Your fume-capture system decides your fan

Before you spec a fume extraction fan, look at how the fume is captured. The hood, hole or enclosure that pulls gas off the process sets the volume, the dilution and the dust-and-acid load the fan will carry for its whole life. Get the capture architecture wrong and the fan is mis-specified from day one — no impeller trim recovers a fan sized against the wrong gas.

Five ways to capture, five different duties

The US EPA (AP-42, Section 12.5) catalogues five named capture configurations for electric-arc-furnace fume — the hottest, dirtiest fume duty in common industry, and a clean worked example for any fume fan. An arc furnace runs on 100 percent scrap in batch heats: iron oxide dominates during melting, and calcium oxide from the slag joins in during refining. Each capture method hands the fan a different gas volume.

Configuration	What it draws	Dilution / gas volume	Captures the roof-off charge?
Fourth hole (direct shell)	Fume straight from the furnace interior	Lowest volume, highest concentration	No — blind while the roof is swung away
Side-draft hood	Off-gas around the electrodes and work doors	Moderate	Yes, across the cycle
Combination hood	Fourth-hole + side-draft merged, with an air gap to burn off CO	Moderate–high	Partly
Canopy hood	Everything rising above the furnace and charge bay	High — most dilution air of the four direct systems	Yes
Full enclosure	The furnace fully surrounded, drawn from the top	Largest design volume of all	Yes — complete capture

The pattern is the point. The same process can hand the fan anything from a lean, high-concentration stream (fourth hole) to the largest diluted volume in the plant (full enclosure) — which is why most new furnaces pair a canopy for the roof-off charge with one of the more efficient systems. “A fume fan” is not a spec. Which configuration, at what gas volume, is. This is the same discipline as specifying the duty point: the number has to trace to a real operating case.

Pre-baghouse or post-baghouse — this picks the wheel

Where the fan sits relative to the filter matters more than almost anything else on the datasheet.

• Post-baghouse (clean side). On new installations the fan is almost always placed after the baghouse, in gas that has been substantially cleaned — direct-shell evacuation plus a charging hood on a common baghouse can cut carbon-steel EAF fume by more than 99.9 percent (EPA AP-42, Table 12.5-1). At roughly 80–150 °C on clean gas, a backward-curved plate wheel is the standard, efficient choice. See choosing the fan wheel.
• Pre-baghouse (dirty side). Legacy and raw-exhaust systems put the fan in the dust stream before the filter. Raw iron-oxide loading will erode a backward-curved blade quickly — the same logic as a heavy dust-extraction duty. Here you want a radial open-paddle or semi-shrouded wheel, and you want the leading edges hardfaced.

Our standing wear treatment on those radial families is chromium-carbide hardfacing laid down by FCAW at roughly 55–60 HRC on the blade leading edges, ranked by the ASTM G65 abrasion-test method rather than by a bare hardness number. More on that trade in fan wear protection & hardfacing.

Acid is a different problem from dust

Dust wears; acid corrodes — and the fixes point in opposite directions. We were called to a chemical-fume customer whose impeller had been eaten alive by highly concentrated acidic fume. The blades had thinned to rust-and-verdigris-stained flags and torn clean through; the wheel was scrap and had to be replaced outright. The replacement was not simply a harder wheel. For an acidic stream the answer is corrosion allowance — a thicker-gauge wheel, or SS316 construction — because the failure mode is chemical attack, not abrasion.

That is the trap worth stating plainly: stainless steel (304/316) sits around 124–147 HBW, barely harder than mild steel. Against abrasion it buys almost nothing. Stainless solves corrosion; hardfacing and AR plate solve wear. An acidic-fume duty and a dusty duty pull the wheel spec two different ways — and a stream that is hot, dirty and acidic needs both decisions made on purpose, not one bundled into the other.

Three questions before you release the RFQ

• Which capture configuration is installed — and does the quoted flow match the gas volume that configuration actually delivers?
• Is the fan post-baghouse (backward-curved plate) or pre-baghouse / raw gas (radial wheel with wear protection)?
• Is the duty dusty, acidic, or both — and does the wheel spec answer each on its own terms (hardfacing for wear, thicker gauge or SS316 for corrosion)?

State the capture system, the fan’s position around the filter, and the nature of the gas, and the wheel almost specifies itself. Leave them out and everyone is guessing. We work backwards from the stated configuration and duty — sized and tested to the IS 4894 / ISO 5801 method — not from a generic “fume fan” number. For the wider list of ways this goes wrong, see seven mistakes buying an industrial fan.

Talk to us about fume extraction →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.