Analysis of Positive and Negative Pressure Management in the Tunnel Kiln firing Process

1. Kiln Pressure:
In the production of clay fired bricks using tunnel kilns, kiln pressure is a key parameter affecting thermal efficiency, product quality, and safe operation. The pressure state is mainly divided into positive-pressure firing and negative-pressure firing modes. Under different modes, the airflow, temperature distribution, atmosphere control, and energy consumption inside the kiln differ significantly.
Reasonable selection and stable control of the pressure state is one of the core tasks in tunnel kiln thermal management.

2. Positive-Pressure Firing：
(1) Definition: When the kiln pressure is higher than the external atmospheric pressure, it is called positive-pressure firing.
(2) Characteristics and Advantages：
• Forms an airflow from the inside to the outside of the kiln, ensuring uniform heat distribution;
• Gas flow direction is usually from top to bottom, helping to maintain stable temperature and atmosphere in the kiln;
• At certain high-temperature firing stages, positive pressure helps reduce cold air inflow, avoiding temperature fluctuations;
• Beneficial for sintering products requiring a stable oxidizing atmosphere.
(3) Risks and Limitations：
• If fuel and air mix unevenly, local high-pressure zones may occur, increasing the risk of explosion or flame ejection;
• Excessive positive pressure may cause flue gas leakage, raising workshop temperature and affecting the working environment;
• High sealing requirements; otherwise, energy loss and safety hazards may arise.
(4) Control Points：
• Positive pressure should be kept within a safe range permitted by the process (e.g., +1 to +3 Pa, depending on kiln type);
• Must be linked with combustion and fan systems to avoid sudden pressure fluctuations;
• Kiln doors, inspection holes, etc., must be well-sealed to prevent hot gas leakage.

3. Negative-Pressure Firing：
(1) Definition: When the kiln pressure is lower than the external atmospheric pressure, it is called negative-pressure firing.
(2) Characteristics and Advantages:
• Airflow direction is from outside to inside, reducing risks of local overpressure and flame ejection;
• Reduces high-temperature flue gas leakage, helping to maintain lower workshop temperatures;
• May lower energy consumption, especially when waste heat recovery and exhaust treatment are optimized;
• Inflowing external air can partially dilute high-temperature flue gases, reducing equipment heat load.
(3) Risks and Limitations:
• Excessive negative pressure may cause large amounts of cold air inflow, leading to temperature fluctuations and reduced thermal efficiency;
• Kiln atmosphere may become oxygen-deficient, affecting combustion efficiency and sintering quality (especially for oxidizing atmosphere products);
• In humid or rainy seasons, external moist air may enter and cause brick rehydration.
(4) Control Points
• Negative pressure should remain stable (e.g., -1 to -3 Pa, depending on process requirements);
• Avoid long-term deep negative pressure to prevent massive cold air inflow;
• In humid climates, combine dehumidification and airflow control to prevent rehydration.

4. Comparative Analysis of Positive vs. Negative Pressure:

5. Comprehensive Recommendations and Application Choices:
(1) For high-quality products requiring a stable oxidizing atmosphere, positive-pressure firing is preferred, combined with precise pressure control systems;
(2) For energy-saving priority and lower thermal load, negative-pressure firing may be considered, provided oxygen deficiency and moisture issues are addressed;
(3) For long tunnel kilns, a segmented approach is often used: slight negative pressure in the preheating zone, slight positive pressure in the firing zone, and negative pressure in the cooling zone, to balance quality and efficiency;
(4) Regardless of pressure mode, kiln door sealing, fan speed regulation, and burner control must form a closed-loop management system.