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 FD fan sits
- Inlet filter and silencer
- FD (combustion air) fan
- Recuperator (if fitted)
- Air ring main
- Burner ratio controls
- Burner rows
The fan sits at ground level and supplies a ring main around the shaft, from which each burner takes air through its own ratio control. Where a recuperator is fitted, the fan pushes cold air through it first. The pressure must cover the recuperator, the ring main, the ratio valves and premix or nozzle-mix burners, typically 400 to 800 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 200 to 350°C where fitted, downstream of the fan |
| Dust | Filtered air |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Turndown | Melting rate varies with caster demand; individual burner rows are switched in and out |
| 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 copper shaft melting furnaces
| Blade form | Backward curved, with a steep pressure curve for stable header pressure |
| 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 ring main pressure |
| Also specified | Inlet filter, inlet silencer and often a standby fan, since loss of air stops melting |
What this FD fan has to cope with
Ratio drift as burners switch
Shaft furnaces change melting rate by switching burners or whole rows on and off. When a row is switched off, ring main pressure rises and the remaining burners receive more air than their gas valves expect, so they run lean and oxidise the copper. A drive holding constant header pressure keeps each burner's ratio where it was set.
Burner backfire on air loss
Premix burners can flash back into the mixer if air pressure falls while gas is still flowing. Low air pressure switches on the ring main, a gas shut-off interlocked to them, and a fan curve with no unstable region at low flow protect the burners when several rows are turned down together.
Noise at the charging floor
Combustion air fans at 400 to 800 mmWC run at two-pole speed and are loud at the inlet, and the fan is often close to the charging platform where operators work all shift. An inlet silencer, a lagged casing and flexible connections to stop duct-borne noise keep levels acceptable.
Energy and control
The combustion air fan runs as long as the furnace melts, often around the clock. Matching fan speed to the active burner rows, instead of throttling with a damper, saves power during reduced melting and holding. The larger saving usually comes from correct air-fuel ratio, which also governs copper quality, so control stability is worth more than a small gain in fan efficiency.
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 Induced Draft (ID) Fan for copper shaft melting furnaces.
Questions buyers ask
What does the FD fan supply on a copper shaft furnace?
It supplies combustion air to the burners around the base of the shaft. On this furnace the FD fan is the combustion air blower, sized for all burner rows firing at full rate plus the losses of any recuperator, the ring main and the ratio valves. It handles only clean ambient air.
Why do shaft furnace burners need such stable air pressure?
The burners run slightly fuel rich to keep oxygen out of the copper. A small change in air pressure shifts the air-gas ratio at every burner, either oxidising the melt or wasting fuel and raising CO. Holding header pressure constant with a speed-controlled fan keeps the ratio steady as burner rows switch.
Is a standby combustion air fan needed on a shaft furnace?
On a furnace that feeds a continuous caster or rod line, many plants fit a standby fan with automatic changeover, because loss of combustion air stops melting within minutes and can freeze metal in the launder and holding furnace. On smaller batch-fed operations a spare impeller and motor may be enough.
Information needed to quote an FD fan for copper shaft 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 arrangement. Number of burner rows and burners, and the air pressure each burner needs.
- Ratio control. Type of ratio control and whether air is preheated.
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.