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Out of curiosity, what constitutes as high density? in kW per volume.
I've been doing some custom watercooling stuff for fun, wondering how thin the cooling blocks should be. So far I've only gone down to (in total) 7 mm thick blocks and they have had problems with flow.
As for liquid cooling it should generally require little (or no) servicing, especially since miners aren't kept running for long.
High Density varies a lot depending on the nature of your deployment, but it can go as high as 18Kw/m3 depending on the cooling you have.
However, i tend to go a little more low there, the densest deployment i have is about 2.7 Kw/m3.
Watercooling isn't only for density, though, it is also for ultra-high humidity deployments, like my country's usual >60% (even near 98% 4 months a year) relative humidity.
7mm block is too small, i'm designing mine with 2cm blocks at least.