How EAF off-Gas waste heat boilers work
An electric arc furnace melts scrap, DRI or hot metal in batches, with oxygen lancing and carbon injection. The primary off-gas is drawn through the fourth hole at 1,200 to 1,600°C with high CO, through a gap where air enters and burns the CO, and into a combustion chamber and drop-out box. A waste heat boiler or evaporative-cooled duct then cools the gas while raising steam, usually into a steam accumulator because heat flow rises and falls with each heat. Below the boiler the gas mixes with canopy and secondary fume, which cools it to about 100 to 130°C, and passes a large bag filter. One or more main ID fans draw the whole system, and the stack follows.
Where the ID fan sits
- EAF fourth hole
- Combustion chamber and drop-out box
- Waste heat boiler
- Mixing with canopy gas
- Bag filter
- Main ID fan
- Stack
The fan usually sits after the bag filter, handling cleaned, mixed gas at 100 to 130°C. Some older systems use a pressure bag house with the fan before the filter, where it sees full dust load. The fan works against the elbow, combustion chamber, boiler, ducts, mixing damper and bag filter, typically 350 to 550 mmWC. It must pull enough at the fourth hole to keep CO burning in the combustion chamber and enough at the canopy to capture charging and tapping fume.
Gas at the ID fan
| Gas temperature, normal | 100 to 130°C after mixing and the bag filter |
| Maximum or upset | Up to 150°C with low canopy dilution or bag filter bypass |
| Dust at the fan | Low after the bag filter; fine zinc and iron oxide fume, high if the fan is ahead of the filter |
| Moisture and dew point | Low; dry gas from scrap melting |
| Load pattern | Cyclic, changing with charging, melting, refining and tapping in each heat |
| Density at the design temperature | About 0.90 kg/m³ at 120°C, 75% 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 EAF off-Gas waste heat boilers
| Blade form | Backward curved or aerofoil, often double inlet for the volume |
| Material of construction | Mild steel; wear liners where the fan precedes the bag filter |
| Drive and arrangement | Direct coupled; one or two large fans in parallel |
| Bearings and cooling | Pedestal bearings with monitoring |
| Flow control | Variable speed drive programmed to melt phase, with inlet damper |
| Also specified | Vibration monitoring, inlet box, outlet silencer for large fans |
What this ID fan has to cope with
Cyclic loading with every heat
Gas flow and temperature rise and fall with charging, melting and tapping. A fan run at full speed throughout wastes a great deal of power, and constant speed changes stress the drive. A variable speed drive with ramp rates matched to the heat cycle handles both.
Boiler fouling and steam accumulator
Dust from the furnace deposits on boiler tubes and raises resistance during a campaign. The fan is selected with margin for fouled tubes. The accumulator smooths steam supply but not gas flow, so the fan still sees the cyclic gas load.
Fan before the bag filter
Where the bag house is the pressure type, the fan sits before the filter and handles raw fume with zinc and iron oxide at several grams per cubic metre. Blades wear and deposits form, so wear liners and periodic balancing are needed. A change to a suction bag house with the fan after the filter is often considered at revamps.
Energy and control
The main ID fan is often the largest single motor in a melt shop after the furnace transformer. Programming the speed to the furnace phase, high during melting and lower during charging and refining, saves a large share of its energy. Sealing leaks in the duct and canopy reduces the volume the fan must move for the same capture.
Questions buyers ask
Where should the main ID fan be placed in an EAF fume system?
Usually after the bag filter, so it handles clean gas and runs much longer between repairs. Older pressure bag houses place the fan before the filter, which exposes it to the full fume load and wears the impeller. When a waste heat boiler is added to an existing shop, the fan position is often reviewed at the same time.
Why does the EAF ID fan need a variable speed drive?
Gas flow and temperature change sharply through each heat, from charging through melting to tapping. A variable speed drive matches suction to the phase of the heat, so the fan runs fast only when the furnace needs it. This saves a large share of the energy and reduces wear on the dampers and on the fan bearings.
Does the waste heat boiler change the ID fan duty?
Yes. The boiler adds resistance, which raises the pressure the fan must develop, and it removes heat, which reduces the dilution air needed to protect the bag filter. The fan is selected for the boiler resistance when fouled and for the mixed gas volume after the canopy extraction joins the primary off-gas.
Information needed to quote an ID fan for EAF off-Gas waste heat boilers
- 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.
- Furnace and heat cycle. Furnace size, tap-to-tap time and the gas flow profile through a heat.
- Fume system. Fourth hole, canopy and other extraction points, and the bag filter type and resistance.
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.