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The role of the heat exchanger in the kiln.
The traditional art of ceramics firing has a long history, and even in contemporary times, the production of ceramics still primarily relies on high-temperature kilns. These kilns can be categorized as continuous (such as tunnel kilns) and intermittent (such as downdraft kilns). They share a common characteristic: the combustion of fuel generates high-temperature flue gas, which flows in the opposite direction to ceramic products inside the kiln. Along the way, heat is transferred to the clay body, kiln furniture, and kiln walls through radiation, convection, and conduction. However, a significant portion of the heat is lost when the high-temperature flue gas is discharged through the chimney, limiting the improvement of overall thermal efficiency.
To overcome heat losses, a heat pipe heat exchanger can be used to recover residual heat from the flue gas, which can then be used to heat the air and serve as a heat source for drying the ceramics. This method effectively conserves energy and brings about significant energy-saving benefits to the ceramic firing process.
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How are the performance of SiC the heat exchanger？
Considering the harsh operating conditions in high-temperature,materials of heat exchangers need to exhibit excellent high-temperature resistance and thermal shock resistance. These materials should withstand the formation of cracks in the temperature range from ultra-high temperatures to room temperature and maintain good corrosion resistance in oxidative and acidic environments.
Silicon carbide ceramics excel in this regard as a superior material. They offer outstanding thermal conductivity, exceptional high-temperature strength, excellent resistance to oxidation and thermal shock, a long lifespan, low maintenance requirements, and reliable and stable performance. Left is a comparison of the thermal conductivity coefficients of several materials, highlighting how silicon carbide’s high thermal conductivity significantly enhances the efficiency of heat exchangers.
What is the working principle of heat exchanger？
The working principle of a silicon carbide ceramic heat exchanger involves placing it near the flue gas outlet, where temperatures are higher. During this process, there is no need to introduce additional cooling air or high-temperature protection measures. When the furnace temperature is maintained within the range of 1250-1450°C, the flue gas outlet temperature typically falls between 1000-1300°C. Ceramic heat exchangers can recover residual heat as high as 450-750°C and introduce the recovered hot air into the furnace, where it is mixed with fuel gas and combusted. This approach directly reduces production costs while enhancing economic benefits.
In comparison to metal heat exchangers, silicon carbide (SiC) heat exchangers excel in handling corrosion resistance and high-temperature resistance issues, making them perform exceptionally well in operation.
Common Issues with Silicon Carbide (SiC) Heat Exchangers in Kiln Usage
When using a silicon carbide(SiC) heat exchanger in a kiln, various situations and common problems may arise. Here are some of the common issues:
- Cracking: Cracks in the heat exchanger material can occur due to factors such as thermal stress, mechanical vibration, material fatigue, rapid temperature changes, and even during transportation where the heat exchange tubes may experience vibration, shocks, or impacts, leading to cracks or fractures and subsequent damage.
- Corrosion: The chemical atmosphere within the kiln can corrode the heat exchanger material, causing damage and corrosion to the material’s surface, potentially reducing its strength and durability.
- Oxidation: At high temperatures, the heat exchanger material may react with oxygen to produce oxides, resulting in surface oxidation. Oxidation can degrade material performance, increase brittleness, and impact the material’s lifespan.
- Clogging: In high-temperature environments, certain components in gases may deposit on the heat exchanger’s surface, forming solid deposits. These deposits can lead to pipe blockages, affecting heat conduction and heat exchange efficiency, thereby reducing the heat exchanger’s performance.
- Deformation and Damage: High temperatures and thermal expansion can cause the heat exchanger material to deform, potentially affecting its connection and sealing performance, or even leading to material damage. Design flaws, such as uneven fluid distribution or concentrated thermal stress, can increase the risk of damage.
- Mechanical Vibration and Mechanical Damage: Mechanical vibrations during kiln operation or accidental collisions and impacts during routine maintenance or operational processes can result in deformation and damage to the heat exchanger material.
- Failed Connections: Poor connections between the heat exchanger and other components can lead to leaks or damage, affecting the normal operation of the heat exchanger.
- Material Aging: Prolonged exposure to high temperatures can cause material aging, leading to a decline in its physical properties, which can, in turn, affect the heat exchanger’s lifespan and performance.
