Electrostatic Precipitation

Particulate matter can be quantitatively removed from air by ESPs. Devices that operate on the same principal but are much larger are frequently used to remove particulate matter from stack gases prior to discharging into the atmosphere. Several commercially available ESPs can be used for air sampling, and all operate on the same general principle of passing the air between charged surfaces, imparting a charge to particles in the air, and collecting the particles on an oppositely charged surface or plate.

In one of widely used commercial devices (see Figure 5.10.6), a high-voltage discharge occurs along a central wire; the collecting electrode is a metallic cylinder placed around the central wire while the air is passing through the tube. An intense corona discharge takes place on the central wire; the particles entering the tube are charged and are promptly swept to the walls of the tube where they remain firmly attached. With this method, collecting a sample for subsequent weighing or chemical analysis and examining the particles and studying their size and shape is possible. However, the intense electrical forces can produce aggregates of particles that are different from those in the sampled air.

ESPs are not as widely used as filters for ambient air sampling because they are less convenient and tend to be heavy due to the power pack necessary to generate the high voltage. Nevertheless, they are excellent instruments for obtaining samples for subsequent analysis and sample, at high-flow rates and low resistance.

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