Welding Fume Capture and Make-Up Air
A fabrication bay measures 20 m × 15 m × 6 m and contains two MIG welding stations. Each station has a local exhaust hood on a 200 mm diameter duct. A recent check with a thermal anemometer showed 7.5 m/s at the duct centerline and only 0.2 m/s at the arc, with the hood positioned about 300 mm from the arc. The bay also has a general exhaust fan rated at 2.0 m³/s, but no dedicated make-up air; the only inlet is a 1 m × 2 m door that is hard to open because of negative pressure. Welders report eye irritation and visible fume drifting to a nearby grinding area. The building owner will allow new ductwork but not a new roof curb; a 4-hour shutdown is available next month. The safety officer must determine whether the local exhaust is adequate and what to fix.
1. Calculate the local exhaust airflow and assess whether the hood is controlling welding fume. Show your assumptions.
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For a 200 mm duct, radius = 0.1 m and area = π × 0.1² = 0.0314 m². At 7.5 m/s centerline velocity, Q = velocity × area = 7.5 × 0.0314 ≈ 0.236 m³/s per hood, or about 236 L/s. Two hoods therefore move about 0.47 m³/s. The duct velocity is in a plausible transport range for fume, but transport velocity is not the same as capture at the arc. The measured capture velocity at 300 mm is only 0.2 m/s. For welding fume, a common design target is on the order of 0.5 m/s at the source, so 0.2 m/s is too low, especially with cross-drafts from the hard-to-open door and general exhaust. The hood is also far from the arc; capture velocity falls rapidly with distance. Visible fume escaping to the grinding area and eye irritation are consistent with inadequate capture. I would treat the local exhaust as not verified, reposition the hood closer to the arc, check for obstructions and leaks, and re-measure capture velocity with a smoke tube and anemometer before relying on airflow alone.
2. Diagnose the room air balance and propose engineering changes that can be completed within the constraints. How would you verify them?
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The bay has about 2.0 m³/s of general exhaust plus about 0.47 m³/s of local exhaust, or roughly 2.47 m³/s total, with no dedicated make-up air. The door is hard to open, which indicates negative pressure and insufficient replacement air. That starves the local hoods, creates cross-drafts, and can pull fume toward other areas. Within the constraints-no new roof curb and a 4-hour shutdown-I would add a dedicated make-up air fan and wall louver sized to supply at least the total exhaust, tempered if needed, and distribute it away from the welding plume. I would also shorten and rebalance the local exhaust runs, move each hood to within about 150 mm of the arc where practical, and consider a fume-extraction welding gun for confined work. Verification should include an air-balance measurement, hood capture velocity and smoke visualization at the arc, checks for door operation and pressure, and personal welding-fume sampling before and after the change. I would not accept the duct velocity reading as proof of control.