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Wearing a skirt...

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Dec 18, 2000
Well, here's another cheesy but potentially helpful tip:

Ever wondered why some HSF come with skirts just below the fan and around the heatsink. The fact is, that fans DO NOT blow air straight on, but rather angled outwards from the fan's axis. So some of the airflow at the heatsink edges is wasted -ie. will not even get to touch any fin or pin. Anyone can easily fit a skirt around a HS's bare butt with some sticky tape and cardboard. Be mindful as to not choke off the airflow from the fan. A rule of thumb is that the skirt should extend down about half the heasinks's height.
It usually doesn't make much differance with the fan blowing as it just ends up causing a slightly higher pressure but not much if any extra airflow. When suching it helps a lot though
I have found that shrouds or skirts help most when the fan is sucking air out of the Heatsink. I think they encourage the air to be drawn in further down toward where the fins/pins are bonded to the baseplate and then travel up the fins/pins to the base of the fan. I have not seen any improvement when the fan blows down into the heatsink.

Yes, true. Skirts around a heatsink help more if you intend to suck air from the base of the heastsink and exhaust it from the top. Air sucked in by fans do not follow as straight a path as the air blown outwards, which have momentum and direction imparted to them by the blades. In fact, without some means of ducting, some of the air blown out will actually get recirculated back into the fan -not good if you want 100% fresh, cool air to cool your heatsink. However, every bit of airflow does help so why not attach a skirt even if the fan is blowing into the heatsink?
Phil (Jun 20, 2001 11:39 a.m.):
It usually doesn't make much differance with the fan blowing as it just ends up causing a slightly higher pressure but not much if any extra airflow. When suching it helps a lot though

The key point here is RECOVERING stray airflow with the use of skirts. Heck, maybe the cooling gains are nominal but it sure is a good placebo for the paranoid overclocker....
I have some aluminum stepping sitting around, I think I'll give it a go. Even if it doesn't do anything it'll still look cool. If I do make a skirt I might want to paint it. Would this be a bad Idea? I painted the frame inside my case with "Ford Blue" engine enamel. It looks cool and it would look really kool if the skirt matched. Do you think the enamel coating on the skirt would affect the heat dissapation i anyway?
Nah, the skirts are in minimal contact with the HS so they receive very little heat from the HS and have very little heat to be carried away by airflow or free convection. So painting your skirt with enamel to match your HS colour or whatever is OK. Note that the skirts themselves are to direct the stray airflow of a fan and are not extensions of the HS's fins.
Kool, I'll give it a go then. Actually what I was concerned about the skirt not allowing heat to dissapate away from the HS correctly as the skirt would prevent it from moving away from the HS and the enamel might worsen this...then I remembered that that is what the fans are there for. *Blue Jester smacks his forhead*-Duh!...what can I say, it was a long day at work yesterday. =]
Hmmm, this is something to consider. A 3'C drop by removing the skirt is quite significant. Heck, when I removed my trusty 80cfm Twinturbo from my casing my mobo and CPU only went up 1'C.
That is something to consider, It'll be a while before I have the time to try this, but when I do I'll make sure to check my temps and post my findings. I'll even check before and after I paint it.