Your CFM requirements are largely a function of how far you let the
smoke disperse. If you can capture the smoke within a few inches of the
weld, you only need a couple hundred CFM. If you let it propagate
throughout the whole shop, you need one several times that and it will
take a while to clear the smoke.

One thing I like is to do a fume hood sort of thing: Assuming fairly
small projects (less than a 2' cube) set up a 3 sided booth around the
weld bench, add some wings to tuck in close to your body, seal the top,
and add your fan. The fresh air will be coming in around you, the total
CF of contaminated air is very small, a bathroom vent fan is quite
sufficient.

Your fan needs to deal with both a CFM rating and a water column rating.
You will find that a lot of the muffin fans are the right size, will
move a lot a air when in free air, but just won't push much of any air
through a vent pipe. Most any centrifugal unit will be MUCH better.

I worry about welding in the basement. The little sparks like to roll
off into the corners, start fires hours later. Unless your weld area is
completely free of junk, dust, sawdust, etc, you are always taking a
chance.

wrote:
I'm thinking of the upcoming winter when I need to, as my wife puts
it, bond with my welder. So that means welding in the basement, for
which I know I need a fume exhaust system. As usual, I intend to
build my own cheap. The surplus store has a pretty good selection of
blowers, but I have no idea what size I need (CFM). Can anyone help
me here?