How biomedical waste incinerators work
Biomedical waste incinerators are usually controlled-air, two-chamber units of about 50 to 500 kg/h at common treatment facilities and large hospitals. Bagged waste is charged in batches or by a ram feeder into the primary chamber, held at about 800°C with limited air, where it dries and gasifies. The gases burn in the secondary chamber at about 1,050°C with at least two seconds residence time, supported by diesel or gas burners. The waste contains plastics, including PVC, along with cotton, fluids and sharps, so the flue gas carries HCl and fine particulate. Gas is cooled in a quench or heat exchanger, cleaned in a venturi and packed scrubber with alkali dosing, and drawn by the ID fan to a stack. Combustion air blowers supply both chambers.
Where the FD fan sits
- Inlet screen
- FD fan
- Air header
- Primary chamber air
- Secondary chamber air
- Chambers
The blower sits beside the incinerator and delivers ambient air into a header with separate dampered branches for the primary chamber and the secondary chamber. Burners on each chamber usually have their own small integral blowers. The combustion air blower works against nozzle and duct resistance, typically 150 to 300 mmWC. On small units one blower serves both chambers; larger units have separate primary and secondary blowers.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the chambers | Ambient, not preheated on most units |
| Dust | Clean air; screen against debris at waste-handling areas |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Air split | Primary chamber sub-stoichiometric; secondary chamber with excess air |
| 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 biomedical waste incinerators
| Blade form | Backward curved or radial for a stable, steep curve at modest flows |
| Material of construction | Mild steel, painted |
| Drive and arrangement | Direct mounted on the motor shaft |
| Bearings and cooling | Motor bearings; no cooling needed |
| Flow control | Motorised dampers on each branch, set by chamber temperatures; variable speed drive on larger units |
| Also specified | Inlet screen, air pressure switch interlocked with the burners and feed |
What this FD fan has to cope with
Too much air in the primary chamber
Excess primary air turns controlled-air gasification into fast combustion, raising primary chamber temperature and carrying more ash to the secondary chamber and scrubber. Separate branch dampers controlled by chamber temperature keep the primary chamber starved of air as designed.
Secondary chamber temperature drop
Wet waste or a large charge can pull the secondary chamber below its required temperature. Enough secondary air for complete burnout, but not so much that it cools the chamber, needs accurate flow control rather than a fixed damper setting.
Blower trip during operation
If the blower stops, combustion becomes incomplete and smoke reaches the scrubber. An air pressure switch that stops waste feeding and holds the burners in a safe state is a standard part of the system.
Energy and control
The combustion air blower on a biomedical incinerator is small, and its energy use is minor compared with burner fuel. The larger saving comes from correct air split: too much air in either chamber cools it and increases support fuel. A variable speed drive with oxygen measurement in the secondary chamber outlet reduces both fuel and the gas volume the ID fan must move.
ID fan and FD fan in biomedical waste incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber, at the stack | Beside the incinerator, feeding both chambers |
| What it handles | Saturated gas with acid mist | Ambient combustion air |
| Temperature at the fan | 60 to 80°C | Ambient, 25 to 45°C |
| Main design concern | Chloride corrosion and charging surges | Primary and secondary air split |
| Control | Primary chamber pressure | Chamber temperatures and oxygen |
See also the Induced Draft (ID) Fan for biomedical waste incinerators.
Questions buyers ask
What is the combustion air blower on an incinerator?
It is the forced draft fan that supplies air for burning the waste, separate from the small blowers built into the support burners. On a two-chamber biomedical incinerator it feeds limited air to the primary chamber and excess air to the secondary chamber.
How much air does the primary chamber of a biomedical incinerator need?
Less than needed for complete combustion, typically 30 to 80% of stoichiometric, so the waste gasifies rather than burns fast. The rest of the air goes to the secondary chamber, where the gases burn completely at high temperature.
Does the FD fan need to be corrosion-resistant?
No. It handles ambient air and never sees flue gas, so painted mild steel is enough. Corrosion-resistant construction is needed on the ID fan, which follows the wet scrubber and handles saturated gas carrying HCl from plastics.
Information needed to quote an FD fan for biomedical waste 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 split. Design air flows to the primary and secondary chambers.
- Burners. Whether the support burners have integral blowers or take air from the main fan.
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.