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What is tunnel kiln?

2026-03-16

A tunnel kiln is a type of kiln used for the continuous firing of materials such as ceramics, bricks, or refractories. It is called a “tunnel kiln” because it has a long, tunnel - shaped structure.

Structure

Firing Zone:

The central part of the tunnel kiln is the firing zone. In this area, the temperature is raised to the required level for the firing process. For example, in a ceramic tunnel kiln, temperatures can reach up to 1,300 - 1,400 degrees Celsius. The length of the firing zone can vary depending on the type of product and the production scale.

Pre - heating Zone:

At the entrance of the tunnel kiln, there is a pre - heating zone. The materials to be fired are gradually heated as they move through this zone. This helps to reduce thermal shock when the materials enter the high - temperature firing zone. The pre - heating is usually achieved by the waste heat from the firing zone, which is a way of energy - saving.

Cooling Zone:

After passing through the firing zone, the products enter the cooling zone. Here, the products are cooled down in a controlled manner. The cooling rate is important as it can affect the quality of the final products. For instance, rapid cooling may cause cracks in ceramics, so a proper cooling curve is designed to ensure the products cool down at an appropriate speed.

Working Principle

The materials to be fired are placed on kiln cars or conveyor belts and are continuously moved through the tunnel kiln at a slow and steady speed. As they move through the different zones, they experience the pre - heating, firing, and cooling processes in sequence. The heat source for the tunnel kiln can be various, such as gas burners, electric heaters, or a combination of both. The hot gases generated in the firing zone flow towards the pre - heating zone, which helps to utilize the waste heat.

Advantages

High Production Efficiency:

Because it is a continuous production process, tunnel kilns can produce a large quantity of products in a relatively short time. Compared to batch - type kilns, the production cycle is more regular and efficient. For example, a well - designed brick - making tunnel kiln can produce thousands of bricks per day.

Good Product Quality Uniformity:

The products in the tunnel kiln experience the same temperature - time curve during the firing process as they move through the kiln at a constant speed. This ensures a high degree of uniformity in product quality. In the production of high - grade ceramics, the tunnel kiln can help to achieve consistent color, density, and strength of the products.

Energy - saving:

The utilization of waste heat in the tunnel kiln is an important feature. The hot gases from the firing zone are used to pre - heat the raw materials, which reduces the energy consumption required for the overall firing process. This makes the tunnel kiln more energy - efficient compared to some other types of kilns.

Applications

Ceramics Industry:

Tunnel kilns are widely used in the production of various ceramic products, including tableware, sanitary ware, and ceramic tiles. For ceramic tile production, the tunnel kiln can ensure that the tiles have a smooth surface, accurate dimensions, and good mechanical properties.

Brick and Tile Industry:

In the brick and tile manufacturing, tunnel kilns play a crucial role. They can fire different types of bricks such as clay bricks, shale bricks, and perforated bricks. The fired bricks have high strength and good durability, meeting the requirements of construction.

Refractories Industry:

For the production of refractory materials, tunnel kilns are used to fire products such as refractory bricks and insulating materials. The high - temperature firing process in the tunnel kiln can improve the refractoriness and thermal insulation performance of these materials, which are essential for applications in high - temperature industrial furnaces.

How to dry and fire clay bricks in clay brick kilns ?

How to produce hollow clay blocks ?

Related questions

Can Low-Grade Coal Gangue Produce High-Quality Fired Bricks?

Coal gangue is a solid waste generated during coal mining and processing, which has long been regarded as a useless industrial residue. High-quality coal gangue with high carbon content, kaolin, alumina or pyrite can be reused for power generation, cement production, ceramic processing and chemical raw material manufacturing. However, most residual coal gangue belongs to low-grade raw materials with low effective component content, traditionally only used for road paving and pit filling, resulting in massive stockpiling and environmental pollution.

