This article's table of contents introduction:
- General Context
- Key Design Features for < 20,000 Pa ID Fans
- Performance & Selection Criteria (for < 20,000 Pa)
- Typical Applications (where < 20,000 Pa is used)
- Common Problems & Solutions for ID Fans in this Range
- Summary Checklist for Selection
- Final Note:
Here is a detailed explanation regarding Boiler Induced Draft (ID) Fans designed for system pressures less than 20,000 Pa (approx. 80 inches WG or 2 meters WG).
This is the most common pressure range for ID fans in industrial and utility boilers (excluding high-pressure supercritical units). The key characteristics and design considerations differ significantly from high-pressure fans (e.g., those used for fluidized bed combustion or high-draft boilers).
General Context
- Function: An ID fan is located at the exit of the boiler (after the economizer, air heater, and dust collector). It creates a negative pressure (suction) inside the furnace to draw flue gas through the entire system and discharge it to the chimney.
- Pressure Range: < 20,000 Pa is considered Low to Medium Pressure in fan engineering.
- Low: < 1,000 Pa (e.g., natural draft support)
- Medium: 1,000 – 3,000 Pa (common for small packaged boilers)
- Medium-High: 3,000 – 15,000 Pa (typical for utility boilers with ESP/baghouse)
- High: > 15,000 Pa (large units with high resistance SCR, wet FGD)
- Gas Conditions: Flue gas is hot (120°C – 180°C typical), dirty (containing fly ash, SOx, NOx), and corrosive, especially with high sulfur fuels.
Key Design Features for < 20,000 Pa ID Fans
A. Impeller Type (Most Critical)
For this pressure range, the Backward Curved / Backward Inclined (BC/BI) Blade design is the industry standard.
| Feature | Advantage for < 20,000 Pa |
|---|---|
| Blade Shape | Curved away from rotation. |
| Pressure Curve | Non-overloading power curve (power peaks at design point, then drops). Prevents motor burnout if system resistance changes. |
| Efficiency | High (80-85% typical). |
| Dust Handling | Moderate. Blades are self-cleaning to some extent due to the curvature. Not ideal for heavy dust, but acceptable for fly ash after ESP. |
| Speed | Usually runs at 980 or 1480 RPM (for 50 Hz) or 1180/1780 RPM (60 Hz). |
Alternative (Less Common):
- Radial Blade (Paddlewheel): Used only if the dust content is extremely high before collection. It’s robust but lower efficiency (60-70%) and noisier.
- Forward Curved: Rarely used for ID fans due to overload risk and lower efficiency.
B. Casing & Shaft
- Casing: Typically heavy-gauge carbon steel with wear plates in areas directly exposed to fly ash erosion (cut-off, scroll tongue). For < 20,000 Pa, the casing thickness is standard (6-10 mm for medium sizes).
- Shaft: Carbon steel (EN8 or 1045) or alloy steel for higher temperatures. Seals are critical to prevent air in-leakage (which reduces boiler efficiency).
- Bearings: Usually outside the casing (overhung design or with a bearing pedestal). Self-aligning spherical roller bearings are standard. Cooling may be needed if gas temp > 150°C (fan shaft cooling disc or forced air).
C. Drive System
- Direct Drive: Common for large fans (high MW). Uses a coupling (flexible or rigid).
- Belt Drive: Common for smaller or variable speed applications (e.g., package boilers). Allows easy speed change.
- VFD (Variable Frequency Drive): Highly recommended for any ID fan in this range. Allows precise furnace draft control (typically -20 to -50 Pa at furnace exit) and significant energy savings (up to 30-40% part-load savings).
D. Material Selection
Since pressure is low but temperature is moderate:
- Impeller: Wear-resistant steel (e.g., Corten, HARDOX, or mild steel with weld overlay on leading edges).
- Shaft: Alloy Steel (e.g., 42CrMo4 or EN19) for strength at temperature.
- Casing: Carbon steel with ceramic tile lining if the dust load is high (e.g., after a cyclone but before ESP) or with hard-facing on the cut-off plate.
Performance & Selection Criteria (for < 20,000 Pa)
| Parameter | Typical Value | Notes |
|---|---|---|
| System Resistance (SP) | 5,000 – 18,000 Pa | Determined by boiler tubes, air heater, duct losses, ESP/baghouse, and stack height. |
| Flow Rate | 50,000 – 2,000,000 m³/h | Depends on boiler capacity (steam generation). |
| Temperature | 120°C – 180°C | Flue gas exit temperature. Must be considered for density correction. |
| Gas Density | ~1.0 – 1.2 kg/m³ | At operating temp (vs 1.2 kg/m³ at 20°C). |
| Efficiency | ≥ 82% | At Best Efficiency Point (BEP). |
| Noise | < 85 dB(A) | Without silencer; silencer often required. |
Typical Applications (where < 20,000 Pa is used)
- Utility Boilers (50-300 MW): After Electrostatic Precipitator (ESP) or Fabric Filter (Baghouse) where pressure drop is < 15,000 Pa.
- Industrial Boilers (10-100 TPH): Coal, biomass, oil, or gas fired.
- Heat Recovery Steam Generators (HRSG): Behind gas turbines (combine cycle).
- CFBC Boilers: Note: CFBC boilers often need higher pressure (20,000-25,000 Pa) due to the bed material. If pressure is exactly < 20,000 Pa, it's likely a smaller CFBC or a circulating fluidized bed with low bed inventory.
Common Problems & Solutions for ID Fans in this Range
| Problem | Cause at < 20,000 Pa | Solution |
|---|---|---|
| Vibration | Dust buildup on blades (imbalance), bearing wear, foundation resonance. | Install online cleaning system (compressed air or acoustic horns); balance rotor annually. |
| Erosion | High fly ash velocity (even at lower pressure, high tip speed). | Apply hard-facing (Tungsten Carbide) on blade leading edges; install wear plates at casing cut-off. |
| Low Efficiency | Damper operation (instead of VFD), fan oversized. | Replace dampers with VFD; impeller trim or replace with smaller fan. |
| Corrosion | Flue gas below acid dew point (for sulfur fuels). | Keep operating temperature >150°C; use Corten steel; avoid condensation (insulate casing). |
| Overload | Too-low system pressure (e.g., new baghouse bypass causing low resistance). | Fan power curve is non-overloading, but check motor nameplate and VFD current limit to prevent motor overload. |
Summary Checklist for Selection
If you are specifying an ID fan for Pressure < 20,000 Pa:
- [ ] Impeller: Backward Curved (BC) or High-Efficiency Aerofoil (if dust is low).
- [ ] Material: Carbon steel impeller with wear protection (hard-facing or ceramic tiles if dust > 50 mg/Nm³).
- [ ] Drive: VFD is mandatory for energy savings and draft control.
- [ ] Bearings: Spherical roller bearings with temperature monitoring (RTDs).
- [ ] Controls: Furnace draft control loop (PID) with pressure transmitter at furnace exit (target: -20 to -50 Pa).
- [ ] Test Block: Must be tested at actual operating conditions (density correction for temperature) in accordance with ISO 5801 or AMCA 210.
Final Note:
If your boiler system has a high-dust environment (e.g., no ESP upstream) or high temperature (>250°C), even though pressure is low, you should consider a Radial Blade Fan (if dust is very high) or special alloy material (if temperature is high). But for 99% of standard cases, a Backward Curved ID Fan with VFD is the optimal, cost-effective solution for pressures under 20,000 Pa.
