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 ID fan sits
- Furnace or burner
- Scrap preheating shaft or bucket
- Post-combustion chamber
- Quench tower
- Bag filter
- ID fan
- Stack
The fan is on the clean side of the bag filter, at 100 to 150°C. Upstream, the gas leaves the scrap at 200 to 600°C, passes a post-combustion chamber at 850°C or more to destroy organics, and is rapidly quenched, often with water spray, to below about 200°C. The fan provides suction for the scrap bed, the post-combustion and quench and the filter, and on EAF systems it also controls furnace pressure.
Gas at the ID fan
| Gas temperature, normal | 100 to 150°C after quench and bag filter |
| Maximum or upset | 200 to 250°C if quench spray fails |
| Dust at the fan | Low after the bag filter; zinc and iron oxides, carbon upstream |
| Condensables | Oil smoke and tars if post-combustion is poor |
| Moisture and dew point | High after water quench; dew point close to the filter temperature at cold starts |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 scrap preheating systems
| Blade form | Backward curved after the bag filter |
| Material of construction | Mild steel; wear-resistant leading edges for bag failure events |
| Drive and arrangement | Direct drive, arrangement 8 |
| Bearings and cooling | Pedestal bearings, no cooling needed |
| Flow control | Variable speed drive on furnace or preheater pressure |
| Also specified | Casing drain, cleaning doors, and vibration monitoring |
What this ID fan has to cope with
Tar and oil deposits
When oily scrap is heated without effective post-combustion, the gas carries tar that condenses in ducts, on filter bags and on the fan. Bags blind and the impeller builds sticky deposits. A properly sized post-combustion chamber with burner support is the main defence; the fan needs access for cleaning.
Dioxin reformation
Dioxins and furans can reform as the gas cools through about 450 to 250°C in the presence of dust and chlorine. Rapid quench through that range, and sometimes activated carbon injection before the filter, are standard. The fan must be sized for the extra volume of quench steam.
Changing resistance of the scrap bed
Light shredded scrap and heavy melting scrap give very different bed resistance, and the resistance changes again as the scrap settles while it heats. The fan must hold suction across both, otherwise gas bypasses the scrap or escapes from the preheater seals. Speed control with pressure margin covers the range.
Energy and control
Scrap preheating can cut melting power significantly, so the system pays for its fan power many times over when it runs well. The ID fan follows the melting cycle: high flow during melting and preheating, lower when charging or tapping. A variable speed drive tied to furnace or preheater pressure avoids running a large fan flat out during the off-gas lulls.
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 Forced Draft (FD) Fan for scrap preheating systems.
Questions buyers ask
Why does scrap preheating need post-combustion?
Scrap carries oil, paint and plastics that vaporise when heated. Without burning these in a post-combustion chamber, the gas carries smoke, tar and organic compounds that foul the bag filter and fan and breach emission limits. Post-combustion also burns CO from an EAF off-gas.
What temperature does the scrap preheater ID fan see?
After quench and bag filter, usually 100 to 150°C. The scrap preheating itself can reach 300 to 600°C, and the post-combustion chamber 850°C or more, but the fan is downstream of the cooling and filter. It should still be rated for 200 to 250°C to cover quench spray failure.
Can an induction furnace shop preheat scrap?
Yes, with burner-fired preheaters in buckets or a chamber, since induction furnaces produce no hot off-gas. These need a combustion air fan, an exhaust fan and usually a filter, because oily scrap gives smoke when heated.
Information needed to quote an ID fan for scrap preheating systems
- 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.
- Preheating method. EAF off-gas or burner-fired, and the preheater type: shaft, conveyor or bucket.
- Scrap quality. Typical scrap mix, oil and plastics content, and moisture.
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.