This area roughly includes the entire upper structure of the melting pool itself, plus the small furnace neck, but does not include the regenerator or any regenerator refractory material. The surface temperature in this area is slightly higher than the surface temperature of the refractory bricks in the glass contact area (generally 66~93℃ higher than the actual glass temperature). The temperature variation is very large, and the highest temperature is generally in the hot spot area in the rear half of the length of the melting pool. In soda-lime glass melting furnaces, this temperature can reach up to 1649℃; in some borosilicate glass melting furnaces, this temperature is higher.
Compared with refractory bricks in contact with glass, the choice of refractory bricks for the upper structure is very different, mainly depending on whether the refractory bricks are eroded horizontally or vertically. Generally speaking, the horizontal surface of fully or partially exposed refractory bricks is subject to both steam erosion and a certain degree of erosion by batch dust or flying solid particles; while refractory bricks with only vertically exposed surfaces are usually mainly affected by corrosive steam.
If the refractory is already molten, steam attack causes the melt to run and drip from the refractory surface. This running and dripping can damage the underlying refractory and cause defects in the final glass product. If the refractory attacked by steam is not damaged (which is often the case), it is important that the surface of the refractory is stable to spalling, delamination, filming and cracking reactions. These reactions make it difficult to homogenize the refractory that settles in the glass, thus causing glass defects.
In the case of soda-lime glass, the types of vapor are generally sodium oxide, sodium sulfate and limited other volatile components of the batch. The main type of vapor in borosilicate glass appears to be sodium tetraborate. The solids attacking the horizontal surface of the refractory are mainly alkaline earth metal oxides and silicon dioxide from the batch.
The temperature fluctuations in various areas of the superstructure are greatly increased compared to the temperature fluctuations in the melting pool itself, but are not excessively large, except in the vicinity of the burner and the nozzle brick itself. The cause of this temperature cyclic fluctuation is the reversal of combustion in a typical regenerative furnace.
Generally, refractory materials with greater resistance to erosion by steam and solid solution should be used near the superstructure, although silica bricks with poor erosion resistance are still used in some places in the glass industry. In places such as the breast wall of the melting furnace, silica bricks are easy to melt and produce silicon-containing flow. Even if such silicon-containing flow occurs, it is easier to homogenize than flow containing alumina or zirconia.
The refractory materials used in various parts of the superstructure are not the same in variety and quality. They are mainly selected based on experience and economic effects, but obviously related to the strict requirements of various production conditions, because these conditions vary from factory to factory and melting furnace to furnace. If the condensation reaction of steam has the opportunity to occur at the joints between the colder refractory bricks in the superstructure, it may also cause the superstructure refractory materials to melt and damage. Insulation measures are widely adopted in almost all areas of the superstructure. For certain refractory materials used in the superstructure, this insulation measure has additional benefits because their resistance to thermal fluctuations is relatively poor, so taking insulation measures can reduce the amplitude of thermal fluctuations in important areas, thereby reducing the tendency of the refractory materials to delaminate, crack, peel, etc.
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