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why static pressure is suited for rad?

 
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post #1 of 9 (permalink) Old 08-16-2013, 11:07 PM - Thread Starter
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Hello fellows Oc,

I've look around for fans to best fit for radiator configuration and they all point to high "static pressure" fan. I don't agree with this but since it's me against a multi-billions dollar company, the doubt are in my part. From what i gathered:

high static pressure (SP): more blades, blades oriented slightly parallel to the fan face, "ideal for rads"
high air flow (AF): fewer blades, blades oriented slightly perpendicular to fan face, "ideal for case"

base on the blades angle, the air blown would be illustrated as below:

please excuse my crude drawing

If the picture shown is correct, then air pressure at any point in an AF should be higher (allow air to go around obstruction, like a rad) then the SP fan. Therefore it would make sense that AF should be chosen for rads. Additionally, AF fans have less rpm (equates to less noise) than SP.

Data I gathered:



Thanks,
X

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post #2 of 9 (permalink) Old 08-16-2013, 11:13 PM
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Your analysis of the two fan types is essentially correct, that the profile of the blades are more "parallel" to the face of the fan, e.g. have a reduced pitch. The part where your logic fails is that the static pressure fans also have less gap between the blades. If viewed axially, you will see that eh SP blades almost meet - and that's the secret to increase static pressure. The air cannot weave it's way back past the next blade as easily as it would if there was a larger gap between the blades.

For high resistance situations, like blowing air through a radiator, the SP blades are superior. The evidence is widely documented.

Greg

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post #3 of 9 (permalink) Old 08-16-2013, 11:14 PM
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I believe the reason static pressure fans are better suited for rads is because they generally have wider fan blades that come very close to the edge of the fan housing, allowoing for steady, constant pressure to be applied in a certain direction. Obviously this allows the most heat to be dissipated across the heatsink and therefore decreasing temperature. It doesn't matter how fast the air moves across the heatsink but rather how much air.

As with rads, its about surface area. More surface area-->more heat dissipated-->lower temps.
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post #4 of 9 (permalink) Old 08-16-2013, 11:18 PM
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You need the air pressure to pull or push air through a Rad... depending on the pitch and the rpm a fan can move a lot of free air as it has not restriction but if the blades are not made to actually pull air then it will not work on a rad... pretty simple thermodynamics. It works the same way with pumps. some pumps will not work unless they are submerged or feed the liquid ( you can feed them by having them at the lowest point same as submerged then). the pitch of the blades has a direct impact on the performance on a rad. same way a prop works on some ships you actually don't increase the speed of the shaft you change the pitch to get more speed or even to reverse the ship...

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post #5 of 9 (permalink) Old 08-16-2013, 11:21 PM
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think of the fans as a car transmission. radiator restriction is comparable to an uphill road.

static pressure fans are like lower transmission gears, not very fast but powerful, able to go up to a very steep hill.
high airflow blades are like higher gears, can go very fast on a flat road, but will have a hard time going uphill.



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post #6 of 9 (permalink) Old 08-16-2013, 11:23 PM
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Quote:
Originally Posted by ghostrider85 View Post

think of the fans as a car transmission. radiator restriction is comparable to an uphill road.

static pressure fans are like lower transmission gears, not very fast but powerful, able to go up to a very steep hill.
high airflow blades are like higher gears, can go very fast on a flat road, but will have a hard time going uphill.

a good simple way to put it!!! I applaud you applaud.gif

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post #7 of 9 (permalink) Old 08-16-2013, 11:31 PM
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Let me put in this way.
Static pressure is actually suited for ANYTHING. In fact the only fans i use have insane static pressure

Static pressure is not "pressure". It's just, how much restriction it can overcome at 0CFM therefore the word "static"

In real situations CFM is always lower than rated even on a RX120 (sub-8fpi) on push-pull Delta AFB1212H which are SP120-beating fans
Usually even for filters static pressure is very important (if you want to have more airflow)
My WFB1212Ms in front of my case have 3.4mmH2O static pressure and 72CFM, how much airflow do i get after my case's filter? (I cut out the plastic as well, duh. even so)
Just slightly less than half ... but if i put a AFB1212M there instead? I would have got more than half even if it delivered less airflow (like 62CFM)

Keep in mind all the fans i use apart from the deepcool fans are SP120 beaters
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post #8 of 9 (permalink) Old 08-17-2013, 12:51 AM
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Its much like pumps and loop restriction if you are familiar with that.

Any radiator ( or any air obstruction .. filter, fan grill) has a certain restriction to airflow. The denser the FPI and the thicker the inner tubes the higher it is (roughly)

This is equivalent to the pressure drop graphs you see for water blocks etc ... the higher the flow, the more resistance.

Fans have a PQ curve that will trace how much pressure they exert on the air from 0 flow rate all the way up to their maximum flow rate that is only seen with 0 restriction.





Fans marketed as "high static pressure" will usually have a curve that starts high on the left side, indicating high pressure at 0 flow and will dive down quite steeply to their max flow rate and 0 pressure.
Fans with very steep blades will start off relatively low on the pressure at 0 flow but the graph will decrease at a much more shallow rate to end up at a higher flow rate at 0 pressure.

