How direct fired thermal oxidisers work
Direct fired thermal oxidisers burn VOCs, odorous gases and hazardous vent streams in a refractory-lined chamber at typically 750 to 1,100°C, with residence time of about 0.5 to 2 seconds depending on the compounds. They suit high VOC concentrations, small or intermittent flows and compounds such as halogenated organics, amines or hydrogen cyanide that need higher temperature than an RTO allows. A burner fired on gas or oil supplies heat not provided by the VOCs. Heat is usually recovered in a recuperative heat exchanger that preheats the incoming process gas, or in a waste heat boiler or thermic fluid heater. Halogenated streams add a quench and scrubber. A combustion air fan supplies the burner, and an ID fan pulls the gas out.
Where the ID fan sits
- Process vent gas
- Recuperator cold side
- Burner and chamber
- Recuperator hot side
- Heat recovery unit
- ID fan
- Stack
The fan sits after the recuperator or heat recovery unit, at the stack, so the chamber and process vents are under suction. This placement is preferred where vent gases are toxic, because a leak draws air in rather than releasing gas. Suction at the fan is typically 200 to 400 mmWC. On halogenated duty, the fan comes after the quench and scrubber and handles wet gas instead.
Gas at the ID fan
| Gas temperature, normal | 250 to 400°C after heat recovery |
| Maximum or upset | Up to 600°C or more if heat recovery is bypassed; capped with dilution air |
| Dust at the fan | Clean in most cases; inorganic fume where vent gases carry silicones or salts |
| Moisture and dew point | Moderate; dew point below the gas temperature |
| Corrosive species | HCl, SO₂ or NOx from halogen, sulphur or nitrogen in the vent gas |
| Density at the design temperature | About 0.60 kg/m³ at 320°C, 50% 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 direct fired thermal oxidisers
| Blade form | Backward curved or radial, depending on size and temperature |
| Material of construction | Heat-resisting steel impeller; mild steel casing with insulation, stainless where acid gases are present |
| Drive and arrangement | Direct coupled with the motor away from the hot casing |
| Bearings and cooling | Pedestal bearings with cooling disc and heat shield |
| Flow control | Variable speed drive on chamber or vent header pressure |
| Also specified | Dilution air damper with temperature interlock, insulation, and temperature trip |
What this ID fan has to cope with
Heat recovery bypass
When the recuperator or boiler is bypassed, at start-up, for maintenance or to protect it from overheating at high VOC loads, gas at 600°C or more can reach the fan. A dilution air damper and a heat-resisting impeller allow the fan to keep running safely.
Variable vent flow
Process vents change with batch operations. The fan must hold suction at the vent header while flow changes, without pulling in so much air that chamber temperature falls. A variable speed drive on header pressure provides this.
Acid gas below dew point
Halogen or sulphur in the vents produces acid gases. If heat recovery cools the gas too far, acid condenses in the fan. Controlling heat recovery outlet temperature, or adding a scrubber, prevents this.
Energy and control
Direct fired oxidisers use more burner fuel than RTOs, so heat recovery is the main energy measure. The ID fan moves gas against the recuperator and chamber resistance; a variable speed drive saves power when vent flows are low. Reducing dilution air, by keeping heat recovery in service, cuts both fan power and fuel.
ID fan and FD fan in direct fired thermal oxidisers
| ID fan | FD fan | |
|---|---|---|
| Position | After heat recovery, at the stack | At the burner windbox |
| What it handles | Hot oxidised gas | Ambient combustion air |
| Temperature at the fan | 250 to 400°C, up to 600°C upset | Ambient, 25 to 45°C |
| Main design concern | Heat resistance and bypass protection | Stable flame across turndown |
| Control | Chamber or vent header pressure | Burner firing and oxygen |
See also the Forced Draft (FD) Fan for direct fired thermal oxidisers.
Questions buyers ask
What temperature does the ID fan on a thermal oxidiser see?
Typically 250 to 400°C after a recuperator or heat recovery unit, and up to 600°C or more if heat recovery is bypassed. Heat-resisting steel impellers and dilution air protect the fan.
When is a direct fired thermal oxidiser used instead of an RTO?
For high VOC concentrations, small or intermittent flows, and compounds such as halogenated organics, amines or HCN that need higher temperature or are unsuited to ceramic beds. The fan duty differs: hotter gas, smaller volume.
Should the thermal oxidiser fan push or pull?
Pulling, with an ID fan at the end, keeps toxic vents under suction. Pushing, with a fan before the oxidiser, keeps the fan cool but means any leak releases untreated gas. For hazardous vents, an ID fan is preferred.
Information needed to quote an ID fan for direct fired thermal 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.
- Vent streams. Flow, VOC composition and range of each vent stream.
- Heat recovery. Recuperator, boiler or thermic fluid heater, and its outlet temperature and bypass.
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.