only a minor effect on the process
The fuel side of a heater used for mild pyrolysis of a fuel oil will be performance.
analyzed. The flowsketch of the process is shown in Figure 8.20, Basic specifications on the process are the total heat release
and the tube arrangement finally decided upon is in Figure 8.21. (102.86 MJ3tu/hr), overall thermal efficiency (75%), excess air
Only the temperatures and enthalpies of the process fluid are (25%), the fraction of the heat release that is absorbed in the
pertinent to this; aspect of the design, but the effect of variation of radiant section (75%), and the heat flux (10,000 Btu/(hr)(sqft).
heat flux along the length of the tubes on the process temperature In the present example, the estimated split of 75% and a