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 FD fan sits
- Inlet filter
- FD (combustion air) fan
- Burner air valve and volatile air dampers
- Afterburner burner
- Afterburner chamber
The fan sits beside the afterburner and supplies air to the burner and, where fitted, to secondary air ports that provide oxygen for burning the volatiles coming from the kiln. Pressure required is typically 300 to 600 mmWC. Any air supplied here becomes part of the loop gas, so the amount affects the oxygen returned to the kiln. For that reason the fan is sized closely to the real duty, with flow measured on each branch.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Dusty scrap handling areas nearby; inlet should be from clean air |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | Burner fires less as coating load supplies more of the heat |
| 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 aluminium scrap delacquering kilns
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with flow measurement to burner and secondary air |
| Also specified | Inlet filter, inlet silencer and separate flow measurement on each air branch |
What this FD fan has to cope with
Too much air raises loop oxygen
Air supplied to the afterburner beyond what the burner and volatiles need passes into the recirculation loop and raises oxygen at the kiln. Measured flow, controlled by afterburner temperature and loop oxygen, keeps this in check. An oversized fan on a damper makes fine control difficult at low flow.
Wide firing range with coating load
With heavily coated scrap the volatiles supply most of the afterburner heat, and the burner falls to low fire. With clean scrap the burner fires hard. The fan must be stable from low to full flow, which a variable speed drive provides better than throttling.
Dust from scrap handling
Shredding and conveying scrap creates dust and fine aluminium particles around the plant. An unfiltered fan inlet draws these in, and they build up on the impeller and burner. An inlet filter or a duct to a clean area keeps the fan and burner reliable.
Energy and control
The afterburner fuel demand falls as the coating load rises, so the combustion air fan spends much of its time at reduced flow. A variable speed drive, with flow measurement and control on afterburner temperature and loop oxygen, avoids wasted air and keeps the loop stable. Every unit of excess air must also be removed by the ID fan, so precise control saves power at both fans.
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 Induced Draft (ID) Fan for aluminium scrap delacquering kilns.
Questions buyers ask
What does the FD fan supply on a delacquering kiln?
It supplies combustion air to the afterburner burner and, in many designs, controlled air for burning the volatile organics released from the scrap. It handles ambient air. The kiln itself is heated by recirculated hot gas, not by a burner supplied from this fan. Its duty falls as the coating load rises.
Why is combustion air control so important on a delacquering kiln?
Air supplied to the afterburner becomes part of the recirculated loop gas. Too much raises the oxygen returned to the kiln, which can let the aluminium oxidise or burn. Measured, controlled air keeps oxygen low while burning the volatiles completely. An oversized fan on a damper makes this control harder at low fire, so speed control is preferred.
Does the delacquering FD fan handle hot gas?
No. It handles ambient air and pushes it into the afterburner. The hot gas is moved by the recirculation fan and, after cooling and filtration, by the ID fan. The FD fan is a standard mild steel design. Its main requirements are stable flow and good control across a wide firing range.
Information needed to quote an FD fan for aluminium scrap delacquering kilns
- 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.
- Afterburner. Burner firing range and the volatile combustion air required.
- Control. Afterburner temperature and loop oxygen set points.
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.