P 760

The factor f is a wire roughness factor, usually between 0.5 and 0.7 (White 1963).

Equation 5.17(8) shows that reducing the size of the corona wire decreases the applied voltage necessary to initiate corona V0. Decreasing the corona starting voltage increases the corona current for an applied voltage.

An analysis similar to that shown for wire-in-cylinder configuration can be applied to the parallel plate and wire electrode configuration. The equations are not as simple because the symmetry of the cylindrical precipitator simplifies the mathematics. The plate-type precipitator also has an additional degree of freedom—corona wire spacing. Qualitatively, if the corona wires are spaced close together, the system approaches the field configuration of a parallel plate capacitor, which yields a constant field strength in the interelectrode space. For an applied voltage, this configuration reduces the electric field near the corona wire. Therefore, using smaller wires or spacing the wires farther apart increases the current density in a region.

In the presence of aerosol particles, the particles get charged with the same sign as the wire. This particle charging decreases the voltage gradient, and therefore the corona starting voltage has a higher value. Another effect is that the electric field is distorted and the electric field near the collector electrode increases due to image forces.

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