Slow Fan Bottlenecking Fast Fan?

[30Valve]

Hello, new user here so please be gentle. I was wondering if it's possible for a slow fan to bottleneck the performance of a faster fan.

For example, I am thinking of purchasing for my heatsink a Noctua NF-F12 Industrial PPC-3000 (110 CFM airflow and 7.63 mmH20 static pressure at full-chooch). Behind it I would have a generic fan from an old Alienware case exhausting at 45 CFM. Would I be limiting the efficiency of my cooler if the hot air behind it can't be dissipated fast enough? Would this create problems with air swirl in that part of the case?

 

I have about 3.5 cm of heatsink infront of and behind the CPU fan, and about 8 cm between CPU and exhaust fan. There are no additional holes in the case around the exhaust fan and no top vents.

[WoodenMarker]

A slow fan or a static fan can definitely impede airflow. A low rpm fan might not affect performance much though. 

If you're running both fans at max rpm, I think it would be better than just running the single NF-F12 in push. 

 

Why get such a high rpm fan though? It's far from quiet and the normal line of noctuas are quieter at the same rpm. 

What cooler are you using and why are you considering buying a new fan for it?

[LogicalDrm]

Two such different fans on CPU cooler might make things worse rather than better. Sure you can have iPPC as heatsink fan and another as exhaust, that could work. But unless you have system in very problematic situation, I would advice getting normal fan, most with 60-70 cfm are quite capable.

[30Valve]

8 hours ago, WoodenMarker said:

Why get such a high rpm fan though? It's far from quiet and the normal line of noctuas are quieter at the same rpm. 

What cooler are you using and why are you considering buying a new fan for it?

 

To preface my answer: everything I'm working with I got for free. I'm using a Tuniq cooler bracketed to an LGA775 socket and a Core 2 Extreme QX6850, which runs very hot when OC'd. The fan that came with the cooler pushes 76.9 CFM at 3.5 mmH20, which I realize isn't half bad, but the fan uses a 3 pin connector and a separate 2 pin connected to a rheostat that sits in a PCI slot bracket (archaic, I know). I don't feel like crawling under my desk and behind my PC to tweak CPU fan speeds on the fly, and if I try to connect that 2 pin header into the fourth CPU fan pin without grounding it, it behaves erratically. I don't have the means to strip wires and crimp on a new connector.

So that's why I'm looking for a new PWM fan. I figured since the iPPC 2000 and 3000 cost the same right now, I'd get the 3000 for more headroom and simply run it slower most of the time. My next option would be a two-pack of Corsair ML120, they are only $3 more than either Noctua on Amazon right now.

[WoodenMarker]

7 hours ago, 30Valve said:

To preface my answer: everything I'm working with I got for free. I'm using a Tuniq cooler bracketed to an LGA775 socket and a Core 2 Extreme QX6850, which runs very hot when OC'd. The fan that came with the cooler pushes 76.9 CFM at 3.5 mmH20, which I realize isn't half bad, but the fan uses a 3 pin connector and a separate 2 pin connected to a rheostat that sits in a PCI slot bracket (archaic, I know). I don't feel like crawling under my desk and behind my PC to tweak CPU fan speeds on the fly, and if I try to connect that 2 pin header into the fourth CPU fan pin without grounding it, it behaves erratically. I don't have the means to strip wires and crimp on a new connector.

So that's why I'm looking for a new PWM fan. I figured since the iPPC 2000 and 3000 cost the same right now, I'd get the 3000 for more headroom and simply run it slower most of the time. My next option would be a two-pack of Corsair ML120, they are only $3 more than either Noctua on Amazon right now.

If you need fan speed control to keep noise down, I'd suggest getting a different fan. iPPC's aren't as quiet as the normal Noctuas even at the same rpm and are very loud at 3000rpm.

I'd suggest going with a NF-A12x25 instead if it doesn't break the budget. 

[30Valve]

2 hours ago, WoodenMarker said:

iPPC's aren't as quiet as the normal Noctuas even at the same rpm and are very loud at 3000rpm.

Thank you for the insight, that is very helpful. I was drawn to the industrial fans for two primary reasons. One being the excellent static pressure performance and two (I hate to be that hater) the black aesthetics. I really don't care for Noctua's traditional color scheme. Noise isn't that big of a factor for me, and the iPPC 2000 outperforms the P12 Redux.

Is the only difference between the iPPC 3000 and 2000 a higher RPM ceiling? Do the fans perform identically up to 2000 RPM? Does the 3000 have a higher minimum RPM floor?

[WoodenMarker]

6 minutes ago, 30Valve said:

Thank you for the insight, that is very helpful. I was drawn to the industrial fans for two primary reasons. One being the excellent static pressure performance and two (I hate to be that hater) the black aesthetics. I really don't care for Noctua's traditional color scheme. Noise isn't that big of a factor for me, and the iPPC 2000 outperforms the P12 Redux.
Is the only difference between the iPPC 3000 and 2000 a higher RPM ceiling? Do the fans perform identically up to 2000 RPM? Does the 3000 have a higher minimum RPM floor?

All NF-F12's perform identically at the same rpm since the structure is the same. Acoustic signature would be the main difference. 

Noctua lists the minimum rpm of 450 for the 2000rpm model and 750rpm for the 3000rpm model. 

I wouldn't recommend getting iPPC's if you care at all about noise. NF-A12x25's perform much better for the noise. That being said, 3000rpm NF-F12's will be better if all you care about is cooling. 

https://www.enlacehw.com/noctua-nf-a12x25-review/3/

https://www.techpowerup.com/reviews/Noctua/NF-A12x25_PWM/5.html

https://ggwptech.com/pc-hardware-reviews/review-noctua-nf-a12x25/

[30Valve]

Thanks for those resources, I appreciate it. Regardless of the CPU fan I get, let's get back to the main topic:

Is there an acceptable minimum ratio of airflow between a CPU fan and an exhaust fan? With my current setup (Alienware case fan and Tuniq CPU fan), the exhaust fan can move about 58.5% of the airflow of the CPU fan. When you builders assemble from scratch, do you consider this difference if using multiple models of fans? If so, where do you find a good balance?

[WoodenMarker]

6 hours ago, 30Valve said:

Is there an acceptable minimum ratio of airflow between a CPU fan and an exhaust fan? With my current setup (Alienware case fan and Tuniq CPU fan), the exhaust fan can move about 58.5% of the airflow of the CPU fan. When you builders assemble from scratch, do you consider this difference if using multiple models of fans? If so, where do you find a good balance?

Specs aren't representative of real life performance where there are obstructions that significantly reduce airflow. It's too complicated to predict with any detail and the best you can get is an educated guess. For the exhaust fan to be of any use, it has to move air more quickly than the air already exiting the cooler by the time it reaches the rear of the case. There's no need to find a balance and it's easier to just aim for as much performance as possible without sacrificing acoustics. 

 

Make sure to quote a post or tag a member when replying or else they won't get a notification that you replied to them.