Application of Heat Pipe Heat Exchangers in Low-Temperature Flue Gas Waste Heat Recovery from Annealing Furnaces

Application of Heat Pipe Heat Exchangers in Low-Temperature Flue Gas Waste Heat Recovery from Annealing Furnaces

Low-temperature flue gas (250–420℃) from annealing furnaces is a core waste heat source in the metallurgical and metal processing industries. Using heat pipe heat exchangers to recover this waste heat can efficiently produce steam, preheat combustion air, or heat process water, achieving energy savings of 18%–25%. The investment payback period is generally shorter than 8 months, making it a mature solution that balances energy saving and cost reduction.

Flue Gas Characteristics

Temperature: Annealing furnace exhaust gas is mostly 250–420℃ (low-temperature range), with some high-temperature sections reaching 700–800℃.

Composition: Contains small amounts of dust, rolling oil mist, SO₂/NOₓ, easily corroding and clogging.

Gas Flow Rate: The flue gas flow rate of cold rolling/galvanizing annealing furnaces is moderate and fluctuates greatly. Traditional heat exchangers are prone to scaling and rapid efficiency degradation.

Heat Pipe Working Principle: A heat pipe is a closed vacuum heat exchange element filled with a working fluid (water/ethanol):

Evaporation Section (Flue Gas Side): Absorbs heat from the flue gas, the working fluid vaporizes → steam flows to the condensation section.

Condensation Section (Water/Air Side): Steam releases heat and condenses → liquid flows back to the evaporation section, repeating the cycle.

Core Advantages: Extremely low thermal resistance, strong isothermal properties, single-pipe thermal power ≥1.5kW, suitable for low-temperature differential heat exchange.

System Process Flow (Taking steam production as an example):

Annealing furnace low-temperature flue gas (300–420℃) → Bypass flue → Heat pipe evaporator (cooled to 150–180℃) → Induced draft fan → Desulfurization → Chimney; Softened water → Deoxygenation → Steam drum → Heat pipe heat absorption → Saturated steam (0.3–0.6MPa) → Process steam/Grid connection.

Core Equipment Configuration

Heat Pipe Evaporator: Finned heat pipe module, flue gas flows outside the pipes, water flows inside the pipes, resistant to dew point corrosion, online soot blowing.

Steam Drum/Steam Distributor: Steam-water separation, pressure stabilization, steam supply pressure 0.3–0.7MPa.

Softening, Deoxygenating, and Water Makeup Skid: Fully automatic soft water + thermal deoxygenation, preventing scaling and oxygen corrosion.

Flue Gas Bypass Valve Assembly: Switches to the original flue during maintenance without affecting main production.

PLC Control Cabinet: Real-time monitoring of temperature/pressure/water level, one-button start/stop + fault alarm.

Key Technical Points

1. Heat Pipe Selection (Low Temperature Dedicated)

Material: Carbon steel/copper-nickel alloy, resistant to low-temperature dew point corrosion (≥150℃ exhaust).

Working Fluid: Deionized water, applicable temperature 200–450℃, high cost-effectiveness.

Fingers: High-frequency welded fins, enhanced heat transfer, prevents ash accumulation, spacing 8–12mm. 2. Corrosion Resistance and Ash Blocking Design

Dew Point Control: Exhaust gas temperature ≥150℃, avoiding acid dew point (120–140℃).

Online Ash Cleaning: Reserved compressed air/steam soot blowing interface for uninterrupted ash cleaning.

Modular Structure: Individual unit failures can be isolated without affecting overall operation.

3. Energy Efficiency Design Indicators

Flue Gas: Inlet 300–420℃, Outlet 150–180℃, Pressure Drop ≤400Pa.

Steam: Steam production 0.3–0.5t/h (0.6MPa saturated steam).

Heat Exchange Efficiency: ≥80%, Energy Saving Rate 18%–25%.