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Thread: 120 mm fan replacement: max air flow, max air pressure or balance?

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    I've always felt that the number of blades should be a prime number, which is not the case for some of the fans you listed.

    How is flow rate measured? for a fan pulls air around the outside (entrainment), and if this is included it will boost the specs.
    Last edited by ashiekh; 07-14-2018 at 06:51 PM.

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    Hm, you may be on to something. The Delta does have seven conventional blades along with a very large hub, so the assumption of it being not the most efficient design would not be unreasonable. There are sites which perform their own air flow measurements, but their reviews as far as I can tell tend to pitch retail fan against each other, and include few (if any) OEM models for comparison sake.

    The local computer store has the Scythe SY1225DB12H (1600 rpm, double ball) in stock for a good price, so perhaps I'll go with that.

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    Quote Originally Posted by Fan-tastic View Post
    Hm, you may be on to something. The Delta does have seven conventional blades along with a very large hub, so the assumption of it being not the most efficient design would not be unreasonable. There are sites which perform their own air flow measurements, but their reviews as far as I can tell tend to pitch retail fan against each other, and include few (if any) OEM models for comparison sake.

    The local computer store has the Scythe SY1225DB12H (1600 rpm, double ball) in stock for a good price, so perhaps I'll go with that.
    Is that a Slip Stream? Scythe Slip Stream fans are known for their high CFM and low static pressure.

    Fan sellers often cheat on their specs (I know this is shocking info). The industrial makers do not, because their customers buy in such bulk they can test them themselves. I have never heard any complaints about Delta or Nidec, for example. As for Sanyo Denki (makers of the "San Ace" line) from personal experience, I think their specs are such that they are guaranteeing that 95% of their fans will meet or exceed those numbers. I cannot speak about other industrial suppliers.

    Other OEMs give you a spec that represents the average of what the fans will do.

    Bottom line: if you are going to replace a fan from specs, go with a fan from an industrial supplier.
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    Quote Originally Posted by ashiekh View Post
    I've always felt that the number of blades should be a prime number, which is not the case for some of the fans you listed.
    While blade count can be important; being a prime number is irrelevant. Blade count in relation to the design of the fan is important. If pressure is your focus you want fewer larger blades, while if airflow is your focus you want more thinner blades.

    After those factors you have to consider pitch, curvature, relation of tip to housing, etc to factor in for noise.
    Quote Originally Posted by ashiekh View Post
    How is flow rate measured? for a fan pulls air around the outside (entrainment), and if this is included it will boost the specs.

    Airflow testing is pretty simple, you buy a dedicated machine, pop the fan in, and then hit the button and wait for a printout to tell you the specs.


    For the rest of us amaturers you need to build (or buy) an airtight box/tunnel that is 1 cubic meter; place your fan on one side blowing in, and then your anemometer on the other side where the air will exhaust. The design / shape of this container is more accurate if it's a "tube" that's relatively the same size as the fan itself (something 120mm in diameter for example) with no obstructions to get the clearest results.



    Quote Originally Posted by Fan-tastic View Post
    Hm, you may be on to something. The Delta does have seven conventional blades along with a very large hub, so the assumption of it being not the most efficient design would not be unreasonable.

    The Delta's blade count and design is to facilitate a balanced approach to airflow and static pressure. To have more static pressure you're going to need the larger "hub" as you need more torque to your motor.


    Quote Originally Posted by Fan-tastic View Post
    The local computer store has the Scythe SY1225DB12H (1600 rpm, double ball) in stock for a good price, so perhaps I'll go with that.
    Bad choice. SlipStream's lack any kind of static pressure which you need for a PSU as the airflow pattern in most PSUs cause the air to take a quick 90 degree turn which kills flow if there isn't proper pressure behind it.



    EDIT: Also, for why reviewers don't touch industrial fan products: They are generally too expensive to go out and buy on your own, even for the general consumer. Companies will provide you fans to review if you ask nicely enough. At least most of the time. Unfortunately fan reviews get VERY few hits. Unless they're RGB and you have some good photography / video coverage.

    Industrial fans have a small market for the consumer space so they don't even register most of the time on people's google searches for "What case fan to buy" or "What heatsink fan should I get." SEO is a bastard to those who like niche things.



    Ed was also right about companies like Delta, Sanyo Denki, & Nidec Servo having upstanding build quality as well as accurate specifications. Other companies can be very "liberal" with how they rate their products or downright deceptive.

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    Quote Originally Posted by Tator Tot View Post
    While blade count can be important; being a prime number is irrelevant.

    If there are 7 blades for example one has harmonics at 7 times the rotational frequency

    If there are 6 blades one has harmonics at 6, 3 and 2 times the rotational frequency

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    It's the Same Issue with Rads, I think it's a little better Pushing the Air on to it. But then you Develop of course the awful build up. It was Meant to be Reversed and Pull.
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    Quote Originally Posted by ashiekh View Post
    If there are 7 blades for example one has harmonics at 7 times the rotational frequency

    If there are 6 blades one has harmonics at 6, 3 and 2 times the rotational frequency
    Then why is the Noctua NF-S12B a six-bladed fan?
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    I thought the Noctua NF-S12B is 7 bladed?

    https://www.amazon.com/Noctua-Bearin...+nf+s12b&psc=1



    But I have seen 9 bladed fans


    Seems that if one spreads the noise over more of the spectrum it is perceived as less noisy by the brain;
    so we have a dilemma... less noise for the specs or less noise for the user.

    Translation: I haven't the foggiest idea; but we are here to figure things out by playing with ideas:

    Lets take a 3,000 rpm fan
    7 blade means 21 kHz and above, beyond the limit of the ear.


    This might suggest that fast fans might benefit from prime (to push the fundamental out of hearing range),
    and slow fans might benefit from non-prime to try and spread the noise over the spectrum.
    Last edited by ashiekh; 07-15-2018 at 10:07 PM.

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    If you're really looking to adjust frequency to avoid causing the human ear to perceive something as noisy then you either move into the route that Noctua has with it's dimples, notches, etc.

    The A20, A12x25, & P12 have 9 blades while the A14, F12, & S12A have 7 blades.

    Or you have blades of varying size and geometry like Silverstone's AP123, which has 9 blades, not a prime number.



    Like I said before; being a prime number isn't relevant on it's own. When it comes down to it, your blade geometry is the most important factor.

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    Thank you very much for the advice, it helps me a lot! It is exactly the kind of specific information I was looking for, and based on your explanations I can see why static pressure is important in a PSU. I kind of suspected this would be the case after looking at the offerings from Enermax and be quiet yesterday, i.e. the two companies I assumed to be most likely to use retail-style fans in their products. As it turns out, even their take on a PSU fan appears to be a pressure-optimized design combined with the strengths of the respective company (bearing, blade geometry) instead of a case fan already in production.

    So, no Slipstreams then. It’s a bit of a shame, since they are very reasonably priced and Scythe’s ball bearings have thus far been decent in my experience. I don’t have time for proper research at the moment, but skimming over the lists again, alternative products appear to be more expensive. Well, aside from the Enermax Magma Advance @ high/1800 rpm. The Antec EA-430 was a budget PSU to begin with (albeit of good quality), and spending 20+ bucks for a fan to be installed in a several years old PSU isn’t really something I could justify.

    If everything else fails, I suppose performing some preventive maintenance on that spare, bearing noise-free D12SM-12 and using it instead would be an option to fall back on.

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