Basics of Filtration

Depth Filtration

A true depth filter allows particles to penetrate the filter matrix and become captured throughout the depth of the medium. As with surface filtration, this only holds true when the particle size/pore relationship is conducive to the process for which the cartridge was designed. The depth filter matrix has a broad pore size distribution; hence, depth cartridges rely on adsorptive retention for a portion of their dirt-holding capacity. Some depth filters, such as the ARD, Nexis and DFT classic, have a gradient pore structure, with tighter pores near the center core, to maximize mechanical retention. In some depth cartridges, such as string wound, the medium is not a fixed pore matrix, as with chemically or thermally affixed pleated media. For this reason, depth cartridges should not be subjected to flows as high as those that are possible for pleated cartridges. Most depth filters are made from extruded melt blown fibers or twisted yarn fibers. Melt blown depth filters are generally made from polypropylene, polyester or nylon and can be made in both absolute and nominal retention ratings. These types of cartridges can be made to filter particles sizes from less than one micron to over 100 microns. Yarn wound cartridges, made with fibrous materials, are often brushed in order to maximize the tortuosity of flow through the filter. They are nominally rated but offer the advantage of being made from a variety of materials.

Depth Versus Surface

Descriptions of depth filters and surface filters usually emphasize the extreme characteristics of each. In reality, the filtration process is somewhere on a scale between the two, leaning predominantly to one end or the other. The filter chosen to perform the task will dictate whether or not surface filtration or depth filtration will predominate. The debate of depth filter versus surface filter often becomes a complex issue that is dependent upon many different factors.

Economics

Generally pleated cartridges cost more per 10" equivalent than do depth cartridges. However, at the lower micron ratings, the higher cost of the cartridge is made up by the greater dirt holding capacity. A comparison of cost and dirt holding capacity for wound cotton DFT Classics versus pleated Duo-fines was made to determine which is more economical.

Cost Per Gram of Contaminant

The higher cost per cartridge of the pleated versus the wound levels out between 3 and 10 microns; below this point it becomes more economical to use pleated cartridges. Conversely, above this level, the wound cartridge is likely to be more economical. Keep in mind, however, that direct cartridge to cartridge replacement cost is not always the only governing factor. Consider an entirely new application in which a system has to be sized from flow data. Due to the ability of the pleated cartridge to flow at a higher rate with a lower PSID, fewer pleated cartridges would have to be incorporated into the system. This would require a smaller housing, fewer replacement cartridges and lower disposal costs. In this case, one would have to weigh the difference of the initial cost and cartridge replacement cost. For example, a 3-micron Duo-fine costs three times more than a polypropylene DFT but holds 5.2 times the contaminant before reaching change out level.