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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

How to Eliminate Spiral & S-Cracks in Clay Bricks？

Spiral cracks and S-cracks are the most frequent structural defects in clay brick manufacturing. These undesirable imperfections lead to high scrap rates, raw material waste, and poor finished brick appearance. This practical guide summarizes mature industrial solutions covering raw material treatment, extrusion optimization, moisture adjustment, equipment maintenance, and kiln processing, helping global brick manufacturers reduce defect rates and maximize production profits.

1. Optimize Raw Material Gradation & Reduce Clay Plasticity

Reasonable particle gradation is the foundation of crack prevention. Manufacturers should mix 20% to 30% coarse aggregates such as grog, shale, and coal gangue into raw clay. Coarse particles enhance interlayer friction and restrain clay internal sliding. Meanwhile, reduce high-plastic clay proportion and add limestone powder or quartz sand to lower drying sensitivity.

2. Balance Extrusion Speed & Optimize Clay Flow

To solve uneven flow velocity, install adjustable resistance bars at the extruder head to slow down the central clay flow and balance overall extrusion speed. Replace ordinary blades with variable-pitch spiral blades to reduce shear difference. Keep the gap between spiral blades and machine cylinder within 2mm for stable extrusion molding.

3. Precise Moisture Control for Raw Clay & Green Bricks

Control the raw clay moisture steadily between 18% and 22% according to local clay properties. Adopt staged slow drying technology for green bricks; keep the initial heating rate below 20°C/h to avoid surface crusting. Uniform moisture removal effectively prevents shrinkage cracks and layered cracks.

4. Standardize Daily Extruder Maintenance & Parts Replacement

Establish regular equipment inspection cycles. Timely replace worn spiral blades and damaged cylinder liners to guarantee stable pushing force. Check extruder head sealing and gaps weekly to avoid disordered clay flow caused by mechanical aging. Scientific maintenance reduces artificial brick defects greatly.

5. Upgrade Drying & Sintering Kiln Curves

Adopt gradient heating mode in drying chambers and tunnel kilns. Control the heating rate between 20°C/h and 30°C/h with sufficient constant-temperature holding time. During quartz crystal transformation (600℃-900℃), slow down heating speed below 40°C/h to relieve internal thermal stress.

Eliminating spiral and S-cracks requires systematic production management from raw materials to finished bricks. Scientific formula proportion, optimized extrusion equipment, precise moisture monitoring, standardized maintenance, and improved kiln technology can reduce brick scrap rates by 5%-10%. Stable product quality helps brick factories occupy more shares in the global construction material market.

How to Boost Brick Factory Output?

For modern brick factories pursuing stable production and high profits

low output and high defective rates have always been troublesome problems. Many brick plant owners blindly upgrade brick making machinery and increase production frequency, but ignore the core bottleneck restricting production efficiency—the drying process. In the actual brick production workflow, the drying section is the key link that determines the qualified rate of green bricks and the daily output of the entire brick factory. Relevant industry data shows that reducing the moisture content of bricks before entering the kiln can directly bring obvious output growth; every 1% reduction in kiln entry moisture can increase brick production by 3% to 5%.

Many brick factories have unreasonable drying settings, resulting in long drying cycles, damp green bricks, and mildewed blanks, which not only reduces the drying qualification rate but also causes adverse effects on the subsequent kiln firing process. To break the drying bottleneck of brick factories, enterprises need to optimize two core indicators: drying temperature and moisture exhaust efficiency.

First, reasonably increase the drying temperature according to the raw material characteristics, and ensure uniform temperature distribution in the drying chamber to avoid uneven drying of green bricks.
Second, strengthen the moisture exhaust system of the drying workshop, timely discharge water vapor generated in the drying process, and prevent water vapor from condensing on the surface of green bricks to cause secondary moisture regain.

In addition to temperature and moisture exhaust optimization, raw material management is also an auxiliary measure to improve drying efficiency. Brick factories need to maintain stable raw material ratio and uniform internal combustion blending. Frequent replacement and proportion adjustment of raw materials will lead to inconsistent moisture characteristics of raw mud, which increases the difficulty of drying control. Uniform internal combustion mixing can avoid local underfiring and reburning of bricks, reduce defective products caused by unreasonable combustion, and further cooperate with the drying process to improve the overall qualified rate of bricks.

For brick making equipment

the operating state of the molding end also indirectly affects the drying effect. The extruder needs to maintain a reasonable running speed while ensuring the compactness of the green bricks. The stable vacuum degree above 0.085 is an essential standard for high-quality brick blanks. Worn reamer blades should be replaced in a timely manner to prevent loose green bricks from being difficult to dry thoroughly. Only when the quality of molded bricks is up to standard can the drying link exert the maximum efficiency advantage.

In the production logic of brick factories, the optimization priority must be clear:

prioritize solving drying problems, then stabilize the firing process, and finally carry out equipment speed increase transformation.

It is necessary to clarify a core principle: the real output of a brick factory refers to the number of qualified bricks. Excessive defective products and reburning bricks will completely offset the production growth brought by equipment acceleration. Optimizing the drying system is the lowest-cost and highest-return way for most brick plants to increase production, which is suitable for small and medium-sized brick factories with limited transformation budgets.

