(3) Crown structure
The crown is used to form a flame space with the breast wall and the front wall. At the same time, it can also serve as a medium for the flame to radiate heat to the material and glass liquid, that is, to absorb the heat released when the fuel is burned and then radiate it to the surface of the glass liquid. The weight of the crown is transferred from the steel crown slag through the upper iron and the vertical column to the steel structure at the bottom of the kiln. The height and characteristics of the crown can be reflected by the span ratio. From a thermal perspective, a lower crown is beneficial, as it can radiate heat to the glass liquid as much as possible. Reducing the height of the crown can be achieved by reducing the height of the breast wall and reducing the crown strands. However, the height of the breast wall is restricted by factors such as the small furnace outlet and the structural strength of the crown; the smaller the strand height, the greater the thrust, and the less heat dissipation. Reducing the crown strands will increase the horizontal thrust of the crown and increase the instability of the crown. Generally, the crown strand span ratio of large float glass melting furnaces is about 1:8. Depending on the length of the melting section, the crown can be divided into several sections, generally at least three. During construction, expansion joints of approximately 100-120 mm are reserved between each section. Wider expansion joints are required at the crown tops of the front and rear gables.
The crown is typically constructed with high-quality silica bricks in a wedge-shaped configuration, with staggered horizontal joints. The size of the mortar joints (also known as mud joints) is determined by the specific requirements of the mortar (also known as mud slurry) used, generally ranging from 1-2 mm. The crown slag of the crown in float glass furnaces is mostly steel, and air cooling is required. The extended lines of the inclined surfaces of the steel crown slag on both sides must pass through the center of the crown arc, and the angle formed by these lines is the crown's central angle. The life of the crown determines the life of the entire furnace. The weak links of the crown in use are the holes such as the temperature measuring holes and pressure measuring holes, the transverse seams of the crown bricks (also known as the top seams), the crown heads of each crown section, and the side crowns of the crown. When the kiln is in normal operation, the kiln is under positive pressure. The various holes on the crown top can easily become larger due to fire penetration. If the side crown is not in close contact with the steel crown slag, it can easily be eroded and burned by the flame. Therefore, these places should use refractory materials with better performance. Currently, sintered zircon bricks are more commonly used.
(4) Structure of the pool wall and pool bottom
The kiln pool consists of two parts: the pool wall and the pool bottom. Both the pool wall and the pool bottom are built with large bricks. The kiln pool is built on steel beams supported by the kiln's lower columns. The weight of the entire kiln pool and the molten glass it holds are borne by the steel structure supported by the columns. The columns of a float glass furnace are generally made of concrete or steel. Atop the columns are I- or H-beam main beams running the length of the kiln. Large float glass furnaces typically have four main beams, with secondary I-beams mounted perpendicular to them. In the past, when kiln floors were not insulated, flat steel was laid directly on the secondary beams, and clay bricks were placed on top of the flat steel. In this case, the secondary beams should avoid the gaps between the bricks, with two flat steels and two secondary beams underneath each brick. With the widespread adoption of insulation technology, the kiln floor structure has evolved accordingly. Channel steel is laid perpendicular to the secondary beams, with brick stacks nestled within the channels. Clay bricks from the tank floor are then laid on top of the brick stacks. Before the bricks are laid, movable steel plate supports are welded to the channel steel, and insulation is built between the brick stacks and above the supports. After the tank depth is reduced and the kiln bottom is insulated, the temperature of the bottom glass liquid rises and its fluidity increases. To reduce corrosion of the tank bottom bricks by the glass liquid, a protective layer is laid on top of the clay bricks. This layer consists of a 25mm thick layer of zircon or zirconium corundum ramming mass, and then a 75mm thick layer of fused zirconium corundum or sintered zirconium corundum bricks on top of this. The tank walls are built on the clay bricks at the bottom. Due to the combustion of fuel and the melting of batch materials on the surface of the molten glass in the melting section, the surface temperature of the glass liquid reaches over 1450°C. The convection of the glass liquid is also strong, and the liquid level fluctuates up and down. As a result, the tank walls are severely corroded, especially near the glass liquid level line, where the wall deteriorates rapidly. In the past, due to investment costs and other factors, the tank walls often adopted a multi-layered structure, with clay bricks in the lower section, fused mullite bricks in the middle section, and fused zirconium corundum bricks in the upper section. This structure resulted in uneven erosion of the tank walls, with erosion being most severe near the liquid surface, significantly impacting the quality of the molten glass.
