Foundry Fume Extraction for Aluminum Melting: Hoods, Fans and Filters

Foundry fume extraction in an aluminum melt shop has two jobs: protect the people at the furnace and keep stack emissions within the permit. Good design starts with knowing where the fume comes from, then follows with hoods, ducts, a fan and a filter sized as one system. This guide covers each piece and the regulatory references worth knowing.

Where fume and dust come from in aluminum melting

Molten aluminum itself produces little visible fume. Most of what you see comes from what goes into the furnace and what you do to the bath.

A U.S. EPA fact sheet on secondary aluminum lists the sources. Air toxics are released from scrap shredding, thermal chip drying and scrap decoating. They also come from furnace operations: melting, holding, refining, fluxing and alloying. Depending on the plant, emissions may include up to 11 metals, organic compounds such as dioxins and furans, and acid gases such as hydrogen chloride and chlorine.

For foundry fume extraction design, the main fume events on the shop floor are:

• Charging: oil, paint, plastics and moisture on scrap burn off at the door.
• Fluxing: salt and fluoride fluxes release fume and acid gases when they react.
• Drossing: skimming and dross handling release hot fine particles.
• Tapping and transfer: open metal streams and ladles release fume and heat.
• Dry pretreatment: shredding, chip drying and decoating add dust and organics.

Flux choice and dosing therefore affect the extraction load. For background on fluoride salts, see our article on cryolite in aluminum production, and match your fluxes and demagging agents to the treatment you really need.

Hood design basics for foundry fume extraction

The hood is where most systems succeed or fail. A powerful fan cannot rescue a hood that is in the wrong place.

The OSHA Technical Manual sets out the basics. First, enclose the operation as far as possible, using enclosures, side baffles or curtains. This reduces the airflow you need. Second, a canopy hood is an overhead hood that receives rising hot air or gas, which suits a hot furnace plume.

For capture hoods, distance is everything. OSHA gives an example: with 3,000 fpm (about 15 m/s) in the duct, air speed one duct diameter in front of the hood is about 300 fpm. Further out, it drops sharply. Accordingly, OSHA’s rules of thumb for simple capture hoods are:

• Keep the source within 1.5 duct diameters of the hood face.
• Do not design for less than 50 fpm (about 0.25 m/s) capture velocity.
• Control cross-drafts, which can cut hood effectiveness by 75%.

The EU BAT conclusions for the sector point the same way. For secondary aluminum, it lists a hood on top of the furnace door and at the taphole, with extraction to a filter. It also lists fume collection enclosures, sealed doors and boosted suction during charging. In short, foundry fume extraction works best when the hood follows the fume event, not the building layout.

Ducts, fans and makeup air

Once captured, the fume must reach the filter without dust settling in the duct. OSHA notes that air flows through ducts at between 2,000 and 6,000 fpm (about 10 to 30 m/s). Every dust has a minimum transport velocity below which particles settle. Therefore, avoid oversized ducts and long horizontal runs that slow the air.

The fan is chosen from air volume, fan static pressure, contaminant type and noise. OSHA also flags two common field problems:

• System effect: keep about six duct diameters of straight duct at the fan inlet and three at the outlet.
• Wrong rotation: a centrifugal fan running backward may deliver only 30 to 50% of rated flow.

Measure hood static pressure about 4 to 6 duct diameters downstream of the hood, in a straight section. Record it when the foundry fume extraction system is commissioned, then trend it. A falling value points to a blocked duct, a loaded filter or a worn fan.

Finally, air removed must be replaced. Without makeup air, the building runs under negative pressure, doors slam and hoods underperform. OSHA notes that exhaust systems are generally interlocked with a dedicated makeup air system. For the fan itself, an industrial extraction fan should be quoted against the system curve, not a catalog airflow figure.

Bag vs cartridge filters in foundry fume extraction

Both are fabric filters cleaned by pulses of compressed air. The difference is the media, the gas temperature they tolerate and the airflow they suit. The EPA fact sheets for pulse-jet baghouses and for cartridge collectors give a useful comparison.

Some warnings apply to both. The EPA notes that a spark or flame entering the collector can cause a fire or explosion with some dusts. Fabric filters also cannot run in moist conditions, and gas must stay above its dew point. As a result, hot furnace gas usually needs dilution air or a cooler, and often a spark arrestor or cyclone ahead of the filter. Where acid gases matter, lime injection ahead of the bags is common.

Our dust collectors are specified per application, including media and temperature limits.

Regulatory context for foundry fume extraction

Rules depend on country, plant size and process, so treat these as references, not a compliance checklist.

• United States: EPA’s secondary aluminum air toxics rule uses particulate matter as a surrogate for metals and HCl for acid gases. Its 2000 fact sheet also announced a review of how it applied to foundries, so check the current text.
• European Union: the BAT conclusions for non-ferrous metals set a dust level of 2 to 5 mg/Nm³ for secondary aluminum furnaces using a bag filter. They also set levels for HCl, chlorine and HF.
• Mexico and Latin America: national workplace and environmental authorities set their own limits. Confirm them with your local consultant and permit.

FAQ

Can one fan serve several furnaces?

Yes, with balanced branches and dampers. Yet each hood must still meet its own capture velocity.

Why does my baghouse catch fire?

Usually sparks or hot particles reach the media. Add cooling and spark separation upstream.

Where should foundry fume extraction start?

At the fume event: furnace door, taphole and fluxing station, then size ducts, fan and filter.

AL-LINX sources dust collectors and extraction fans from verified factories in China, with inspection at origin. Each item is quoted to your airflow, temperature and dust data, and several can go in one request. Request a quote for your fume extraction system.