Fans are the lungs of a boiler system. Without properly sized Forced Draft (FD) fans and Induced Draft (ID) fans, combustion efficiency drops, emissions rise, and energy costs soar. In this guide, we’ll break down the step-by-step process of calculating FD and ID fan capacity, making it easy for engineers, students, and plant operators to understand.
🌬️ Step 1: FD Fan Capacity – Supplying Combustion Air
The FD fan pushes fresh air into the boiler for combustion. To size it correctly:
Calculate total air requirement
- Start with theoretical air (stoichiometric requirement based on fuel composition).
- Add excess air (typically 10–20% for oil/gas, higher for coal/biomass).
Convert mass flow to volume flow
- Express air in mass units (kg/s or lb/h).
- Multiply by the specific volume of air at standard temperature (20°C).
- Correct for ambient temperature and altitude (since density changes with elevation and climate).
👉 Example: If your boiler requires 10 kg/s of air, at 20°C and sea level, the volume flow is about 8.3 m³/s. At higher altitudes, this increases due to lower air density.
📊 Step 2: FD Fan Discharge Pressure
The FD fan must overcome all air-side pressure drops in the system:
- Air preheater
- Ductwork resistance
- Burner wind box and registers
Add these pressure drops together, then correct for altitude. Fan vendors often design for slightly higher air temperatures than ambient to ensure reliability.
🌪️ Step 3: ID Fan Capacity – Handling Flue Gas
The ID fan pulls flue gases out of the boiler and maintains proper draft.
Calculate flue gas flow
- Based on fuel input and combustion air supplied.
- Convert to volumetric flow (m³/s or ft³/min).
Account for flue gas pressure drops
- Radiant superheater panels
- Reheater
- Convective superheater panels
- Boiler bank
- Economizer banks
- Gas side of air preheater
- Dust collection systems (scrubbers, cyclones, bag filters, ESPs)
👉 Remember: Always correct flue gas pressure drops for altitude.
⚖️ Step 4: Balanced Draught vs. Package Boilers
Balanced Draught Boilers
- FD fan handles air-side pressure drops.
- ID fan handles flue gas-side pressure drops.
Package Boilers / Oil & Gas Fired Units
- FD fan handles all pressure drops (both air and flue gas).
📈 Step 5: Add Margins for Reliability
Fans lose efficiency over time due to wear, fouling, and system changes. To ensure long-term performance:
- Add 15% margin on flow
- Add 30% margin on pressure drops
These safety factors are common in industrial oil and gas-fired boilers.
✅ Key Takeaways
- FD fan = supplies combustion air, sized by air requirement + pressure drops.
- ID fan = removes flue gas, sized by flue gas volume + pressure drops.
- Always correct for altitude and temperature.
- Add design margins to ensure reliability over time.
📌 Conclusion
Understanding FD and ID fan capacity calculation is essential for boiler engineers, energy managers, and students in mechanical engineering. Correct fan sizing improves combustion efficiency, reduces operating costs, and ensures compliance with emission standards.
By following these steps, you’ll not only master the fundamentals but also gain practical knowledge that applies directly to real-world boiler systems.