Currently, float glass furnace tank walls are constructed using single, large bricks—typically knife-handle bricks laid vertically and dry-laid. These bricks are typically made of AZS33 fused bricks. This type of tank wall has no horizontal joints, offers a higher quality, slower erosion, less contamination to the molten glass, and a longer service life, making it widely used. Tank wall thickness has been reduced from 300mm to 250mm. As expectations for furnace life continue to rise, research into tank wall structures has continued. After 2000, knife-handle-shaped tank wall bricks were adopted and popularized in float glass furnaces. These bricks are made of AZS33 and AZS36 fused bricks, with some companies also using AZS41 fused bricks. However, AZS41 fused bricks have poor thermal stability and are prone to cracking during kiln baking. Therefore, the thinner the pool wall thickness, the better the cooling effect of the cooling air. The use of knife-shaped bricks allows for two brick ties and slows erosion, thus greatly extending the life of the pool wall (to over 10 years).
03 Neck and cooling section
The neck is located between the melting section and the cooling section. It is used to install a cooling water bag and agitator to isolate the influence of the melting section airflow on the glass forming in the cooling section. Because the viscosity of the melted glass liquid is low and not suitable for forming, it must be cooled to make its viscosity reach the viscosity range required for forming. Therefore, a cooling section is set up. The structure of the cooling section is basically the same as that of the melting section. It is also divided into two parts: the upper space and the lower tank kiln. The difference is that the height of the breast wall is lower than that of the melting section, and the depth of the tank bottom is shallower than that of the melting section. The cooling method generally adopts natural cooling, which mainly relies on the uniform heat dissipation from the glass liquid surface and the tank wall and bottom to the outside for slow cooling. (1) Neck structure Since the birth of the float glass process in China, the commonly used neck structures are mainly low-pitched structure and hanging wall structure.
① Low Wall Structure
The earliest low walls used in domestic float glass production lines had the same or very similar spans and heights for the rear gable crown of the melting section, the neck crown, and the front gable crown of the cooling section. The parapet was not very high, and some neck crown ballast bricks were directly placed on the tank wall to minimize the space opening (this is often referred to in the industry as a "neck structure without agitators"). With technological advancements and increasing demands for glass quality, agitators were gradually installed at the neck. Agitators come in two types: vertical and horizontal. Vertical agitators are inserted through a pre-recorded hole at the top of the neck crown. This type of agitator has no specific height requirement for the neck parapet. Horizontal agitators are inserted from the parapets on either side of the neck and are installed in pairs. This configuration does not require a hole in the crown top, but requires a hole approximately 300mm high and long enough in the neck parapet to facilitate agitator insertion. Therefore, the parapet must be elevated. This structure also creates the conditions for moving large water pipes from the end of the melting section to the breastplate.
② Suspended Wall Breastplate Structure
Due to concerns about the safety of the low crown structure, the span-to-stretch ratio cannot be too small. Consequently, the space opening is relatively large, and the partitioning effect is not very good. This is especially true with the use of horizontal agitators and the increased breastplate height, which further deteriorates the performance. Therefore, the neck structure with a suspended wall has emerged. This structure allows for a larger span-to-stretch ratio, improving safety, while the space partitioning is achieved by the suspended wall. This type of suspended wall is currently available both domestically and internationally. The refractory materials used for the suspended wall are primarily high-quality silica bricks and sintered mullite bricks. The bricks are shaped in an I- or W-shaped pattern, and the entire wall is constructed by interlocking individual bricks, secured by steel plates on both sides. In addition to the two aforementioned neckplate structures, various other neckplate structures have been introduced from abroad in recent years, including U-shaped suspended crowns, double L-shaped suspended crowns, and suspended flat crowns. These neckplate structures are complex and require significant investment, and have been adopted and promoted in some high-end glass and extruded glass production lines in China.
