Precipitator Design

Precipitator design involves determining the sizing and electrical parameters for an installation. The most important parameters are the precipitation rate (migration velocity), specific collecting area, and specific corona power (White 1984). In addition, the design includes ancillary factors such as rappers to shake the dust loose from the plates, automatic control systems, measures for insuring high-quality gas flow, dust removal systems, provisions for structural and heat insulation, and performance monitoring systems.

The design engineer should determine the size distribution of the dust to be collected. Based on this information, the engineer can calculate the migration velocity (also known as the precipitation rate) Vp using Equation 5.17(14) for each size fraction. The engineer calculates the number of charges on a particle n using Equation 5.17(11) or 5.17(13), depending on whether field or diffusion charging is predominant. Diffusion charging is the dominant charging mechanism for particles less than 0.2 /m, while field charging is predominant for particles greater than 1 / m. For particles of intermediate sizes, both mechanisms are significant. The engineer can also calculate Vp empirically from pilot-scale or full-scale precipitator tests. The value Vp also varies with each installation depending on resistivity, gas flow quality, reentrainment losses, and sec-tionalization. Therefore, each precipitator manufacturer has a file of experience to aid design engineers in selecting a value of Vp. A high migration velocity value indicates high performance.

After selecting a precipitation rate, the design engineer uses the Deutsch-Andersen relationship, Equation 5.20(15) or 5.20(18), to determine the collecting surface area required to achieve a given efficiency when handling a given gas flow rate. If Equation 5.20(18) is used, the engineer can choose the value of the reentrainment factor f empirically from pilot-scale studies or previous experience or set it to zero as an initial guess. The quantity A/Qg is called the specific collection area.

The corona power ratio is Pc/Qg, where Pc is the useful corona power. The design engineer determines the power required for an application on an empirical basis. The power requirements are related to the collection efficiency and the gas volume handled. Figure 5.17.7 plots the collection efficiency versus the corona power ratio. At high efficiencies, large increments of corona power are required for small increments in efficiency. The precipitation rate (migration velocity) is related to corona power as follows:

where k is an empirical constant that depends on the application. The Deutsch-Andersen equation can therefore be expressed as follows:

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