understanding correlation between fan air pressure and air flow

[m4rtin]

I read a review of Cooler Master Hyper 212 Evo CPU cooler and according to this fan of Hyper 212+ provides 3.90 mmH2O air pressure and 77CFM air flow while fan of Hyper 212 Evo provides 2.7mmH2O air pressure and 83CFM air flow. Both fans rotate at the same RPM and are with same diameter. How is it possible that while the air pressure is lower, the amount of moved air is larger?

[suicidaleggroll]

You're reading those specs as an AND, when it really should be more of an OR.  Think of the open-circuit-voltage/short-circuit-current specs for a solar panel.  The 212+ can generate more pressure and push more air into a restrictive load (dense heatsink), but it can't flow as much air into the open.  The 212 Evo is the opposite, it can flow more air into open space, but can't generate as much pressure and therefore can't flow as much air into a dense heatsink.

Which one to choose depends on the heatsink and the restriction it creates.  A dense heatsink would benefit more from a fan that can generate more pressure, rather than one that can push more air into open space.

In other words, you're just looking at the fan specs, but it's the fan/heatsink combination that matters.  The 212+ is the successor to the 212 Evo.  My guess is Cooler Master increased the fin density on the 212+, and swapped to a fan with a blade pitch that's more efficient at higher pressure, because they discovered that that combination increases the total cooling capacity of the heatsink/fan combination.

http://www.tomshardware.com/answers/id-2192463/high-air-flow-static-pressure-case-fans.html

[IanB]

In the same way that electrical power is voltage times current, the power consumed by a fan is approximately pressure rise times air flow. For a given power in the motor, if you move more air you can have less pressure, or if you move less air you can have more pressure (the difference is in the shape of the fan blades).

You could also compare this to a transformer. If two transformers are the same size, the one with the higher output voltage will have a lower output current.

[fivefish]

Physics... correlation between pressure and air flow brings back to Bernoulli's principle/equaltion, which is related to conservation of energy.

[Simon]

If you look at a graph for a fan on the Y you tend to have the pressure and on the x the volume of air. The line will start top left and drop down to top right, sometimes straight sometimes with a bit of a hump.

At maximum pressure no air is moving, at maximum flow you get no pressure. So your careful when reading headline specs becase the flow can drop quickly with some pressure drop that will always be there.

[Armxnian]

Science aside, Cooler Master's specs are usually fishy. For example, I remember having their 200mm fans a while back. The spec sheet claimed 19db of noise, which is basically inaudible during the day. But they certainly were not silent. I would have put them at 30db at minimum running on 12v.

As for the fan, I don't actually believe it has 3.9mm H20 of static pressure, especially at only 2000rpm. I've tested quite a few fans including Gentle Typhoons, multiple Corsair fans, San Aces, other coooler masters and etc, all with lower static pressure ratings, yet all with more pressure than the fan the CM units come with. You don't need a fancy air chamber to do this, can just use your hand on open air or on the other side of a radiator.

Anyway, the evo and 212+ are great performing and the fan is fairly quiet.

The 212+ is the successor to the 212 Evo. 

Other way around. I had the 212+ when the evo was not yet released.

[suicidaleggroll]

The 212+ is the successor to the 212 Evo. 

Other way around. I had the 212+ when the evo was not yet released.

Sorry, was going off of this on their site:

Hyper 212 Plus
Succeeding the popular Hyper 212, the Hyper 212 Plus carries on the legacy of providing a great balance of performance and noise level during high and low speed operations.

I thought they were talking about the evo, didn't realize they had just a regular "212" before either of them.

[m4rtin]

If you look at a graph for a fan on the Y you tend to have the pressure and on the x the volume of air. The line will start top left and drop down to top right, sometimes straight sometimes with a bit of a hump.

At maximum pressure no air is moving, at maximum flow you get no pressure. So your careful when reading headline specs becase the flow can drop quickly with some pressure drop that will always be there.