How to Prevent Clay Bricks from Collapsing in Tunnel Kilns？

1. Pre-firing Material Constraints (The "Foundation" Stage)

Collapse often begins before the bricks even enter the kiln if the green body lacks physical integrity.

Moisture Threshold: The residual moisture content must be kept below 6%. High moisture levels drastically reduce the compressive strength of the bricks, causing the bottom layers to buckle under the weight of the stack.
Material Aging: Clay requires at least 3 days of aging to ensure uniform plasticity and water distribution. Insufficient aging leads to internal stresses and a fragile structure.
Mechanical Density: Ensure an extrusion pressure of ≥40kg/cm² to increase the density of the green body, making it more resistant to deformation at high temperatures.

2. Strategic Stacking Techniques (Mechanical Stability)

Stacking is not just about volume; it is about managing gravity and thermodynamics.

The "Four-Point" Standard: Stacks must be level, stable, vertical, and straight. Any minor deviation in the center of gravity will be amplified as the bricks soften in the heat.
Airflow Optimization: Follow the principle of "Dense Edges, Sparse Centers" and "Dense Tops, Sparse Bottoms." This balances the temperature across the kiln cross-section, preventing the edges from over-firing while the center remains under-fired.
Load Management: Due to the high sensitivity of clay, limit the stacking height to 12 layers or fewer. This minimizes the static pressure on the base bricks.

3. Dehumidification in the Preheating Zone (The "Critical" Stage)

This is the most common zone for collapses. If moisture is not evacuated efficiently, the bricks effectively "steam" and lose their rigidity.

Inlet Temperature Control: Keep the initial drying air below 116°C. Temperatures above this threshold cause the surface to harden too quickly, trapping steam inside and creating internal pressure.
Heating Rate: Maintain a steady rise of 6–8°C/h. Sudden temperature spikes, especially in winter, can cause thermal shock and structural failure.
Ventilation and Pressure: Ensure the exhaust fan provides sufficient negative pressure. Poor ventilation causes moisture to linger and re-condense on the bricks, leading to "soggy" bricks that collapse instantly.

4. Firing Zone Temperature Management (Thermodynamic Control)

Once the bricks reach high temperatures, preventing them from entering a pyroplastic state (melting) is vital.

Anti-Overfiring Measures: Strictly monitor the sintering peak. Exceeding the clay's softening point leads to viscous flow, where the bricks begin to behave like liquid and slump.
Internal Fuel Ratio: Control the amount of internal additives (coal powder or gangue). Excessive internal fuel generates uncontrollable heat within the stack, causing the bricks to "melt from the inside out."
Visual Monitoring: Use inspection holes to watch for "white-out" conditions or "shimmering/swaying" stacks, which are immediate warning signs of imminent collapse.

5. Infrastructure and Mechanical Integrity (Environmental Factors)

The physical environment of the kiln must remain consistent to prevent mechanical triggers.

Track Leveling: Regularly inspect kiln car tracks. Uneven rails cause vibration and jolting, which can topple a stack that is already weakened by heat.
Kiln Structure Maintenance: Check for sagging roof bricks or protruding exhaust ports. Mechanical obstructions are a frequent cause of "domino-effect" collapses during car movement.

How to Crusher Output in Sintered Brick Plants Efficiently？

In sintered brick production lines, the output and quality are often restricted by four key pieces of equipment: crushing equipment, belt conveyor equipment, vacuum extruders, and kiln thermal equipment. Among them, jaw crushers and hammer crushers, as common primary and secondary crushing equipment, directly determine the overall production efficiency of the entire line. Many brick plant operators are eager to maximize crusher output while ensuring the particle size of crushed materials—here are practical and actionable tips to achieve this goal.

First, ensure proper feeding. To make the jaw plates wear evenly and reduce operating costs, gangue or hard shale should be evenly distributed along the feeder inlet and fill the crushing chamber completely. Uneven feeding will not only accelerate jaw plate wear but also reduce crushing efficiency, leading to unnecessary energy waste.

Second, adjust the feeder amplitude reasonably. During normal use of the feeder, you can adjust the amplitude through the knob on the control box within the rated amplitude range according to the required productivity, so as to achieve stepless adjustment of the feeder. Sufficient amplitude ensures that materials enter the crushing chamber continuously and stably, avoiding gaps that affect output.

Third, pay attention to feeding precautions. It is crucial to prevent iron blocks from entering the crushing chamber, as iron blocks can damage jaw plates and other key components. The height of the materials to be crushed should not exceed the fixed jaw plate, and the maximum feed particle size should be smaller than the feed inlet—large blocks are likely to block the crushing chamber and reduce crushing efficiency.

Fourth, set a reasonable discharge port size. The discharge port is the distance between the two jaw plates at the lower end of the crushing chamber. Too small a discharge port will cause blockages and excessive energy consumption, leading to serious damage to the crusher; too large a discharge port will increase the load of the second crushing. Finding the optimal size according to the production needs is the key to improving output.

