Cascade impactor characteristics

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) ... 

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. 

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). 

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. 

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...

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. 

Due to their simplicity of construction and use and the relatively sharp cut-off characteristics, cascade impactors have been widely used for the size classification and size-classified chemical analysis of aerosols. Table 6.1 lists the most important integrating sampling methods with their main characteristics. Table 6.2 gives the most important differential, size-resolving methods used to sample and measure atmospherie aerosol particles. The section of the particle size distribution and the modes that dominate the sensitivity of the methods are indicated. The upper and lower size limits are nominal values for the most commonly used forms of the techniques. Cost, complexity of operational requirements, calibration problems, and the demands of the particular evaluation to be used also affect the choice of methods. For example, chemical analysis usually requires that a sample be collected, then taken to the evaluation device. 

The impaction characteristics of the cascade impactors are governed by the fluid s velocity flow field, which in turn is specified by the Navier-Stokes equations. A solution of the Navier-Stokes equations (Marple et al 1974) reveals that the velocity flow field is a function of the physical configuration of the impactor and the Reynolds number. Re, of the flow passing through the nozzle. The Reynolds number, based on the hydraulic diameter of the nozzle, is defined as... 

Physical characteristics, flow parameters and experimental results of a compact multistage cascade impactor... 

---