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High Static Pressure


amaskedman
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Lately, I am seeing fans sold as High Static Pressure that have a Static Pressure spec of under 2. This is a link to corsair RGB fans (new ones)

 

http://www.corsair.com/en-us/ml120-pro-rgb-led-120mm-pwm-premium-magnetic-levitation-fan-3-fan-pack-with-lighting-node-pro

 

Now this is a link to the non RGB fan sold as High Static Pressure. Rated at 3.1

 

http://www.corsair.com/en-us/air-series-sp120-high-performance-edition-high-static-pressure-120mm-fan

 

Did the way pressure is calculated change, or is the baseline for what is considered high static pressure get lowered?

 

What is the rating to be considered a high static pressure fan?

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The term "static pressure designed" in reference to the blade or fan as a whole is general term, like sports car, deluxe or anything else. In real terms, a static pressure designed blade will be flatter, wider, and usually have a fewer number of blades compared to an airflow designed fan. The airflow prioritized fan usually has more blades, they are thin, and steeply raked. This makes it more efficient at moving air and with less air noise, but they fall flat when faced with moderate resistance and have wide conical dispersion.

 

The problem is the real meaning of the static pressure value gets run over when people talk about it. The static pressure number you see on fan specifications is the amount of negative pressure it takes to make a fan come to a complete stop at XXXX speed. It is the last part you need to remember. No matter how well designed a fan is for a radiator, the static pressure needed to stop it at 100 rpm is small. The reverse is also true and a 3 blade razor thin AC Delco churner will have a rather flattering static pressure rating at 3000 rpm. That doesn't mean you want it on your radiator. No radiator provides enough resistance to stop a fan, at any speed. So that is not the worth of the metric. It is a rough efficiency prediction of how efficient a fan will move air when faced with some measure of resistance. However, you must equalize the values by speed/RPM.

 

Take the SP120 page you linked. It shows two variants for the SP120 -- a high speed and "Quiet speed" model. It is the same fan with a different motor to create the speed range. That blade makes 3.1 mm at 2350, but only 1.29 at 1450. The high speed variant will also make 1.29 at 1450. Same blade, same values at different RPMs. Any fan, 140 or 120mm will have a very low looking static pressure value under 1000 rpm. The speed has a massive bias on the end number, but that does not mean a SP120 is bad fan when under 1000 rpm. It means it takes less pressure to stop it at 900 than at 1800 and of course moves less air at lower speeds. Not really a surprise.

 

The example I would often cite is this little Blacknoise fan I have that makes 1.66 mm H20. Not bad and better than a lot of 140mm fans. Want it for your radiator? Probably not. It only moves 5.4 cfm and needs 4000 rpm to do so. It is a 40x40x20mm fan. Ultimately, it is still the amount of air you can move that matters. A higher static pressure may indicate better performance on a radiator, but it isn't quite that straightforward. When speeds are low, those higher static pressure fans with wide blades will move the most air, but at those low speeds differences can be small. As speeds increase, the airflow thinner blade fan overcomes its deficit and gains ground back. At 3000 rpm, the airflow fan likely moves much more air than the static pressure fan. However, who wants a 3000 rpm fan running. A lot of new products are taking the middle ground with a hybrid design, something in between the two types. They have enough blade to do the job at low speed, but are no so wide and heavy they become limited at higher speeds. I would say the ML series falls into that category. They can be both case fan or radiator fan and get the job done. HD and SP-RGB models are similar in that respect.

Edited by c-attack
Lost a “not”
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Thank you for the reply. I find it interesting that many manufacturers make two model of RGB fans, a high airflow and static pressure. If they are going for a hybrid fan, why have two models? Why not just put RGB on their existing high static pressure fans thus separating the two fan models?
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By no previous standards are the current crop of RGB High Static Pressure Fans actually High Static Pressure Fans. Thats my point of confusion.

 

I think its a marketing slight of hand that lowers the bar. I am googling to see fi anyone has done real world testing.

