Flue Gas Heat Exchanger: Complete Guide
A flue gas heat exchanger is a thermal recovery device that captures waste heat from exhaust gases (flue gases) and transfers it to another medium (water, air, or process fluid), improving energy efficiency and reducing fuel costs.
How Flue Gas Heat Exchangers Work
- Hot Flue Gases (200–600°C) from boilers, furnaces, or engines enter the exchanger.
- Heat Transfer occurs through tubes, plates, or fins to:
- Preheat combustion air (recuperator)
- Heat water/steam (economizer)
- Warm process fluids (thermal oil, glycol)
- Cooled Exhaust exits at a lower temperature (100–200°C), reducing energy waste.
Types of Flue Gas Heat Exchangers
1. Recuperators (Air Preheaters)
- Function: Preheats combustion air using exhaust heat.
- Design: Shell & tube / plate heat exchangers.
- Efficiency: 5–15% fuel savings.
- Applications: Boilers, kilns, furnaces.
2. Economizers (Water/Steam Heaters)
- Function: Heats feedwater before it enters the boiler.
- Design: Finned tube bundles.
- Efficiency: 5–12% fuel reduction.
- Applications: Power plants, industrial steam systems.
3. Condensing Heat Exchangers
- Function: Recovers latent heat by cooling flue gases below dew point.
- Efficiency: 10–20% higher than standard economizers.
- Material: Corrosion-resistant (stainless steel, polymers).
- Applications: High-efficiency boilers, biogas plants.
4. Waste Heat Boilers
- Function: Generates steam from high-temperature exhaust (e.g., gas turbines).
- Output: 5–50 tons/hr of process steam.
- Applications: Refineries, chemical plants.
5. Thermal Oil Heaters
- Function: Transfers heat to thermal oil for indirect heating.
- Advantage: No water required, operates at high temps (up to 400°C).
- Applications: Food processing, textile industries.
Key Benefits
✔ Energy Savings – Recovers 10–30% of wasted heat.
✔ Lower Emissions – Reduces fuel consumption → fewer CO₂/NOx emissions.
✔ Faster Payback – ROI in 1–3 years due to fuel cost savings.
✔ Process Optimization – Stable temperatures improve product quality.
Design Considerations
| Factor | Importance |
| Flue Gas Temp. | Determines heat recovery potential |
| Flow Rate | Sizing the exchanger correctly |
| Corrosion Resistance | Stainless steel for acidic flue gases (H₂SO₄, HCl) |
| Fouling Risk | Soot/ash buildup reduces efficiency (self-cleaning designs help) |
| Pressure Drop | Should not restrict exhaust flow |
Top Applications
- Power Plants (Boiler exhaust heat recovery)
- Cement Industry (Kiln preheating)
- Steel Mills (Blast furnace gas heat capture)
- Chemical Plants (Process heating optimization)
- Biomass Boilers (Condensing heat recovery)