HRSG and gas path
Finned tube and harp tube
Also known as finned tubes, harp tube, extended-surface tube, HRSG harp.
A finned tube carries helically-wound (or stud-welded) metal fins on its outside surface, multiplying the gas-side heat-transfer area by 5–10× compared with a bare tube. Finned tubes are universal in HRSGs because gas-side heat transfer (low-pressure exhaust gas) is the limiting factor — adding fins is the standard way to compensate.
A harp tube is the assembled vertical bundle of finned tubes that forms one tube bank inside the HRSG, named for its resemblance to a harp string array. Multiple harps in series make up the economiser, evaporator and superheater sections.
Why finned surfaces foul easily
The narrow gap between fins (3–10 mm typical pitch) is geometrically sensitive: even a thin deposit on the fin face significantly restricts the gas flow path between fins. Particulate that would pass through a bare-tube bank settles between fins and consolidates over time.
Cleaning
Sonic horns installed across HRSG harps keep the fin gaps clear. Sound waves penetrate between fins more effectively than steam-jet sootblowers, which struggle to project energy into the narrow inter-fin space. Combined sonic-and-sootblower cleaning regimes maintain HRSG heat transfer through the operating campaign.
Related terms
Related terms
- Heat Recovery Steam GeneratorAn HRSG recovers heat from a gas turbine's exhaust to generate steam, the second cycle of a combined-cycle plant. Finned-tube ash deposition and ABS fouling are the main cleaning concerns.
- 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.
- 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.