Electrostatic precipitator field charging

According to this approximation, the drift velocity is proportional to the square of the electric field. This is a clear indication of the importance of the electric field inside an electrostatic precipitator. Equation (13.60) is a valid approximation for large particles [dp > 0.5 m), provided that particle charge is close to the saturation level. In the case of small particles, the effect of diffusion charging must be taken into account. 

Electrostatic precipitators devices used to trap fine dust particles (usually in the size range 30 to 60 p,m) that operate on the principle of imparting an electric charge to particles in an incoming airstream which are then collected on an oppositely charged plate across a high-voltage field. 

The particles in a flue gas can pick up an electric charge when they move through an electrical field. The operation of an electrostatic precipitator makes use of this property. After passing the flue gas through the field, the device exposes the charged particles to plates carrying the opposite charge, which attract them. The particles stick to the plates and are periodically scraped off for disposal. 

The complete process of particle removal by electrostatic precipitation commonly includes three steps (1) particle charging (2) particle migration in an external electric field and (3) particle discharging and collection. This removal process is illustrated in Fig. 7.10. 

Example 7.2 Consider a dust-laden gas passing through an electrostatic precipitator. Assume that all particles have the same charge-to-mass ratio, q/m, and the electric field is uniform. Use the Deutsch equation to estimate the overall collection efficiency if the particle size distribution based on weight percentage (wt%) can be expressed by the mass density function, /M, as... 

Example 13.3 An electrostatic precipitator sampler consists of a 0.020-indiameter wire placed along the axis of a 1.5-in-diameter tube. What is the maximum field strength (assuming negligible space charge) at the outer edge of the tube when the precipitator voltage is 20 kV ... 

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