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 Post subject: 120mm suction duct mod
PostPosted: Wed Apr 16, 2003 7:23 pm 
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Location: Virginia
I've seen a few HSF-to-rear fan duct mods, but they all seem to be made to blow onto the heatsink. Has anyone made an exhaust duct from their PAL 8045/8942 to a rear 120mm fan?

Negative pressure in a duct could be difficult to maintain than positive pressure. I'm thinking I'll have to use something more rigid than drier exhaust ducting, maybe plexi or rubbermaid bin lid plastic. Would be so nice to have a flexible elbow though...

Maybe get an 80mm Badong duct ($20!), shorten it to 2-3 inches (to minimize flow resistance) and attach it to a rigid 120 mm square-ish duct (tall tupperware?) hot-melt-glued to a fan frame. Then mount the frame on the inside of the top 120 exhaust and put the 120mm Panaflow exhaust on the outside of the exhaust hole.

I wonder how much diameter decrease I would get due to sides of flexible duct getting sucked in by neg pressure, and how much of an air leak I would have through the various couplings and thru the duct plastic itself.

Anyone brave enough to chime in with ideas?

As a backup plan I got a Zalman fan bracket for a Panaflo 92mm to blow down onto the Alpha, (like in the bitbenderforums.com article) but that's not an elegant solution.


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PostPosted: Wed Apr 16, 2003 8:19 pm 
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What HSF/fan combo are you using now?

Are you thinking of using your duct idea to eliminate the fan on the CPU/HSF? Rather than having a fan at each end of the duct.

Have you considered finding a way to mount the fan on/near the Alpha, pulling air across the Alpha and blowing into the duct, rather than mounting the fan on the case wall at the far end of the duct? This would create positive pressure in your duct. If you are undervolting the fan, it might move the noise further inside the case, too (always a plus).

I guess you've seen Rusty's Quiet In-Desk PC. His fan set up sounds similar to what you are looking at doing.


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 Post subject:
PostPosted: Wed Apr 16, 2003 10:40 pm 
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I'm assembling a system w/ an Alpha PAL 8942 in a LiteOn FS020 (2 rear panel 120mm fans). Would like to use one of the rear 120mm Panaflo's as the CPU fan, no fan at all on the Alpha, just duct and shroud delivering suction.

The idea of blowing across a horizontally-finned heatsink (not possible with an Alpha) is interesting but has never been successfully implemented in an aftermarket HSF. Check Dan's Data for a few of the horizontal-flow HSF's. They're all poor performers. I don't think that it is possible to provide enough cooling to the lower portions of the heatsink fins, where it's needed. The only way horizontal flow would be effective would be with heatpipes distributing the base plate heat evenly thru the fins.


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PostPosted: Wed Apr 16, 2003 10:46 pm 
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Location: Tampere, Finland
I've done similar setup than what Aphonos here is saying. I have fan attached to Alpha PAL8045 blowing into duct and out from back of the case. The duct is made out of this. I bought it from local store, it's not really cheap but its such a short that it isn't much of a problem. 30cm costed me about 5$.

It's littlebit too stiff but at the time it was all I could think of and I haven't had the energy to go and find something that would be better. I was thinking about these aluminum pipes(?) that are used in cars that are easily pendable, but haven't given it too much of thought where to find big enough pipe. Or maybe doing it from cardboard or thick enough paper (drawindpaper maybe).

And the bottomline here is that in my case :lol: , I could turn the fans to little bit slower than without the duct allthough the effect wasn't as good as I hoped it to be, only couple C's for Case/CPU/HDD. At full fanspeed, change is little bit more. And most dramatic change in temperatures happened in PSU exhaust temps, down from ~39 to ~35.

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 Post subject:
PostPosted: Thu Apr 17, 2003 1:25 am 
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Location: Cambridgeshire, England
I've been considering a similar setup for my machine. I have an Antec Sonata (120mm exhaust L1A), and a Alpha 8942.
I've thought about using an 90deg bend 120mm diameter smooth-wall aluminum ducting to attach between the 120mm exhaust fan to a 120mm->80mm adapter mounted to the Alpha to draw case air across the heatsink (in suck mode) and exhaust the hot air out the back. This would eliminate the panaflo 80mm L1A I have on the Alpha now.

The only drawbacks to the design that I can forsee are these:

1) The majority of the case air would be drawn into the heatsink, thereby making it a coarse filter and catching loads of dust (if any).

2) Since the velocity of the air would increase with any decrease in duct diameter, the noise produced at the heatsink would be much greater than the two fans at present.

Example: 120mm L1A at 7V pulls approx 40CFM, that same volume of flowing air restricted to 80mm (50% reduction) would cause the velocity to increase proportionally, to approx 60 CFM (50% increase). 60 CFM flowing through the heatsink would cause lots of turbulence (noise).
Granted that the turbulent air would be at the heatsink and not at the leading-edge of the spining fan blades. The air entering the fan should be at the same turbulence level as without the duct.

I suppose if you wrapped the ducting in acoustical foam or something it shouldn't be too bad.

Somebody please correct me if my assumtions are wrong.

