After the furnace masonry is completed, the next step is the baking process of the furnace. Before baking the furnace, it is necessary to clarify the insulation problem of the furnace cover. The furnace cover is cold insulation (referred to as "cold insulation") or hot insulation (the furnace is heated to above 1500 ° C and then begin insulation,it’s "hot insulation"). At present, there are mainly two manners in the industry.
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More than 20 years ago, the traditional method was "hot insulation", and the hot insulation was started when the furnace was baked to above 1450°C, so that you could see whether the "brick drop" phenomenon occurred in the furnace cover, and if there was a "brick drop" easy to handle. In a large glass factory in Shanghai, the furnace cover was built for insulation when it was cold, and only the 1m-wide area in the middle of the furnace cover was not built for insulation (the insulation should be done after the kiln was finished), and aluminium silicate fibre felt was used to cover it. The purpose of aluminum silicate fiber felt is to reduce the temperature difference between the upper and lower ends of the brick body as much as possible and facilitate inspection. Because in the process of baking the furnace, the cracks caused by the thermal expansion of the furnace cover are mainly concentrated in the middle part of the furnace cover. After the kiln is finished, clean the aluminum silicate fiber felt in the middle part, then grouting (or filling with very fine quartz sand or siliceous refractory mud), and finally pad the insulation material in the middle part.
This practice also impacted our traditional thinking and code of conduct at the time. When we visited, we were confused about what to do if the bricks of the furnace cover "dropped bricks". After repeated comparisons, I finally figured out the principle. After the furnace cover is kept in a cold state, the temperature difference between the inside and outside of the furnace cover during the baking process is small, and the upper and lower expansions are uniform, so it is not easy to turn the bricks (we think that using the "beating mud method" for masonry, the larger the brick seam is the premise conditions), and "hot insulation", although the furnace cover is also covered with fiber felt when the kiln is baked, but after all, the heat preservation is thin, and the temperature difference between the upper and lower sides is large, especially after the fiber felt is removed to clean the furnace cover, the temperature difference between the inside and outside of the furnace cover is 1000 ℃~1200℃.
In this state, after the insulation layer of the furnace cover is done, the surface temperature of the furnace cover will rise rapidly, the outer end (outer arc) will have a large amount of expansion, while the inner end face will basically expand in place, and the outer arc will expand. The amount of expansion should be absorbed by the cracks on the upper end face, but these cracks are filled with quartz sand or siliceous refractory mud. If the top wire is loosened at this time, the furnace cover may form a "down opening".If the top wire is not loosened, the furnace cover will rise, forming a slight "upper opening", but it is covered by the heat preservation of the furnace cover and cannot be seen. Larger cavitation will occur in the force of the whole furnace top wire and fixture. Especially when the furnace cover is kept warm, the top climbing sign has been removed (even if it is not removed, it will collide and shift and lose its guiding function). Because there is no pressure hand and other testing equipment, it is not completely reliable based on human feeling. The stress of each part is inconsistent, and the uneven stress will generate stress, which has an impact on the life of the brick body. In addition, during the "heat protection", the ambient temperature is too high, the hot air and radiant heat on the furnace cover make people feel suffocated, the heat conduction under the feet, the labor insurance leather shoes are baked and deformed, the feet are hot and sweaty, and the workers can't stay on the furnace cover. In this working environment, it is impossible to do the heat preservation work of the furnace cover very carefully.
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Through actual work experience and a large number of case comparisons, the effect of "cold insulation" of the furnace cover is better than that of "hot insulation", and the probability of problems is smaller (if there is a fire through the furnace cover, a considerable part of the reason is that the bricks before and after the same row of bricks are in the same row. For example, the 115m2 furnace cover of an enterprise in Sichuan is "cold insulation", which has been used for nearly 13 years. From the outside, it is still very good, and there has been no fire penetration phenomenon.
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