Comprehensive Analysis Of Seven Parts Setting, Three Parts Firing in Brick Kilns

Most professionals in the brick and tile industry have heard the saying, "Seventy percent stacking, thirty percent firing." However, merely knowing this phrase does not equate to truly understanding it. Those who can translate the meaning behind this saying into concrete actions and diligently apply it to stacking work are exceptionally rare. Therefore, in the practical execution of stacking, when scrutinizing the "seventy percent stacking," how exactly should it be done? It is likely that very few can articulate this clearly, let alone implement it in actual operations.

In real-world production, if the meaning of "seventy percent stacking" remains unclear, and the discussion is limited to trivial matters unrelated to stack design and operation, the impact may be negligible. However, the problem becomes severe when those who cannot clarify the meaning of "seventy percent stacking" include kiln design units, individuals responsible for formulating stacking plans, manufacturers of stacking machines tasked with developing stacking schemes, and even some kiln masters with the authority to decide stacking methods.

To discuss stacking, one must first understand its purpose. The goal of proper stacking is to produce high-quality, compliant products efficiently, quickly, and cost-effectively. In other words, it is about devising every possible means to ensure that the stacked bakes consume minimal energy while achieving optimal and rapid firing. To realize the objectives of high-quality, fast, and low-energy firing, airflow management is key. Ultimately, the core issue to address in stacking is how to achieve the most rational distribution of airflow through the stacked bakes, thereby enabling optimal drying and firing.

Thus, to stack well, one must first comprehend the flow state of air inside the kiln from start to finish. What kind of stack can ensure that airflow maintains uniform temperature, humidity, and pressure across the entire cross-section of the drying chamber while minimizing vertical temperature stratification? What kind of stack can facilitate the rational distribution of airflow in the firing kiln, achieving balanced temperature across the entire cross-section and promoting uniform combustion? These are the critical questions to resolve.

01 Importance of Gas Flow Inside the Kiln

Currently, the vast majority of tunnel kilns for brick firing in China adopt internal combustion firing processes, predominantly utilizing the single-firing (drying and firing) technique. The green brick stacks loaded onto kiln cars already carry most or all of the heat required for drying and firing. The brick stacks formed on kiln cars according to predetermined stacking patterns complete the firing process through heat generated by the combustion of internal fuel pre-mixed within the green bricks.

For the fuel inside the green bricks to combust, oxygen is required. This oxygen is supplied by air drawn into the firing kiln from the kiln tail through fans, which flows through the cooling and heat preservation zones while being heated. The high-temperature gases produced by brick combustion are then drawn by exhaust fans through the preheating zone of the firing kiln, gradually heating the newly loaded green bricks on kiln cars.

This high-temperature gas assumes the responsibility of completing tasks unfinished in the drying chamber: First, it must gently and thoroughly remove any residual moisture that the drying chamber failed to eliminate (moisture removal at this stage carries greater risks and hazards than in the drying chamber - while collapsed or damp bricks from the drying chamber can be reprocessed, problems in the high-temperature preheating zone of the firing kiln will turn carefully produced, high-cost green bricks into worthless waste). Second, it must completely remove chemically bound water from the mineral components of the green bricks. Third, it must facilitate gradual heating of the green bricks to prepare them for safe entry into the high-temperature firing zone.

After firing completion, cooling is necessary for unloading, achieved by drawing cool air into the firing kiln. This cooling air then transforms into high-quality hot air free of moisture and pollutants, which is subsequently delivered by fans to the drying chamber for wet brick drying. Thus, from drying to firing after extrusion, every stage of brick production critically depends on air flow management.

To accelerate the firing speed, promote rapid oxidation and combustion of the internal fuel in the green body, and enhance the preheating rate of the green body as well as the cooling speed of the fired products, a substantial air volume is required at each of these stages. However, simply increasing the air volume is not the fundamental solution to all issues. To ensure the air entering the kiln functions effectively, two key measures must be implemented: first, guaranteeing that the gas fed into the kiln can flow smoothly and permeate unimpeded between the green bodies; second, ensuring rational air distribution across all sections of the setting. Only by achieving these conditions can the flame front across the entire cross-section advance uniformly to the greatest extent possible.

