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 FD fan sits
- Inlet filter
- FD fan
- Air hose or swivel joint
- Cover burner manifold
- Tundish burners
The blower is usually on the caster platform or the preheating station structure, feeding the cover burners through a flexible hose or a rotary joint so that the cover can lift. Air is cold on most tundish heaters; a few have recuperators in the cover exhaust. The blower pressure covers the burner, hose and valves, typically 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, often 35 to 55°C on the caster platform |
| Air at the burner | Usually ambient; preheated only on recuperative designs |
| Dust | Mould powder, refractory and scale dust from the casting floor |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Duty | One heat-up per casting sequence, one to three hours each |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 tundish preheaters
| Blade form | Backward curved, high pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, compact frame |
| Bearings and cooling | Standard bearings; motor sized for hot platform ambient |
| Flow control | Variable speed drive following the preheat curve |
| Also specified | Inlet filter, heat shield, and low pressure trip to the burner controls |
What this FD fan has to cope with
Spalling from fast heat-up
Firing at full rate on a cold, damp lining can spall it. The blower and burners must run steadily at low rates during the first part of the curve. A blower with speed control and a burner rated for turndown let the preheat follow the refractory supplier's curve.
Hot ambient
Air on the caster platform can be well above normal ambient, which reduces the mass flow the blower delivers and loads the motor. Sizing for the real intake temperature avoids an air-starved burner when the tundish is wanted most.
Hose damage
The flexible connection to a moving cover gets hot and is often damaged. A burst hose starves the burners and blows dust around. Routing, heat protection and a low pressure trip are part of the duty.
Energy and control
The blower runs only during preheat, and for part of that time at low rate, so its annual energy is modest. Speed control that follows the preheat program saves power and gives a steadier flame than a damper. Controlled excess air also shortens preheat time and saves fuel, because less heat leaves the tundish with surplus air.
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 Induced Draft (ID) Fan for tundish preheaters.
Questions buyers ask
What blower is used on a tundish preheater?
A high pressure centrifugal combustion air blower, usually a backward curved fan with direct drive and a variable speed drive. It supplies air to the burners in the tundish cover and is sized from the total burner firing rate, the air-fuel ratio and the burner pressure, with allowance for hose and valve losses.
How long does tundish preheating take?
Typically one to three hours to bring the working lining to about 1,000 to 1,200°C, depending on the lining type and the caster practice. The blower must run steadily at low and high firing through that time.
Does the tundish preheater FD fan need a standby?
Usually not, because the preheat can be restarted once a blower is changed, though the casting start is delayed. Plants with tight sequence schedules, or with only one preheating position, often keep a spare blower ready for quick exchange, or a standby blower on a common header serving two stations.
Information needed to quote an FD fan for tundish preheaters
- 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.
- Burners. Number of burners in the cover, firing rate and air pressure needed.
- Preheat curve. Required heating rate and final lining 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.