Case fan sizing

[Craig Palme]

I'm going to be replacing my case fan. The one I have is too loud. At least
I think so. At times it's annoying other times not so much. What I'm
interested in knowing is the amount of air flow, cfm, should I look for as a
minimum. Is there some general rule of thumb other than the max you can get.
I know some of the quieter fans are that way because the speed is slower and
the air flow is less. I don't want to undersize the fan.

Thanks

Craig

 

[Paul]

I'm going to be replacing my case fan. The one I have is too loud. At least
I think so. At times it's annoying other times not so much. What I'm
interested in knowing is the amount of air flow, cfm, should I look for as a
minimum. Is there some general rule of thumb other than the max you can get.
I know some of the quieter fans are that way because the speed is slower and
the air flow is less. I don't want to undersize the fan.

Thanks

Craig

There is a formula here:

http://web.archive.org/web/20030215003614/http://www.chassis-plans.com/cooling_and_noise.html

CFM = 3.16 x Watts / allowed_temp_rise_deg_F

The watts is not your power supply watts, it is the amount of power
actually being used. That can vary from say 100W on an older computer,
to a lot more on a newer computer (like one with SLI video cards).

If the room temp is 25C, a well cooled case might be 32C, which
is a 7C or 10F temperature rise. The primary reason for cooling
the computer case, is to keep the disk drives cool. They are
probably the least able to take high temperatures in the long
term, and hard drives will fail faster if they are not kept
at a reasonable temperature. A secondary reason, is some
processors throttle back their computing rate if they get over
70C, and by keeping the case air temp down, it makes it easier
for the CPU cooler to do its work.

If you have something to control fans with, such as a Zalman
Fanmate2, or one of those drive bay boxes with the adjustment
knobs on the front, you can adjust the voltage fed to the case
fan, and slow it down a bit. Using the motherboard temperature
sensor, and knowing the room temperature, you should be able
to adjust the fan to achieve a compromise between noise
and heat.

A fan doesn't have to work as hard, if there is adequate vent
space for air input. On my Antec Sonata, I had to remove some
of the plastic from the front of the computer, in order to
get good cooling. There is no sense in a fan spinning like it
was running a vacuum cleaner - to be effective, a fan with a
lot of CFMs rating, needs an equally large vent area for the
air to get into the box. With decent vent space on the front,
it may mean the rear exhaust fan can be slowed down a bit,
without causing the case temperature to rise.

So the formula above doesn't tell you about the other little
details.

Paul

 

[Dave]

I'm going to be replacing my case fan. The one I have is too loud. At
least I think so. At times it's annoying other times not so much. What I'm
interested in knowing is the amount of air flow, cfm, should I look for as
a minimum. Is there some general rule of thumb other than the max you can
get. I know some of the quieter fans are that way because the speed is
slower and the air flow is less. I don't want to undersize the fan.

Thanks

Craig

OK, you need either balance, or -positive- airflow. Overall you want more
air pushed into your case than is pulled out of it (don't forget the power
supply is pulling air out)

So before you look for an answer to your question, you have to consider
whether the fan you are replacing is sucking in or blowing out, and how does
that relate to other fans in your case?

Theoretically, if the fan you want to replace is the only case fan, and air
flow of that fan is OUTWARD, then you've got a problem. That's because you
already have an imbalance of airflow in the wrong direction. So you need a
higher CFM fan than what you've got (or more than one), and it will have to
be installed somewhere else.

But then, if the fan you want to replace is pulling air into the case, and
that is the ONLY case fan, then you can replace it with just about anything,
except that you want to find one with a CFM rating equal to or higher than
the one you are currently using. This makes your task difficult if your
goal is to reduce noise.

But your third option is to replace one noisy fan (pulling air into the
case) with 2 QUIET fans (pulling air into the case). The reason this will
work is, one noisy fan can easily create more noise than 2 quiet fans. So
you can increase the number of fans and DECREASE the overall noise level.
But that would assume that your case will support adding more fans than you
currently have. My most recent build, I chose a case with two cooling fans
on the SIDE, and replaced them with two quiet cooling fans. But then, I
chose the case at the same time as the cooling fans. Your case might not
have side mounts for a pair of matched (quiet) cooling fans.

In general . . . you only NEED one case fan, so it should be a quiet one
(80mm or 120mm) located in the front of your case, pushing air into the
case. This will balance the airflow of the cooling fan on the power supply,
which is sucking air out of the case. If there is an imbalance, you want
more air pushed into the case than is pulled out. Having negative pressure
inside the case will suck dust through your vital components . . .
especially external drives. -Dave

 

[Terry]

In general . . . you only NEED one case fan, so it should be a quiet one
(80mm or 120mm) located in the front of your case, pushing air into the
case. This will balance the airflow of the cooling fan on the power supply,
which is sucking air out of the case. If there is an imbalance, you want
more air pushed into the case than is pulled out. Having negative pressure
inside the case will suck dust through your vital components . . .
especially external drives. -Dave

I don't quite follow this argument. If you have positive pressure, it
will suck that same dust in through the fan and then push it out
through your vital components ...

