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Unusual question about AF/SP fans ;)


4wk_
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Hi there,

 

I didn't find anywhere on the web the answer to my question so I took the liberty to contact you guys ;)

 

I know a lot of things about computer fans. I'm pretty familiar with the AF model, and the SP one.

1) I understand that a SP fan is more effective to PUSH the air trough a radiator, or a dust filter, or a hard-drive cage. => SP push less air, but harder, stronger.

2) I understand that an AF fan is more effective to move the air when there is - according to your specification - at least 3 cm clearance after the fan. => AF push more air (in volume)

 

But, speaking about DRAWING IN the air ONLY, is there a difference between the AF and the SP fans ? In others words, do they "suck" the same amount of air ?

 

I want to know if it’s better to use a SP or an AF fan for an intake fan on the front of a computer case with a dust filter (for exhausting the air through a dust filter, I know, it’s a SP fan the best, but for drawing in from a dust filter ?).

 

Here is an ugly diagram of my case (Vengeace C70, with HDD cage removed) :

[Front of the case] | [dust filter] | [the fan] | [LARGE room, no resistance]

 

Thank you for your time,

Cheers,

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1) At any given RPM in free air, an AF fan will move more air than an SP fan at the same speed.

 

2) An AF fan may still be move more air when faced with *some* level of resistance. The greater the resistance, the more the SP fan closes the airflow gap. Whether the resistance is in front or behind the fan, does not matter for airflow. However, a dust filter or screen that nearly touches a steeply raked blade (like on an AF model) may have a greater impact than expected. The fan tries to draw air in from the sides of the fan and at that angle, the surface area of the filter can act as a larger barrier (more surface area at the angle).

 

3) At lower speeds, an AF fan is more susceptible to the effects of resistance. At faster fan speeds, the AF may still move more air when faced with resistance. If you have dust filters and really want to also keep fan speeds low (for noise purposes), you likely want SP fans. If you need as much airflow as possible and like gaming with headphones (and thus are less concerned about fan noise) several AF fans at high speed will move more air than the same number of SP at like speeds.

 

Unfortunately, it's hard to write a golden rule. Some fans react badly with particular cases or exact distances between barriers. You may also need to account for honeycomb coverings. Those tend to be more restrictive than a wire frame. You don't really know for sure until you try. I've never set-up a C70, so I cannot definitively answer your question. However, there is another alternative. The ML and new HD/SP RGB series fans are all 'hybrids' of the AF/SP dichotomy. They are something in between and represent a compromise of both designs. This may be your most optimal set-up for the front. Even in my 540's with no front filter and a very open air pathway, I usually prefer the hybrid style 120mm fan in the front of case instead of a steeply raked AF style. I have enough airflow, but the more focused direction of the hybrid style is more effective at displacing some heat around the motherboard, yet it still provides plenty of airflow. I just put some other fans (3x120 AF like style) in the front of a new 740. Even with increased overall airflow in the case, I have noticed a few components actually register slightly higher temps than with the hybrid model. Most of the time, you are not doing anything more than exchanging the air inside the case (current out, new in). Small differences in airflow do no make much difference in the overall health and temperature. You are more likely to be put off or irritated by a loud fan than a 2C difference in ambient temperature. I would choose along those lines unless you have some very heavy big watt hardware to manage.

Edited by c-attack
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