Discharge Electrodes

The discharge electrodes in most U.S. precipitator designs (prior to the 1980s) are thin, round wires varying from 0.13 to 0.38 cm (0.05 to 0.15 in.) in diameter. The most common size diameter for wires is approximately 0.25 cm (0.1 in.). The discharge electrodes are hung vertically, supported at the top by a frame and held taut and plumb by a weight at the bottom. The wires are usually made from high-carbon steel, but have also been constructed of stainless steel, copper, titanium alloy, and aluminum. The weights are made of cast iron and are generally 11.4 kg (25 lb) or more. Discharge wires are supported to help eliminate breakage from mechanical fatigue. The wires move under the influence of aerodynamic and electrical forces and are subject to mechanical stress. The weights at the bottom of the wire are attached to guide frames to help maintain wire alignment and to prevent them from falling into the hopper in the event that the wire breaks (Figure 1).

Weights that are 11.4 kg (25 lb) are used with wires 9.1 m (30 ft) long, and 13.6 kg (30 lb) weights are used with wires from 10.7 to 12.2 m (35 to 40 ft) long. The bottom and top of each wire are usually covered with a shroud of steel tubing. The shrouds help minimize sparking and consequent metal erosion by sparks at these points on the wire.

The size and shape of the electrodes are governed by the mechanical requirements for the system, such as the industrial process on which ESPs are installed and the amount and properties of the flue gas being treated. Most U.S. designs have traditionally used thin, round wires for corona generation. Some designers have also used twisted wire, square wire, barbed wire, or other configurations, as illustrated in Figure 2.

European precipitator manufacturers and most of the newer systems (since the early 1980s) made by U.S. manufacturers use rigid support frames for discharge electrodes. The frames may consist of coiled-spring wires, serrated strips, or needle points mounted on a supporting strip. A typical rigid-frame discharge electrode is shown in Figure 3. The purpose of the rigid frame is to eliminate the possible swinging of the discharge wires. Another type of discharge electrode is a rigid electrode that is constructed from a single piece of fabricated metal and is shown in Figure 4. Both designs are occasionally referred to as rigid-frame electrodes. They have been used as successfully as the older U.S. wire designs. One major disadvantage of the rigid-frame design is that a broken wire cannot be replaced without removing the whole frame.

One U.S. manufacturer (United McGill) uses flat plates instead of wires for discharge electrodes. The flat plates, shown in Figure 5, increase the average electric field that can be used for collecting particles and provide an increased surface area for collecting particles, both on the discharge and collection plates. The corona is generated by the sharppointed needles attached to the plates. These units generally use positive polarity for charging the particles. The units are typically operated with low flue gas velocity to prevent particle reentrainment during the rapping cycle (Turner, et al. 1992).

                                           Figure 1 - Guide frames and shrouds for discharge wires

                                              Figure 2 -Typical wire dischare electrodes

                                      Figure 3 -Rigid frame discharge electrode design

                                Figure 4 - Typical rigid discharge electrode

                             Figure 5 - Flat-plate discharge electrode (United McGill design)