Radiant preheating

Rapid flame spread. As we know, radiant preheating of a material can cause its surface temperature to approach its piloted ignition temperature. This causes a singularity in simple flame spread theory that physically means that a premixed mixture at its lower flammability limit occurs ahead of the surface flame. Hence, a rapid spread results in the order of 1 m/s. 

Just as in the reflow soldering oven, there are multiple solder-wave preheater styles, but only two are in prevalent use radiant preheaters (direct and indirect IR) and forced-air convective preheaters. Both are effective and both have their advantages. In fact, the best configuration is a combination of the two. Some wave-soidering machines can be equipped with both top and bottom preheaters. This can be advantageous for thermally massive boards. 

Chemical Regeneration. In most MHD system designs the gas exiting the toppiag cycle exhausts either iato a radiant boiler and is used to raise steam, or it exhausts iato a direct-fired air heater and is used to preheat the primary combustion air. An alternative use of the exhaust gas is for chemical regeneration, ia which the exhaust gases are used to process the fuel from its as-received form iato a more beaeftcial oae. Chemical regeaeratioa has beea proposed for use with aatural gas and oil as well as with coal (14) (see Gas, natural Petroleum). 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

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. 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

Combustion may be induced by lower pressures by preheating the strand, by providing a sufficient incident radiant flux, by adding certain catalytic agents, and by adding small amounts of various fuels. 

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. 

Figure 8.8 Pyrolysis-front propagation velocity vp, and flame-tip propagation velocity Vf, as functions of the incident radiant energy flux, for a preheat time p of 2 min on a wood sample [13]...

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. 

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.

Qr is the enthalpy absorbed in the radiant zone, Qa is the enthalpy of the entering air, Q, that of the entering fuel, QL is the enthalpy loss to the surroundings, Qg is the enthalpy of the gas leaving the radiant zone Qa and Q,are neglected if there is no preheat, and OjQn is about 0.02-0.03 Q is the total enthalpy released in the furnace... 

One of the most effective reformer modifications is to use heat from the convection section to preheat radiant section feed. This will reduce radiant section duty and firing rate. The effectiveness of this option is limited only by the risk of coking in the preheat coil, the metallurgy of the preheat coil and the metallurgy of the radiant inlet system. This option has been used to increase capacity by 10% without increasing the arch temperature in the radiant section86. 

The furnace is divided into a convection section and a radiant section. The tubular reactor, generally 3 to 5 inches in diameter and over 150 feet in length, is situated in the radiant section. Feed enters the convection section where it is preheated and mixed with dilution steam. 

Improvements in overall thermal efficiency have been coupled with the introduction of extensive air preheat to decrease the fired fuel requirements. Additional decreases in fired fuel requirements can be obtained by materially improving the radiant box efficiency through changes in radiant box geometry. Usually a balance was struck between the fired fuel requirement to meet the thermal reactor duty, and the overall need for additional heat for generation of high pressure steam required for the process as a reactant or for driving pumps, compressors, or... 

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