10-2 10-1 100 101

Particle Diameter, /m ---Impaction -Combined ---Diffusion

FIG. 5.18.13 Fractional efficiency for combined impaction and diffusion.

10-2 10-1 100 101

Particle Diameter, /m ---Impaction -Combined ---Diffusion

FIG. 5.18.13 Fractional efficiency for combined impaction and diffusion.

dissipated in operating the scrubber (termed contacting power) and little affected by scrubber size, geometry, or the manner in which the power is applied.

In this text, contacting power is the power per unit of the gas volumetric flow rate used in contacting the aerosol to be cleaned. This power is ultimately dissipated as heat. The power per unit of the gas volumetric flow rate is considered an effective friction loss and represents friction loss across the scrubbing unit, neglecting losses due to kinetic energy changes in the flowing gas stream or losses due to equipment operating dry (Semrau 1960, 1977).

Contacting power is stated in the following contacting power rule:

where Nt is the number of transfer units. Transfer units are related to the fractional collection efficiency as follows:

In Equation 5.18(3), the terms a and y are empirical constants that depend on the properties of the dust being collected. Table 5.18.2 lists the values for a and y for various dust plus scrubber combinations. The contacting power PT is calculated from the following equation (Semrau 1960):

where: Pg

= the power input required to overcome the gas pressure drop across the collector PL = the power input required to produce droplets through the spray nozzles PM = the power input required to drive any rotor, if present.

Figure 5.18.14 plots NT as a function of contacting power for various dusts. In the preceding equations, the power terms are expressed in kWhr/1000 m3. Table 5.18.2 gives the factors for converting power to either hp/1000 cfm, or effective friction loss (inches or centimeters of water).

Using the contacting power approach is best when some knowledge of the scrubber's performance for a specific aerosol is available. For example, when pilot plant designs are scaled up to full-size units for a dust control problem, contacting power is helpful in predicting the overall efficiency of the new, full-size unit. Or, if a scrubbing unit is to be replaced with a new and different design, the contacting power approach is helpful in estimating the performance of the new unit. However, in determining contacting power, a design engineer must insure that only the portion of energy input representing energy dissipated in scrubbing is used (Semrau 1977). Also, little data exists relating gas-phase and mechanical contacting power. Since the contacting power approach is an empirical approach, extrapolating the results into areas with little data requires caution.

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