---
title: "Carry-over"
description: "Carry-over is the entrained molten or partly-molten smelt droplets and ash particles that are lifted from the recovery boiler furnace upward into the convective pass instead of falling to the boiler bottom. Carry-over is the dominant fouling agent on recovery-boiler superheater, generating-bank and economiser tubes."
canonical_url: "https://sylio.co/glossary/carry-over"
last_updated: "2026-06-28T02:29:35.857Z"
---

**Carry-over** is the entrained molten or partly-molten [smelt](/glossary/smelt) droplets and ash particles that are lifted from the [recovery boiler](/glossary/recovery-boiler) furnace upward into the convective pass instead of falling to the boiler bottom. Carry-over is the dominant fouling agent on recovery-boiler [superheater](/glossary/superheater), [generating-bank](/glossary/generating-bank) and [economiser](/glossary/economiser) tubes.

## Why carry-over is so problematic

- Particles arrive on the tubes still partly molten or sticky
- They bond on contact, producing a deposit that resists steam sootblowing
- The deposit composition (sodium sulphate + carbonate + sulphide) is alkali-rich and corrosive
- Build-up accelerates if not actively dislodged early

## Cleaning

[Sonic horns](/glossary/sonic-horn) and [infrasonic cleaners](/glossary/infrasonic-cleaner) on recovery boilers target carry-over deposits before they consolidate. The combination of continuous acoustic action and periodic [IK retract sootblowing](/glossary/ik-long-retract-sootblower) is what allows modern recovery boilers to extend run-time targets to 12–18 months between [chill-and-blow](/glossary/chill-and-blow) campaigns.

## Related terms

- [Recovery boiler](/glossary/recovery-boiler)
- [Smelt](/glossary/smelt)
- [Fume](/glossary/fume)
- [Superheater](/glossary/superheater)
- [Generating bank](/glossary/generating-bank)
- [Sonic horn](/glossary/sonic-horn)