let’s say…compared to your normal 30" box fan… I have no idea how fast 275 CFM is…this fan is 6" across…it runs at 3050 RPM if that matters…
I need it to speed the evaporation of a body of water below a huge amount of lighting…
Hmmm. Well for comparison, your average bathroom ceiling fan moves about 100cfm and your average clothes dryer moves about 200 cfm. So if you’ve ever felt the amount of air coming out of the vents of these two fans, you can use that for comparison.
The site gives the specs for a 20" box fan:
http://www.atrendyhome.com/airki20debox.html
Even on low its moving 1300 cfm…
Just out of curisoity, is this for a fish tank?
yes, it is
I’m researching a lot and will be piecing things together over time and learning as much as I can – probably delaying the actual setup of the tank by over a year from the time I acquired the tank…
I figure it’s the right thing to do…but 275CFM sounds pretty weaksauce, so I’ll look at other fans.
thanks
Can you use an 8" fan?
Uh, no, it’s not weak at all. 275CFM is near the upper limit of what you’re gonna find. Of all square fans (excluding blowers) between ~5" and 7" in diameter carried by Digikey and designed to run on 115V nominal AC, the most powerful puts out 300 CFM and costs $72.90 each (and is also available in 240V AC, 24V DC and 48V DC (just in case your aquarium is gonna go in a telephone central office, I guess. :))
If you’re on a budget, Marlin P Jones and Associates has some surplus (used) 230 CFM 6" 120V AC fans for $9.95, and new 282 CFM 6.75" 24V DC fans for $15.95 each. They also have an exactly-six-inches 200CFM 24V DC fan for $12.95.
That last fan also has a stall-sensor output, so it would be very easy to make a simple alarm circuit for it it (just a self-driven piezo, a pull-up resistor, a 5V source, and a transistor).
Make that two resistors–one pull-up and one for the base of the transistor (probably a darlington pair, actually). Nevermind the details–suffice it to say it would be simple. 
Perhaps you could post more details about your setup. A fan may not even be necessary.
2x250w MH top lights about 10" off the water’s surface, no glass in the way, just the chromed-plastic grating(like you probably see in your office) to add a nice shimmer across the water and to keep the fish in the tank
2x65w PC actinics
I figure the actinics will come on 2h before the MH’s then the MH’s will burn for about 6h a day, then the actinics will shut off 2h later… I may have the actinics turn off during the MH’s photoperiod, as I’m not sure that 630w is really needed for a 75g tank
advice is more than welcome, since it’ll be months until this is up and running
also! if I do get the 4-pin DC computer fans…should I just run them off a computer power supply? or do you have other recommendations?
I forgot the part about the house being 80 degrees
Well, that’s a little tricky.
Long answer:
A typical ATX computer power supply is gonna have +5, +3.3V, +12V, -12V, and -5V. The -12V output can only sink a small fraction of the current the +12V can source, so that rules out 24V fans. The 12V output is relatively beefy though (IIRC, it’s used by the disk drives and other electromechanical parts), and should be able to drive plenty of fans for you (but do check the datasheet for the supply!)
Also, such power supplies usually need to source a decent amount of current from the +5V output in order to work, so you may have to stick a power resistor from +5V to ground.
Short answer:
Yes, but it will be some work.
I’d instead recommend buying a non-computer power supply. If you buy from the surplus’ish places, like Marlin P Jones and Associates, you can get these pretty cheap.
For example, here’s a used +24.5V (that’ll be well within spec for a +24V fan), 5A enclosed switching supply for only 14.95.
That sucker will be able to power up to 4 of these $15.95 282 CFM fans.
With that power supply, you’ll want to build a simple enclosure. If you don’t need that many fans, there are plenty of enclosed +24V power supplies available (like this 4A one for $49.95 or this 2A one for 14.95). Personally, I’d be safe and go for something like the $49.95 one.