Impactor characteristics

The characteristic behavior of impactors depends on factors such as nozzle-to-plate distance, nozzle shape, flow direction, and Reynolds numbers for both the jet and the particle. Other factors of importance include the probability that the particles will stick to the impaction surface and particle loss to the walls of the impactor. It is not surprising that with such a variety of possible variables it is quite difficult, if not impossible, to accurately predict impactor characteristics on purely theoretical grounds. 

Impactor Characteristics. The impactor characteristics can be calculated if the densities of the particles are known. However, since this study was only for the size-fractionation of particles, the distribution was measured by optical microscopy. Ambient air was sampled for 1 hr and the particles were collected on glass slides. The particles were matched to the size circle on the calibrated Porton graticule. Approximately 100 particles were counted on each stage and the size distributions were generated. 

Cascade Impactor Characteristics. If the cascade impactor is to be used to obtain information on the diameters of particles in the atmosphere, then the distribution of the particle sizes on each stage must be known. Two approaches were used to obtain these data. The first used an empirical equation derived for this impactor (26,27,28) ... 

The electrical low-pressure impactor was used to measure the number concentrations of diesel exhaust particles. The particle size distribution ranges from 30 nm upward were then determined using the aerodynamic diameter as the characteristic dimension. ... 

Figure 8B shows the characteristic theoretical and practical collection efficiency curve. The x-axis may be plotted as particle diameter, in which case a series of curves at ever-decreasing sizes representing different stages of the impactor would exist. Converting the data to the square root of Stokes number overlays each of the particle size curves. The characteristic Stokes number for circular jets is 0.22. 

Improved control devices now frequently installed on conventional coal-utility boilers drastically affect the quantity, chemical composition, and physical characteristics of fine-particles emitted to the atmosphere from these sources. We recently sampled fly-ash aerosols upstream and downstream from a modern lime-slurry, spray-tower system installed on a 430-Mw(e) coal utility boiler. Particulate samples were collected in situ on membrane filters and in University of Washington MKIII and MKV cascade impactors. The MKV impactor, operated at reduced pressure and with a cyclone preseparator, provided 13 discrete particle-size fractions with median diameters ranging from 0,07 to 20 pm with up to 6 of the fractions in the highly respirable submicron particle range. The concentrations of up to 35 elements and estimates of the size distributions of particles in each of the fly-ash fractions were determined by instrumental neutron activation analysis and by electron microscopy, respectively. Mechanisms of fine-particle formation and chemical enrichment in the flue-gas desulfurization system are discussed. 

The reader is cautioned to keep in mind that atmospheric particles are not all spherical nor even necessarily simple in shape. Thus, as discussed in Chapter 9.A, the term size cannot be uniquely defined for atmospheric particles. As a result, a measurement of the distribution of sizes using an impactor that is based on inertial characteristics, for example, may not give the same results as a size measurement based on optical techniques that use light scattering. With this caveat in mind, let us examine the most commonly used... 

A second way to achieve collection of coarse- and fine-mode particles onto filters is through tandem filtration through the stacked filter unit (SFU) (4, 5, 6). In these devices, the convenient filtration characteristics of filters (Nuclepore) allow a 2.5- xm cut point on the basis of pore size and the face velocity of the airstream. Such devices are very compact and inexpensive and have been heavily used in remote-area networks (7, 8, 9). Again, however, the limitations of the method limit the number and sharpness of the size cuts so that almost all units are operated at 2.5 xm and give coarse and fine fractions very much like those of the virtual impactor. Examples of the 2.5- xm cut points of VI, SFU, and impactors are shown in Figure 2. However, cyclones, virtual impactors, and stacked filter units cannot give the sharp, multiple cut points of impactors as shown in Figure 1. 

Typical cascade impactors consist of a series of nozzle plates, each followed by an impaction plate each set of nozzle plate plus impaction plate is termed a stage. The sizing characteristics of an inertial impactor stage are determined by the efficiency with which the stage collects particles of various sizes. Collection efficiency is a function of three dimensionless parameters the inertial parameter (Stokes number, Stk), the ratio of the jet-to-plate spacing to the jet width, and the jet Reynolds number. The most important of these is the inertial parameter, which is defined by Equation 2) as the ratio of the stopping distance to some characteristic dimension of the impaction stage (10), typically the radius of the nozzle or jet (Dj). 

Table IV. Characteristics of Three Reduced Pressure Impactors...

Figure 1 Hypothetical collection efficiency of a stage of an impactor. Fraction deposited plotted against particle size. The step function indicates the theoretical deposition, and the curve indicates the practical characteristics.

It should also be mentioned that the majority of microbial airborne particles are dead or noncultivable , which means that they do not grow under the laboratory conditions chosen. This is, for example, true for microorganisms hke Stachybotrys chartarum. To fully survey the indoor microbial situation and to avoid overlooking fungal species which may be indicators of damage, additional samples have to be taken using methods hke direct microscopy which are not dependent on hving, cultivable cultures. Here, for example, microscopic slides with an adhesive surface are inserted in special impactor samplers. The airborne particles collected are stained in the laboratory. EspeciaUy spores with a characteristic shape hke those of S. chartarum are easily detected under the microscope. 

Guo C, Gillespie SR, Kauffman J, et al. Comparison of delivery characteristics from a combination metered-dose inhaler using the Andersen cascade impactor and (he next generation pharmaceutical impactor. J Pharm Sci 2007 97(8) 3321-3334. 

The data upon which these curves are based were obtained by use of a cascade impactor, which sizes particles according to their aerodynamic characteristics. The aerodynamic diameter of a particle is the diameter of a spherical particle of unit density which, when falling, reaches the same terminal velocity as the particle in question. This will be discussed further below. 

Impactor testing is very labour intensive, and some robot systems are in use for this purpose. These are often very expensive, as they mechanically take the impactor apart, wash off the drug for assay and then dry and reassemble the equipment. An industry consortium has been formed to design and develop a new multistage impactor with improved operating characteristics, which will be capable of easy automation at a reasonable price. 

An impactor, illustrated in Fig. 29.6, resembles a heavy-duty hammer mill except that it contains no grate or screen. Particles are broken by impact alone, without the rubbing action characteristic of a hammer mill. Impactors are often primary-reduction machines for rock and ore, processing up to 600 ton/h. They give particles that are more nearly equidimensional (more cubical ) than the slab-shaped particles from a jaw crusher or gyratory crusher. The rotor in an impactor, as in many hammer mills, may be run in either direction to prolong the life of the hammers,... 

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. 

Table 2.1 Characteristics of Some Typical Cascade Impactors...

Wang HC and John W (1988) Characteristics of the Berner impactor for sampling inorganic ions. Aerosol Sci Technol 8 157-172. 

Fig. 6.26. Anderson cascade impactor a — suctional view, b — characteristics of the separation of aerosol particles at different stages (1-6). A — air flow, B — health hazard region, C — safe area, penetration to lung is not likely...

Rao AK, Whitby KT. Non-ideal coUection characteristics of single stage and cascade impactors. Am Ind Hyg Assoc J 1977 38(4) 174-179. 

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