How tundish preheaters work
Before each casting sequence on a continuous caster, the tundish is preheated so that the first steel does not freeze in the nozzles. The working lining is usually a dry vibratable mass or a sprayed or gunned layer, bonded with resin or other organic binders. The tundish sits on its car or in a preheating station with a cover carrying two to four burners, fired with natural gas, LPG, coke oven gas or oil, and the lining is taken to about 1,000 to 1,200°C over one to three hours. Submerged entry nozzles and stopper rods are heated with it or in separate small furnaces. Flue gas leaves through cover openings and the tundish nozzles. On many casters a hood and exhaust fan above the preheating station collect the gas and binder smoke.
Where the ID fan sits
- Tundish cover burners
- Tundish lining
- Cover openings and nozzles
- Exhaust hood
- Exhaust (ID) fan
- Roof stack
The hood sits above the tundish car at the preheating position, open at the sides so that the cover and car can move. The exhaust fan, on the caster platform or roof, draws flue gas and a large share of ambient air through it, which brings the temperature down to about 150 to 300°C. On some newer casters the cover has a dedicated flue connection and the fan works more like a conventional ID fan, with a smaller volume at higher temperature.
Gas at the ID fan
| Gas temperature, normal | 150 to 300°C after hood dilution |
| Maximum or upset | 400°C or more with the hood close and dilution low |
| Dust at the fan | Fine refractory dust from the dry vibe lining; moderate |
| Condensables | Resin and organic binder smoke in the first part of the heat-up, forming tar on cool surfaces |
| Moisture | Steam from sprayed and gunned linings during dry-out |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 tundish preheaters
| Blade form | Radial tipped or backward inclined plate blades, tolerant of sticky deposits |
| Material of construction | Mild steel, with a heat-resisting impeller where the fan can see over 400°C |
| Drive and arrangement | Belt or direct drive, arrangement 8 with cooling disc |
| Bearings and cooling | Pedestal bearings outside the gas stream, heat slinger |
| Flow control | Variable speed drive, with reduced speed between preheats |
| Also specified | Inspection and cleaning doors on casing and inlet box, casing drain |
What this ID fan has to cope with
Tar build-up on the impeller
Binder smoke condenses where the gas cools, coating ducts and the impeller with a sticky, uneven layer. The fan goes out of balance and the deposit can catch fire. Keeping the gas warm enough, using a blade form that sheds deposits, and regular cleaning access are the design response.
Short, frequent cycles
A tundish is preheated once per casting sequence, and the fan may start and stop several times a day while the burners cycle from cold. Repeated thermal cycling loosens shrink fits and warps light impellers. Robust construction with keyed or bolted hubs suits this duty.
Smoke escaping onto the casting floor
If the hood volume is too low, smoke spills onto the platform where operators work. The fan is sized for the peak smoke release and the movement of the cover, not just the burner flue gas.
Energy and control
The exhaust fan only needs full flow while the tundish is on preheat, and most of the smoke comes in the first part of the heat-up. A variable speed drive that drops to low speed between sequences, and is linked to the burner status, saves most of the running power on a caster that casts long sequences.
ID fan and FD fan in tundish preheaters
| ID fan | FD fan | |
|---|---|---|
| Position | Above the preheating station, on the hood | On the platform, feeding the cover burners |
| What it handles | Flue gas, binder smoke and dilution air | Ambient platform air |
| Temperature at the fan | 150 to 300°C | Ambient, often hot |
| Main design concern | Tar deposits and smoke capture | Turndown for the preheat curve |
| Control | Hood exhaust volume | Burner firing rate |
See also the Forced Draft (FD) Fan for tundish preheaters.
Questions buyers ask
Why does a tundish preheater need an exhaust fan?
The burners in the tundish cover produce flue gas, and the lining binder releases smoke as it burns out. On an open caster platform these would spill onto the working floor. An exhaust hood and fan collect them and send them to a stack or, on some plants, to a filter.
What temperature does a tundish exhaust fan handle?
With an open hood and dilution air, usually 150 to 300°C. If the cover is ducted directly to the fan with little dilution, the gas can be 400 to 600°C and the fan needs heat-resisting construction or a dilution damper.
How do you stop tar fouling the tundish exhaust fan?
Keep the duct and fan above the condensation temperature of the binder smoke during the critical period, use a blade shape that does not trap deposits, and provide doors for regular cleaning. Insulating the duct also helps.
Information needed to quote an ID fan for tundish preheaters
- 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.
- Lining and binder. Tundish lining type, binder system and preheat curve.
- Hood arrangement. Hood dimensions, distance from the cover and whether the cover is ducted directly.
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.