Static Pressure for computers

[MDCOX]

Hello everyone.

Lately I have been doing alot of research on computer fans. I have been testing CFM and in the process the comment "static pressure is more important" keeps coming up.

Here is my two part question:

If the idea of a computer fan is to move HOT air OUT of the case and to move COLD air INto the case, wouldn't CFM be more important?

And it seems static pressure is measured mostly in air ducts or (from research) animal holding facilities. So I can agree that the computer case itself is a duct of sorts but the primary goal is to move hot air away and from the components and get cool air to them. So is static pressure more important in this situation?

Just FYI:

Most people setup the fans so they have 1-2 in the front of the case as intakes pulling fresh air in, 1 intake on the bottom of the case, 1-2 exhaust setup on the back panel and 1-2 exhaust on the top of the case.

So fresh air flows in from the front and bottom while hot air exits the back and top.

I hope I articulated all that sufficiently. If not bare with me and I'll get it right.

Thanks in advance for any help.

 

[MDCOX]

Any one?

 

[Compositepro]

You need cfm for heat removal. But without sufficient static pressure capability of the fan or blower to overcome flow resistance in your system, you cannot get adequate cfm. So both numbers are important. Fan cfm is rated at a certain differential pressure, typically 0.1" of water.

Every fan, blower, or pump has an operating curve for output pressure versus flow rate. As output pressure increases (due to a resistance placed in the flow) the flow rate decreases.
Every system (your computer) has a operating curve for pressure required to get a certain flow. As pressure increases flow rate increases. The actual pressure and flow rate will be where these two curves intersect.

 

[MDCOX]

Is there a way for me to test this at home? I currently have an Extech AN100 anemometer that I use to test CFM but I'm wondering about testing static pressure and would I do it in the case somehow or fans out of the case.

The biggest thing for the computer guys is the heat removal. FWIW we also have a basic understanding of positive and negative pressure in our systems. Most agree that even though it requires a little more house cleaning negative pressure is the best option.

I greatly appreciate the help.

 

[rb1957]

trying experiments sounds a reasonable way to go ...

the CPU will tell you it's temperature, so you can see the effect of the fan. you could put a clean plastic cover on the box (so you can see inside) and introduce smoke to see where it goes. you could seal the box to control airpaths. remember those heat sinks (like on the video card) generate a lot of heat, which needs to be directed out and away from teh CPU.

the advice i got from computer guys was Always control the airflow in one direction (suck at the front, and blow at the back).

 

[Compositepro]

The difference in behavior of airflow on the suction side of the fan versus the discharge side of the fan has nothing to do with "positive" or "negative" pressure. It is about the momentum of air flow. On the suction side, motionless air accelerates toward an area of lower pressure. On the discharge side air is flowing at its maximum velocity and it will keep moving in the same direction unless something deflects it. So you can have a "jet" of air on the discharge but not on the suction. I often see this misconception in dust handling systems where someone thinks that by pointing a duct in a certain direction they can pick-up air from that direction.