---
title: "Subcritical / supercritical / ultrasupercritical"
description: "Subcritical, supercritical and ultrasupercritical (USC) describe steam-condition classes for utility boilers. The classification refers to whether the working fluid is operated below or above the critical point of water (22.064 MPa, 373.95 °C)."
canonical_url: "https://sylio.co/glossary/subcritical-supercritical-ultrasupercritical"
last_updated: "2026-06-28T02:29:26.399Z"
---

**Subcritical**, **supercritical** and **ultrasupercritical (USC)** describe steam-condition classes for utility boilers. The classification refers to whether the working fluid is operated below or above the critical point of water (22.064 MPa, 373.95 °C).

<table>
<thead>
  <tr>
    <th>
      Class
    </th>
    
    <th>
      Steam pressure
    </th>
    
    <th>
      Steam temperature
    </th>
    
    <th>
      Plant efficiency (LHV)
    </th>
  </tr>
</thead>

<tbody>
  <tr>
    <td>
      Subcritical
    </td>
    
    <td>
      < 22.1 MPa
    </td>
    
    <td>
      540–565 °C
    </td>
    
    <td>
      36–39%
    </td>
  </tr>
  
  <tr>
    <td>
      Supercritical
    </td>
    
    <td>
      22.1–25 MPa
    </td>
    
    <td>
      560–600 °C
    </td>
    
    <td>
      41–43%
    </td>
  </tr>
  
  <tr>
    <td>
      Ultrasupercritical (USC)
    </td>
    
    <td>
      25–28 MPa
    </td>
    
    <td>
      600–620 °C
    </td>
    
    <td>
      44–47%
    </td>
  </tr>
  
  <tr>
    <td>
      Advanced USC (A-USC, developmental)
    </td>
    
    <td>
      30+ MPa
    </td>
    
    <td>
      700 °C+
    </td>
    
    <td>
      50%+ target
    </td>
  </tr>
</tbody>
</table>

## Fouling implications

Higher steam conditions concentrate value in every operating hour: a 1 percentage-point efficiency loss from [convective-pass](/glossary/convective-pass-backpass) fouling on a USC plant costs measurably more in fuel than on a subcritical one. The economic case for [sonic-horn](/glossary/sonic-horn) installation on [economisers](/glossary/economiser), [air heaters](/glossary/air-heater) and [reheaters](/glossary/reheater) rises with steam-condition class.

## Related terms

- [Boiler](/glossary/boiler)
- [PC boiler](/glossary/pc-boiler)
- [Heat rate](/glossary/heat-rate)
- [Superheater](/glossary/superheater)