Explain the exact mechanism by which an electrostatic precipitator removes particulate matter from an air stream.

An electrostatic precipitator (ESP) removes particulate matter, which are small solid or liquid particles suspended in an air stream, by using electrical forces. The exact mechanism involves three main stages: particle charging, particle collection, and collected dust removal.

The core components of an ESP include high-voltage discharge electrodes and grounded collection plates. The air stream containing particulate matter flows between these electrodes.

First, for particle charging, a very high direct current (DC) voltage, typically tens of thousands of volts, is applied to the discharge electrodes. This creates an intense electric field around the discharge electrodes. This strong field causes a phenomenon called corona discharge, where air molecules (like nitrogen and oxygen) near the discharge electrodes are ionized, meaning they lose or gain electrons to become electrically charged ions. Specifically, a large number of free electrons are generated. These electrons, being very light, are accelerated rapidly towards the grounded collection plates. As these high-speed electrons travel through the air stream, they collide with the particulate matter suspended in the air. Upon collision, the electrons transfer their negative charge to the individual particulate particles, effectively making them negatively charged.

Second, for particle collection, once the particulate matter becomes negatively charged, it is now subject to the strong electric field established between the discharge electrodes and the collection plates. Since the discharge electrodes are highly negatively charged and the collection plates are grounded (which acts as the positive side of the electric field relative to the negative discharge electrodes), there is a powerful attractive force. The negatively charged particulates are repelled by the negative discharge electrodes and strongly attracted to the positively charged (grounded) collection plates. This electric force causes the charged particles to drift rapidly and deposit onto the surface of the collection plates, where they adhere due to electrical forces and cohesion, forming a layer of dust.

Third, for collected dust removal, over time, a significant layer of collected particulate matter, known as a dust cake, accumulates on the collection plates. To maintain the precipitator's efficiency and prevent the collected dust from being re-entrained into the air stream, this dust cake must be periodically removed. This is typically achieved by mechanical devices called rappers or vibrators. These devices periodically strike or vibrate the collection plates, causing the accumulated dust cake to dislodge from the plate surfaces and fall by gravity into collection hoppers located at the bottom of the precipitator. From these hoppers, the collected particulate matter is then conveyed away for disposal or further processing, while the cleaned air, now significantly free of particulates, exits the ESP.