How ladle preheating stations work
Steel ladles are dried and preheated before tapping so that the refractory lining does not chill the steel or crack from thermal shock. A ladle preheating station has a burner mounted in a swinging or lifting lid, horizontal or vertical, fired with natural gas, LPG, coke oven gas or oil. New linings follow a slow drying curve over many hours; ladles in circulation are reheated to about 1,000 to 1,200°C on the hot face between heats. On a simple station the flue gas escapes through the gap between lid and ladle rim. More efficient stations use recuperative burners, or regenerative burner pairs, that draw the flue gas back through the lid to preheat the combustion air, and these need an exhaust (ID) fan. Oxy-fuel preheaters need no air fans.
Where the FD fan sits
- Inlet filter
- FD fan
- Reversing valve
- Regenerator bed or recuperator
- Lid burner
- Ladle
The blower is mounted on or near the preheater structure and connected to the lid by a flexible hose or a duct through the lid pivot. On regenerative heaters it pushes air through the hot regenerator bed, which adds pressure drop and heats the air to 700 to 900°C before the burner. Typical blower pressure is 400 to 900 mmWC, higher on regenerative designs.
Air at the FD fan
| Air temperature at the fan | Ambient, 30 to 50°C in the melt shop |
| Air at the burner | Ambient on cold air units, 700 to 900°C after a regenerator |
| Dust | Melt shop dust and kish graphite; filter needed |
| Pressure at the fan outlet | Typically 400 to 900 mmWC |
| Turndown | Wide, from slow dry-out firing to full reheating rate |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 ladle preheating stations
| Blade form | Backward curved, high pressure design |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, compact frame for the preheater structure |
| Bearings and cooling | Standard bearings; heat shielding from radiant heat off nearby ladles |
| Flow control | Variable speed drive on burner air demand, with ratio control |
| Also specified | Heavy duty inlet filter and protection against radiant heat and metal splash |
What this FD fan has to cope with
Melt shop dust
Air near the converter or arc furnace carries iron oxide fume, lime dust and graphite flakes. These load the inlet filter and, on regenerative units, the regenerator beds. A fan with pressure margin and a filter sized for the environment keeps the burner air steady.
Turndown for dry-out
Drying a new lining may call for a small fraction of the full air flow for many hours. A fan without speed control throttled to that level runs unstably and the burner flame becomes unreliable. A drive with a wide speed range and a burner able to turn down are both needed.
Radiant heat on the fan
Preheating stations stand close to hot ladles, and the blower motor and bearings can receive a lot of radiant heat, along with occasional metal and slag splash when a ladle is moved. Motors overheat and grease breaks down. Heat shields, placing the blower behind the station structure and a motor rated for the hot ambient protect them.
Energy and control
Ladle preheaters often run around the clock and are notorious for heating ladles that then wait. A blower on speed control that follows firing demand saves power, but the larger saving comes from not overheating idle ladles and from using regenerative burners, which need a correctly sized combustion air fan to push air through the beds.
ID fan and FD fan in ladle preheating stations
| ID fan | FD fan | |
|---|---|---|
| Position | After the regenerator or recuperator | Feeding air to the lid burner |
| What it handles | Flue gas drawn back from the ladle | Ambient melt shop air |
| Temperature at the fan | 150 to 250°C, spikes on reversal faults | Ambient |
| Main design concern | Upset temperature and exhaust balance | Wide turndown and dusty intake |
| Need | Only on heat recovery heaters | On every air-fuel preheater |
See also the Induced Draft (ID) Fan for ladle preheating stations.
Questions buyers ask
What is the combustion air blower on a ladle preheater?
It is the forced draft fan that supplies air to the burner in the preheater lid. On a cold air unit it feeds the burner directly; on a regenerative unit it pushes air through the hot regenerator so that the burner gets preheated air.
What pressure does a ladle burner blower need?
Cold air ladle burners usually need 300 to 600 mmWC including hose and valve losses. Regenerative units need more to overcome the regenerator bed and reversing valve, often 600 to 900 mmWC. The burner supplier's data decides the exact figure.
Can one blower serve several ladle preheaters?
It can, but stations operate independently, one drying a new lining at low fire while another reheats at full rate, and a shared blower makes pressure and ratio control harder. A blower per station, or per pair of stations with a common header and pressure control, is the usual arrangement in melt shops.
Information needed to quote an FD fan for ladle preheating stations
- 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 rating. Maximum and minimum firing rate and the burner air pressure.
- Regenerator. Air side pressure drop of the regenerator beds and reversing valve, if fitted.
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.