How much CFM do I need for my case fan?

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pannayar
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How much CFM do I need for my case fan?

Post by pannayar » Sat Apr 04, 2009 11:28 pm

Dell Dimension 4600, Pentium 4.

Stock fan (92 mm) got very noisy, I have been using a Scythe Kama Flex 1600 rpm, 30 dB fan - $13. Quiteness is my main criteria now. I tried the silenx 32 CFM 11 dB fan, it is slightly quiter but expensive ($27). I am tempted to get the silenx 14 dB 42 CFM fan...

My question is, how much airflow / CFM is sufficient? Is 30 CFM enough? I dont plan to use the PC for extended periods of time.

Thanks!

dukla2000
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Post by dukla2000 » Sun Apr 05, 2009 12:23 am

Q = (1.76 * H) / T

where Q is airflow in cfm, H is Heat dissipated in Watts & T is Temperature rise in Centigrade.

Note headline fan airflow specs are in free air (no back pressure) - in reality your config has back pressure. To cause an argument assume your fan airflow will be half its free air spec.

pannayar
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Post by pannayar » Sun Apr 05, 2009 12:30 am

I don't know what to plugin for H & T in that formula.

What I interested in is just "how much case fan CFM is ok" for the average user...?

pcy
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Post by pcy » Sun Apr 05, 2009 1:28 am

Hi,


30 CFM is a good working figure...

dukla2000 wrote:Q = (1.76 * H) / T

where Q is airflow in cfm, H is Heat dissipated in Watts & T is Temperature rise in Centigrade.

Note headline fan airflow specs are in free air (no back pressure) - in reality your config has back pressure. To cause an argument assume your fan airflow will be half its free air spec.

A while ago I spent a happy hour googling away to find the physical constants necessary to calculate this formula, and came up with more like:

Q = (1.79 x H) / T

but of course it depends on the temperature, pressure humidity of the air. I actually use

Q = (1.8 x H) / T

because that's quite accurate enough, and I need to do this conversion in my head...


However, I don't agree that a case fan will deliver only half it's "free air" CFM. The air through a CPU cooler is another matter - 20% to 33% of the free air figure is quite normal there - because a cooler really does offer substantial airflow resistance. The backpresure caused by airflow through the case is negligable in my experience.



But... back to the question.


H is the heat output of the non-terminal components of your PC, where by "terminal component" I mean a component whose heat output exhausts directly out of the case.

If you have a conventional cooling setup, with the CPU cooler blowing down onto the motherboard, and you don't have a mobster gaming graphics card, then the only terminal component is the PSU, so H is the total power consumed by your computer (as distinct from the power the PSU draws from the mains - the difference being the heat produced by the PSU, which is indeed exhausted straight out the back).

I don't know the value for H on your comuter, but assuming it's a typical P4 with a modest graphics card and a single HD, 160W when running flat out seems a reasonable guess, or about 290 CFMC.

Note: CFMC is power expressed in airflow (CFM) x Temp rise (C).


That means that if you had 30 CFM of air blowing through the case, the air temp near the exit point would be just under 10C above the air inlet (i.e room) temperature.

The problem is that this is the temperature of the air being used to cool the CPU, Mobo and Graphics Cards. You want to keep that down as much as possible...

However, there is a law of diminishing returns here - if you double the airflow the temperature increase will halve (60 CFM and you get a 5C rise). Even at 60CFM you are moving out od the area of "quiet computing" and beyond that the improvement in temperature is marginal, whereas the increase in noise is huge.

In addition this figure of 290CFMC is an over-estimate, for two reasons:

1. It represents the power used when running the computer flat out
2. The typical arrangement of a PC, with exaust fans close to the CPU at the back, means that some of the hot air coming off the CPU does exhaust straight out of the case and only some re-circulates. So the CPU is partially a "terminal component".

Put these two factors to-gether (which is a fair assumption so long as you don't run CPU intensive tasks for minutes or hours at a time) and the value of H could halve. In that case 30 CFM would give you a an air temperature in the back of the case around 5C above room temperature, and you could possibly consider dropping the airflow throught the case to 20 CFM.



Peter

pannayar
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Post by pannayar » Sun Apr 05, 2009 3:51 pm

Thanks for the detailed explanation. I appreciate it. I think I understood it to some extent...

In summary, 30 CFM is an acceptable flow for a standard desktop PC? Btw, if the spec on the fan says 30 CFM will it be pretty close to that or could it be way off like 20 or so...?

Tephras
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Post by Tephras » Sun Apr 05, 2009 4:27 pm

No, don't go by the fan spec. Look at the fan reviews by SPCR and you will see that there always are a difference between the measured CFM and the airflow rating. An example: Scythe Slipstream 1200RPM have a rating of ~68CFM but the measurement says 46CFM.

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Post by NeilBlanchard » Sun Apr 05, 2009 6:30 pm

Hi,

The specs from the manufacturer are not very accurate. You have to go by the results. The Scythe fan is probably the best bet -- the other's noise ratings are dodgy, and the price is no guaranty of anything other than you'll spend a lot of money.

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Post by thejamppa » Sun Apr 05, 2009 8:16 pm

less objects there is front of air flow ( cables etc ) less air flow you need to adequately cool components. If you have restrictive intakes ( filters etc ) then higher pressure fan might be better than adding cfm's.

It all really needs to be tested in your case. My main rig is easily cooled off with two undervolted Slipstream 800 fans. Including passively cooled HD 3850.

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Post by mesasone » Thu Jul 02, 2009 1:47 am

I'm just going to bump this thread instead of starting a new one, hope nobody minds.

I am curious about what kind of air flow I should be aiming for in my new build. I will be using the Antec 300 case and will be adding two front intake fans of the Scythe Slipsteam variety. I am trying to decide between the 800 and 1200 RPM versions, which move roughly 40 and 68 cfm at 10 and 24 dBA respectively.

I will be running an i7 chip and would like to overclock it a decent amount - however high I can get it while keeping temps and noise under control (I'd like to hit 4 ghz; doesn't hurt to be a dreamer, does it? :o ) The goal is to be approaching "whisper quiet", or more realistically moderately quiet. I'm just not sure which fan will meet my needs, as I'm not sure what kind of ballpark figures I should be looking at for cfm. I'm using a Sapphire 4870 x-vapor for graphics, which is supposed to run at between 20 and 30 dBA depending on fan speed, so I'm guessing the noisiest component will be the PSU. Would the PSU, Antec EA 650, effectively drown out the noise of the 1200rpm slipstreams to the point where I would see little or no reduction in total noise by going with the 800 rpm version? Will two 40 cfm fans move enough air to meet my needs?

Thanks in advance for any input.

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Post by MikeC » Thu Jul 02, 2009 10:04 am

mesasone --

The original question of this thread always struck me as unrealistic and impractical. There's no way of replying accurately. Fan specs only state CFM w/o any impedance. Even if "realistic" CFM data was available, you'd have to be looking at it on an impedance/airflow curve , and how could you determine how much impedance you had in your intended usage? Yes, you could be scientific, but it would require far more specific data than you have or can pull together.

Take the realistic approach: If overclocking is a goal, go for the best CPU heatsink and mate it with a quiet fan that can also run fast. Ditto the case fans. Add a fan speed controller if your mobo doesn't give you good enough control. Now you're equipped with high cooling capability but you can dial down your fan speeds if lower noise is desired or if the cooling doesn't require as high airflow. It's a tweaking game.

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