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 Post subject: Airflow basics
PostPosted: Sat Dec 21, 2013 3:32 am 
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I am highly confused by the way current cases and CPU heatsinks are designed wrt airflow.

I think the basic idea of air cooling is to make sure that cold air passes over hot components, so that it receives heat with those components and takes it away, cooling the components.

If that basic idea is right (is it?) all the systems should be designed either to push cold air into hot components or to pull hot air away, hoping that cold air will replace it. But this basic idea seems to contrast with the way systems are actually designed.

Horizontal CPU heatsinks like the Noctua NH-L12 push hot air over the CPU. The top fan of the NH-L12 pulls cold air and pushes through the heatsink, where it is heated and then pushed by the bottom fan to the CPU. Why?

In most tower cases, GPU fans pushes hot air downwards, while hot hair physically goes upwards. This means that the hot air is being pushed against its natural flow. Shouldn't this make the cooling worse because you are pushing down air what will soon came back up? This is made even worse by the fact that some cases have bottom fans that pushes even more air against the GPU. There are cases where the mainboard is rotated so to have the GPU fans point upwards, but the SPCR reviews of these cases say that GPU will be cooler when pointing downwards.

Most current cases like the Fractal Design Defini Mini have the PSU under the GPU, so the GPU pushes hot air toward the PSU. Why aren't the PSU positioned so that there is no hot air going against it?

Most current cases have front fans that pushes air towards the back of the case, but the PSU is in the back and pushes air towards the front. Doesn't this create a turbolence inside the cases, especially cases that are not very deep?


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 Post subject: Re: Airflow basics
PostPosted: Sat Dec 21, 2013 3:52 am 
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I think there's a few things here where you may be confused.

silentbobbo wrote:
Horizontal CPU heatsinks like the Noctua NH-L12 push hot air over the CPU.

The air going into the CPU cooler is not hot. What has it already been used to cool? In most systems if the airflow is coming through the front it might go over a disk drive, then over the RAM and some motherboard components but the heat it will pick up from them is trivial so the air is still relatively cool. Look at the motherboard temperature sensors and you'll generally see some pretty low temps that show this. So long as the air going through the CPU heatsink is cooler than the CPU is, the CPU will be cooled so this setup works well.

silentbobbo wrote:
In most tower cases, GPU fans pushes hot air downwards, while hot hair physically goes upwards.

Most GPU coolers do not blow air downwards. Why did you think that? They generally push air upwards and backwards/sideways through the GPU heatsink. With designs that expel hot air out of the back this works well for overall system cooling. The fact hot air rises makes little difference as chimney effect only really makes a big difference when you have tall chimneys over which there can be a big temperature gradient.

silentbobbo wrote:
Most current cases like the Fractal Design Defini Mini have the PSU under the GPU, so the GPU pushes hot air toward the PSU.

Leading on from my previous point, this won't happen as the GPU does not blow hot air downwards.

silentbobbo wrote:
Most current cases have front fans that pushes air towards the back of the case, but the PSU is in the back and pushes air towards the front.

What PSUs have you seen that pull air in the back? Every PSU I know with a fan pushes/pulls air through it and out of the back.

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 Post subject: Re: Airflow basics
PostPosted: Sat Dec 21, 2013 4:19 am 
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edh wrote:
I think there's a few things here where you may be confused.


Thanks for your answers, they really helped.

Just one thing:

edh wrote:
silentbobbo wrote:
Horizontal CPU heatsinks like the Noctua NH-L12 push hot air over the CPU.

The air going into the CPU cooler is not hot. What has it already been used to cool? In most systems if the airflow is coming through the front it might go over a disk drive, then over the RAM and some motherboard components but the heat it will pick up from them is trivial so the air is still relatively cool. Look at the motherboard temperature sensors and you'll generally see some pretty low temps that show this. So long as the air going through the CPU heatsink is cooler than the CPU is, the CPU will be cooled so this setup works well.


Why isn't the air going onto the CPU not hot?

From I understand it (and from what I read in these forums), the top fan of the Noctua NH-L12 pushes the cold air of the case towards the heatsink that is hot because it is dissipating the heat produced by the CPU. Doesn't this cold air pushed from above become warm once it goes thought the heatsink's fins? Then the bottom fan takes this heated air and pushes it down to the CPU. Isn't the final effect that hot air ends up begin pushed towards the CPU?


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 Post subject: Re: Airflow basics
PostPosted: Sat Dec 21, 2013 5:18 am 
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Cooling a computer is not rocket science. Direct some air at the hot stuff, make sure the hot air has a place to go. Making fans push against natural convection is usually not a big deal and delta-T between "hot" and "cold" air is also usually small. In your example with the Noctua NH-L12, the hot air that gets pushed down is actually quite cool. You would have to bend the laws of physics to make it hotter than the CPU. :)

Different cases direct airflow in different ways and while the basics are all the same, pure theoretical reasoning in a very general way is not that useful in the specific case. You usually get one type of case for the high end gaming system, another one for the light desktop system etc.

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 Post subject: Re: Airflow basics
PostPosted: Sat Dec 21, 2013 7:46 am 
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Also, forced convection (aka fans) in a PC trumps natural convection every time. Even the slowest case fan will disrupt any natural convection - this is why fanless top vents are just a source of noise in a system with front to back cooling.

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