Heat Transfer Enhancement by Pulse Combustion in Industrial Processes

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total heat-tran~fer area 2f this combustor is therefore g.3 M (56.8 Ft). Firing rate is 293 Kw (1 x 10 Btu/Hr) at 10 percent excess air. Data from Battelle's tests and elsewhere in the literature(8) show that a high heat flux is possible with pulse combustion even though the flue gases leave the system at low temperatures. It is possible for a system such as the one shown in Figure25 to produce a heat flux of ~1.5 to 37.8 Kw/M (10,000 to 12,000 Btu/Hr-Ft). This heat flux is sufficient to deliver enough heat to the tank to provide 80 percent efficiency without the need for a second pass. A return tube pass is shown in Figure 5 for the purposes of ducting the exhaust gases back to the same side of the tank as the inlet. The return pass also provides additional heat­transfer area for start-up conditions, and to recover from the sudden addition of fresh material in the tank. The conventionall~ designe2 heater needed a surface area of 12.7 M (137 Ft ) to accomplish the same goals. Therefore, the advantages of pulse-combustion technology for this application are manifested primarily in compact design. A secondary advantage is that the pulsations will cause a slight surface vibration on the burner. These vibrations must be accommodated in the design from a fatigue viewpoint, but will also serve to maintain a clean heat-transfer surface, which could be important in some applications. RADIANT TUBES - Radiant-tube heaters are used in applications where the combustion gases should not contact the material being heated. An example of this type of operation would be a steel reheat furnace where a carefully controlled atmosphere is maintained in the furnace. Heat transfer by radiation is strongly controlled by the surface temperature of the radiator and receiver and by the furnace geometry and heater surface characteristics. Inside the tube, heat transfer to the high-temperature radiating surface is due to a combination of convection and radiation in the flue gases. Radiation from the flame is low because of the transparent nature of natural gas flames(9)(10), so therefore, the controlling factor is the convective coefficient between the flue products inside the radiant tube and the tube wall. Efforts to increase this heat transfer rate have included increasing the flame temperature via combustion-air preheat. Many radiant tubes have been deSigned to include a self-recuperative feature to provide the needed air preheat. The recuperator approach is now receiving considerable attention because energy consumption is greatly reduced by using air preheat. Development issues being addressed involve the material selection and use problems encountered with high air-preheat temperatures. This difficulty is resolved for the most part by the use of high-performance ceramics for the recuperator(11). Some developments have been directed towards improving the internal heat transfer in radiant tubes through the use of axial fins(12). There 42 has not been any significant attempt to incre:~~a the convective coefficient however, and th h iS e a~ is where pulse-combustion technology can av important impact. The following discussion illustrates the advantages of pulse-combustion technology when applied to this important ~ield. Advantages of Pulse-Co~ust1on - Rad1ant: tube heaters operate at elevated temperatures 1n high-temperature furnaces. The predominant mode of heat transfer from the tube surface is by thermal radiation. The details of the radiation pattern etc. are dependent on the individual application, and will not be discussed here. Instead, the general thermal performance of these types of tubes will be considered. A conventional radiant tube operating without recuperation has a flue-gas temperature that is strongly dependent on tube temperature. Figure 6 shows some typical operating temperatures and efficiencies for these tubes(13). The average convection coefficient inside the tube for the2cases shown on F~gure 6 is approximately 57 W/M -c (10 Btu/Hr-Ft -F), assuming an average flame temperature of about 1790 C (3250 F). Applying pulse-combustion technology to radiant tubes will enhance the convective heat­transfer coefficient inside the tube by up to 100 percent. The actual enhancement depends on tube geometry, firing rate, and other factors. The net effect of increasing the heat-transfer coefficient is to raise tube efficiency which results in a lower flue-gas temperature. Figure 7 shows this effect. Flue-gas temperature is plotted against percent enhancement for heat­transfer enhancements ranging up to 100 percent. Tube efficiency increases from 36 percent with conventional technology to 44 percent using pulse-combustion technology. There is a secondary benefit derived from pulse-combustion technology shown in Figure 7. This benefit is that the gains obtained via heat­transfer enhancement replace air preheat by a significant amount. The effective air preheat curve shown on Figure 7 corresponds to the degree of air preheat needed to match the pulse combustors efficiency with conventional technology. For example, with a 50 percent enhancement, the efficiency of the system is 42 percent. Using conventional technology, this would require an air-preheat temperature of 163 C (325 F). Although this degree of air preheat is rather modest, it is important to note that it represents the preheat obtained at the recuperator discharge. In other words, because the pulse combustor delivers lower flue gas temperature than a conventional system, it in effect operates as a pre-recuperator. This benefit could be important in high­temperature systems where air preheat temperatures of 815 (1500 F) or higher are needed. If a system is being deSigned for operation with a ceramic recuperator capable of providing 815 C (1500 F) preheat, using a pulse­combustion radiant tube could reauce the air preheating requirements to the range of 570 to