How scrap preheating systems work
Scrap preheating heats the charge before melting to cut electricity use and tap-to-tap time. On electric arc furnaces the hot off-gas is passed through the scrap, in a shaft above the furnace, a horizontal conveyor tunnel or the charging bucket, taking it to about 300 to 600°C. On induction furnace melt shops, which have no hot off-gas of their own, burner-fired preheaters heat scrap in buckets or a chamber with natural gas, LPG or oil. Scrap carries oil, paint, plastics and moisture, which release volatile organics and smoke as it heats. The gas is therefore post-combusted, quenched and cleaned in a bag filter before the ID fan, with attention to dioxin reformation as it cools through about 450 to 250°C.
Where the FD fan sits
- Inlet filter
- FD fan
- Air main
- Preheater burners or post-combustion air nozzles
- Scrap or post-combustion chamber
On a burner-fired preheater the fan sits beside the preheating station and feeds the burners in the bucket cover or chamber, usually with cold air at 300 to 700 mmWC. On an EAF shaft or conveyor system, the post-combustion air fan supplies air to nozzles in the off-gas duct or chamber, plus air for support burners. In both cases the fan handles clean ambient air and never sees the off-gas.
Air at the FD fan
| Air temperature at the fan | Ambient, 30 to 50°C in the melt shop |
| Air at the burner | Ambient on most preheaters |
| Dust | Melt shop dust; inlet filter needed |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Load pattern | Follows charging and melting cycle; organic load peaks early in preheating |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 scrap preheating systems
| Blade form | Backward curved, medium to high pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; shielding from radiant heat near the furnace |
| Flow control | Variable speed drive on burner demand or on post-combustion oxygen |
| Also specified | Heavy duty inlet filter and low air pressure trip |
What this FD fan has to cope with
Air demand peaks with organics
Oily scrap gives off most of its volatiles early in the heat-up. If the air supply is set for the burner alone, the organics leave unburnt. Controlling post-combustion air on measured oxygen or CO, with a fan having margin for the peak, keeps combustion complete.
Melt shop dust
Intake air near an arc or induction furnace carries iron oxide fume, lime dust and graphite that clog filters, settle in the burner and wear the fan. As the filter loads, air delivery falls and combustion of the organics suffers. A well-placed intake and a filter sized for the environment, with a pressure alarm, are needed.
Burner turndown
Light scrap heats quickly and needs less firing; heavy scrap needs full firing for longer, and wet scrap needs extra heat to drive off water first. A speed-controlled fan with good turndown matches all these cases without an unstable flame, and the burner must be chosen for the same turndown range.
Energy and control
The combustion or post-combustion air fan uses modest power compared with the ID fan, but it decides whether the organics burn. Controlling air on oxygen or CO measurement, rather than running a fixed high air rate, avoids heating excess air that the ID fan must then move and cool. Speed control saves power and gives finer control.
ID fan and FD fan in scrap preheating systems
| ID fan | FD fan | |
|---|---|---|
| Position | After the quench and bag filter | Feeding burners or post-combustion nozzles |
| What it handles | Cleaned off-gas from the scrap | Ambient melt shop air |
| Temperature at the fan | 100 to 150°C | Ambient |
| Main design concern | Tar, dioxin control and scrap bed resistance | Air for organic peaks |
| Control | Furnace or preheater pressure | Burner demand or measured oxygen |
See also the Induced Draft (ID) Fan for scrap preheating systems.
Questions buyers ask
Does an EAF scrap preheating system have an FD fan?
Not in the usual sense, because the EAF off-gas provides the heat. It does have a post-combustion air fan that supplies air to burn CO and organics in the off-gas, and often a combustion air fan for support burners. These do the job an FD fan does elsewhere.
What fan does a burner-fired scrap preheater need?
A combustion air fan for the burners, typically backward curved at 300 to 700 mmWC, and an exhaust or ID fan to draw the gas through a post-combustion chamber and filter. The combustion air fan handles ambient air only.
How is post-combustion air controlled?
Usually on measured oxygen or CO after the post-combustion chamber. The fan speed is increased when the organics or CO rise and reduced when they fall, so that combustion stays complete without excessive air.
Information needed to quote an FD fan for scrap preheating systems
- 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.
- Burner or post-combustion duty. Burner rating, or the post-combustion air quantity and nozzle pressure.
- Control signal. Whether air is controlled on burner demand, oxygen or CO.
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.