How mill scale and fines dryers work
Rolling mills, forging shops and continuous casters generate mill scale, a dense iron oxide flake with typically 0.5 to 3% oil and grease from the mill and 5 to 15% water from the scale pits. Steel plants also collect flue dust, sludge and ore fines. Before briquetting, sintering or sale, this material is dried in a rotary dryer heated by an oil, gas or coal fired hot air generator, usually to below 1 to 2% moisture. The dryer exhaust leaves at about 100 to 150°C, carrying water vapour, heavy iron oxide dust and oil vapour or smoke. It is cleaned in a cyclone and a bag filter, or a cyclone and wet scrubber, before the ID fan and stack.
Where the ID fan sits
- Hot air generator
- Rotary dryer
- Cyclone
- Bag filter or wet scrubber
- ID fan
- Stack
The fan follows the final collector. After a bag filter it handles warm, dry gas with little dust; after a wet scrubber it handles saturated gas with oily sludge. The fan provides the suction for the dryer, cyclone and collector, typically 250 to 500 mmWC, and holds the dryer feed end at slight suction to keep dust from escaping at the breeching and seals.
Gas at the ID fan
| Gas temperature, normal | 100 to 150°C after a bag filter; 60 to 75°C after a wet scrubber |
| Maximum or upset | 200 to 250°C if feed stops while the HAG fires |
| Dust at the fan | Low after a bag filter; heavy iron oxide fines if the filter is bypassed |
| Condensables | Oil vapour and smoke from the scale |
| Moisture and dew point | High water vapour; dew point typically 50 to 60°C |
| 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 mill scale and fines dryers
| Blade form | Radial tipped or backward curved with wear liners, shaped to shed oily deposits |
| Material of construction | Mild steel with wear liners; stainless steel or coating for wet scrubber duty |
| Drive and arrangement | Belt or direct drive, arrangement 8 or 9 |
| Bearings and cooling | Pedestal bearings outside the gas stream |
| Flow control | Variable speed drive on dryer suction |
| Also specified | Cleaning doors, casing drain, and spark detection upstream of the filter |
What this ID fan has to cope with
Oily deposits on the impeller
Oil vapour condenses where the gas cools and forms a sticky film that catches iron oxide dust. The deposit grows unevenly and unbalances the fan. Keeping the gas above the oil condensation range, using a blade form that sheds deposits and regular cleaning are the design response.
Bag filter fire
Oil-soaked dust on filter bags can ignite from a hot particle or overheating. A fire damages the bags and can send burning material to the fan. Spark arrestors, temperature monitoring, and a fan that can be isolated quickly are needed.
Abrasive iron oxide
Mill scale fines are dense, hard iron oxide, several times heavier than most process dusts, so they strike the blades with high energy. A torn bag or a blocked cyclone sends them straight to the fan, and the leading edges and scroll cut-off wear through in weeks. Bolted wear liners, a moderate tip speed and broken bag detection on the filter limit the damage.
Energy and control
Mill scale dryers often run in campaigns as scale accumulates, at varying feed rate and moisture, and may stand idle for days between them. A speed-controlled ID fan follows the load and saves power. Draining the scale well before drying reduces the water to evaporate, and so the gas volume and fan power, more than any fan change.
ID fan and FD fan in mill scale and fines dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or scrubber | Feeding the HAG burner |
| What it handles | Moist exhaust with oil vapour and iron oxide dust | Ambient air |
| Temperature at the fan | 100 to 150°C dry, 60 to 75°C wet | Ambient |
| Main design concern | Oily deposits, fire and abrasion | Close firing control for oily feed |
| Control | Dryer suction | Dryer outlet temperature |
See also the Forced Draft (FD) Fan for mill scale and fines dryers.
Questions buyers ask
Why does mill scale need drying before briquetting?
Wet mill scale is difficult to handle, mix with binder and briquette, and the water adds freight weight and consumes heat in the sinter plant. Drying to below about 1 to 2% moisture gives a free-flowing material that briquettes properly and can be sent to the sinter plant, a melt shop or sold.
What makes a mill scale dryer ID fan different from other dryer fans?
The oil in mill scale vaporises in the dryer and condenses downstream, coating the fan and filter with a sticky film that traps heavy iron oxide dust. A mill scale dryer ID fan therefore has to handle deposits, fire risk and abrasion together, where most dryer fans face only one of them.
Can mill scale dryer exhaust go through a wet scrubber?
Yes, many smaller plants use a cyclone and wet scrubber, which also removes the fire risk of oily dust on filter bags. The ID fan then handles saturated gas at 60 to 75°C with oily sludge, so it needs corrosion protection, casing drains, wash nozzles and a blade form that resists deposits.
Information needed to quote an ID fan for mill scale and fines dryers
- 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.
- Feed. Material mix, oil content and moisture of the mill scale and fines.
- Collector. Bag filter or wet scrubber, with spark and fire protection arrangements.
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.