Posted:
Fri May 28, 2004 7:24 pmby Greg
I tried a similar setup a while back but found that these type of fans, although they will pull a large amount of air at a low depression are not able to pull much higher depression even when the intake is almost fully closed off.
Mine is a 2kW 3 phase motor on it but i think the fan has far too much clearance around it and the air just recirculates in the fan rather than build to a higher depression.
Then again, the only way to really know is to test it. Try some plates with different size holes in them against the intake and measure what depression you are getting.
Posted:
Fri May 28, 2004 7:28 pmby 84-1074663779
For a centrifugal blower, in very rough terms, flow goes up directly with RPM, and pressure goes up with RPM squared.
Drive power goes up with RPM increase CUBED. This works for small increases, but if you get really greedy the internal flow areas of the blower are going to restrict output capability, but the drive power certainly still goes up. The lost drive power comes back to you as hotter discharge air.
If you could run it at three times rated speed you might expect maybe 1200 CFM and 22" or something like that, but it would then need a ten horsepower motor to turn it. Always assuming it does not fly to bits at that speed.
I would look for something with more pressure capability and a bit lower rated flow.
My forge blower was originally rated at 250CFM and 17" at 2850 RPM with a one horsepower (50Hz) single phase motor. Run at twice the shaft speed I can easily reach 500 CFM at up to 63" with a 7.5 BHP motor run up at its maximum rated current.
I suggest you look for something with a larger diameter rotor with higher tip speed and rated pressure, and a smaller inlet eye size (lower original flow rating).
But you are definitely on the right track.
I think you will get a lot more air at fewer amps than using multiple vacuum cleaner motors, and a much nicer smoother pressure/flow characteristics as well. But it might cost more initially to do it with one big blower.
Having done this myself I would never go back to vacuum cleaner motors again.