How catalytic oxidisers work
A catalytic oxidiser destroys solvent vapours in process exhaust from coating lines, printing presses, pharma reactors and dryers. The solvent-laden air, usually below 25% of the lower explosive limit, is collected by a process fan and passed through a plate or shell-and-tube heat exchanger, where it is preheated by the treated gas. A small gas or electric preheater lifts it to the catalyst light-off temperature, typically 250 to 350°C, and the VOCs oxidise on a precious metal or metal oxide catalyst, raising the gas by 20 to 30°C for every gram per cubic metre of solvent. The hot, clean gas returns through the exchanger and leaves at 150 to 250°C. Units use either a forced draft process fan ahead of the exchanger, an induced draft fan after it, or both on larger systems.
Where the FD fan sits
- Process vents and hoods
- Filter or knock-out
- FD fan
- Heat exchanger, cold side
- Preheater
- Catalyst bed
The fan sits between the collection ductwork and the oxidiser inlet. A filter ahead of it catches overspray and dust that would blind the catalyst. Its suction side is the capture system, and it delivers into the heat exchanger, preheater and bed, typically 250 to 400 mmWC. A fresh air damper on the inlet allows dilution when solvent concentration rises towards the alarm limit.
Air at the FD fan
| Air temperature at the fan | Ambient to about 60°C, depending on the process exhaust |
| Flammable vapour | Usually designed below 25% LEL, with alarm and dilution above that |
| Dust | Overspray, resin mist or powder, reduced by an inlet filter |
| Condensable solvents | Heavier solvents and plasticisers can condense in the casing at low temperature |
| Pressure at the fan outlet | Typically 250 to 400 mmWC through exchanger and bed |
| 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 catalytic oxidisers
| Blade form | Backward curved, in a non-sparking construction |
| Material of construction | Mild steel casing with an aluminium or brass rubbing ring at the inlet; stainless where vapours are acidic |
| Drive and arrangement | Direct drive with a flameproof motor suited to the area classification |
| Bearings and cooling | Standard bearings outside the gas stream; no cooling needed |
| Flow control | Variable speed drive on hood suction, with fresh air and bypass dampers on the LEL controller |
| Also specified | Shaft seal to limit vapour leakage, earthing of all parts and a casing drain for condensed solvent |
What this FD fan has to cope with
Ignition risk in the fan
The fan handles vapour that can burn if concentration rises. A rub between impeller and inlet cone, or a static discharge, is a credible ignition source. Non-sparking rubbing materials, generous running clearances, earthing and a flameproof motor reduce the risk, and the LEL monitor trips to dilution before the fan sees a flammable mixture.
Solvent condensation and resin build-up
High-boiling solvents, plasticisers and resins condense on the cool casing and impeller, building sticky deposits that unbalance the rotor and narrow the inlet clearance. A drain at the lowest point, inspection doors, an inlet filter matched to the coating process and a scheduled wash keep the build-up manageable and the vibration within limits.
Vapour leakage at the shaft
Because the fan discharge is under positive pressure, a worn shaft seal lets solvent vapour escape into the fan room, where it can collect near the motor. A proper shaft seal, a ventilated fan enclosure, gas detection near the fan and regular seal checks avoid odour complaints and stop a hazardous zone forming around the drive.
Energy and control
The process fan runs whenever any connected line is running, but collection volume should follow the number of open hoods. Speed control from hood or header suction, with dampers closing on idle lines, keeps the oxidiser from heating unnecessary air. Concentrating the exhaust as much as safety allows raises the energy released on the catalyst and reduces the preheater fuel, which is usually a bigger saving than the fan power itself.
ID fan and FD fan in catalytic oxidisers
| ID fan | FD fan | |
|---|---|---|
| Position | After the heat exchanger, before the stack | Before the heat exchanger and catalyst |
| What it handles | Treated, clean gas | Solvent-laden process air |
| Temperature at the fan | 150 to 250°C, higher on bypass | Ambient to about 60°C |
| Main design concern | Upset temperature and fouled-bed resistance | Spark resistance and vapour tightness |
| Control | Hood suction | Hood suction and LEL dilution |
See also the Induced Draft (ID) Fan for catalytic oxidisers.
Questions buyers ask
Can a standard fan handle solvent-laden air to a catalytic oxidiser?
Not without changes. The process fan on a catalytic oxidiser should have non-sparking rubbing materials at the inlet, a flameproof motor suited to the hazardous zone, earthing of all parts and a shaft seal. The plant must also keep the solvent concentration below the LEL limit with continuous monitoring and an automatic fresh air dilution damper.
What pressure does the process air fan need on a catalytic oxidiser?
The process air fan has to overcome the heat exchanger, preheater and catalyst bed, usually 250 to 400 mmWC in total, plus the capture ductwork and inlet filter if the fan is not close to the hoods. The resistance of a fouled bed and a dirty filter should be included, or the fan will run short as the catalyst ages.
Is a forced draft or induced draft fan better for VOC oxidisers?
An induced draft fan keeps leaks inward and handles destroyed gas, which suits toxic or odorous solvents. A forced draft fan is smaller and cooler but sees flammable vapour and puts the oxidiser under pressure. The choice depends on the solvent hazard, the plant layout and how far the oxidiser is from the capture points.
Information needed to quote an FD fan for catalytic oxidisers
- 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.
- Area classification. Hazardous zone at the fan location and the required motor protection.
- Capture points. Number of hoods or vents, their individual flows and how often lines run at the same time.
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.