Page 243 - Chemical Process Equipment - Selection and Design

8.11. FIRED HEATERS 213
Figure 8.19(d) also provide some flexibility. In many operations, air compared with 10-20% for conventional burners. Heaters
about 75% of the heat is absorbed in the radiant zone of a fired equipped with radiant panels cost more but provide better control
heater. of temperatures of reactions such as pyrolysis of hydrocarbons to
Horizontal tube supports are made of refractory steel to ethylene for instance.
withstand the high temperatures. Hangers for vertical tubes make Distances between tube banks are of the order of 20 ft or so. A
for a less expensive construction per unit of tube surface. Furnaces rough guide to box size is about 4cuft/sqft of radiant transfer
are Lined with shaped light weight refractory brick 5-8 in. thick. A surface, but the ultimate criterion is sufficient space to avoid
1 in. layer of insulating brick is placed between the lining and the impingement of flames on the tubes. Some additional notes on
metal shell. dimensions are stated with the design procedure of Table 8.18.
Differences of opinion exist among designers with respect to Tubes are mounted approximately one tube diameter from the
housing shapes and tube arrangements. Nelson (PetroZeurn Refinery refractory walls. Usual center-to-center spacing is twice the outside
Engineering, McGraw-Hill, New York, 1958, p. 587), for example, tube diameter. Wider spacings may be employed to Bower the ratio
describes a dozen types. The most common are cylindrical shells of peak flux at the front of the tube to the average flux. For single
with vertical tubes and cabin or box types with horizontal tubes. rows of tubes. some values of these ratios are
Figures 8.19 and 17.16 are of typical constructions. Convection
zones are most commonly at the top. Process fluid goes first through Center-to-center/diameter 1 1.5 2 2.5 3
the convection section and usually leaves the radiant tubes at the Max flux/avg flux 3.1 2.2 1.8 1.5 1.2
top, particularly when vaporization occurs in them. In the more
complex flow pattern of Figure 8.19(b), some of the convection Less is gained by extending the ratio beyond 2.0. Excessive fluxes
may damage the metal or result in skin temperatures that are
tubes are used for preheat and the remainder to maintain the harmful to the process fluid.
process fluid at a suitable reaction temperature that was attained in A second row of tubes on triangular spacing contributes only
the radiant tubes. Some of the convection zone also may be used for
steam generation or superheating or for other heat recovery services about 25% of the heat transfer of the front row. Accordingly, new
in the plant. furnaces employ only the more economical one-row construction.
Capacities of 10-200 MBtu/hr can be accommodated in heaters Second rows sometimes are justifiable on revamp of existing
with single radiant chambers, and three to four chambers with a equipment to marginally greater duty.
common convection section are feasible. Stoichiometric combustion
air requirements of typical fuels are tabulated: HEAT TRANSFER

Performance of a heater is characterized by the average heat flux in
Combustion Air
LHV the radiant zone and the overall thermal efficiency. Heat fluxes of
Fuel (Btu/lb) Ib/lb lb/1000 Btu representative processes are listed in Table 8.15. Higher fluxes
make for a less expensive heater but can generate high skin
Methane 21,500 17.2 0.800 temperatures inside and out. Thermal sensitivity of the process
Propane 19,920 15.2 0.763 fluid, the strength of the metal and its resistance to corrosion at
Light fuel oil 17,680 14.0 0.792
Heavy fuel oil 17,420 13.8 0.792 elevated temperatures are factors to be taken into account in
Anthracite 12,500 4.5 0.360 limiting the peak fiux. Because of the refractory nature of water,
however, allowable fluxes in steam boilers may reach 130,00OBtu/
Burners may be located in the floor or on the ends of the (hr)(sqft), in comparison with a maximum of about 20,000 in hydro-
heaters. Liquid fuels are atomized with steam or air or carbon service. Example 8.13 is a study of the effect of tube spacing
mechanically. A particularly effective heater design is equipped with on inside film peak temperatures.
radiant panel (surface combustion) burners, illustrated in Figure A certain amount of excess air is needed to ensure complete
17.16(a), (b). The incandescent walls are located 2-3ft from the combustion. Typical minimum excess requirements are 10% for
tubes. The furnace side of the panel may reach 2200°F whereas the gaseous fuels and 15-20% for liquids. Radiant panel burners may
outer side remains at 120°F because of continual cooling by the get by with 2-5% excess air.
air-gas mixture. Radiant panel burners require only 2-5% excess Efficiency is the ratio of total heat absorbed in radiant,

TABLE 8.15. Typical Radiant Fluxes and Process Temperatures

Average Radiant Rate
(Btu/hr/ftz) Temperature
Service (Based on OD) (“F)
Atmospheric crude heaters 10,000-14,000 400-700
Reboilers 10,000-12,000 400-550
Circulating oil heaters 8000-1 1,000 600
Catalytic reformer change and reheat 7500-1 2,000 800-1000
Delayed coking heater 10,000-1 1.000 925
Visbreaker heaters-heating section 9000-1 0,000 700-950
Soaking section 6000-7000 950
Lube vacuum heaters 7500-8500 850
Hydrotreater and hydrocracker charge heaters 10,000 700-850
Catalytic-cracker feed heaters 10,000-1 1,000 900-1050
Steam superheaters 9000-13,000 700-1 500
Natural gasoline plant heaters 10,000-12,000 -
Ethvlene and DroDvlene svnthesis 10,000-15,000 1300-1 650