With mature and innovative firing technology, low-grade coal gangue can be fully utilized to produce high-quality fired bricks that meet national industrial standards. Different from high-value coal gangue used for chemical and building material raw materials, low-grade coal gangue relies on scientific testing, optimized batching and precise sintering processes to realize resource regeneration without secondary waste discharge.

The premise of low-grade coal gangue brick production is comprehensive physical and chemical performance testing and thermal sintering experiments. Manufacturers need to detect key indicators including chemical composition, calorific value, plasticity index, particle gradation, drying and firing shrinkage, water absorption, lime bursting and efflorescence. Notably, the plasticity index test requires crushing all samples to below 1mm to ensure accurate data, avoiding detection errors caused by excessive particle size.

Since there is no national standard for coal gangue calorific value detection, the industry uniformly adopts the national standard GB/T 213-2003 for coal calorific value testing instead of empirical industrial methods, which guarantees reliable parameter basis for subsequent production process design. Through systematic testing, enterprises can confirm the applicable brick types (solid bricks, hollow bricks, porous blocks) and match exclusive production equipment and kiln processes, eliminating blind investment and production risks.

Low-grade coal gangue fired bricks adopt internal combustion firing technology as the core process. The raw material calorific value of 450×4.17kj/kg is the most ideal standard for full coal gangue internal combustion brick production, requiring no additional fuel or auxiliary materials. For raw materials with excessive or insufficient calorific value, targeted adjustment measures such as blending inert materials, adding a small amount of clean coal or low-temperature decarbonization can balance the combustion heat value, realizing stable and high-yield production of high-strength fired bricks above WU20 grade.

Why Airflow Is Critical in Tunnel Kiln Brick Firing？

Why Airflow Is Critical in Tunnel Kiln Brick Firing: The Hidden Energy Carrier in Every Kiln

In tunnel kiln brick firing systems, airflow is not only the carrier of oxygen required for combustion but also the most important medium for heat transfer and moisture removal throughout the entire firing process.

Cold air enters from the discharge end of the kiln and passes through the cooling zone and insulation zone. During this process, it absorbs residual heat from fired bricks, gradually increasing in temperature. This preheated air then enters the firing zone, where it supplies oxygen for coal combustion, ensuring maximum heat release and stable firing conditions.

As the hot airflow continues forward into the preheating zone, it transfers heat to cooler green bricks, raising their temperature evenly. At the same time, it promotes evaporation of internal moisture, which is then carried away as water vapor through exhaust channels in the kiln walls.

In once-fired tunnel kilns, moist hot air also passes through the drying zone, further assisting in uniform dehydration of bricks. Exhaust vents (also known as “air holes”) discharge moisture-laden air into the atmosphere.

Modern tunnel kilns with waste heat recovery systems reuse part of this airflow to preheat and dry green bricks, significantly improving thermal efficiency and reducing energy consumption.

Airflow in tunnel kiln operation has several key characteristics:

1. Air follows paths of least resistance.
2. Air tends to move in straight channels.

The moisture-carrying capacity of air depends heavily on temperature. At 100°C, 1 m³ of air can carry about 800.99 g of water, while at 0°C it carries only 4.84 g. This means hot air is over 100 times more effective in moisture transport than cold air.

For stable kiln operation, exhaust gas temperature is typically controlled below 40°C, and relative humidity is kept below 80% to prevent condensation and collapse of brick stacks.

In practice, 30–40 m³ of air is required to remove 1 kg of water safely.

Airflow must maintain close contact with brick surfaces to enable effective heat and mass exchange. However, only a small portion of airflow directly contacts fuel particles, while most serves as a heat and moisture transport medium.

Therefore, the actual air supply in tunnel kilns is far greater than the theoretical combustion requirement. The ratio is called the excess air coefficient, typically 5–6 in tunnel kiln systems.

Proper airflow control is essential. Insufficient air leads to incomplete combustion and higher coal consumption, while excessive air increases heat loss and reduces efficiency.

How to Avoid Kiln Car Burnout and Stack Collapse in Tunnel Kiln?

