---
title: "Re-entrainment"
description: "Re-entrainment is the recapture of just-released dust by the flue-gas stream before it can fall into the ESP hopper. It is the dominant cause of opacity spikes on rapped ESPs and a major reason continuous acoustic cleaning is increasingly preferred over (or alongside) mechanical rapping."
canonical_url: "https://sylio.co/glossary/re-entrainment"
last_updated: "2026-06-28T02:29:31.238Z"
---

**Re-entrainment** is the recapture of just-released dust by the flue-gas stream before it can fall into the [ESP hopper](/glossary/esp-hopper). It is the dominant cause of [opacity](/glossary/opacity) spikes on rapped ESPs and a major reason continuous acoustic cleaning is increasingly preferred over (or alongside) mechanical [rapping](/glossary/esp-rapper).

## How re-entrainment happens

When a [rapper](/glossary/esp-rapper) impacts a [collecting plate](/glossary/collecting-electrode), a sheet of dust detaches and slides down the plate. Some of this falling dust is caught by the horizontal gas flow and carried out of the field instead of reaching the hopper. The faster and harder the rap, the larger the released sheet and the worse the re-entrainment.

## Why sonic horns reduce re-entrainment

[Sonic horns](/glossary/sonic-horn) firing every few minutes deliver small, frequent dust releases instead of large, occasional ones. The released particles are smaller in aggregate per event, fall more gently and have time to settle into the hopper before being picked up. Plants that retrofit horns to back-corona- or re-entrainment-limited ESPs commonly see opacity reductions of 20–40% with no other process change.

## Related terms

- [Electrostatic precipitator](/glossary/electrostatic-precipitator)
- [ESP rapper](/glossary/esp-rapper)
- [Collecting electrode](/glossary/collecting-electrode)
- [Opacity](/glossary/opacity)
- [Sonic horn](/glossary/sonic-horn)