How aluminium scrap delacquering kilns work
A delacquering kiln removes paint, lacquer, oil and plastic from shredded used beverage cans and coated scrap before melting. The shredded scrap passes through a rotary kiln, or a belt or fluidised system, where hot gas at about 480 to 550°C with low oxygen, typically 6 to 8%, volatilises the coatings without letting the aluminium burn. The organic-laden gas goes to an afterburner, or thermal oxidiser, at 750 to 850°C, where the volatiles are burned and often supply much of the heat. Part of the afterburner gas is tempered and recirculated to the kiln by a hot recirculation fan. The surplus gas passes a heat exchanger or waste heat boiler, lime injection for HCl, a bag filter and the ID fan.
Where the ID fan sits
- Delacquering kiln
- Afterburner
- Recirculation take-off
- Heat exchanger or waste heat boiler
- Lime injection
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter and removes only the surplus gas from the system: the combustion products and volatiles burned in the afterburner, plus air leakage. The much larger recirculating stream is handled by a separate hot recirculation fan. Suction at the ID fan is typically 250 to 400 mmWC, and its speed sets the pressure at the kiln seals and the oxygen level in the loop.
Gas at the ID fan
| Gas temperature, normal | 150 to 200°C after heat recovery and the bag filter |
| Maximum or upset | Up to 250°C when coating load is high or heat recovery is bypassed |
| Dust at the fan | Traces of lime, salts and fine pigment ash after the filter |
| Moisture and dew point | Combustion moisture from burned coatings and fuel; HCl raises the acid dew point |
| Corrosive species | HCl from PVC and chlorinated coatings; SO₂ in small amounts |
| Organics | Should be destroyed in the afterburner; unburnt organics if it is under temperature |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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 aluminium scrap delacquering kilns
| Blade form | Backward curved on the clean side of the bag filter |
| Material of construction | Mild steel with insulation; SS 316 at cold spots where HCl condensation is possible |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with shaft cooling disc |
| Flow control | Variable speed drive on kiln or loop pressure |
| Also specified | Casing drain, insulation, inspection door and interlock with afterburner temperature |
What this ID fan has to cope with
Loop pressure and oxygen control
If the ID fan pulls too hard, the kiln seals draw in air, oxygen in the loop rises and the aluminium can start to burn, with a risk of kiln fire. If it pulls too little, the kiln goes positive and smoky gas escapes. Fine speed control on loop pressure, tied to oxygen measurement, keeps the kiln safe.
Varying coating load
Different scrap carries different amounts of paint and plastic. A heavily coated batch releases a surge of volatiles that raises afterburner temperature and surplus gas volume. The fan must have capacity for that surge without losing pressure control, and the afterburner temperature limits set the operating window.
Tar deposits from an under-temperature afterburner
If the afterburner falls below its operating temperature, unburnt organics reach the heat exchanger, filter and fan, where they condense as tar. Tar binds lime and dust into a sticky layer on the blades. An interlock that stops scrap feed when the afterburner is cold prevents this, and the fan has an access door for cleaning.
Energy and control
A delacquering kiln burns the coatings it removes, and in steady operation the afterburner needs little extra fuel. The ID fan removes only surplus gas, so excess air ingress directly raises fan power and fuel use. Tight kiln seals and a variable speed drive on loop pressure keep the surplus volume, and the fan power, at the minimum the process allows.
ID fan and FD fan in aluminium scrap delacquering kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Beside the afterburner |
| What it handles | Surplus afterburner gas with HCl traces | Ambient air for the afterburner |
| Temperature at the fan | 150 to 200°C | Ambient |
| Main design concern | Loop pressure and oxygen control | Controlled air into the loop |
| Control | Kiln or loop pressure | Afterburner temperature and oxygen |
See also the Forced Draft (FD) Fan for aluminium scrap delacquering kilns.
Questions buyers ask
What is the difference between the recirculation fan and the ID fan on a delacquering kiln?
The recirculation fan moves the large hot gas stream, at around 500 to 600°C, from the afterburner back to the kiln. The ID fan removes only the surplus gas after heat recovery and the bag filter, at 150 to 200°C. They differ greatly in temperature, volume and construction.
Why must oxygen be kept low in a delacquering kiln?
At the kiln temperature, thin aluminium scrap can oxidise or ignite if there is too much oxygen. Keeping oxygen at about 6 to 8% lets the coatings volatilise without burning the metal. The ID fan speed controls air leakage into the loop and so helps hold oxygen low.
Why does the delacquering ID fan get tar on it?
Tar comes from organic coatings that were not fully burned in the afterburner. When the afterburner runs below its set temperature, the volatiles pass through and condense on cooler surfaces such as the filter and fan. Keeping the afterburner at temperature, with a feed interlock, prevents this.
Information needed to quote an ID fan for aluminium scrap delacquering kilns
- 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.
- Scrap and coating. Scrap type, throughput and typical coating content by weight.
- Gas system. Afterburner temperature, recirculation ratio and heat recovery arrangement.
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.