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 FD fan sits
- Inlet filter
- FD fan
- Cold air main
- Recuperator
- Hot air main
- Zone burners
The fan sits near the charging end, close to the recuperator, and feeds a cold air main into it. Hot air, typically 300 to 450°C, then goes to the heating and soaking zone burners. Pusher furnaces usually have two or three fired zones, fewer than a walking beam furnace, so the air system is simpler, but the fan pressure must still cover recuperator loss and burner pressure, typically 400 to 800 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 300 to 450°C, downstream of the fan |
| Dust | Mill bay dust; inlet filter recommended |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Load pattern | Long holds at low fire between rolling campaigns |
| 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 pusher type reheating furnaces
| Blade form | Backward curved, with a steep curve for stable pressure at low flow |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings; heat shield if mounted close to the recuperator |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter and silencer; low air pressure interlock to the burners |
What this FD fan has to cope with
Poor atomisation at low fire
With low pressure air atomising oil burners, the air pressure at the burner sets the droplet size. If the fan is throttled heavily during holds, pressure falls, atomisation suffers and the flame smokes and deposits soot in the recuperator. Holding air main pressure by fan speed, not by closing the main damper, keeps atomisation good.
Recuperator air side leakage
Cracked recuperator tubes let combustion air leak into the flue gas side. The fan then delivers more air than reaches the burners, wasting power and overloading the ID fan or chimney. Comparing fan delivery with burner demand reveals the leak and avoids the mistaken conclusion that the fan is too small.
Dust in the mill bay
Pusher furnaces in small mills often sit in dusty bays with scale and coal dust in the air. Without an inlet filter the dust builds up on the impeller and enters the recuperator. A simple inlet filter with a pressure drop gauge prevents imbalance and recuperator fouling.
Energy and control
Pusher furnaces often hold at low fire for long periods while the mill changes rolls or waits for orders. A combustion air fan running at full speed against a nearly closed damper wastes most of its power during these holds. A variable speed drive on air main pressure, together with correct air to fuel ratio, saves both power and fuel.
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 Induced Draft (ID) Fan for pusher type reheating furnaces.
Questions buyers ask
What pressure combustion air blower is needed for a pusher furnace?
For a pusher reheating furnace with a recuperator and low pressure air atomising oil burners, the forced draft fan typically delivers 400 to 800 mmWC. The figure depends on burner type, recuperator air side loss and the length of the air mains, and should be confirmed from the burner supplier data.
Why does our pusher furnace smoke at low firing?
Often because the combustion air pressure falls when the fan is throttled, and oil atomisation suffers. Running the combustion air fan on a variable speed drive that holds air main pressure, rather than closing a damper, keeps atomising air pressure steady at low fire. Cleaning the burner tips and checking the oil temperature help as well.
Should the combustion air fan be replaced when the recuperator is upgraded?
Check it first. A new recuperator with higher air preheat often has a higher air side pressure drop, and the hotter air needs larger burner passages or more pressure. If the existing fan cannot deliver full air at full firing with the new losses, it should be uprated or replaced.
Information needed to quote an FD fan for pusher type 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 and atomising system. Burner type, oil atomising method and air pressure required at the burner.
- Recuperator. Air side pressure drop and 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.