Terry

 

[nos1eep]

It is further alleged that on or about Sun, 12 Mar 2006 23:53:42 GMT,
in alt.comp.hardware.pc-homebuilt, the queezy keyboard of Terry
<[email protected]> spewed the following:

|
|>In general . . . you only NEED one case fan, so it should be a quiet one
|>(80mm or 120mm) located in the front of your case, pushing air into the
|>case. This will balance the airflow of the cooling fan on the power supply,
|>which is sucking air out of the case. If there is an imbalance, you want
|>more air pushed into the case than is pulled out. Having negative pressure
|>inside the case will suck dust through your vital components . . .
|>especially external drives. -Dave
|
|I don't quite follow this argument. If you have positive pressure, it
|will suck that same dust in through the fan and then push it out
|through your vital components ...

It is quite easy to install a HEPA filter in front of the intake fan.
This solves the problem. I fail to see how this applies to external
drives, but then I am not Dave.

 

[Mike T.]

I don't quite follow this argument. If you have positive pressure, it
will suck that same dust in through the fan and then push it out
through your vital components ...

Terry

To a point, that's true. The difference is, the dust will tend to settle
close to it's entry point. That's how you get big piles of dust in the
bottom front of the case, in cases that have a cooling fan installed in that
location, pulling air in. In any case with cooling fans, dust is going to
get into everything, eventually. But positive pressure will tend to keep
more of the dust closer to the cooling fans. (Cooling fans are cheap and
relatively easy to clean)

Or put another way, it's easier to put a dust filter on the inlet for a
cooling fan than it is to put a dust filter over the front of your external
drives. -Dave

 

[Terry]

To a point, that's true. The difference is, the dust will tend to settle
close to it's entry point. That's how you get big piles of dust in the
bottom front of the case, in cases that have a cooling fan installed in that
location, pulling air in. In any case with cooling fans, dust is going to
get into everything, eventually. But positive pressure will tend to keep
more of the dust closer to the cooling fans. (Cooling fans are cheap and
relatively easy to clean)

Dust will tend to settle when the air velocity drops. Just like snow
settles behind a snow fence. I would not expect dust to settle behind
an intake fan -- that is likely to be the highest velocity air in the
case. It might settle behind a large card or component, but I would
most expect to see it build up at the back of the floppy and CD drive,
as the air slows down on it's way out. Not much better than the
original situation (output fans).

Or put another way, it's easier to put a dust filter on the inlet for a
cooling fan than it is to put a dust filter over the front of your external
drives.

That's true, and if you did use a filter you would see much less dust
inside the case. In the "old days", most electronics including
computers came with input fans, with washable filters. But nobody ever
changed or washed the filters, so they ended up restricting air flow
and causing overheating. And if you did clean them, after about 5
years the foam started to disintegrate, and suddenly you were pulling
little bits of foam into the equipement. It's pretty unusual today
to see even laboratory or medical equipment with filters, because the
manufactureers know nobody ever pays any attention to them. And I
can't recall seeing a fan plus filter for sale as a component for PC
builders (though I admit I haven't gone looking for one).

It's not that I have anything against intake fans, if you want to use
them. But a blanket statement about the best way to cool cases is with
a single input fan (without even mentioning a filter) is bogus. Most
systems are set up with output fans, and this is often a better choice
simply becuase the case can often accomodate a 120 mm output fan (at
the back) but only an 80 m intake fan (at the front). That's less
noise, and the noise emanates from the back of the case rather than
the front.

By the way, you don't mean external drives. External drives, as the
term is commonly used, have a USB or firewire connection, and won't
have anything to do with case air flow. You mean internal CD and
floppy drives.

Terry

 

[Craig Palme]

OK, you need either balance, or -positive- airflow. Overall you want more
air pushed into your case than is pulled out of it (don't forget the power
supply is pulling air out)

So before you look for an answer to your question, you have to consider
whether the fan you are replacing is sucking in or blowing out, and how
does that relate to other fans in your case?

Theoretically, if the fan you want to replace is the only case fan, and
air flow of that fan is OUTWARD, then you've got a problem. That's
because you already have an imbalance of airflow in the wrong direction.
So you need a higher CFM fan than what you've got (or more than one), and
it will have to be installed somewhere else.

I'll have to check that. Make sure it's drawing air into the case instead of
out.

But then, if the fan you want to replace is pulling air into the case, and
that is the ONLY case fan, then you can replace it with just about
anything, except that you want to find one with a CFM rating equal to or
higher than the one you are currently using. This makes your task
difficult if your goal is to reduce noise.

But your third option is to replace one noisy fan (pulling air into the
case) with 2 QUIET fans (pulling air into the case). The reason this will
work is, one noisy fan can easily create more noise than 2 quiet fans. So
you can increase the number of fans and DECREASE the overall noise level.
But that would assume that your case will support adding more fans than
you currently have. My most recent build, I chose a case with two cooling
fans on the SIDE, and replaced them with two quiet cooling fans. But
then, I chose the case at the same time as the cooling fans. Your case
might not have side mounts for a pair of matched (quiet) cooling fans.

Never thought about using two fans.

one case fan, so it should be a quiet one