(2) Structure of the cooling section
The function of the cooling section is to cool the melted glass evenly. The structure of the cooling section is basically the same as that of the melting section, and also includes the crown, crown slag, breast wall, pool wall and pool bottom and corresponding steel structure. However, the pool depth can be the same as that of the melting section or slightly lower. The span of the crown is smaller than that of the melting section, so the structure is slightly simpler, but the refractory materials used vary according to the requirements of the glass quality. The cooling section pool wall and pool bottom paving bricks of high-grade glass are generally made of α-βAl2O3 bricks, and the ramming layer under the paving bricks is made of α-βAl2O3 ramming material. These materials have a zero foaming index and a zero pollution index, so they do not pollute the glass liquid. It is better to use high-quality silica bricks for the breast wall and crown.
04 Port and regenerators
Port and regenerators are the main components of the melting furnace structure. The port and regenerator structure combination of float glass melting furnaces has two forms according to the fuel type, namely box combination and semi-box combination. Furnaces burning oil and natural gas adopt box combination, while furnaces burning producer gas adopt semi-box combination. The port and regenerators of float glass melting furnaces are arranged symmetrically on both sides of the tank furnace. Depending on the scale of the melting volume, 4 to 10 pairs of ports are set.
1. Port
(1) Types of Port
Float glass melting furnaces have different types according to the fuel used. When the fuel is producer gas, its combustion equipment is called a port, and the port mouth is called a flame nozzle. When the fuel is heavy oil or other liquid fuel, a nozzle (i.e., burner) is used, and the port mouth should be called a jet nozzle.
(2) Function of the port
The port is an important component of the glass melting furnace. It is a device that preheats and mixes the fuel and air and organizes combustion. The flame must ensure a certain flame length, brightness, rigidity, and sufficient coverage, without drifting or stratification, while also meeting the required temperature and atmosphere within the kiln. Gas and air are preheated in the regenerator, then flow through vertical channels (ascending channels) and horizontal channels, respectively, into the precombustion chamber. There, they mix and partially combust, then are injected into the kiln at a specific direction and velocity for continued combustion. The flue gas then enters the port opposite, thus serving as an air and smoke duct. However, the structure of the small furnace plays a crucial role in both heat transfer within the kiln and the glass melting process. Currently, most domestic float glass melting furnaces with a production capacity of 400 t/d or more utilize six pairs of ports, while those with a capacity of 700 t/d or more employ seven pairs, with as many as ten pairs. The characteristics of fuel oil, coal, or gas determine the differences in their technical parameters during furnace design. These include the ratio of the total area of the furnace's outlet to the melting area, and the downward inclination angle of the furnace's slant arch.
(3) Structure of a small port
The port consists of a top arch, side walls and a pit bottom. The arch connecting the small furnace to the melting furnace is called the port flat arch, the arch connecting to the regenerator is called the rear flat arch, and the arch in the middle is the inclined arch. The arch, side walls and pit bottom form the port space. The flat arch of the float glass melting furnace adopts an inserted structure, made flat on the top and curved on the bottom, and matched with the breast wall of the melting furnace. The measure to prevent the breast wall from tilting inward is to design the breast wall surface inward, and the arch bricks of the large arch are directly pressed against the breast wall. The inclined arch of the small furnace is an important part of the small furnace and is also a part that is easily burned. The design of the inclined arch must match the corresponding port flat arch structure.