In addition, regular inspection and replacement of jaw plates, proper lubrication of bearings, and scientific adjustment of the discharge port opening are also essential links to ensure stable and high output of the crusher. By following these tips, sintered brick plants can effectively improve crusher production capacity while ensuring product quality.

Why Poor Drying & Dehumidification Causes Cracked Clay Bricks?

Clay sintered bricks are widely used in construction projects around the world, favored for their durability, thermal insulation, and environmental friendliness. However, many construction teams and brick manufacturers often encounter a frustrating problem: clay bricks crack, crumble, or have uneven texture after sintering. What most people don’t realize is that the root cause of these quality issues is often inadequate drying and dehumidification technology—a key step that is easily overlooked in the brick-making process.

As a professional engaged in the brick-making industry for years, we often receive questions like: “Why do my clay bricks crack after sintering?” “How to make clay bricks stronger and more durable?” Today, we will popularize the critical role of drying and dehumidification technology in clay sintered brick production, and tell you how to avoid common quality problems completely.

First, let’s understand the basic principle: clay raw materials contain a lot of moisture, and if this moisture is not fully removed before sintering, it will expand rapidly when heated in the kiln, generating huge internal pressure. This pressure will directly cause the bricks to crack, burst, or even break into pieces—just like the “tofu bricks” that have been exposed in some quality incidents, which are mostly related to insufficient drying and dehumidification before sintering. In addition, uneven drying will lead to inconsistent moisture content in different parts of the brick blank. During sintering, the shrinkage degree of each part is different, resulting in uneven surface, low compressive strength, and poor weather resistance of the finished bricks.

Many small and medium-sized brick factories still use traditional natural drying or simple hot air drying methods. These methods have obvious defects: natural drying is greatly affected by the weather, and it is easy to cause the brick blanks to get damp again or dry unevenly; simple hot air drying often leads to too fast surface drying and too slow internal moisture diffusion, forming a “dry shell” on the surface, which traps the internal moisture and eventually causes cracks during sintering. These problems not only reduce the qualified rate of bricks but also increase production costs and affect project progress.

The good news is that these quality problems can be completely solved with advanced drying and dehumidification technology. Our company’s clay sintered brick making machine is equipped with a high-efficiency intelligent drying and dehumidification system, which perfectly solves the pain points of traditional drying methods. The system can accurately control the temperature, humidity, and air flow during the drying process, realizing uniform drying of the brick blanks from the inside out. It can quickly and thoroughly remove the moisture in the clay, ensuring that the moisture content of the brick blanks before sintering is controlled at the optimal range of 2% or less—which is the key to ensuring the quality of sintered bricks.

With our machine, you don’t have to worry about cracked, crumbly, or uneven clay bricks anymore. The sintered bricks produced by our equipment have uniform texture, high compressive strength, strong weather resistance, and no cracks or defects. They fully meet the international construction quality standards and are widely used in residential buildings, industrial workshops, and municipal engineering. Whether you are a brick manufacturer or a construction enterprise, our machine can help you improve production efficiency, reduce waste, and create higher economic benefits.

Which Ceramic Clay Split Face Brick Making Machine Adapts to Various Overseas Regions and Raw Materials?

Overseas brick factories are distributed in different regions, with great differences in raw material types, voltage standards and transportation conditions. Many buyers buy brick making machines that are not suitable for local conditions—either they cannot adapt to local ceramic clay and other raw materials, or they cannot be used normally due to voltage mismatch, or they are difficult to transport due to large volume. So, which ceramic clay split face brick making machine can adapt to various overseas regions and raw materials?

Our Ceramic Clay Split Face Brick Red Brick Making Machine is designed with "global adaptability" as the core, perfectly solving the problem of regional mismatch. In terms of raw material adaptation, it has strong compatibility, and can process not only pure ceramic clay, ordinary clay, but also mixed raw materials such as clay mixed with sand, tailings and coal gangue. It can automatically adjust the mixing ratio according to the characteristics of local raw materials, ensuring stable product quality without additional raw material processing equipment.

In terms of voltage adaptation, we can customize the motor voltage according to the local voltage standards of different countries and regions—whether it is 220V (North America, Southeast Asia), 380V (Europe, Africa), or other special voltages, we can meet the requirements, avoiding equipment failure caused by voltage mismatch and ensuring normal production.

In terms of transportation, the machine adopts a detachable design, which can be disassembled into small parts for transportation, greatly reducing the volume and transportation cost. For regions with inconvenient transportation (such as remote areas in Africa and Southeast Asia), it can be easily transported to the factory and assembled quickly—only 1-2 days are needed to complete the installation and commissioning, and it can be put into production immediately.

In addition, the machine is designed with a dust-proof and moisture-proof structure, which can adapt to different climate conditions—whether it is the high temperature and dryness in Africa, the humid and rainy in Southeast Asia, or the low temperature in Europe, it can operate stably without being affected by the climate. Up to now, our machine has been used in more than 60 countries and regions around the world, adapting to various complex working conditions and winning unanimous recognition from local customers.

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