Burners are a critical part of the thermal system in many gas-fired tunnel kilns. Unlike batch kilns, a tunnel kiln operates continuously or semi-continuously, with products moving through different thermal zones while the kiln maintains a controlled temperature profile.
This makes burner design particularly important.
The burner system must provide the right amount of heat at the right location and at the right stage of the firing cycle. It also needs to work together with kiln geometry, product loading, combustion air, exhaust, kiln pressure, and temperature control.
A typical thermal process can be represented as:
Preheating → Firing → Soaking → Controlled Cooling
Burners are primarily responsible for supplying and controlling thermal energy in the heating and firing sections.
The objective is not simply to make the kiln hot. It is to create a stable, controllable, and uniform thermal profile along the entire kiln.
A tunnel kiln is a continuous or semi-continuous industrial kiln in which products move through a long chamber while passing through different temperature zones.
Depending on the application, products may be transported by:
Kiln cars
Rollers
Shelves
Conveyors
Other transport systems
A typical tunnel kiln contains several functional areas:
Preheating Zone
Firing Zone
Soaking or Holding Zone
Cooling Zone
The temperature profile changes gradually along the direction of product movement.
This differs from many batch kilns, where the products remain stationary while the kiln temperature changes over time.
Burners are normally distributed along the heating and firing sections rather than concentrated at a single point.
Depending on kiln design, burners may be installed:
Along both side walls
On one side of the kiln
At different elevations
In multiple firing zones
At different angles
A simplified arrangement is:
Product Movement →
Preheating Zone → Main Firing Zone → Soaking Zone → Cooling Zone
Burners are distributed according to the heat demand of each zone.
This arrangement allows the kiln to establish a controlled temperature profile rather than relying on one large heat source.
A tunnel kiln is usually much longer than a conventional heating chamber.
A single burner would concentrate heat in one area and make it difficult to maintain a controlled temperature profile throughout the kiln.
Multiple burners allow thermal energy to be distributed along the firing section.
This provides several potential benefits:
Better temperature distribution
More precise zone control
Flexible heat input
Better adaptation to changing process requirements
Reduced localized overheating
More effective use of combustion heat
However, simply adding more burners does not automatically improve performance.
The number, capacity, spacing, and orientation of burners must match the kiln's thermal design.
A tunnel kiln is designed around a temperature curve.
For example:
Ambient Temperature
↓
Gradual Preheating
↓
Rapid or Controlled Heating
↓
Peak Firing Temperature
↓
Soaking
↓
Controlled Cooling
The exact profile depends on the ceramic material and process.
Burners are mainly responsible for controlling the heating portion of this curve.
Each burner zone contributes a defined amount of thermal energy.
The objective is to make the actual kiln temperature profile closely follow the required process profile.
The preheating zone gradually raises the temperature of the incoming products.
Burner output in this section is generally coordinated with the required heating rate.
The purpose is not simply to heat the product as quickly as possible.
Rapid or uneven heating can cause thermal stress, depending on the material.
Therefore, the preheating section may require carefully controlled heat input and gas circulation.
Burner output can be adjusted according to:
Product characteristics
Production speed
Required heating rate
Kiln temperature
Gas-flow conditions
The main firing zone is where the kiln reaches the required high-temperature conditions.
This section typically represents the highest thermal demand.
Burners must provide sufficient heat while maintaining:
Stable combustion
Appropriate flame characteristics
Uniform temperature
Controlled atmosphere
Suitable kiln pressure
The burner arrangement is particularly important because the firing zone needs to provide consistent thermal conditions across the product width and along the kiln.
The soaking or holding zone is used to maintain the required thermal conditions for a defined period.
The required heat input may be lower than during the peak heating stage because the kiln is primarily maintaining temperature rather than continuously increasing it.
Burner turndown becomes important here.
If burners cannot operate stably at lower output, the system may experience:
Temperature fluctuations
Excessive cycling
Local overheating
Unnecessary fuel consumption
Appropriate burner modulation or staging can help maintain stable conditions.
The primary function of burners is heating, so the cooling section generally does not operate like the firing section.
