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The Nuances of Paper Filtration in Coolant Filtration Systems

Dave Semersky is a 40-year veteran of industrial fluid control and purification.  In his experience, industrial filtration and coolant filtration systems are definitely places where knowing a little can trip you up worse than knowing nothing.  

As an example, Semersky points to paper media filters.  There are several factors at play here, which conspire to cause people to waste a lot of time and money, and yet never see the fluid performance they deserve.

For starters, paper filters can be a little counterintuitive.  In contrast to something more familiar–like a car’s air filter–these actually work better when they’re “dirty.”  

A paper filter rated for “50µm filtration” doesn’t catch 50µm fines right out of the box (i.e., when it is still so-called “naked paper”).  Disposable paper filters like these are designed to perform optimally only after they’ve built up a “filter cake.” This is an initial coating of solids that its captured from your fluids.  That filter cake can seem quite substantial–perhaps a quarter of an inch thick. Only after you build up a nice filter cake will your filter perform as rated.  So, a filter can look awful–but actually, be performing at its peak.

Interlocking Factors Decrease Paper Filter Performance

In general, coolant filtration systems are designed and developed under relatively straight-forward conditions.  If your filter is meant to catch particulates in a water-based coolant, then the filter’s designers almost certainly set filled up a few tanks with water-based coolant, dumped in fines of various types and sizes, and started recirculating that fluid through their filters to see how they performed.  

However, in your particular operation, it’s almost certain that there is more mixed into that coolant than just fines.  For example, it might be totally normal for some hydraulic fluid to enter your stream. Tramp oil in a water-based coolant will blind off the paper in your filter.  From the system’s perspective, the flow seems constricted–which it interprets as meaning the filter cake has gotten too thick. It indexes for new paper. And if you look at the paper filter you’ve pulled, it certainly looks dirty.  There’s no reason to doubt what the system says. But, in reality, you’ve probably barely begun to see that filter performing as specced. It seems like your whole filter system just doesn’t work–even though you’re “doing everything right.”

“So,” Semersky says, “there’s an example of a very good product, properly made, properly deployed–but you have to also recognize that this imperfect world has contaminants that that filter wasn’t made to handle.  When we get that offending oil out of the way, then that filter works like it’s supposed to, it filters better, there are fewer fines circulating, they get better filter performance and therefore better fluid performance.”

At Trucent, we are coolant filtration experts, and we don’t use paper filters.