How walking beam reheating furnaces work
A walking beam furnace reheats billets, blooms or slabs to rolling temperature, around 1,150 to 1,250°C, by lifting the stock on water-cooled moving beams and stepping it forward through preheat, heating and soaking zones. It is fired with natural gas, fuel oil, mixed gas or producer gas through side or roof burners. The flue gas leaves the charging end at 800 to 950°C and passes a central recuperator that preheats the combustion air to 400 to 550°C. On many furnaces the chimney provides the draft; where the recuperator, a waste heat boiler or long flues add resistance, an ID fan is used, often with a dilution air connection to protect the recuperator from overheating.
Where the ID fan sits
- Heating and soaking zones
- Charging end flue
- Recuperator
- Waste heat boiler (if fitted)
- ID fan
- Chimney
The fan sits after the recuperator, which brings the gas down from about 900°C to 350 to 500°C. With a waste heat boiler downstream, the gas reaches the fan at 180 to 250°C. The fan works with the stack damper to hold the furnace at a small positive pressure, about +1 to +3 mmWC at hearth level, so that cold air is not drawn in through the doors and skids while flame does not blow out of them.
Gas at the ID fan
| Gas temperature, normal | 350 to 500°C after the recuperator; 180 to 250°C after a waste heat boiler |
| Maximum or upset | Up to 600°C when dilution air fails or the recuperator is bypassed |
| Dust at the fan | Low; fine oxide scale and refractory dust, usually below 100 mg/Nm³ |
| Moisture and dew point | Natural gas firing gives 15 to 18% water vapour, but the gas stays well above its dew point |
| Corrosive species | SO₂ with fuel oil or producer gas firing; matters only if the fan is cooled below the acid dew point |
| Density at the design temperature | About 0.49 kg/m³ at 450°C, 41% 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 walking beam reheating furnaces
| Blade form | Backward curved or backward inclined, for efficiency on clean hot gas |
| Material of construction | Heat-resisting steel impeller (such as SA 387 or stainless grades above 450°C); mild steel casing with external insulation |
| Drive and arrangement | Direct coupled, arrangement 8, with the casing free to expand on sliding supports |
| Bearings and cooling | Shaft cooling disc and heat shield; water- or oil-cooled bearings above about 400°C |
| Flow control | Variable speed drive on furnace pressure, with a fast-acting stack damper |
| Also specified | Turning gear or slow rotation on shutdown to prevent shaft bow; insulated inlet box |
What this ID fan has to cope with
Shaft bow after a hot stop
If the fan stops while full of hot gas, the upper side of the shaft heats more than the lower and the shaft bows. Slow turning after shutdown, or keeping the fan running at low speed until the gas cools, avoids heavy vibration at restart.
Thermal growth and alignment
At 450°C the casing grows several millimetres. Sliding feet, a fixed point at the drive end and flexible ductwork connections keep the coupling aligned and stop the casing loading the bearings.
Creep and loss of strength
Mild steel loses strength rapidly above 400°C. The impeller is designed for stress at the maximum upset temperature, not the normal one, using heat-resisting steel where the gas can exceed about 450°C.
Furnace pressure hunting
A fan that responds too slowly lets the furnace swing between suction and pressure, with scale and fuel losses on one side and refractory damage on the other. Speed control with a well-tuned loop and a fast damper gives steady pressure.
Energy and control
The ID fan on a reheating furnace is often oversized and throttled hard by the stack damper. Variable speed control, matched to furnace pressure, cuts fan power and improves pressure control at the reduced firing rates seen during mill delays. Running the furnace at the right pressure also reduces air ingress, which saves fuel; the fuel saving is usually larger than the fan power saving.
ID fan and FD fan in walking beam reheating furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator, before the chimney | Before the recuperator, feeding the burners |
| What it handles | Hot flue gas with little dust | Cold, filtered air |
| Temperature at the fan | 350 to 500°C, up to 600°C upset | Ambient |
| Main design concern | Heat-resisting materials and thermal growth | High, steady pressure across a wide turndown |
| Control | Furnace pressure | Air main pressure and air-fuel ratio |
See also the Forced Draft (FD) Fan for walking beam reheating furnaces.
Questions buyers ask
Why does a reheating furnace need an ID fan if it has a chimney?
A tall chimney can provide enough draft for the furnace alone. Once a recuperator, a waste heat boiler or long flue runs add resistance, the natural draft is not enough to hold the furnace pressure at full firing, and an ID fan is added.
What material is used for a high temperature ID fan impeller?
Up to about 400°C mild steel or boiler quality plate is acceptable with proper stress design. Above that, heat-resisting steels or stainless steel are used, selected for strength at the maximum temperature the fan can see.
What is dilution air on a reheating furnace?
Dilution air is ambient air injected ahead of the recuperator to stop the gas temperature exceeding what the recuperator tubes can withstand. It increases the gas volume, so the ID fan must be sized including the dilution air.
Information needed to quote an ID fan for walking beam reheating furnaces
- 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.
- Recuperator and waste heat recovery. Gas temperature before and after the recuperator, dilution air quantity, and any waste heat boiler.
- Furnace pressure control. Present control method and the furnace pressure set point.
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.