Fluidized Boiler Stainless Steel Forced 960r/min Induced Draft Fan

This article's table of contents introduction:

1. The Application: Fluidized Bed Boiler (CFB/FBC)
2. The Specification: 960 RPM
3. Most Likely Fan Type: Centrifugal (Radial or Backward Inclined)
4. Engineering Data You Need to Confirm (Critical to Ordering)
5. Operational Considerations for 960 RPM
6. Summary Recommendation

This is a technical request for a specific industrial fan. Based on your wording, you are looking for an Induced Draft (ID) Fan for a Fluidized Bed Boiler (CFB or BFB) , made of Stainless Steel, operating at a speed of 960 RPM.

Here is a breakdown of what this specification means, the engineering considerations, and the likely type of fan you need.

The Application: Fluidized Bed Boiler (CFB/FBC)

In a Fluidized Bed Boiler, the Induced Draft (ID) fan is located at the exit of the boiler (after the baghouse/ESP and scrubber). Its job is to pull the flue gas through the boiler system, maintaining a slight negative pressure (draft) in the furnace.

Why Stainless Steel?

• Corrosion: Flue gas from burning high-sulfur coal or biomass (especially with circulating fluidized bed (CFB) technology) contains sulfurous and nitric acids. When the gas temperature drops below the acid dew point (typically 90°C-120°C), condensation occurs, causing rapid corrosion.
• Erosion: Fluidized bed boilers have high particulate loading (fly ash). Even though the fan is after the dust collector, a high-chrome or duplex stainless steel (e.g., SS 316L, 904L, or Duplex 2205) is often specified to resist both chemical attack and particle erosion at the blade tips.

The Specification: 960 RPM

960 RPM is the synchronous speed of a 6-pole motor (50 Hz grid: $120 \times 50 / 6 = 1000$ RPM – slip reduces to ~960–970 RPM; or 60 Hz grid: $120 \times 60 / 8 = 900$ RPM – slip reduces to ~960? Likely 50 Hz).

• Speed Analysis: 960 RPM is a medium speed for a large fan.
  • Too fast? For very large boilers (>100 MW), 960 RPM would require a massive impeller and dangerously high tip speeds (high stress, high wear).
  • Too slow? For smaller boilers, 960 RPM is suitable.
• Implication: This suggests a smaller to mid-sized fluidized bed boiler (e.g., 20–80 MW thermal). The fan is likely a centrifugal fan (radial or backward curved) rather than an axial fan.

Most Likely Fan Type: Centrifugal (Radial or Backward Inclined)

For this specific combination (fluidized bed, stainless steel, 960 RPM), the most common designs are:

Judgment: At 960 RPM, a Backward Curved (BC) or Radial Tip fan is most common. A Radial (Paddlewheel) fan at 960 RPM would have a very large diameter to generate sufficient pressure, making it expensive. A Backward Inclined Airfoil is too fragile for fluidized bed ash unless the dust collector is highly efficient.

Engineering Data You Need to Confirm (Critical to Ordering)

To provide a specific fan model or quotation, the manufacturer will need:

1. Flow Rate: m³/s or CFM (At actual flue gas temperature).
2. Pressure Rise: Total pressure in Pa or mmWC. (For a CFB ID fan, this is typically high – 6,000 to 12,000 Pa).
3. Gas Temperature: Normal temperature (e.g., 140°C) and maximum emergency temperature (e.g., 200°C). This dictates the thermal expansion allowance.
4. Dust Loading: mg/Nm³. (Tells you if you need wear plates).
5. Material Grade: SS 304? SS 316L? Duplex 2205? (Specify exactly). The housing and impeller may be different grades.
6. Motor Data:
   • Power (kW) – (Calculate from flow $\times$ pressure / efficiency).
   • Voltage (e.g., 400V, 3.3kV, 6.6kV).
   • Frequency (50 Hz or 60 Hz). Note: 960 RPM typically implies 6-pole at 50 Hz.

Operational Considerations for 960 RPM

• Vibration: At 960 RPM (approx. 16 Hz), the fan is below the standard 25Hz/50Hz line frequency. This is an advantage for balancing – lower running speed allows for thicker, heavier impellers.
• Critical Speed: Ensure the fan rotor assembly (shaft + impeller) has a 1st lateral critical speed significantly above 960 RPM (e.g., >1500 RPM) to avoid resonance during startup/shutdown.
• Damper: ID fans need heavy-duty inlet box dampers (radial or louver type) to control draft.

Summary Recommendation

You are specifying a Centrifugal Induced Draft Fan, Backward Curved or Radial Tip, Stainless Steel (SS316L or better), 960 RPM (6-pole @ 50Hz), for a Fluidized Bed Boiler.

Next Steps:

1. Check your boiler size. If it's a large utility unit (>100 MW), 960 RPM is too slow (fan would be huge). You would need a higher speed (e.g., 1480 RPM / 4-pole).
2. Confirm the stainless steel grade. For flue gas with acid dew point issues, use SS316L or SAF 2507 (Super Duplex) for the impeller wheel.
3. Request shaft seals. Specify high-temperature mechanical seals or labyrinth seals to prevent flue gas leakage.

If you need a quotation, you must provide the flow, pressure, temperature, and dust load. Without those numbers, no manufacturer can size a fan for a fluidized bed boiler at 960 RPM.