How carbon anode baking furnaces work
Carbon anodes for aluminium smelting are baked in open-top ring furnaces. Green anodes made of petroleum coke and coal tar pitch are packed in deep refractory pits, separated by hollow flue walls, and covered with packing coke. A fire of several sections moves slowly around the ring: burner ramps fire natural gas or fuel oil into the flues, pitch volatiles released from the anodes burn in the flues, and the anodes reach about 1,100 to 1,200°C. Blowing ramps push air into the cooling sections, where it is preheated by the baked anodes before reaching the fire. An exhaust ramp draws the flue gas into a ring main at 150 to 250°C and on to the fume treatment centre, then the ID fan.
Where the FD fan sits
- Inlet
- FD (blowing ramp) fan
- Blowing ramp manifold
- Flue walls in cooling sections
- Fire zone
Blowing ramp fans are mounted on portable ramps placed over the flue wall openings of the cooling sections, a few sections behind the fire. They push ambient air into the flues, where it is heated by the baked anodes before reaching the burner ramps. Each ramp is moved with the fire. Pressure required is moderate, typically 150 to 400 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, but warm above the furnace, 35 to 55°C |
| Dust | Packing coke and alumina dust over the furnace |
| Pressure at the fan outlet | Typically 150 to 400 mmWC |
| Turndown | Steady, set by fire progression and cooling requirement |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 carbon anode baking furnaces
| Blade form | Backward curved, compact for portable ramps |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft for compact ramps |
| Bearings and cooling | Motor bearings; motor rated for the hot furnace hall |
| Flow control | Variable speed drive on flow or flue pressure |
| Also specified | Inlet screen against packing coke dust, lifting points for crane handling |
What this FD fan has to cope with
Hot, dusty furnace hall
Air above the furnace is warm and carries packing coke and alumina dust. The fan takes this air, which lowers its mass flow and builds dust on the impeller. Inlet screens and motors rated for the hall temperature keep the ramps reliable. Periodic cleaning restores flow.
Handling and damage
Blowing ramps are lifted and moved by crane at every fire move. Rough handling damages fans and connections. Robust frames, lifting points and flexible connections reduce damage. Fans that are light and compact for their duty make crane handling easier, and a simple electrical plug connection speeds the changeover at each fire move. Spare ramps allow a damaged fan to be repaired without delaying the fire.
Air distribution to the flues
Uneven air to the flue walls gives uneven cooling and combustion. Measured flow or pressure at each ramp, and fans with matched performance, keep the air distribution even. Poor distribution shows up as variation in anode baking level between pits, which affects smelter performance. Regular checks of flue pressure at each wall show where the air is uneven.
Energy and control
Blowing ramp fans run continuously as part of each fire. Speed control linked to flue pressure, rather than a fixed damper, matches air to the fire and saves power. The air they supply is preheated by the anodes, so the right amount of air also improves fuel economy and anode quality.
ID fan and FD fan in carbon anode baking furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the fume treatment centre | On portable ramps over the cooling sections |
| What it handles | Treated flue gas with tar and fluoride traces | Warm hall air |
| Temperature at the fan | 100 to 130°C | 35 to 55°C |
| Main design concern | Tar deposits and acid corrosion | Robust portable design and even air |
| Control | Ring main suction | Flue pressure at each ramp |
See also the Induced Draft (ID) Fan for carbon anode baking furnaces.
Questions buyers ask
What is a blowing ramp on an anode baking furnace?
It is a portable manifold with fans that sits over the flue walls of the cooling sections and blows ambient air into them. The air cools the baked anodes and is preheated on its way to the fire zone, where it supports combustion. It is the forced draft side of the furnace.
Why do blowing ramp fan motors fail?
The air above the furnace is hot and dusty, and the ramps are moved often by crane. Motors in high ambient temperature, with dust and handling damage, fail early. Motors rated for the hall temperature, inlet screens and careful handling extend their life. Spare motors are usually kept.
Do blowing ramp fans handle hot gas?
No. They push ambient air into the flue walls. The air heats up downstream inside the furnace as it cools the anodes. The fans themselves see warm hall air, so mild steel construction is used. The motor is rated for the hall temperature, and the fan is sized for the reduced air density.
Information needed to quote an FD fan for carbon anode baking 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.
- Ramp arrangement. Number of ramps per fire and flue walls per section.
- Air requirement. Air flow and pressure per flue wall.
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.