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what is a "good" amount of CFM's?

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dito

Member
Joined
May 28, 2009
Location
Vacaville, CA
what is a "good" amount of CFM's to go through a case for "proper" cooling? I guess what I'm asking is how much CFM's should I be putting into a mid-tower case? Also, should it be more exhaust CFM's than intake or vice-versa?:shrug::confused:
 
Excellent question! ...I have no idea =D. However, I'd probably say that although its important to have good airflow (i.e. intake to flow through), that it's probably more important to have better exhaust. So where your GPU/CPU are located, should be where the most exhaust lies. Eg, for myself, I run a Antec 300, with 2 fans very close to my CPU, the default ones running both on low. When I swapped into this case, (without changing anything else) I noticed a drop of 5 degrees. I added some basic intake fans, and the difference was less, 0-2 degrees, however they're at low cfm (5 volt of a 2 pin molex) for the sake of noise. XD

Really its up to you and your hardware, and also where its located in the room! If you know your stuff gets really hot, higher cfm = better naturally, if your stuff is pretty cool as it is, then jsut a decent amount is fine, unless you're hardcore about those low temps :p. My 3 cents, yep, 3, better than 2! Ha! Lol :p
 
I would have to say it depends upon the amount of heat produced in your case to determine that, so it would be different for everyone. And as to the more intake or exhaust, that is an ago old question that has quite the discussion... Here.
 
As much airflow as possible .......... as mentioned above positive or negative is arguable with no real answer but as far as cfm's are concerned you'll never cool your system too much and as long as you dont have too little then the only real factor would be noise .......... Deltas very noisy lots of air being moved :) the fans i'm going to get are rated at about 80 cfm and a few of them in my estimation should be enough to cool a simple system :)
 
Totally depends on case, cleanliness of case (less wire management will = more need for more flow), as well as what is in the case producing heat.

a P1 would need a single 80 at most.

a quad at 4ghz with 10 SATA drives would need substantially more airflow.

A properly done case might not need as much CFM if the setup is done correctly.

Personally, I just put enough that the case should feasibly be able to fly around the room on its own.
 
Ditto from my personal hovercraft, hehe, mines louder than I like but oh well. If you count GPU fans etc think about 13 in here now. I usually have headphones on or the amp reasonably loud when watching movies and its off to the side of the room from the big screen, so not too bad.

:)

Need some wings and landing gear on this thing.

:beer:
 
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My case is older, and only has 4x80mm fan mounts for case fans... so my soon to be overclocked quad, x1950xt, and 6 hard drives are hurting for cooling. There are as a result a few fans inside the case just to blow air on things.

In total, I have... 12 fans in my case... But I need ALL of them I SWEAR!!!!
 
Personally, I try to balance CFM's with dBa produced. I do not like a loud case, but I also do not like an overheated system. Depending on what kind of grills your fans have in front of them, you will also want fans that have a decent static pressure to get the air through the grill (what is a decent static pressure? I have no clue :p). I run two 120mm's (front intake/rear exhaust) that I think are rated around 48 cfm and one 80mm (side intake) along with the 120mm in the PSU, the 120mm on the CPU and the 8800GTS OEM cooling fan.
 
I have 3 120mm fans + the 120mm CPU fan. The 3 case fans are near death and need replacing and all provided less than 50 CFM when they were new. My system runs at 35C idle and peaks at 47C. Once I get some time and can pick out 3 replacement fans I suspect those numbers will drop a bit. I always shoot for around 35-50 CFM per fan ... well based on the specs we're provided anyway.
 
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exactly, the question should be how much noise can you tolerate :D

the more cfm, the louder your setup.. i have 3x 38x120mm panaflo nmb-mat with 115cfm a piece lined up in a row, that alone is like sitting beside a vaccuum lol ;)

you get used to it sure, but i can see the appeal of water :)
 
exactly, the question should be how much noise can you tolerate :D

I don't if this is a very good answer. :)

As stated above, it depends on the PC case and its components. However, we can do a simple calculation to approximate how much airflow we will need. For example, lets assume that we have a fairly powerful PC that is putting out 300watts of heat. Lets also say that we want our PC to operate at an average temperature of 32 Celsius at steady state (constant heat input, constant temp, constant airflow).

So if we operate at steady state we know that the heat generated by the PC must be removed at the same rate by the airflow, or

Qgenerated = Qremoved.

We know the amount of heat being generated, but we need to find the heat removed. The amount of heat removed will be the mass flow rate of the air multiplied by its specific enthalpy, or

Qrem = mass flow rate (M) x change in enthalpy (dh).

Since there are so significant changes in pressure this equation can be stated as;

Qrem = density (p) x volumetric flow rate (V) x specific heat of air (cp) x change in temperature (dT)

The volumetric flow rate, V, is what we are solving for. The change in temperature is a function of what we want our PC's average temperature to be at (32C) and the ambient air we are blowing into the case (25C). So solving for V we get,

V = Qrem / (p x Cp x dT)

Plugging in all our values we get,

V = 300 J/s / [1.184 kg/m^3 x 1012 J/kg*K * (32-25C)] = 0.0358 m^3/s

or 75.9 CFM.

So to keep our 300 watt space heater at 32 C we only need about 76 CFM of airflow. There are obviously a lot of assumptions that are made with this model but it gives you an idea of the minimum amount of airflow required to keep your PC cool. If you have a good airflow setup I would probably multiply this number by 1.5-2 to give yourself a good factor of safety. For anyone that has a calculator handy, what happens to the required airflow when you want to cool the PC by another 2 degrees? How about 4 and 6 degrees? :confused:
 
lol ok :D

i dont think 79cfm is very much :)

if you think thats enough to cool your pc, then go for it!