02 What are the resistances that affect the smooth flow of wind inside the kiln?

(1)Frictional resistance: The wind is drawn by the fan from the natural air outside the kiln and delivered into the kiln through preset air ducts. Regardless of the shape or material of the air ducts used to transport the wind, there will always be some degree of resistance. All friction generated between the gas and the duct walls, kiln walls, kiln roof, kiln cars, and green bodies is referred to as "frictional resistance." When frictional resistance occurs, the flow velocity and direction of the airflow remain unchanged.

(2)Local resistance: When airflow moves in one direction and suddenly encounters a sudden change in velocity that alters the airflow direction, cross-sectional area size, or shape, the resulting obstruction is referred to as local resistance. These obstructions could include protrusions on the kiln walls, scattered debris on the kiln car surface, a sudden reduction in the cross-sectional area of the air duct, bricks placed to stabilize unstable stacks (known as "pressure-seam bricks"), or horizontal brick layers within the stack. All such obstacles can abruptly redirect smoothly flowing air.

Horizontal brick layers inherently exhibit high resistance, and irregular stacking causes the brick ends to extend two centimeters beyond the aligned bricks. This two-centimeter extension not only narrows the already tight gaps between brick stacks but also obstructs the airflow entering the duct. Blocked by this protruding obstacle, the airflow cannot smoothly pass through to enter the brick gaps or seamlessly flow into the stack gaps. Instead, it is forced to change direction, flowing upward, downward, or sideways. As a result, the originally useful airflow becomes wasted, merely skirting the edges.

03The lifting force of wind during heating

When air is heated and its temperature rises, its density decreases. Naturally, lower density means lighter mass. Therefore, the buoyancy of the heated air, which is lighter and less dense, surrounded by colder air, becomes greater than that of the surrounding cold air, thereby causing the sky lantern to rise. This is also the principle behind why taller chimneys have greater draft. The taller the chimney, the greater the temperature difference between the air at the top and bottom inside the chimney, resulting in a stronger draft.

Now, let us analyze the specific state of airflow within the kiln during its movement for discussion:

The gas enters the firing kiln from the kiln tail not by natural flow but is forced into the kiln under the suction force of the fan. After entering through the cooling zone, the airflow passes through the cooling zone - insulation zone - firing zone - preheating zone. By the time the gas reaches the high-temperature firing zone, it has been gradually heated to its highest temperature. The heated high-temperature gas is then transported to the preheating zone of the firing kiln under the suction force of the fan.

After the air enters the kiln from the kiln tail, in addition to the horizontal suction force from the fan at the front of the firing kiln, the gradually heated and temperature-elevated gas also generates an upward lifting force. Moreover, the higher the temperature of the air, the greater the lifting force.

The mode of supplying hot air and exhausting moisture in artificial drying chambers generally involves introducing hot air from both sides at the rear of the drying chamber. The air flows counter-current toward the front of the drying chamber, where the low-temperature, high-humidity moist air is extracted by exhaust fans installed at the top of the drying chamber.

Due to the characteristic of high-temperature hot air having an upward lifting force, the hot air introduced into the drying chamber is drawn by the exhaust fans and flows toward the front of the drying chamber. During this flow, the hot air gradually loses heat as it passes through the green bodies, causing its temperature to decrease progressively. As the flowing hot air cools, it carries away the moisture released from the green bodies.

During the airflow movement, the portion of air with lower density, lighter mass, and higher temperature always travels along the upper section. The low-temperature air carrying a large amount of moisture becomes increasingly colder as the flow path lengthens, and the moisture it carries continues to accumulate. As the temperature of this low-temperature, high-moisture airflow drops, its density increases, and its mass grows larger.

