What is an Air Preheater?
An air preheater (APH) is a heat exchanger that recovers waste heat from flue gases and uses it to preheat combustion air entering boilers, furnaces, or kilns. This improves combustion efficiency and reduces fuel consumption by 5-15%.
How Air Preheaters Work
- Hot Flue Gases (250-400°C) exit the boiler/furnace and pass through the APH.
- Cold Combustion Air (ambient temp) flows in the opposite direction (counter-flow design).
- Heat Transfer occurs through:
- Rotating heat-absorbing elements (regenerative type)
- Stationary tubes/plates (recuperative type)
- Preheated Air (150-300°C) enters the combustion chamber, requiring less fuel to reach ignition temperature.
Types of Air Preheaters
1. Recuperative APH (Tubular)
- Design: Stationary shell-and-tube heat exchanger
- Advantages:
- No moving parts → low maintenance
- Handles high dust loads (cement/steel industries)
- Disadvantages:
- Larger footprint
- Lower efficiency than regenerative types
2. Regenerative APH (Ljungström/Rotary)
- Design: Rotating matrix absorbs/releases heat
- Advantages:
- Compact design
- Higher efficiency (up to 90% heat recovery)
- Disadvantages:
- Air leakage risk
- Requires maintenance of rotating parts
3. Plate-Type APH
- Design: Stacked steel plates create alternating gas/air passages
- Best For:
- Medium-temperature applications
- Corrosive flue gases (with coated plates)
4. Heat Pipe APH
- Design: Sealed pipes with evaporating/condensing fluid
- Advantages:
- No cross-contamination between air/flue gas
- Passive operation (no moving parts)
Key Benefits
✔ Fuel Savings – 5-15% reduction in fuel costs
✔ Higher Combustion Efficiency – Preheated air burns fuel more completely
✔ Lower Emissions – Reduces CO₂, NOx, and SOx
✔ Improved Boiler Performance – Stabilizes combustion temperatures
Design Considerations
| Factor | Importance |
| Flue Gas Temperature | Determines heat recovery potential |
| Air Leakage | Critical for rotary designs (seal quality) |
| Corrosion Resistance | Stainless steel for acidic flue gases |
| Fouling Potential | Soot/ash buildup reduces efficiency |
| Pressure Drop | Impacts fan power consumption |
Top Applications
- Power Plants – Boiler efficiency improvement
- Cement Kilns – Preheating secondary/tertiary air
- Steel Industry – Blast furnace air heating
Chemical Plants – Process heating optimization