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 FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Burner windbox
- Combustion chamber
The fan sits beside the burner and delivers ambient air to the windbox, sometimes through a preheater. It works against burner and duct resistance, typically 200 to 500 mmWC. Where vents are nitrogen-blanketed or low in oxygen, the fan must also supply the air needed to oxidise the VOCs, which can make it much larger than the burner alone needs.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | Up to 200 to 400°C where fitted, downstream of the fan |
| Dust | Clean air; filtered inlet on chemical sites |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Load profile | Wide turndown with vent flow and VOC load |
| 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 direct fired thermal oxidisers
| Blade form | Backward curved or high-pressure radial for stable pressure across turndown |
| Material of construction | Mild steel; coated for corrosive site atmospheres |
| Drive and arrangement | Direct mounted or direct coupled; flameproof motor where the area classification requires |
| Bearings and cooling | Standard motor or pedestal bearings |
| Flow control | Variable speed drive on burner firing with oxygen trim |
| Also specified | Airflow measurement, air pressure interlock and inlet filter |
What this FD fan has to cope with
Oxygen shortage with inert vents
Nitrogen-blanketed vents bring VOCs with little oxygen. If combustion air covers only the burner fuel, the VOCs leave unburnt. The fan is sized to supply air for both, with oxygen measurement at the chamber outlet.
Flame stability at low fire
When VOC loads are high, the burner turns down to low fire. The fan must hold stable air pressure at low flow so the flame does not lift or go out.
Site atmosphere hazards
Chemical sites may have flammable or corrosive atmospheres near the oxidiser. Fan materials, coating and motor rating are chosen for the area classification, and the air intake is located in a safe, non-classified area away from process vents.
Energy and control
Combustion air above what the burner and VOCs need cools the chamber and increases fuel use, and fuel is the main running cost of a direct fired oxidiser. A variable speed drive with oxygen trim keeps excess air low across the turndown. Preheating combustion air from the flue gas also reduces fuel, though the fan itself stays on the cold side of the preheater.
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 Induced Draft (ID) Fan for direct fired thermal oxidisers.
Questions buyers ask
What does the FD fan do on a thermal oxidiser?
It is the combustion air fan for the burner. It supplies air to burn the support fuel and, where vent gases contain too little oxygen, the air needed to oxidise the VOCs as well.
Why does a thermal oxidiser need extra combustion air for nitrogen-blanketed vents?
Vents from nitrogen-blanketed reactors and tanks contain VOCs but little oxygen. The VOCs need air to burn, so the combustion air fan must supply it, in addition to the burner's own air.
What pressure does a thermal oxidiser burner fan need?
Typically 200 to 500 mmWC at the fan, depending on the burner design, the windbox and any preheater on the air side. The burner supplier states the air pressure needed at the burner, and the fan is sized to that plus the duct losses.
Information needed to quote an FD fan for direct fired thermal 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.
- Vent oxygen. Oxygen content of each vent stream.
- Burner. Burner type, fuel and turndown range.
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.