Most small and medium-sized brick and ceramic factories suffer from unplanned downtime caused by kiln car burnout and green body stack collapse every year. Most faults are not caused by equipment aging, but irregular daily operation, neglected sealing maintenance and unreasonable thermal parameter setting. This article focuses on operable daily operation standards, emergency handling steps and low-cost prevention schemes for the two typical tunnel kiln faults, suitable for frontline operators and workshop management staff.

1. Daily Judgment Standard for Kiln Car Burnout Risk

Frontline operators can judge burnout risk without professional detection equipment: if bearing noise increases, kiln car propulsion resistance rises obviously, and local high temperature appears at car bottom in firing and cooling zones, it means upper-lower kiln pressure imbalance and sealing failure. The core prevention logic is balancing pressure and isolating high-temperature flue gas.

Low-cost daily maintenance rules for anti-burnout:

Fixed-shift air cooling inspection: Check the wind volume of bottom cooling fans every shift to ensure uniform heat dissipation;
Fixed-quantity sand adding: Each shift adds sealed sand to sand grooves by fixed weight, avoid sand missing or sand accumulation failure;
Regular appearance inspection: Check skirt plate flatness weekly, repair deformed parts before high-temperature firing period;
Dynamic pressure adjustment: Adjust exhaust fan frequency timely to keep car bottom pressure slightly equal to upper kiln pressure.

Note: Partial enterprises only focus on kiln upper firing temperature but ignore car bottom pressure, which will cause cumulative bearing burnout and plate deformation within 1-2 months, bringing high replacement cost of kiln car accessories.

2. On-site Classification Emergency Handling for Green Body Stack Collapse

Defined as car reversing in kiln workshop, green body collapse has differentiated disposal plans based on fault location, which can minimize production loss:

Cooling zone slight collapse: No kiln shutdown required. Push the faulty kiln car to kiln outlet directly, clean broken green bodies offline and reuse the empty kiln car;
Preheating zone head collapse: Stop heating of local combustion chambers, isolate high-temperature flue gas, open access doors, and adopt reverse traction to withdraw faulty cars; resume feeding after kiln internal temperature stabilizes;
Firing zone collapse: Forbid blind traction to prevent secondary damage to high-temperature shed frames. Use reserved side fault holes to clean collapsed fragments and adjust offset green body stacks;
Whole-section severe collapse: Stop kiln firing and cooling air supply slowly, implement gradient cooling, drag out all kiln cars from kiln tail after temperature drops to safe range.

3. Six Daily Operation Rules to Prevent Car Reversing

Implement double-inspection for loading: Workers check stack stability, managers recheck cushion brick fastening degree before kiln entering;
Control green body moisture strictly: Limit incoming moisture to factory standard value to eliminate burst risk in preheating stage;
Stabilize preheating temperature: Control preheating zone vertical temperature difference to avoid uneven heating cracking;
Optimize firing curve: Avoid over-firing and long-time constant high temperature to prevent green body softening deformation;
Regularly replace aging shed frames: Eliminate fracture risk of low high-temperature resistant shed accessories;
Calibrate kiln car walking track monthly: Rectify derailment and unbalanced traveling hidden dangers in advance.

Critical operation taboo during fault disposal: Rapid cooling is forbidden. Sudden cold air inflow will crack integral kiln refractory lining, which needs long-term overhaul and affects long-term production.

How to Overcome High-Hardness Material Extrusion Challenges in ECP Wall Panel Production？

In the modern prefabricated construction industry, Extruded Cement Panel (ECP) wall panels have become a mainstream choice due to their superior structural properties. However, for manufacturers, processing advanced raw materials—such as high-end ceramic materials, cement fiber products, and semi-hard plastic viscous materials—poses a significant technical challenge. Ordinary extruders often fail due to insufficient extrusion pressure, leading to structural defects in the panels and costly production line downtime.

To solve this industry bottleneck, investing in an advanced ECP Wall Panel Vertical Vacuum Extruder built to rigorous European standards has proven to be the most effective strategy.

