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 ID fan sits
- VOC capture hoods
- Heat exchanger, cold side
- Preheater
- Catalyst bed
- Heat exchanger, hot side
- ID fan
- Stack
In the induced draft arrangement the fan is the last item before the stack. Upstream it has the capture ductwork, both passes of the heat exchanger, the preheater and the catalyst bed, whose resistance rises as the catalyst ages or blinds with particulate. Total suction is usually 250 to 450 mmWC. Placing the fan here keeps leaks inward, so solvent-laden air cannot escape into the plant, and keeps the fan away from flammable vapour, which by this point has been destroyed.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C after the heat exchanger |
| Maximum or upset | 350 to 450°C if a solvent slug overheats the bed or the exchanger is bypassed |
| Dust at the fan | Very low; fine catalyst attrition dust and ceramic fibre from insulation |
| Moisture and dew point | A few percent water vapour from combustion of the solvent; well above dew point |
| Corrosive species | HCl or HBr if chlorinated or brominated solvents reach the unit; acid condensation at shutdown |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 catalytic oxidisers
| Blade form | Backward curved, for efficiency on clean, warm gas |
| Material of construction | Mild steel for normal duty; SS 304 or SS 316 where halogenated solvents produce acid gas |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8, casing insulated and free to expand |
| Bearings and cooling | Shaft cooling disc and heat slinger; plain rolling bearings below about 250°C |
| Flow control | Variable speed drive holding suction at the capture hoods or a set point upstream of the exchanger |
| Also specified | Rated for the upset temperature; inspection door and casing drain for acid condensate |
What this ID fan has to cope with
Rising pressure drop as the catalyst blinds
Silicones, phosphorus and fine particulate coat the catalyst and the exchanger plates, and the bed resistance climbs over months. A fan selected at clean-bed resistance runs out of pressure and capture at the hoods falls off. Selecting with a margin for the fouled bed, and running on speed control, lets the fan hold capture through the cleaning interval.
Temperature excursion on a solvent spike
A sudden rise in solvent concentration can push the bed outlet well above design and, with the exchanger bypass open, send hotter gas to the fan. The impeller stress and the bearing cooling are checked at the bypass temperature, and the fan is interlocked with the high-temperature dilution damper.
Acid condensate from halogenated solvents
Where dichloromethane or other chlorinated solvents reach the oxidiser, the exhaust carries HCl. On shutdown the casing cools below the dew point and pitting starts at the bottom of the scroll. Stainless construction, a casing drain and a post-purge with warm air reduce the attack.
Energy and control
Process exhaust volume often follows the production line, with coaters, presses or dryers coming on and off. A variable speed drive that holds hood suction lets the fan follow the real exhaust, and lower flow raises the residence time and reduces preheater fuel. When solvent load is high the oxidiser can be self-sustaining, and every unnecessary cubic metre of air pulled through it costs heat and fan power.
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 Forced Draft (FD) Fan for catalytic oxidisers.
Questions buyers ask
Does a catalytic oxidiser need an ID fan or an FD fan?
Either can work. An induced draft fan after the heat exchanger keeps the unit and the ducts under suction and handles destroyed, clean gas. A forced draft fan ahead of the unit handles cold, solvent-laden air and puts the oxidiser under pressure. Larger catalytic oxidisers often use both, with the ID fan controlling hood suction.
What temperature does the exhaust fan on a catalytic oxidiser see?
After a recuperative heat exchanger the treated gas from a catalytic oxidiser is usually 150 to 250°C at the fan. The fan must also be checked at the bypass condition, where hot gas from the catalyst bed goes straight to the stack, which can reach 350 to 450°C for short periods during a solvent spike.
Why does catalytic oxidiser fan pressure rise over time?
Catalyst poisons such as silicones and phosphorus, and fine particulate that passes the inlet filter, gradually coat the catalyst bed and the exchanger surfaces. The system resistance rises until the catalyst is washed, regenerated or replaced. The fan pressure should include a fouled-bed allowance so that capture at the hoods holds through the cleaning interval.
Information needed to quote an ID fan for catalytic oxidisers
- 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.
- Solvent and capture data. Solvent types, typical and peak concentration as % LEL, and any halogenated or silicone-bearing compounds.
- Heat exchanger and bypass. Exchanger pressure drop clean and fouled, and the gas temperature with the hot-side bypass fully open.
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.