Static Pressure vs Airflow...(Please Read)

[JaZoN_XD]

So this is not your average day "what's the difference between these two types of fans?" but rather why an airflow fan would still not work well when in a "push" setup through a radiator.

 

Reason I ask is:

 

For example, if fan A (140mm) is static pressure optimized, and has 1.5 mmH2O at around 45 CFM and fan B (140mm) at only 0.75 mmH2O but more airflow at 85 CFM. If you were to put both fans in the open and have no restriction in front of them at all, air would be pushed faster from the back of fan B because it has to move larger quantity of air using the same fan size compared to fan A. Therefore, the air movement will be physically faster. (Right?)

 

So if you were to use fan B on a radiator as an exhaust (basically no restriction in the front of the fan), shouldn't it work better than the static pressure fan? I say this because both of the fans are still moving AIR which has the same density...etc. Since fan B is able to move the air faster to begin with, shouldn't the air be able to push through the fins of the radiators more effectively than the static pressure fans?

 

I don't have radiators to test my question on, so I'll consult some of the LTT fluid dynamics PHDs

[Enderman]

there is very little difference between airflow and SP fans for that reason

jayztwocents uses airflow fans on his radiators and they work fine

 

i think SP fans would be quieter though since the air is being pushed through with less turbulence

 

btw SP rating of mmH2O is measured at 0cfm

radiators do not block 100% of airflow so the pressure will actually be a lot less

in the end its very close to the cfm of an airflow fan

 

ill look for some graphs to prove what i mean and post them in a min

[Enderman]

Just now, KenKraken said:

how are they different though? don't they both "push" air? how they any different? 

heres an example

 

look at the cyan SP120 line and compare it to the green Scythe line

 

the SP120 clearly has more mmH2O when you look on the left

but the scythe has more cfm when you look on the bottom

 

a radiator is neither 0 resistance nor 100% resistance, its actually somewhere in between

if you look at the center at about 1.75mmH2O you can see both lines cross over eachother

 

at this point both fans perform exactly the same because they are pushing the same amount of air with the same force

 

if youre installing a fan in a place with no resistance, then its better to get an airflow fan since the CFM is greater when mmH2O is close to 0

 

for a radiator, you can use both fans as long as the radiator isnt too restrictive, due to the fact that radiators are not a solid piece of metal, air actually goes through them

 

if youre installing a fan in a very restrictive place, like in front of a bunch of hard drives, then a SP fan will perform better because it will have higher CFM at high mmH2O compared to the AF fan

[JaZoN_XD]

8 minutes ago, Enderman said:

there is very little difference between airflow and SP fans for that reason

jayztwocents uses airflow fans on his radiators and they work fine

 

i think SP fans would be quieter though since the air is being pushed through with less turbulence

 

btw SP rating of mmH2O is measured at 0cfm

radiators do not block 100% of airflow so the pressure will actually be a lot less

in the end its very close to the cfm of an airflow fan

 

ill look for some graphs to prove what i mean and post them in a min

yeah i've tried to look at some pq graphs and i kinda get it but still. I think that generally, static pressure fnas are actually louder becuase they tend to throw air off to the sides, and that is amplified when

[JaZoN_XD]

1 minute ago, Enderman said:

heres an example

 

look at the cyan SP120 line and compare it to the green Scythe line

 

the SP120 clearly has more mmH2O when you look on the left

but the scythe has more cfm when you look on the bottom

 

a radiator is neither 0 resistance nor 100% resistance, its actually somewhere in between

if you look at the center at about 1.75mmH2O you can see both lines cross over eachother

 

at this point both fans perform exactly the same because they are pushing the same amount of air with the same force

 

if youre installing a fan in a place with no resistance, then its better to get an airflow fan since the CFM is greater when mmH2O is close to 0

 

for a radiator, you can use both fans as long as the radiator isnt too restrictive, due to the fact that radiators are not a solid piece of metal, air actually goes through them

 

if youre installing a fan in a very restrictive place, like in front of a bunch of hard drives, then a SP fan will perform better because it will have higher CFM at high mmH2O compared to the AF fan

what about a fan IN FRONT of harddrive bays where there are no restrictions in FRONT of the fa (0 mmH20) but restriction BEHIND the fan?

