How copper shaft melting furnaces work
A copper shaft furnace melts cathode, and clean scrap or reverts, continuously. Charge is fed at the top of a vertical refractory shaft and descends against hot gas from several rows of natural gas or LPG burners around the base. Each burner is fired slightly fuel rich so that the molten copper does not pick up oxygen, which leaves CO and hydrogen in the gas. Molten copper at about 1,120 to 1,150°C runs to a holding furnace and on to a continuous caster or rod mill. Because the gas gives up its heat to the cold charge, it leaves the top at a modest 250 to 500°C. An afterburner or the charging hood burns the CO with air before the gas goes to a recuperator or stack, often through an ID fan.
Where the ID fan sits
- Burner rows
- Shaft and charge
- Charging hood
- Afterburner
- Recuperator (if fitted)
- ID fan
- Stack
The fan sits after the afterburner and recuperator, or after a dilution air inlet where no heat recovery is fitted. It must hold the charging door under a small suction so that CO does not spill on to the charging platform, while not pulling so much air into the shaft that the burners lose their reducing atmosphere. Suction at the fan is modest, typically 100 to 200 mmWC, but control must be fine.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after afterburner and dilution |
| Maximum or upset | Up to 500°C at low charge level or with a hanging charge |
| Dust at the fan | Low; copper oxide fines and refractory dust, more with scrap or reverts |
| Moisture and dew point | About 15 to 18% water vapour from natural gas firing, well above dew point |
| Combustible species | CO and hydrogen from reducing burners; must be burned out before the fan |
| 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 copper shaft melting furnaces
| Blade form | Backward curved, suited to the clean, moderately hot gas |
| Material of construction | Mild steel or boiler quality plate for normal duty; heat-resisting steel impeller where upset temperature exceeds about 400°C |
| Drive and arrangement | Direct coupled, arrangement 8, casing on sliding feet |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive on charging hood pressure |
| Also specified | Gas-tight shaft seal, explosion relief in the duct ahead of the fan, and a purge interlock before burner light-up |
What this ID fan has to cope with
Unburnt CO reaching the fan
If the afterburner flame fails or its air supply is short, CO-rich gas passes into the duct and fan, and an air leak at a flange or door can form an explosive mixture. The fan is interlocked with afterburner flame proof, the ductwork carries explosion relief panels, and the system is purged with air before any burner is relit.
Temperature rise at low charge level
When charging stops for a crane delay or a cathode shortage, hot gas passes straight through the half-empty shaft with little heat taken out, and the fan inlet temperature climbs quickly. The impeller is stress-rated for this upset temperature, and a dilution air damper opens automatically on a high temperature signal.
Oxygen pickup in the copper
Too much draft pulls cold air through the charging door and down into the shaft, where it meets the melting cathode. Oxygen in the melt rises and rod or strip quality suffers. Fine speed control on hood pressure and a hood designed for a small, steady suction prevent the fan from overdrawing.
Energy and control
Shaft furnaces run continuously for long periods, with the melt rate set by the caster. The gas volume tracks the burner firing, so a variable speed drive following hood pressure saves energy at reduced melting rates and over weekends on hold. A recuperator that preheats burner air raises furnace efficiency, and its gas side resistance is part of the ID fan duty.
ID fan and FD fan in copper shaft melting furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the afterburner and recuperator | At the base, feeding the burner ring main |
| What it handles | Burned-out shaft gas with little dust | Clean ambient air |
| Temperature at the fan | 250 to 350°C, up to 500°C upset | Ambient |
| Main design concern | CO safety and fine hood control | Stable ratio at every burner |
| Control | Charging hood pressure | Ring main pressure |
See also the Forced Draft (FD) Fan for copper shaft melting furnaces.
Questions buyers ask
Does a copper shaft furnace need an ID fan?
Small shaft furnaces with a tall stack and no heat recovery can run on natural draft. Where a recuperator, afterburner or fume treatment adds resistance, or the charging area must be kept reliably free of CO, an ID fan gives controlled hood suction that a stack damper alone cannot hold as the charge level changes.
Why does the shaft furnace exhaust contain CO?
The burners are fired slightly fuel rich so that the copper melts without picking up oxygen, which would spoil rod and strip quality. The unburnt CO and hydrogen leave the top of the shaft with the gas and are burned with air in an afterburner or the charging hood before the gas reaches the ID fan.
What temperature should a shaft furnace ID fan be designed for?
Normally 250 to 350°C after the afterburner and dilution air, which suits mild steel or boiler quality plate. The impeller must also be checked for the upset temperature at low charge level, which can reach about 500°C, and heat-resisting steel is used if that upset is likely to last.
Information needed to quote an ID fan for copper shaft 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.
- Melting rate and burner rows. Melting rate in t/h, number of burners and the firing range.
- Gas treatment. Afterburner, recuperator and dilution arrangement before the fan.
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.