Cascade Impactor

For particles to be classified into various sizes, individual impactors are connected together in series to form a cascade impactor (Figure 13.43b). The individual impactors, or stages of the cascade are arranged in order, with the largest cut diameter (cut diameter is the smallest aerodynamic diameter retained by the stage) being first and the smallest last. The cut diameter is reduced in stages by changing the orifice diameter or the number of orifices and the distance of the impaction plate from the orifice plate. The impaction plates are demountable, so that the collected particles can be weighed and analyzed. After the last stage, there is usually a filter to collect particles smaller than the last-stage cut diameter. It is assumed that at each stage of a cascade impactor all the particles larger than its cut diameter are collected. A practical impactor collection efficiency curve is shown in Figure 13.43c). The cumulative mass of the particles is normally plotted on the particle size distribution graph as a function of the upper limit of the particle size range corresponding to each stage. Using a cascade impactor, emission gas particle samples can be classified into 12 fractions (aerodynamic diameter ranging from 0.15 to over 7 μm).

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Impactors utilize aerodynamic forces to separate the various particle size classes. This is consistent with the particles’ aerodynamic diameter. The operation principle of an impactor is that the particle is carried through an orifice with a gas stream directed against a plate (Fig. 13.43a). This is the impaction or collection plate. It changes the flow into an abrupt 90° turn. Particles, which are too slow, are not capable of following the change in the direction and collide with the plate. The cross-section of the impactor is shown in Fig. 13.43a.

Cascade Impactor