1. Definition and Core Functions
An industrial boiler heat exchanger is the central component for thermal energy transfer. It transfers heat from a high-temperature medium (such as flue gas or steam) to a low-temperature medium (such as water or air) through a solid wall or direct contact, meeting process temperature requirements.
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Main Objectives:
- Improve boiler thermal efficiency (traditional boilers operate at ~80% efficiency; with heat exchangers, this can exceed 90%);
- Recover waste heat (e.g., flue gas heat recovery can reduce exhaust temperature to below 100°C, minimizing thermal losses);
- Reduce fuel consumption (for every 10°C decrease in exhaust temperature, fuel savings of approximately 1% can be achieved).
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Typical Applications:
- Chemical industry: heating reactor feedstock and controlling reaction temperatures;
- Power generation: increasing steam parameters (from saturated to superheated steam) to enhance turbine efficiency;
- Food industry: precise temperature control during sterilization and drying processes.
2. Main Types and Technical Details
(1) Water Wall
- Structure: Composed of densely arranged vertical steel tubes lining the furnace walls, with water flowing internally for cooling.
- Functions:
- Absorbs radiant heat from the flame (accounting for 50%-60% of total boiler heat), converting water into a steam-water mixture;
- Protects the furnace walls from high-temperature erosion (above 1300°C), extending service life.
- Technical Parameters:
- Material: Typically 20G boiler steel, capable of withstanding pressures of 15–20 MPa;
- Design pressure: Must exceed boiler operating pressure by 10%-15% to ensure safety.
(2) Superheater
- Types:
- Low-Temperature Superheater: Convective type, located in the flue duct, with serpentine tubes heated by flue gas cross-flow;
- High-Temperature Superheater: Radiant or semi-radiant type, positioned at the top of the furnace, directly absorbing radiant heat from the flame.
- Functions:
- Heats saturated steam (100°C) to superheated steam at 300–600°C, increasing steam enthalpy (e.g., from 2676 kJ/kg to 3500 kJ/kg);
- Prevents erosion damage to turbine blades caused by wet steam.
- Design Considerations:
- Steam velocity must be optimized to prevent scaling; tube wall temperature must be monitored to avoid overheating and tube rupture.
(3) Reheater
- Application: Used in reheat cycle systems (e.g., supercritical units).
- Functions:
- Reheats steam discharged from the high-pressure turbine (around 300°C) back to over 550°C before sending it to the intermediate and low-pressure turbines for additional work;
- Increases thermal efficiency (reheat cycles are 15%-20% more efficient than standard Rankine cycles);
- Reduces steam moisture content (from ~10% to below 1%), protecting turbine blades.
(4) Economizer
- Structure: Consists of multiple serpentine tube bundles installed in the boiler’s rear flue duct.
- Working Principle:
- Flue gas (300–400°C) flows over the tubes, transferring heat to feedwater inside (heating it from 150°C to 200°C);
- Reduces exhaust gas temperature (from 200°C to 100°C), minimizing heat loss.
- Energy-Saving Effects:
- For every 10°C reduction in exhaust temperature, approximately 1% fuel savings can be achieved;
- Installing an economizer can improve boiler efficiency by 5%-10%.
(5) Air Preheater
- Types:
- Tubular Type: Heat transfer occurs through metal tube walls between flue gas and air; simple structure but lower efficiency;
- Regenerative (Rotary/Junkers) Type: A rotating rotor alternately contacts hot flue gas and cold air, achieving high heat transfer efficiency (up to 80%), though regular soot cleaning is required.
- Functions:
- Preheats combustion air from 20°C to 300°C, improving combustion conditions (increasing theoretical combustion temperature);
- Reduces heat loss (exhaust temperature drops from 150°C to below 100°C).
(6) Heat Pipe Heat Exchanger
- Working Principle:
- Utilizes phase-change heat transfer (evaporation-condensation cycle) to achieve highly efficient conduction under minimal temperature differences;
- Equivalent thermal conductivity can be thousands of times higher than that of conventional metals.
- Advantages:
- Compact size and lightweight (about one-third the weight of traditional heat exchangers);
- Resistant to low-temperature corrosion (by controlling wall temperature above acid dew point);
- Suitable for flue gas waste heat recovery (e.g., reducing 120°C flue gas to 80°C).
3. Physical Mechanisms of Heat Transfer
- Conduction: Heat transfers through solid walls from the hot side to the cold side, following Fourier’s Law (heat flux proportional to temperature gradient).
- Convection: Heat is transferred between fluid and surface via molecular motion, including natural convection (e.g., in air preheaters) and forced convection (e.g., in economizers).
- Radiation: High-temperature flames emit infrared radiation toward water walls, transferring energy in the form of electromagnetic waves, governed by the Stefan-Boltzmann Law (radiant heat flux proportional to the fourth power of absolute temperature).
4. Application Value and Case Studies
- Energy Efficiency Benefits:
- A chemical plant reduced flue gas temperature from 180°C to 120°C after installing an economizer, saving 1.2 million RMB annually in fuel costs;
- Heat pipe heat exchangers recovered waste heat, increasing boiler efficiency from 82% to 88%.
- Environmental Benefits:
- Lowering exhaust temperature reduces NOx and SO₂ emissions (a 50°C reduction decreases NOx emissions by 15%);
- Complies with China’s Boiler Atmospheric Pollutants Emission Standards (GB 13271-2014).
- Equipment Longevity:
- Water walls with anti-wear coatings extended service life from 5 to 10 years;
- Installing soot blowers on superheaters reduces efficiency loss due to ash buildup.
5. Maintenance and Troubleshooting
- Scaling Issues:
- Water-side scaling (CaCO₃, Mg(OH)₂) reduces heat transfer coefficient; periodic acid cleaning (every two years) is required;
- Flue gas-side ash accumulation increases flow resistance; sonic soot blowers should be installed.
- Corrosion Protection:
- Low-temperature corrosion (sulfuric acid dew point corrosion): Maintain tube wall temperature above the acid dew point (typically above 120°C);
- High-temperature corrosion (sulfide corrosion): Use corrosion-resistant alloys (e.g., Inconel 625) or protective coatings.
- Performance Monitoring:
- Regular thermal efficiency testing per standards such as GB/T 10180-2016;
- Install online monitoring systems to track heat transfer coefficient, pressure drop, and other key parameters in real time.
II. Selection and Design Considerations
- Thermal Load Calculation: Determine required heat transfer based on process needs (e.g., Q = m·c·ΔT, where m is mass flow rate, c is specific heat capacity, and ΔT is temperature difference).
- Material Selection:
- High-temperature zones (superheater): Use 12Cr1MoVG alloy steel;
- Low-temperature zones (economizer): Use 20G carbon steel.
- Structural Design:
- Tube diameter selection: Balance flow velocity and pressure drop (e.g., economizer tubes typically Φ32mm);
- Flow arrangement: Parallel flow (higher efficiency but larger temperature difference) or counter-flow (smaller temperature difference but slightly lower efficiency).