Float glass furnaces are structurally similar to other flat glass furnaces. They are shallow, horizontally flamed tank furnaces. However, float glass furnaces are significantly larger in scale. Currently, the highest daily melting capacity of float glass furnaces worldwide can reach over 1,100 tons (usually expressed as 1,000 tons/day). While float glass furnaces differ from other flat glass furnaces, their structures share commonalities. The structure of a float glass furnace primarily includes the charging system, melting system, heat source supply system, waste heat recovery system, and flue gas supply system.
01 Charging Pool
The charging pool is located at the beginning of the furnace. It is a small rectangular pool protruding from the outside of the furnace and connected to the furnace pool. The charging port consists of two parts: the charging pool and the upper retaining wall (front wall). Batch materials are fed into the furnace through the charging port.
1. The size of the feeding pool
Feeding is one of the important process links in the melting process. It is related to the melting speed of the batch material, the hot spot position of the melting zone, the stability of the bubble boundary, and ultimately affects the quality and output of the product. Since the melting volume of the float glass melting furnace is large, a horizontal flame tank furnace is used, and its feeding pool is set at the front end of the melting pool. The size of the feeding pool varies with the size of the melting pool, the state of the batch material, the feeding method and the number of feeding machines. The state of the batch material is powdery, granular and slurry (currently powdery is generally used); the feeding method is determined by the selected feeding machine, which includes spiral, ridge, roller, reciprocating, wrap-in, electromagnetic vibration and inclined blanket. (Currently, ridge and inclined blanket feeding machines are mostly used).
(1) The size of the feeding pool using a ridge feeding machine The width of the feeding pool using a ridge feeding machine depends on the number of feeding machines selected, and the length of the feeding pool can be determined according to the process layout and the structural requirements of the front wall.
(2) The size of the feeding pool of the inclined blanket type feeding machine The inclined blanket type feeding machine has been widely used in the market. Its feeding method is similar to that of the ridge type feeding machine, except that the feeding surface is much wider than that of the ridge type feeding machine. Therefore, the size of its feeding pool is not much different from that of the feeding pool of the ridge type feeding machine in design. It is still determined by the width of the melting tank and the requirements of the feeding surface. With the maturity of glass melting technology and the update of melting process, the width of the feeding pool of float glass melting furnace is getting larger and larger. Because the heat absorbed by the batch material is proportional to its coverage area, the wider the feeding pool, the larger the coverage area of the batch material, the more conducive to improving thermal efficiency and energy saving, and the more conducive to improving the melting rate. Therefore, in the design of large float glass melting furnaces, the feeding pool and the melting tank are all equal or quasi-equal width. As the width of the charging pool continues to increase, large inclined blanket charging machines have emerged. For furnaces with a melting and charging pool width of 11 meters, two inclined blanket charging machines can meet production and technical requirements.
02 Melting Section
The melting section of a float glass furnace is where the batch materials are melted and the molten glass is clarified and homogenized. The melting section consists of a melting zone and a clarifier zone, with the upper and lower sections divided into an upper flame space and a lower furnace pool. The upper space, also known as the flame space, is a flame-filled space enclosed by the front wall, the glass surface, the large crown of the kiln roof, and the breast wall of the kiln wall. The lower furnace pool consists of the tank bottom and walls. The melting section is responsible for converting the batch materials into molten glass through physical and chemical reactions at high temperatures, while the clarifier zone is responsible for quickly and completely discharging bubbles from the resulting molten glass to achieve the required glass quality. 1. Flame space The flame space is filled with hot flame gas supplied by the heat source. The flame gas uses its own heat to melt the batch material and also radiates it to the glass liquid, kiln wall and kiln roof. The flame space should be able to meet the complete combustion of the fuel, ensure the supply of heat required for glass melting, clarification and homogenization, and minimize heat dissipation. 2. Pool kiln The pool kiln is the part where the batch material is melted into glass liquid and clarified and homogenized. It should be able to supply a sufficient amount of completely melted transparent glass liquid. In order to ensure that the kiln pool reaches a certain service life, the pool wall thickness is generally 250-300mm, and the pool bottom thickness varies according to its insulation conditions. If no insulation belt is used, the pool bottom thickness is generally 300mm.
(1) Front wall structure
The front wall is the front end wall of the melting section flame space, spanning the upper part of the charging pool to block the escape and heat radiation of hot gas (including flame) at the charging port at the front end of the melting furnace. Because the front wall is easily damaged by flames and erosion from material powder, and is prone to deformation during hot air kiln heating, most domestic float glass manufacturers currently use L-shaped suspended walls.
