CDU Filtration, Explained: Why One Filter Was Never Going to Be Enough
Quick myth to bust: there's no such thing as "the CDU filter."
If you've ever looked at a Coolant Distribution Unit spec sheet, filtration gets listed like it's one line item. It's not. It's three separate filters, each doing a job the other two literally cannot do. Mixing them up or skipping one to save money is how a liquid cooling system runs great for a year, then quietly starts falling apart.
Here's the breakdown.
๐น Primary - the front door
Sits where facility water enters the loop, before it hits the heat exchanger. Job: stop the coarse stuff sediment, pipe scale, general grit from the building's water lines.
Rated around 200 microns. Not fine at all, on purpose. It's catching big debris, not fine particulate. Usually a washable strainer install once, clean forever.
Skip this one and you're feeding building debris straight into your heat exchanger.
๐น Secondary - the one that actually matters
This is the filter everyone pictures when they think "CDU filter." Sits between the heat exchanger and your GPU/CPU cold plates.
Cold plate microchannels can be 100โ300 microns wide close to the width of a human hair. Anything past this filter goes straight for the narrowest, most expensive part of the system.
Rated 25โ50 microns. Tighter (25ยตm) for dense in-rack setups, looser (50ยตm) for in-row where pressure drop matters more. Built as hot-swappable dual chambers, so maintenance never means downtime you're not shutting down an AI cluster to change a filter.
๐น Side-stream - the one everyone forgets
Not always considered "required," which is exactly why it's the one worth caring about most.
Diverts 5โ10% of coolant flow through a 0.2โ5 micron membrane, continuously, forever. It's not catching debris it's catching the slow stuff: oxidation buildup, biological film, ultra-fine particulate that accumulates over months.
Systems without this stage aren't broken. They just develop problems slowly, quietly, with no single obvious cause which is honestly worse, because nobody catches it until it's already expensive.
Why you need all three, not just your favorite one
Primary protects the heat exchanger. Secondary protects your actual hardware. Side-stream catches what the other two structurally can't. None of them substitute for each other because none of them are solving the same problem.
A CDU running two out of three filtration positions isn't running a lighter version of the system it's running a system with a specific, predictable gap that shows up eventually as unexplained thermal drift, usually right after the warranty expires.
As AI rack density keeps climbing, coolant has to move faster to keep up, and there's less room for a partial CDU filtration setup to get away with cutting a corner.
