How oil fired re-rolling mill furnaces work
Secondary steel re-rolling mills reheat billets, ingots or cut pieces, typically 10 to 40 tonnes per hour, in small pusher or batch furnaces fired with furnace oil, LSHS or light diesel oil. The oil is heated to reduce viscosity and burned in low pressure air atomising burners, where the combustion air from the blower also breaks up the oil, or in pressure jet burners. Flue gas leaves at the charging end at 700 to 900°C, often passing a simple metallic recuperator, and goes to a chimney. Many such furnaces run on natural draft alone; others need an ID fan once a recuperator, a wet scrubber or a cyclone for particulate control is added. Furnace oil can carry 2 to 4% sulphur, so the flue gas is acidic.
Where the ID fan sits
- Furnace
- Charging end flue
- Recuperator
- Cyclone or scrubber (if fitted)
- ID fan
- Chimney
On a furnace with only a recuperator, the fan sits after it and sees about 300 to 400°C gas. Where a wet scrubber has been added to meet particulate limits, the fan goes after the scrubber and handles saturated gas at 60 to 80°C, which changes its construction entirely. The suction is modest, typically 50 to 150 mmWC without a scrubber and 150 to 300 mmWC with one.
Gas at the ID fan
| Gas temperature, normal | 300 to 400°C after the recuperator; 60 to 80°C after a wet scrubber |
| Maximum or upset | Up to 600°C if the recuperator is bypassed |
| Dust at the fan | Soot and oil ash, heavier with poor atomisation |
| Corrosive species | SO₂ and SO₃ from furnace oil sulphur, acid dew point often 130 to 160°C |
| Moisture | Saturated with droplets after a scrubber |
| Density at the design temperature | About 0.57 kg/m³ at 350°C, 47% 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 oil fired re-rolling mill furnaces
| Blade form | Backward inclined or radial tipped, simple plate blades that tolerate soot |
| Material of construction | Boiler quality plate for dry hot gas; SS 316 or rubber-lined steel after a wet scrubber |
| Drive and arrangement | Belt driven on small furnaces, direct coupled on larger units |
| Bearings and cooling | Cooling disc and heat shield on dry hot duty |
| Flow control | Variable speed drive or outlet damper on furnace pressure |
| Also specified | Cleaning door, casing drain and insulation against acid condensation on dry duty |
What this ID fan has to cope with
Acid corrosion from furnace oil sulphur
Furnace oil sulphur forms SO₃ that condenses as sulphuric acid below about 130 to 160°C. Fans and ducts in re-rolling mills often run cold during long holds or overnight banking, and corrosion attacks the casing and the impeller root welds. Insulation, keeping the gas above the dew point and, after a scrubber, stainless or lined construction prevent it.
Soot from poor atomisation
Low oil temperature, worn burner tips or low atomising air pressure leave unburnt carbon in the flue gas, which builds up on the impeller and in the recuperator. The fan goes out of balance and the recuperator chokes. Correct oil heating, burner maintenance and steady air pressure reduce the soot at its source.
Scrubber retrofit with the old dry fan
When a wet scrubber is added for pollution control, the old dry fan is sometimes reused after it. It then handles wet, acidic gas in mild steel and corrodes within months. A fan after a scrubber should be in stainless or rubber-lined steel, with drains and a mist eliminator upstream.
Energy and control
Re-rolling mills often run one or two shifts, with long holds and frequent stoppages for roll changes. A variable speed drive on the ID fan saves power during holds and gives tighter furnace pressure control, which reduces cold air ingress, fuel use and scale loss. On small furnaces even a two-speed motor brings much of the benefit.
ID fan and FD fan in oil fired re-rolling mill furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator or scrubber, before the chimney | Beside the furnace, feeding the burners |
| What it handles | Sulphur-bearing flue gas with soot | Ambient air |
| Temperature at the fan | 300 to 400°C dry, 60 to 80°C after a scrubber | Ambient |
| Main design concern | Acid corrosion and soot | Stable pressure for atomisation |
| Control | Furnace pressure | Air header pressure |
See also the Forced Draft (FD) Fan for oil fired re-rolling mill furnaces.
Questions buyers ask
Does an oil fired re-rolling furnace need an ID fan?
Not always. Many oil fired re-rolling mill furnaces run on natural draft with a chimney. An induced draft fan is needed when a recuperator, a cyclone or a wet scrubber adds resistance the chimney cannot overcome, or when tighter furnace pressure control is required to save fuel.
Why does our re-rolling furnace ID fan corrode so quickly?
Furnace oil sulphur forms acid that condenses when the fan or ducts run below the acid dew point, often 130 to 160°C. Long holds, overnight banking and wet scrubbers all bring the gas below that point. Insulation, keeping the fan warm, or stainless construction after a scrubber are the remedies.
What material should an ID fan after a wet scrubber be?
After a wet scrubber the gas is saturated and acidic, so the fan should be SS 316 or rubber-lined steel, with a casing drain and a mist eliminator upstream. A mild steel fan designed for dry hot gas will corrode rapidly in this service. Mild steel also fails quickly at the casing drains.
Information needed to quote an ID fan for oil fired re-rolling mill furnaces
- 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.
- Fuel oil. Type of oil fired, its sulphur content and the hourly consumption.
- Pollution control. Cyclone, wet scrubber or bag filter fitted or planned, and its 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.