[SOLVED] Barefoot empiricism concerning fans

[gn842a]

I was getting worked up about fans yesterday because I was having this thought:

If you take a case like my 5 year old cooler master, with one intake (front 200 mm) and two exhaust (one four inch, one 200 mm),

So let's say the 200 mm intake is 70 cfm and the 200 mm exhaust is 200 mm and 70 cfm and we have a four inch fan also set to exhaust which is 40 cfm.

So the exhaust fans would be pushing out (or trying to) a combined 110 cfm and the intake would be supplying only 70 cfm.

One could make the argument in these conditions that the intake fan is actually slowing down the movement of air through the case.

So I decided to do a crude test. I took a paper napking which was almost the size of the 200 mm exhaust fan and put it on top of the hole. There was enough wind to life the paper napkin, it didn't want to stay in place. So I taped it into place on one side, making a sort of hinge along one side. That way I could measure the air flow by the height to which the edge of the napkin rose, with one side taped down. To the best of my ability all the various holes in the case have been masked off: the holes at the bottom that the PSU does not cover, the holes at the back where the unused removable slots have holes in them, other small holes such as I found here and there including at the top of the case.

1. Measured height of napkin with all fans going and the side removed from the computer case: 1 1/2 inches
2. Measured height of napkin with all fans going and the side put back on: 7/8 inch
3. Measured height of napkin with front fan disabled and two exhaust fans on: 1/2 inch

Discussion: The highest reading was of course with the case open (#1). The exhaust fans had infinite air available at only a few inches of distance and so they could increase their flow to close to their theoretical maximum. So the lower rating of (2) was not a surprise since the fans now have to work to pull the air through the case and up against gravity. As it turns out you can move a great deal of mass with fans and it can add up to a lot of power: When I had fans running in every room of my house in the summer, my electric bill went up noticeably. Anyhow if you think of the air as a fluid you're trying to pull it up against gravity so obviously if the intake is a hole about 24 inches away the fans are going to have to work harder to pull the air in and then push it up and out.

But where I directly addressed my theoretical concern was with case #3, which showed decisively that the intake fan was significantly improving the movement of the air through the system.

I haven't figured out a way to control the individual fans or I would do more studies such as turn off the top fans and leave on the front intake fan. Speedfan used to give me such control on my own build but on this new one speedfan doesn't even detect all the fans. In any case the intake fan helps, but what would really be interesting is to be able to get something like speed fan and measure air throughput with the different options. --Greg N

 

[CountMike]

Well, no air flow will actually cool any dry material just try to equalize cooler's temperature with intake air temps.
What is ideal is to have as much airflow as possible over a given element and fast enough to take heat away but over certain air speed that element can't transfer heat to the air fast enough so speeding it up even more it will not help any.
Gases and liquids follow same laws of thermodynamics and because of liquid's slower heat transfer are easier to notice. Take an example of car engine cooling, if you take thermostat right out it will most probably overheat at higher revs because of water flowing too fast and not being able to soak heat from engine and transfer to radiator for same reason.

 

[Paperdoc]

Clever thinking to find a way to measure air flow in your experiments. The results make sense. I suggest you do not need to worry a lot.

First of all, air intake flow always equals outflow - there's no other way to balance. But POTENTIAL air flow can be different if the various fans have potentially unbalanced flow. How that really shows up is that the interior of the case may have a very slight positive air pressure inside compared to the outside (if intake capacity is greater than exhaust) or a negative pressure if the balance is the other way. You can start this estimation by looking at the specs of each fan. For example, you have two 200 mm fans - one for intake, one for exhaust. If they are the same model and air flow specs, using those two ONLY would produce matched flows and no net internal pressure. Next, recognize that the INTAKE fan should have a dust filter in front of it so its actual performance will be less flow that its max spec, and we have a small negative pressure inside the case. Then add the fact that there really is a third fan in an exhaust position, so that's even more exhaust capacity and hence a negative internal pressure. HOWEVER, the addition of that third fan still increases the total air flow forces compared to the two-fan scenario, so it IS helping.

In my thinking, the "ideal" fan arrangement has slightly MORE intake capacity than exhaust for two reasons. First, the intake fan dust filters reduce their performance slightly. More importantly, all cases have small openings and cracks through which air will leak. If the case's internal pressure is above the exterior, then that leak will cause air to flow from inside the case outwards. In the opposite situation (negative pressure) air will be sucked into the case at those leak points, BUT there are NO dust filters there, so this can cause some dust intake with the leaked air. Thus my preference for small positive case pressure (more intake capacity than exhaust) is based on dust prevention, and NOT on total air flow. No matter which way the flow balance is, in general more fans generate more air flow and better cooling.

OP, IF you could do it, I would suggest changing the arrangement so the 120mm fan now in an exhaust position becomes an intake fan with dust filter, like the front 200 mm fan. But that won't change the total air flow or the cooling capacity significantly. So if you cannot do that, leave it the way it is.