Precipitation, electrical

Fig. 13. Operating principle of two-stage electrical precipitator. From Ref. 205,...

The exhaust gases are generally discharged into dust and fume knockdown equipment to avoid contamination of the atmosphere. Gas-cleaning equipment includes cyclones, setthng chambers, scrubbing towers, and electrical precipitators. Heat-recoveiy devices are utilized both within and outside the lain. These result in an increase in lain capacity or a decrease in fuel consumption. Waste-heat boilers, grates, coil systems, and chains are used for this purpose. 

B. Spacing between wire and plate, or between rod and curtain, or between parallel plates in electrical precipitators m ft ... 

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

Electrical Precipitators When particles suspended in a gas are exposed to gas ions in an electrostatic field, they will become charged and migrate under the ac tion of the field. The functional mechanisms of electrical precipitation may be listed as follows ... 

There are two general classes of electrical precipitators (1) single-stage, in which ionization and collection are combined (2) two-stage, in which ionization is achieved in one portion of the equipment, followed by collection in another. Various types in each class differ essentially in the details by which each function is accomplished. 

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). 

TABLE 17-15 Performance Data on Typical Single-Stage Electrical Precipitator Installations ... 

Power Supply Electrical precipitators are generally energized by rectified alternating current of commercial frequency. The voltage is stepped up to the required value by means of a transformer and then... 

Electrical precipitators are probably the most versatile of all types of dust collectors. Veiy high collection efficiencies can be obtained regardless of the fineness of the dust, provided that the precipitators are given proper maintenance. The chief disadvantages are the high initial cost and, in some cases, high maintenance costs. Furthermore, caution must be exercised with dusts that are combustible in the carrier gas. 

Nature of a Fog. Fog, like smoke, is a colloid. Once a fog is formed, it is very difficult to knock down. It will go right through packed columns, mist eliminators, or other such devices. Special devices are required to overcome a fog, such as an electric precipitator with charged plates. This can overcome the zeta potential of the charged particles and make them coalesce. 

The electrical precipitator is a dry dust or liquid mist removal unit which utilizes the ionization of the process gas (usually air) to impart electrical charges on the suspended entrained particles and effect particle collection by attraction to an oppositely charged plate or pipe. This type of unit is in use in services which are difficult for other types of entrainment removal equipment. Figures 4-79, 4-80, and 4-81 illustrate the usual fundamental action of these units. 

Figure 4-81. Electrical precipitator principle of operation. Courtesy of Sturtevant Div. Westinghouse Electric Corp.

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