How API process residue incinerators work
API manufacturing leaves process residues that are neither clean solvents nor ordinary solid waste: distillation still bottoms, tarry residues, spent carbon and filter aids, salt cakes from neutralisation and off-specification intermediates. Captive incinerators at API sites burn these in a static hearth, a small rotary kiln or a combined chamber with liquid injection for pumpable residues, followed by a secondary chamber at 1,100°C or more. The residues often contain nitrogen, sulphur, chlorine, bromine and fluorine, and some carry spent catalyst metals. Gas is quenched, scrubbed in a venturi and packed alkali column, passed through a mist eliminator and drawn by the ID fan. Combustion air fans serve the primary and secondary chambers.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Primary chamber air
- Secondary chamber air
- Chambers
The fan sits beside the incinerator, feeding a header with branches to the primary chamber or kiln, the liquid residue burner and the secondary chamber. It works against burner and port resistance, typically 250 to 500 mmWC. On many pharma sites the incinerator is close to solvent storage, so the fan and motor are selected for the hazardous area classification and the intake is taken from a safe area.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the chambers | Ambient on most plants |
| Dust | Clean air; filtered inlet on pharma sites |
| Pressure at the fan outlet | Typically 250 to 500 mmWC |
| Load profile | Peaks after each batch charge of residue |
| 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 API process residue incinerators
| Blade form | Backward curved or high-pressure radial |
| Material of construction | Mild steel; coated against acid vapours in the site atmosphere |
| Drive and arrangement | Direct mounted or direct coupled, flameproof motor where required |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive with branch dampers, trimmed on oxygen |
| Also specified | Airflow measurement, air pressure interlock and inlet filter |
What this FD fan has to cope with
Oxygen loss after charging
A charge of solvent-wet residue can release volatiles faster than the air supply can burn them. Combustion air must increase quickly, so a drive tied to oxygen measurement and margin on the fan are needed.
Secondary chamber cooling
Excess air after the volatiles have burnt off cools the secondary chamber below its required temperature and raises support fuel use. Separate branch control lets the operator reduce secondary chamber air independently of the primary chamber, holding temperature through the burnout phase.
Site atmosphere corrosion
Acid vapours from process vents and scrubbers on chemical sites corrode unprotected fans inside and out. A coated casing and impeller, and an intake located away from process vents, extend fan life.
Energy and control
The combustion air fan is small, but excess air from it raises support fuel use and the gas volume that the quench, scrubber and ID fan must handle. Oxygen trim and a variable speed drive keep excess air low between charges and raise it only when a fresh charge releases volatiles, which suits the batch feeding pattern of most API residue incinerators.
ID fan and FD fan in API process residue incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the alkali scrubber | Beside the incinerator, feeding all chambers |
| What it handles | Saturated gas with HCl, HBr and HF | Ambient combustion air |
| Temperature at the fan | 60 to 80°C | Ambient, 25 to 45°C |
| Main design concern | Mixed halogen corrosion | Oxygen after batch charging |
| Control | Chamber pressure | Oxygen and chamber temperature |
See also the Induced Draft (ID) Fan for API process residue incinerators.
Questions buyers ask
What does the combustion air fan do on an API waste incinerator?
It supplies the air for burning residues in the primary chamber and for completing combustion in the secondary chamber, and air to any liquid residue burner. It handles ambient air.
Why does an API residue incinerator smoke after charging?
Volatile residues release gas faster than the air supply can burn it, so the chamber runs short of oxygen. More combustion air at the moment of charging, faster control on oxygen, or smaller and more frequent charges usually solve it.
Does the FD fan need a flameproof motor?
It depends on the area classification at the fan location. Pharma incinerators are often near solvent storage, so this needs to be checked against the site's hazardous area drawing.
Information needed to quote an FD fan for API process residue incinerators
- 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.
- Air branches. Design air flows to primary chamber, residue burner and secondary chamber.
- Area classification. Hazardous area zone at the fan location, if any.
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.