---
title: "Finned tube / harp tube"
description: "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."
canonical_url: "https://sylio.co/glossary/finned-tube-harp-tube"
last_updated: "2026-06-28T02:29:33.975Z"
---

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](/glossary/heat-recovery-steam-generator) 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](/glossary/economiser), evaporator and [superheater](/glossary/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](/glossary/sonic-horn) 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

- [Heat Recovery Steam Generator (HRSG)](/glossary/heat-recovery-steam-generator)
- [Economiser](/glossary/economiser)
- [Superheater](/glossary/superheater)
- [Sonic horn](/glossary/sonic-horn)