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1、CHAPTER EIGHTELECTROSTATIC PRECIPITATORS（ ESPs） ACKNOLEDGESBASIC IDEA OF ESPs THE STRUCTURE OF ESPs THE BASIC PARAMETERS OF ESPs INTRODUCTION If gravity settlers and centrifugal separators are devices that drive particles against a solid wall, and if neither can function effectively (at an industria
2、l scale) for particles below about 5 in diameter,INTRODUCTION then for wall collection devices to work on smaller particles, they must exert forces that are more powerful than gravity or centrifugal force. INTRODUCTION The electrostatic precipitator (ESP) is like a gravity settler or centrifugal sep
3、arator, but electrostatic force drives the particles to the wall. It is effective on much smaller particles than the previous two devices.INTRODUCTION In all three kinds of devices, the viscous (Stokes law) resistance of the particle to being driven to the wall is proportional to the particle diamet
4、er. INTRODUCTION For gravity and centrifugal separators, the force that can be exerted is proportional to the mass of the particle, which, for constant density, is proportional to the diameter cubed. INTRODUCTION Thus the ratio of driving force to resisting force is proportional to (diameter cubed/d
5、iameter) or to diameter squared. INTRODUCTION As the diameter decreases, this ratio falls rapidly. In ESPs the resisting force is still the Stokes viscous drag force, but the force moving the particle toward the wall is electrostatic. INTRODUCTION This force is practically proportional to the partic
6、le diameter squared, and thus the ratio of driving force to resisting force is proportional to (diameter squared/diameter) or to the diameter. INTRODUCTION Thus it is harder for an ESP to collect small particles than large ones, but the difficulty is proportional to (1 / D) rather than to (1 / D2),
7、as in gravitational or centrifugal devices.1 BASIC IDEA OF ESPs 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. 1 BASIC IDEA OF ESPs In one type of E
8、SP, called a two stage precipitator, charging and collecting are carried out in separate parts of the ESP. This type, widely used in building air conditioners, is sometimes called an electronic air filter. 1 BASIC IDEA OF ESPs However, for most industrial applications the two separate steps are carr
9、ied 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. 2 THE STRUCTURE OF ESPs Figure 8.1 shows in simplified form a wireandplate ESP with two plates.
10、2 THE STRUCTURE OF ESPs 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. 2 THE STRUCTURE OF ESPs The power is obtained by transforming ordinary alternating current to a high volt
11、age and then rectifying it through some kind of solid state rectifier. 2 THE STRUCTURE OF ESPs 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. 2 THE STRUCTURE OF ESPs On the plates the part
12、icles 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. 8.1.2 THE STRUCTURE OF ESPs Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic
13、 rapper; that strikes a vertical or horizontal blow on the edge of the plate. 2 THE STRUCTURE OF ESPs Through science, art, and experience designers have learned to make rappers that cause most of the collected cake to fall into hoppers below the plates (not shown on Fig. 8.1). 2 THE STRUCTURE OF ES
14、Ps Some of the cake is always re entrained, thereby lowering the efficiency of the system.2 THE STRUCTURE OF ESPs There are many types of ESPs; Fig.8.2 shows one of the most common in current use in the world. Gas flow is from right to left. 2 THE STRUCTURE OF ESPs The gas enters at the right throug
15、h an inlet diffuser (not shown) in which the flow spreads out from the much narrower duct to the perforated gas distribution plate that distributes the gas evenly across the entrance face of the precipitator. 2 THE STRUCTURE OF ESPs Each point in space has some electrical potential V. If the electri
16、cal potential changes from place to place, then there is an electrical field, , in that space. If we connect two such points with a conductor, then a current will flow. 2 THE STRUCTURE OF ESPs This V is the voltage we are all familiar with, and E is its gradient in any direction; the units of E are V/m.2 THE STRUCTURE OF ESPs In a typical wireandplate precipit
