How pusher type reheating furnaces work
In a pusher reheating furnace a hydraulic pusher at the charging end shoves a continuous line of billets or blooms along water-cooled skid pipes, so each new piece pushes the whole row forward and one piece drops out at the discharge end. The furnace has a long unfired or lightly fired preheat zone, followed by heating and soaking zones fired from the side, roof or end with furnace oil, producer gas, mixed gas or natural gas. Because the cold stock absorbs heat in the preheat zone, the flue gas leaves the charging end cooler than in many other designs, at about 650 to 850°C. It usually runs down an underground flue to a recuperator and a tall brick or steel chimney, with an ID fan added where the chimney cannot cope.
Where the ID fan sits
- Soaking and heating zones
- Unfired preheat zone
- Charging end downtake
- Underground flue
- Recuperator
- ID fan
- Chimney
The fan is usually placed beside the chimney base, after the recuperator, and often in a bypass so the furnace can fall back on natural draft if the fan stops. The gas reaches it at about 300 to 450°C. The fan works against the downtake, the underground flue, the recuperator and its dampers, typically 60 to 150 mmWC, and must hold a slight positive pressure at hearth level near the discharge door.
Gas at the ID fan
| Gas temperature, normal | 300 to 450°C after the recuperator |
| Maximum or upset | 550 to 650°C when the recuperator is bypassed during repairs |
| Dust at the fan | Low; mill scale flakes and refractory dust from the skids and flue, under about 150 mg/Nm³ |
| Air ingress | Significant on old underground flues, diluting and cooling the gas |
| Corrosive species | SO₂ from furnace oil or producer gas; acid risk only at cold spots |
| Density at the design temperature | About 0.53 kg/m³ at 400°C, 44% 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 pusher type reheating furnaces
| Blade form | Backward inclined, plate bladed, robust for occasional scale and hot running |
| Material of construction | Boiler quality plate up to about 400°C; heat-resisting steel impeller where bypass operation is expected |
| Drive and arrangement | Direct coupled or belt drive, arrangement with the motor well clear of the hot casing |
| Bearings and cooling | Cooling disc and heat shield; water-cooled bearing housings above about 350°C |
| Flow control | Variable speed drive on furnace pressure, with the chimney damper as fallback |
| Also specified | Bypass and isolation dampers for natural draft operation; insulated casing |
What this ID fan has to cope with
Leaking underground flue
Old brick flues admit cold air and ground water through cracks. The air dilutes the gas and adds volume the fan must handle, and the water can flood the flue. Measuring the actual gas flow and temperature at the fan inlet, rather than calculating from fuel alone, avoids buying a fan that is too small for the real system.
Retrofit fan fighting the chimney
When an ID fan is added to a furnace that also has a tall chimney, poor damper arrangement lets the fan recirculate gas or fight the natural draft. A clear bypass and isolation damper scheme, with the fan curve selected for the combined system, lets the two work together and allows a safe fallback to natural draft.
Recuperator bypass running hot
Recuperator tubes on pusher furnaces are often repaired while the mill keeps running, with the gas bypassed straight to the chimney. If the fan stays in service during this period it can see 550°C or more. The impeller material and bearing cooling must be chosen for that, or the fan must be isolated and the furnace run on natural draft.
Energy and control
Pusher furnaces in merchant mills often run one or two shifts, with long hold periods at reduced firing between rolling campaigns. A variable speed drive on the ID fan, holding hearth pressure, cuts power heavily during holds and reduces cold air drawn into the furnace, which saves fuel and scale loss. Sealing the underground flue and recuperator joints reduces the fan duty further.
ID fan and FD fan in pusher type reheating furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator, at the chimney base | At the charging end, before the recuperator |
| What it handles | Flue gas with scale fines and flue leakage air | Ambient air |
| Temperature at the fan | 300 to 450°C, hotter on bypass | Ambient |
| Main design concern | Fitting into an existing natural draft flue | Steady pressure for atomisation at low fire |
| Control | Hearth pressure, with natural draft fallback | Air main pressure |
See also the Forced Draft (FD) Fan for pusher type reheating furnaces.
Questions buyers ask
Can an ID fan be added to an existing pusher reheating furnace?
Yes. It is a common upgrade on pusher reheating furnaces when a recuperator is added or output rises. The fan is usually placed near the chimney base with bypass dampers. The duty should be based on measured gas flow, temperature and suction at the fan location, since old flues leak more air than drawings suggest.
What causes black smoke and flame at the pusher furnace doors?
Flame and smoke at the doors mean the furnace is under too much positive pressure, often because the chimney or fan cannot remove the gas at full firing. A properly sized induced draft fan with pressure control holds the hearth at a small positive pressure without driving flame out of the doors.
How hot does the ID fan run on a pusher furnace?
After a metallic recuperator the gas is typically 300 to 450°C. During recuperator bypass it can reach 550°C or more. The fan design temperature, impeller material and bearing cooling should be chosen for the highest temperature at which the plant intends to keep the fan running.
Information needed to quote an ID fan for pusher type reheating 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.
- Existing flue and chimney. Chimney height and diameter, flue length and condition, and present damper arrangement.
- Furnace output. Present and planned output in tonnes per hour and the fuel fired.
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.