How brass and bronze melting furnaces work
Brass and bronze are melted in coreless induction furnaces, oil or gas-fired crucible furnaces, or small rotary and reverberatory furnaces for scrap. Melting runs at about 950 to 1,150°C. Zinc boils at 907°C, so brass melting gives off dense white zinc oxide fume, with lead oxide from leaded alloys and smoke from oily turnings. Induction furnaces have no flue gas at all; the fume is captured by a hood or lip extraction. Fuel-fired furnaces add combustion products at 900 to 1,100°C. In both cases the gas is cooled by dilution air or a cooling duct, passed through a spark arrestor and a bag filter, and drawn out by the ID fan.
Where the ID fan sits
- Furnace hood or lip extraction
- Dilution air inlet
- Cooling duct
- Spark arrestor
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter because zinc oxide fume is submicron and builds up quickly on any impeller placed on the dirty side. Upstream, the hood draws a large amount of room air along with the fume, which cools the gas to a level the bags can take. Suction at the fan covers the hood, ducts, spark arrestor and filter, typically 150 to 250 mmWC.
Gas at the ID fan
| Gas temperature, normal | 80 to 130°C after dilution and the bag filter |
| Maximum or upset | Up to 180°C during fuel-fired melting with the hood close down |
| Dust at the fan | Traces of submicron zinc and lead oxide after the filter |
| Moisture and dew point | Low moisture with induction melting; combustion moisture with oil or gas firing |
| Other relevant species | Oil smoke and hydrocarbons from turnings; chlorides from fluxes |
| Density at the design temperature | About 0.93 kg/m³ at 110°C, 77% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for brass and bronze melting furnaces
| Blade form | Backward curved on the clean side of the bag filter |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9 |
| Bearings and cooling | Standard pedestal bearings; cooling disc where the gas exceeds about 120°C |
| Flow control | Variable speed drive, with hood dampers to switch extraction between furnaces |
| Also specified | Inspection door, casing drain and an acoustic enclosure or silencer if near the shop floor |
What this ID fan has to cope with
Poor fume capture at the hood
Brass fume rises fast and spreads across the bay when the furnace lid or crucible is open for charging, skimming and pouring. If the hood face velocity is too low the fume enters the shop and the crane runway. The fan is sized on the hood design for the open condition, not only on the gas made by the melt.
Sticky deposits from oily scrap
Turnings and swarf carrying cutting oil produce dense smoke when charged. The oil mist coats bags and ducts and can reach the fan as a tarry film mixed with zinc oxide. Drying or burning off the scrap before melting, and a fan with smooth, accessible blades and a cleaning door, keep deposits under control.
Spark and fire in the filter
Burning zinc particles and oily fume can carry live sparks into the filter, where they ignite the bags or the dust cake. A spark arrestor and enough duct length before the filter, a temperature alarm at the filter inlet, and a fast-closing damper to starve a filter fire of air are standard practice.
Energy and control
Foundries melt in heats, and the fume load is high only when the furnace is charged, melting and pouring. With several furnaces on one fan, hood dampers that close on idle furnaces and a variable speed drive that holds duct suction cut fan power substantially. Fume extraction systems are often oversized for the worst furnace; measuring hood velocities before a retrofit helps choose the correct duty.
ID fan and FD fan in brass and bronze melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | On or beside the furnace, feeding the burner |
| What it handles | Zinc fume with room air or combustion gas | Ambient air |
| Temperature at the fan | 80 to 130°C | Ambient, often warm near the furnace |
| Main design concern | Fume capture at the hood | Correct flame and clean inlet air |
| Control | Duct suction with hood dampers | Burner ratio |
See also the Forced Draft (FD) Fan for brass and bronze melting furnaces.
Questions buyers ask
Why does brass melting produce white fume?
Zinc in brass boils at about 907°C, below the usual melting and pouring temperature, so zinc vapour escapes from the melt surface and burns in air to zinc oxide. This forms a dense white submicron fume that must be captured at the furnace mouth and collected in a bag filter before the air is discharged.
Does an induction furnace melting brass need an ID fan?
It needs a fume extraction fan, which acts as the ID fan of the system. There is no flue gas from induction melting, but the zinc and lead fume must still be captured at the furnace mouth with a hood or lip extraction and drawn through a spark arrestor and bag filter by this fan.
Can the fume extraction fan be placed before the bag filter?
It is not advisable. Zinc oxide fume is very fine and sticks to blades, especially with oily scrap, causing unbalance, vibration and loss of performance within weeks. Placing the fan on the clean side of the bag filter keeps it running reliably between filter maintenance stops.
Information needed to quote an ID fan for brass and bronze melting furnaces
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Furnace type and number. Induction, crucible or rotary furnaces, their capacities, and how many share the extraction system.
- Hood design. Hood type, open area during charging and pouring, and the required capture velocity.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.