M. C. Gourdine
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Proposed use of EGD in precipitators is a substantially different means for fume confrol ince being introduced in 1908, the Cottrell precipi-
S'tator has dominated the solutions of industrial flue-gas problems. Though improvements, through refinement, have been made, the basic Cottrell system has remained essentially unchanged. Recently, however, applications of the principles of electrogasdynamics have yielded a new type of fume precipitation system offering mme distinct advantages over existing techniques. This paper describes the applications with respect to a particular device, the electrogasdynamic (EGD) precipitator. 26
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
Electrogasdynamics is concerned with the acceleration or deceleration of a flowing gas containing unipolar ions with the expenditure or extraction of electrical energy. The idea is usually associated with power generation but may be applied to other problems. The EGD precipitator consists of three basic components: an ionizing section, a dielectric section, and a collector section. The entrance to the precipitator is a high velocity duct in which the ionization of particulate matter occurs. The particles leave the ionizer through a dielectric section a t approximately the same velocity and expand through a decelerating diffuser into a collector. The dielectric section separates and electrically insulates the collector from the ionizer and allows the buildup of a strong space-charge field. Repulsion of the particles,
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due to like charges, drives them to the walls of the collector where they are precipitated. The ionizing section has two important requirements. It must charge the particulates to a level close to saturation but with no effective precipitation on the ionizer electrodes. A simple wire-cylinder ionizer may be used to attain near-saturation charging, but, because the particles have a relatively short residence time in the ionizer, intense fields must be used. T h e second requirement is a high axial flow velocity relative to the radial drift velocity of the charged particles. The charged particles must now be transported to the low velocity collector region. This is achieved in the dielectric duct, or generator section, which also has two requirements. It must insulate the ionizer from the high virtual potentials of the space-charge cloud that is being driven through the generator section to the collector. T h e second function of the generator is to transport the charged particles up the potential gradient to the collector without precipitation on the duct walls by space-charge fields. This is accomplished by keeping the generator cross-sectional area small. T h e radial field intensity due to space-charge effects is minimized by the small radial duct dimensions, thereby maintaining particle drift velocities at a minimum. Efficient charge transport is aided by the high axial flow velocity through the generator, which is usually a continuation of the ionizer and, for most applications, is identical in cross-sectional geometry. However, in all applications the length is dictated by the spacecharge fields generated. I t is interesting to note that by driving the charged cloud against the axial potential gradient, the gas flow does work, thereby taking a pressure drop which is the EGD generator effect (this is small in comparison to the frictional pressure drop). The flow finally enters the collector section through a decelerating diffuser which reduces the flow velocities. T o achieve efficient precipitation in the collector a high radial space-charge field must be attained. The intensity of the radial field is directly proportional to the charge density and the radial distance from the axial center of the collector. The length of the collector required is a function of the number of charged particles, radial drift velocity, and the axial flow velocity. T h e charge concentration in the collector decreases as the flow moves downstream owing to precipitation of the charged particles a t the collector walls. Precipitation efficiency is therefore a function of initial charge concentration and the collector length. A single-stage EGD precipitator is capable of achieving efficiencies above 90%. High pressures and temperatures do not affect the EGD precipitator adversely. A distinct advantage to space-charge collection is that precipitation is not affected as the collected particles build up on the collector walls, since the drift velocities are due to space-charge repulsion. Therefore, the efficiency is not affected by dielectric dusts. T h e higher collector velocities and large diameter collectors considerably reduce the total material weight as well as capital costs. I t should also be noted that the small
ionizer dimensions require moderate voltages which minimize insulation and large power supply problems. However, it must be noted that this precipitator has several design parameters that are extremely critical. Particularly critical is the geometry of the decelerating diffuser since a large pressure drop can be experienced in this precipitator if the diffuser efficiency is not high.
SPACE-CHARGE COLLECTION One of the significant features of the EGD precipitation technique is the use of space-charge fields to effect particle collection. For simplicity the theoretical treatment will deal with a cylindrical geometry in all cases. The charged particles emerge from the ionizer and generator sections and enter the collector tube as shown in the figure. The rate of precipitation of particulates is controlled by the magnitude of the space-charge field in the collector region which may be expressed by Poisson's equation in cylindrical coordinates.
In cylindrical geometry, E, is constant and zero, and b/bO (EB) = 0. For a long cylindrical configuration where d / b x (E,)
