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Show required for the operation and maintenance of the equipment. The electric units have classically higher initial capital costs and higher operating and maintenance costs, compared to conventional gas fired equipment. The conventional combustion systems currently being used are designed and manufactured with standard Nickel and Chromium alloys. These alloys result in relatively shorter service life when operating in excess of 1750 degrees F. Typically, metallic radiant tubes, when operated at 1750 degrees F., will have a service life of less than one year and in some cases, even shorter, depending upon the process environment. Many major corporations are currently evaluating the metallurgical aspects of higher temperature processing. Some of the areas of concern, as a result of higher temperature processing temperatures are: (1) Potential in grain growth, alteration of core properties and changes in fatigue strength. The initial studies have indicated that results are very favorable for high temperature processing. There is, however, no production equipment available to even put this technology into practice. As a result, WB Combustion, Inc. has initialed research activities through a Gas Research contract to design and develop a high temperature resistant recuperative combustion system. Preliminary design concepts and test results are discussed in this paper. SYSTEMS DESCRIPTION - A) Metallic single ended recuperative system - A single ended radiant tube recuperative combustion system consists of a recuperator, a nozzle mix burner, a combustion tube and a radiant tube, all assembled in a compact unit, see Figure #1. Combustion is initiated with the help of an ignition wand, which sparks at the tip of the nozzle where natural gas diffuses into air. The resulting flue products travel down and return back toward the exhaust port. During this process, the hot gases release heat to the incoming combustion air via the recuperative section as shown in Figure #1. The mode of heat transfer is dominated by radiation against convection. The degree of preheat of combustion air is highly dependent on the firing rate and the length of the recuperative section. The SER can be fired with high-low, on-off and pulse firing mode. B) Preliminary design of a ceramic single ended radiant recuperative combustion system - Preliminary design 72 of a ceramic SER is similar to the metallic, with a few exceptions, i.e., both radiant tube and combustion tube are of mullite material. Due to different size and shape of the tubes collars, a new set of flanges and rings were fabricated to ensure better assembly. The tubes dimensions are indicated in Figure #2. Inner tube within outer tube, was centered with an external circular thermocouple wire having three pieces of ceramic pieces. C) Multitube plug recuperator, as shown in Figure #3, is a single pass counter flow heat exchanger, except that the air flows in a counter and parallel manner against flue products. In this design, air flows through the metallic inner tube covered with ceramic (reaction bonded silicon carbide) outer tube over which hot flue gases pass. The recuperator assembly is pushed in the exhaust leg of the radiant tube ("U", "w" or Trident). The size of the recuperator varies as a function of firing rate, preheated air requirement for the burner used, refractory thickness of the furnace and air pressure available to the combustion system. The number of tubes used in a multitube plug could vary depending upon the size of the radiant tube. RESULTS AND DISCUSSION - A) Metallic single ended recuperative radiant tube combustion system - Metallic SER has been commercially applied to different types of applications, such as batch type and continuous heat treat furnaces, using three different sizes 3-1/4" aD, 4-1/4" aD and 6" aD radiant tube. These tubes are fired with a nozzle mix burner to result in a maximum heat flux of 55 - 60 BTU/HR/SQ.IN, at an excess air level of 15 - 20% from stoichiometry. Based upon field data, it is observed that temperature gradient on the radiant tube is within 35 - 40 degrees F. Temperature uniformity changes depend upon the tube diameters and lengths. A new combustion system is under investigation for a radiant tube, effective length 60" longer, particularly for a smaller diameter tube. New nozzle design would result in a little intense flame and longer length to provide a better temperature uniformity. SER fires in all three modes of control, i.e., pulse, on-off and high-low, depending upon the process requirements. B) Preliminary test results on a ceramic single ended recuperative radiant tube combustion system - Ceramic SER was fired with natural gas using a nozzle mix burner similar to a metallic |