Radiant section

The primary reformer is essentially a process furnace in which fuel is burned with air to indirectiy provide the heat of reaction to the catalyst contained within tubes. This area of the furnace is usually referred to as the radiant section, so named because this is the primary mechanism for heat transfer at the high (750—850°C) temperatures required by the process. Reforming pressures in the range 3 —4 MPa (30,000—40,000 atm) represent a reasonable compromise between cost and downstream compression requirements. 

Fig. 13. Radiant section of a Kellogg box-type steam reforming furnace.

In the radiant section, the hydrocarbon mixture undergoes reactions involving free radicals (51). These mechanisms have been generalized to include the molecular reactions shown below ... 

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. 

Efficiency. Since only 35 to 50% of fired duty is absorbed in the radiant section, the flue gas leaving the radiant chamber contains considerable energy that can be extracted efficiently in the convection section of the furnace. In the convection section, the feed is preheated along with dilution steam to the desired crossover temperature. Residual heat is recovered by generating steam. The overall thermal efficiency of modem furnaces exceeds 93%, and a value of 95% is not uncommon. 

FIG, 5-22 Thermal perfo rmance of weU-stirred fiirnace chambers reduced efficiency as a function of reduced firing density D and reduced sink temperature Tj. (a) Radiant section, oil tube stills, cracking cods, (h) Domestic boiler combustion chambers, (c) Open-hearth furnaces, (d) Soaking pits. 

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). 

Honzontal-tube cabin heaters position the tubes of the radiant-section-coil horizontally along the walls and the slanting roof for the length of the cabin-shaped enclosure. The convection tube bank is placed horizontally above the combustion chamber. It may be fired From the floor, the side walls, or the end walls. As in the case of its vertical cylindrical counterpart, its economical design and high efficiency make it the most popular horizontal-tube heater. Duties are 11 to 105 GJ/h (10 to 100 10 Btu). 

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). 

The tubes that are around the flame get most of their heat energs t rom radiation. The tubes in the top of the chamber get their heat from com ec-tion as the hot exhaust gases rise up through the heater and heat ihc process fluid in the tubes. The principal classification of fired heaters relates to the orientation of the heating coil in the radiant section. The tube coils of vertical fired heaters are placed vertically along the walls of the combustion chamber. Firing also occurs vertically from the Hoor of the heater. All the tubes are subjected to radiant energy. 

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).]...

The radiant section of an industrial boiler may typically contain only 10 per cent of the total heating surface, yet, because of the large temperature difference, it can absorb 30-50 per cent of the total heat exchange. The mean temperature difference available for heat transfer in the convective section is much smaller. To achieve a thermally efficient yet commercially viable design it is necessary to make full use of forced convection within the constraint of acceptable pressure drop. 

In the radiant section of a boiler the fourth power of the wall temperature is typically less than 2 per cent of the fourth power of the mean flame and gas temperature. The effects of waterside conditions and wall thickness on the heat transfer rate are therefore negligible. 

Even the presence of a dangerous layer of waterside scale reduces the heat flux only by a few per cent. Although this means that scale has little effect on radiant section performance, it also indicates that the metal temperature escalation due to the presence of scale is not self-limiting but is almost proportional to scale thickness. 

A type of tube bundle superheater located in the radiant section of a WT boiler. 

Heat transfer to the tubes on the furnace walls is predominantly by radiation. In modem designs this radiant section is surmounted by a smaller section in which the combustion... 

The burners are positioned at base or sides of radiant section. Gaseous and liquid fuels are used. The combustion air may be preheated in tubes in the convection section. 

Between 50 to 70 per cent of the total heat is transferred in the radiant section. 

The heat flux to the tubes in the radiant section will lie between 20 to 40 kW/m2, for most applications. A value of 30 kW/m2 can be used to make a rough estimate of the tube area needed in this section. 

A small amount of heat will be transferred to the tubes by convection in the radiant section, but as the superficial velocity of the gases will be low, the heat transfer coefficient will be low, around 10 Wm-2oC-1. 

The lower tubes in the shield bank in the convection section will receive heat by radiation from the radiant section. This can be allowed for by including the area of the lower row of tubes with the tubes in the radiant section. 

The pressure drop in the radiant section will be small compared with that across the convection section and can usually be neglected. 

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... 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

Radiant Section Tubes Horizontally or Vertically Mounted... 

In preliminary design, the heat duty and furnace efficiency are the prime considerations. However, if the tube area needs to be specified, a preliminary estimate can be obtained from an assumed flux. In the radiant section, this usually lies in the range of 45,000 W m-2 to 65,000 W m 2 of tube surface, with a value of around 55,000 W m 2 most often used. The heat flux is particularly important if a reaction is being carried out in the furnace tubes. Overall heat transfer coefficients in the convection section are in the range 20 to 50 W m-2 K-1. 

A typical air preheater will reduce the fuel required to liberate a given amount of heat by 10 percent. The debit for this improved thermal efficiency is a higher flame temperature, and the possibility of overheating the radiant section. The only instance where there is a clear advantage to fit an air preheater to an existing furnace is when the firebox of that furnace is running below a desirable maximum firebox temperature. Three types of air preheaters are in common use ... 

Let us finally look at a situation where one might wish to transfer heat duty from the firebox to the radiant section. At the beginning of the discussion on air preheaters earlier in this chapter, we noted how fitting an air preheater to an existing furnace would increase the flame temperature and possibly overheat the firebox. 

Here is what happened. Preheating the air by 300°F had raised the burner flame temperature by 300°F. The hotter flames then radiated more heat per pound of fuel consumed, and as a result, the firebox became much hotter. The same factor that reduces fuel consumption in the radiant section also reduced the flow of flue gas to the convective section, thus reducing the convective-section outlet temperature. 

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