How aluminium reverberatory melting furnaces work
An aluminium reverberatory melting furnace is a large rectangular hearth fired by gas or oil burners over the bath. Ingot, returns and scrap are charged through doors or a side well, and the metal is melted at about 700 to 780°C with the furnace atmosphere at 1,000 to 1,200°C. Chloride and fluoride fluxes are added for cleaning, and contaminated scrap brings oil, paint and plastics. With cold air burners the flue gas leaves at 900 to 1,100°C; recuperative burners lower this to about 500 to 700°C, and regenerative burners to 200 to 300°C. The gas then passes dilution or cooling, dry lime or sorbent injection for HCl and HF, a bag filter and the ID fan.
Where the FD fan sits
- Inlet filter and silencer
- FD (combustion air) fan
- Air header
- Recuperator or regenerator (if fitted)
- Burners
- Furnace
The fan is placed beside the furnace and supplies an air header that feeds each burner through its control valve. With regenerative burners, air passes through the regenerator bed of the burner not firing, picking up heat on the way to the burner that is firing. The pressure needed covers the header, valves, regenerator or recuperator and the burner, typically 400 to 900 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the regenerator or recuperator | Up to 800 to 1,000°C with regenerative burners, downstream of the fan |
| Dust | Clean air if the inlet is away from the charging area |
| Pressure at the fan outlet | Typically 400 to 900 mmWC |
| Turndown | High fire while melting, low fire while holding and fluxing |
| 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 reverberatory melting furnaces
| Blade form | Backward curved, with a stable pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled at two-pole speed for the higher pressures |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive holding air header pressure, with burner ratio valves |
| Also specified | Inlet filter to protect the regenerator media, inlet silencer |
What this FD fan has to cope with
Dust in the regenerator media
Regenerative burners use beds of ceramic balls or honeycomb. Dust in the combustion air adds to the dust from the flue gas and clogs the beds, raising pressure drop. An inlet filter on the fan helps keep the bed clean between changes. The filter is placed so that it is easy to change without stopping the furnace for long.
Header pressure during cycling
Regenerative burners switch between firing and exhausting every minute or so. Each switch disturbs the air header, and a fan with a steep curve and speed control holds a steadier pressure than a damper. Without that control, the burner pairs see different air flows on each half of the cycle, which upsets the ratio and the flame shape over the bath.
High fire capacity
Melting rate depends on reaching full firing quickly after charging. If the combustion air fan lacks pressure margin, the burners cannot reach full fire, and melting slows. The fan is sized for full firing with fouled regenerators. Fouling of the media raises the resistance steadily between changes, so the pressure margin should reflect the end of the bed life.
Energy and control
Combustion air demand varies widely between melting and holding. A variable speed drive on header pressure saves fan power at low fire, and correct ratio control saves far more in fuel. Recuperative and regenerative burners reduce fuel consumption, and their extra air side pressure drop is a small cost against that saving.
ID fan and FD fan in aluminium reverberatory melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Beside the furnace, feeding the burner header |
| What it handles | Cooled flue gas with flux salts and HCl traces | Ambient air |
| Temperature at the fan | 110 to 160°C | Ambient |
| Main design concern | Chloride corrosion and door capture | Header pressure for high fire |
| Control | Furnace pressure | Header pressure and burner ratio |
See also the Induced Draft (ID) Fan for aluminium reverberatory melting furnaces.
Questions buyers ask
What does the FD fan do on an aluminium melting furnace?
It is the combustion air fan for the burners. It supplies ambient air to the burner air header and, where fitted, through recuperators or regenerators that preheat the air. It handles clean air, and its job is steady pressure from high fire to holding. Mild steel construction is normal, since the fan never sees the hot preheated air or flue gas.
How much pressure does a regenerative burner combustion air fan need?
Typically 400 to 900 mmWC, more than for cold air burners, because the air must pass through the regenerator bed and the burner nozzle. The exact figure comes from the burner maker's data at full firing with fouled media. The fan should have margin over that figure for bed fouling.
Should the combustion air fan have an inlet filter?
Yes, especially with regenerative burners. Dust drawn in with the air settles in the regenerator media and raises its pressure drop. A filter on the fan inlet keeps the media cleaner and the burner air flow steadier. It also protects the impeller from dust build-up and unbalance.
Information needed to quote an FD fan for aluminium reverberatory 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 system. Burner type, number and firing rate, and required air pressure.
- Heat recovery. Recuperator or regenerator air side pressure drop, clean and fouled.
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.