How aluminium holding furnaces work
Holding furnaces keep molten aluminium at 680 to 760°C for die casting, gravity casting or a billet or slab caster. They are reverberatory, crucible or tower-type units fired by gas or oil, or electrically heated with radiant elements. Firing rates are low compared with a melter, since the furnace only makes up heat loss and the heat of small additions, and the burner spends much of the time on low fire. Flue gas leaves at 800 to 1,000°C in small volume. On many installations it goes to a stack on natural draft; where furnaces share a common flue, a recuperator is fitted, or degassing and fluxing fume must be captured, an ID fan with dilution air is used.
Where the FD fan sits
- Inlet screen
- FD (combustion air) fan
- Recuperator (if fitted)
- Air control valve
- Burner
- Holding chamber
The blower is usually mounted on the furnace frame or next to it, feeding the burner through a short air line and control valve. On recuperative burners the air passes the burner's own recuperator. Pressure required is modest, typically 150 to 400 mmWC. In die-casting cells space is tight, and the blower is often close to the casting machine. Ducting its inlet from outside the cell is often worthwhile.
Air at the FD fan
| Air temperature at the fan | Ambient, often 40 to 50°C near the casting cell |
| Dust | Foundry dust and die release mist in the cell |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Turndown | Mostly low fire; high fire only after cold additions |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 aluminium holding furnaces
| Blade form | Backward curved or radial blade, small direct-driven blower |
| Material of construction | Mild steel or cast aluminium housing |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; motor rated for the cell ambient |
| Flow control | Modulating valve or variable speed drive with burner ratio control |
| Also specified | Inlet screen and inlet ducting from clean air, low noise design |
What this FD fan has to cope with
Burner cycling at low fire
A blower sized for high fire with an on-off or high-low burner makes the furnace cycle around the set point, and the metal temperature swings. A modulating burner with a fan that stays stable at low flow holds metal temperature tightly, which matters for casting quality.
Die release mist at the inlet
Die-casting cells carry mist from die release agents and lubricants. Drawn into the blower, it coats the impeller and screen. A duct to a clean air source, or a simple inlet filter, keeps the blower clean. The film builds unevenly and can unbalance a small, high speed wheel, so cleaning intervals shorten if the inlet stays inside the cell.
Noise in the casting cell
Small, high speed blowers close to operators are noisy. A low noise design, inlet silencer or a remote location with a longer air line keeps cell noise within limits. Noise from a holding furnace blower is constant through the shift, unlike melting equipment, so it matters for operator exposure. Acoustic lagging on the casing and a flexible connection to the air line also help.
Energy and control
A holding furnace burner fires at a fraction of its capacity for most of the day, so the blower runs throttled most of the time. A variable speed drive, or a smaller blower matched to the real holding load with enough pressure for occasional high fire, saves power. Recuperative burners reduce fuel use, which is where most of the energy goes.
ID fan and FD fan in aluminium holding furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After dilution or recuperator, before the stack | On or beside the furnace, feeding the burner |
| What it handles | Small volume of diluted hot flue gas | Ambient air |
| Temperature at the fan | 250 to 350°C | Ambient, often warm |
| Main design concern | Fine pressure control to limit dross | Stable air at low fire |
| Control | Furnace pressure | Metal temperature through burner ratio |
See also the Induced Draft (ID) Fan for aluminium holding furnaces.
Questions buyers ask
What does the FD fan do on a holding furnace?
It is the combustion air blower for the burner that keeps the aluminium at casting temperature. It handles ambient air and supplies the burner through a control valve. Most of the time it runs at low flow, since the furnace only makes up heat losses.
Why does my holding furnace metal temperature swing?
Often because the burner cycles on and off or between high and low fire, which is common with oversized blowers and simple controls. A modulating burner with a blower stable at low flow keeps the temperature within a narrow band, improving casting quality. Door discipline also helps.
Can one blower serve several holding furnaces?
It is possible with a common air main and individual control valves, but each furnace then depends on one blower, and a fault stops all of them. Most die-casting shops prefer one blower per furnace for reliability and independent control. Where one blower is shared, a standby unit is advisable.
Information needed to quote an FD fan for aluminium holding 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, firing range and air pressure required at high and low fire.
- Installation. Space available in the casting cell and any noise limit.
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.