Instead, controlled cooling relies on:
Cooling air
Air circulation
Exhaust
Heat recovery
Controlled airflow
However, the combustion and exhaust systems in the firing zone can influence the overall gas-flow pattern through the kiln.
The cooling section therefore needs to be considered as part of the complete tunnel-kiln thermal system.
Burner arrangement determines how combustion energy is distributed along the kiln.
Poor burner arrangement can create:
Local hot spots
Cold zones
Excessive wall heating
Uneven product heating
High exhaust temperatures
The system may then require additional fuel to compensate for poor heat distribution.
A properly designed arrangement can improve the use of combustion heat by directing thermal energy toward the areas where it is needed.
Therefore:
Tunnel-kiln efficiency depends not only on burner combustion efficiency, but also on how effectively the kiln transfers and retains the heat produced by the burners.
Burner spacing affects the interaction between individual flames and hot-gas flows.
If burners are too close together, they may produce excessive local heat concentration or undesirable flame interaction.
If burners are too far apart, cold zones may develop between heating areas.
The correct spacing depends on:
Flame length
Flame width
Burner momentum
Kiln width
Product arrangement
Heat-load distribution
Burner spacing should therefore be determined as part of the thermal design.
Burner angle determines the initial direction of flame and combustion gases.
A properly selected angle can help generate controlled circulation within the kiln.
It can influence:
Heat distribution
Gas velocity
Flame-wall interaction
Product exposure
Heat-transfer efficiency
For example, burners on opposite sides of a tunnel kiln may be arranged to create a controlled flow pattern rather than allowing each flame to travel directly toward the opposite wall.
The correct configuration depends on the kiln geometry and process.
Flame length must be compatible with the available space.
A flame that is too long may:
Reach the opposite wall
Interact with products
Interact with adjacent flames
Create excessive local heating
A flame that is too short may concentrate heat close to the burner.
For this reason, the required flame length should be evaluated against:
Kiln width
Kiln height
Burner spacing
Product location
Burner arrangement
Temperature uniformity is particularly important in tunnel kilns because products continuously move through the firing zone.
If one side of the kiln is hotter than the other, products may experience different thermal conditions depending on their position.
Burner flame characteristics can influence the temperature distribution across the kiln.
The goal is to create a sufficiently uniform thermal environment across the effective product zone.
Temperature uniformity can directly affect product quality.
Uneven temperature can contribute to:
Different firing degrees
Uneven shrinkage
Dimensional variation
Surface defects
Different material properties
Increased rejection rates
For a tunnel kiln, temperature uniformity needs to be considered in at least two directions:
Left-to-right temperature differences.
Temperature differences between different thermal zones.
A well-designed burner system addresses both.
Burners can be controlled individually or by heating zones.
A typical control relationship is:
Temperature Sensor
↓
Controller
↓
Fuel/Air Adjustment
↓
Burner Output
↓
Kiln Temperature
As the kiln temperature changes, burner output can be adjusted.
Multi-zone systems may use independent control loops for different sections.
This allows the burner system to respond to the thermal requirements of each zone.
Tunnel kilns operate continuously, but their thermal demand is not always constant.
Heat demand can change because of:
Production rate
Product type
Product loading
Ambient conditions
Startup conditions
Process changes
A burner with an appropriate turndown ratio can reduce its output while maintaining stable combustion.
This can help avoid:
Excessive burner cycling
Temperature overshoot
Unstable low-load combustion
Unnecessary fuel consumption
Tunnel kiln production speed determines how long products remain in each thermal zone.
If production speed increases:
Shorter Residence Time → Higher Required Heat Transfer Rate
The burner system may need to provide more effective heat transfer to achieve the same product temperature within a shorter time.
If production speed decreases, thermal requirements may change accordingly.
Therefore, burner capacity and control strategy should be considered together with kiln throughput.
The product load determines how much thermal energy is absorbed.
A heavily loaded kiln requires more useful heat than a lightly loaded kiln under otherwise similar conditions.
Important variables include:
Product mass
Product dimensions
Product spacing
Production rate
Initial product temperature
The burner system should be designed around realistic production conditions rather than an empty kiln.
A wider kiln creates a larger area that must be heated uniformly.
