The basic message of this article is - if you are using micron ratings as the primary parameter to select media for closed loop MWF filters the same way you are using micron ratings for hydraulic filters, you may be asking for trouble. It could lead to selecting a fabric which is too tight, yielding a short life and high operating cost.
Selecting filter media based on micron ratings for hydraulic filters usually picks the size of the openings which are small enough to intercept the smallest particle the fluid will encounter. For closed loop metalworking fluids filters, if micron ratings are going to be used, (more often other parameters are used) pick the size of openings which will intercept the largest particle. Here’s why:
Filter elements for hydraulic oils are selected where it is necessary to rely on the initial tightness of the basic fabric to perform the needed particle retention. The filter’s job is to keep “clean” fluid clean. The contaminants they encounter are from either outside foreign sources or minor wear and tear of the system’s components.
MWF applications have a different concept. The filters are on systems to clean “dirty” fluid. Micron ratings are not as significant because the media is not the primary barrier to intercept the particles. The media is the foundation upon which a “cake” of particles is held and the cake performs the needed filtration function. This works well because the application has a continuous input of contaminants to create a permeable cake for depth filtration. It stays permeable longer to intercept particles smaller than the openings in the media. The cake will vary in thickness and density depending on the application.
Figure 1 shows a cross-section of the four types of cakes that are generally experienced.
(a) Random Media Openings; an application where a thick cake has a wide range of particle sizes. The media openings are random in size and relatively large. The fabric is selected to intercept the larger particles first to provide depth filtration which will continuously trap increasingly finer material.
(b) Tighter Material; depicts a situation where the particle size range is relatively narrow and the filter will be fitted with a tighter media. The rate of flow through the filter is selected to still rely on the cake filtration.
(c) Oil Effect; reveals what happens when foreign tramp oil enters the waterbase coolant serving a metalworking application. The oil coats the particles and acts as a sealing agent which prevents the cake from becoming thicker. The film is still of some use, but will not accumulate a significant cake.
(d) Blended Material; exhibits a media structure with a thicker initial barrier for depth filtration within is own fibers. This is an advantage where there is a relatively narrow range of particle sizes or where it is desirable to intercept larger particles in the “pile” of the media to give it a longer life.
Note that these points are applicable to closed loop systems and the degree of openness will depend upon the application and type of filter. Given that all set-up conditions are optimal, a relatively open media can achieve significant clarity levels with relatively low usage and operating costs.
James J. Joseph is the principal/owner of Joseph Marketing, located in Williamsburg, VA. You can reach him at email@example.com. He is the author of Coolant Filtration, 2nd Edition. Click here to read the book review.
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