(4) Structural characteristics of gas-fired ports
In addition to the above differences in structure between gas-fired ports and oil-fired ports, the most important difference is the furnace tongue. Typically, the protruding length of a port tongue is 400-450 mm. The height of a typical gas-fired furnace is 400-500 mm, with a span-to-rise ratio of 1:10. There are currently two types of sloping arches for gas-fired regenerator furnaces: a straight-through type and a trumpet-shaped type. The advantages of the straight-through type are: the gas exits the furnace in a flat ascending duct, easily mixing with the combustion air. This minimizes erosion of the mixed gas against the furnace sidewalls, and the furnace structure is simple and easy to construct. The trumpet-shaped type also has the advantage of forcing the flame into a diffuse pattern, increasing flame coverage and improving the poor maintenance environment caused by the narrow spacing of the gas ascending ducts.
2. Regenerator
The regenerator is essentially a waste heat recovery device—part of the exhaust gas waste heat utilization system. It utilizes refractory materials (called checker bricks) as heat storage elements to store some of the heat from the exhaust gas, which is then used to heat the air entering the kiln. When the high-temperature exhaust gas in the kiln flows through the lattice of the regenerator, it heats the lattice bricks. During this process, the temperature of the lattice bricks gradually increases. The heat stored in the lattice heats the gas or air flowing through the lattice after the flame turns, thereby ensuring that the flame has a high enough temperature to meet the needs of glass melting. During this process, the temperature of the lattice gradually decreases, and the cycle continues. Therefore, the role of the regenerator is to absorb and store the heat contained in the exhaust gas through the lattice bricks, and then transfer it to the air and gas, heating them to a certain temperature to achieve the purpose of saving fuel and reducing costs. The temperature of the exhaust gas discharged from the glass melting furnace is about 1400-1500℃. The gas can be preheated to 800-1000℃ and the air can be preheated to 1000-1200℃. The temperature of the exhaust gas discharged from the regenerator is about 600℃.
(1) Structure of the regenerator
The regenerator consists of a top arch, inner and outer walls, end walls, partition walls, lattice and grate bars. The thickness of the top of the regenerator of a float glass melting furnace is generally equal to or greater than 350 mm. It is built with high-quality silica bricks, with a center angle of 90° to 120°, depending on the specific situation. The side walls, end walls, and partition walls are generally 580 mm thick. Generally, the lower part is built with low-porosity clay bricks, the middle and upper parts are built with alkaline refractory materials, and the upper part is also built with silica materials.
(2) Forms of regenerators
In order to improve the heat storage performance and service life of the regenerator, there are many forms of regenerators at home and abroad. However, as far as domestic float glass melting furnaces are concerned, the most common ones are connected structures, separated structures, semi-separated structures, two-small furnace connected structures, two-stage structures, and fully connected structures. The connected structure is that the air regenerator under the small furnace on one side of the melting furnace is a connected chamber, and the gas regenerator is also a connected chamber. This form is prone to local overheating due to uneven airflow distribution, which causes the checker bricks to burn out quickly. It is rarely used now. The partitioned structure separates the regenerator into individual furnaces, preventing gas from flowing between chambers. Gas distribution is regulated by dampers on the branch flues within each chamber. This structure offers advantages in terms of convenient gas distribution and control, and facilitates lattice repairs. However, the numerous partitions reduce the volume of the lattice, resulting in a smaller heat exchange area and lower thermal efficiency. The semi-partitioned structure separates the flues above the regenerator grate into individual furnaces, while the regenerator itself remains unpartitioned. The gas distribution dampers remain on the branch flues. The two-furnace interconnected structure separates each furnace into a chamber, with each furnace receiving a branch flue to regulate gas distribution. Compared to the partitioned structure, this design reduces the number of partitions, increases the heat exchange area within the lattice, and improves thermal efficiency. However, this reduction in partitions reduces the stability of the side walls. Furthermore, the interconnected nature of the two regenerators makes lattice repairs difficult, requiring both furnaces to be repaired simultaneously, significantly impacting production. This type of regenerator is currently widely used in large float glass furnaces. The two-stage structure divides a single regenerator into two chambers, separated by a partition wall and connected by a vertical passageway. This divides the regenerator into a high-temperature zone and a low-temperature zone. This structure primarily prevents corrosion of the checker bricks from the gas-liquid-solid transformation of sodium sulfate, confining the transformation within the connecting passageway and extending the life of the checker bricks. Due to its complex structure, this design is now rarely used. The fully connected structure connects the entire regenerator on one side of the furnace into a single chamber, with branch flues configured for each furnace to regulate gas distribution. This regenerator maximizes the heat exchange area of the latticework and offers high thermal efficiency. However, the lack of partition walls results in less stable sidewalls, making thermal repair impossible if checker bricks collapse or become blocked. This type of regenerator is also currently used in large float glass furnaces.