I also found one fan graph to illustrate your post:



However, this all seems to me bit counter-intuitive. Or should I think of this air pressure as an air pressure at the fan intake, i.e. in case there is no fan blade at all and simply the 120x120mm fan frame the pressure would be zero and if the large fan blades rotates fast, then this does not allow air to easily pass and creates pressure?

[suicidaleggroll]

If you look at a graph for a fan on the Y you tend to have the pressure and on the x the volume of air. The line will start top left and drop down to top right, sometimes straight sometimes with a bit of a hump.

At maximum pressure no air is moving, at maximum flow you get no pressure. So your careful when reading headline specs becase the flow can drop quickly with some pressure drop that will always be there.

I also found one fan graph to illustrate your post:



However, this all seems to me bit counter-intuitive. Or should I think of this air pressure as an air pressure at the fan intake, i.e. in case there is no fan blade at all and simply the 120x120mm fan frame the pressure would be zero and if the large fan blades rotates fast, then this does not allow air to easily pass and creates pressure?

The pressure is at the fan outlet, and is being created by whatever the fan is trying to push the air through.  If there is nothing after the fan, just open space, then you'd be looking at the "0" line on the y-axis, and the fan will generate the highest CFM possible (~120).  If there is a brick wall after the fan, so the air can't go anywhere, then you'd be looking at the far left side of the plot, with zero air flow and high pressure.  The actual operating point will depend on the restriction in the heatsink you have attached to the fan (assuming there is a heatsink).  The denser the heatsink, the more restriction it creates, the higher the operating pressure, and the lower the air flow rate.

[m4rtin]

If you look at a graph for a fan on the Y you tend to have the pressure and on the x the volume of air. The line will start top left and drop down to top right, sometimes straight sometimes with a bit of a hump.

At maximum pressure no air is moving, at maximum flow you get no pressure. So your careful when reading headline specs becase the flow can drop quickly with some pressure drop that will always be there.

I also found one fan graph to illustrate your post:



However, this all seems to me bit counter-intuitive. Or should I think of this air pressure as an air pressure at the fan intake, i.e. in case there is no fan blade at all and simply the 120x120mm fan frame the pressure would be zero and if the large fan blades rotates fast, then this does not allow air to easily pass and creates pressure?

The pressure is at the fan outlet, and is being created by whatever the fan is trying to push the air through.  If there is nothing after the fan, just open space, then you'd be looking at the "0" line on the y-axis, and the fan will generate the highest CFM possible (~120).  If there is a brick wall after the fan, so the air can't go anywhere, then you'd be looking at the far left side of the plot, with zero air flow and high pressure.  The actual operating point will depend on the restriction in the heatsink you have attached to the fan (assuming there is a heatsink).  The denser the heatsink, the more restriction it creates, the higher the operating pressure, and the lower the air flow rate.

I understand now, thanks! So when vendors publish air-pressure specs(for example 3.90 mmH2O or 2.7mmH2O), then this depends on radiator design? For example in case of Cooler Master Hyper 212 Evo, the fan pushes air against the radiator fins and creates 2.7mmH2O air-pressure:



In case of same fan but another radiator with for example wider gap between the fins the air-pressure would be different?

[Kilrah]

So when vendors publish air-pressure specs(for example 3.90 mmH2O or 2.7mmH2O), then this depends on radiator design?

It would rather be the capability of the fan alone, "if it had to it could push up to a pressure of X".

[m4rtin]

So in a nutshell, high air pressure rating for a fan is mainly thanks to blade design(usually larger blades with shallow angle) and by keeping the blades close to the fan frame so that air pushed through the fan can not easily "bounce" back through the fan if there is a restriction(for example an heatsink) right after fan outlet? And fan with high CFM and low mmH2O operates in a way that if there is a significant amount of restriction in front of the fan outlet, then lot of air "bounces" back through the fan and this causes fan to have low air pressure?

[IanB]

So in a nutshell, high air pressure rating for a fan is mainly thanks to blade design(usually larger blades with shallow angle) and by keeping the blades close to the fan frame so that air pushed through the fan can not easily "bounce" back through the fan if there is a restriction(for example an heatsink) right after fan outlet? And fan with high CFM and low mmH2O operates in a way that if there is a significant amount of restriction in front of the fan outlet, then lot of air "bounces" back through the fan and this causes fan to have low air pressure?

