I design blowers and exhausters for a small company here in Pennsylvania. Sorry nothing small enough to help (they will literally blow the whole garage away, they are the size of a two story house) Any way, what size of fan were you thinking of using? If you are thinking of a regular household box fan, it could work, but where are you planning for the air to go once you scrub it thru your filters? If you suck the welding vapor thru a filter, the filter will only scrub what you can see, the outlet of the fan will still send the micro-toxins out the discharge side of your set up (ie. it will still stink up the inside of the garage). What you need to do is vent the air to the outside of the garage as many of the others have indicated, you also have to pull fresh air in (no matter what you thing air has to come in from somewhere, cracks, holes seams (the better or more finished the garage is the worse your welding vapor will be... for example an old block garage will "leak" more than a new finished dry walled garage). My thoughts are build a box around the 6" vent in the wall where the heater used to vent. Put the fan in exhausting towards the 6" hole, then build a transition down to half the size of the fan (ex. fan is 24" in diameter then build a transition down to a 12"sq. (if it is a square box fan) then build a square duct at least 2 times the diameter in length approx. 4ft. long (48"). you can incorporate the filter at any point in the duct...
This is the reasoning... it does not matter what the hole size is in the garage wall (although getting any smaller than 6" will be to small) with it being boxed in you are creating a point of pressure build up, the atmosphere outside will be less than what will build up in your box on the inside of the garage, thus the air will want to scavenge out the hole (kind of like. the burnt air full mixture escaping out the exhaust valve a combustion engine, with the exception of the air is not super heated). The pressure that will build up will be due to the induced draft you are creating thru the 48"lg. duct. when the fan is running, experiment with a window at the opposite side of the garage, open it or close it as you weld until you get the proper mix of air flow... You will know when you find the right mixture. I exhaust my paint fumes thru a 7" diameter duct thru the roof of my garage in this manor. My whole set up cost me around $20.00 (using the duct an old 6" personal fan and an old wood burner exhaust pipe with cap etc.) I have to open one of my garage windows about 6" to get the flow right... of course a little less in the summer a little more in the winter due to the density of the air (hot to cold).
Now, I know I am long winded, but if your fan is round you could use light gage sheet metal pop-riveted together, you could make a round to round reduction with the light gage material (if you need I can layout what you want), if your fan is in a box you could build a box from old paneling and some 1"sq. wood stock (just seal up the cracks with latex caulk). next, you want at least 48" of duct run. A straight run is alway's best but you could build it vertical, transition from the box to the ceiling then make your run etc. If I get a chance, I'll draw a quick sketch to illustrate what I am speaking of.
Remember, I only design fans, I am not an engineer yet (currently going back to school to become one someday) I have 13yrs of experience, but I hold no responsibility or liability, this is just merely a suggestion (mine works fine).
On a windy day, the wind blowing by helps to create a greater suction which makes me have to turn up my gas on my welder to keep my gas from "blowing away".
Last edited by Festive57; 08-14-2005 at 10:16 PM.