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Fan direction

Hi,

I rarely design anything that needs active cooling so when it comes to fans, my approach is largely inconsistent (read that as: "varies with day of week and phase of moon!).

When it comes to *case* fans ("CPU"s, external disk drives, printers, etc.) I tend to opt to push air

*into* the case drawing it through a filter, first (i.e., pulling air *out* usually results in drawing all sorts of dust and crud into the case from the area surrounding the device -- since most inlets are not filtered).

Of course, that depends on what's inside the case "behind" the fan. As well as the roles of any other fans in the case.

I'm adding a couple of (tiny) fans to some external disk enclosures and found myself facing this question, yet again. :-/

So, can anyone offer a nice mnemonic / *rule* that I can just commit to memory and never have to waste time thinking about crap like this again?? :)

Thanks!

--don

Reply to
D Yuniskis
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You're underthinking it. Fans are all about getting air where you want it. You need to control the airflow in and out to make sure the hot things get cooled. A general rule of thumb#1 is "plugged filters don't cool nuthin'". Rule of thumb#2 is "All filters get plugged".

Reply to
mike

If possible, always preferred positive pressure.

The only time I might vary from that would be some physical reason the hottest device can be located behind some vents but the fan must be located somewhere else. Then, negative pressure will draw external air onto the heat source.

Reply to
1 Lucky Texan

When one has a supply in a case that is to be mounted within a case, sometimes one will see the other circumstance.

Typically, however, one wants positive pressure to keep the dust build up down and more controllably located.

Reply to
TheGlimmerMan
[snip]

Dust coated heat sinks and other components don't colvery well either. One filter is easier to clean than the entire innards a PC or test gear.

--
Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.
Reply to
Paul Hovnanian P.E.
[snip]

Sorry. No rule. But in addition to the filter logic, think about turbulence. Air flow is air flow and logically should cool just as well going in either direction. But that flow might be relatively laminar coming from one direction and quite turbulent when leaking in through a bunch of small openings going the other way. But which way is which one can't say without inspecting the equipment.

This advice is only applicable for equipment that has been carefully designed from a thermal point of view when someone decides that reversing the fan direction isn't significant.

--
Paul Hovnanian  paul@hovnanian.com
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Have gnu, will travel.
Reply to
Paul Hovnanian P.E.

Dust is democratic. It doesn't much care what direction the air is blowing.

There is a big difference in the air flow at the inlet and outlet sides of a fan. At the inlet, air drifts into the fan gently from all directions. At the outlet, there's a focused stream of air, very directional, turbulent, often with a hole in the middle. In some situations, the focused air flow will whack one hot gadget and miss others. Sorry, it's not simple.

I usually have the fan exhaust an enclosure, and arrange air inlets to meter various amounts of air to the things that need it. But then, I don't use filters.

ftp://jjlarkin.lmi.net/Chimera.JPG

Air is the most peverse substance on the planet [1]. Air flow is usually nothing like what one expects. Things near a fan can be in still air. Flows are often in the opposite direction from the obvious. Heatsinking in an air flow is often bizarre. What we do is build physical mockups and analyze them with air flow meters and incense sticks.

John

[1] which is why weather and climate predictions are generally useless, or worse.
Reply to
John Larkin

Push the air in, and as Paul pointed out, Laminar air flow for best cooling. Or Impingement cooling for pin fin heat sinks, see

Also, inlet area should be 1/2 the outlet area.

Cheers

Reply to
Martin Riddle

Em 27/12/2010 23:21, Martin Riddle escreveu:

Where in the Earth does this really happen? In order to decrease the air to heat sink resistance *turbulent* air flow must be insured.

--
Cesar Rabak
GNU/Linux User 52247.
Get counted: http://counter.li.org/
Reply to
Cesar Rabak

Absolutely. Laminar flow is great for lift, not so good for cooling.

Reply to
Jim Stewart

I stand corrected, 'Turbulent air flow' is correct. ( its what I was thinking, but not typing ;( )

Cheers

Reply to
Martin Riddle

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