14cm fans versus 12cm fans - diminishing returns?

[dan]

hi,
i've been looking enviously at the coolmax PSU with 14cm fan and the superflower PSU with 14cm fan versus the seasonic tornado/s12 with a "mere" 12cm fan.

clearly a 14" pizza is nearly 33% more pizza than a 12" pizza.

if all else equal (i.e electrical power, amount of rotations per minute, turbulence, model, etc.,) wouldn't a 14cm fan be more effective (i.e 33% more effective) in pushing air than a mere 12 cm fan?

i am designing my system so that the 12 cm or 14 cm PSU fan is the only fan, and the coolmax and superflower 14 cm-fanned PSU are much less expensive than the mere 12 cm fan of the seasonic.

i would imagine that i could either push the same amount of air with a 14cm PSU fan as a 12CM PSU fan at lower RPM and lower noise and turbulence, OR push more air at the same RPM, all else equal.

[ferdb]

Theoretically you are correct. However when you try to apply that in the real world you run into problems. Such as that fans larger than 120mm almost invariably have ball bearings which constitute a large noise source if you are running things slow. The larger fans also are usually designed to flow a great deal of air, since that's usually why you are willing to put up with a larger more expensive fan. With high airflow as the primary design goal the motors are strong and are designed to be electrically efficient. Unfortunately these parameters tend to make for noisy motors. Many of the larger fans also have fan blade designs that are optimized for fairly high RPM and tip speed. Running them at low speeds and their back pressure capabilities drop off rapidly, more than a blade design with more cross sectional coverage, lower blade pitch, and blade overlap. All these things tend toward canceling out the advantages of a larger fan size.
This does not mean it's impossible to find something better than 120mm fans. It just tends to be harder than you would think at first glance. There are no guarantees at all that any given 140mm fan is going to be better than the available 120mm alternatives. Depending on the application it's possible you will not be able to find any larger fans on the market that will outperform the 120mm.

Because of the number of real world variables that creep into these things the only real way is to get the fans and test them yourself. Assumptions don't get you very far when looking for quiet fans.

[dukla2000]

i would imagine that i could either push the same amount of air with a 14cm PSU fan as a 12CM PSU fan at lower RPM and lower noise and turbulence, OR push more air at the same RPM, all else equal.

Yes

Because of the number of real world variables that creep into these things the only real way is to get the fans and test them yourself.

There's the rub! My kneejerk in response to the thread topic is stick to 12cm because at least you have a choice out there in the aftermarket - ferdb put it better. In the (likely) event the 14cm fan that comes free with your psu wont do what you need, you have very few options to replace it. At 12cm you have Nexus (Yate Loon Slow), Papst 4412FGL, SuperRed and upwards through Evercool to get the rpm/voltage you need.

[Tibors]

And with a bit of extra work, you can use all the 12cm fans in the 14cm fanned PSU. Try it and report back.

If everybody only uses the recommended components, then we never find new interesting things.