Fired heaters convection section

Figure 3-21. Six bask designs used in horizontal-lube fired heaters. Radian section coil is horLmtai. (a) Cabin, (b) Two-cell box. (c) Cabin with dividing bridgewall, (d) End-fired box. e) End-fired box, with side-mounted convection section, (f) Horizontal-tube, single-row, double-h red. [From Chem. Eng., 102-103 (June 19, 1978).]...

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

In the horizontal-tube box heater with side-mounted convection tube bank, the radiant-section tubes run horizontally along the walls and the flat roof of the box-shaped heater, but the convection section is placed in a box of its own beside the radiant sec tion. Firing is horizontal from the end walls. The design of this heater results in a relatively expensive unit justified mainly by its abihty to burn low-grade high-ash fuel oil. Duties are 53 to 210 GJ/h (50 to 200 10 Btu/h). 

FIG. 27-51 Representative types of fired heaters a) vertical-tube cylindrical with cross-flow-convection section (h) horizontal-tube cabin (c) vertical cylindrical, helical coil, from Berman, Chem. Eng. 85 98-104, June 19, 1978.)... 

External Combustor (experimental). The heat exehanger used for an external-combustion gas turbine is a direct-fired air heater. The air heater s goal is to achieve high temperatures with a minimum pressure decrease. It consists of a rectangular box with a narrow convection section at the top. The outer casings of the heater consist of carbon steel lined with lightweight blanket material for insulation and heat re-radiation. 

The radiant section tube coils of horizontal fired heaters are arranged horizontally so as to line the sidewalls and the roof of the combustion chamber. In addition, tliere is a convection section of tube coils, winch are positioned as a horizontal bank of tubes above the combustion cham her. Nonnally the tubes are fired vertically from the floor, but they can also be fired horizontally by side wall mounted burners located below the tube coil. Tins economical, high dficiency design currently represents the majority of new horizontal-tube-t1icd heater installations. Duties run from 5 to 250 MMBtu/hr. Six types o) horizontal-tube-fired heaters arc-shown in Figure 3-21. 

Fired heaters radiant rate, 12,000 Btu/(hr)(sqft) convection rate, 4000 cold oil tube velocity, 6 ft/sec approx equal transfers of heat in the two sections thermal efficiency 70-75% flue gas temperature 250-350°F above feed inlet stack gas temperature 650-950°F. 

A simple vertical cylindrical heater has vertical tubes arrayed along the walls of a combustion chamber fired vertically from the floor. This type of heater does not include a convection section and is inexpensive. It has a small footprint but low efficiency, and it is usually selected for small-duty apphcations [0.5 to 21 GJ/h (0.5 to 20 10 Btu/h)]. 

In the vertical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convection bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

For process liquid and gas stream heating, most designs heat the process stream as it flows through tubes that pass through fireboxes, convection sections, or combustion gas stacks, although a few fire-tube heaters exist. 

Fire-resistive insulation should be provided as passive fire protection for critical load bearing heater supports. Critical supports include those for the firebox, the convection section, any "breeching" (combustion exhaust gas connections to a... 

Figure 8.19. Some types of process fired heaters (See also Fig. 17.16 for a radiation panel heater), (a) Radiant, shield, and convection sections of a box-type heater, (b) Heater with a split convection section for preheating before and soaking after the radiant section (Lobo and Evans, 1939). (c) Vertical radiant tubes in a cylindrical shell, (d) Two radiant chambers with a common convection section.

The fuel consumed in a fired heater can be estimated from the fired heater duty divided by the furnace efficiency. The furnace efficiency will typically be about 0.85 if both the radiant and convective sections are used (see Chapter 12) and about 0.6 if the process heating is in the radiant section only. 

Figure 12.71. Fired heaters, (a) Vertical-cylindrical, all radiant, (b) Vertical-cylindrical, helical coil, (c) Vertical-cylindrical with convection section.

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