1. Definition

An air preheater (APH) is a heat exchange device installed in the tail flue of a boiler. It utilizes waste heat from flue gases to preheat the combustion air before it enters the boiler, improving thermal efficiency, reducing fuel consumption, and lowering exhaust gas temperatures to minimize energy losses.

Air preheaters are primarily used in coal-fired power plant boilers and can be categorized into tubular (fixed) type and rotary type. The rotary type can be further divided into rotating-hood type and rotating-heat-surface type, with the latter being more commonly used in power plant boilers. Based on the compartment structure, air preheaters can be classified into two-compartment, three-compartment, and four-compartment designs, among which the four-compartment design is widely used in circulating fluidized bed (CFB) boilers due to its better adaptability to varying operational conditions.

2. Working Principle

The working principle of an air preheater is based on heat exchange through temperature differences between high-temperature flue gas and low-temperature air, thereby raising the air temperature and enhancing combustion efficiency.

According to the heat transfer mechanism, air preheaters are classified into conductive (tubular) type and regenerative (rotary) type:

1. Tubular Air Preheater (Fixed Type):

   
   

   • Constructed with thin-walled steel tubes, typically arranged in a cubical structure.

   • The tubes are vertically staggered, with both ends welded to upper and lower tube plates, and an intermediate tube plate inside the housing.

   • Flue gas flows up and down outside the tubes, while air moves horizontally through the tube bundle, enabling heat transfer.

   • Suitable for medium- and small-sized boilers, offering compact structure and excellent sealing performance.

   
   

2. Rotary Air Preheater (Regenerative Type):

   
   

   • Consists of a cylindrical rotor with multiple sector-shaped passages for flue gas and air.

   • The rotor rotates slowly, sequentially exposing heat transfer elements to hot flue gas and cold air, facilitating indirect heat storage and transfer.

   • Commonly used in large-scale power plant boilers, providing higher heat transfer efficiency, though regular maintenance is required to prevent ash accumulation and wear.

     
     

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By integrating an air preheater, the air temperature can typically be increased to around 300°C (572°F), which improves combustion conditions, reduces fuel consumption, and prevents condensation-related corrosion in the boiler system.

3. Key Features

Air preheaters offer several advantages in boiler systems, including:

1. Enhanced Boiler Efficiency:

   • Preheating the combustion air enables more complete fuel combustion, improving thermal efficiency and reducing unburned carbon loss.

   • Improved efficiency leads to lower fuel consumption, reducing operational costs.

2. Lower Exhaust Gas Temperature:

   • Extracting heat from flue gas minimizes thermal energy losses, reducing exhaust temperature.

   • Helps lower thermal pollution and improves overall energy utilization.

3. Excellent Sealing Performance:

   • Optimized sealing structures minimize air leakage, improving heat exchange efficiency.

4. High Heat Transfer Efficiency:

   • Designed with high thermal conductivity materials and optimized airflow pathways to maximize heat transfer.

5. Ease of Manufacturing and Maintenance:

   • Simple yet robust design allows for cost-effective fabrication, easy installation, and convenient maintenance, ensuring long-term reliability.

     
     

4. Applications

Air preheaters are widely used in various industries, including:

1. Power Plant Boilers:

   • Essential for coal-fired power stations, optimizing combustion efficiency and reducing coal consumption.

2. Industrial Boilers:

   • Commonly used in industries such as metallurgy, chemical processing, building materials, and paper production to improve thermal efficiency.

3. Waste Heat Recovery Systems:

   • Utilized in industrial heat recovery applications, reducing energy wastage and enhancing cost savings.

4. Environmental Protection Projects:

   • Plays a key role in reducing emissions and lowering flue gas temperatures, contributing to sustainable energy use and environmental compliance.