Those fans are generally used for duct booster fans in a house, helping the main blower at the furnace do its job - at y junctions and to help flow to the farthest runs in the house.. As soon as you put anything that makes static pressure on it, or try to use it alone in ducting, the flow rate drops dramatically. Like Xcrispi said, hes got 2 and they are barely doing the job. I tried to use a couple of 6" ones for my cooltube (2x400W) and had to return them to the store, didn't hardly do anything.

In the fan world, powerful AND quiet = $$$$$. I found that a good fan is one of the more expensive parts of any setup, but totally worth it in the long run.