Compared to traditional multi-panel designs, L-shaped suspended walls offer advantages such as extended front wall service life, enhanced energy efficiency, improved on-site environment, protection for the feeder, increased melting speed, reduced dust generation, and increased lattice life. When designing the front wall, careful attention should be paid to selecting a reasonable distance from the centerline of the melting furnace. Too small a distance will accelerate front wall burnout, reduce batch preheating, and increase furnace burnout and blockage. Too large a distance can result in low feed tank temperatures, resulting in melting of the batch pile and difficulty in advancing the material. Currently, the distance between the front wall and the centerline of the melting furnace in domestic float glass production lines generally ranges from 3.2 to 4.3 meters, depending on the fuel and tonnage.
① Arched arch front wall
This type of front wall consists of two or three arches and refractory bricks built on top of them. A fire barrier is added to the arched opening below the front wall to block flames, saving fuel and protecting the feeder. The fire barrier's load is provided by a large water bag spanning the feeder tank. Knife-handle-shaped refractory bricks are mounted on the bag to prevent direct flame contact with the bag, and strip bricks are stacked on top of the knife-handle bricks. For safety reasons, the span ratio of this type of front wall is limited, so its span should not be too large, generally not exceeding 7 meters. Even so, the front arch and fire barrier are easily damaged by flame damage and erosion by the alkaline atmosphere. Damage to the fire barrier and water bag can be repaired and replaced hot, but severe damage to the front arch requires cold repair by draining water. Therefore, this type of front wall structure is being phased out in float glass furnaces, but it is still used in flat glass furnaces other than float glass furnaces. Conventional arch-shaped front walls are limited by span and safety considerations. Further increasing the melting area requires widening the charging pool and expanding the feeding surface. To address this conflict, L-shaped ceiling walls were developed.
②L-shaped ceiling wall structure: Large float glass furnaces widely utilize an L-shaped ceiling wall. This ceiling wall is independently suspended, and its height from the glass melt surface can be adjusted using mechanical jacks. Constructed of heat-resistant steel and refractory materials, the L-shaped ceiling wall's structural safety is unaffected by its width. The L-shaped ceiling wall can be as wide as the melting pool, meeting the requirements of equal or quasi-equal-width charging pool designs. The L-shaped ceiling wall, combined with a longer charging pool, not only reduces dust but also enhances pre-melting of the batch material. The L-shaped ceiling wall consists of a straight section and an L-shaped section. The straight section is constructed of high-quality silica bricks, while the nose section utilizes sintered mullite and sintered zirconium jade. The exterior of the ceiling wall is insulated with ceramic fiber felt, and a water bag is located at the front of the nose to provide a seal after cooling.
(2) Breast wall structure
Since the erosion conditions and thermal repair time of each part of the float glass melting furnace are different, in order to separate the most severely damaged parts for thermal repair, the breast wall, the arch and the kiln pool are divided into three separate supporting parts, and finally the load is transferred to the steel structure at the bottom of the kiln. The load-bearing of the breast wall is transmitted to the columns by the breast wall support plate (made of cast iron or angle steel) and the lower iron palm, and finally to the steel structure at the bottom of the kiln.
The design of the breast wall must ensure sufficient strength under high temperature. Among them, the hook brick is the key part. The hook brick is provided at the bottom of the breast wall to block the flame in the kiln and prevent it from passing through and burning the breast wall support plate and the iron palm. Generally, the breast wall of the melting zone adopts AZS33 electric melting bricks, the upper gap bricks adopt low creep and crack resistant sintered zircon bricks, and the breast wall of the clarification zone generally adopts high-quality silica bricks.
The height of the breast wall depends on factors such as the type and quality of the fuel, the melting rate, the melting heat consumption, the scale of the melting furnace, the heat dissipation, and the thickness of the gas layer. Theoretically, as long as the refractory material used in the breastwork ensures corrosion resistance, the breastwork should not be a critical component affecting the life of the furnace. However, in practice, many furnaces have experienced shortened breastwork life due to inward tilting of the melting zone breastwork. Some have even experienced breastwork collapse in later stages due to untimely discharge. This is primarily due to the inward tilting of the breastwork support plates (higher on the outside and lower on the inside) caused by the tightening of the bracing after the arch masonry is completed. Another factor is that after the bricks are tied to the tank wall, the breastwork support plates are exposed to the flame space, causing deformation and inward tilting. To reduce or prevent this phenomenon, an improved breastwork design has been developed. This design eliminates gap bricks, positions the arch foot directly against the breastwork, lowers the breastwork support plates, and intentionally tilts the upper breastwork inward. The arch side bricks use three layers of zircon bricks, and the hook design for the melting zone hook bricks is eliminated. This prevents inward tilting of the breastwork due to the quality issues of fused AZS hook bricks, which can cause hook brick breakage. In addition, some large melting furnaces have replaced 50mm thick ordinary carbon steel support plates with 60mm thick medium silicon ductile iron support plates, which has also achieved good results.
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