I'm in the midst of building a new welding fume extractor setup - this will be vented via a 4" duct to outside. I'm having a HVAC sheetmetal place build a collector backstop that will vent out the top, go up, & through a blower, then make the turn & go outside. I need advice on what type of 4" blower (puller) to use. I obviously want all steel construction, but not sure about how many cfm would be best or what type. Anybody with any experience in this? Advice appreciated.