(3) Furnace bars The furnace bars are refractory structures that bear the weight of the lattice body. In fact, they are also arch structures, but they are made of single arch bricks. There are gaps between the bars for ventilation, so they are called furnace bar arches. Since the furnace bar arch is an arch that bears the weight of the lattice body (with lattice bricks stacked on it), the top of the arch must be leveled. There are two ways to level the arch. One is to use climbing bricks to level the top of the arch, and the other is to directly use arch bricks that are flat on the top and curved on the bottom. The width and height of the furnace bar arch should be determined based on the weight of the lattice body that the furnace bar bears. Generally, the width is not less than 150mm, the height is not less than 300mm, and the spacing between each furnace bar is not less than 150mm. In order to increase the stability and integrity of the single furnace bar, two reinforcing bricks are usually added to the furnace bar arch. The refractory material of the grate bar is generally built with low-porosity clay bricks.
(4) Grid body
The grid body is the heat transfer part of the regenerator and is the most important component of the regenerator structure. Whether the structure of the grid body is reasonable not only affects the service life of the regenerator, but also directly affects the heat storage efficiency of the regenerator, and thus affects the thermal efficiency of the entire melting furnace. Therefore, the refractory material that constitutes the grid body is required to be resistant to high temperature, erosion, store a lot of heat, transfer heat quickly, and have good thermal vibration stability, and the entire grid body is required to have good structural stability.
05 Flue
(1) Function and classification of flue
The flue is a gas channel, which is the function of the flue. The exhaust gas after the fuel is burned in the kiln goes down from the small furnace to the regenerator, and then discharged into the atmosphere through the flue and chimney. In addition to being used for exhausting smoke and supplying air, the flue can also be used to adjust the gas flow and pressure in the kiln by setting a gate. The function of the flue is to use its height to generate a certain amount of suction to overcome the resistance of the kiln system, including the chimney itself, so that air can be sprayed into the kiln at a certain speed and the combustion products can be discharged outside the kiln. The flue system includes air flue, gas flue, air branch flue, gas branch flue, intermediate flue, main flue and flue leading to the waste heat boiler.
(2) Structural form of the flue
The flue is an arch structure above the flue. The center angle of the flue is generally 90 degrees, the thickness of the flue is 230 mm, and the bottom is a rectangular cross-section. Generally, the height is slightly greater than or equal to the width. Since the exhaust gas temperature passing through the flue is relatively high (500-600℃), the inner wall is built with refractory clay bricks, the outer wall is built with red bricks, and the bottom is made of concrete as the foundation. In order to prevent the concrete temperature from being too high, diatomaceous earth insulation bricks are generally laid. The top and side walls of the ground flue or outdoor flue are generally insulated to prevent excessive temperature drop.
(3) Flue layout
① Heavy oil or natural gas combustion
The flue layout of the float glass melting furnace that burns heavy oil or natural gas is relatively simple. The flue is arranged inside the heat storage chamber, that is, below the kiln pool, and consists of a main flue, a branch flue, and a branch flue. The branch flue is equipped with a flue gas damper and a combustion air inlet. The branch flue is equipped with an air (smoke) exchange damper (commonly known as a large damper or a reversing damper). The main flue is equipped with a rotating damper to adjust the kiln pressure. A damper is provided at the base of the chimney to adjust the suction force.
② Gas combustion
The float glass melting furnace that burns gas has two flues, air and gas, and a gas reversing jump cover, so its flue layout is more complicated.
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