If you're venting to the outside then you need to draw fresh air in somewhere or the vacuum in the shop will eventually overpower the fan and nothing will move. Also, without drawing outside air the same vacuum will suck in any fumes/exhaust from any heat source - like a furnace, wood stove, etc.. Not good. And then, if you draw in outside air, you're going to freeze your ass off in winter.

A proper fume and smoke extractor will move a LOT of air. I have a standalone recirculating unit and it has an 8" diameter hose. The large hose actually serves to slow down the suction - enough to suck the smoke but not enough to suck away your sheilding gas. It has a large canister filter that has a flame retardant in case it sucks in sparks. I weld on average 15 hours a week and I wash the filter out annually.

My advice, if you haven't already blown a ton of money trying to reinvent the wheel ... invest in a recirculating unit and get on with it. You have to spend money to make money !! And yes, I know how much they are !!!