Cut-off diameter

For a cyclone of fixed dimensions, the collection efficiency may be improved by increasing the fluid flow rate through the cyclone. As a consequence of this, the pressure drop over the cyclone increases. It can be shown that the relationship between the cut-off diameter and the fluid flow rate Q may be approximated as follows ... 

Figure 10A depicts a stacked Andersen eight-stage impactor with calibrated cut-off diameters of 9.0, 5.8, 4.7, 3.3, 2.1, 1.1, 0.7, and 0.4 pm and a lower absolute filter (0.22 pm) when operated at 28.3 L/min. Figure 10B depicts a four-stage liquid impinger with... 

Inertial impaction is most widely applied for the characterization of inhalation systems. The principles of particle separation on the basis of inertial and drag forces have been well described for many different applications. Theoretical cut-off diameters (for particles with 50% collection efficiency) of impactors can be calculated on the basis of Stokes numbers for nozzles of a particular design [8,120]. Many different designs are available, but only a few are described in the United States and European Pharmacopoeia [121,122]. 

Plot the fractional efficiency curve and estimate the cut-off diameter of this cyclone. 

Cascade impactors are typically characterized based on the nozzle geometry, number of stages, flow rate capacity, and the range of effective cut-off diameters of the impaction stages. Characteristics of some most commonly used cascade impactors are exemplified in Table 2.1. 

Based on the observed lack of bioconcentration of hexabromobenzene, hepta- and octachloronaphthalene and octachlorodioxin, it has been suggested that molecules with certain spatial dimensions are not able to cross the membrane-water interface and enter the membrane (Opperhuizen et al. 1984, 1985). These molecules, which were suggested to have minimal internal cross sections (MICS) of 0.96 nm or larger, are thus believed to have a low or zero bioconcentration factor. From a mechanistic point of view it seems conceivable that a cut-off diameter exists for passive membrane diffusion. However, what this cut-off diameter exactly is remains unclear since in a recent unpublished bioconcentration experiment, brominated biphenyls with the same MICS as hexabromobenzene were accumulated in guppies. The obvious test of this effect is to perform bioconcentration experiments with conservative chemicals varying in their spatial dimensions. 

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