Exhaust air waste heat recovery method and efficiency

Table of Contents[hide]

• 1. Method of recovering waste heat from exhaust air
• 2. Factors affecting recycling efficiency
• 3. Typical efficiency range
• 4. Optimization Suggestions
• 5. Case reference

Recovery of waste heat from exhaust air is an important technology for recovering low-temperature heat energy in coal mine ventilation systems. It is mainly used to improve energy utilization efficiency and reduce energy consumption. The following are common methods for recovering waste heat from exhaust air and their efficiency analysis:

1. Method of recovering waste heat from exhaust air

1. Heat pipe heat exchange technology
   • principle: Utilize the efficient thermal conductivity of heat pipes to transfer low-temperature heat in the exhaust air to the refrigerant (such as water or air) for heating or hot water production.
   • advantage: Compact structure, high heat transfer efficiency, and adaptable to low temperature difference environment.
   • Application Scenario: Heat exchange between mine exhaust air and ambient air or water.
   • efficiency: The heat recovery efficiency of the heat pipe heat exchanger is generally between 60% and 80%, depending on the operating conditions and design.
2. Heat Pump Technology
   • principle: Through a heat pump system (such as an air source or water source heat pump), low-grade thermal energy is extracted from the lack of air and upgraded to high-grade thermal energy for heating or cooling.
   • advantage：The energy efficiency ratio (COP) is high, and 1 unit of electrical energy can be driven to recover 3-5 units of thermal energy.
   • Application Scenario: Mine heating, wellhead antifreeze or domestic hot water.
   • efficiency：The COP of the heat pump system is usually between 3.0-5.0, and the heat recovery efficiency can reach 70%-90%.
3. Plate heat exchanger technology
   • principle: The heat of the exhaust air is transferred to the cold side medium (such as water or air) through the metal plate.
   • advantage: Simple structure, easy maintenance, suitable for large-scale heat exchange.
   • Application Scenario: Heat exchange between exhaust air and heating system.
   • efficiency：The heat recovery efficiency is generally between 50% and 75%, which is greatly affected by the heat exchange area and temperature difference.
4. Spray heat exchange technology
   • principle: The exhausted air is directly heat exchanged by spraying water, and the water absorbs the heat and is used for heating or other purposes.
   • advantage: Suitable for high temperature, high humidity and lack of air, with high heat exchange efficiency.
   • Application Scenario: Heat recovery from exhaust air in high temperature mines.
   • efficiency: The heat recovery efficiency can reach 65%-85%, but the water treatment cost needs to be considered.
5. Thermal storage technology
   • principle: Use heat storage materials (such as phase change materials or solid thermal storage bodies) to store the heat of exhaust air and release it when needed.
   • advantage：It can realize the temporal and spatial transfer of heat, which is suitable for intermittent heating needs.
   • Application Scenario: Mine heating or process heat.
   • efficiency: The heat recovery efficiency is generally between 60% and 80%, which is limited by the performance of the thermal storage material.

2. Factors affecting recycling efficiency

1. Ventilation temperature and humidity：The higher the exhaust air temperature and the greater the humidity, the greater the heat recovery potential. Usually the exhaust air temperature is 15-25℃ and the humidity is close to saturation.
2. Heat exchange equipment performance：The material, structural design and heat exchange area of the heat exchanger directly affect the efficiency.
3. Environmental conditions: When the outside temperature is low, the COP of the heat pump system is higher and the recovery efficiency is better.
4. System compatibility: The degree of match between the heat recovery system and the mine heating demand affects the overall efficiency.
5. Operation and maintenance: Fouling, blockage or improper operating parameters of the heat exchanger will reduce efficiency.

3. Typical efficiency range

• Single technology: The heat recovery efficiency is usually between 50%-90%, and the heat pump system has the highest efficiency.
• Composite system：Combining heat pump with heat pipe or thermal storage technology can increase the overall efficiency to 80%-95%.
• Practical Application: Due to working conditions, the actual heat recovery efficiency is mostly between 60%-80%.

4. Optimization Suggestions

1. Multi-technology coupling: Such as heat pump + heat pipe combination to improve heat recovery efficiency.
2. Intelligent Control: Optimize operating parameters and reduce energy losses through sensors and automation systems.
3. Regular maintenance: Clean the heat exchanger to prevent efficiency loss.
4. Cascade utilization of waste heat：The recovered heat is graded for heating, hot water and process heat to maximize its utilization.

5. Case reference

• A coal mine uses an air source heat pump to recover waste heat from exhaust air. The exhaust air temperature is 18°C, the heating COP is 4.2, and the heat recovery efficiency is about 85%.
• A mine uses a heat pipe heat exchanger to recover the heat of exhaust air for wellhead antifreeze, with a heat recovery efficiency of about 70%.

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