Fused cast AZS bricks are among the most important glass furnace refractories used in modern glass melting furnaces. In many glass plants, the melting furnace is one of the most expensive thermal units, and its service life directly affects production stability, repair cost, and the overall furnace campaign.
Since fused cast AZS refractory bricks became widely used, they have become the preferred material for glass furnace tank sidewalls. However, with rising costs of raw materials, fuel, electricity, and refractory production, fused cast AZS bricks have become increasingly expensive. In many glass furnaces, fused cast refractories account for more than 50% of the total refractory cost.
More importantly, the service life of AZS sidewall bricks often determines the operating cycle of the entire furnace. Therefore, proper use, cooling, inspection, and maintenance of fused cast AZS bricks are essential for extending furnace life and reducing unplanned shutdown risks.
This article uses a real horseshoe-flame glass furnace as a case study to explain practical methods for reducing AZS brick corrosion and extending the service life of glass furnace sidewall refractories.
The furnace discussed in this case was a 23.96 m² coal-fired horseshoe-flame glass melting furnace. Except for the throat cover brick and the charging port corner brick, which used AZS 41 bricks produced by the oxidation process, the melting tank sidewall used AZS 33 fused cast AZS bricks produced by the oxidation process.
The furnace was commissioned in August 2000 and operated until May 2007. During this period, only one minor cold repair was carried out. The repair mainly involved replacing the throat cover brick, charging port corner brick, several AZS sidewall bricks, and checker bricks in the regenerator. Most of the other refractory materials remained in service throughout the furnace campaign.
The furnace was used for melting high-white glass. Its main operating parameters were:
Through a series of cooling, patching, and daily maintenance measures, the service life of the furnace was significantly extended.
Reducing the corrosion rate of AZS sidewall bricks is the key to extending the life of glass furnace refractories. One of the most effective ways to slow down corrosion is to reduce the operating temperature of fused cast AZS bricks.
However, the same cooling method is not suitable for every furnace area. Different glass furnace zones, different operating periods, and different corrosion conditions require different cooling strategies.
The upper part of the glass furnace tank sidewall, especially the top 270 mm area without external insulation, is usually exposed to severe corrosion.
In a horseshoe-flame furnace, the flame heats the glass surface directly. As a result, the temperature of the upper glass layer is much higher than that of the lower layer. In addition, silicate formation, fining bubbles, and strong thermal convection inside the molten glass are more intense near the upper tank area.
This makes the upper AZS sidewall brick area one of the fastest-corroding zones in the glass melting furnace.
For this reason, a properly designed forced air cooling system is necessary. The cooling fan and air duct arrangement should ensure that each sidewall area receives the required air volume.
The cooling air system should be installed and started during furnace heat-up when the temperature reaches about 250–300°C. As the furnace temperature increases, the air volume should be increased gradually.
Some plants only install or start sidewall cooling during the middle or later stage of furnace operation, often after the AZS sidewall brick already shows a visible light-red color. At that point, part of the brick has already been corroded.
Cooling should start early, but it must be increased slowly and steadily. Sudden cooling may create excessive thermal stress due to a sharp temperature gradient, which can damage fused cast AZS bricks.
Based on practical glass furnace operation experience, AZS refractory bricks may generally last three to five years under normal operating conditions.
After about two years of operation, the furnace usually enters the middle-to-late campaign period. At this stage, localized problems may appear due to differences between individual AZS bricks, internal cavities, large joints formed during heat-up, or improper joint treatment. These weak areas may begin to leak molten glass.
For this type of localized leakage, single-point concentrated forced air cooling can be effective.
In this case study, molten glass leakage appeared at an expansion joint near a sidewall corner after two years and three months of operation. After two years and six months, another large joint in the glass furnace sidewall also began to leak glass.
The plant used concentrated forced air cooling at these leakage points. This method helped increase the viscosity of the glass near the leakage area and reduce the working temperature of the affected AZS bricks. After this treatment, no serious problems appeared in these areas until the end of the furnace campaign.
Some areas of a glass melting furnace are especially difficult to protect. The throat area is one example. It is one of the fastest-corroding furnace zones and is also difficult to repair during operation.
