Electrostatic precipitators
Re-entrainment
Also known as rapping re-entrainment, dust re-entrainment.
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.
How re-entrainment happens
When a rapper impacts a collecting plate, 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 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
Related terms
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.
- ESP rapperAn ESP rapper is the mechanical hammer or magnetic impulse device used to dislodge accumulated dust from ESP plates and discharge electrodes. Sonic horns complement and partly replace this duty.
- Collecting electrodeThe collecting electrode is the grounded plate or tube on which charged particulate accumulates inside an ESP. Dust must be released to hoppers without re-entraining into the gas stream.
- Opacity (stack)Opacity is the percentage of light obscured by particulate in stack flue gas. The headline visual KPI for ESP performance; continuously monitored and permit-limited.
- Sonic hornA sonic horn is a pneumatically-driven low-frequency sound emitter (typically 60–400 Hz at 140–180 dB SPL) used to dislodge particulate fouling from boilers, ESPs, baghouses and process vessels.