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 FD fan sits
- Inlet filter and silencer
- FD fan
- Recuperator
- Hot air main
- Zone control valves
- Burners
The fan sits at the charging end, close to the recuperator, and delivers air into a cold air main. The air passes the recuperator and is distributed through an insulated hot air main to each zone, where control valves set the air flow to match the fuel flow. The fan must provide the pressure needed by the burner at full firing plus the losses of the recuperator, hot main and valves, often 500 to 900 mmWC in total.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 400 to 550°C, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 500 to 900 mmWC |
| Turndown | Zones fire from about 20% to full, so total air varies widely |
| 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 walking beam reheating furnaces
| Blade form | Backward curved, at a tip speed suited to the high pressure |
| Material of construction | Mild steel; high strength impeller plate where tip speed is high |
| Drive and arrangement | Direct coupled at two-pole or four-pole speed |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive holding air main pressure, with zone air valves for distribution |
| Also specified | Inlet filter, inlet silencer, and often a standby fan for continuous mills |
What this FD fan has to cope with
Pressure dip during load changes
When several zones open their valves together, main pressure falls and burners run short of air. A fan with a steep pressure curve and a drive controlling main pressure keeps every burner supplied.
Recuperator protection
Losing combustion air while the furnace is hot can overheat the recuperator. The fan duty and controls must cover the minimum air flow that keeps the tubes cool, and a standby fan or interlock is common.
Noise
High pressure combustion air fans are noisy at the inlet. An inlet silencer and acoustic insulation on the casing are often needed to meet limits in the mill bay.
Energy and control
Combustion air fans on reheating furnaces run continuously, so efficiency is worth paying for. A variable speed drive holding the air main pressure, instead of a throttled inlet damper, saves substantially at the reduced firing rates during mill delays. A correctly set air-fuel ratio matters more for fuel cost than any fan saving, and the fan must supply enough pressure for the ratio control to work at every zone.
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 Induced Draft (ID) Fan for walking beam reheating furnaces.
Questions buyers ask
Is the FD fan on a reheating furnace the same as the combustion air blower?
Yes. On a reheating furnace the forced draft fan supplies all the combustion air to the burners, and it is often called the combustion air blower or combustion air fan.
Does the combustion air fan handle hot air from the recuperator?
No. In the usual layout the fan pushes cold air into the recuperator. Placing the fan after the recuperator would mean a much larger, hot fan, so this is avoided.
Why is the combustion air pressure so high on a reheating furnace?
The air must overcome the recuperator, the long insulated hot air main, the zone control valves and the burner itself, and high velocity burners need substantial pressure at the nozzle to mix the air and fuel.
Information needed to quote an FD fan for walking beam 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. Number of burners per zone, burner type and the air pressure each needs at full firing.
- Recuperator. Air side pressure drop of the recuperator and the hot air 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.