I built the unit linked below several years ago. I wanted to rebuild it to utilize a electrostatic filter and bag filter. Right now I just run 2 filters (11 merv) before the fans and 1 filter (11 merv) after the fans. I know things like design, filters, etc all work into the calculations of this, but I am wondering if I would be shooting myself in the foot by adding the electrostatic filter and bag filter. Right now the filter sizes are 10"x24". Would be going to 12"x24" Each fan is rated at 340 cfm........so to me all 3 running would be more CFM. One commercially available units run unrestriced at 1500cfm using a squirrel cage blower, then with the filters in place drop down to 1000cfm. Another one unrestricted runs at around 1000cfm and filtered 750cfm. http://lumberjocks.com/projects/392 I have a big furance blower that I was thinking of using if this one doesn't cut it, but that would have to be on the ground and now mounted. All I know right now is the unit as it stands sucks up a lot of junk and I can see the particles of dust heading towards the unit. Thanks all!