How wire patenting and annealing furnaces work
A patenting line pulls a band of 20 to 60 carbon steel wires in parallel through a long open-fired or muffle furnace, where they are austenitised at about 900 to 980°C, and then straight into a molten lead bath or fluidised bed held at 500 to 600°C to form fine pearlite. Wire annealing lines use the same strand furnace layout at lower temperature, or batch pit furnaces. The furnaces are usually fired with natural gas, LPG or producer gas through rows of burners along the side walls, often slightly rich of stoichiometric to limit scale and decarburisation on the wire. Flue gas leaves through roof or end flues at 700 to 900°C, passes a recuperator that preheats the combustion air, and is drawn to the chimney by an ID fan where natural draft is not enough.
Where the FD fan sits
- Inlet filter
- FD fan
- Recuperator
- Hot air main
- Zone ratio valves
- Side-wall burners
The fan sits near the recuperator at the flue end and blows ambient air into it. Preheated air at 300 to 450°C runs through an insulated main along the furnace, with a ratio valve for each zone of burners. The fan pressure covers the recuperator air side, the main, the valves and the burner, typically 300 to 600 mmWC depending on burner type.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 300 to 450°C, downstream of the fan |
| Dust | Clean air through an inlet filter; mill dust near the drawing bay can load it |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | Moderate; firing follows wire size and line speed rather than swinging hourly |
| 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 wire patenting and annealing furnaces
| Blade form | Backward curved, for efficiency and a stable pressure curve |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard grease-lubricated bearings |
| Flow control | Variable speed drive on air main pressure, zone valves for ratio control |
| Also specified | Inlet filter with differential pressure gauge, and a low air pressure trip interlocked to the fuel valves |
What this FD fan has to cope with
Ratio drift as the filter loads
Drawing and pickling areas make dusty, sometimes acidic air. As the inlet filter clogs, fan delivery falls and burners drift richer than intended, producing soot and unburnt gas in the flue. A pressure-controlled drive and a filter alarm keep the ratio where the metallurgist set it.
Recuperator protection at low fire
At low line speed the air flow through the recuperator drops while the flue gas is still hot. The fan must not be turned down below the flow that keeps the recuperator tubes cool, or a bleed to atmosphere is needed.
Burner pressure margin
Older patenting furnaces were often converted from producer gas to natural gas with new high-velocity burners needing more air pressure. Reusing the old low pressure fan leaves zones starved at full speed. The fan is checked against the new burner data, not the nameplate of the old one.
Energy and control
The combustion air fan runs for the whole campaign, so its efficiency counts. Speed control on air main pressure, rather than an inlet damper held half shut, saves power when the line runs finer wire at lower firing. The air-fuel ratio matters more than the fan power: each per cent of excess air heats nitrogen for nothing and thickens the scale on the wire.
ID fan and FD fan in wire patenting and annealing furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator, before the chimney | Before the recuperator, feeding side burners |
| What it handles | Hot flue gas with burnt-out CO | Ambient filtered air |
| Temperature at the fan | 300 to 450°C | Ambient |
| Main design concern | Fine furnace pressure control at the wire slots | Steady air main pressure for tight ratio control |
| Control | Furnace pressure | Air main pressure |
See also the Induced Draft (ID) Fan for wire patenting and annealing furnaces.
Questions buyers ask
What is the FD fan on a patenting furnace?
It is the forced draft or combustion air fan that supplies air to the furnace burners, usually through a recuperator. On a wire patenting furnace it is sized for the burner pressure plus recuperator and main losses, and its delivery must be steady because the air ratio is set close to stoichiometric.
Why do patenting furnaces run slightly rich?
A small air deficit leaves CO and hydrogen in the furnace atmosphere, which slows oxidation and decarburisation of the hot wire. The combustion air fan supplies slightly less than the stoichiometric air at the burners, and the remaining combustibles are burnt with extra air in the flue.
Can one combustion air fan serve two patenting lines?
It can, if the air main is sized so that one line changing its firing does not disturb the other. In practice a fan per furnace with a common standby is easier to control, because each line has its own speed, wire size and ratio settings.
Information needed to quote an FD fan for wire patenting and annealing 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.
- Burners. Number of burners per zone, burner type and air pressure required at full fire.
- Air ratio. Normal air-fuel ratio and whether the burners are ever run with excess air.
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.