so i do understand that the sp fans make high pressure but move less air and the af fans make low pressure but move much more air, but this is on the outlet, what about the inlet? Ive got a case side door with the dust filter, THEN the fan, then a lot of open space inside the case(no grill after a fan, no cages, nothing). So no need for pressure inside. The thing that confuses me is that i dunno if the pressure fans make the air come through the dust filter BEHIND them with the same strong force that appears then infront of them (thus helping the air to come through the dustfilter behind) or they dont work backwards? Because if they dont, the af fan would be much better cus it would move more air through the filter at the same rpm. Wonder if theres any aerodynamics guy that can explain it. Thanks a lot
does static pressure fan create pressure on both sides?
- Thread starter ckooii
- Start date
I don't know about this, but If a fan lists it moves X amount of air, than that is what it does. Static pressure just means air flow is not being wasted and airflow is more directed. So yes if air goes out than the air inside the case is moving along also.
I am sure that whether the (very small) air flow impediment of a dust filter is on the intake or exit side of a fan makes NO real difference to its performance. For that fan location, an Air Flow design is right. The only time a dust filter becomes a real restriction on air flow is when it is so dirty it is clogged, and by then it should have been cleaned already.
a stronger fan will push more air no doubt, but the question is whether the sp fan will create more pressure on its back too compared to af thus making it easier for air to move thru the filter.
and the air flow impediment of a dust filter is far from low, please dont just guess or assume things
again, some misunderstanding may occur so il try to make the topic question clearer.
im asking whether the sp fan will move the air with more pressure through a filter thats positioned behind it (on it's inlet). Behind. Does it have same good pressure behind it. Or is it same as af fan. Does it SUCK (not push) air with a stronger PRESSURE (not volume) than the af fans.
and the air flow impediment of a dust filter is far from low, please dont just guess or assume things
again, some misunderstanding may occur so il try to make the topic question clearer.
im asking whether the sp fan will move the air with more pressure through a filter thats positioned behind it (on it's inlet). Behind. Does it have same good pressure behind it. Or is it same as af fan. Does it SUCK (not push) air with a stronger PRESSURE (not volume) than the af fans.
I'm gonna say yes it will create more pressure behind it since the air has to come from somewhere. It also depends if you have your case setup for positive or negative airflow. I will say that my noctua industrial 140mm fan has so much CFM that it is creating a very strong negative pressure in the case. If it put a piece paper anywhere that isn't a direct inlet (like holes on the side) the paper gets sucked up.
It is not so much a question about pressure from the fan itself, as a single fan in a open environment will not create any sort of real pressure since it has infinite surrounding air to pull from. Once you put a case intself a case than it has limited air and therefore can create that negative pressure.
It is not so much a question about pressure from the fan itself, as a single fan in a open environment will not create any sort of real pressure since it has infinite surrounding air to pull from. Once you put a case intself a case than it has limited air and therefore can create that negative pressure.
OK, to be more detailed, for any given fixed air flow through a filter, the pressure drop across the filter will be the SAME no matter whether that filter is on the intake or discharge side of the fan. So the question really is, looking at it from a slightly different perspective, will the reduction of air flow that the fan produces because of the pressure drop across that filter be the same if a specified pressure drop is on the intake side, or on the discharge side. I'm not a fan design engineer, but my guess is the difference will be very small.
The way I read the comparisons between Static Pressure and Air Flow fans is this. All fans will deliver reduced air flow as the backpressure is increased. At very low backpressures, the Air Flow fans will delivery more than Static Pressure ones. But as backpressure is increased, the reduction of air flow delivered is greater for Air Flow designs than for Static Pressure ones; the result is that at the higher backpressures usually resulting from finned heatsinks and radiators the Static Pressure fans will deliver reduced air flow but still MORE that the Air Flow designs will. At some intermediate back pressure (not specified anywhere) the air flow delivered by the two designs will be almost equal.
You do need to bear in mind that the pressure we are talking about is small, enough that there is REALLY minimal effect from the density of the air that if flowing. The typical backpressure used for measuring the air flow generated by a computer case fan is between 1.5 and 2 mm water. On VERY high-flow fans I've seen specs of 5 to 10 mm water. For reference, the "standard" atmospheric air pressure at sea level is 760 mm Mercury or 33.9 FEET of water - that's 10,333 mm of water. So a typical "backpressure" faced by a fan is <2/10,000th of normal air pressure and that small a pressure difference has virtually NO impact on the density of the air.
The way I read the comparisons between Static Pressure and Air Flow fans is this. All fans will deliver reduced air flow as the backpressure is increased. At very low backpressures, the Air Flow fans will delivery more than Static Pressure ones. But as backpressure is increased, the reduction of air flow delivered is greater for Air Flow designs than for Static Pressure ones; the result is that at the higher backpressures usually resulting from finned heatsinks and radiators the Static Pressure fans will deliver reduced air flow but still MORE that the Air Flow designs will. At some intermediate back pressure (not specified anywhere) the air flow delivered by the two designs will be almost equal.
You do need to bear in mind that the pressure we are talking about is small, enough that there is REALLY minimal effect from the density of the air that if flowing. The typical backpressure used for measuring the air flow generated by a computer case fan is between 1.5 and 2 mm water. On VERY high-flow fans I've seen specs of 5 to 10 mm water. For reference, the "standard" atmospheric air pressure at sea level is 760 mm Mercury or 33.9 FEET of water - that's 10,333 mm of water. So a typical "backpressure" faced by a fan is <2/10,000th of normal air pressure and that small a pressure difference has virtually NO impact on the density of the air.
okay, THIS was really helpfull and thanks for understanding me 
Just in case sent a letter to Noctua to see if they can make the even more clear with their fans.