Although design measures such as sinking and inclination can help improve the working environment, the throat area still suffers from strong mechanical erosion and gas drilling corrosion. Before cold repair, the throat cover brick in this furnace had been in service for only about 2.5 years and was already severely damaged.
Forced air cooling had been used from the beginning, but it was not enough to achieve the expected cooling effect.
During the cold repair, the throat area was changed to a semi-insulated structure. Instead of leaving the AZS brick surface fully exposed, the surface of the fused cast AZS brick was surrounded with high alumina bricks and tightened with a steel structure.
After the furnace was restarted, air-water mist cooling was applied. This method combines air and atomized water mist to improve cooling efficiency. It is simple to fabricate, stable to control, and provides a significant cooling effect.
Compared with direct water cooling, air-water mist cooling reduces the risk of refractory damage caused by improper water application. This is one of the main reasons many glass plants are cautious about using direct water cooling on AZS refractories.
After this improvement, the same throat area operated for more than four years after cold repair.
Atomized water shower cooling is mainly suitable for the middle and late stages of furnace operation, especially for the middle and upper areas of the glass furnace tank sidewall.
In the early stage of furnace operation, these areas usually do not show obvious problems because the middle and upper sidewall areas are often partially insulated and reinforced externally with steel plates. The corrosion rate is also relatively lower than in the most exposed upper sidewall area.
However, in the third year after cold repair, glass leakage gradually appeared in the middle and upper sidewall areas. These leakage points had one common feature: most of them occurred near the upper edge of the external insulation layer.
This indicated that molten glass had formed a channel between the fused cast AZS sidewall brick and the insulation layer, then flowed outward through that channel. It also showed that the AZS bricks in these areas had already been seriously corroded.
Because these areas were covered by external insulation, forced air cooling was not effective. Removing the backup bricks required significant labor and created safety risks. Direct water cooling was also difficult to control.
After comparison, atomized water shower cooling was selected. This method allowed the affected AZS sidewall areas to continue operating safely until the furnace campaign ended.
The method works by atomizing normal-temperature tap water and spraying it directly onto the leakage area. The main operation points are as follows:
Compared with air-water mist cooling, atomized water shower cooling is simpler. The system is easy to operate, easy to adjust, low in failure rate, and does not require additional complex equipment. It also produces little noise.
From actual use, the atomized water consumption was not high, while the cooling effect was good. It also avoided the uncontrolled water flow problem that can occur when direct water spray is not properly controlled.
External brick patching is a common and effective method used by many glass plants to extend glass furnace service life.
In the later stage of the furnace campaign, the plant applied external brick patching to several areas, including the charging port corner and the upper sidewall opposite the charging end.
When using this method, all patching bricks should be preheated before installation. This is especially important when fused cast AZS bricks are used, because sudden temperature differences may lead to thermal shock or cracking.
Good daily management is also essential for extending the life of fused cast AZS sidewall bricks. Even the best cooling design cannot work well without stable operation and continuous inspection.
The following measures were used in this case:
In this case study, different maintenance measures were applied to different glass furnace sidewall areas at different stages of furnace operation, especially after the minor cold repair. These measures included forced air cooling, concentrated point cooling, air-water mist cooling, atomized water shower cooling, and external brick patching. As a result, the service life of the glass melting furnace was significantly extended.
The use of atomized water as a cooling medium proved effective when properly controlled. Compared with direct water cooling, water mist cooling provides a more moderate and stable cooling effect on fused cast AZS bricks.
However, water-based cooling must be applied carefully. If atomization is poor or the control system is unstable, excessive cooling water may cause overcooling of the refractory. Minor damage may include microcracks. In more serious cases, especially when the remaining AZS brick thickness is less than 50 mm, a large temperature gradient and strong thermal stress may cause brick cracking.
Overcooling may also reduce the local glass temperature too much, increase viscosity, and form a stagnant glass layer. If this glass later enters the forming flow, it may cause product defects such as cords, streaks, or lines. In addition, large cooling areas can increase heat loss and affect furnace thermal efficiency.
Further work is still needed to improve atomizer performance. The key is to make the water droplets finer, more uniform, and better mixed with air. A good atomizing system should also be easy to control, allowing fused cast AZS bricks to be cooled properly without excessive thermal shock.
