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 ID fan sits
- Oven zones
- Exhaust collection duct
- Tar knockout or filter
- Exhaust (ID) fan
- Thermal oxidiser
- Stack
The fan sits on the exhaust duct that collects bleed air from each oven zone, upstream of the thermal oxidiser in most layouts, so it pushes the exhaust into the oxidiser. It works against the zone balancing dampers, the duct and any tar knockout or filter, typically 100 to 250 mmWC. On some lines an ID fan after the oxidiser pulls the whole system instead, which keeps the oxidiser under suction but puts the fan in hot, clean gas.
Gas at the ID fan
| Gas temperature, normal | 230 to 300°C oven exhaust air |
| Maximum or upset | Up to 350°C during a runaway exotherm or fibre fire in a zone |
| Dust at the fan | Little dust; fibre fragments and condensed tarry oligomers |
| Toxic and flammable species | HCN, ammonia and CO, kept well below lower explosive limits by the bleed rate |
| Condensation | Tars condense on surfaces below about 200°C, so cold spots in the duct and fan foul quickly |
| Density at the design temperature | About 0.66 kg/m³ at 260°C, 55% 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 carbon fibre oxidation ovens
| Blade form | Backward curved or radial bladed, with open blade passages that tolerate some tar build-up |
| Material of construction | Stainless steel impeller and casing, commonly SS 304 or SS 316, with welded gas-tight casing joints |
| Drive and arrangement | Direct coupled with the motor outside the gas stream; arrangement with a heat-isolated bearing pedestal |
| Bearings and cooling | Pedestal bearings with cooling disc; shaft seal of the purged or multi-ring type to stop leakage |
| Flow control | Variable speed drive on oven pressure, with zone dampers for balance |
| Also specified | Insulated and trace-heated casing to avoid tar condensation, cleaning doors, and spark-resistant construction where required by the hazard study |
What this ID fan has to cope with
Tar build-up on the impeller
Oligomers and tars condense on any surface below their dew point and bake into a hard layer. On the impeller this causes unbalance and loss of flow. Insulating the casing, keeping the gas hot through the fan and providing access for scheduled cleaning keep the fan in service.
Toxic gas leakage at the shaft
HCN at the shaft seal or casing joints is a direct hazard to operators. Welded casing seams, gasketed access doors and a purged or multi-lip shaft seal make the casing gas-tight; gas detection near the fan is normal practice.
Exotherm or fibre fire
A stopped line or broken tow can lead to a runaway reaction in a zone, raising exhaust temperature sharply. The fan and its bearings must survive the upset, and the exhaust control should hold suction rather than shut off and let toxic gas into the building.
Energy and control
Oxidation ovens run continuously for long campaigns, and the exhaust rate is set by safety limits on HCN and the need to remove reaction heat. Holding oven pressure with a variable speed drive avoids throttling losses, and keeping the bleed at the minimum safe rate reduces the fresh air that has to be heated and the gas that the thermal oxidiser must burn, which is usually the larger energy cost.
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 Forced Draft (FD) Fan for carbon fibre oxidation ovens.
Questions buyers ask
What does the exhaust fan on a carbon fibre oxidation oven handle?
Hot process air at about 230 to 300°C carrying hydrogen cyanide, ammonia, CO, water vapour and tarry oligomers from the stabilisation of PAN fibre. It is mostly air, not combustion gas, but it is toxic and fouling, so the fan must be gas-tight and kept hot.
Why is a stainless steel fan used on oxidation oven exhaust?
Condensate from the exhaust contains ammonia, cyanide compounds and organic acids that attack mild steel, and stainless surfaces are easier to clean of tar. Stainless steel also keeps its strength at exhaust temperatures and during exotherm upsets.
How do you stop tar fouling in a carbon fibre oven exhaust fan?
Keep the gas above the tar dew point all the way to the oxidiser by insulating and, where needed, trace heating the duct and fan casing. Provide cleaning doors and plan regular cleaning, because some deposition is unavoidable over a long campaign.
Information needed to quote an ID fan for carbon fibre oxidation ovens
- 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.
- Exhaust composition. Expected HCN, ammonia and CO concentrations and the design bleed rate per zone.
- Oxidiser arrangement. Whether the fan pushes into the thermal oxidiser or pulls after it.
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.