breadth of application. These devices are simple and almost maintenance-free. Because any medium-sized welding shop can make one, the big suppliers of pollution control equipment, who have test data on the effects of small changes in the internal geometry, have been unwilling to make these data public. The same basic device as the cyclone separator is used in other industrial settings where the goal is not air pollution control, but some other kind of separation. When it is used to separate solids from liquids it is generally called a hydroclone. A cyclone called an air-swept classifier is attached to many industrial grinders. It passes those particles ground fine enough, and collects those that are too coarse, returning them to the grinder 9.1.3 Electrostatic Precipitators(ESP) If gravity settlers and centrifugal separators are devices that drive particles against a solid wail and if neither can function effectively(at an industrial scale) for particles below about 5 Ix in diameter, then for wall collection devices to work on smaller particles, they must exert forces that are more powerful than gravity or centrifugal force. The electrostatic precipitator(ESP) is like a gravity settler or centrifugal separator, but electrostatic force drives the particles to the wall. It effective on much smaller particles than the previous two devices The basic idea of all esps is to give the particles an electrostatic charge and then put them in an electrostatic field that drives them Das.-colection to a collecting wall. This is an suppon one type of ESP, calleda two-stage precipitator, charging and collecting are carried out in of the esr this type, widely used in building air conditioners, is sometimes called an electronic air filter. However Dirty gas for most industrial applications the two separate steps are carried Corona dis part of the ESP. The charging E function is done much more the collecting function. and the size of the esp is largely determined by the Collected dust collecting functio Ground Dust renoved from Fig 9.2 shows in simplified form a wire-and-plate ESP with two Fig. 9-2 Diagrammatic sketch of a simplified ESP with two plates four plates. The gas passes between the vires, and one low channel Industrial-size ESPs have many such plates, which are electrically channels in parallel grounded (i.e, voltage =0) Between the plates are rows of wires, held at a voltage of typically -40 000 volts The power is obtained by transforming ordinary alternating current to a high voltage and then rectifying it through some kind of solid-state rectifier. This combination of charged wires and grounded plates produces both the free electrons to charge the particles and the field to drive them against the plates. On the plates the particles lose their charge and adhere to each other and the plate, forming a"cake. The cleaned gas then passes out the far side of the precipitator as shown in Fig. 9.3 9-29-2 breadth of application. These devices are simple and almost maintenance-free. Because any medium-sized welding shop can make one, the big suppliers of pollution control equipment, who have test data on the effects of small changes in the internal geometry, have been unwilling to make these data public. The same basic device as the cyclone separator is used in other industrial settings where the goal is not air pollution control, but some other kind of separation. When it is used to separate solids from liquids it is generally called a hydroclone. A cyclone called an air-swept classifier is attached to many industrial grinders. It passes those particles ground fine enough, and collects those that are too coarse, returning them to the grinder. 9.1.3 Electrostatic Precipitators (ESP) If gravity settlers and centrifugal separators are devices that drive particles against a solid wail, and if neither can function effectively (at an industrial scale) for particles below about 5 Ix in diameter, then for wall collection devices to work on smaller particles, they must exert forces that are more powerful than gravity or centrifugal force. The electrostatic precipitator (ESP) is like a gravity settler or centrifugal separator, but electrostatic force drives the particles to the wall. It is effective on much smaller particles than the previous two devices. The basic idea of all ESPs is to give the particles an electrostatic charge and then put them in an electrostatic field that drives them to a collecting wall. This is an inherently two-step process. In one type of ESP, called a two-stage precipitator, charging and collecting are carried out in separate parts of the ESR This type, widely used in building air conditioners, is sometimes called an electronic air filter. However, for most industrial applications the two separate steps are carried out simultaneously in the same part of the ESP. The charging function is done much more quickly than the collecting function, and the size of the ESP is largely determined by the collecting function. Fig 9.2 shows in simplified form a wire-and-plate ESP with two plates. The gas passes between the plates, which are electrically grounded (i.e., voltage = 0). Between the plates are rows of wires, held at a voltage of typically -40 000 volts. The power is obtained by transforming ordinary alternating current to a high voltage and then rectifying it through some kind of solid-state rectifier. This combination of charged wires and grounded plates produces both the free electrons to charge the particles and the field to drive them against the plates. On the plates the particles lose their charge and adhere to each other and the plate, forming a "cake." The cleaned gas then passes out the far side of the precipitator as shown in Fig. 9.3. Fig. 9-2 Diagrammatic sketch of a simplified ESP with two plates,four wires, and one flow channel. Industrial-size ESPs have many such channels in parallel