Re-entrainment

Re-entrainment is the return of collected particulate to the gas stream before it reaches permanent storage or discharge. In ESPs, it commonly happens when rapping dislodges dust from collecting plates but the dust cloud is swept back into the flue gas instead of falling cleanly into the hopper.

Re-entrainment is not the same as low collection efficiency at the inlet field. The particles may have been successfully charged and captured, then lost during cleaning or hopper handling. Net emissions are therefore controlled by both collection and dust removal from the collector.

Where it occurs

In an ESP, re-entrainment can occur at collecting plates, discharge electrodes, hoppers, turning vanes, and outlet fields. High gas velocity, poor gas distribution, weak hopper evacuation, excessive rapping intensity, ash with poor cohesion, or dust layers that release in sheets can all increase it.

Baghouses can also experience re-entrainment when pulse cleaning launches dust back into upward gas flow, when hoppers are aerated too aggressively, or when material bridges and collapses. In ductwork, dust knocked from ledges can be picked up again if the hopper or drop-out point is not effective.

Operating symptoms

ESP re-entrainment often appears as opacity spikes that coincide with rapper operation. It may also show as high outlet particulate despite acceptable electrical conditions. Hopper-level alarms, dust backing up into the casing, or visible puffs during cleaning events are practical clues.

The cure is not always stronger cleaning. Harder rapping may clean plates better but can increase dust clouds and reduce net performance. Better tuning may mean lower intensity, different timing, staggered fields, improved hopper heaters, better ash discharge, or inlet flow correction.

Acoustic cleaning context

Sonic horns are used to reduce reliance on high-impact rapping and to keep dust layers thinner between rapper events. Continuous or frequent acoustic cleaning can move dust in smaller increments, which may reduce opacity spikes. The system still needs enough hopper capacity and discharge reliability; otherwise acoustic cleaning only moves dust to the next bottleneck.

Where it appears

Re-entrainment appears in ESP fields when rapping lifts dust back into the gas stream, in baghouses when pulses release too much cake at once, in hoppers when falling dust is swept back upward, and in ducts when settled dust is disturbed by velocity changes. It can also occur during acoustic cleaning if released material has no clear path to a hopper or if gas velocity is high enough to carry it downstream.

Operators look for outlet particulate spikes linked to rapper or pulse timing, dust deposits beyond the collector, uneven hopper loading, or emissions events during start-up and load change. A high-efficiency collector can still show poor stack performance if collected dust repeatedly returns to the gas path.

Controls and maintenance context

Controls include lower hopper gas velocity, better inlet distribution, staggered cleaning sequences, anti-sneakage baffles, correct rapper intensity, pulse tuning, and reliable hopper discharge. Maintenance checks focus on full hoppers, failed rotary valves, air leaks, broken baffles, worn bags, distorted ESP plates, and plugged ash transport. Acoustic cleaning should be set to move dust at a rate the downstream collection and discharge equipment can handle. More energy is not always better; gentle frequent release can be cleaner than infrequent slugs.

Measurement context

The clearest way to identify re-entrainment is to align particulate or opacity spikes with cleaning events, hopper discharge failures, or load changes. Short-term data matter because daily averages can hide repeated release events. In baghouses, triboelectric leak detectors can show pulse-linked spikes. In ESPs, field-by-field trends and rapper timing can show whether dust is being lifted faster than it settles.

Acoustic cleaning should be tuned for release rate. The aim is to keep material mobile enough to discharge steadily, not to suspend a large dust cloud. If emissions rise after cleaning starts, the next checks are gas velocity, hopper evacuation, sequence timing, and whether loosened dust is crossing into a high-velocity zone.

Related terms

• Electrostatic precipitator
• ESP rapper
• Collecting electrode
• Opacity
• Sonic horn