If burners are installed only along one side, the opposite side may receive less direct thermal input.
This can create:
Hot Side → Center → Cold Side
Multiple burners positioned across both sides can sometimes provide more balanced heat distribution.
The actual arrangement depends on the kiln's gas-flow design.
Kiln height affects the vertical distribution of heat and combustion gases.
Hot gases naturally tend to rise, while burner momentum can influence their movement.
Burner elevation and angle should therefore be considered when designing the vertical thermal field.
This is especially important when the product occupies a significant portion of the kiln's vertical space.
The exhaust system controls the removal of combustion gases and influences kiln pressure.
If exhaust flow is too high, the system may:
Remove useful heat too quickly
Increase fuel consumption
Draw unwanted cold air into the kiln
If exhaust flow is too low, combustion gases may not move through the kiln properly.
The burner system and exhaust system should therefore be designed together.
Kiln pressure affects gas movement and combustion behavior.
An inappropriate pressure condition can cause:
Air infiltration
Temperature instability
Excessive exhaust
Changes in flame behavior
The objective is to maintain a stable pressure profile appropriate for the kiln design.
This is particularly important in long tunnel kilns because pressure conditions can vary along the kiln.
Combustion air affects both combustion and gas flow.
The burner requires an appropriate air-to-fuel ratio to maintain stable combustion.
Excessive air can increase:
Exhaust gas volume
Exhaust heat loss
Fuel consumption
Insufficient air can result in:
Incomplete combustion
Increased CO
Flame instability
Air control should therefore be coordinated with burner output and kiln temperature.
The combustion process influences the atmosphere within the kiln.
Depending on the process, the kiln may require specific oxidation or reduction conditions.
Burner operation can influence:
Oxygen availability
Combustion-gas composition
Air-to-fuel ratio
Gas circulation
Therefore, burner control is not only about temperature. It can also be part of controlling the thermal atmosphere required by the ceramic process.
Oversized burners can create several problems.
Potential issues include:
Excessive heat input
Difficulty operating at low load
Temperature overshoot
Poor temperature stability
Increased fuel consumption
An oversized burner may appear attractive because it provides additional capacity, but unused capacity does not necessarily improve kiln performance.
The burner should be selected for the actual operating range.
Undersized burners may not provide sufficient thermal energy for the required production rate.
Potential consequences include:
Failure to reach target temperature
Excessively long heating zones
Reduced production speed
Longer firing cycles
Poor process stability
Burner capacity should therefore consider both maximum and normal operating conditions.
Several factors should be considered together.
Avoid excessive oversizing and insufficient capacity.
Distribute heat according to the required thermal profile.
Consider flame length, width, momentum, and shape.
Maintain an appropriate air-to-fuel ratio.
Avoid unnecessarily high exhaust temperatures and excessive air infiltration.
Maintain stable operation across different thermal loads.
Allow burner output to follow the actual temperature profile.
This can result in poor low-load performance.
Different zones have different thermal requirements.
The flame must fit within the available space.
Poor spacing can create hot and cold zones.
Flame direction strongly affects gas circulation.
Pressure problems can affect combustion and heat distribution.
Real production conditions can differ significantly from empty-kiln conditions.
Exhaust flow directly affects burner and kiln performance.
A practical selection process can follow these steps.
Determine:
Product type
Product dimensions
Product mass
Production rate
Determine:
Target temperature
Heating rate
Residence time
Soaking requirements
Atmosphere requirements
Consider:
Product heating
Kiln heat loss
Exhaust loss
Startup requirements
Determine:
Kiln length
Kiln width
Kiln height
Burner locations
Exhaust locations
Evaluate:
Capacity
Turndown
Flame length
Flame shape
Momentum
Fuel requirements
Determine:
Burner quantity
Spacing
Angle
Elevation
Heating zones
Ensure the complete gas-flow system is compatible.
Coordinate:
Temperature measurement
Fuel modulation
Air control
Burner staging
Flame detection
Safety interlocks
DYDTEC Combustion develops industrial burners and combustion-system solutions for industrial heating applications, including applications where burner arrangement and thermal-zone control are important.
