Surface Filtration implies that particles are intercepted mostly on the surface of the media, forming a layer of material “filter cake” that increases the efficiency or fineness of particles retained. Generally speaking, this type of filter media is referred to as having a “nominal retention”; perhaps initially being 60% to 70% efficient at retaining the targeted particle size and as the “cake layer” develops eventually becoming nearly 100% efficient. Nominally rated media is the most common and is less expensive than Depth Media.
Depth Filtration refers to a thicker media or multiple layers of media, forming a torturous path to retain particles. This type of engineered media ideally retains larger particles at the surface and progressively finer particles through the thickness or layers. Although there are nominally rated depth filtration media, the more complex designs are often rated at 95 to 99% efficiencies and therefore not reliant on a filter cake for their efficiency.
The cost differences of various filter bags and cartridges are often a reflection of the thickness and design of the media, ultimately related to its efficiency.
It is interesting to note that there isn’t a “universal” or industry standard for “nominally” rated filter media. Each manufacturer selects or manufactures the core material based upon their specifications and definitions of “nominal” and therefore if you are comparing pricing and one filter bag is considerably more expensive than another there is a good chance that it is a reflection of the material’s thickness and complexity. Depth filtration media will usually last longer than surface filtration media, although it also costs more, so your return on investment calculation must also include the applicable incoming freight costs, disposal costs and labor costs associated with filter cartridge and bag changes.