ID, FD and PA fans

Industrial boilers use several large fans to control gas and air movement. The names describe where the fan acts in the pressure balance of the boiler:

The ID fan normally sets furnace draft by maintaining slight negative pressure in the furnace. FD and PA fans supply the air needed for combustion, fuel transport and air staging. Poor fan performance can therefore show up as unstable furnace pressure, reduced load, high oxygen trim, poor mill performance, flame instability or emissions drift.

Why fans foul

Fan fouling is a combination of particle loading, sticky chemistry and local aerodynamics:

• ID fan blade deposits from fly ash, ammonium bisulphate, unburned carbon, cement dust or biomass ash cause imbalance and vibration.
• Inlet-guide-vane build-up changes the flow angle into the impeller and reduces control authority.
• PA fan fouling can occur where coal fines, mill rejects or moisture carry into the air stream.
• FD fan fouling is usually atmospheric dust, filter bypass or corrosion products rather than process ash.
• Downstream deposits in silencers, dampers and expansion joints increase system resistance and fan power.

ID fans on coal, cement, biomass and waste-to-energy plants are the most common problem because they handle dirty flue gas near the end of the gas path. Even a small mass of uneven deposit on a large impeller can push vibration beyond alarm limits.

Operating and maintenance implications

Operators track fan health through vibration, bearing temperature, motor current, damper position, static pressure, flow, differential pressure and draft stability. A fouled fan may appear as a gradual rise in power demand, a need to open dampers further for the same load, or a sudden vibration alarm after a deposit breaks loose.

Maintenance options include online washing where permitted, outage cleaning, balancing, blade inspection, erosion repair and inlet duct cleaning. Repeated imbalance events should trigger an upstream fouling review, not only a fan repair.

Sonic horns on fan housings

Sonic horns are usually installed upstream of the fan, on the inlet duct, transition or fan housing. The aim is preventive: keep dust from settling on inlet vanes and blade leading edges before a hard, unbalanced layer forms. Cement preheater ID fans, biomass boiler ID fans and WtE induced-draft systems are typical candidates.

Acoustic cleaning is most effective where deposits are dry, powdery or lightly sticky. It is less likely to solve deposits caused by condensation, tar, oil mist, severe ammonium-bisulphate stickiness or poor gas-temperature control. In those cases the cleaning system must be paired with process correction.

Reliability and inspection detail

An ID fan is a rotating machine, so small process changes can become mechanical problems. Dust build-up on one side of a blade changes balance and raises vibration. Corrosion at the inlet cone or casing can release scale that damages the impeller. Erosion can thin blade leading edges, especially where the upstream collector is leaking or where ash has a high quartz content. Operators therefore watch vibration, bearing temperature, motor current, damper position, inlet pressure, outlet pressure and casing temperature together rather than treating the fan as only an airflow device.

Maintenance checks include blade thickness, balance weights, inlet-vane freedom, shaft seals, expansion joints, drains, access-door leakage, bearing lubrication and the condition of variable-frequency-drive cooling. After a trip, ash can settle in the casing and create a rough restart. Where the gas is near acid dew point, low-load operation and cold starts can accelerate casing corrosion.

For acoustic cleaning, the useful target is usually upstream deposition that causes fan imbalance or draft restriction. Horn cycles should be coordinated with hopper evacuation and fan operating state so loosened material is carried away safely instead of collecting in the scroll.

Related terms

• Boiler
• Convective pass / backpass
• Sonic horn