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 FD fan sits
- Inlet screen
- FD (combustion air) fan
- Air control valve
- Burner
- Crucible or furnace chamber
The blower is usually mounted on or beside the furnace and connected to the burner through a short air line with a control valve. On a tilting crucible furnace it is mounted on the frame or connected through a flexible hose that follows the tilt. The pressure needed is modest, typically 150 to 400 mmWC, covering the valve and burner. Because the blower is so close to the furnace, the air it takes in is often hot and dusty.
Air at the FD fan
| Air temperature at the fan | Ambient, but often 40 to 55°C close to a melting furnace |
| Dust | Foundry dust and fume around the furnace |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Turndown | High fire for melting, low fire for holding and between heats |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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. |
FD Fan selection for brass and bronze melting furnaces
| Blade form | Backward curved or radial blade, depending on size and pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; motor rated for high ambient temperature near the furnace |
| Flow control | Linked air and fuel valves with high-low or modulating control |
| Also specified | Inlet screen and inlet ducting from a cooler, cleaner area |
What this FD fan has to cope with
Hot, dusty inlet air
A blower placed close to the furnace draws air at 50°C or more carrying zinc fume and foundry sand. The warm air is less dense, so mass flow drops, and the dust builds up on the impeller and screen. Taking the inlet from outside the melting bay through a short duct restores the duty and keeps the wheel clean.
Incomplete combustion and more fume
Too little air makes the flame smoky and adds soot to the zinc fume load on the bag filter; too much air oxidises the melt, raises zinc loss and wastes fuel. A blower with enough pressure margin and a proper linked or modulating ratio control keeps the flame correct through melting and holding.
Motor overheating
Motors beside melting furnaces run in high ambient heat and radiant heat from the open lid. Insulation ages quickly and thermal trips stop the burner mid-heat. A motor with a suitable temperature class and a heat shield, or a remote fan location with a longer air line, avoids these trips.
Energy and control
Crucible and small rotary furnaces cycle between high fire and holding many times a shift. Modulating ratio control saves both fuel and blower power compared with a fixed high fire setting. Where several small furnaces are being replaced by induction melting, the combustion air blowers are retired and the fume extraction system becomes the only fan 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 Induced Draft (ID) Fan for brass and bronze melting furnaces.
Questions buyers ask
Does an induction furnace for brass need an FD fan?
No. An induction furnace heats the metal electrically and has no burner, so there is no forced draft fan or combustion air blower. The only fan duty is the fume extraction fan that captures zinc and lead fume at the furnace mouth and draws it through the bag filter.
What pressure does a brass crucible furnace burner need?
Typically 150 to 400 mmWC at the blower outlet, depending on the burner type, the fuel and the length of the air line. The burner maker's data sheet gives the required air pressure at full firing; the blower is selected for that plus the valve and line losses, with a modest margin.
Why does my combustion air blower lose flow over time?
Usually because the impeller and inlet screen are coated with foundry dust and zinc fume drawn in from around the furnace, and because the air it takes in is hot. Cleaning the wheel and screen and moving the inlet to a clean, cool air source normally restores the original flow.
Information needed to quote an FD fan for brass and bronze melting furnaces
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Burner. Burner type, fuel and firing rate, and required air pressure.
- Furnace type. Crucible, rotary or reverberatory, and whether the burner moves with a tilting body.
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.