How forging furnaces work
Forging furnaces heat steel billets, bars and blooms to about 1,150 to 1,250°C for hammers, presses and ring rolling. Common types are batch box or slot furnaces with a front door, rotary hearth furnaces feeding press lines, and small continuous pusher furnaces. They fire natural gas, LPG, furnace oil or light diesel oil through high velocity or flat flame burners. Because the furnace runs close to forging temperature and the stock is loaded and unloaded through open doors many times an hour, the flue gas leaves at 1,000 to 1,200°C, much hotter than from a long reheating furnace. Smaller furnaces vent through the door and a canopy hood; larger ones use a flue, a recuperator protected by dilution air, and an ID fan or chimney.
Where the FD fan sits
- Inlet filter
- Combustion air (FD) fan
- Recuperator
- Air header
- High velocity burners
The fan is usually mounted beside the furnace and feeds an air header to the burners, often through a recuperator. High velocity burners need air pressure of about 300 to 600 mmWC at the burner, so the fan discharge, with recuperator and pipe losses, is typically 500 to 900 mmWC. The fan is small but sees fast changes in demand.
Air at the FD fan
| Air temperature at the fan | Ambient, often a hot forge shop at 35 to 50°C |
| Air after the recuperator | 300 to 500°C, downstream of the fan |
| Dust | Scale and forge shop dust; inlet filter recommended |
| Pressure at the fan outlet | Typically 500 to 900 mmWC |
| Turndown | Wide and fast, following door openings and holds |
| 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 forging furnaces
| Blade form | Backward curved, steep curve for stable pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings; motor rated for the hot shop ambient |
| Flow control | Variable speed drive on air header pressure |
| Also specified | Inlet filter and silencer; vibration isolation from hammer foundations |
What this FD fan has to cope with
Hot shop ambient
Forge shops are hot, and air drawn at 45 to 50°C is lighter than standard air. A fan sized at 20°C delivers less mass flow and pressure than expected. The fan should be selected at the actual shop temperature, or the inlet ducted from outside the building.
Vibration from hammers
Fans near forging hammers pick up ground vibration through their foundations, which can damage bearings even when the fan is stationary. Isolating the fan base and mounting it away from hammer foundations prevents false brinelling of the bearings. Flexible connections on the air duct stop hammer shocks travelling into the fan through the ductwork as well, and a vibration check at commissioning confirms the isolation works.
Slow recovery after door opening
After the door closes, the burners must fire hard to recover temperature. If the fan cannot provide full pressure quickly, recovery is slow and output falls. A fan with margin and a fast speed loop supports quick recovery. Burners with good turndown and an air header large enough to avoid pressure drop to the far burners help further, and recovery time is a useful figure to measure before and after any change.
Energy and control
Forging furnaces spend much time at low fire during die changes and between batches. A variable speed combustion air fan holding header pressure saves power, while correct air to fuel ratio reduces fuel use and scale. Ducting the inlet from outside the hot shop also improves fan output and burner performance.
ID fan and FD fan in forging furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After dilution and recuperator, or on the door hood | Beside the furnace, feeding the burners |
| What it handles | Hot diluted flue gas | Ambient air from a hot shop |
| Temperature at the fan | 350 to 500°C | Ambient, often 35 to 50°C |
| Main design concern | High temperature and door-opening pressure swings | Fast turndown and recovery |
| Control | Furnace pressure with door feedforward | Air header pressure |
See also the Induced Draft (ID) Fan for forging furnaces.
Questions buyers ask
What size combustion air blower is needed for a forging furnace?
It depends on furnace firing rate and burner type. High velocity burners typically need 300 to 600 mmWC at the burner, so the forced draft fan delivers 500 to 900 mmWC including recuperator and pipe losses. The flow is set from the maximum firing rate with excess air.
Why does the forging furnace take long to recover temperature?
Often because the combustion air fan cannot deliver full air quickly after the door closes. A fan with pressure margin, a fast speed control loop and good burner turndown shorten recovery time. Checking that the recuperator is not fouled, and that the air header pressure holds at full fire on the burner furthest from the fan, usually finds the cause.
Should the forge furnace blower take air from inside the shop?
Taking air from outside is often better. Shop air is hot, so a fan sized for standard conditions delivers less. Ducting the inlet from outside gives cooler, denser air and more consistent combustion. The duct must be short and well supported, with a rain hood and a filter at the outside inlet so that rain and dust do not reach the fan.
Information needed to quote an FD fan for forging 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 type. Burner make or type, number and the air pressure each needs.
- Shop conditions. Shop ambient temperature near the fan and whether an outside air inlet is possible.
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.