Here is why i was making the topic - im trying to build an utterly silent pc, and considering the fan blades design - as the pitch increases so does the noise. So taking into account that the DEMCifilters im supposed to use claim to resist 20% of the total airflow its probably not gonna be an insane amount of loss, and im better off with silent airflow optimised fans? More precisely, the NF-S12A ULN 8.6dB 43.73 CFM than the louder and more static pressure optimised NF-A14 ULN 11.9dB 47CFM. Whats ur opinion on this?
heres an advice from another forum btw
"It is perfectly ok to use a case fan for just moving air through your case as there isn't much chance for building a pressure gradient (cut out back fan grill/holes and use a wire one). A blower will not give any better results cfm versus cfm and are typically much noisier for the same cfm ratings."
Just in case sent a letter to Noctua to see if they can make the even more clear with their fans.Here is why i was making the topic - im trying to build an utterly silent pc, and considering the fan blades design - as the pitch increases so does the noise. So taking into account that the DEMCifilters im supposed to use claim to resist 20% of the total airflow its probably not gonna be an insane amount of loss, and im better off with silent airflow optimised fans? More precisely, the NF-S12A ULN 8.6dB 43.73 CFM than the louder and more static pressure optimised NF-A14 ULN 11.9dB 47CFM. Whats ur opinion on this?
heres an advice from another forum btw
"It is perfectly ok to use a case fan for just moving air through your case as there isn't much chance for building a pressure gradient (cut out back fan grill/holes and use a wire one). A blower will not give any better results cfm versus cfm and are typically much noisier for the same cfm ratings."
Thanks for Best Solution.
Yes, I do recommend you go with the airflow-optimized design. I can see iamacow's point about using liquid cooling for reducing noise, but the expense is a factor. On the other hand, among standard case fans Noctua produces some of the lowest-noise units, so you may find that will give you all you need.
You should be aware of a wrinkle in comparing fans, and there's very little you can do for this point. As an example, look at those specs you quoted for two different Noctua units. The larger fan produces more noise and more air flow when both are run at top speed. What they cannot ell you is what the noise from that larger fan will be when it is slowed down to produce the SAME air flow (and hence the same cooling performance) as the smaller fan.
I should also point out something about similar fans. The Noctua ULN line of fans are designed to run slower for the same signals as their "regular" version of the same fan. This means they produce less noise AND less air flow. What is hard to decide is this: if you bought the "regular" version of that same fan, what noise would it generate if it were running NOT at top speed, but at a reduced speed to produce the same (lower) air flow as that ULN unit? You MAY be able to guess at that because in their specs, Noctua often tells you air flow and noise figures at three different speeds, So maybe you can use those to answer the question of comparative noise at equal AIR FLOW rates. You see, the entire point of using mobo-based automatic fan controls is that the system will reduce the fan speeds to just the right speed to produce the AIR FLOW needed for the cooling requirements. So for any given heat load, if you used a different fan the system would change its speed to give you the SAME air flow and cooling, and this would result in a new noise level.
What a ULN fan does is limit the maximum cooling you can achieve, since it will never run as fast as a "regular" fan of that family.
Yes, I do recommend you go with the airflow-optimized design. I can see iamacow's point about using liquid cooling for reducing noise, but the expense is a factor. On the other hand, among standard case fans Noctua produces some of the lowest-noise units, so you may find that will give you all you need.
You should be aware of a wrinkle in comparing fans, and there's very little you can do for this point. As an example, look at those specs you quoted for two different Noctua units. The larger fan produces more noise and more air flow when both are run at top speed. What they cannot ell you is what the noise from that larger fan will be when it is slowed down to produce the SAME air flow (and hence the same cooling performance) as the smaller fan.
I should also point out something about similar fans. The Noctua ULN line of fans are designed to run slower for the same signals as their "regular" version of the same fan. This means they produce less noise AND less air flow. What is hard to decide is this: if you bought the "regular" version of that same fan, what noise would it generate if it were running NOT at top speed, but at a reduced speed to produce the same (lower) air flow as that ULN unit? You MAY be able to guess at that because in their specs, Noctua often tells you air flow and noise figures at three different speeds, So maybe you can use those to answer the question of comparative noise at equal AIR FLOW rates. You see, the entire point of using mobo-based automatic fan controls is that the system will reduce the fan speeds to just the right speed to produce the AIR FLOW needed for the cooling requirements. So for any given heat load, if you used a different fan the system would change its speed to give you the SAME air flow and cooling, and this would result in a new noise level.
What a ULN fan does is limit the maximum cooling you can achieve, since it will never run as fast as a "regular" fan of that family.
okay just in case you wondering what Noctua said
"thanks for contacting Noctua!
A computer dust filter is nothing to worry of, the Airflow Optimized fan will be clearly better and quieter."
so i guess it applies to the filter on the intake and on the exaust sides similarly
"thanks for contacting Noctua!
A computer dust filter is nothing to worry of, the Airflow Optimized fan will be clearly better and quieter."
so i guess it applies to the filter on the intake and on the exaust sides similarly
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