[Enderman]

1 minute ago, JaZoN_XD said:

yeah i've tried to look at some pq graphs and i kinda get it but still. I think that generally, static pressure fnas are actually louder becuase they tend to throw air off to the sides, and that is amplified when

how loud the fan itself is actually depends on the blade design, bearing, and motor

not AF or SP

[Enderman]

Just now, JaZoN_XD said:

what about a fan IN FRONT of harddrive bays where there are no restrictions in FRONT of the fa (0 mmH20) but restriction BEHIND the fan?

it doesnt matter where the restriction is

either way the air has to pass through the fan

drive cages in front will be just as restrictive as drives in the back

[Enderman]

Just now, KenKraken said:

i understand that they perform different im asking how? they both have fan blades and the same structure  

shape of blades, torque of motor, type of bearing, speed, etc...

usually the shape of the blades is the main deciding factor on SP or AF

[JaZoN_XD]

3 hours ago, Enderman said:

it doesnt matter where the restriction is

either way the air has to pass through the fan

drive cages in front will be just as restrictive as drives in the back

Right, and so I pose this question, since the restriction is behind the fan, the air has therefore already passed through the fan and is moving at a pretty high speed. Once all that fast moving air hits the restriction, shouldn't it be able to go through that restriction easier than an SP fan because the speed of air coming from an SP fan will be slower?

[Enderman]

1 hour ago, JaZoN_XD said:

Right, and so I pose this question, since the restriction is behind the fan, the air has therefore already passed through the fan and is moving at a pretty high speed. Once all that fast moving air hits the restriction, shouldn't it be able to go through that restriction easier than an SP fan because the speed of air coming from an SP fan will be slower?

in the case of high restriction, no

air is very light

that means it carries very little momentum

even if its going fast, it will be extremely slowed down bu the restrictive drives in the way

 

a static pressure fan moves the air slower, but due to the shape of the blades it does a better job of "squeezing" the air forward and into the cracks between the drives

 

thats why SP fans get better cfm than airflow fans in those extremely restrictive cases

[JaZoN_XD]

17 hours ago, Enderman said:

in the case of high restriction, no

air is very light

that means it carries very little momentum

even if its going fast, it will be extremely slowed down bu the restrictive drives in the way

 

a static pressure fan moves the air slower, but due to the shape of the blades it does a better job of "squeezing" the air forward and into the cracks between the drives

 

thats why SP fans get better cfm than airflow fans in those extremely restrictive cases

Sorry to keep bothering you but, that "squeezing" thing is precisely what I'm confused about

 

Although air is very light, if an airflow fan moves the air faster than a SP fan, the air will undoubtedly have more momentum. I can see how restrictions in front of an SP fan will allow it to take advantage of the wide and close blades compared to an AF, but if there was no restriction in the front, SP fans at 0 mmH2O usually has less CFM than AF fans. Since both of these fans are still pushing the same medium (air) through them, the only way I can think of "squeezing" air through those cracks is by pushing it faster.

[agent_x007]

Pressure vs. Airflow ?
Delta/San Ace all the way

Loudness :
99% speed dependent (if U lower RPM's low enough, every fan will become "silent") and 1% quality dependent.

Personal note :
True airflow fans should become useless if U had ANY metal mesh/air filter/metal heatsink, behind or before it.

