Discussion on Pressure Regime in Brictec Tunnel Kiln Firing

During tunnel kiln firing, the rationality of the pressure regime has a significant impact on firing results. The kiln pressure regime refers to the distribution pattern of static pressure along the length of the tunnel kiln. By plotting the pressure data of different car positions, a “pressure curve” is obtained.

The pressure regime directly determines the flow state of gases inside the kiln, which in turn affects:
1. Heat exchange in the kiln;
2. The amount of combustion air required and the exhaust gas discharged;
3. The uniformity of pressure and temperature distribution inside the kiln.

I. Formation of the Pressure Regime
The pressure distribution inside the kiln is the result of multiple gas flows acting together:
1. Cooling zone: A large amount of cold air enters, while heated air is extracted.
2. Firing zone: Fuel combustion generates gases, with the addition of primary air and atomized fuel. Pressure in this zone is usually higher than atmospheric pressure, forming positive pressure.
3. Preheating zone: A large amount of flue gas and water vapor is discharged by the exhaust fan. Kiln pressure is lower than atmospheric pressure, forming negative pressure.
During the transition from positive pressure to negative pressure, there exists an interface where the internal kiln pressure equals the external atmospheric pressure. This is called the zero-pressure plane (or zero point/zero position).
(1) Before the zero-pressure plane (preheating zone) → Negative pressure zone, beneficial for discharging flue gas and water vapor, but may cause cold air infiltration, leading to thermal stratification and increased temperature difference.
(2) After the zero-pressure plane (firing zone) → Positive pressure zone, prevents cold air entry and maintains stable, uniform temperature.

II. Control of the Zero-Pressure Position
In practical operation, most tunnel kilns maintain the firing zone under slight positive pressure, and set the zero-pressure plane at:
1. The middle of the firing zone;
2. Or around 1/3 of the coal-feeding rows toward the insulation zone;
3. Or slightly behind this position.
This allows high-temperature gases to evenly fill the entire brick stack, ensuring uniform heating and insulation of bricks at high temperatures.

III. Key Points for Controlling Positive Pressure
1. Advantages of moderate positive pressure: Stable kiln atmosphere, uniform temperature, prevention of cold air entry.
2. Risks of excessive positive pressure: Significant hot gas leakage → increased heat loss;
3. If kiln car bottom sealing is insufficient → hot gases flow downward, possibly leading to:
(1) Deformation and damage of kiln car skirts;
(2) Wear of sand seal grooves;
(3) Evaporation of bearing lubricating oil, causing bearing damage;
(4) Deterioration of the working environment.
Therefore, tunnel kiln operation should avoid excessive positive pressure and maintain only slight positive pressure.