Quote from: chiguireitor on August 23, 2015, 06:53:04 PM

with >60% humidity points here on my country, high density only happens with water blocks).

I don't understand that statement, heat sinks don't care about humidity to cool, they care about temperature differential. They do NOT use evaporative cooling like humans do.

Ignore "heat index", that's only an estimate of how hot a HUMAN feels due to humidity level reducing the ability of a human body to cool itself, NOT the same mechanics as for an item cooled by heatsink-to-air heat transfer.

Quote

Using a PCI-hardware backplane, even if the signal is still USB, really isn't any better than a PCB with a securely-mounted heatsink and a fifty-cent cable. By my consideration, it's substantially worse based on cost and longevity.

Cost more, definitely so when you include the cost of a backplane - though not as much as you think, passive PCI backplanes do exist and have been used for a long time in some hardware and aren't exactly rare. No need to reinvent the wheel there.

Mount really isn't any more complex than mounting those heatsinks to the case or however you're planning to mount them.

Longevity, IME PCI connections tend to last longer than the hardware they are being used by, Heck, I've got "ancient" ISA based gear that still connects reliably after 20+ YEARS of usage. I do NOT see a longevity advantage for the typical cheap connector used on any USB setup - though I doubt it would average much if any worse, BOTH will probably outlast Bitcoin mining.

The size disadvantage I can see POSSIBLY being an issue, especially since you're trying to limit board length for a better balance on front-to-back cooling.

Just had a thought - but I can see mounting issues getting "interesting". Make the hash boards horizontal, instead of vertical, then mount the power supplies to one sire of the case. Would probably need a subframe mounted inside the case, or spacers between the boards, to keep the boards from flexing too much. Would give fewer but larger boards, so would be a bit less "flexable" about incrimental upgrades. It WOULD make the cooling issues on the hash boards easier to manage.

Second thought - why limit it to 4U? As I recall BitFury and Avalon both made rack-mount 6U miners, which would make space management a LOT easier inside the case.
6x 120mm fans on the front would generally be more airflow per square inch than 3x140 too while using a LOT more common size of fan with a LOT more options available.

Delta, for example, does not list ANY 140mm 12V fans on their website, but they have a TON of 120mm 12V options (the 140mm fans they DO list are 24V and UP).