How animal carcass incinerators work
Animal carcass incinerators dispose of dead livestock, slaughterhouse condemned material and veterinary waste. Most are batch units: a large primary chamber with a hearth on which whole or cut carcasses are loaded, fired by diesel or gas burners at about 800 to 850°C, and a secondary chamber or afterburner at about 1,000 to 1,100°C. Carcasses are about two-thirds water and contain fat, so burn time is long and fat release causes sudden flare-ups. Smaller farm units rely on natural draft. Larger units add tempering air or a heat exchanger to cool the gas, then a cyclone or scrubber, and an ID fan to the stack. A combustion air blower supplies the chambers.
Where the ID fan sits
- Primary chamber
- Afterburner
- Tempering air inlet
- Cyclone
- ID fan
- Stack
The fan sits after the tempering air inlet and any cyclone or scrubber, close to the stack. Tempering air drawn in through a controlled damper cools the gas from above 1,000°C to typically 250 to 350°C, which allows a steel fan. Suction at the fan is modest, typically 80 to 200 mmWC, without a wet scrubber. Natural draft units without a fan cannot hold suction during fat flare-ups, which is why larger units use an ID fan.
Gas at the ID fan
| Gas temperature, normal | 250 to 350°C after tempering air |
| Maximum or upset | Up to 450°C during a fat flare-up before tempering air responds |
| Dust at the fan | Light bone ash and soot, a few hundred mg/Nm³ after a cyclone |
| Moisture and dew point | High water vapour from carcass moisture; dew point well below the gas temperature |
| Odour and organics | Unburnt fat vapour and odour if the afterburner is under temperature |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 animal carcass incinerators
| Blade form | Radial or backward curved, robust against soot build-up |
| Material of construction | Heat-resisting steel impeller for upset temperatures; mild steel casing with insulation |
| Drive and arrangement | Belt drive or direct coupled, motor kept away from the hot casing |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive or damper on primary chamber pressure; tempering damper on fan inlet temperature |
| Also specified | Temperature trip, cleaning door, and casing drain |
What this ID fan has to cope with
Fat flare-ups
As carcasses heat, rendered fat runs onto the hearth and burns suddenly, raising gas flow and temperature. The ID fan needs pressure margin to keep the primary chamber negative, and the tempering damper must react quickly to protect the fan from the temperature spike.
Soot and grease deposits
If the afterburner runs cool, unburnt fat condenses with soot on the impeller and casing, causing unbalance and a fire risk. Keeping the afterburner at temperature, and regular cleaning through an access door, prevent this.
Too much tempering air
Cooling the gas with air increases the gas volume, so the fan must be larger. A heat exchanger or water spray reduces the volume, but at higher cost. The choice affects fan size, power and capital cost and should be made at the design stage.
Energy and control
Carcass incinerators run in batches with a long burnout. The ID fan can slow down once the main burn is over and fat release ends. A variable speed drive on chamber pressure saves power through the burnout and cool-down stages. Reducing tempering air by controlling its damper on fan inlet temperature, rather than leaving it fixed open, also cuts the volume the fan must move.
ID fan and FD fan in animal carcass incinerators
| ID fan | FD fan | |
|---|---|---|
| Position | After tempering and cyclone, at the stack | Beside the incinerator, feeding chamber ports |
| What it handles | Tempered hot gas with ash and soot | Ambient combustion air |
| Temperature at the fan | 250 to 350°C, up to 450°C upset | Ambient, 25 to 45°C |
| Main design concern | Heat resistance and fat flare-ups | Air matched to the burn phase |
| Control | Primary chamber pressure | Chamber temperatures |
See also the Forced Draft (FD) Fan for animal carcass incinerators.
Questions buyers ask
Does an animal carcass incinerator need an ID fan?
Small farm units often run on natural draft. Larger units, or any unit with a cyclone, scrubber or heat exchanger, need an ID fan to hold the primary chamber negative, particularly during fat flare-ups, and to overcome the extra resistance.
How is the gas cooled before the ID fan on a carcass incinerator?
Most commonly by tempering air drawn in through a controlled damper, which cools the gas to about 250 to 350°C. Some units use a heat exchanger or a water spray quench instead, which reduces gas volume but adds cost.
What temperature can an incinerator ID fan handle?
Mild steel impellers are limited to about 350°C. Above that, heat-resisting steel impellers are used. Carcass incinerator fans are usually specified for upset temperatures of 400 to 450°C to cover fat flare-ups.
Information needed to quote an ID fan for animal carcass incinerators
- 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.
- Batch size. Carcass load per batch, burn cycle time and chamber volume.
- Gas cooling. Tempering air, heat exchanger or quench, and target fan inlet temperature.
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.