No, it's not caused by air going backwards through the fan. If this happens it is called "surging" and on big fans or compressors it can be very undesirable. It leads to unstable air flows that can cause shaking and vibration.

It really just comes down to the size and shape of the fan blades and the power of the motor.

[m4rtin]

It really just comes down to the size and shape of the fan blades and the power of the motor.

I understand this, but when two fans with different blade design rotate at the same RPM and are with the same diameter and first one creates less pressure while has higher CFM rating and other fan creates more pressure and has lower CFM, then where does this air disappear for the second fan? If this doesn't go backwards through the fan then are fans with high pressure simply with very channelized, but narrow air flow? And fans with low air pressure and high air flow simply spread the air "pumped" through the fan into wider area?

[suicidaleggroll]

It really just comes down to the size and shape of the fan blades and the power of the motor.

I understand this, but when two fans with different blade design rotate at the same RPM and are with the same diameter and first one creates less pressure while has higher CFM rating and other fan creates more pressure and has lower CFM, then where does this air disappear for the second fan? If this doesn't go backwards through the fan then are fans with high pressure simply with very channelized, but narrow air flow? And fans with low air pressure and high air flow simply spread the air "pumped" through the fan into wider area?

Again, you're reading the specs as an AND, when it should be an OR.  The fan does not dictate both the pressure and the air flow.  The relationship between the pressure and air flow is decided by the restriction of whatever follows the fan.  You should be reading those numbers as two separate operating limits, similar to voltage and current limits of a bench power supply, or the short circuit current and open circuit voltage of a solar panel.

[SL4P]

Also look up 'cavitation' which occurs when a fan or pump is moving a fluid (air/water etc)...
If you overspec a fan for cfm or pressure, you may reduce the effectiveness, unless you slow the rotation speed to eliminate this problem!

[Mechatrommer]

How is it possible that while the air pressure is lower, the amount of moved air is larger?

pressure is not a function of air volume alone, but density ie air mass as well per unit volume. the capability of creating high density region will depend on the efficiency of the impeller design. fan is a very inefficient impeller and a good impeller is a very inefficient fan. different fan has different design and hence efficiency... in an open space like a fan, there will be a dramatic spatial pressure differences on the verge of the impeller, err fan surfaces. the OP is asking in ee forum an advance subject of fluid mechanics that only can be tested in the lab...

[SL4P]

P.S. a common method for reducing fan noise, while retaining airflow - is to have larger, heavily pitched blades, running slower.

[IanB]

Also look up 'cavitation' which occurs when a fan or pump is moving a fluid (air/water etc)...
If you overspec a fan for cfm or pressure, you may reduce the effectiveness, unless you slow the rotation speed to eliminate this problem!

More details:

Cavitation applies to centrifugal pumps moving liquid. It occurs when the suction pressure is too low and the liquid gets below its vapour pressure. This can cause tiny bubbles to form and collapse on the blades of the impeller, causing pitting and erosion. Cavitation by definition cannot happen when moving gases like air. In fact, throttling the suction is a safe and practical way to reduce the flow of gas through a compressor or fan, but it is a complete no-no with liquids.

Surging is what happens to a compressor or fan if you restrict the outlet too much. It is an unstable flow pattern when the gas stops flowing smoothly and "piles up" against the blades, causing vibration and potential damage to the machine. Compressor control systems are nearly always put in place to protect against surging. Although it is not normally good to throttle the outlet of a compressor for this reason, it is OK to throttle the outlet of a pump. Since liquid is incompressible a pump is not generally prone to surging.

(Note: all of the above applies to turbomachinery. Reciprocating machines have different characteristics.)

[SL4P]

Thanks for the detail!
I remember an example of cavitation I heard many (many) years ago- was a boat propeller that was not specked properly - effectively pushing more water/air than the boat could displace in forward movement ...  The prop is then effectively thrashing in a'bubble' of already displaced water.
Comments?

