A tunnel kiln is a type of continuous kiln used for firing ceramic products, bricks, and other materials, moving the product through different temperature zones on cars.
Tunnel kilns operate as a continuous process, allowing materials to be fed in at one end and exit at the other, passing through distinct temperature zones. This design ensures efficient production and consistent results.
The Continuous Process
Unlike batch kilns where the material is loaded, fired, and cooled in one chamber, a tunnel kiln features a long, insulated tunnel. Products, often stacked on refractory cars, move slowly through this tunnel on rails. The kiln is divided into three main zones, each maintained at a specific temperature:
Key Zones
- Preheating Zone: As the cars enter the kiln, they move through a progressively hotter zone. Here, moisture is removed from the material and organic matter is burned out. This zone is heated, in part, by the hot flue gases coming from the firing zone.
- Firing Zone: This is the hottest section of the kiln where the material reaches its peak firing temperature. Combustion takes place in this central zone.
- Cooling Zone: After passing through the firing zone, the cars enter the cooling zone, where the fired products gradually cool down before exiting the kiln.
Heat Flow and Energy Recovery
A key aspect of a tunnel kiln's efficiency is the counter-current flow of heat and product.
- The hot flue gases, generated in the firing zone after combustion, do not simply exit the kiln immediately. Instead, they travel towards the car entrance end, flowing in the opposite direction to the incoming green (unfired) bricks. As they travel, these gases lose heat, losing a part of the heat to the green bricks entering the kiln, effectively preheating them.
- Similarly, cool air is often introduced at the cooling zone exit and flows towards the firing zone, recovering heat from the cooling product and providing preheated air for combustion.
- Furthermore, hot air/gases are extracted from the tunnel kiln at several points along the length of the kiln. This extracted heat, which would otherwise be wasted, is a valuable energy source and are supplied to the drying tunnel/chamber, providing necessary heat for the drying process before the materials even enter the kiln.
Benefits of Tunnel Kilns
Tunnel kilns offer several advantages for industrial-scale production:
- Energy Efficiency: The counter-current heat exchange and recovery of waste heat significantly reduce energy consumption.
- Consistency: The controlled movement of product through stable temperature zones results in uniform firing quality.
- High Throughput: Continuous operation allows for large volumes of material to be processed efficiently.
- Automation: The process is well-suited for automation, reducing labor costs.
In essence, a tunnel kiln works by moving materials through a carefully controlled temperature gradient, utilizing efficient heat exchange and recovery processes to achieve consistent firing results on a large scale.
