Inertial impactors

For determination of the aerodynamic diameters of particles, the most commonly apphcable methods for particle-size analysis are those based on inertia aerosol centrifuges, cyclones, and inertial impactors (Lundgren et al.. Aerosol Measurement, University of Florida, Gainesville, 1979 and Liu, Fine Paiiicles—Aerosol Generation, Measurement, Sampling, and Analysis, Academic, New York, 1976). Impactors are the most commonly used. Nevertheless, impactor measurements are subject to numerous errors [Rao and Whitby, Am. Ind. Hyg. A.s.soc.]., 38, 174 (1977) Marple and WiUeke, "Inertial Impactors, in Lundgren et al.. Aerosol Measurement and Fuchs, "Aerosol Impactors, in Shaw, Fundamentals of Aerosol Sci-... 

A. Hickey, Factors influencing aerosol desposition in inertial impactors and their effect on particle size characterization, Pharm, Tech. (Sept. 1990), pp. 118-120. 

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). 

Inertial impactors fractionate aerosol particles aerody-namically according to their inertia, which increases with size. Inertial impaction is the most widely used method for sizing MDI aerosols, partly because a large sample is measured, consisting of the total aerosol generated from a number of actuations. Methods of this type have the advantage that the aerodynamic... 

Rgure 12.16 (a) The multistage liquid impinger. (Reproduced from G. W. Hallworth. Br. J. Clin. Pbarm., 4, 689 (1977).) (b) The principal features affecting the performance of inertial impactors air flow, the dimensions 5 and T, the diameter of the jet, D, the density, p, and the diameter of the particles, d. The trajectory of particles which are too small to deposit is shown. 

The principle on which inertial impactors operate is based on the aerodynamic behavior of aerosol particles. Fig. 6 shows a schematic diagram of particle collection by an inertial collection device. When the direction of a gas flow changes, the suspended particles continue to move in the original direction of... 

Marple, V. A., and K. Willeke (1976). Inertial impactors Theory, design and use. In Fine Particles, Aerosol Generation, Measurement, Sampling and Analysis (B. Y. H. Liu, ed.), pp. 411-446. Academic Press, New York. 

To study the potential health effects of pollutants in crowded spaces for recreational activities, particle mass versus size distribution of an ambient aerosol have been measured using a 10-stage Quartz Crystal Microbalance cascade impactor system (Junker et al. 2000). This impactor system consists of a series of 10 aerodynamic inertial impactors capturing particles at a mid-point cut-off ranging from 0.07 to 35 mm, arranged in a cascade with jets, which segregate the larger aerosol particles on top. 

The size distributions of colloidal suspensions of nanoparticles 74 nm to 14 nm in diameter are analyzed on-line. The sols are first diluted in water seeded with enough TFA to attain electrical conductivities in the range of 0.01 S/m. The solution is then finely dispersed into an atmosphere of CO2 via a Taylor cone-jet. The resisting electrospray of ultrafine droplets dries, transferring the solution particles virtually uncontaminated into the gas. There they are sized by means of a differential mobility analyzer and an inertial impactor of unusually high resolution. The technique is first tested successfully with previously calibrated monodisperse polystyrene latex (PSL) spheres 74 to 21 nm in diameter. It is then used to size a solution of colloidal silica with particle diameters nominally between 10 and 14 nm. 

A Size-Spectrometer based on an Inertial Impactor. Inertial impactors are devices where an aerosol is accelerated through a nozzle, forming a jet wluch impacts perpendicularly against a collector plate (21) This simple scheme has the remarkable property of acting as a low-pass filter, in which particles smaller than a critical diameter remain in suspension, while those supercritical impact on the surface and are captured. The parameter governing the subcritical-supercritical transition is generally called the Stokes number S ... 

Cascade impactors are instruments which have been extensively used for sampling and separating airborne particles to determine the aerodynamic size classification of aerosol particles. There are three kinds of cascade impactors inertial impactors, virtual impactors and particle trap impactors. 

Inertial impactors have been used extensively to measure aerosol size distribution and to collect samples for further chemical analysis. They consist of an acceleration nozzle and a flat plate. In inertial impactors, particles larger than the cutoff diameter of the impactor will impact on the collection plate, while particles smaller than the cutoff diameter will follow the streamlines and not be collected on the plate. Many researchers have investigated inertial impactors to collect and separate aerosol particles and found inherent problems which were particle bounce and re-entrainment (Dzubay and Rickel, 1978 Markowski, 1984 Marple, 1970 Marple and Liu, 1974). 

Even though inertial and virtual impactors have been widely used for aerosol collection and separation, particle trap impactors have not been used extensively. So far, studies on particle trap impactors were not carried out sufficiently. In addition, most particle trap impactors employ the design of inertial impactors for their design parameters, though actually the design of particle trap impactors is different from that of inertial impactors. Therefore, there still remains the necessity for study of the accurate design parameters for particle trap impactors. 

Data from inertial impactors are normally presented as 50% effective cutoff diameters. For the Andersen cascade impactors containing round jets and flat collection surfaces, the 50% effective cutoff diameter would yield a value of 0.14 for the inertial impaction parameter K. The Cunningham slip correction factor is equal to... 

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