How reheating furnace waste heat boilers work
A pusher or walking beam reheating furnace in a rolling mill heats billets or slabs to 1,150 to 1,250°C, firing furnace oil, natural gas or producer gas. Its flue gas leaves the recuperator at typically 450 to 650°C, which is still high enough to raise useful steam. A waste heat boiler placed in the flue between the recuperator and the chimney recovers this heat, cooling the gas to about 180 to 220°C. The boiler adds resistance that the chimney cannot overcome, so an ID fan after the boiler provides the draft. A bypass flue with dampers lets the furnace run on chimney draft when the boiler is out, and the furnace pressure control works through the fan and dampers together.
Where the ID fan sits
- Reheating furnace
- Recuperator
- Waste heat boiler
- ID fan
- Chimney
The fan sits after the boiler, at the base of the chimney or in a new stack, with a bypass flue from the recuperator outlet to the chimney. Normally it pulls the gas through the boiler at 180 to 220°C against a suction of typically 80 to 150 mmWC. Its speed or damper is controlled from the furnace hearth pressure, which must stay slightly positive so cold air does not enter. When the boiler trips, the bypass opens and the fan either stops or briefly handles hotter gas until the dampers settle.
Gas at the ID fan
| Gas temperature, normal | 180 to 220°C after the boiler |
| Maximum or upset | Up to 450 to 600°C during bypass damper change-over or boiler tube failure |
| Dust at the fan | Low; fine scale and refractory dust |
| Moisture and dew point | Low to moderate; acid dew point with furnace oil firing |
| Corrosive species | SO₂ and SO₃ from furnace oil sulphur; low on gas firing |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 reheating furnace waste heat boilers
| Blade form | Backward curved |
| Material of construction | Mild steel for normal duty; heat-resisting steel impeller where the fan must run through the hot bypass case |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc, sized for the upset temperature |
| Flow control | Variable speed drive on furnace hearth pressure, coordinated with boiler and bypass dampers |
| Also specified | Isolation dampers, dilution air damper for overtemperature, casing insulation |
What this ID fan has to cope with
Furnace pressure upsets on change-over
Switching between boiler and bypass changes the draft available at the furnace in seconds. If the fan and dampers are not coordinated, the furnace swings from positive to negative pressure, drawing in cold air and upsetting the heating. A control sequence that ramps the fan with the dampers keeps hearth pressure steady.
Hot gas reaching the fan
During a damper fault or boiler trip, gas at 450 to 600°C can reach the fan before the bypass is fully open. A mild steel fan sized for 200°C can distort. Either the fan is built for the hot case, or a dilution air damper and an interlock protect it.
Air ingress in long flues
Older flues between the recuperator and the chimney leak and draw in cold air, especially at expansion joints and cleaning doors. This lowers the gas temperature entering the boiler, cuts steam output, and adds volume the fan must move. The fan is sized on measured gas flow at its inlet, including the ingress, and flue sealing is part of the project.
Energy and control
The reheating furnace firing follows mill production, with frequent delays and rolling changes. A variable speed drive on hearth pressure follows these quickly and uses far less power than a damper. Sealing flue leaks raises the steam output of the boiler and reduces the ID fan load at the same time.
Questions buyers ask
Why does a reheating furnace waste heat boiler need an ID fan?
The boiler adds resistance and cools the gas, so the chimney alone can no longer provide enough draft for the reheating furnace. The ID fan restores the draft and becomes the means of controlling furnace hearth pressure. Without it the furnace would run positive, push flame out of the doors and lose heat.
What happens to the ID fan when the waste heat boiler trips?
The bypass damper opens and the furnace switches to chimney draft. The fan either stops or runs at low speed. It must be designed for the hot gas it may see during the change-over or be protected by dilution air.
How is furnace pressure controlled with a waste heat boiler?
By varying the ID fan speed, or its damper, to keep a small positive pressure at the furnace hearth, usually measured near the discharge end. The control is coordinated with the boiler inlet and bypass dampers, so the pressure stays steady during change-overs and the furnace does not draw cold air through the doors.
Information needed to quote an ID fan for reheating furnace waste heat boilers
- 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.
- Furnace and recuperator. Flue gas flow and temperature after the recuperator, and the fuel fired.
- Bypass arrangement. Bypass flue and damper layout, and whether the fan must run during bypass.
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.