Boilers
Convective pass and backpass
Also known as convective pass, backpass, boiler backpass, back pass.
The convective pass (also backpass) is the downstream section of a boiler where heat transfer is by conduction across tube banks rather than radiation from a flame. The convective pass contains, in order of decreasing gas temperature: the finishing superheater, the reheater, the primary superheater, the economiser, and finally the air heater.
Why the convective pass is the prime sonic-horn zone
Three reasons:
- Deposits are dry, not molten. Ash arriving at convective surfaces has cooled below its sticking temperature; it deposits as a friable layer that acoustic energy can lift.
- Surfaces are extensive and partly inaccessible to retract sootblowers — perfect for non-contact cleaning.
- Heat-rate sensitivity is high. Every degree of approach temperature loss in the economiser or air heater translates directly into fuel cost.
A typical large utility boiler benefits from 8–20 sonic horns distributed across the convective pass, complementing existing steam sootblowers.
Sequencing
Horns are fired in a programmed sequence that respects compressed-air supply, avoids overlapping firing on adjacent fields, and times their action between sootblower cycles to maintain continuous low-level dust release.
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.
- EconomiserAn economiser is the final tube bank in a boiler's convective pass that recovers heat from the flue gas by preheating feedwater. Ash bridging in the economiser is a routine cleaning challenge.
- SuperheaterA superheater is a tube bank that raises steam temperature beyond the saturation point using flue-gas heat. Sticky alkali ash and slag deposits are the dominant fouling concerns.
- ReheaterA reheater is a tube bank in the boiler's convective pass that re-superheats steam returning from the HP turbine before it enters the IP turbine.
- Air heaterAn air heater (also air preheater, APH) recovers low-grade heat from flue gas to preheat combustion air. Cold-end fouling and corrosion are the dominant operational challenges.
- 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.