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 FD fan sits
- Inlet filter
- FD (combustion air) fan
- Flexible air line
- Door-mounted burner
- Rotary drum
The fan sits on the floor near the furnace and feeds the burner on the swinging door through a flexible hose or articulated duct, since both door and furnace move. The pressure required is typically 300 to 600 mmWC. With oxy-fuel burners, a small fan may still provide cooling air to the burner block when it is not firing or during door opening.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Salt fume and dust in the furnace bay if the inlet is local |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | High fire after charging; low fire during salt treatment and pouring |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 rotary tilting furnaces
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with burner ratio control |
| Also specified | Flexible connection rated for the door movement and radiant heat; inlet from outside the bay |
What this FD fan has to cope with
Air line damage at the door
The burner hose flexes every time the door opens and the furnace tilts, and it sits in radiant heat from the open drum. Hoses crack and leak, starving the burner. A properly routed articulated duct or heat-resistant hose, sized for the fan pressure, and regular inspection prevent this.
Salt fume at the fan inlet
The furnace bay air carries salt fume and fine dust from charging and pouring. A fan drawing this air builds up salt on the impeller, which corrodes it and causes unbalance. Taking the inlet from outside the building keeps the fan clean and reliable. An inlet filter is a second option.
Oversized fan after oxy-fuel conversion
Converting the burner to oxy-fuel leaves the air fan oversized and throttled, or idle. If cooling air is still needed, a small dedicated fan is more efficient and quieter, and the larger fan can be removed or kept as a spare for air-fuel firing. The burner supplier's cooling air figure sets the new duty.
Energy and control
Firing varies through the batch, and furnaces often wait between batches. Speed control on the combustion air fan saves power at low fire and on standby. The bigger energy decision is between air-fuel and oxy-fuel firing, which changes fuel use, flue gas volume and the size of both fans; the fan arrangement should follow that choice.
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 Induced Draft (ID) Fan for aluminium rotary tilting furnaces.
Questions buyers ask
Does an aluminium rotary tilting furnace need an FD fan?
Only if it fires air-fuel, when the combustion air fan supplies the burner. Many rotary tilting furnaces fire oxy-fuel, and then the burner uses oxygen and the main forced draft duty disappears. A small air fan may still be kept for burner cooling when the door is open or the burner is off.
How is air supplied to a burner on a moving door?
Through a flexible hose or articulated duct that follows the door swing and the furnace tilt. It must withstand radiant heat from the open furnace and the fan pressure, and it is routed so that it does not kink or rub as the furnace moves through its range.
Why does the combustion air fan corrode in an aluminium scrap plant?
Air drawn from the furnace bay carries salt fume and chlorides from fluxing. These deposit on the impeller and casing and absorb moisture, forming a corrosive film. Ducting the inlet from outside air and keeping the fan clean prevent most of this corrosion. Periodic washing removes what remains.
Information needed to quote an FD fan for aluminium rotary tilting 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.
- Firing method. Air-fuel or oxy-fuel, burner firing range and required air pressure.
- Door and tilt. How the burner moves with the door and the hose route available.
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.