Once this high-humidity, low-temperature airflow reaches the dew point temperature during its forward movement, the substantial moisture it carries condenses into water droplets, which are absorbed by the green bodies along the flow path. This is the reason why the upper layers of the green stack in the damp collapse remain largely intact while the lower layers disintegrate.

04 Brick stacking defects caused by kiln design errors

At the initial stage of kiln design, the first priority is to clarify what types of products are in demand in the local building materials market. Moreover, it is highly likely that the local market requires more than just one type of brick. This necessitates comprehensive consideration in the design of the drying chamber, firing kiln, as well as the height, width, and length of kiln cars, striving to accommodate the reasonable stacking height of all brick types within the kiln.

The upper edge of the top green bricks is 460mm away from the ceiling of the drying chamber. It would even be feasible to stack three more layers of green bricks vertically in this overhead space. The lamentable reality, however, is that the height of the firing kiln is lower than that of the drying chamber. While the brick stack maintains a 460mm clearance from the drying chamber ceiling, this distance shrinks to merely 100mm from the kiln roof after transfer. The upper section of dried green bricks shows only 2% moisture content, whereas the middle and lower sections retain a post-drying moisture level merely 3% lower than their initial forming moisture. This demonstrates that after twenty hours of drying, only 3% moisture gets removed. The detrimental impact of excessive overhead space thus becomes glaringly apparent!

Upon interviewing this "vernacular" designer, it was revealed that his intentional design of such high clearance aimed to facilitate smooth upward movement of humid air into the exhaust chimney. He theorized that greater overhead space would promote more efficient moisture discharge. The operational truth, however, proves otherwise: insufficiently dried bricks entering the kiln may withstand slow firing, but any accelerated firing process inevitably causes either stack collapse or product crumbling.

Through the above analysis of the airflow state inside the kiln, the following conclusions can be drawn: First, hot air possesses an upward buoyancy. The highest-temperature airflow travels along the upper section immediately beneath the kiln crown, with progressively lower-temperature airflows beneath it. Second, the air supply layout in the artificial drying chamber adopts a countercurrent design, meaning high-temperature air is introduced from the rear of the drying chamber and flows toward the front under the action of the exhaust fan. Consequently, the highest-temperature hot air rapidly rises to the top of the drying chamber upon entry, with little to no passage through the base of the brick setting before being directly extracted by the exhaust fan from the upper section and discharged through the exhaust chimney. Excessive gaps at the top of the brick setting will cause the highest-quality hot air to be rapidly expelled without effectively drying the green bodies. When brick cars with such stacking patterns enter the firing kiln, underfired edges and overfired centers occur. This happens because most of the airflow follows the path of least resistance-through the top gaps, setting gaps, and edge gaps-failing to penetrate the interior of the brick setting. As a result, heat within the brick setting cannot be carried away by the airflow, leading to excessive temperature concentration in the central region. Meanwhile, the excessive airflow volume at the edges and top creates the peculiar phenomenon of underfired peripheral products alongside overfired, adhered, or scorched central products in the brick setting.

Both of these flawed brick stacking methods are caused by design flaws in the kiln structure, demonstrating that such so-called "local experts" fundamentally lack understanding of brick stacking modulus. When initially designing a kiln, it is essential to comprehend the local construction market's demands and requirements for brick types. No market in any region will require only a single product variety. Therefore, from the very beginning of kiln and kiln car design, consideration must be given to the differing airflow requirements through the brick stacks in both the drying chamber and firing kiln for various products. While it may be impossible to achieve optimal combinations for all products, every effort should be made to accommodate the airflow needs of all brick types to the greatest extent possible.

