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Show British Gas Corporation has successfully demonstrated that a straight, silicon nitride bonded silicon carbide tube can be effectively used for indirect heating. The development of this ceramic radiant tube was conducted at Midlands Research Station. As illustrated in Figure 3, the radiant tube consists of a plain ceramic tube (Refrax 20 from Carborundum) held between refractory lined metallic tubular extensions. An axial compressive force is applied via a spring loaded plate, to reduce tensile stresses in the ceramic seal. A recuperative burner is used to provide high efficiency burner operation and an inner ceramic tube recirculates the combustion products to provide a uniform temperature distribution. The overall specification of the burner system is described in Table 4. This development is covered by two British Patents (#1,331,427 and 1,404,578). Table 4 Ceramic Radiant Tube Indirect Heating System British Gas Corporation Radiant tube outside diameter 170mm (6 3/4 inches) Working length of tube 1.07-1.37m (42-54 inches) Tube operating temperature 13000C (maximum) Process temperature range 980-12500C Heat flux from tube surface 39.4 kw/m2 (12,500 Btu/ft2 hr) Tube efficiency 50% at 12000C 55% at 10000C Burner rating High fire 58 kw Low fi re 12 kw A similar technology was introduced by Holcroft, a division of Thermo Electron Corporation, and was developed in cooperation with a consortium of gas companies. This technology was targeted for aluminum melting furnace applications. In this effort, a furnace was retrofitted with silicon nitride bonded silicon carbide radiant tubes and appeared to show significant efficiency and productivity gains when compared to the original furnace design. The most important benefit discovered in this test effort was a reduction in metal loss due to oxidation. (5) A second design (Figure 4) utilizes a closed-one-end tube which is known as a singled-ended radiant tube or SER. (1) The 308 original development of the SER was in England and dates back to 1968. The bulk of the development from 1975 to 1978 resulted in a commercial product known as Supertube 80. The metallic single-ended radiant tube for indirect heating applications is currently marketed by Eclipse, Inc. and WB Combustion, Inc. In cooperation with these companies, GRI will be evaluating the viability of the ceramic version of the SER design. GRI has cooperated with Midland Ross in the evaluation of a single-ended ceramic radiant tube design. Emphasis in this design was on the integration of high temperature burners and high effectiveness heat exchangers with ceramic tubes. Laboratory tests with this new system have demonstrated that heat flux can be increased by 200% compared to a conventional burner (6). The third design (Figure 5) is a conventional "u" tube design. This design can effectively utilize a plug-in recuperator or other air to air finned plate heat exchanger to optimize fuel efficiency. Because large ceramic component manufacturing technology is still in the developmental stage, cost effective, reliable ceramic components are not commercially available for application in this design. GRI, in cooperation with North American Manufacturing Company, is evaluating the viability of a prototype ceramic "u" tube configured burner. The design concept utilizes regenerative twin-bed burners on both ends of the tube which fire alternately. This design currently appears very promising. It is clear that the most critical component for all three design concepts is the ceramic tubular component. This component must be commercially available, effective, durable and highly reliable. CANDIDATE MATERIALS Silicon-base ceramics, (ie. silicon carbide, silicon nitride), toughened oxide ceramics, and ceramic matrix composites have potential for application as radiant tubes. The required configurations (i.e. straight tube, single-ended tube and U-tube) and sizes (3 1/2" to 6" O.D., 70" to 120" length and 1/16" to 3/8" thickness) have been identified for the near-term (1-2 year) and long-term (3-5 years) configurational needs of candidate materials for radiant tube applications. Table 6 summarizes the current status of various candidate materials. Even with the partial success of monolithic ceramic tubes (such as the silicon nitride bonded SiC tubes), it can be accurately stated that currently available technical ceramics appropriate for radiant tube application lack uniformity, reproducibility, durability, reliability and effective life cycle cost. It can be further stated that the most serious impediment to the early exploitation of ceramic materials as structural |