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 ID fan sits
- Furnace chamber
- Flue outlet
- Dilution air inlet
- Recuperator
- ID fan
- Stack
On recuperative forging furnaces the fan sits after the recuperator. Gas leaves the furnace at 1,000 to 1,200°C and is mixed with dilution air before the recuperator to protect its tubes, so it reaches the fan at about 350 to 500°C. On door-vented furnaces the fan instead serves a canopy over the door and handles flue gas heavily diluted with shop air, at 150 to 250°C. The suction is modest, typically 50 to 150 mmWC.
Gas at the ID fan
| Gas temperature, normal | 350 to 500°C after dilution and recuperator |
| Maximum or upset | 600 to 700°C if dilution air fails |
| Dust at the fan | Light scale and refractory dust |
| Flow variation | Large; gas flow swings with door openings and burner turndown |
| Corrosive species | SO₂ only with oil firing |
| 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 forging furnaces
| Blade form | Backward inclined plate blades, robust at high temperature |
| Material of construction | Heat-resisting steel impeller; mild steel casing with internal or external insulation |
| Drive and arrangement | Direct coupled, with a long shaft and the motor well clear of the hot casing |
| Bearings and cooling | Cooling disc and heat shield; water-cooled bearings at the higher temperatures |
| Flow control | Variable speed drive on furnace pressure, with a fast damper for door events |
| Also specified | Dilution air damper interlocked with recuperator inlet temperature; slow turning after shutdown |
What this ID fan has to cope with
Pressure swings at every door opening
Each time the door opens to load or unload a billet, the furnace loses pressure and cold air rushes in; when it closes, pressure jumps. A slow fan control lets the furnace oscillate, wasting fuel and increasing scale. Fast speed response, a quick damper, and door-linked feedforward to the controller keep the furnace near its set point.
Dilution air failure
If the dilution air damper fails closed, gas at furnace temperature reaches the recuperator and then the fan. The recuperator tubes can be damaged and the fan impeller overstressed. An interlock that trips the burners on high recuperator inlet temperature, and a fan designed for the upset temperature, are needed.
Shaft bow on short stops
Forging shops often stop furnaces between shifts or for die changes. A hot fan stopped full of gas bows its shaft and vibrates on restart. Slow turning after shutdown, or running at low speed until the casing cools, prevents this. The same applies after a power failure, when a UPS-backed barring drive or a small auxiliary motor is worth its cost on larger fans.
Energy and control
Forging furnaces run in a stop-start pattern tied to the press or hammer, with holds during die changes. A variable speed ID fan holding furnace pressure avoids drawing cold air in through door gaps during holds, which saves more fuel than fan power. Good door seals and tighter pressure control also reduce scale loss on the forgings.
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 Forced Draft (FD) Fan for forging furnaces.
Questions buyers ask
Does a forging furnace need an ID fan?
Small forging furnaces often vent through the door and a canopy hood, with no ID fan on the furnace itself. Larger forging furnaces with a recuperator usually need an induced draft fan, because the recuperator and dilution add resistance and the furnace pressure must be held tightly as the doors open and close.
Why is the flue gas from a forging furnace so hot?
Forging furnaces are short and run close to forging temperature, and the stock is loaded hot or near hot, so little heat is taken out of the gas before it leaves. Flue gas typically leaves at 1,000 to 1,200°C. Dilution air is added before the recuperator and fan to protect them.
How do I control forging furnace pressure when the door opens?
Use a fast-responding ID fan speed control, a quick-acting damper and, where possible, a signal from the door mechanism to the controller so it anticipates the door event. This keeps the furnace near a small positive pressure and reduces cold air ingress and scale. Good door seals help as much as the controller.
Information needed to quote an ID fan for forging 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.
- Furnace and door cycle. Furnace type, door size, door openings per hour and the forging temperature.
- Dilution and recuperator. Flue gas temperature at furnace outlet, dilution air quantity and recuperator inlet limit.
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.