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 FD fan sits
- Inlet filter
- FD fan
- HAG burner
- Combustion chamber
- Dilution air
- Rotary dryer
The fan sits beside the hot air generator at the feed end of the dryer. It supplies combustion air to an oil, gas or coal burner, and often a separate stream of dilution air that mixes with the flame gas to bring it down to 400 to 600°C before the dryer. Fan pressure is typically 200 to 600 mmWC, covering the burner, the combustion chamber and the dryer inlet. The fan handles ambient air only.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air at the burner | Ambient |
| Dust | Scale yard dust; filtered intake recommended |
| Pressure at the fan outlet | Typically 200 to 600 mmWC |
| Turndown | Follows feed rate and moisture |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 mill scale and fines dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on firing rate and dryer outlet temperature |
| Also specified | Inlet filter, flame safeguard and low air pressure trip |
What this FD fan has to cope with
Overheating oily feed
If firing does not drop when feed falls, the dryer overheats, the oil on the scale smokes heavily and the dust in the filter can ignite. Air and fuel must follow dryer outlet temperature closely, with a high temperature cut-out. The fan therefore needs a drive that turns down smoothly to low firing without an unstable flame.
Dust at the intake
Mill scale yards and loading points are dusty, and the dust is heavy, magnetic and abrasive. Drawn into the fan, it wears the impeller and settles in the burner air register, upsetting the air ratio. An intake filter and an intake placed away from scale handling and screening protect the fan and burner.
Dilution balance
Excess dilution air increases the exhaust volume, the ID fan load and the heat lost up the stack; too little overheats the dryer shell and the lifters near the inlet. A controlled dilution damper, or a separate dilution fan on its own loop, keeps the dryer inlet temperature steady as firing changes.
Energy and control
Firing follows the evaporation load, which changes with how well the scale has drained and with throughput. A speed-controlled combustion air fan saves power at the low firing rates common in this duty and gives better ratio control than a damper. Good ratio and dilution control also reduce the exhaust volume and the ID fan power that follows it.
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 Induced Draft (ID) Fan for mill scale and fines dryers.
Questions buyers ask
What is the FD fan on a mill scale dryer?
It is the forced draft or combustion air fan of the hot air generator that heats the dryer. It handles ambient air and supplies the burner, and sometimes dilution air for temperature control. The hot gas and the moisture driven off the mill scale are drawn through the dryer, cyclone and filter by the ID fan, which is the larger and harder duty.
What temperature should mill scale be dried at?
The hot gas entering the dryer is usually 400 to 600°C, and the exhaust leaves at 100 to 150°C. Running hotter dries faster but makes the oil smoke and raises the fire risk in the filter, so firing is controlled on dryer outlet temperature and the inlet temperature is limited by dilution air.
What fuel is used for mill scale dryers?
Furnace oil, natural gas, LPG, coal or steel works gas, depending on what the plant has available. Gas firing gives the easiest temperature control on oily feed. The combustion air fan is sized for the chosen burner and fuel, since air demand and pressure differ between an oil burner and a low calorific value gas burner.
Information needed to quote an FD fan for mill scale and fines dryers
- 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. Fuel, burner rating and air pressure.
- Temperature control. Dryer inlet and outlet temperatures and how firing is controlled.
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.