TABLE OF CONTENTS Sutherland and Purchas [12] describe four fundamental filtration mechanisms: surface straining, depth straining, depth filtration, and cake filtration. Following is a brief description of each one:
Surface straining: The particle is larger than the pores and simply cannot pass through. Particles, smaller than the pore diameters, pass through the medium and are not separated. This type of separation is generally not associated with nonwoven fabrics but rather with media that has uniform pore openings. Examples are woven mesh fabrics, screens and membrane materials where the openings are uniform in diameter.
Depth straining: This applies to felts and nonwoven materials that are relatively thick compared to pore diameters, and where the pore diameters are quite variable in their length. The particles penetrate the pores until they reach a necking point where the diameter becomes smaller than the particle and at this point the particle is trapped in the pore.
Depth filtration: Depth filtration is different from depth straining, It involves mechanisms for removing a particle from a fluid even though the particle may be smaller than the diameter at any point in the pore structure. The mechanisms for doing this are discussed in Section 2.1.2.
Cake filtration: Cake (or surface) filtration involves the capture of particles on the surface (or near the surface) of a filter medium so that the build-up, of particulate matter into a layer of filter cake, participates in the filtration process. Surface modified needlefelts...
