Interception and impaction

Typical mechanisms for aerosol removal from gas streams by filters are diffusion to surfaces, interception and impaction. Very large particles can be removed by gravitational settling. These mechanisms are quite dependent on the particle size and it is usually found that conventional filters have a minimum in filter efficiency for particles in a narrow size range less than 1 im. When the gas is hot relative to the filter, thermophoresis can enhance particle removal. When the aerosol laden gas stream contains elevated concentrations of steam that condenses within the filter, difflisiophoresis will enhance particle removal. These phoretic enhancements of filtration are attractive because filtration efficiencies by these mechanisms are not especially dependent on the aerosol particle size. Washed Venturi scmbbers involve the injection of water droplets into the aerosol laden gas and these water droplets act much like spray water droplets to remove aerosol particles. Electrostatic precipitation is, in principle, a very attractive decontamination process, but it is difficult to assure that the necessary power will be available to operate the precipitators under accident conditions. 

Particles measuring 5-30 pm in diameter deposit in the nasopharynx via interception and Impaction. 

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. 

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. 

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. 

In the case of multiparticle blockage, as the suspension flows through the medium, the capillary walls of the pores are gradually covered by a uniform layer of particles. This particle layer continues to build up due to mechanical impaction, particle interception and physical adsorption of particles. As the process continues, the available flow area of the pores decreases. Denoting as the ratio of accumulated cake on the inside pore walls to the volume of filtrate recovered, and applying the Hagen-Poiseuille equation, the rate of filtration (per unit area of filter medium) at the start of the process is ... 

Once the information is processed and yon (consciously or unconsciously) have decided what it means, yonr nervous system coordinates a response. This is the task of execution. The loop from sensation to processing to execution can occur at many levels. The simplest level is the reflex arc. As noted earlier, when your family doctor strikes your knee with a rubber hammer, the nerves at your knee sense the impact and transmit that information. This information is intercepted and processed well before it ever reaches your consciousness. A reflex center interprets the sensation as a possible threat and automatically executes a command to straighten your knee. The result is a reflex action that protects your leg from injury by kicking away the perceived threat. This sensation to processing to execution loop is completed without any involvement of yonr brain. 

With fibrous filters, airborne particles are collected by the mechanisms of impact on, interception, and diffusion. 

Based on the combined mechanisms of impaction, interception, and diffusion, a minimum efficiency should result for spheres depositing on cylindrical collectors. 

Filtration is a physical separation whereby particles are removed from the fluid and retained by the filters. Three basic collection mechanisms involving fibers are inertial impaction, interception, and diffusion. In collection by inertial impaction, the particles with large inertia deviate from the gas streamlines around the fiber collector and collide with the fiber collector. In collection by interception, the particles with small inertia nearly follow the streamline around the fiber collector and are partially or completely immersed in the boundary layer region. Subsequently, the particle velocity decreases and the particles graze the barrier and stop on the surface of the collector. Collection by diffusion is very important for fine particles. In this collection mechanism, particles with a zig-zag Brownian motion in the immediate vicinity of the collector are collected on the surface of the collector. The efficiency of collection by diffusion increases with decreasing size of particles and suspension flow rate. There are also several other collection mechanisms such as gravitational sedimentation, induced electrostatic precipitation, and van der Waals deposition their contributions in filtration may also be important in some processes. 

Wet scrubbing uses liquid droplets to remove fine dust in a gas stream. In all types of wet scrubbing, the basic cleaning mechanism involves the attachment of particles to the droplets. The function of the droplets in scrubbers is similar to that of spherical fibers in filtration. Likewise, the primary collection mechanisms in scrubbing are similar to those in filtration, i.e., inertial impaction, interception, and diffusion [e.g., Fan, 1989]. Secondary collection mechanisms include thermophoresis due to temperature gradients, coagulation of particles due to particle electrification, and particle growth due to liquid condensation. 

Deposition mechanisms in the nose include inertial impaction, sedimentation, diffusion, interception and electrostatic attraction. The structure and physiology of the nasal cavity, with the small cross-section for airflow and sharp curves, suggests that inertial impaction is the most significant mechanism for drag deposition in the nasal cavity. 

Five mechanisms govern particle deposition in lung airways, namely, inertial impaction, gravitational sedimentation, diffusion, interception, and electrostatic attraction.f Electrostatic charges enhance deposition by increasing attractive forces to airway surfaces. Melandri et found that the deposition of... 

For Stokesian particles, rvi o impaction regimes are similar when the Stokes, interception, and Reynolds numbers are the same. The impaction efficiency. as in the case of diffusion, is defined as the ratio of the volume of gas cleared of particles by the collecting element to the total volume swept out by the collector. (Refer to Fig. 4.5 for the case of the cylinder.) If all panicles coming within one radius of the collector adhere, then we obtain... 

Previous work suggests that sedimentation is an important mechanism for relatively smooth, flat surfaces (15, 16, 20). One would therefore expect particle size to be a key parameter influencing the deposition rate. It is of interest that the slope of the flat plate curve is 1.7, close to the value expected for sedimentation, although it must be cautioned that the deposition velocities reported are also substantially greater than those predicted solely by sedimentation. This is most likely due to the use of MMD for the x-axis in the figure in reality, the small fraction of large particles is probably responsible for most of the mass deposition. If impaction, interception, and other mechanisms are important, these mechanisms would increase the measured deposition velocities above that expected on the basis of sedimentation alone. 

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