Decreased gas consumption of a fluidized bed furnace
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INTRODUCTION
F L U I D I Z E D bed furnaces, in comparison with conventional gas furnaces, possess such advantages as a higher uniform distribution of temperature, a more rapid rate of heating the workpiece, and a briefer period required for heat treatment. Utilization of a fluidized bed furnace in heat treatment of steel is becoming increasingly popular in both Europe and America. tl-41 A fluidized bed furnace comprises a heat-resistant retort in which a medium of refractory particles, such as A1203, SiOz, etc., is heated while being fluidized by a gas flowing through a porous distributor plate which is located at the bottom of retort. The fluidizing gas depends on the process to be carried out in the furnace. Nitrogen is employed for neutral heating, nitrogen/ammonia mixtures for nitriding, and nitrogen/propane/air or nitrogen /methanol/methane mixtures for carburizing. A fluidized bed furnace is generally operated in an open system, i.e., gases are introduced from the bottom and then exhausted to the air from the top of the furnace. The operating procedure is illustrated as follows, with an example of carburization being cited. Parts are lowered into the fluidized bed which is operated on nitrogen, usually at a working temperature of 900 ~ to 1000 ~ The nitrogen flow continues until the parts reach the carburization temperature. At this time, methanol is introduced into the furnace together with nitrogen and a small quantity of methane. The carbon potential of furnace atmosphere is controlled by the additional level of methane. At the end of carburization, the atmosphere is purged with nitrogen for about 2 minutes; the parts are then quenched in oil. Y O N G - C H W A N G CHEN, Associate Professor, is with the Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan 10764, Republic of China. Manuscript submitted November 30, 1992. METALLURGICAL TRANSACTIONS B
The gas flow of a fluidized bed furnace, as compared with that of a conventional gas furnace, needs maintaining at a relatively higher level to make the refractory particles become fluidized. The gas consumption of a fluidized bed furnace consequently exceeds that of a conventional gas furnace. This property is the major shortcoming of a fluidized bed furnace. Recycling of the atmospheres of fluidized bed furnaces was investigated for the purpose of decreasing gas consumption. For general approaches, the exhaust gas of a fluidized bed furnace was recirculated with a gas pump after this exhaust gas had been mixed with an appropriate amount of enriching gas, such as CH4, C3H8, e t c . The recycling percentage of atmospheres for this method is relatively small, ranging from 50 to 80 pct. ~41Control of the carbon potential is relatively complicated, because the carbon potential of the recycled atmosphere depends not only on the amount of enriching gas but also on the composition of the exhaust gas. The gaseous mixture generated by reacting humid air with hot charcoal has been found to possess a carburizing capability for steel, lSl
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