How aluminium rotary tilting furnaces work
A rotary tilting furnace melts aluminium dross, swarf, painted and mixed scrap in batches, with or without a salt flux. The drum rotates to mix the charge, and tilts on its axis to charge through the single door and to pour metal and salt slag. Firing is usually oxy-fuel, sometimes air-fuel, from a burner mounted in the door, and the flue gas leaves through the same door opening into a hood at 800 to 1,100°C. The gas carries salt fume, alumina and fine metallic aluminium dust, and smoke from oil and coatings. It passes a hood, an afterburner where organics are heavy, a cooler or dilution, lime and activated carbon injection, a bag filter and the ID fan.
Where the ID fan sits
- Door and tilting hood
- Afterburner (if fitted)
- Cooler or dilution
- Lime and carbon injection
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. The hood around the door must follow the furnace as it tilts, so its open area and the room air drawn in change through the batch, and the fan sees much more air than the burner gas alone. Suction at the fan covers the hood, any afterburner, the cooler and the filter, typically 300 to 450 mmWC.
Gas at the ID fan
| Gas temperature, normal | 110 to 160°C after the bag filter |
| Maximum or upset | Up to 200°C when burning heavily coated scrap |
| Dust at the fan | Traces of salt fume, alumina and lime; fine metallic aluminium if a bag fails |
| Moisture and dew point | Combustion moisture and moisture from damp dross; chlorides raise the dew point |
| Corrosive species | HCl and chloride salts from flux; HF from fluoride additives |
| Organics | Oil, paint and plastic smoke from scrap, burned in an afterburner where fitted |
| Density at the design temperature | About 0.86 kg/m³ at 140°C, 72% 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 rotary tilting furnaces
| Blade form | Backward curved on the clean side of the bag filter |
| Material of construction | Mild steel with insulation; SS 316 wetted parts where chloride condensation is likely |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive with set points for charging, melting and pouring |
| Also specified | Non-sparking construction considerations for metallic dust, earthing, casing drain and access door |
What this ID fan has to cope with
Hood capture as the drum tilts
The furnace tilts back for charging and forward for pouring, and the door moves relative to the hood each time. Gaps open and fume escapes, especially with smoky scrap. A hood designed for the full tilt range, and a fan with enough margin and speed control to raise suction during charging and pouring, keep the bay clear.
Aluminium dust explosion risk
Dross and swarf melting gives fine metallic aluminium dust as well as oxide. It is a combustible dust, and a failed bag can carry it to the fan. Earthing, avoiding rubbing contact, keeping the fan on the clean side and including it in the explosion protection plan of the filter are essential.
Organic smoke and sticky deposits
Painted, oily or plastic-coated scrap releases volatile organics that condense as tar on cool surfaces if not burned. They bind salt and lime into a sticky layer on bags, ducts and fan blades. An afterburner or controlled burn-off in the furnace, and keeping the gas warm, prevent the deposits.
Energy and control
Each batch passes through charging, melting, salt treatment and pouring, and the gas volume needed at the hood changes at each stage. A variable speed drive with stage set points, or linked to furnace tilt and door position, cuts fan power considerably. Oxy-fuel firing reduces the burner gas volume, so most of the fan duty is hood air, which good hood design can reduce further.
ID fan and FD fan in aluminium rotary tilting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | On the floor, feeding the door burner |
| What it handles | Flue gas and hood air with salt fume traces | Ambient air |
| Temperature at the fan | 110 to 160°C | Ambient |
| Main design concern | Hood capture and combustible dust | Flexible air supply to a moving burner |
| Control | Hood suction by batch stage | Burner ratio |
See also the Forced Draft (FD) Fan for aluminium rotary tilting furnaces.
Questions buyers ask
Why is the ID fan on a rotary tilting furnace so large for its burner?
The fan handles not only the burner flue gas but also the room air drawn into the door hood, which must capture fume at every tilt position. With oxy-fuel firing the burner gas is small and hood air dominates. Better hood sealing reduces the fan size and power required.
Is aluminium dust from a rotary furnace explosive?
Fine metallic aluminium dust is combustible and can be explosive in a dust cloud. Dross and swarf melting produces it along with oxide and salt. The bag filter and fan must be included in the plant's explosion protection design, with earthing and the fan kept on the clean side.
Does salt-free melting change the ID fan on an aluminium rotary furnace?
It reduces chloride fume, which reduces corrosion and salt deposits at the fan. The gas volume is set mainly by the burner and hood, so the duty changes little unless the firing method also changes. Organics from scrap remain the main deposit risk. Lime and carbon dosing can usually be reduced.
Information needed to quote an ID fan for aluminium rotary tilting 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.
- Charge and flux. Charge type, share of dross and coated scrap, and whether salt flux is used.
- Hood and tilt range. Hood design and tilt angles for charging and pouring.
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.