A boiler economizer is a heat exchanger device that recovers waste heat from boiler flue gases and uses it to preheat the feedwater entering the boiler. This improves the overall efficiency of the boiler system by reducing fuel consumption and lowering emissions
How a Boiler Economizer Works
- Hot flue gases (200–300°C) exit the boiler and pass through the economizer.
- Cold feedwater (returning from the system) flows through tubes inside the economizer.
- Heat transfer occurs: The flue gases lose heat, warming the feedwater before it enters the boiler.
- Preheated water reduces the energy needed to reach boiling temperature, saving fuel.
Types of Boiler Economizers
Non-Condensing Economizer
- Recovers sensible heat only (does not cool flue gases below dew point).
- Typically increases boiler efficiency by 5–10%.
- Made of carbon steel or stainless steel.
2. Condensing Economizer
- Cools flue gases below dew point, recovering latent heat from water vapor.
- Can improve efficiency by 10–15% (higher in natural gas/biogas systems).
- Requires corrosion-resistant materials (stainless steel, fluoropolymer coatings).
3. Finned Tube Economizer
- Uses finned tubes to increase heat transfer surface area.
- Compact design, suitable for small to medium boilers.
4. Fire Tube vs. Water Tube Economizers
- Fire Tube: Flue gases pass through tubes, water surrounds them (common in small boilers).
- Water Tube: Water flows inside tubes, flue gases pass outside (better for high-pressure boilers).
Benefits of Using an Economizer in Biogas Boilers
✔ Fuel Savings – Reduces fuel consumption by 5–15%.
✔ Lower Emissions – More efficient combustion = less CO₂, NOx, and SOx.
✔ Quick Payback – Typically pays for itself in 1–3 years due to energy savings.
✔ Extended Boiler Life – Preheating reduces thermal stress on the boiler.
Key Design Considerations
- Flue Gas Temperature – Must be high enough for heat recovery (usually >150°C).
- Feedwater Temperature – Too cold can cause acid condensation (corrosion risk).
- Material Selection – Biogas may contain H₂S, requiring stainless steel or coated tubes.
- Pressure Drop – Should not restrict flue gas flow excessively.
- Cleaning & Maintenance – Soot build-up reduces efficiency; access for cleaning is important.
Applications in Biogas Systems
- Biogas-fired boilers (steam/hot water production).
- Combined Heat and Power (CHP) plants.
- Industrial process heating.
- Waste-to-energy plants.
Economizer vs. Recuperator
| Feature | Economizer | Recuperator |
| Function | Preheats feedwater | Preheats combustion air |
| Heat Source | Boiler flue gases | Exhaust gases (engines, furnaces) |
| Efficiency Gain | 5–15% | 5–10% |
| Common Materials | Carbon/stainless steel | Stainless steel, ceramics |
Installation Tips
- Install after the boiler but before the chimney/flue gas treatment.
- Use corrosion-resistant materials if biogas contains H₂S or moisture.
- Monitor flue gas temperature to avoid acid condensation.