Electrode discharge

Schematic diagram of a thermospray ion. source. This source, of current design, also incorporates (a) a discharge electrode so that the source can be operated in plasmaspray mode and (h) a repeller electrode to induce fragmentation. The vaporizer itself can be used as a discharge electrode.

Fig. 11. Types of (a) collecting plate and (b) discharge electrode designs.

Opera.tingPrinciples. No collection will occur in a precipitator, and no current will flow in the secondary circuit, until gas ionization starts around the discharge electrode. This process is known as corona formation. The starting voltage for corona in air in a tube precipitator is given as... 

D, Outside diameter of wire or discharge electrode of concentric-cylinder type of electrical precipitator m ft ... 

Electric Wind By virtue of the momentum transfer from gas ions moving in the electrical field to the surrounding gas molecules, a gas circiilation, known as the electric or ionic wind, is set up between the electrodes. For conditions encountered in electrical precipitators, the velocity of this circulation is on the order of 0.6 m/s. (2 ft/s). Also, as a result of this momentum transfer, the pressure at the collecting eleclrode is slightly higher than at the discharge electrode (White-head, op. cit., p. 167). 

Ozone produced by the negatively charged discharge electrode during gas ionization... 

Tests were conducted using an approximately 30-cm-diameter, 80-/rm-thick polycarbonate film with brass backing plate. A 50-mm-diameter spherical discharge electrode was used to initiate discharge of the charged film... 

Many of the typical features found in present-day electrostatic precipitators are based on work by W. A. Schmidt. One of his most important applications is the electrostatic precipitator that was installed at the Riverside Portland Cement Company in 1912. This plant handled a gas flow of 470 ni/s at the temperature of 400-500 °C. This was the first precipitator in which thin wire was used as discharge electrode. 

A Degarive discharge electrode attracts positive ions and forces them to impact on its surface. These impacts provide an addirional source of electrons which contribute to the process. Ultraviolet light generated by the cormu glow causes photoelectric emission of electrons from the electrode surfaces, which further enhances the formation of free electrons. 

Corona current per unit length of the discharge electrode, j , is given by... 

Back corona is caused by the electrical breakdown of gas in the dust layer. This breakdown produces positive ions, which drift toward the negative discharge electrode. The presence of ions with opposite polarity causes a reduction in the particle-charge and -collection efficiency. To avoid this problem, several methods are used. These include... 

To improve the efficiency of collection, several units can be installed in series. The plate type unit is the most common design for dry dust removal, while pipe design is mainly for removal of liquid or sludge particles and volatilized fumes. The plates/pipes are the collecting electrodes, with the discharge electrodes suspended between the plates or suspended in the pipes [41,53,57],... 

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