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Ideal RPM/CFM for a CPU Heatsink – Physics Question : P

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July 14, 2010 5:50:11 PM

Hi,

I just registered with the site but have been a long time observer… : )

1)I wondering if its true that a lower RPM with high CFM cpu heatsink fan is better for cooling the cpu than a fan that has a high RPM/CFM do to ‘turbulence’ (see quoted passage below before answering)?

I saw this claim on another post, quoted below:

“I'm using an Xigmatek HDT-s1283 with a Scythe Slipstream 110 CFM 1900RPM fan all in a case that cools like an Antec 900. Your heatsink should be pretty close in performance to mine, try reseating or buy a new fan. The best fans to put on heatsinks are fans with the high CFM and low RPM ratings, because if the RPMs are too high it will cause turbulence between the fins of the heatsink and wont perform well.”

Source:  http://www.tomshardware.com/forum/256144-29-1156-core-o...


I’m trying to OC my i5-750 to 4.0ghz on my Asus P55 Deluxe with Hyper 212 cooler.

I have 4gb of gskill ripjaw 1600 ram, but upgrading to 8gb soon…( not sure how this will affect my OCing)

I currently have a Delta PWM 3400 RPM, 113 CFM fan on the 212 that I regulate with ASUS fan xpert around 2000-2500 rpm.

I’m idling at 35 degrees C, and prime95 shoots my core Temps into the 90s! : /
(Vcore ~ 1.32)

Im wondering if I may need to reseat the heatsink or get a new fan…

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2)However running the delta at 2000 probably lowers the CFM so I was wondering if getting a 1900RPM, 110CFM Slipstream would be a better fan for cooling my cpu?

(I don’t have any room on my fan controller so it would have to be hooked up to a 3 pin slot on the Mobo.)

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3)I like the PWM cpu fan because I was able to control it with asus’s program, will I lose this ability with a 3 pin Sycthe Slipstream?

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4)What are the pros/cons of using a PWM fan vs a 3 pin case fan for the CPU cooler?

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I know this is getting long but I could find anything this specific on the forums so I decided to post.


Thanks for your help!

-Statz22
a c 97 à CPUs
a b K Overclocking
July 15, 2010 5:45:47 PM

Welcome to the forum!

You asked for Physics - how about some Thermo Dynamics? Heat transfer is a maximum when the speed is supersonic. The flow must be Laminar (Streamlined). For laminar flow in hydraulics, the Reynolds number must be less than 2000 - don't know what the corresponding number is for air.

Therefore, for maximum cooling, you must have the maximum air flow and the maximum speed of the air flow in a laminar flow condition.

If you can plug in the fan connector to the motherboard, you can control it thru BIOS settings

PWM is a type of speed control - The Width of the electrical Pulses are Modulated to control the amount of power going to the device (fan). This is generally accepted as a better way of speed control (Lasers are precisely controlled this way).

What does all this boil down to? Just do some trials and check the temps. Forget about Laminar flow and Reynold's number, etc.
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a b à CPUs
a c 100 K Overclocking
July 15, 2010 6:20:13 PM

The laminar flow depends on the fin spacing inside the heat sink. Maybe I can pull out my books at home later...

I think the biggest reason for lower RPM is the lower noise, IMO. CFM is a direct result of surface area and speed, so if it's more CFM in the same sized fan, the air itself is moving faster even if the fan is slower. Just means the fan probably has bigger fins that are at bigger angles to push more volume.

As for your temps, the idle is ok but yeah those load temps are way too high. At 1.32V my temps are maybe 70. You should start by reseating the heat sink. If you don't mind voiding your warranty, CPU and HS lapping can help quite a bit in some cases. I lapped my CPU and dropped the temps big time because the CPU wasn't flat, so there was a large area of little to no contact.
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July 15, 2010 7:06:49 PM

I heard static pressure is more important than cfm on heatsinks. Look for high static pressure fans.
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October 8, 2010 7:27:59 PM

Best answer selected by Statz22.
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