300w is nothing tho, my pc chews alot more then that when its loaded :thup:
 
lol ok :D

i dont think 79cfm is very much :)

if you think thats enough to cool your pc, then go for it!

300w is nothing tho, my pc chews alot more then that when its loaded :thup:

I can only imagine what your rig sounds like. lol I've been down that road and won't ever be going back. You do sort of get used to it but once you go quieter you can't really go back.
 
Agree with that. Two systems ago when I had my watercooling rig, I actually used a 120v 120mm fan to cool my radiator. That thing could push some serious air but it was like a jet engine beside me. Once I got a case that could handle 120mm fans normally, I was overjoyed.
 
I don't if this is a very good answer. :)

As stated above, it depends on the PC case and its components. However, we can do a simple calculation to approximate how much airflow we will need. For example, lets assume that we have a fairly powerful PC that is putting out 300watts of heat. Lets also say that we want our PC to operate at an average temperature of 32 Celsius at steady state (constant heat input, constant temp, constant airflow).

So if we operate at steady state we know that the heat generated by the PC must be removed at the same rate by the airflow, or

Qgenerated = Qremoved.

We know the amount of heat being generated, but we need to find the heat removed. The amount of heat removed will be the mass flow rate of the air multiplied by its specific enthalpy, or

Qrem = mass flow rate (M) x change in enthalpy (dh).

Since there are so significant changes in pressure this equation can be stated as;

Qrem = density (p) x volumetric flow rate (V) x specific heat of air (cp) x change in temperature (dT)

The volumetric flow rate, V, is what we are solving for. The change in temperature is a function of what we want our PC's average temperature to be at (32C) and the ambient air we are blowing into the case (25C). So solving for V we get,

V = Qrem / (p x Cp x dT)

Plugging in all our values we get,

V = 300 J/s / [1.184 kg/m^3 x 1012 J/kg*K * (32-25C)] = 0.0358 m^3/s

or 75.9 CFM.

So to keep our 300 watt space heater at 32 C we only need about 76 CFM of airflow. There are obviously a lot of assumptions that are made with this model but it gives you an idea of the minimum amount of airflow required to keep your PC cool. If you have a good airflow setup I would probably multiply this number by 1.5-2 to give yourself a good factor of safety. For anyone that has a calculator handy, what happens to the required airflow when you want to cool the PC by another 2 degrees? How about 4 and 6 degrees? :confused:



What :confused: which school did you go to they never taught us anything like that where I went :beer:
 
I don't if this is a very good answer. :)

As stated above, it depends on the PC case and its components. However, we can do a simple calculation to approximate how much airflow we will need. For example, lets assume that we have a fairly powerful PC that is putting out 300watts of heat. Lets also say that we want our PC to operate at an average temperature of 32 Celsius at steady state (constant heat input, constant temp, constant airflow).

So if we operate at steady state we know that the heat generated by the PC must be removed at the same rate by the airflow, or

Qgenerated = Qremoved.

We know the amount of heat being generated, but we need to find the heat removed. The amount of heat removed will be the mass flow rate of the air multiplied by its specific enthalpy, or

Qrem = mass flow rate (M) x change in enthalpy (dh).

Since there are so significant changes in pressure this equation can be stated as;

Qrem = density (p) x volumetric flow rate (V) x specific heat of air (cp) x change in temperature (dT)

The volumetric flow rate, V, is what we are solving for. The change in temperature is a function of what we want our PC's average temperature to be at (32C) and the ambient air we are blowing into the case (25C). So solving for V we get,

V = Qrem / (p x Cp x dT)

Plugging in all our values we get,

V = 300 J/s / [1.184 kg/m^3 x 1012 J/kg*K * (32-25C)] = 0.0358 m^3/s

or 75.9 CFM.

So to keep our 300 watt space heater at 32 C we only need about 76 CFM of airflow. There are obviously a lot of assumptions that are made with this model but it gives you an idea of the minimum amount of airflow required to keep your PC cool. If you have a good airflow setup I would probably multiply this number by 1.5-2 to give yourself a good factor of safety. For anyone that has a calculator handy, what happens to the required airflow when you want to cool the PC by another 2 degrees? How about 4 and 6 degrees? :confused:

Very nice answer, but it does not apply directly as you did state that it might not. What are the hot spots? usually around the drives and back side of the MOBO. It does not matter how much air is moved all you get are eddies. A good study of the dynamics of air flow is what is needed.

1. positive v/s negative (pushing in and pulling out respectively): a filtered positive air flow system keeps the case cleaner hence less dust on HS.

2. decide where you want the air to go. (you cant put 500 CFM into a case that cant efficiently evacuate it and your fans dont appreciate either) Pop out all of the blanks in the front of the case (leave the outer covers on) to cool HDD, as well as some of the pop outs or expansion slot covers near the video card. To prevent an insect infestation put some screen in using goop or another hard drying glue.

3. deflect the hot air from the PSU up as to prevent it from re-entering the case.

4. if you can find a decent vented cover pop a hole in the top of the case above or near the HDD.

5. install as many fans as you can (with filters kinda like these: http://www.quietpcusa.com/Fansis-Washable-120mm-Fan-Dust-Filter-P298C0.aspx)

6. Antec spot cool can help with the dead spots: http://www.antec.com/Believe_it/product.php?id=ODA=

And that is all there is to air cooling you case with out using my olde favorite The Directed Air AC mod :cool:
 
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Very nice answer, but it does not apply directly as you did state that it might not.

Correct. What I stated was the MINIMUM amount of airflow that is required. A "good" amount will obviously be higher unless you have an extremely well designed case.

What which school did you go to they never taught us anything like that where I went

I'm currently getting my masters in mechanical engineering. All I did was just use the first law of thermodynamics. Very simple first year stuff really.
 
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