Conventional barrier filtration – the principle that bag filtration works on – can be classified as either surface filtration or depth filtration.

Surface filtration, as the name implies, stops particulate on the surface of the filter media. The filtration rating of these media can be accurately determined through direct observation -using a microscope- of the size of the pores in the filtration media. For basic woven filtration fabrics, the open space between intersecting yarns of the filtration fabric defines the pore, and therefore the filtration rating of the fabric. Particles that are too large to fit through the opening are stopped, and particles that are smaller than the opening pass through the filter surface.

One distinct advantage to surface filtration is that the accumulated particulate can often be removed from the filtration surface with a cleaning process, and the filter can be re-used. The effectiveness of cleaning a surface filter is contingent on the composition of the filter media, the retentate, and the permeate.

Here are several examples of surface filter medias: plain-woven fabrics and metal meshes, filtration papers and non-woven spunbond and wet laid medias; and synthetic membranes like PTFE, nylon, polyethersulfone (PES), and polysulfone (PS).

Membrane filtration is generally not applicable for bag filtration, but it is a form of surface filtration that relies on osmosis. Large format membrane filters are almost always manufactured into filter cartridges.

If you are looking for membrane filtration cartridges, we offer Shelco products here.

Depth filtration allows the particulate that is entrained in the fluid to enter the cross-section of the filter media where it becomes entangled in the tortuous path of the filter structure. Unlike surface filtration, it is much harder to assess the filtration rating of a depth filtration through direct observation because the filtration characteristics change throughout the cross-section of the media. There is no uniform pore-size on the visible surface of the filter that can be quantified.

One benefit to depth filtration is that the utilization of the filter media cross section increases the amount of particulate (retentate) that can be removed from the process fluid for a given amount of filtration area. However, depth filters are much harder to clean because a significant percentage of the retentate is permanently embedded within the cross-section of the filter media. Therefore, depth filters are generally considered single use filters. However, with proper sizing and system integration, the change-out life of the filters can be extremely long.

Examples of depth filters are: felted and twill-woven fabrics, melt-blown polypropylene, high-loft fiberglass and synthetics (often polyester) medias; string wound filters; fiberglass mats and fabrics, and cellulose medias with a cross-section greater than 1/32” (0.8mm).

Where surface filtration can be thought of as occurring in two-dimensions (on a plane, or combination of planes), depth filtration occurs in three-dimensions.

Filter Products Company offers a wide variety of filtration medias that can be used for the development of application specific filter bags. Use the form below to tell us about your application. We are happy to provide guidance on media selection.

Depth vs. Surface Filtration

Depth vs. Surface Filtration

Depth Filtration Media

Surface Filtration Media

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