How steel reheating furnaces work
Steel reheating furnaces heat billets, blooms and slabs to about 1,100 to 1,250°C before rolling. The main types are pusher furnaces, where the stock is pushed along skids, walking beam and walking hearth furnaces, where it is lifted and stepped forward, and batch or rotary hearth furnaces for special products. Fuels range from furnace oil and producer gas in re-rolling mills to natural gas and mixed works gases in integrated plants. Heat recovery varies just as widely: none at all, a metallic recuperator, regenerative burners, or a waste heat boiler. The furnace type, fuel and recovery method decide the temperature at the ID fan and the pressure the FD fan must deliver, and in some smaller furnaces a chimney alone provides the draft.
Where the FD fan sits
- Inlet filter
- FD fan
- Recuperator or reversing valves
- Air main
- Zone valves
- Burners
The fan draws ambient air and delivers it to the recuperator or, on regenerative furnaces, to the reversing valves and regenerator beds. On simple oil-fired furnaces the air may go straight to the burners. The fan pressure covers burner nozzle pressure, recovery equipment and distribution, from about 300 mmWC on simple furnaces to 700 to 1,000 mmWC on high velocity or regenerative firing.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after preheating | 300 to 550°C after a recuperator; up to about 1,000°C after regenerators |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | About 300 to 1,000 mmWC |
| Turndown | Wide, following mill delays and zone firing |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 steel reheating furnaces
| Blade form | Backward curved, at a tip speed matched to the pressure |
| Material of construction | Mild steel; high strength plate on high pressure fans |
| Drive and arrangement | Direct coupled at two-pole speed for high pressure duties |
| Bearings and cooling | Standard bearings, no cooling |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet silencer and filter; standby fan on continuous mills |
What this FD fan has to cope with
Insufficient pressure after burner changes
Upgrading from low pressure oil burners to high velocity gas burners, or adding regenerative burners, raises the pressure needed at the fan considerably. An old fan kept in service then cannot deliver full air at full firing, and the furnace runs short of air in the hottest zones. The fan must be rechecked whenever burners change.
Recuperator overheating on air failure
Losing combustion air while the furnace is hot leaves a metallic recuperator with hot flue gas on one side and no cooling air on the other. Tubes can distort or crack within minutes. Low air flow interlocks, dilution air and a reliable fan, with a standby on continuous mills, protect it.
Noise in the mill bay
High pressure combustion air fans running at two-pole speed are loud, mainly at the inlet. Where several furnaces share a bay the noise adds up. Inlet silencers, acoustic lagging on the casing and a quiet blade selection are commonly specified on reheating furnaces built close to the rolling mill floor.
Energy and control
The combustion air fan runs whenever the furnace is hot. Variable speed control on air main pressure saves power at the low firing rates that follow mill delays. Correct air to fuel ratio across all zones has the larger effect on fuel cost, and the fan must give the ratio control enough pressure to work.
ID fan and FD fan in steel reheating furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | End of heat recovery, before chimney | Before recuperator or regenerators |
| What it handles | Flue gas, low dust | Ambient air |
| Temperature at the fan | 150 to 500°C by furnace type | Ambient |
| Main design concern | Matching material to the recovery scheme | Pressure for the burner type |
| Control | Furnace pressure | Air main pressure and air to fuel ratio |
See also the Induced Draft (ID) Fan for steel reheating furnaces.
Questions buyers ask
Is the FD fan on a reheating furnace the combustion air blower?
Yes. On a reheating furnace the FD fan and the combustion air blower are the same machine. The forced draft fan supplies all the air for combustion, preheated through a recuperator or regenerators where fitted, and on oil-fired furnaces it may also provide the atomising air for low pressure burners.
How much pressure does a reheating furnace combustion air fan need?
It depends on the burners and the heat recovery. Simple oil burners with no recuperator may need 300 to 500 mmWC at the fan; furnaces with recuperators and high velocity burners, or regenerative systems with their reversing valves and ceramic beds, often need 700 to 1,000 mmWC or more.
Does the FD fan see hot air on a reheating furnace?
Normally no. The fan handles ambient air and pushes it through the recuperator or regenerators, so the hot air is downstream of it. Hot air fans placed after the recuperator are rare, because they would be larger, need heat-resisting construction and cost more to run for the same air mass.
Information needed to quote an FD fan for steel reheating furnaces
- 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.
- Burner data. Burner type, number per zone and air pressure needed at full firing.
- Air path losses. Pressure drop through the recuperator, reversing valves or regenerators.
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.