How oil fired re-rolling mill furnaces work
Secondary steel re-rolling mills reheat billets, ingots or cut pieces, typically 10 to 40 tonnes per hour, in small pusher or batch furnaces fired with furnace oil, LSHS or light diesel oil. The oil is heated to reduce viscosity and burned in low pressure air atomising burners, where the combustion air from the blower also breaks up the oil, or in pressure jet burners. Flue gas leaves at the charging end at 700 to 900°C, often passing a simple metallic recuperator, and goes to a chimney. Many such furnaces run on natural draft alone; others need an ID fan once a recuperator, a wet scrubber or a cyclone for particulate control is added. Furnace oil can carry 2 to 4% sulphur, so the flue gas is acidic.
Where the FD fan sits
- Inlet filter
- Combustion air (FD) fan
- Recuperator (if fitted)
- Air header
- Low pressure oil burners
The blower is mounted beside the furnace and feeds an air header to each burner, either directly or through the recuperator. Low pressure air atomising burners typically need about 500 to 750 mmWC at the burner, so the blower discharge must cover that plus recuperator and pipe losses. Pressure jet burners need less air pressure, since the oil pump does the atomising.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | 200 to 400°C where a recuperator is fitted |
| Dust | Mill bay dust and scale; inlet filter recommended |
| Pressure at the fan outlet | Typically 600 to 900 mmWC for low pressure air atomising burners |
| Load pattern | Frequent holds and restarts |
| 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 oil fired re-rolling mill furnaces
| Blade form | Backward curved or radial, with a steep curve for stable pressure |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on header pressure, with burner air valves |
| Also specified | Inlet filter, low air pressure interlock with the oil valve |
What this FD fan has to cope with
Smoky flame at low fire
When the blower is throttled for low firing, the air pressure at the burner falls and atomisation becomes coarse. The flame smokes, soot fouls the recuperator and fan, and fuel use rises. A blower that holds header pressure through speed control, rather than throttling, keeps atomisation good across the range.
Recuperator retrofit without blower check
Adding a recuperator adds air side pressure drop. With the old blower, the burner pressure falls and atomisation suffers, so the fuel saving expected from preheated air is partly lost. The blower duty should be checked whenever a recuperator is added. In most cases a modest increase in fan speed or a new impeller restores the pressure, but this must be checked against the motor rating and the impeller tip speed limit.
Wear on oil burner orifices
Worn burner tips change the air to oil relationship. If the blower is adjusted to compensate, the system drifts away from correct ratio. Maintaining burners and setting the blower from header pressure keeps combustion consistent. Worn tips also spray unevenly, so part of the oil burns late in the flue and adds soot to the recuperator and ID fan. Regular tip replacement is cheaper than compensating with more air.
Energy and control
In small re-rolling mills the combustion air blower often runs at full speed throughout, including long holds. A variable speed drive on header pressure saves power, but the main saving comes from correct air to fuel ratio and good atomisation, which reduce furnace oil consumption and soot. Fitting an inlet filter and keeping it clean also keeps the blower on its design curve.
ID fan and FD fan in oil fired re-rolling mill furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator or scrubber, before the chimney | Beside the furnace, feeding the burners |
| What it handles | Sulphur-bearing flue gas with soot | Ambient air |
| Temperature at the fan | 300 to 400°C dry, 60 to 80°C after a scrubber | Ambient |
| Main design concern | Acid corrosion and soot | Stable pressure for atomisation |
| Control | Furnace pressure | Air header pressure |
See also the Induced Draft (ID) Fan for oil fired re-rolling mill furnaces.
Questions buyers ask
What pressure combustion air blower does a re-rolling mill furnace need?
For low pressure air atomising burners the forced draft blower typically delivers 600 to 900 mmWC, covering about 500 to 750 mmWC at the burner plus pipe and recuperator losses. Pressure jet burners need less. Burner supplier data should set the exact figure. Too little pressure gives poor atomisation; too much wastes power.
Why is the flame smoky when our furnace runs at low fire?
On low pressure air atomising burners, atomisation depends on air pressure. If the blower is throttled, pressure drops and the oil is poorly atomised. Holding air header pressure with a variable speed blower keeps atomisation and combustion good at low fire. Oil temperature and burner tip wear should also be checked.
Can the same blower be used after adding a recuperator?
Only if it has enough pressure margin. The recuperator adds air side resistance, and without extra pressure the burners lose atomising air. Check the blower curve against the new system resistance before reusing it. In many cases a new impeller or a modest speed increase is enough, provided the motor and tip speed allow it.
Information needed to quote an FD fan for oil fired re-rolling mill 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. Low pressure air atomising or pressure jet burners, and their number and capacity.
- Recuperator. Fitted or planned, and its air side pressure drop.
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.