Collection electrodes

There are four basic variations of the linear MHD channel (Fig. 5) which differ primarily in their method of electrical loading. The simplest is the two-terrninal Faraday or continuous electrode generator, Figure 5a, where a single pair of current-collecting electrodes spans the channel in the axial direction, short-circuiting the channel from end to end. Hence, for this configuration, = 0, andj can be obtained from equations 21 and 22 ... 

Current Flow Corona discharge is accompanied by a relatively small flow of electric current, typically 0.1 to 0.5 mA/m" of collecting-electrode area (projected, rather than actual area). Sparking usually involves a considerably larger flow of current which cannot be tolerated except for occasional periods of a fraction of a second duration, and then only when smtable electrical controls are provided to hmit the current. However, when suitable controls are provided, precipitators have been operated continuously with a small amount of sparking... 

Type of precipitator Type of dust Gas volume, cu ft/ min Average gas veloc- ity, ft/sec Collecting electrode area, sq ft Over-all collection efficiency, % Average particle migration velocity, ft/sec... 

The choice of size, shape, and type of electrode is based on economic considerations and is usually determined by the characteristics of the gas and suspended matter and by mechanical considerations such as flue arrangement, the available space, and previous experience with the electrodes on similar problems. The spacing between collecting electrodes in plate-type precipitators and the pipe diameter... 

Maximum normol occelerotion of collecting electrode plote produced by normol rop ( q")... 

A unit is available in which electrostatic precipitation is combined with a dry-air filter of the type shown in Fig. 17-64Z . In another unit an electrostatic field is superimposed on an automatic filter. In this case the ionizer wires are located on the leading face of the unit, and the collecting electrodes consist of alternate stationary and rotating parallel plates. Cleaning in this case is automatic and continuous. 

Fig. 29-9. Wet-wall ESP with tubular collection electrodes. Source Oglesby, S., Jr, and Nichols, G. B., Electrostatic precipitators, in "Air Pollution," 3rd ed., Vol. IV (A. C. Stern, ed.), p. 238, Academic Press, New York, 1977.

A very important parr of the gas-deatimg process is the removal of the collected particles from the cleaning system. This should be as controlled as possible in order to avoid particle reenrrainmenr to the gas flow. This can be accomplished in the case of liquid particles such as acid fume or tar or oil smoke. olid particles are normally removed by periodic rapping of discharge and collection electrodes. Solid particles can also be removed with the aid of water, as is done in wet electrostatic precipitators. 

Most of the results presented in the previous chapters are based on idealized conditions. In practice, the performance of an electrostatic precipitator can be significantly influenced by the dust layers on discharge and collection electrodes i.e., dust layers may alter the electrical properties of the system. It is also possible that dust layers are not stable i.e., collected particles become loose, increasing the particle concentration in the outlet of the precipitator. These problems play a much smaller role if the surface collection electrode is continuously flushed with water. These wet elearostatic precipitators, however, cannot be used in all applications. 

Pipe-type electrostatic precipitators are used to collect liquid aerosols (e.g., mists and fogs). They are also used in applications which require water flushing of collection electrodes. The diameter of precipitator pipes is typically in the range of 15 0 cm, and the length is in the range of 3-6 m. The number of pipes depends on the total gas flow. The gas-flow rates in pipe-type electro static precipitators is normally much lower than in duct-type precipitators. 

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],... 

H2O2 Production Rate Based on the collection efficiency A at the Pt collecting electrode located downstream from the working electrode, we calculated the ratio of the H2O2 production rate to the overall ORR rate, P(H202) ... 

The pipe-type units are usually built in a cylindrical housing having a header sheet near the inside top. In this header sheet are nested pipes, which act as collecting electrodes. The discharge electrodes are supported above the... 

Electric Wind By virtue of the momentum transfer from gas ions moving in the electrical field to the surrounding gas molecules, a gas circulation, 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 electrode is slightly higher than at the discharge electrode (White-head, op. cit., p. 167). 

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