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 ID fan sits
- Strand furnace
- Roof or end flue
- Post-combustion air port
- Recuperator
- ID fan
- Chimney
The fan stands at the end of the flue run after the recuperator, usually at floor level beside the furnace entry end. Because the burners run slightly rich, the flue gas carries a few per cent of CO and hydrogen, and air is admitted at a port in the flue so that these burn out before the recuperator. The suction the fan works against is modest, typically 40 to 100 mmWC across the flue, burn-out zone and recuperator, but it must be trimmed in steps of a fraction of a millimetre at the furnace.
Gas at the ID fan
| Gas temperature, normal | 300 to 450°C after the recuperator |
| Maximum or upset | Up to 550 to 600°C if post-combustion runs into the recuperator or dilution air fails |
| Dust at the fan | Very low; fine iron oxide scale and refractory fibre, usually below 50 mg/Nm³ |
| Combustibles | CO and H₂ from rich firing, burnt out ahead of the recuperator when the air port is set correctly |
| Corrosive species | Negligible on natural gas or LPG; some H₂S and SO₂ with producer gas firing |
| Density at the design temperature | About 0.53 kg/m³ at 400°C, 44% 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 wire patenting and annealing furnaces
| Blade form | Backward inclined, for a flat power curve on clean hot gas |
| Material of construction | Heat-resisting steel impeller where gas can exceed about 400°C; insulated mild steel casing |
| Drive and arrangement | Arrangement 8 direct drive, or belt drive on small furnaces, with a cooled shaft and heat slinger |
| Bearings and cooling | Pedestal bearings outside the hot zone; water cooling where the gas runs above about 400°C |
| Flow control | Variable speed drive on furnace pressure measured near the wire entry slot |
| Also specified | Tapping for a dilution air damper ahead of the fan, and a pressure test point at the fan inlet |
What this ID fan has to cope with
Scale and decarburisation from air ingress
If the fan pulls the furnace negative, cold air streams in through the wire slots and burns the protective rich atmosphere. The wire picks up thicker scale and a decarburised skin, which shows up later as breaks at drawing. Holding a slight positive pressure, with the fan on speed control, is the design response.
Afterburning in the recuperator
Unburnt CO meeting air leaks at the recuperator can burn inside the tubes and overheat them, and the fan then sees gas well above its rating. A defined burn-out port, temperature trip before the recuperator and a fan rated for the upset temperature cover this.
Fibre and oxide build-up
Ceramic fibre linings shed fine fibre that mixes with oxide dust and settles on the blades in a thin felt. On a hot fan this bakes on unevenly and causes slow vibration growth. Inspection doors and periodic cleaning keep it under control.
Energy and control
Patenting lines run long continuous campaigns at a fairly steady throughput set by wire size and line speed. When the line changes to finer wire, firing drops and the ID fan should drop with it rather than sit throttled by a damper. A variable speed drive on furnace pressure saves fan power, but the larger gain is fuel: a furnace held at the right pressure does not burn its atmosphere or heat infiltrated air.
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 Forced Draft (FD) Fan for wire patenting and annealing furnaces.
Questions buyers ask
Does a wire patenting furnace need an ID fan?
Short, simple strand furnaces sometimes manage on chimney draft alone. Once a recuperator, a burn-out chamber and a long horizontal flue are added, the natural draft cannot hold a steady furnace pressure across the line speed range, and an ID fan with speed control is fitted. It also allows a lower chimney.
What furnace pressure should a patenting furnace run at?
Most operators hold the furnace slightly positive, a fraction of a millimetre to about 1 mmWC at the wire slots, so that a small amount of combustion gas escapes rather than air leaking in. The exact set point depends on slot size and furnace atmosphere and is tuned on site.
Is the lead bath fume handled by the same ID fan?
Normally not. Lead bath fume is captured by hoods over the bath and sent to a separate filter with its own exhaust fan. Mixing it with the furnace flue gas would load the recuperator with lead oxide and make furnace pressure control harder.
Information needed to quote an ID fan for wire patenting and annealing 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.
- Firing and atmosphere. Fuel, burner air ratio and whether CO is burnt out in the flue before the recuperator.
- Wire slots and pressure. Number and size of strand openings and the furnace pressure set point.
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.