---
title: "Acid dew point"
description: "The acid dew point is the temperature at which sulphuric acid (H₂SO₄) begins to condense from flue gas containing SO₃ and water vapour. For typical coal-fired flue gas the acid dew point sits in the 120–160 °C range, depending on SO₃ concentration and moisture content. Higher SO₃ raises the dew point; in extreme cases it reaches 180 °C."
canonical_url: "https://sylio.co/glossary/acid-dew-point"
last_updated: "2026-06-28T02:29:25.405Z"
---

The **acid dew point** is the temperature at which sulphuric acid (H₂SO₄) begins to condense from flue gas containing SO₃ and water vapour. For typical coal-fired flue gas the acid dew point sits in the 120–160 °C range, depending on SO₃ concentration and moisture content. Higher SO₃ raises the dew point; in extreme cases it reaches 180 °C.

## Why it matters

Operating any cold-end surface — [air heater](/glossary/air-heater) baskets, [economiser](/glossary/economiser) tubes, ducting — below the acid dew point allows condensed sulphuric acid to attack the metal, causing [cold-end corrosion](/glossary/cold-end-corrosion-dew-point-corrosion). The dew point sets the practical floor on cold-end metal temperature.

## SCR-related complication

Boilers with upstream [SCR](/glossary/selective-catalytic-reduction) face a double challenge: the SCR catalyst converts a fraction of SO₂ to SO₃ ([SO₂/SO₃ conversion](/glossary/so2-so3-conversion)), raising the dew point, *and* unreacted [ammonia slip](/glossary/ammonia-slip) combines with SO₃ to form [ammonium bisulphate](/glossary/ammonium-bisulphate) that condenses and fouls cold-end surfaces in the same temperature window.

## Operational implications

- Cold-end air-heater inlet temperature is normally controlled at least 10–15 °C above the calculated dew point
- Steam coil air heaters or hot-air recirculation raise inlet air temperature during low-load operation
- Periodic dew-point measurement campaigns confirm the calculated value

## Related terms

- [Cold-end corrosion / dew-point corrosion](/glossary/cold-end-corrosion-dew-point-corrosion)
- [Air heater](/glossary/air-heater)
- [Ammonium bisulphate](/glossary/ammonium-bisulphate)