Within the distance quoted for its capture velocity, typically a nozzle diameter or two from the arc for common designs. Capture velocity falls approximately with the square of distance, so a nozzle left a little too far back can be almost ineffective while appearing to be in use. Repositioning as the weld progresses is part of the method.

Temperature and sparks. It must be flame-retardant and rated for the hot gas drawn from an arc, resist burn-through from spatter and grinding sparks, and survive being dragged and crushed on a shop floor. Ordinary flexible ducting fails on all three counts and becomes a fire path back to a filter loaded with combustible dust.

A flange or hood around the nozzle opening blocks air being pulled in from behind the hood, so more of the airflow comes from in front where the fume is. It measurably increases the capture distance for the same fan duty, which means either better capture or a smaller extractor for the same result.

Holding the nozzle on ferrous work close to the joint without an arm or stand, so the welder can reposition it in seconds as the seam progresses. It is the most practical arrangement for general fabrication. On non-ferrous work, or where the surface is coated, a clamp, stand or articulated arm is needed instead.

It captures at the arc itself, so it needs far less airflow for the same removal and cannot be left in the wrong place. The costs are added torch weight and bulk, reduced access in tight joints, and care to avoid disturbing shielding gas coverage. Many shops use it for long production runs and nozzles for varied work.

Whatever the extractor and nozzle are specified for - bore sets the airflow the system can move and its resistance. Undersizing throttles the fan and cuts capture; oversizing can drop transport velocity so fume and dust settle in the duct. Follow the manufacturer's pairing rather than fitting whatever hose is on the shelf.

Considerably. Every metre of flexible hose, and every bend, adds resistance and reduces the flow reaching the nozzle, and flexible convoluted hose is far worse than smooth duct for its size. Keep runs as short and straight as the work allows, and account for the actual installed length when sizing the extractor.