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Thank you for the reply. I find it interesting that many manufacturers make two model of RGB fans, a high airflow and static pressure. If they are going for a hybrid fan, why have two models? Why not just put RGB on their existing high static pressure fans thus separating the two fan models?

 

The term "hybrid" is one I use and I don't know of any one who markets their fans with that tag. You are going to market the fans as "high airflow" or "pressure focused" mostly because that's what advertising does - focus on key words. In reality, most every 120mm fan on the market is a hybrid fan. Corsair just happens to have a clear dichotomy with the SP and AF sereis which make for good illustrative examples. I do not think Corsair is being disingenuous with their marketing nor really any one else. All fans will work on a radiator. There is no commercially available fan that can draw power that will not help move heat off a radiator, even if it does have 15 thin little blades. Same thing with case fans and a SP120 PWM will work just fine in that role. However, a lot of us like to min/max and pursue elusive last tenths of performance because, well just because. If you know you are in that group, then it is also on you to do the homework, experiment, and learn the true value of the specifications.

 

The number one reason to make a hybrid instead of separate AF/SP-like versions of a RGB fan is production and storage costs. You can have one fan do the job in nearly every application and do it well. You don't need the separates. Personally, I consider the AF and SP series to be niche tools. If you are trying to set up a really quiet case, a bunch of AF case fans will suit you well. Yes, it may lose some airflow when restricted with a dust filter and it may not be good for direct cooling, but it is quiet with low blade noise and that is priority one. The SP120 PWM is the classic low speed drive cooling or spot cooling fan. I do not like them at moderate speed at all and I never used them on my radiators. However, in applications with lots of fans where you don't need speed, they are ideal. I always think of a 900D with 480mm radiator across the bottom and 8 SP120s just strolling along at minimum speed and doing their job.

 

The other thing to remember is a "pressure orientated" fan may not be the best cooling solution for you. It is still about airflow and that makes the answer RPM dependent. The SP120 should be better (slightly) at low speeds, but the ML120 will pull away and put more air through a radiator at moderate and better speeds. The hybrid fan is better because it can move more air through the radiator than the fan with the higher static pressure number at maximum. That is part you need to be careful of. Static pressure does not equal airflow, in fact often the opposite (PV=nRT). Take a look at this snippet from the Thermal Bench website and this testing of Corsair 120mm fans. Note this is something like an efficiency rating with linear airflow at a specific decibel levels. So, how loud does it have to be to put XXX amount of air through the radiator.

 

http://i.imgur.com/ZtUIwNTl.jpg

 

The ML fan blows every thing else away in terms of dB per linear feet per minute, particularly at the lower end. I think a lot of that relates the relative quiet at lower speeds with that special bearing type. However, also note the standard SP120 RGB (not the ring fan) takes the lead near the top of its radiator speed max around 1300 rpm. The hybrid fans are winning. You can see the SP120 QE above, as both the loudest per air speed. The full 2350 rpm version is not graphed, but I know what it looks like. It will not surpass the ML later. It dovetails off and become limited at the higher speeds. Be careful when comparing static pressure numbers. You need to adjust for speed as the figure is very RPM dependent. Going back to the SP120 QE (the same fan as the PE edition), it will make 1.29 mm H20 at 1450. That is less than the SP120 RGB at 1400 (1.45mm). Less than the 1.56 I am estimating for the ML120 RGB at 1400. Less than the 1.82 mm I estimate for the HD120. Frankly, I don't think the pressure number matters much at 1300+ RPM and the real reason those fans perform better is they are able to move more air. This will be true both in free air and on the radiator. You can also compare this to an AF120 that should make about 0.8mm H20 at 1400.

 

So what is a significant difference in terms of performance? It's not 1.4 vs 1.5 or 2.9 vs 3.1 Those fans will all have similar performance on a radiator and graph out much like the linked one above. 1.4 vs 0.8 is significant. 3.1 vs 2.1 would be significant, IF those figures both come at the same speed. Most likely they would not. To see 3.0 mm H20 or better, you are likely looking at 2000 rpm+ and again, pressure becomes a diminishing factor as the speeds increase.

Edited by c-attack
numerous typographical errors
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