---
title: "Heat-transfer surface fouling"
description: "Heat-transfer surface fouling is the engineering term for tube fouling viewed from the thermodynamic-impact angle. A fouling layer adds a thermal-resistance term in series with the underlying tube wall and the inside/outside film coefficients, reducing the overall heat-transfer coefficient (U) for the tube."
canonical_url: "https://sylio.co/glossary/heat-transfer-surface-fouling"
last_updated: "2026-06-28T02:29:32.190Z"
---

**Heat-transfer surface fouling** is the engineering term for [tube fouling](/glossary/tube-fouling) viewed from the thermodynamic-impact angle. A fouling layer adds a thermal-resistance term in series with the underlying tube wall and the inside/outside film coefficients, reducing the overall heat-transfer coefficient (U) for the tube.

## Quantifying the effect

The added fouling resistance R_f is reported in m²·K/W (or h·ft²·°F/Btu in US units). Typical published values:

<table>
<thead>
  <tr>
    <th>
      Service
    </th>
    
    <th>
      R_f (m²·K/W)
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Clean steam-side
    </td>
    
    <td>
      0
    </td>
  </tr>
  
  <tr>
    <td>
      Clean coal-fired boiler gas-side
    </td>
    
    <td>
      ~0.0005
    </td>
  </tr>
  
  <tr>
    <td>
      Fouled coal-fired economiser
    </td>
    
    <td>
      0.001–0.003
    </td>
  </tr>
  
  <tr>
    <td>
      Heavily-fouled biomass / WtE superheater
    </td>
    
    <td>
      0.005+
    </td>
  </tr>
  
  <tr>
    <td>
      Acid-dew-point-corroded air heater
    </td>
    
    <td>
      severe + corrosion
    </td>
  </tr>
</tbody>
</table>

Doubling R_f roughly halves the *useful* heat-transfer coefficient for the surface, with proportional impact on [heat rate](/glossary/heat-rate).

## Why sonic horns matter here

[Sonic horns](/glossary/sonic-horn) keep R_f close to its design value over the operating campaign by preventing the friable-to-bonded transition that drives R_f up. Plants commonly report 1–3% heat-rate improvement on retrofitting horns to a unit with established fouling drift.

## Related terms

- [Tube fouling](/glossary/tube-fouling)
- [Fouling](/glossary/fouling)
- [Heat rate](/glossary/heat-rate)
- [Sonic horn](/glossary/sonic-horn)