By Josh Cosford, Contributing Editor
A few years ago, I brought my vehicle into the dealership for servicing, and while I waited, I watched the TV hanging from the wall behind the counter. It was a reel of service schedules, recommended replacement items, and some other upsell commercials.
Once, such a video showed a cutaway of a vehicle HVAC system, first with a brand-new cabin filter, and an animation of fresh, baby-blue, breezy air flowing freshly into the cabin. They contrasted it with a clogged filter apparently pulled from a dump, where the air flowing into the cabin appeared more like green puffs of toxic dog farts.
When I mentioned to the service advisor that filters don’t work that way and that a clogged filter would likely yield fresher air, she looked at me like I was reciting the first few thousand prime numbers. When I broke the long silence with, “…maybe it would flow less air, though,” she responded dryly with, “What’s the best number we can reach you at?”
Regardless, it’s easy for the fluid power layperson to misunderstand the fundamentals of filtration, because a clogged filter is actually more efficient at removing both more and finer particles from your hydraulic oil. A partially loaded filter accumulates debris on the surface or in the depths of the filter element, blocking pores and reducing the number of paths for particle-containing fluid to pass. A bunch of 6 micron particles blocking a bunch of 5 micron pores will prevent anything larger from passing and then likely get stuck on the surface.
As contamination builds to high levels, a layer of cake can form, and the cake will trap even more fine particles. Of course, cake increases pressure drop exponentially, so it’s likely that a return line filter would have gone into bypass long before this. A filter bypass valve is a spring-offset check valve that starts to open as pressure drop increases due to dirt resisting flow across the filter media.
If your filter is in bypass, even just a little bit, you’re passing entirely unfiltered fluid. In this case, a clogged filter is much worse, so it’s important to monitor a filter’s differential pressure using pressure gauges, indicators, or electrical switches/lights. Differential pressure is a specific type of pressure measurement that compares two locations, in this case, the pressure before the filter and after the filter. (For more on the dangers of clogged filters, read What are the symptoms of a clogged hydraulic filter?)
We often casually refer to the above differential pressure measuring devices as their clogging indicators, which are designed to signal that the filter should be changed when the pressure reading is around two-thirds of what the bypass valve will open at. This gives you time to purchase a spare filter element and have it installed. Just keep in mind that cold hydraulic systems with cold hydraulic oil will appear to show a more clogged filter during machine startup.
So, as long as you’re regularly monitoring your filter’s bypass indicator, it’s actually beneficial to leave your filter partially loaded rather than arbitrarily changing it out under the assumption that a new filter is better. And the next time the technician at the lube shop pulls out your cabin filter and shows you “how dirty it is,” just tell him to put it back because you like it dirty.
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