Tunnel Kiln Automatic Control System: Functions of Core Equipment for Fired Brick Manufacturing
In modern fired brick production lines, a tunnel kiln relies on a complete set of auxiliary facilities and an automatic control system to stabilize firing quality, adjust temperature curves, and optimize energy consumption. Every component, from the exhaust fan, hot air extraction fan, kiln car bottom, sand seal, balance fans to hatch dampers, plays an irreplaceable role in airflow distribution and thermal balance.
The exhaust fan has two primary missions. First, it draws air into the tunnel kiln to supply oxygen for combustion and delivers hot flue gas to the drying chamber. Air passes through the cooling zone, soaking zone, firing zone and low-temperature preheating zone, and is extracted via hatch dampers. The airflow direction runs opposite to the travelling direction of green bricks. Second, the extracted hot flue gas is transported to dry green bricks inside the drying kiln.
In the low-temperature preheating zone, heat transfers from flue gas to green bricks until the internal mixed coal reaches its ignition point. Once ignited, the coal burns and pushes the temperature up to the target firing level such as 1000℃. The air volume of the exhaust fan must match the internal coal dosage to maintain proper oxygen concentration in the firing zone. Around 70%–80% of the embedded coal should finish combustion in this zone before bricks move into the soaking and cooling zones.
In the soaking and cooling zones, hot bricks release heat to the passing air, which then flows back to support combustion in the firing zone. Improper airflow directly ruins firing conditions. Insufficient airflow leads to incomplete combustion, low heat output, slow front fire and unstable rear fire; excessive airflow carries away too much heat, resulting in low overall kiln temperature even if the fire line moves fast.
Different from the exhaust fan, the hot air extraction fan pulls hot air only from the cooling zone and sends it to dry green bricks, without supplying combustion oxygen. Larger extraction air volume cools down the cooling zone, followed by temperature drops in the soaking, firing and preheating zones. Reduced hot extraction air raises temperatures across these zones. The airflow direction of the hot air extraction fan is opposite to the exhaust fan. Hot air extraction consumes part of the exhaust airflow and may lower kiln temperature. If the firing zone sits close to the hot air extraction outlet, over-extraction will stop fire advancement and cause overfiring in the center of stacked bricks. With fixed air supply from kiln door cooling fans, excessive hot extraction can trigger oxygen-deficient combustion.
The kiln car bottom forms a continuous air duct for pressure balancing. Sand seals and wind baffles on both sides separate the car-bottom air channel from the inner kiln atmosphere. When the pressure of the car bottom balances with kiln internal pressure, car-bottom air cannot leak upward into the kiln to cause underfired bricks, and high-temperature flue gas cannot leak down to burn out the kiln car structure. Normally, car-bottom temperature should be controlled between 70℃ and 100℃.
Coarse sand the size of mung beans is filled through sand feeding ports for sand seals. Fine sand is easy to be blown away by airflow, while coarse sand is carried by kiln car baffles to fully cover the sand trough, blocking cross air leakage and protecting both brick quality and kiln cars.
The car-bottom balance fan and pressure monitoring system are designed to realize pressure equilibrium. For fully negative-pressure tunnel kilns without independent cooling blowers, balance fans work in suction mode. For kilns equipped with cooling blowers at the discharge door, the preheating zone stays under negative pressure while the cooling zone runs under positive pressure. This setup requires two pairs of balance units: one suction fan and one blower fan. Two groups of pressure sensors are installed, and variable-frequency drives (VFDs) adjust fan output dynamically to match car-bottom pressure with kiln pressure in both front and rear sections.
A car-bottom balance gate creates negative pressure mechanically, yet it is not suitable for automatic closed-loop regulation and cannot be lifted in positive-pressure sections.
Low-temperature preheating hatch dampers serve positions 1 to 10, covering the temperature range from brick entry up to 560℃. The total opening degree of these dampers decides the overall airflow volume, while different opening combinations shape the temperature distribution among these kiln positions.
The maximum lifting height of each hatch damper equals its radius. The quantity of fully open dampers follows the square of the ratio between the main flue diameter and single damper diameter. For example, if the main flue is 1.2 m and each damper is 0.4 m, the ratio is 3, meaning 9 dampers for full opening. If running at half opening, 18 dampers (9 pairs) are required to avoid excessive pressure loss on dampers.
A critical rule for VFD fan adjustment: fully open the ventilation area of hatch dampers so that pressure energy is not wasted at dampers. Damper pattern and opening degree control fire speed and daily output. Separate damper settings are required for standard bricks, perforated bricks, and hollow bricks. Settings also differ for wet green bricks and pre-dried green bricks entering the kiln. If the fan runs at its minimum speed for dryer demand while fire speed remains too fast, operators must tune damper pattern and opening to stabilize drying airflow and pressure.
The core logic of damper adjustment is straightforward: lifting a hatch damper increases the hot flue gas passing through that kiln position. Larger and longer flue flow brings more heat exchange with green bricks and raises the local temperature.
FAQ
Q1: What causes underfired bricks in tunnel kiln production?
A1: Common reasons include unbalanced pressure between kiln interior and kiln car bottom, sand seal leakage, mismatched exhaust fan airflow and coal dosage, and unreasonable hatch damper opening.
Q2: What is the difference between exhaust fan and hot air extraction fan in a tunnel kiln?
A2: The exhaust fan provides oxygen for brick firing and recovers waste heat for drying. The hot air extraction fan only collects waste heat from the cooling zone for drying and does not participate in combustion oxygen supply.
Q3: How to set hatch dampers for hollow bricks and perforated bricks?
A3: Hollow bricks, perforated bricks and standard bricks need independent damper curve settings. Operators should adjust damper pattern and opening according to green brick moisture, kiln speed and target firing temperature.
