Air-to-Air Recuperator for Steel Reheat Furnaces (Rolling Mills)

Steel reheat furnaces in rolling mills are among the most energy-intensive units in the metallurgical industry. Large volumes of flue gas are discharged at temperatures often exceeding 800–1,000 °C, carrying substantial recoverable thermal energy. An air-to-air recuperator is a proven and robust heat recovery solution designed to capture this waste heat and reuse it to preheat combustion air, significantly improving furnace efficiency and reducing operating costs.

What Is an Air-to-Air Recuperator?

An air-to-air recuperator is a heat exchanger that transfers thermal energy from hot exhaust gases to incoming cold combustion air without mixing the two streams. In steel reheat furnace applications, the recuperator is typically installed in the flue gas duct downstream of the furnace and upstream of the stack.

Hot flue gas passes through one side of the heat exchanger, while fresh combustion air flows through the other side in counterflow or crossflow arrangement. Heat is transferred across metallic or alloy heat transfer surfaces, raising the temperature of combustion air before it enters the burners.

Why Recuperators Are Critical for Rolling Mill Reheat Furnaces

Reheat furnaces operate continuously or in long campaigns and consume large quantities of fuel. Preheating combustion air provides multiple technical and economic benefits:

• Fuel consumption reduction: Preheated air improves flame temperature and heat transfer efficiency, lowering fuel demand.

• Higher thermal efficiency: Overall furnace efficiency can increase by 10–30%, depending on exhaust temperature and recuperator design.

• Lower CO₂ and NOₓ emissions: Reduced fuel firing directly translates into lower greenhouse gas emissions and improved environmental compliance.

• Improved temperature uniformity: Stable and hotter combustion air enhances billet or slab heating consistency.

• Short payback period: Energy savings typically allow payback within 6–18 months.

Design Features for Steel Industry Applications

Air-to-air recuperators for reheat furnaces must withstand harsh operating conditions. Key design considerations include:

• High-temperature materials: Heat-resistant carbon steel, stainless steel, or high-alloy steels (e.g., Cr-Ni alloys) for continuous operation at elevated temperatures.

• Robust construction: Welded tube, plate, or finned-tube designs to handle thermal cycling and vibration.

• Low pressure drop: Optimized flow paths to minimize fan power increase.

• Dust and scale tolerance: Design allowances for mill scale and particulate-laden flue gas.

• Ease of maintenance: Access doors and modular sections for inspection and cleaning.

Typical Configuration in a Rolling Mill

1. Flue gas exits the reheat furnace at high temperature

2. Gas passes through the air-to-air recuperator

3. Combustion air is preheated before entering burners

4. Cooled flue gas is discharged to stack or downstream equipment

This configuration requires no additional fuel or complex controls, making it highly reliable for steel plant environments.

Case Study: Air-to-Air Recuperator for a Steel Rolling Mill Reheat Furnace

Project Background
A medium-sized steel rolling mill operated a continuous walking-beam reheat furnace firing natural gas. The furnace exhausted flue gas at approximately 950 °C directly to the stack, resulting in high fuel consumption and rising energy costs.

Challenges

• Excessive fuel usage per ton of reheated billets

• Increasing pressure to reduce CO₂ emissions

• Limited space for additional energy recovery equipment

Solution Implemented
A customized air-to-air tubular recuperator was installed in the furnace exhaust duct. The unit was designed to handle high dust loading and thermal cycling typical of rolling mill operation.

Key Design Parameters

• Flue gas inlet temperature: ~950 °C

• Combustion air inlet temperature: ~30 °C

• Combustion air outlet temperature: ~420 °C

• Heat exchanger material: High-temperature alloy steel

• Configuration: Counterflow tubular design

Results Achieved

• Fuel savings: Approximately 18% reduction in natural gas consumption

• CO₂ emissions reduction: Over 2,500 tons per year

• Improved furnace thermal stability and billet heating uniformity

• Payback period: Less than 12 months

Customer Feedback
The rolling mill reported stable long-term operation with minimal maintenance and confirmed that the recuperator significantly improved overall furnace efficiency.

Conclusion

An air-to-air recuperator for steel reheat furnaces in rolling mills is a highly effective waste heat recovery solution that delivers immediate and measurable benefits. By reclaiming energy from high-temperature flue gases and preheating combustion air, steel producers can reduce fuel costs, lower emissions, and enhance furnace performance without disrupting production.