Forced Convection

Doin' a fan controller. Someone on the project mentioned the need for the project to work over altitude.

My Google-Fu is failing me; searches seem to lead to a lot of irrelevant information. Anyone happen to know of a good article that might illuminate the subject of fan speed vs. cooling with altitude effects taken into account?

Thanks in advance...

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Reply to
Tim Wescott
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I never use Google, but these seem to be relevant:

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Reply to
krw

cooling should be directly related to mass flow, that's how hot wire mass f low meters work in cars etc.

easy to look up density vs. altitude, but I don't know how you'd get the vo lume moved by the fan vs. density other than measuring

I guess as long as you are around the working range of the fan the power vs . mass moved is sorta constant

Reply to
Lasse Langwadt Christensen

On one recent design, we used a box exhaust fan that had about 3x the required air flow. The uP could sense PCB temperature and control the fan speed. That makes things quiet in normal use but cranks up as needed. It also sort of stabilizes PCB temperature so reduces worst-case drifts a bit.

How high do you need to go? 6000 feet is only a 10%ish density drop.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

On a sunny day (Sat, 03 Jun 2017 08:54:57 -0700) it happened John Larkin wrote in :

But it is also colder up there:

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Reply to
Jan Panteltje

Well, I discovered something the hard way. We needed to cool about 100 photomultiplier tube bases, and keep it light-tight. So, I worked out how much air volume I had to move, and we bought this insane regenerative blower. The equipment was tightly packed in an aluminum housing. Well, the discovery was that the air turbulence transported most of the heat to the case, so we could have done with about 1/4 the blower pressure and volume.

With decreasing air pressure, the blower will run faster, and you need to move more air volume through the housing to provide equivalent heat removal. It is mostly about air MASS.

Jon

Reply to
Jon Elson

The blower motor type matters, of course; if it's a synchronous motor, the speed is NOT gonna change with load. DC fans have all sorts of motor character. Centrifugal fans, too, deliver pressure according to the density of the air... not so for blade fans, which deliver velocity.

Reply to
whit3rd

Is it legal to force convection?

Reply to
John S

No, only conduction.

Reply to
krw

to conduct a convection inversion is irreversible bad behaviour.

Reply to
Sjouke Burry

Yes, but it's not legal to coerce convection.

I'm unsure about convection of coercion though.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

John S wrote on 6/4/2017 4:50 PM:

Certainly not if it is underage...

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Rick C
Reply to
rickman

Arguably, but it is perfectly legal to arrange convection.

Reply to
Tom Gardner

------------------

** There is always *someone* who thinks there is a problem to be solved when in fact there is not. High altitudes have not only lower air density but also very LOW temperatures.

At 18000 feet the air density is halved, but the air temp is about 40C cooler than at sea level.

Forget it.

.... Phil

Reply to
Phil Allison

It takes a lot of assumptions to justify that statement. One might argue that the reason you need a fan is because the air being fanned is a lot hotter than that. I'd want more info before I jumped to that conclusion.

If the cooling is related to mass flow, you're gonna need twice the velocity to get the same mass flow at half the ambient pressure. Sounds like a fan aerodynamics problem.

Reply to
mike

---------------

** Which statement ?

** Fraid that is gobbldegook.

** Cooling depends strongly on the air temp.

Try thinking this through and posting in normal English in full sentences.

.... Phil

Reply to
Phil Allison

over altitude? like an airplane? less drag in the thin air, so greater fan speed. no, think about blowing across a cup of hot coffee. fast, lower pressure air. blow harder, coffee cools even faster. BRB...

Reply to
Good Guy

This may be true for people climbing mountains, but why would they carry with them equipment consuming so much power that would require fans ?

Millions of people live permanently above 4000 m and usually they have some heating systems for keeping the indoor temperature at around

+20C. Thus, the fan input temperature would be +20 C.
Reply to
upsidedown

snipped-for-privacy@downunder.com wrote on 6/6/2017 3:31 AM:

If someone can't figure this out on their own, do you really thing explaining it will help? Well, in the case of most, yes. But, Phil? Really?

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Rick C
Reply to
rickman

( aka a colossal bullshit artist)

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** And all kinds of unpressurised aircraft and remote monitoring locations.

( snip utter s**te)

** Really ?

Like in the high Andes and Himalayas.

** Like dung fuelled stoves and live goats in their beds.

That what you have at home ?

Wot a steaming great f****it.

.... Phil

Reply to
Phil Allison

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