- Thermal Shock: Rapid temperature changes can induce thermal shock in the material, leading to cracks and damage.
- Leakage: Poor connections, material aging, or cracks can result in heat exchanger leakage, impacting heat exchange efficiency and the working environment.
- Material Embrittlement: High temperatures and corrosive environments can cause certain materials to become brittle, losing their toughness and strength, and making them prone to fracture.
Analysis of the Causes of Damage in Silicon Carbide Heat Exchangers Used in Kilns
After years of silicon carbide（SiC） heat exchanger production, we have summarized several possible reasons for damage:
- Thermal Stress: High temperatures can create significant temperature differences between the inside and outside of the heat exchanger, leading to thermal stress. This stress may cause the material of the heat exchanger to crack or rupture.
- Chemical Corrosion: Corrosive gases or chemicals may exist within the kiln, and these substances can react chemically with the heat exchanger material, resulting in material damage.
- Improper Operation: Incorrect assembly, operation, or maintenance practices can lead to heat exchanger damage, such as excessive localized heating, rapid cooling, improper cleaning methods, and more.
- Aging: Prolonged exposure to high temperatures can lead to material aging, causing it to lose its original physical properties and making it more susceptible to damage.
- Uneven Temperature: Non-uniform temperature distribution can lead to stress concentration in specific areas, exacerbating material fatigue and cracking.
- Design Flaws: The design of the heat exchanger may have flaws, such as an unreasonable structure or connection method, which can lead to stress concentration and subsequent damage.
- Inappropriate Material Selection: Selecting materials that are not suitable for the specific working environment, such as mismatched temperature tolerances, atmospheres, or chemical compositions, can result in material corrosion, oxidation, or failure.
- Thermal Shock: Rapid heating or cooling of the kiln can induce thermal shock in the heat exchanger material, causing cracking or damage.
- Environmental Impact: Factors such as pollutants and humidity in the surroundings of the kiln can also affect the material performance of the heat exchanger, accelerating material damage.
To avoid these adverse situations, material selection for the heat exchanger should consider durability in high-temperature, corrosive, and oxidizing environments. The design should minimize stress concentration, accurate temperature control systems should be employed to avoid exceeding material temperature tolerances, and adequate support and fixation of heat exchange pipes should be ensured to reduce vibration and mechanical stress. Proper operation and maintenance are crucial, including regular inspections and upkeep to ensure the heat exchanger’s normal operation. Customized improvement measures should be taken based on the specific requirements of the equipment and process for different usage scenarios.
Actual Cases of Silicon Carbide Heat Exchangers in Kiln Usage
1.Specific Damage Description of Heat Exchanger Damage
The figure right illustrates an actual case of damage to silicon carbide heat exchangers occurring in an industrial kiln. These heat exchangers developed cracks and ultimately fractures at the connection points with the burner nozzles after only a few usage cycles. It’s worth noting that even though this industrial kiln had replaced different heat exchangers several times, similar issues continued to arise.
This case highlights the challenges that can be faced when using silicon carbide heat exchangers in industrial kilns. Possible reasons for this damage may include but are not limited to thermal stress induced by high-temperature cycling, mechanical stress at the connection points between the heat exchanger and burner nozzles, and material fatigue properties. Addressing such issues may require in-depth engineering analysis, including material performance testing, stress analysis, and optimization of design and operation. By implementing improved designs and operational controls, it is possible to reduce the occurrence of such damage events, thereby enhancing the stability and reliability of industrial processes.
2.Cause analysis of Heat Exchanger Damage
Heat ExchangerThe main heat exchange components of this heat exchanger are made of silicon carbide ceramic material. In this structure, high-temperature flue gas flows on the outside of the pipes, while the heated gas flows on the inside of the pipes, creating a counterflow heat exchange process. Silicon carbide pipes are highly temperature-resistant and can withstand temperatures of up to 1540°C, allowing high-temperature flue gas to directly enter the heat exchanger for heat exchange.
The high-temperature flue gas here emits strong radiative heat, and silicon carbide pipes can fully utilize this characteristic through radiative heat transfer from the high-temperature gas and flame, enabling efficient heat exchange. This design effectively leverages the high-temperature resistance and radiative heat transfer capability of silicon carbide material, thus achieving efficient heat exchange in high-temperature environments.
