Glossary
HRSG and gas path
ID, FD and PA fans
Also known as induced draft fan, forced draft fan, primary air fan, ID fan, FD fan, PA fan.
Industrial boilers use three principal fans to manage gas and air movement:
| Fan | Function | Location |
|---|---|---|
| ID (Induced Draft) | Pulls flue gas through the convective pass | Downstream of the ESP or baghouse, before the stack |
| FD (Forced Draft) | Pushes combustion air into the burners | Ahead of the air heater air-side inlet |
| PA (Primary Air) | Conveys pulverised coal from mills to burners | Between the coal mills and the burner deck |
Why fans foul
- Fly-ash deposition on ID fan blades unbalances the impeller, causing vibration and bearing wear
- PA fan blade build-up from sticky coal fines
- FD fan inlet vane fouling from atmospheric dust accumulating on the air-intake filter or vane assembly
ID fans on coal-fired and biomass plants are particularly prone to blade fouling; a trip-causing imbalance is a regular outage risk.
Sonic horns on fan housings
Sonic horns installed on the upstream ducting and at the fan inlet keep the blades clean by preventing the dust from settling onto them in the first place. Cement preheater ID fans are a particularly common installation.
Related terms
Related terms
- BoilerA boiler is a vessel that converts fuel chemical energy into steam by heating water. Coal-fired, biomass, oil, gas and recovery boilers all foul; sonic horns clean heat-transfer surfaces.
- Convective pass and backpassThe convective pass is the downstream section of a boiler where heat transfer is by conduction across tube banks: superheater, reheater, economiser. The primary zone for sonic-horn cleaning.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.