Hello,
I have a small setup based on a fan. Basically I just want to suck air through a
small hole in the wall. I started with a 120 nice silent fan:
http://www.ebay.co.uk/itm/NEW-Scythe-Kaze-Jyuni-500-RPM-Slip-Stream-Fan-120mm-/311004556633?pt=UK_Computing_Case_Fans&hash=item486950ed59
Which is really a nice and silent. It moves a lot of air (high CFM). But when I put it to use I noticed that it sucks near zero air. It just spins in a vortex, and there is even some flow reversal (it seems to do the opposite then what supposed!). I researched, learned about fan stall, flow separation, and ultimately static pressure.
Static pressure is a very important concept when designing air systems. Unfortunately the static pressure performance of most regular advertized computer fan is usually not disclosed. That's probably because they are pretty low, especially for those fans that have a high CFM (move lots of air).
CFM (cubic feet per minute), which is what is advertized, is the capacity of a fan to move air in an optimal free air setting; if you, for example, hold it in your hand and make sure the inlet and outlet are totally free of any obstacles. In such a setting, such a fan moves lots of air. But as a soon as it starts encountering some resistance, its performance decreases dramatically, until it ultimately (in cases or high resistance) reaches a stall and flow reversal configuration, where it is just spinning air around, making sound, consuming electricity but not doing anything useful.
Static pressure is the capacity of a fan of pulling (or pushing) air through resistance and obstacle, such as small openings.
A simple analogy to help you understand: It's a bit like a car. If you put it in 1st gear, it will pull lots of weight, but won't go too fast. If you put it in 5th gear, it will go fast, but if you start it in 5th gear, and in a steep uphill climb (resistance) it just won't move.
So these high CFM performance fan are especially weak in the static pressure department.
So I got myself some (2nd hand) proper, serious rack server fans:
http://www.ebay.co.uk/itm/Dell-Poweredge-860-Dual-Fan-Assy-HH668-KH302-/150638939507?pt=UK_Computing_Servers&hash=item2312c7cd73
Those have so much static pressure, you make a proper vacuum cleaner out of these if you run them together at max power. I'm not kidding. Problem is they also sound exactly like a vacuum cleaner when you run them at full speed.
These are centrifugal fans, as you can see by their shape. The general characteristic of centrifugal fans (also called "blower") is that they have a high static pressure but lower CFM than axial fans.
So as you can see, I'm a fan geek now, or I'd rather say, a fans fan :) .
Putting aside beyikh jokes and getting excited about computer fans, what I'm getting to what I wanted to ask originally.
I originally ordered a variable potentiometer to go with the Scythe fan. One of those small cheap pots that are rated for like 1W (or less maybe, I forgot). I tried it on the 120mm skythe fan, and it was sparking and producing fumes, and this is where I also learned about amps and power rating. The Skythe fan is sub 0.1 or .2 amp, not much, but still over 1 watts, which is over the rating of the pot. I don't even want to try it on the centrifugal fans, which can pull like 300 W each.
In the meantime, I had also bought some telephone (or ADSL) cable, because it's the cheapest kind of cable that I had found, and used it to extend the power cable to the fan.
I noticed that adding using the telephone cable would reduce the speed to the fan. So since my pot was not working, I was still able to modify the speed of any fan by modifying the length of the phone cable segment that I was using to get power to it. It works pretty well and have never noticed the cable getting hot.
So a phone cable (or is it a DSL cable?) can be used to as a poor man's resistance, Mc Gwyver style. Is that OK? And why is that? Anyone knows more about this topic?