1. Engineering the Solution for High-Hardness Materials

Traditional horizontal extruders frequently struggle with dense, semi-hard plastic viscous materials because the feeding and de-airing processes are disrupted by gravity inconsistencies. A vertical vacuum extruder design inherently optimizes material flow. By integrating increased extrusion pressure, the machinery ensures that even high-hardness materials like shale and coal gangue are densely compacted without structural voids or micro-cracks.

2. European Technical Standards: The Parametric Evidence of Reliability

When evaluating an ECP wall panel extruder, long-term operational consistency is determined by its design framework. Our system is manufactured using advanced European technologies and processes, strictly adhering to European standards across four critical dimensions:

Technical Structure: Optimized for shale and gangue applications, ensuring the physical configuration resists structural deformation under peak mechanical loads.
Electrical and Operating Systems: Equipped with a high degree of automation and excellent operability, allowing engineers to calibrate vacuum levels and pressure metrics precisely in real-time.
Robust and Durable Design: Engineered for long service life and reduced maintenance costs, eliminating the risk of unexpected component failure during high-intensity shifts.

3.Minimizing Overhead with Smart Integration

Beyond performance, operational footprint and maintenance cycles directly impact a plant's profitability. This equipment features a compact footprint with an integrated water cooling system. The water cooling system ensures the mechanical components maintain thermal stability during continuous operation, preventing thermal degradation of the viscous raw materials. Furthermore, the stable performance is paired with easy machine cleaning, drastically cutting down on routine sanitation downtime.

Upgrading to a European-standard ECP wall panel vertical vacuum extruder is the defining factor between a high-scrap factory and a high-yield automated plant.

Are you looking to enhance your ECP wall panel output or switch to high-hardness raw materials? Contact our senior application engineers today for a complete technical proposal and custom quote.

How to Effectively Improve the Output of Crushers in Fired Brick Production Lines?

In the daily operation of fired brick production lines, equipment performance directly restricts product yield and quality. Among them, crushing equipment, belt conveyor equipment, vacuum brick extruders and kiln thermal control equipment are the core factors affecting production efficiency. As the key coarse and fine crushing equipment in the crushing system of brick factories, jaw crushers and hammer crushers determine the overall operating efficiency of the entire production line.

To maximize the crusher’s production capacity while ensuring qualified crushed material particle size, standardized and scientific operation and maintenance measures are essential.

First of all, standardized feeding is the foundation of efficient crushing. Coal gangue and hard shale materials should be evenly distributed along the feeder inlet and fully fill the crushing cavity. This method can realize uniform wear of the jaw plate and effectively reduce the equipment operating cost.

Secondly, it is necessary to ensure the feeder operates with sufficient amplitude. According to the actual production capacity demand, operators can adjust the knob of the control box within the rated amplitude range to steplessly adjust the feeding amplitude, so as to match the feeding speed with the production load and improve crushing continuity.

In the feeding process, strict precautionary measures must be implemented. It is forbidden for iron blocks to enter the crushing cavity to avoid damage to the jaw plate and other core components. Meanwhile, the height of materials to be crushed shall not exceed the fixed jaw plate, and the maximum feeding particle size must be smaller than the size of the feed inlet. Excessively large materials are prone to block the crushing cavity, which will seriously reduce crushing efficiency.

The reasonable setting of the discharge opening is crucial to balance crushing quality and efficiency. An excessively small discharge opening will cause material blockage, increase energy consumption and even cause severe damage to the crusher. In contrast, an overlarge discharge opening will lead to coarse crushed materials and increase the load of secondary crushing.

The opening of the discharge opening can be adjusted by increasing or decreasing the adjusting gaskets behind the swing rod base plate. During the size measurement, the position of the upper eccentric shaft must be calibrated to keep the bottom ends of the fixed jaw plate and the movable jaw plate at the closest position. The maximum diameter of the circle tangent to the two jaw plates at this position represents the material particle size, which needs to be measured with the standard ring gauge equipped with the machine. After adjustment, the tension spring shall not be excessively compressed to avoid tight arrangement without gaps. Since the support rod and tension spring bear long-term fatigue stress, they need to be replaced every 2 to 3 years.

