How slag drying plants work
Steel plants dry slag for sale or reuse: granulated blast furnace slag before grinding or dispatch, and crushed steelmaking slag fines before magnetic separation or as aggregate. Wet granulated slag carries about 8 to 15% moisture; steelmaking slag fines carry less water but contain free lime and metallic iron. Drying is done in rotary or fluid bed dryers heated by a hot air generator, which in a steel plant often burns blast furnace gas, coke oven gas or mixed gas, sometimes with oil or coal. Hot gas at 400 to 700°C enters the dryer and leaves at 90 to 130°C with water vapour and a heavy load of abrasive, glassy dust, which is cleaned in a cyclone and bag filter before the ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner and HAG
- Dilution air ports
- Hot gas duct
- Dryer
The fan sits beside the hot air generator and supplies combustion air to the burner, often with a second branch or separate fan for dilution air. On blast furnace gas firing the air volume per unit of heat differs from natural gas, and the burner is larger. Fan pressure is typically 300 to 700 mmWC. 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 | Slag yard dust; filtered intake needed |
| Pressure at the fan outlet | Typically 300 to 700 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 slag drying plants
| 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 burner firing and dryer inlet temperature |
| Also specified | Inlet filter, low pressure trip and flame safeguard interlock |
What this FD fan has to cope with
Slag yard dust at the intake
Slag handling areas are among the dustiest in a steel works, and the dust is abrasive and alkaline. An unfiltered intake wears the fan impeller, coats the burner air register and upsets the air ratio. A filter sized for the dust load and an intake placed away from slag handling, crushing and loading points protect it.
Fuel gas swings
Blast furnace gas calorific value and pressure vary. If combustion air is not adjusted, the flame becomes unstable or the gas burns incompletely. Ratio control with an oxygen trim, and a fan with margin, keep combustion stable.
Dilution air balance
Too much dilution air lowers the hot gas temperature, increases the volume the ID fan must handle and carries more heat out of the stack; too little overheats the dryer shell and the feed end seals. A separate controlled dilution fan, or a well-designed damper on its own temperature loop, keeps the balance as firing changes.
Energy and control
Hot air generators on slag dryers run with varying feed moisture and output, since granulated slag drains differently in wet and dry seasons, so a fixed fan speed wastes power at lower duty. Speed control on firing demand saves power, and controlling excess air and dilution air reduces the exhaust volume and the ID fan power that follows.
ID fan and FD fan in slag drying plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | Feeding the HAG burner |
| What it handles | Moist exhaust with slag dust | Ambient air |
| Temperature at the fan | 90 to 130°C | Ambient |
| Main design concern | Abrasion and condensation | Fuel gas variability and turndown |
| Control | Dryer suction | Burner firing and dryer inlet temperature |
See also the Induced Draft (ID) Fan for slag drying plants.
Questions buyers ask
What is the FD fan on a slag dryer?
It is the combustion air fan for the hot air generator burner. It handles ambient air and supplies the burner with air for combustion, sometimes with dilution air for temperature control. The hot gas itself is drawn through the dryer by the ID fan.
Can a slag dryer burn blast furnace gas?
Yes. Many steel plant slag dryers use blast furnace gas, coke oven gas or mixed gas, which makes use of a by-product fuel. The burner and combustion air fan must be designed for the low calorific value and the larger gas volumes involved, and pilot or support firing is often added for flame stability.
What pressure does the slag dryer combustion air fan need?
Typically 300 to 700 mmWC, depending on the burner design and the hot air generator. The burner supplier's air pressure requirement at full firing sets the fan pressure, with a margin added for the inlet filter as it loads and for any dilution air taken from the same fan.
Information needed to quote an FD fan for slag drying plants
- 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.
- Fuel. Fuel or gas blend, calorific value and burner rating.
- Dilution. Dilution air quantity and dryer inlet temperature.
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.