How carbon fibre oxidation ovens work
Carbon fibre is made by first stabilising PAN precursor tow in air at about 200 to 300°C, in a series of oxidation ovens where the tow passes back and forth many times on rollers. The reaction is exothermic and releases hydrogen cyanide, ammonia, carbon monoxide, water vapour and tarry oligomers. Large recirculation fans drive hot air across the tow at controlled velocity, and the ovens are heated by gas burners or electric heaters, often indirectly. To keep toxic and flammable species far below safe limits, a bleed of oven air is continuously exhausted and replaced with fresh air. The exhaust (ID) fan takes this bleed to a thermal oxidiser, and a fresh air supply fan, which serves as the FD fan, replaces it.
Where the FD fan sits
- Inlet filter
- FD fan
- Heat recovery exchanger
- Supply air duct
- Zone make-up dampers
- Oven zones
The supply fan sits outside the oven enclosure, takes ambient air through a filter and pushes it through the heat recovery exchanger, where the air is preheated, then into the make-up ports of each zone. The oven's own recirculation fans then move the air across the tow. The supply fan works against the filter, exchanger and duct, typically 100 to 250 mmWC. Supply and exhaust are balanced so the oven stays slightly negative.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C, filtered |
| Air after heat recovery | 100 to 200°C, downstream of the fan |
| Dust | Filtered air; fibre fly from the line and outdoor dust must not reach the tow |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Balance requirement | Supply set slightly below exhaust so the oven stays negative |
| 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 carbon fibre oxidation ovens
| Blade form | Backward curved for efficiency on clean, filtered air |
| Material of construction | Mild steel, painted, or galvanised; stainless steel where the duct is shared with exhaust heat recovery |
| Drive and arrangement | Direct mounted or direct coupled |
| Bearings and cooling | Standard motor or pedestal bearings; no cooling needed |
| Flow control | Variable speed drive tracking the exhaust flow to hold oven pressure |
| Also specified | Inlet filter with pressure drop monitoring, airflow measurement and a non-return damper |
What this FD fan has to cope with
Dust and fibre in the supply air
Particles in the make-up air land on the tow and become defects in the fibre. Fine filtration on the inlet and a fan that does not shed paint or rust into the air protect product quality.
Imbalance between supply and exhaust
If the supply fan delivers more than the exhaust removes, the oven goes positive and HCN escapes at the tow slots. Flow measurement on both fans and a control that tracks exhaust flow keep the oven negative.
Exchanger leakage
A leak in the heat recovery exchanger can pass oxidiser exhaust into the supply air. Pressure on the supply side is kept above the exhaust side so any leak goes outwards, and the exchanger is inspected at each shutdown.
Energy and control
The supply fan runs for the whole campaign at a flow set by the exhaust bleed. Its power is modest; the larger saving is in the heat needed to bring fresh air to oven temperature. Recovering heat from the oxidiser exhaust into the supply air, and keeping the bleed at the minimum the safety limits allow, reduce gas or electric consumption more than any change at the fan itself.
ID fan and FD fan in carbon fibre oxidation ovens
| ID fan | FD fan | |
|---|---|---|
| Position | On the exhaust bleed, ahead of the thermal oxidiser | On the fresh air supply, ahead of heat recovery |
| What it handles | Hot air with HCN, ammonia and tars | Filtered ambient air |
| Temperature at the fan | 230 to 300°C | Ambient, 25 to 45°C |
| Main design concern | Gas tightness and tar fouling | Clean air and supply-exhaust balance |
| Control | Oven pressure | Tracks exhaust flow |
See also the Induced Draft (ID) Fan for carbon fibre oxidation ovens.
Questions buyers ask
Is there a forced draft fan on a carbon fibre oxidation oven?
Not in the boiler sense. The equivalent is the fresh air supply fan that replaces the exhaust bleed. Where the ovens are gas-fired, small combustion air fans feed the burners as well. The large fans inside the oven are recirculation fans, which are a separate duty.
Does the supply air need to be preheated?
It is usually preheated by recovering heat from the oxidiser or oven exhaust. The fan stays on the cold side, ahead of the exchanger, so it handles ambient air and the heat duty falls on the exchanger.
Why must the oven stay at negative pressure?
The process releases hydrogen cyanide and other toxic gases. Negative pressure makes air leak inwards at the tow slots and doors rather than letting oven gas leak into the workplace.
Information needed to quote an FD fan for carbon fibre oxidation ovens
- 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.
- Make-up rate. Fresh air flow per zone and the total exhaust bleed it must balance.
- Filtration. Required filter grade on the supply 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.