Particle removal direct interception

An aerosol with particles in the micron size range flows around a smooth solid sphere a few millimeters in diameter. At sufficiently high Reynolds numbers, a laminar boundary layer develop.s over the sphere from the stagnation point up to an angle of about 110 at which. separation takes place. The removal of particles by direct interception can be calculated from the velocity distribution over the forward. surface of the sphere, up to 90 from the forward stagnation point (Fig. 4.P4). 

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 Particle removal from a gas stream by a filter with geometry such that the particulate matter does not deviate from the fluid flow lines. 

The often expressed intuitive belief that small particles are more difficult to remove From a ga.s than laige ones is usually not correct. The particles most difficult to collect are those in the size range corresponding to the transition from diffusional to inertial deposition, usually between 0.1 and 1 iim. The transition may be strongly influenced by direct interception (finite particle diameter effect) depending on the dimensions of the system. 

In general, if particulate matter is in the size and density range where it can be removed by inertial effects, one can expect to find an increase in filtering efficiency with an increase in vapor velocity. On the other hand, a decrease in filtering efficiency with an increase in vapor velocity, usually indicates that diffusion is the primary mechanism of particle removal. Increasing the vapor velocity would have little effect on the filtering efficiency due to direct interception. 

Clarification processes are normally used to separate the suspended cells from a fluid stream primarily by direct interception and sieving, where the particles are caught in or on the surface of the filter medium. In addition, some liquid filters removal capabilities can be achieved or enhanced by charge effects, in which case they may retain some molecules by adsorption. ... 

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