[Enderman]

3 hours ago, JaZoN_XD said:

Sorry to keep bothering you but, that "squeezing" thing is precisely what I'm confused about

 

Although air is very light, if an airflow fan moves the air faster than a SP fan, the air will undoubtedly have more momentum. I can see how restrictions in front of an SP fan will allow it to take advantage of the wide and close blades compared to an AF, but if there was no restriction in the front, SP fans at 0 mmH2O usually has less CFM than AF fans. Since both of these fans are still pushing the same medium (air) through them, the only way I can think of "squeezing" air through those cracks is by pushing it faster.

no, as i said air is very light and barely has any momentum

 

think of the fan blades as a screw

 

this type of screw goes in very fast, since you only need to turn it a few times

 

 

now here is a fine thread screw

this other type of screw goes in very slow but it is a lot stronger at pushing into things because moving the same distance forward requires more turns, and more turns = more work done

due to conservation of energy, that mechanical work is converted into "pressure" pushing the screw forward

 

 

fans work similarly due to the blade design

"flatter" and larger blades act like a finer screw, and it takes more turns, but pushes with more force

 

 

airflow fans have steep and coarse blades, which move the air faster but with less pushing strength

[JaZoN_XD]

4 hours ago, Enderman said:

~~snip~~

ahh, i see. Thank you for your explanation. It reminds me of engine torque or gear ratios. Longer gears have higher speed but less torque, whilst shorter gears have more mechanical torque. So SP fans are in some sense like 1st gear?

[Enderman]

18 minutes ago, JaZoN_XD said:

ahh, i see. Thank you for your explanation. It reminds me of engine torque or gear ratios. Longer gears have higher speed but less torque, whilst shorter gears have more mechanical torque. So SP fans are in some sense like 1st gear?

yup!

[wildgg]

On 7-3-2016 at 7:04 PM, JaZoN_XD said:

So this is not your average day "what's the difference between these two types of fans?" but rather why an airflow fan would still not work well when in a "push" setup through a radiator.

 

Reason I ask is:

 

For example, if fan A (140mm) is static pressure optimized, and has 1.5 mmH2O at around 45 CFM and fan B (140mm) at only 0.75 mmH2O but more airflow at 85 CFM. If you were to put both fans in the open and have no restriction in front of them at all, air would be pushed faster from the back of fan B because it has to move larger quantity of air using the same fan size compared to fan A. Therefore, the air movement will be physically faster. (Right?)

 

So if you were to use fan B on a radiator as an exhaust (basically no restriction in the front of the fan), shouldn't it work better than the static pressure fan? I say this because both of the fans are still moving AIR which has the same density...etc. Since fan B is able to move the air faster to begin with, shouldn't the air be able to push through the fins of the radiators more effectively than the static pressure fans?

 

I don't have radiators to test my question on, so I'll consult some of the LTT fluid dynamics PHDs

''shouldn't it work better than the static pressure fan?''

this also depends on the fin density of the radiator.

 

the reason some people use airflow fans on radiators is because the fin density is very low, so the airflow fan doesn't need much pressure to push air through it. (or pull)

but if you have a high fin density radiator you want to use high pressure fans.

 

not sure if this helped at all but i thought i would hop in on this

 

 

[ThomasD]

13 hours ago, JaZoN_XD said:

ahh, i see. Thank you for your explanation. It reminds me of engine torque or gear ratios. Longer gears have higher speed but less torque, whilst shorter gears have more mechanical torque. So SP fans are in some sense like 1st gear?

Much closer to this.

 

SP and air volume are not independent aspects, they are both elements of a single function ( which is why you can draw those fan curves pictured above.) 

 

As a rough general rule assume that, for any give set of fan specs (which are all reported as if under ideal conditions,) the fans with the higher reported SP will retain more of it's maximum stated airflow as resistance to flow (ie obstructions) increases.

 

But there are no guarantees, real world answers require real world use.

 

Nose level is even more complex, because turbulence can become the dominant factor, particularly when there are combinations of high flow, multiple fans, and multiple flow restrictions.  What should be nice and steady can quickly start to form oscillations.