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 ID fan sits
- Holding chamber
- Flue outlet
- Dilution air inlet
- Recuperator or common flue (if fitted)
- ID fan
- Stack
The fan sits after a dilution air inlet that cools the gas to a temperature suited to mild steel, or after a recuperator. In die-casting shops, several holding furnaces often share one flue and fan, each with its own damper. Suction is low, typically 30 to 100 mmWC, but must be finely controlled, since holding furnaces have small doors and ladling wells that leak easily.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after dilution |
| Maximum or upset | Up to 450°C if dilution air fails |
| Dust at the fan | Low; alumina and flux traces |
| Moisture and dew point | Combustion moisture; well above dew point |
| Corrosive species | Minor; chlorides from fluxing or degassing salts if used |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 aluminium holding furnaces
| Blade form | Backward curved, small and efficient at low flow |
| Material of construction | Mild steel or boiler quality plate; heat-resisting steel impeller if dilution cannot be guaranteed |
| Drive and arrangement | Direct coupled, arrangement 8 or 4 for small fans |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive on furnace pressure, with dilution air damper on temperature |
| Also specified | Insulated casing and individual furnace dampers on a common flue |
What this ID fan has to cope with
Over-draft and metal oxidation
Holding furnaces have a large open metal surface and frequent door openings. Too much draft pulls cold air in over the bath, cooling it and forming more dross. A fan selected for the actual small gas flow, with speed control on furnace pressure, avoids the over-draft common with oversized fans.
Common flue imbalance
When several furnaces share one fan, a furnace that opens its door or goes to high fire disturbs the draft at the others. Individual furnace pressure dampers, and a fan holding constant suction in the common flue, keep each furnace stable. The fan curve should be flat enough near the duty point that one furnace changing its demand does not shift the suction for the rest.
Dilution air failure
If the dilution damper sticks closed, the gas reaching the fan can exceed 400°C. Mild steel loses strength rapidly at that temperature. A high temperature interlock and an impeller rated for the upset temperature protect the fan. A dilution damper that fails open, rather than closed, is a simple further safeguard worth specifying on new systems.
Energy and control
Holding furnaces run continuously but at low fire for most of their time. A small, correctly sized fan on a variable speed drive uses little power, while an oversized fan throttled hard wastes it. Holding furnace energy is dominated by fuel; reducing air ingress through correct pressure control and tight doors saves more than the fan itself.
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 Forced Draft (FD) Fan for aluminium holding furnaces.
Questions buyers ask
Does an aluminium holding furnace need an ID fan?
Not always. A single holding furnace with a short flue often works on natural draft. An ID fan is used when several furnaces share a common flue, a recuperator adds resistance, or fume must be drawn to a filter. It then gives steadier pressure control than a stack damper alone.
Why does my holding furnace make so much dross?
One common cause is too much draft. If the ID fan or stack pulls cold air in through doors and ladling wells, the air oxidises the metal surface and forms dross. Holding the furnace at a slight positive pressure with a correctly sized, speed-controlled fan reduces this.
What temperature does a holding furnace ID fan see?
The flue gas leaves the holding chamber at 800 to 1,000°C, but it is small in volume and is usually cooled with dilution air to 250 to 350°C before the fan. The fan is checked for the upset temperature if dilution fails. The upset is usually short, so heat-resisting steel is needed only where dilution cannot be relied on.
Information needed to quote an ID fan for aluminium holding 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.
- Number of furnaces. Number of holding furnaces on the flue, their firing rates and door arrangements.
- Pressure control. Required furnace pressure and the present control method.
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.