This graph is from the Noiseblocker eloop datasheet.
The yellow curve is very roughly the same yellow curve from the graph above for the Black Ice rad in converted units
The Black dots are roughly the curve for the Scythe GT AP15 from above converted to the units for this graph.
The straight red line is an imaginary High static pressure fan
The straigh blue line is an imaginary high air flow / low static pressure fan



We can see that the AP 15 and eloop B12-3 (green curve) cross the line of the rad curve at very close to the same point, even though the eloop is rated to some 20 cubic meters an hour higher flow. At full speed both fans would produce about 60 m3/h air flow rate.
The eloop B12-ps (dashed blue line) is rated to the same max flow rate as the AP15 but the higher static pressure of the AP15 means it crosses the rad curve at a much higher flow rate. 50 vs 65ish m3/h air flow.

The imaginary fan lines are just to show how it is possible for fans with very different manufacturer specs to end up being relatively close to the same flow rate through a radiator or grill.

A more restrictive rad will have a steeper restriction curve that will favour fans that stick to the high pressure parts of graphs. Lower restriction rads have a more gently sloping restriction curve and are a little more tolerant of different fan types as their curves will intersect at flow rates that are not too far apart.

Obviously this isn't meant to be a scientific comparison of different fans on rads ... just to outline the concept and how it works, so go easy.
The only reason these two fans were used is that they are both made by the very few companys that actually publish proper data.
The radiator air restriction graph is a very old one too so don't think that it applies exactly to modern rads.

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post #9 of 9 (permalink) Old 08-17-2013, 02:44 AM
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Quote:
Originally Posted by Jakusonfire View Post

Warning: Spoiler! (Click to show)
Its much like pumps and loop restriction if you are familiar with that.

Any radiator ( or any air obstruction .. filter, fan grill) has a certain restriction to airflow. The denser the FPI and the thicker the inner tubes the higher it is (roughly)

This is equivalent to the pressure drop graphs you see for water blocks etc ... the higher the flow, the more resistance.

Fans have a PQ curve that will trace how much pressure they exert on the air from 0 flow rate all the way up to their maximum flow rate that is only seen with 0 restriction.





Fans marketed as "high static pressure" will usually have a curve that starts high on the left side, indicating high pressure at 0 flow and will dive down quite steeply to their max flow rate and 0 pressure.
Fans with very steep blades will start off relatively low on the pressure at 0 flow but the graph will decrease at a much more shallow rate to end up at a higher flow rate at 0 pressure.

This graph is from the Noiseblocker eloop datasheet.
The yellow curve is very roughly the same yellow curve from the graph above for the Black Ice rad in converted units
The Black dots are roughly the curve for the Scythe GT AP15 from above converted in to the units for this graph.
The straight red line is an imaginary High static pressure fan
The straigh blue line is an imaginary high air flow / low static pressure fan



We can see that the AP 15 and eloop B12-3 (green curve) cross the line of the rad curve at very close to the same point, even though the eloop is rated to some 20 cubic meters an hour higher flow. At full speed both fans would produce about 60 m3/h air flow rate.
The eloop B12-ps (dashed blue line) is rated to the same max flow rate as the AP15 but the higher static pressure of the AP15 means it crosses the rad curve at a much higher flow rate. 50 vs 65ish m3/h air flow.

The imaginary fan lines are just to show how it is possible for fans with very different manufacturer specs to end up being relatively close to the same flow rate through a radiator or grill.

A more restrictive rad will have a steeper restriction curve that will favour fans that stick to the high pressure parts of graphs. Lower restriction rads have a more gently sloping restriction curve and are a little more tolerant of different fan types as their curves will intersect at flow rates that are not too far apart.

Obviously this isn't meant to be a scientific comparison of different fans on rads ... just to outline the concept and how it works, so go easy.
The only reason these two fans were used is that they are both made by the very few companys that actually publish proper data.
The radiator air restriction graph is a very old one too so don't think that it applies exactly to modern rads
.

Jakusonfire always to the rescue with the extensive explanations complete with graphs and examples.*Golf clap*


Quote:
Originally Posted by xconwing View Post

Hello fellows Oc,

I've look around for fans to best fit for radiator configuration and they all point to high "static pressure" fan. I don't agree with this but since it's me against a multi-billions dollar company, the doubt are in my part. From what i gathered:

high static pressure (SP): more blades, blades oriented slightly parallel to the fan face, "ideal for rads"
high air flow (AF): fewer blades, blades oriented slightly perpendicular to fan face, "ideal for case"
Warning: Spoiler! (Click to show)
base on the blades angle, the air blown would be illustrated as below:

please excuse my crude drawing
If the picture shown is correct, then air pressure at any point in an AF should be higher (allow air to go around obstruction, like a rad) then the SP fan. Therefore it would make sense that AF should be chosen for rads. Additionally, AF fans have less rpm (equates to less noise) than SP.
Warning: Spoiler! (Click to show)
Data I gathered:

Thanks,
X

Your assumption that airflow is better for radiators is correct. Simply put, higher airflow means the air doesn't have as much time to heat up, resulting in lower deltaTemperatures. On the other hand higher rated airflow fans will not always actually have more airflow than high SP fans once you introduce notable amounts of restriction. SP fans will usually end up having MORE airflow through radiators because they can handle the restriction better. For more detail on that, let us go back to Jakusonfire's post. biggrin.gif

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