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 ID fan sits
- Furnace and side well
- Flue and recuperator (if fitted)
- Dilution or cooling
- Lime or sorbent injection
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. On a cold air burner furnace, large dilution air volumes are needed to cool the flue gas before the bags, so the fan is large for the heat input. With regenerative burners, part of the flue gas passes through the burner regenerators and is drawn by the burner system's own exhaust fan, and the main ID fan handles door and well extraction. Suction is typically 250 to 400 mmWC.
Gas at the ID fan
| Gas temperature, normal | 110 to 160°C after the bag filter |
| Maximum or upset | Up to 200°C during heavy fluxing or with cooling problems |
| Dust at the fan | Traces of chloride salts, alumina and lime after the filter |
| Moisture and dew point | Combustion moisture; HCl raises the acid dew point |
| Corrosive species | HCl and HF from chloride and fluoride fluxes; chlorides as hygroscopic salt |
| Organics | Oil and paint smoke from contaminated scrap |
| 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 reverberatory melting furnaces
| Blade form | Backward curved on the clean side of the bag filter |
| Material of construction | Mild steel with insulation; SS 316 or coated 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 on furnace pressure and door hood suction |
| Also specified | Casing drain, insulation, and an access door for cleaning salt deposits |
What this ID fan has to cope with
Chloride salt deposits
Chloride fluxing gives submicron salt fume that passes in traces through the bags and deposits as a hygroscopic film on the fan. In humid weather or after a stop, the deposit becomes wet and corrosive. Insulation, keeping the gas above dew point, stainless steel at cold spots and periodic washing keep the fan sound.
Door opening and charging fume
When the charging door opens, cold air rushes in and furnace pressure collapses, while fume from contaminated scrap escapes above the door. Door hoods and a fan with enough margin to handle the open-door flow, under speed control, keep the bay clear. The door hood branch usually carries a damper that opens with the door.
Burner change alters the fan duty
Converting from cold air burners to recuperative or regenerative burners reduces flue gas temperature and volume. The original ID fan is then oversized and runs throttled. A new impeller, a lower speed or a smaller fan should follow a burner upgrade. Measuring the actual gas flow after the upgrade gives the right basis for the change.
Energy and control
Reverberatory furnaces cycle between charging, melting, fluxing and tapping, with the gas flow varying through each cycle. A variable speed drive on furnace pressure, with door switches to raise speed when doors open, avoids wasting power during holding. Burner upgrades to regenerative or recuperative firing cut fuel use substantially and also lower ID fan power because less dilution air is needed.
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 Forced Draft (FD) Fan for aluminium reverberatory melting furnaces.
Questions buyers ask
Why does the ID fan on an aluminium melting furnace corrode?
Chloride and fluoride fluxes produce HCl, HF and fine chloride salts. Traces pass the bag filter and deposit on the fan. When the casing is cold or the air humid, the deposits absorb water and attack the steel. Keeping the casing warm, stainless steel at cold spots and washing the fan reduce it.
Does a regenerative burner system change the ID fan duty?
Yes. With regenerative burners most flue gas is drawn through the burner regenerators by the burner system's own exhaust fan, and leaves at 200 to 300°C. The main ID fan then handles less gas, mainly from the doors and side well, and is usually smaller.
What temperature does an aluminium reverberatory furnace ID fan handle?
After cooling and the bag filter, typically 110 to 160°C. The flue gas leaving the furnace is much hotter, up to about 1,100°C with cold air burners, but it is cooled by dilution or a cooler before the bag filter and fan. The impeller is still checked for the upset temperature.
Information needed to quote an ID fan for aluminium reverberatory melting 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.
- Burner system. Cold air, recuperative or regenerative burners, and firing rate.
- Charge and fluxing. Share of contaminated scrap and the fluxes used.
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.