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This duct setup does have some adjustment, both in/out at the Alpha heatsink end, as well as some twisting available. At the fan end there is also some adjustment. I first used an 80mm fan with a rubber connector (Plumb-Quick flexible downspout connector, PDSC 43, HomeDepot $3). The final setup uses a 92mm fan. If you cut off the four mount tabs on a 92mm fan, with a little filing, it slips into the other end of the duct. The hole setup attaches to the case using the four fan holes. You do have to drill holes in the case to match the new fan location.
Now we get to the hard part: How to achieve passive cooling. Well almost anyway. The whole setup is a negative pressure case. There is somewhat of a vacuum being created inside this Lanboy by the PSU output fan and the output of the lower case fan (the subject of part three in this series). All the vent holes, the drives, the ports, and every other possible air-entry hole has been sealed. So the only way air enters the case is through the Alpha duct. This allows enough airflow to cool the P4 2.66 very well, without the duct fan even running. Using the A-Open SilentTek utility, the duct fan never turns on till the CPU temp reaches 39C, which is rare for me. Even with ambient temps around 27/28C, about the only way the duct fan turns on, is to run CPUBurn.
There's a good reason, not very obvious, why the CPU runs so cool... it's in the pictures.
The duct is solid aluminum and it's a snug fit on the Alpha end. Heat from the Alpha is transferred into the aluminum of the duct by conduction and by convection also. If you feel the duct on the Alpha end, it gets warm. This actually makes the duct itself, an extension of the Alpha heatsink. Now look at the power supply in the pictures: it has a 92mm fan drawing air over the hot end of the duct. Also when the duct fan is not running, this same fan is sucking air through the Alpha duct. Remember, the Alpha duct with it's filter is the only way air enters the case. The lower fan does the same thing: blows out and sucks it's air through the Alpha duct. Another thing, the air entering the duct and hitting the Alpha is ambient temperature. This computer can be run for hours with normal usage, without the duct fan even running. The CPU never exceeds 36C.
The definition of "passive cooling" probably varies for everyone. But as far as I'm concerned, this setup has a passive cooled CPU over 90% of the time. What that means for me is I have only two quiet fans blowing out of the case most of the time, as well as no fan at all on the CPU. It's a quiet computer.
Bluefront's Lanboy, Part 3: Acoustically Muffled Case Fan Box
Bluefront's Lanboy, Part 1: Muffled, Filtered & Ducted
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