Turning my Alpha into a dust trap that needs cleaning every week isn't part of my plan. :lol:

The good thing about the design is that it would reduce the total number of fans inside the case and at the same time increase the velocity and volume of air flowing through the heatsink for greater cooling capacity.

Dell has come up with a similar plan in it's Optiplex tower series but they have the fan mounted halfway between the CPU and the rear opening.

Maybe if I can find some ducting locally I'll give it a shot and find out for myself.

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 Post subject:
PostPosted: Thu Apr 17, 2003 2:44 am 
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The easiest way to keep your case clean is to blow into the case, with the incoming air being filtered. You can take this one step further and with a duct, blow the clean, cool air directly on the heatsink. If you place the fan on the inside end of the duct, the overall noise level is reduced. Depending on your setup, you may be able to use this one duct/fan for the whole cooling thing. This senario is working for me....Antec 2600, Celeron 2.0, alpha 8942.


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 Post subject:
PostPosted: Thu Apr 17, 2003 4:14 am 
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Mr_Smartepants wrote:
Example: 120mm L1A at 7V pulls approx 40CFM, that same volume of flowing air restricted to 80mm (50% reduction) would cause the velocity to increase proportionally, to approx 60 CFM (50% increase).


No

The CFM will not increase, it will decrease a little due to the higher resistance. Only the velocity will increase.

Do you really need both fans, why not remove the 120mm one, and just keep the 80mm on the heat sink, ducted to where the 120mm used to be. That is what I am planning on my new PC in a few weeks time.


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 Post subject:
PostPosted: Thu Apr 17, 2003 7:20 am 
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efcoins wrote:
Mr_Smartepants wrote:
Example: 120mm L1A at 7V pulls approx 40CFM, that same volume of flowing air restricted to 80mm (50% reduction) would cause the velocity to increase proportionally, to approx 60 CFM (50% increase).


No

The CFM will not increase, it will decrease a little due to the higher resistance. Only the velocity will increase.

Do you really need both fans, why not remove the 120mm one, and just keep the 80mm on the heat sink, ducted to where the 120mm used to be. That is what I am planning on my new PC in a few weeks time.


Doh! :o
Yeah, you got me. I didn't think it through enough.
The 120mm fan will only pull 'X' amount of air under ideal circumstances. If you restrict the intake (120mm -> 80mm) then you restrict the amount of air the fan can pull thereby reducing it's effectiveness. Only the velocity will increase.
Thanks for the correction.

The way my temps are at the moment, I'd be hesitant to remove the 120mm fan since it's contributing more than it's fair share of moving air. If anything, I'd remove the 80mm L1A since at 12V it only pulls about 24CFM (not a whole lot in comparison).

I know there's a thread somewhere in here where someone tried this same setup. Anyone know where?

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 Post subject:
PostPosted: Thu Apr 17, 2003 9:19 am 
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gbass01 wrote:
I'm assembling a system w/ an Alpha PAL 8942 in a LiteOn FS020 (2 rear panel 120mm fans). Would like to use one of the rear 120mm Panaflo's as the CPU fan, no fan at all on the Alpha, just duct and shroud delivering suction.

The idea of blowing across a horizontally-finned heatsink (not possible with an Alpha) is interesting but has never been successfully implemented in an aftermarket HSF.


I suppose I wasn't very clear. :oops: I wasn't referencing Rusty's PC so much for the idea of pulling air across a hoizontally-finned HSF as I realize this will not work with the Alpha 8942 (I have one in my case), but to show the idea of how he constructed a duct out out plexi with a 120mm fan mounted on the case (desk?) wall to pull air across the HSF. I thought this was similar to the design you mentioned in your original post...

gbass01 wrote:
Has anyone made an exhaust duct from their PAL 8045/8942 to a rear 120mm fan?


I was thinking that rather than try to pull air across horizonatal fins (as in Rusty's case) you would be pulling air up through the Alpha--with one end of your duct mounted on the Alpha's shroud (maybe you could even omit the shroud and just put the duct over the top of the Alpha in place of where the shroud was. I'm not sure you can secure the shroud to the Alpha without a fan mounted on the Alpha unless you use custom-length screws...but I digress) and the other end of your duct attached to the 120mm fan on your back case wall (a la Rusty). Have I got your idea correct?

I wouldn't be able to tell you if it generates sufficient air to cool the HSF or not. You could always try it by making a rough draft of a duct out of something simple like cardboard and duct tape and then checking your temps.

One other thought on your original post...

gbass01 wrote:
I wonder how much diameter decrease I would get due to sides of flexible duct getting sucked in by neg pressure, and how much of an air leak I would have through the various couplings and thru the duct plastic itself.


If you are using any type of quiet fan and/or undervolting for quietness, I doubt your fans will be strong enough to suck in the sides of anything you would use for a duct (might suck/collapse notebook paper, but I think you'd use something stronger).


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 Post subject:
PostPosted: Thu Apr 17, 2003 11:53 am 
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Location: Cambridgeshire, England
Here's a link to Bluefront's mod.
http://forums.silentpcreview.com/viewtopic.php?t=3939
Although, he has his duct draw air from outside and blows onto his Alpha. It'd be easy enough to reverse the fan direction though.

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