Tunnel Kiln Heating, Thermal Shock, and Phase Transformation: Analysis and Process Optimization

I. Core Concepts and Phenomena
1. Thermal Shock and Heating Rate:
(1) Thermal shock refers to stress caused by local rapid heating and temperature gradients, which can lead to cracks or failure.
(2) In brick production, if the tunnel kiln heats up too quickly, the temperature difference between the brick’s surface and its interior increases. Combined with the rapid evaporation of residual moisture causing volume shrinkage, cracks or even brick explosions can easily occur.
2. Residual Moisture and Cracks Caused by Rapid Moisture Removal:
If residual moisture in the brick stack is not fully removed, entering a rapid heating phase will cause the water to turn into steam quickly at high temperature. The resulting volume expansion intensifies internal shrinkage stress beyond the body’s strength, leading to cracks or explosions—a typical manifestation of “thermal shock.”
3. Quartz Inversion and Expansion:
Brick raw materials usually contain 55–70% silica (quartz). At about 573°C, quartz undergoes a β→α structural transformation accompanied by significant volume expansion. If heating is too fast during this stage, thermal expansion stress may cause brick explosions. This phenomenon was known in traditional earth kilns and has been confirmed by modern theory.

II. Importance of Drying Quality: The Root Determines Success
The saying “Those who master drying, master the market” has become a consensus in the brick industry, emphasizing the decisive role of drying in subsequent firing quality:
1. Residual moisture is the root of cracks and explosions: If drying is insufficient, high temperatures will cause steam expansion, producing destructive stress in the brick body.
2. Surface–interior temperature difference increases thermal shock: Uneven drying and stacking patterns cause uneven heating, worsening uneven thermal stress.
3. Multi-layer stacking increases uneven heating: Compared with the single-layer firing of roller kilns, the multi-layer stacking in tunnel kilns leads to uneven heating and requires slower heating to ensure uniformity.

III. Comparison of Kiln Types: Revealing Heating Strategy Differences

Although roller kilns use similar raw materials and firing temperatures, their single-layer structure ensures temperature uniformity, allowing much shorter cycles. Tunnel kilns, however, require slow heating due to stacking, ventilation, and complex heat flow paths to avoid damage.

IV. Optimization Measures and Process Control Recommendations
1. Stage-by-Stage Drying:
(1) Ensure all bricks are fully dehydrated, with residual moisture below 6–8%.
(2) Use “layered drying” instead of “stacked drying” to reduce shrinkage differences between layers.
2. Segmented Heating Rate Control:
(1) Preheating Stage (up to ~120°C): Keep heating rate below 30–50°C/h to avoid network cracks.
(2) Firing Stage (mid–high temperature): Further limit heating rate to 20–30°C/h to prevent surface over-melting (“bread bricks”).
(3) Holding Stage (600→500°C): Avoid rapid cooling to prevent “dumb bricks” (hidden cracks).
3. Optimize Heat Flow Structure and Ventilation:
(1) Use side-firing, fire walls, and flame shields to prevent direct flame or hot air impact on brick surfaces.
(2) Implement precise ventilation and temperature control to ensure uniform heat distribution.
4. Real-Time Temperature Monitoring and Thermal Analysis:
Introduce systems such as PhoenixTM with thermocouples and telemetry to record kiln temperatures in real time.
5. Different Strategies for Different Products:
Common red bricks allow some size deviation; high-grade facing bricks require higher dimensional accuracy and should use single-layer drying to reduce shrinkage differences.

V. Summary Recommendations
1. Drying quality is key: Efficient, thorough drying is the first defense against cracks and explosions.
2. Phased rhythm control: Heating should be segmented to avoid thermal shock while ensuring efficiency.
3. Structural and firepath optimization: Ensuring thermal uniformity is crucial for quality improvement.
4. Technical monitoring supports decisions: Real-time temperature monitoring helps detect and correct issues promptly.
5. Adjust strategies by product type: Different products require tailored controls to meet appearance and performance needs.