[Mechatrommer]

Cavitation applies to centrifugal pumps moving liquid.

any blades applied not just centrifugal type, as long as it fit the bills. boat propeller design also meet this kind of problem. except you dont see cavitation in the air because cavitation in water itself is an air. in air we have vice versa.. .water vapor cone only occured at transonic speed. since this thread is about cpu fan, i prefer toward low pressure high CFM model since this is about air flow cooling. more cfm more heatsink cooling, less pressure less hot air coming in...fwiw..

[m4rtin]

I read about Bernoulli's principle:



..and is it correct to think that high air-flow fan creates a narrow channalized air-flow because of its fan-blades design and thus it has lower pressure? And high-pressure fan "pumps" air into wider area and thus creates a higher pressure?

[IanB]

..and is it correct to think that high air-flow fan creates a narrow channalized air-flow because of its fan-blades design and thus it has lower pressure? And high-pressure fan "pumps" air into wider area and thus creates a higher pressure?

No, that's not it.

The power of a fan can be measured as pressure rise times air flow (just as electrical power can be measured as voltage times current flow).

If a fan motor has a given power rating, then it can either move more air through a lower pressure rise, or less air through a higher pressure rise, so that the product remains about the same. The shape of the fan blades can be designed to optimize for one case or the other.

[timb]

I read about Bernoulli's principle:



..and is it correct to think that high air-flow fan creates a narrow channalized air-flow because of its fan-blades design and thus it has lower pressure? And high-pressure fan "pumps" air into wider area and thus creates a higher pressure?

Bernoulli's principle in a nutshell: As velocity increases, pressure decreases.

This is how airplanes fly. The faster they go, the faster air is channeled over the wing, the lower the pressure around the wing, creating lift! A helicopter is just two wings rotating very fast, causing vertical lift. A propeller is the same thing, it just creates lift on a different axis, which pulls the plane forward (ultimately causing the plane to go fast enough to create lift across the wings as above).

So, the amount of static pressure a fan can push air into is directly related to the design of the blades and the torque of the motor.


Sent from my Tablet

[IanB]

Bernoulli's principle in a nutshell: As velocity increases, pressure decreases.

This is how airplanes fly.

This is a common misconception. Bernoulli's principle applies to flow in pipes and closed channels where the flow is constrained by the walls.

Air flow over a wing is unconstrained and open on all sides bar one. Bernoulli's principle does not apply here, and therefore airplanes fly for other reasons.

[m4rtin]

..and is it correct to think that high air-flow fan creates a narrow channalized air-flow because of its fan-blades design and thus it has lower pressure? And high-pressure fan "pumps" air into wider area and thus creates a higher pressure?

No, that's not it.

The power of a fan can be measured as pressure rise times air flow (just as electrical power can be measured as voltage times current flow).

If a fan motor has a given power rating, then it can either move more air through a lower pressure rise, or less air through a higher pressure rise, so that the product remains about the same. The shape of the fan blades can be designed to optimize for one case or the other.

Ok, thanks! I didn't know that Bernoullis principle works only in case the flow is constrained by the walls.
When a fan has higher pressure and slower air-flow, then how do air molecules move differently than in case the fan has lower pressure and faster air-flow?

[IanB]

When a fan has higher pressure and slower air-flow, then how do air molecules move differently than in case the fan has lower pressure and faster air-flow?

Fan designers use computer simulation software that models the exact flow of air through the fan (a bit like a circuit simulator, but modeling fluid flow instead of electricity).

If you want to know what the equations look like, see here: https://en.wikipedia.org/wiki/Navier%E2%80%93Stokes_equations

These equations, together with a description of the shape and movement of the fan blades, allow the precise performance of a fan to be predicted. All you have to do is solve the equations and you can tell how the air molecules move. Simple! 

I don't design fans, so I can't tell you more than that. The simple answer is it depends on the shape, size and geometry of the fan. If you examine a fan carefully, you will see that the blades have a complex curved shape. They are not simply flat rectangles.

