How aluminium billet homogenising furnaces work
Cast aluminium billets are homogenised before extrusion by soaking them at about 550 to 600°C for several hours, then cooling them at a controlled rate. Batch furnaces take a load of billets on a car or in baskets; continuous furnaces move billets through heating, soaking and cooling sections. Heat is supplied by gas burners firing into a recirculating air stream, by indirect radiant tubes or by electric heaters, and large high temperature circulation fans drive the hot air across the billets. Products of combustion and excess air are vented through an exhaust fan or stack. The cooling chamber then uses large air fans, sometimes with water mist, to cool the billets uniformly.
Where the FD fan sits
- Inlet filter
- FD (combustion air) fan
- Air header
- Recuperator or recuperative burner
- Burners
- Heating chamber
The fan sits outside the furnace casing and supplies an air header that feeds each burner through a ratio valve. Recuperative burners, or a central recuperator, preheat the air with exhaust heat. Pressure required covers the header, ratio valves, recuperator and burners, typically 300 to 700 mmWC. The fan handles only ambient air, as the preheating takes place downstream, so it is a compact mild steel fan even though the burners fire with hot air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 250 to 450°C where fitted, downstream of the fan |
| Dust | Clean air |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Turndown | Full firing during heat-up; low firing during soak |
| 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 billet homogenising furnaces
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive holding header pressure |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Wide range between heat-up and soak
Heat-up of a cold billet load needs full firing, while soak needs only a small fraction. A fan with a damper sized for heat-up runs heavily throttled during the long soak. Speed control holds header pressure efficiently across the whole range. It also reduces noise during soak.
Ratio drift at low fire
At low fire, a small change in air flow makes a large change in the air-fuel ratio. Excess air wastes fuel and cools the furnace atmosphere. A fan stable at low flow, with good ratio control, avoids this drift. Oxygen measurement in the furnace gives a direct check of the ratio.
Recuperator pressure drop
Recuperative burners add pressure drop on the air side, and fouling raises it over time. The fan must have margin for the fouled recuperator, or full firing during heat-up cannot be reached. Measuring the pressure drop at each annual shutdown shows when cleaning is due and confirms that the fan still has enough reserve for heat-up.
Energy and control
Homogenising cycles spend hours at soak, where firing is low. A variable speed drive on the combustion air fan saves power for most of the cycle. Fuel saving through recuperative burners, correct air-fuel ratio and minimum exhaust venting are much larger than fan power savings, and the fan should support those measures.
ID fan and FD fan in aluminium billet homogenising furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | On the furnace exhaust outlet, before the stack | Outside the casing, feeding the burner header |
| What it handles | Hot furnace atmosphere and combustion products | Ambient air |
| Temperature at the fan | 400 to 550°C | Ambient |
| Main design concern | Heat-resisting construction and shaft bow | Turndown between heat-up and soak |
| Control | Furnace pressure | Header pressure and burner ratio |
See also the Induced Draft (ID) Fan for aluminium billet homogenising furnaces.
Questions buyers ask
Is the FD fan on a homogenising furnace a combustion air fan?
Yes. It supplies combustion air to the burners that heat the furnace, either directly into the recirculating air or into radiant tubes. It handles ambient air and is separate from the recirculation fans and the cooling chamber fans. It is sized for full firing during heat-up of a cold load.
How much turndown does a homogenising furnace burner air fan need?
Typically from full firing during heat-up down to a small fraction during soak. A variable speed drive with ratio valves covers this range better than a damper, and keeps the air-fuel ratio stable at low fire. The exact range is set by the load size and the heat-up time the plant requires.
Does the combustion air fan need to be high temperature?
No. It pushes ambient air into the recuperator or recuperative burner. The air is heated downstream, so the fan is a standard mild steel design. Only the exhaust and recirculation fans need high temperature construction. This keeps it compact and inexpensive to maintain. Bearings and motor are standard.
Information needed to quote an FD fan for aluminium billet homogenising furnaces
- 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.
- Burners. Number and type of burners, firing range and required air pressure.
- Recuperation. Recuperator or recuperative burner air side pressure drop.
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.