Various physical mechanisms contribute to a filter's effectiveness in capturing particles. The most predominant mechanisms are: Inertial Impaction, Interception, Diffusion, and Electrostatics. Overall filtration efficiency can be influenced by any one of these mechanisms or by all of them simultaneously.
Inertial Impaction
Inertial impaction occurs when a particle is so large that it is unable to quickly adjust to the abrupt changes in streamline direction near a filter. The particle, due to its inertia, will continue along its original path and hit the filter. This type of filtration mechanism is most predominant when high gas velocity and/or dense packing of the filter media is present.
When particle size and the number of particles increase, so does the probability of collision. As the dust particles collect, they themselves become part of the filter media, thereby increasing efficiency by adding to the number of possible collisions for other suspended particles. Similarly, when the travel distance through the filter increases, so does the probability for collision. Thus the efficiency of removing particles from an air stream by impaction is a function of particle size, particle concentration, and the travel distance through the filter (equivalent filter depth).
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