Fired heaters types

Fired heaters Type, tube pressure and temperature, duty, radiant and convective heat transfer area, MOC... 

Reboilers need to be located next to the tower they serve, except for the pump-through types, which can be located elsewhere. Fired heater reboilers are always located away from the associated tower and use a pump to circulate the bottoms. Ketde-type reboders are preferred from an operational and hydraulic standpoint because they can be designed without the worry of having to ensure sufficient head for circulation required by thermosyphon reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

Air recirculation. Prevailing winds and the locations and elevations of buildings, equipment, fired heaters, etc., require consideration. All air-cooled heat exchangers in a bank are of one type, i.e., all forced-draft or all induced-draft. Banks of air-cooled exchangers must be placed far enough apart to minimize air recirculation. 

In the vei tical-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 convec tion 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). 

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.)... 

In addition to the distillation of crude oil coming into the refinery, stills of various designs are used in other types of service throughout the refinery. Cracked products are separated in distillation equipment which is very similar to an atmospheric crude pipe still. The principal difference is that these products are hot from the cracking operation, so that a fired heater is not required. 

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. 

Figure 3-20. Vertical-tube-fired heaters con be identified by the vertical arrangement of the radiant-section coil, (a) Vertical- lindrical all radiant, (b) Vertical-cylindrical helical coil, (c) Vertical-cylindrical, with cross-flow-convection section. d) Vertical-cylindrical, with integral-convection section, (e) Arbor or wicket type, (f) Vertical-tube, single-row, double-fired. [From Chem. Eng, 100-101 (June 19, 1978).]...

In fire tube type heaters, the coils are immersed in a bath of water. The water is heated by a fire tube that is in the bath below the coils. That is, the fire tube provides a heat flux that heats the water bath. The water bath... 

Another method of increasing the overall cycle efficiency is to use the waste heat energy in the exhaust air to heat process fluids as depicted in Figure 16-10. This is a direct savings in fuel gas that would otherwise be consumed in direct-fired heaters. Overall thermal efficiencies can be as high as 50 to 60% in this type of installation. 

Radiation is not generally considered in conventional heat transfer equipment except for direct gas/oil-fired heaters and cracking units. These later types are not a part of this chapter, because they are specialty items of their own as far as design considerations are concerned. 

Where gas or oil appliances are used for heating and installed within the heated space, between 70 per cent and 90 per cent of the total energy content of the fuel input will be converted into useful heat. Table 27.15 gives particulars of some gas-fired equipment types and Table 27.16 gives similar details for some oil-fired heaters. The first three types of equipment detailed in Table 27.15 and the first two in Table 27.16 are usually used for local warming of individuals rather than to provide a particular temperature throughout the space. 

In such cases, radiant heat transfer is used from the combustion of fuel in a fired heater ox furnace. Sometimes the function is to purely provide heat sometimes the fired heater is also a reactor and provides heat of reaction. The special case of steam generation in a fired heater (a steam boiler) will be dealt with in Chapter 23. Fired heater designs vary according to the function, heating duty, type of fuel and the method of introducing combustion air. However, process furnaces have a number of features in common. A simple design is illustrated in Figure 15.19. The chamber where combustion takes place, the radiant section... 

The procedure begins with a material factor that is a function only of the type of chemical or chemicals used. This factor is adjusted for general and special process hazards. These adjustments or penalties are based on conditions such as storage above the flash or boiling point, endo- or exothermic reactions, and fired heaters. Credits for various safety systems and procedures are used for estimating the consequences of the hazard, after the fire and explosion index has been determined. 

Process furnace or direct-fired heater Design type, absorbed heat duty, pressure, tube material, capacity... 

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.

Cost of direct-fired heaters, cylindrical type with vertical tubes. 

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