The initial investigation focused on the silicon carbide heat exchanger itself. After strict production processes and process supervision, including precise measurements of the heat exchange pipe wall and confirmation of physical performance parameters, no evidence of non-compliance in production or significant quality issues with the silicon carbide heat exchanger were found.
Additionally, given the brittle nature of silicon carbide, the structural design of the heat exchanger must adhere to the design principles for brittle materials to ensure its safety and reliability. In the actual design and usage process, several key issues need to be considered:
- Sealing of Components.
- Differential Thermal Expansion: Silicon carbide ceramic heat exchangers have a significantly different coefficient of thermal expansion compared to metal parts. Therefore, they must have good thermal expansion compensation capabilities.
- Tensile Strength: Brittle materials typically have lower tensile strength compared to compressive strength, so structural design should ensure that silicon carbide heat exchange pipes work under compressive loads.
- Uniformity of High-Temperature Distribution: Paying special attention to the uniformity of high-temperature distribution within the heat exchanger is essential.
By adhering to these principles, safety and reliability can be improved in the design and use of silicon carbide heat exchangers, addressing potential issues and ensuring the stable operation of industrial processes.
Through on-site investigations, it was discovered that the industrial kiln violated the design principles of silicon carbide heat exchangers during assembly and usage, with the following specific issues:
- Design Flaws Leading to Sealing Issues: There were sealing defects in the design of the heat exchanger with the internal burner device. The burner device was embedded too deeply into the heat exchanger, causing internal heat flow to enter the top of the heat exchanger along the gap where they connected. Due to the concentrated impact of a large amount of heat flow, localized embrittlement occurred, leading to cracks and eventual rupture.
- Unaccounted Thermal Expansion Differences: The design of both the heat exchanger and the internal burner device did not account for the differences in thermal expansion. This caused the internal metal device to affect the heat exchanger during thermal expansion, leading to cracks under the influence of expansion.
- Uneven Operating Pressure: The stability of gas pressure in the furnace chamber is crucial for controlling the temperature and gas pressure of the heat exchanger. The working pressure in the combustion area of the industrial kiln was uneven, with a significant negative pressure inside the kiln, allowing more cold air to leak in. On one hand, this resulted in lower temperatures and severe gas stratification with large temperature differences between the upper and lower parts. On the other hand, it was challenging to maintain uniform gas pressure in the combustion area. This led to uneven pressure on the silicon carbide heat exchanger, potentially affecting its brittle characteristics.
- Inadequate Temperature Control: Inappropriate temperature control in the industrial kiln could result in rapid heating or cooling, impacting the brittle nature of the heat exchanger material. Uneven temperature distribution could concentrate thermal stress in specific areas of the heat exchanger, exacerbating issues of material fatigue and cracking.
In summary, these issues directly or indirectly affected the performance and stability of the silicon carbide heat exchanger. Addressing these problems requires actions in multiple areas, including design, assembly, operation, and maintenance, to ensure compliance with the characteristics and principles of brittle materials and achieve the safe and efficient operation of the heat exchanger.
3.Improvement Measures for Heat Exchanger Damage
In response to the heat exchanger damage issue, here are some improvement measures:
- Adjust Burner Device Position: During the assembly process, adjust the position of the burner device to ensure it is parallel to the connecting point of the heat exchanger, reducing the concentration of thermal stress. This can improve the thermal stress issues that may arise from improper alignment.
- Standardize Furnace Operation: Standardize the operation of the furnace, stabilizing the temperature and pressure inside the furnace to mitigate the impact of temperature and pressure on the heat exchanger. Ensure that temperature and pressure variations remain within the heat exchanger’s tolerance range.
- Upgrade Packaging: While the silicon carbide heat exchanger had no quality issues upon leaving the factory in this case, uncertainties during transportation could subject the heat exchanger to impacts. Upgrade the packaging method for shipping, using sturdy packaging materials such as foam, wood, bubble wrap, etc., and ensure secure reinforcement to effectively protect the heat exchanger from external forces.
Conclusion
In conclusion, if Silicon carbide Heat Exchangers encounter various issues such as breakage during use, it is influenced by multiple factors. Specific investigations and determinations need to be made based on the on-site conditions and the characteristics of the product itself. If you are looking for Silicon carbide Heat Exchangers products, feel free to browse our related products or contact us directly.
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