Thanks Dave, It's nice to hear from you again!
The dimension stated (.0226" (0.57 mm)), is the ID of the tubing that is used in the CPU backside cooler. It's a set of very small tubes that run under the CPU. There's a plastic tube that runs between the pins, and a copper tube that's thermally epoxied to the backside.
I figure that filtering for 1/4 of that dimension should keep those tubes clear, so I go from 0.57mm (aka 570 microns) down to ~150 microns.
N8 mentionned that these (CPU backside cooler) tubes would probably clog over time, from various deposits. Personally, knowing that my loop would be closed, I don't expect any new particles to appear. However... I can't say for sure if the coolant flow would eventually start flaking things off, or if any corrosion (copper,brass and solder only) would add any particulates. In other words, no, I don't expect a steady load of particles.
Also, if there are any deposits, I would expect them to appear in the "tanks" to the CPU backside cooler: a 1/2" PVC extension (aka manifold). That's a project that I'm still not sure I want to go ahead and execute, so the filtering is otherwise for the "Cascade" style block, which uses an array of small openings (~1mm ?).
So off hand, I'm more interested in one time filtering: i.e. filter the water, then remove the filter to let it run. If I don't go with the backside cooler, I'd consider leaving a 250 micron mesh filter in, but I don't see the purpose: all I'm putting in the coolant, is distilled water (a new gallon), and a corrosion inhibitor (Silkolene ProCCA, which is 50% mineral oil, mixed at 1%). I might add windshield wiper fluid, when I get close to building the #Rotor chiller.
I like the backwash idea, and I'd implement it, if I had room for it! I'll have to backwash it externally, that's all.:shrug:
Thanks again Dave! BTW, is there such a thing as a filter element that's in the form of a sheet, and rolled slowly, so it's continously "fresh"? Just curious.