05Brick stacking defects caused by errors of the brick stacking machine manufacturer

Reputable manufacturers of brick setting machines typically have standardized technical departments responsible for formulating and designing brick setting methods. Some even hire industry experts specifically to provide guidance on brick setting design, ensuring the developed solutions are generally sound. The most frequent issues arise from low-end imitators. Compounding this problem is the fact that most brick factory owners lack fundamental understanding of proper brick setting, simplistically believing that merely loading green bricks onto kiln cars will produce finished products. Due to this lack of expertise, they cannot identify any problems with the brick setting solutions provided by these manufacturers. Consequently, the market has seen various unimaginable brick setting schemes emerge.

Drying Chamber Example: Corresponding Air Supply Outlets: The drying chamber adopts a side-supply hot air and top-exhaust moisture removal configuration. The air supply outlets on the side walls of the drying chamber are evenly spaced at 1100mm intervals in pairs. The dimensions of each air supply outlet are height × width = 200 × 120mm.

However, the actual gaps in the stacked green bricks on the kiln car corresponding to the air inlets are as follows: The air intake channels at both ends of the kiln car are 30mm wide. The central air intake channel of the 1100mm-long kiln car has a width of 60mm. Meanwhile, the gaps in the other two brick stacks without any air inlets are unexpectedly 98mm wide! Such air channels fundamentally fail to ensure the entry of drying hot air into the interior of the brick stacks. Without airflow reaching the middle of the stacks, drying the green bricks becomes impossible.

The result is that, apart from a barely satisfactory dried layer on the top and edges of the bricks on the kiln car, the interior bricks remain entirely wet. This drying chamber is part of a production line where the drying chamber and firing kiln are of equal length. The kiln is 68m long. When the kiln car loaded with wet green bricks enters the shorter firing kiln, it is quickly pushed into the high-temperature zone. The moisture-laden bricks either collapse under high heat or are fired into defective, unusable products.

In tunnel kilns, the arrangement of green bricks should ideally have minimal or no gap between the kiln wall and the brick stacks. However, since kiln cars in a tunnel kiln operate intermittently during production, a safe distance must be maintained between the kiln wall and both the kiln car and its loaded brick stacks to prevent scraping.

Additionally, the draft ports (Hafeng) in tunnel kilns are typically positioned at the base of the sidewalls near the kiln car deck, where the fan's suction force is strongest at the edges. The draft force within the kiln is greatest where airflow encounters minimal resistance and is closest to the draft ports.

The central region of the brick stacks on the kiln car presents the highest resistance to airflow. Due to its distance from the draft ports and increased resistance, the central area lacks sufficient airflow to regulate temperature, preventing uniform cross-sectional heat distribution. Meanwhile, the edges experience excessive airflow, leading to over-drafting. This imbalance results in overfiring and clinker formation in the center while the edges remain underfired due to excessive airflow.

Theoretically, this explains why brick stacking should follow the principle of "denser at the edges, sparser in the center" and "denser at the top, sparser at the bottom."

From a practical standpoint, aside from maintaining a necessary safety clearance along the edges for kiln car movement, transverse stacks could be arranged with only expansion gaps between them, eliminating dedicated air channels between stacks. The saved space could then be evenly redistributed among the green bricks themselves. This approach would reduce recirculating airflow while enhancing through-flow ventilation between individual bricks.

Summary

In summary, the principle of brick stacking is to strive for balanced resistance across the upper, lower, left, and right cross-sections of the drying chamber and firing kiln, ensuring uniform ventilation throughout the entire stacked brick structure. This is because we know that areas with stronger airflow experience better drying effects (though drying quality depends not only on airflow but also on temperature and humidity), and consequently, faster firing progression. Effective drying establishes the foundation for firing, naturally leading to improved output and quality. To achieve uniform airflow penetration across the entire cross-section, it is essential to thoroughly understand the design of the kiln and kiln cars, ensuring the stacked brick structure perfectly matches them. This demands meticulous effort from operators in stacking techniques. The brick stacking plan must be tailored according to the specific conditions of the kiln and airflow channels. Simply stacking bricks neatly on the kiln car is far from sufficient-such an approach would create significant challenges for subsequent drying and firing processes, critically compromising both product quality and production yield.

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