Jaw plate condition is a key factor affecting crushing capacity. The tooth-shaped jaw plates with flat sections are reversible and interchangeable, which can be installed on both movable and fixed jaw plates. Regular inspection of jaw plate wear is required to carry out inversion, interchange or replacement in a timely manner. When the bottom of the movable jaw plate is worn by 1/3 and the bottom of the fixed jaw plate is worn by 2/3, the two jaw plates shall be inverted. When the top and bottom of the movable jaw plate are worn by 1/3 and the middle part is worn by half, while the top and bottom of the fixed jaw plate are worn by 2/3, the two jaw plates need to be interchanged. When the top and bottom of both jaw plates are completely worn, all jaw plates must be replaced in time.

In addition, high-quality lubrication is the core guarantee for stable crusher operation. As the key component of crusher operation, the service performance and service life of the eccentric shaft bearings on the base bearing box and movable jaw directly affect the crushing efficiency. The crusher is equipped with a labyrinth sealing device to ensure the cleanliness of internal grease. Each of the four bearings is equipped with a grease nipple. Before greasing, the oil nipple and grease gun must be cleaned thoroughly to prevent dust from entering the bearing box and causing component wear.

How to Control Tunnel Kiln Temperature and Prevent Brick Stack Collapse in Brick Production？

In modern brick and tile production, precise temperature control of tunnel kilns is the key to qualified finished products and efficient production. The thermal system adjustment of tunnel kilns is a dynamic optimization process. Production personnel adjust multiple flexible production factors according to product specifications and process requirements to optimize the internal thermal system of the kiln. The conventional temperature adjustment methods in actual production include fan frequency modulation for smoke exhaust, heat dissipation and kiln door ventilation, pipeline gate opening adjustment, kiln car feeding speed control, and kiln top coal feeding regulation.

Only when the tunnel kiln meets stable operating conditions can accurate temperature control and continuous production be guaranteed. Firstly, the internal combustion heat of brick blanks must be consistent without obvious deviation, which is the foundation of stable kiln temperature. Secondly, the temperature of each functional zone of the kiln (preheating, firing, cooling) must be controlled within the standard fluctuation range to avoid under-firing or over-firing of products. Thirdly, maintain a stable feeding rhythm, fix production varieties and internal combustion proportioning parameters, ensure uniform kiln car entry intervals, and stabilize the heat absorption efficiency of brick blanks in key firing zones. Fourthly, keep the physical and chemical properties of raw brick blanks stable to ensure consistent firing conditions.

Brick stack collapse is a major bottleneck restricting tunnel kiln production efficiency. Once the stack collapses in the high-temperature operating section of the kiln, forced kiln shutdown is inevitable. Manual treatment in high-temperature environments not only reduces production efficiency but also brings serious safety hazards to operators. The root causes of stack collapse can be summarized into four categories: improper stacking leading to unstable stack structure and loose displacement during operation; excessive residual moisture of brick blanks after drying; track deformation and settlement causing kiln car inclination and stack collapse; and falling refractory linings of kiln walls and roofs causing mechanical jamming and stack damage.

To solve the problem of brick stack collapse and realize stable temperature control, targeted improvement measures must be implemented in production management. First, strengthen the quality control of the drying process, strictly detect and control the moisture content of brick blanks before entering the kiln to eliminate collapse caused by excessive moisture. Second, standardize manual and mechanical stacking operations, strictly implement the flat, straight and stable stacking standards to enhance the overall firmness of brick stacks. Third, establish a regular kiln equipment inspection mechanism, regularly check the flatness of kiln operation tracks and the integrity of kiln wall and roof refractory bricks, and timely handle deformed, damaged and fallen parts to avoid secondary faults. Meanwhile, once feeding abnormalities are found on site, operation suspension and fault inspection shall be carried out immediately to prevent fault expansion.

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