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 ID fan sits
- Ladle interior
- Lid exhaust port
- Regenerator bed or recuperator
- Reversing valve
- Exhaust (ID) fan
- Stack
On a regenerative ladle heater the fan is downstream of the reversing valves, drawing gas through whichever regenerator bed is in exhaust mode. The bed cools the gas to about 150 to 250°C before the fan. On a recuperative station the fan follows the lid recuperator at 300 to 450°C. The fan typically draws back 70 to 90% of the products of combustion; the rest leaves round the lid and keeps cold air out of the ladle.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C after a regenerator; 300 to 450°C after a recuperator |
| Maximum or upset | 400 to 600°C if a reversal is missed or a regenerator bed is bypassed |
| Dust at the fan | Refractory and slag dust from the ladle lining; higher on fresh linings |
| Moisture | High water vapour during drying of new monolithic linings |
| Cycle | Regenerator reversal every 30 to 90 seconds, so temperature at the fan cycles |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 ladle preheating stations
| Blade form | Backward inclined, rugged plate blades |
| Material of construction | Mild steel for regenerative exhaust; heat-resisting impeller for recuperative or upset duty |
| Drive and arrangement | Direct coupled, arrangement 8, compact for mounting on the lid structure or nearby |
| Bearings and cooling | Pedestal bearings with heat slinger; cooling where gas may exceed about 400°C |
| Flow control | Variable speed drive or inlet damper on exhaust ratio |
| Also specified | High temperature trip at the fan inlet tied to the burner controls |
What this ID fan has to cope with
Temperature spikes on reversal faults
If the reversing valve fails to switch, one regenerator bed is driven past its capacity and hot gas reaches the fan. A fan rated for the upset temperature and a temperature trip that stops the burner prevent impeller damage.
Steam during dry-out of new linings
Fresh castable or gunned linings release a large volume of steam in the first hours. The exhaust gas then carries much more moisture and the volume at the fan rises. The fan is checked for this case, and drains on the casing prevent condensate pooling at cold starts.
Lid seal and exhaust balance
Drawing back too much gas pulls cold air into the ladle round the lid and cools the rim; drawing too little lets hot gas burn the lid structure. Setting the exhaust ratio, and holding it with a control loop as the ladle heats up, is part of commissioning.
Energy and control
A ladle preheater cycles constantly as ladles come and go, and spends long periods at low fire holding a ladle in waiting. The exhaust fan should follow the burner firing rate rather than run at full speed with a damper. The main energy benefit comes from regeneration itself, which can cut fuel use sharply compared with a cold air burner; a well-controlled exhaust fan is what makes the regenerator work.
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 Forced Draft (FD) Fan for ladle preheating stations.
Questions buyers ask
Does a ladle preheater have an ID fan?
Only some. Simple cold air ladle heaters vent round the lid and have just a combustion air blower. Regenerative and recuperative ladle heaters need an exhaust fan to pull flue gas back through the heat recovery equipment.
What temperature does a regenerative ladle heater exhaust fan see?
Normally 150 to 250°C, because the regenerator bed absorbs most of the heat before the gas reaches the fan. The fan must still tolerate short excursions to 400°C or more when a reversal fails or a bed is saturated.
How fast does a ladle need to be preheated?
Ladles in circulation are typically reheated to about 1,000 to 1,200°C on the hot face between heats. New linings follow a slower curve over many hours, set by the refractory supplier, so the fans must also run steadily at low fire.
Information needed to quote an ID fan for ladle preheating stations
- 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.
- Heater type. Cold air, recuperative or regenerative burner, and the share of flue gas drawn back.
- Duty cycle. Drying curve for new linings and the normal reheating cycle between heats.
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.