Round tubes suffer from laminar flow though, and it's hard to make sure you get good flow through ALL the tubes :shrug: . The best bet is very thin & wide flat tubes and turbulators(HC's tubes usualy zig~zag [in> \/\/\/\/\/\/ >out] to create turbulance) so that as many water molecules are in contact with the copper as is humanly possible without it being too restrictive. Then you have to add as much surface area (fins) as possible to those tubes for the air to 'force convect' the heat away from, round is'nt the best for surface area, in fact it's the worse
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You can't beat a heatercore for size~performance~cost. If you're doing it for fun: Good! If you're looking for performance: Buy a HC...