I have small squirrel cage fan salvaged from and old disc drive from times when they had 4 or 5 12" plus removable discs on a single spindle, about 5 mB IIRC model BK5 I think. I have been using this in a homebuilt cyclone dust (wood) collector with quite good results.
I think I may be able to improve the fans through put. The intake area around the motor is 5.7 sq.in. The exhaust port is 3.75 sq.in. Originally there was a rectangular opening about half way around the spiral housing with area 2.625 sq.in., this has been blocked. The total exhaust area would have been 7.375 sq.in. I guess the pressures would have been different. Should the exhaust port be opened up to equal the intake area? Something different better?
I experimented with the clearance between the fan and the start of the spiral housing at the close edge of the exhaust port, closing it up seems to make little difference to the air flow, however the noise goes up considerably as the fan is approached, a la air raid siren I suppose.
Comments, advice please, John.