The company has developed 100+ burner models covering 200+ application scenarios, allowing burner configurations to be considered according to different kiln structures, thermal loads, fuels, and process requirements.
For tunnel kiln applications, the combustion system can be evaluated around:
Burner capacity
Flame length
Flame shape
Burner spacing
Burner angle
Kiln geometry
Combustion air
Kiln pressure
Exhaust conditions
Temperature uniformity
Multi-zone control
DYDTEC Combustion was established in 2012 and has R&D and manufacturing bases in Shanghai and Yangzhou.
For OEM kiln manufacturers, this system-level approach is important because the burner cannot be separated from the kiln's thermal structure, airflow, exhaust, and temperature-control strategy.
The burner system can influence the physical and thermal design of the kiln.
If burner requirements are considered only after the kiln has been designed, the OEM may have to modify:
Burner openings
Burner spacing
Fuel piping
Combustion-air piping
Exhaust arrangement
Control zones
Refractory structures
Early coordination allows:
Kiln Design + Burner Design + Air System + Exhaust + Control
to be developed as one integrated system.
This can reduce commissioning difficulties and make future production adjustments easier.
Before final burner selection, an OEM should ideally provide:
Kiln length
Kiln width
Kiln height
Insulation structure
Burner installation locations
Exhaust locations
Product type
Product dimensions
Product mass
Loading arrangement
Production rate
Target firing temperature
Heating rate
Residence time
Temperature uniformity
Atmosphere requirements
Fuel type
Fuel pressure
Fuel availability
Combustion-air pressure
Combustion-air temperature
Exhaust conditions
Desired kiln pressure
This information provides the foundation for determining burner capacity, quantity, arrangement, and control strategy.
Burners provide and control the thermal energy required in the heating and firing zones while helping maintain the required temperature profile.
Because tunnel kilns are long and require heat to be distributed across multiple zones. Multiple burners allow thermal input to be distributed and controlled more precisely.
They are commonly installed along the side walls or at other strategically selected locations within the heating and firing zones. The exact arrangement depends on kiln geometry and process requirements.
Burner arrangement affects heat distribution, gas circulation, temperature uniformity, and exhaust heat loss. A well-designed arrangement can improve the utilization of combustion heat.
Yes. Burner angle affects flame trajectory and hot-gas circulation and can influence temperature distribution.
Yes. Flame length needs to be compatible with kiln width, burner spacing, product position, and internal geometry.
Capacity should be based on the actual thermal load, including product heating, kiln heat loss, exhaust losses, production rate, and startup requirements.
Tunnel kilns operate under changing thermal conditions. Adequate turndown allows burners to reduce heat input while maintaining stable combustion and temperature control.
Yes. Excessive burner quantity can increase system complexity and may create unwanted flame interaction or excessive local heat concentration.
Yes. Too few burners may concentrate heat and make it difficult to achieve uniform temperature distribution across the kiln.
Yes. Exhaust flow influences kiln pressure, gas circulation, heat retention, and combustion behavior.
Yes. Natural gas is widely applicable to industrial heating systems when the burner, fuel system, air system, and controls are designed for the required operating conditions.
Burner requirements should ideally be considered during the early kiln-design stage so that burner openings, spacing, airflow, exhaust, and control zones can be coordinated.
Kiln dimensions, product information, production rate, firing temperature, firing cycle, fuel conditions, air conditions, exhaust conditions, and temperature-uniformity requirements are particularly important.
Burners in tunnel kilns are used to do much more than generate high temperatures.
They form part of a coordinated thermal system that controls:
Heat Input
→ Flame Development
→ Gas Circulation
→ Temperature Profile
→ Product Firing
→ Controlled Exhaust
The most effective tunnel-kiln burner system is therefore one that matches the burner capacity, flame characteristics, burner arrangement, kiln geometry, airflow, exhaust, and control strategy to the actual ceramic process.
For OEM manufacturers, burner design should be incorporated into the tunnel-kiln design from the beginning.
Ultimately:
A successful tunnel-kiln burner system is not simply a collection of burners. It is an integrated thermal system designed to deliver the right amount of heat, to the right zone, at the right time, while maintaining stable and uniform firing conditions.
