This invention relates to a heating furnace which is employed at a temperature higher than 1,800.degree. C. for the carbonization of carbon fibers for example, and which is constructed by graphite or carbon structures.
Generally in a heating furnace of the kind mentioned above which is utilized at a temperature higher than 1,800.degree. C., refractory structures made from alumina can not be used, as their melting points are too low to stand against such high temperature. Therefore, thermal insulation interior structures such as muffle cases, and heaters employed in such heating furnaces are made from graphite or carbon materials. And, it is conventional that such heating furnaces are consisted of a furnace core tube through which commodities to be heat-treated are passed, heaters located circumferentially outside of the tube, inner frames for keeping insulation materials at a desired configuration in order to make spaces necessary to radiate heat from heaters.
Graphite or carbon tubes are generally employed to make such inner frames, which tubes or plates being conventionally thick in order to have a necessary strength, and having usually a thickness of 10 to 30 mm. According, such inner frames become heavy. And, structural works for assembling the tubes into the inner frames become also heavy, whereby heat capacity of a heating furnace is disadvantageously wasted by those heavy tubes and heavy structural works.
It is also disadvantageous that since such graphite or carbon tubes, porosity of which is as much as 15-22%, readily absorb ambient gases and discharge the gases from themselves at an initial stage of operation when the furnace is heated, the furnace has to be operated idly for a comparatively long period of time.