Electrostatic precipitators
ESP rapper
Also known as rapper, collecting plate rapper, discharge electrode rapper.
An ESP rapper is a mechanical device used to dislodge accumulated dust from the collecting and discharge electrodes of an electrostatic precipitator. Two principal designs dominate: tumbling-hammer rappers, favoured in European-style ESPs, and magnetic-impulse-gravity (MIGI) rappers, favoured in American-style ESPs.
How rapping is sequenced
Rappers are fired in a programmed sequence — usually one rapper at a time per field — to avoid simultaneous releases that would overwhelm the hopper. The interval depends on dust load: every few minutes on heavily-loaded inlet fields, every 20–60 minutes on lightly-loaded outlet fields. Tuning the rap interval is a perennial trade-off between low opacity (frequent rapping) and high re-entrainment (also frequent rapping).
Sonic horns vs rappers
| Attribute | ESP rapper | Sonic horn |
|---|---|---|
| Mechanism | Mechanical impact | Acoustic vibration |
| Release pattern | Large, periodic | Small, frequent |
| Re-entrainment risk | High | Low |
| Hopper coverage | Plates only | Plates and hoppers |
| Wear / fatigue | Discharge-electrode breakage, hammer-shaft failure | Diaphragm replacement every 3–5 years |
| Cost | Hardware + ongoing maintenance | Lower lifecycle cost in retrofit |
In practice, modern ESPs increasingly use both: rappers handle the heavy bottom of the plate, sonic horns handle the upper plate area, the discharge electrodes and the hopper. The combination outperforms either alone.
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.
- Tumbling-hammer rapperA tumbling-hammer rapper uses a rotating shaft and weighted hammers that strike anvils on the ESP plate frame. It is the dominant rapper design in European-style ESPs.
- Magnetic-impulse-gravity rapperA MIGI rapper lifts and drops a steel plunger by electromagnet onto an anvil rod connected to the ESP collecting plate. Standard design in American-style ESPs from B&W, Mitsubishi and Hamon.
- 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.
- Re-entrainmentRe-entrainment is the recapture of just-rapped dust by the flue-gas stream before it falls into the hopper. It causes opacity spikes and is the main reason continuous sonic cleaning is preferred.
- 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.