[Mechatrommer]

Ditto.. bernoulli's and aerofoil's myth are only for kindergarden storytelling.. if u are serious about this, go for navier stoke equation. Its not a one answer solution, i'm guessing its like maxwell's equation... trying to understand air pressure and flow is like trying to understand the shape and strength of a magnetic field around a working cicuit on a pcb..

[Mechatrommer]

Let alone the linear and angular forces that act on every tiny segments of a blade for high efficiency blade design that only can be simulated through FEA.

[m4rtin]

IanB and Mechatrommer, thank you for your answers!  However, it is counterintuitive to imagine that air molecules with higher speed have smaller penetration(for example, penetration to radiator with thin fin gaps) ability than the ones with lower speed.. Or is it once again because of complexity of air flow dynamics and indeed the air flow with slower speed has for some (weird) reason better penetration abilities?

[f5r5e5d]

yes there are better sites for fluid mechanics - the airfoil/lift relations are popular: http://www.av8n.com/how/htm/airfoils.html#sec-airfoils for some good physics

3.16  Consistent (Not Cumulative) Laws of Physics

We have seen that several physical principles are involved in producing lift. Each of the following statements is correct as far as it goes:
• The wing produces lift “because” it is flying at an angle of attack.
•The wing produces lift “because” of circulation.
•The wing produces lift “because” of Bernoulli’s principle.
•The wing produces lift “because” of Newton’s law of action and reaction.

We now examine the relationship between these physical principles. Do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way.

There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli’s principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton’s laws.

See section 19.2 for additional discussion of how Newton’s laws apply to the airplane and to the air.

[Mechatrommer]

• The wing produces lift “because” it is flying at an angle of attack.

99% true

•The wing produces lift “because” of Bernoulli’s principle.

99% myth...

https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html
http://warp.povusers.org/grrr/airfoilmyth.html

99% of lift is generated by the net drag or friction forces acting on the blade/wing surface... and the proper angle of attack is one important role in this case...

However, it is counterintuitive to imagine that air molecules with higher speed have smaller penetration(for example, penetration to radiator with thin fin gaps) ability than the ones with lower speed.. Or is it once again because of complexity of air flow dynamics and indeed the air flow with slower speed has for some (weird) reason better penetration abilities?

you need to get the imagination right... and this can be quite complicated... first, air is a compressible fluid, not like water or electrons that are assumed to be incompressible, ie changes at one point will result on changes on another point instantaneously, air is not like that. kinetics from adjacent air molecules will create drag or suction effect, thats what you see as "low pressure" effect. penetration is not proportional to spatial pressure but more to kinetics energy, you move faster, you penetrate, you stay still nothing happened, only pressure... in ee we have skin effect, in fluid mechanics we have exactly that. air adjacent to aluminum fin are stand still, the farther they are from the surface the faster they go pushed by adjacent molecules, they form parabolic shape in term of speed.... to summarize.... its complicated...

[Mechatrommer]

if you want the poorman's way... you can blindly design a fan, put the outgoing air into a enclosed box with pressure sensor in it, you read the pressure and write it down in your spec. to get the airflow speed spec, you put the outgoing air in an open tunnel with flow meter in it, voila! if you are not happy with the spec, redesign the fan and redo until satisfactory...

[f5r5e5d]

argue with Denker http://www.av8n.com/how/htm/author.html - after reading his really excellent site

[suicidaleggroll]

When a fan has higher pressure and slower air-flow, then how do air molecules move differently than in case the fan has lower pressure and faster air-flow?

Once again...the fan does not control the relationship between pressure and air flow, the heatsink does.  The fan specs just [indirectly] tell you the heatsink density that fan has been optimized for.

Think of the fan like a 10W power supply.  You have unit A rated at 1V and 10A, and unit B rated at 10V and 1A.  Sure they're both "10W" supplies, but they're designed for very different loads.  If you stick a 10 ohm load on unit A, you won't get anywhere near the rated 10W, and similarly of you stick a 0.1 ohm load on unit B you won't either.  Just replace current with air speed and voltage with pressure, and it's basically the same as these "high pressure" and "high flow" fans.