Inertial impaction

Inhaled air follows a tortuous path as it makes its way through the airways. Whether the airborne particles follow the path taken by the air at each turn depends on the balance between the force required to cause the particle to change direction and the fluid drag available to provide that force. If the drag force is sufficient to cause the required change in direction, the particle will follow the gas and will not be deposited. 

Consider a particle of diameter x travelling at a velocity lip in air of viscosity p within an airway of diameter D. Let us consider the extreme case where this particle is required to make a 90° change in direction. The necessary force Fr is that required to cause the particle to stop and then be re-accelerated to velocity Up. The distance within which the particle must stop is of the order of the airway of diameter D and so  

The force available is the fluid drag. Stokes law [Equation (2.3)] will apply to the 

The rationale for using Up as the relevant relative velocity is that this represents the maximum relative velocity that would be experienced by the particle as it attempts to continue in a straight line. 

The ratio of the force required Fr to the force available Fq is then  

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Mechanisms of Filter Retention. In general, filtrative processes operate via three mechanisms inertial impaction, diffusional interception, and direct interception (2). Whereas these mechanisms operate concomitantly, the relative importance and role of each may vary. 

Inertial impaction involves the removal of contaminants smaller than the pore size. Particles are impacted on the filter through inertia. In practice, because the differential densities of the particles and the fluids are very small, inertial impaction plays a relatively small role in Hquid filtration, but can play a major role in gas filtration. 

Includes cyclonic, dynamic, filtration, inertial impaction (wetted targets, packed towers, turbulent targets), spray chambers, and venturi. 

Chaiacteiistics of tfie pads vaiy slighdy witfi mesh density, but void space is typically 97—99% of total volume. Collection is by inertial impaction and direct impingement thus efficiency will be low at low superficial velocities (usually below 2.3 m/s) and for fine particles. The desireable operating velocity is given... 

Scmbbers make use of a combination of the particulate coUection mechanisms Hsted in Table 5. It is difficult to classify scmbbers predominantly by any one mechanism but for some systems, inertial impaction and direct interception predominate. Semrau (153,262,268) proposed a contacting power principle for correlation of dust-scmbber efficiency the efficiency of coUection is proportional to power expended and more energy is required to capture finer particles. This principle is appHcable only when inertial impaction and direct interception are the mechanisms employed. Eurthermore, the correlation is not general because different parameters are obtained for differing emissions coUected by different devices. However, in many wet scmbber situations for constant particle-size distribution, Semrau s power law principle, roughly appHes ... 

Acid mist eliminators use three aerosol collection mechanisms inertial impaction, interception, and Brownian motion. Inertial impaction works well for aerosols having particle diameters larger than 3 p.m Brownian motion and interception work well with aerosols having smaller particle diameters. 

In drying towers of sulfur-burning plants, mesh pads or inertial impaction-type mist eliminators are usually adequate. High efficiency mist eliminators are usually used in drying towers of spent acid or metallurgical plants. 

FIG. 14-130 Calve rt s refined particle ciit-size/power relationship for particle inertial impaction wet collectors. Ref. (R-19) by permission. 

The collection technique involves the removal of particles from the air stream. The two principal methods are filtration and impaction. Filtrahon consists of collecting particles on a filter surface by three processes—direct interception, inertial impaction, and diffusion (5). Filtration attempts to remove a very high percentage of the mass and number of particles by these three processes. Any size classification is done by a preclassifier, such as an impactor, before the particle stream reaches the surface of the filter. 

Direct interception occurs when the fluid streamline carrying the particle passes within one-half of a particle diameter of the filter element. Regardless of the particle s size, mass, or inertia, it will be collected if the streamline passes sufficiently close. Inertial impaction occurs when the particle would miss the filter element if it followed the streamline, but its inertia resists the change in direction taken by the gas molecules and it continues in a... 

PM Impingement-plate tower collection efficiencies range from 50 to 99 percent, depending upon the application. This type of scrubber relies almost exclusively on inertial impaction for PM collection. Therefore, collection efficiency decreases as particle size decreases. Short residence times will also lower scrubber efficiency for small particles. Collection efficiencies for small particles (< 1 fim in aerodynamic diameter) are low for these scrubbers hence, they are not recommended for fine PM control. 

The retention efficiency of membranes is dependent on particle size and concentration, pore size and length, porosity, and flow rate. Large particles that are smaller than the pore size have sufficient inertial mass to be captured by inertial impaction. In liquids the same mechanisms are at work. Increased velocity, however, diminishes the effects of inertial impaction and diffusion. With interception being the primary retention mechanism, conditions are more favorable for fractionating particles in liquid suspension. 

Inertial impaction This is the predominant mechanism used in all particle collection devices. 

The principle of inertial impaction is employed to sample aerosols aerodynamically for characterization of particle size and will be dealt with theoretically later in this chapter. 

Inertial impaction is the method of choice for evaluating particle or droplet size delivery from pharmaceutical aerosol systems. This method lends itself readily to theoretical analysis, ft has been evaluated in general terms [39] and for specific impactors [40]. Inertial impaction employs Stokes law to determine aerodynamic diameter of particles being evaluated. This has the advantage of incorporating shape and density effects into a single term. 

There have been attempts to conduct in vitro experiments in a manner that gives more meaningful data with regard to lung deposition. These methods, which are loosely based on inertial impaction, utilize inspiratory flow cycles rather than fixed flow... 

YA Marple. Simulation of respirable penetration characteristics by inertial impaction. J Aerosol Sci 9 125-134, 1978. 

CA Dunbar, AJ Hickey. Selected parameters affecting characterization of nebulized aqueous solutions by inertial impaction and comparison with phase-doppler analysis. Eur J Pharm Biopharm 48 171-177, 1999. 

Research Centers (IUCRC), 24 395 Inelastic mean free path (IMFP), 24 87 Inert fluids, 11 877 properties of, 11 879 Inert gas dilution, 11 456 Inert gases, 13 456 17 376-377. See also Helium- group elements Noble gases narcotic potency and solubility of, 17 377 Inert gas generators, 17 280 Inertial confinement fusion targets, microcapsules as, 16 460 Inertial impaction, in depth filtration theory, 11 339... 

Deposition efficiencies for particles in the respiratory tract are generally presented as a function of their aerodynamic diameter (e.g. [8,12]). Large particles (> 10 pm) are removed from the airstream with nearly 100% efficiency by inertial impaction, mainly in the oropharynx. But as sedimentation becomes more dominant, the deposition efficiency decreases to a minimum of approximately 20% for particles with an aerodynamic diameter of 0.5 pm. When particles are smaller than 0.1 pm, the deposition efficiency increases again as a result of dif-fusional displacement. It is believed that 100% deposition due to Brownian motion might be possible for particles in the nanometer range. 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

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]. 

Inertial impaction has many inaccuracies and limitations and there are also some relevant differences between deposition in vitro (impactor) and in vivo (respiratory tract) which cause poor correlation between impactor data and lung deposition data. The most important difference is that deposition in vitro is by inertial impaction only, whereas deposition in vivo is by sedimentation and diffusional deposition as well. Except for the possible passage of the final stage (by the finest particles), particle collection in vitro is almost 100% efficient. In con-... 

It is dispersed by wind and removed by gravitational settling (sedimentation), dry deposition (inertial impaction characterized by a deposition velocity), washout by rain (attachment to droplets within clouds), and rainout (scrubbing action below clouds) (Schroeder et al. 1987). The removal rate and distance traveled from the source depends on source characteristics (e.g., stack height), particle size and density, and meteorological conditions. 

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