How bogie hearth furnaces work
A bogie hearth or car bottom furnace is a large batch furnace whose hearth is a refractory-lined bogie that runs out on rails for loading heavy castings, forgings, fabrications or coils. It is used for annealing, normalising, stress relieving, hardening and tempering, typically between about 550 and 1,100°C. Burners on the side walls, usually high velocity gas or oil burners, fire into the chamber, often with a programmed cycle of ramp, soak and controlled cooling lasting many hours. Flue gas leaves through roof or sidewall flues, sometimes through a recuperator, and the temperature at the flue follows the cycle. Controlled cooling is often done by running the combustion air blower with burners off, or with separate cooling air fans.
Where the FD fan sits
- Inlet filter
- Combustion air (FD) fan
- Air header
- Burner air valves
- High velocity burners
- Furnace chamber
The fan is mounted beside the furnace and feeds a header to the side wall burners. During firing it delivers combustion air at the pressure the burners need, typically 400 to 700 mmWC at the fan. During forced cooling the burner air valves open fully with fuel off, and the fan delivers much more air at lower pressure. The fan must work at both points on its curve.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Ambient; few bogie furnaces preheat air |
| Dust | Shop dust; inlet filter recommended |
| Pressure at the fan outlet | Typically 400 to 700 mmWC during firing |
| Cooling duty | High flow at lower pressure during forced cooling |
| 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 bogie hearth furnaces
| Blade form | Backward curved, with a curve that covers both firing and cooling points |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on header pressure during firing and on cooling rate during cooling |
| Also specified | Motor sized for the cooling flow, inlet filter |
What this FD fan has to cope with
Motor overload during cooling
With burner valves wide open for cooling, the system resistance falls and the fan runs far out on its curve. A motor sized only for the firing duty overloads and trips. The motor should be sized for the cooling point or the speed limited during cooling.
Cooling rate too fast or too slow
Heat treatment specifications set cooling rates. Too much air cools the load unevenly and risks distortion; too little extends the cycle. Variable speed control on measured cooling rate keeps it within limits. Distributing the cooling air evenly across the burners, rather than through a few, also reduces temperature differences across large or thick-walled parts.
Turndown at soak
At soak the burners fire at low rate. A fixed speed fan throttled hard gives poor air control at the burners. Speed control on header pressure gives stable low fire. Where burners are pulse-fired at soak, the fan must also hold steady header pressure as the burners switch between high and low fire, so the speed loop needs to be fast.
Energy and control
Bogie hearth furnaces run long cycles with most of the time at low fire. A variable speed combustion air fan saves considerable power at soak, and allows controlled cooling at the right rate without a separate cooling fan. Correct air to fuel ratio across the cycle saves the most fuel.
ID fan and FD fan in bogie hearth furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | End of flue, after dilution | Beside the furnace, feeding the burners |
| What it handles | Flue gas, then cooling air | Ambient air for firing and cooling |
| Temperature at the fan | 250 to 400°C, varying through the cycle | Ambient |
| Main design concern | Thermal cycling fatigue | Two duty points, firing and cooling |
| Control | Furnace pressure through the cycle | Header pressure, then cooling rate |
See also the Induced Draft (ID) Fan for bogie hearth furnaces.
Questions buyers ask
Can the combustion air blower be used for cooling a bogie hearth furnace?
Yes. On many bogie hearth furnaces the forced draft fan blows air through the burners with the fuel off for controlled cooling. The fan and motor must be selected for both the firing duty and the higher flow during cooling. Variable speed control then sets the cooling rate required by the heat treatment specification.
Why does our bogie furnace blower trip during cooling?
During cooling the burner valves open fully and system resistance drops, so the fan delivers more air and draws more power. If the motor was sized for firing only, it overloads. Limiting speed during cooling or uprating the motor solves this. The cooling and firing points should both be checked on the fan curve.
What pressure does a bogie hearth furnace combustion air fan need?
High velocity burners typically need 300 to 500 mmWC at the burner, so the fan delivers 400 to 700 mmWC including header losses. The burner data sheet gives the exact requirement. The flow is set from the maximum firing rate with excess air, and the forced cooling flow is checked as a second duty point on the same curve.
Information needed to quote an FD fan for bogie hearth 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.
- Firing and cooling duties. Air flow and pressure for firing, and the flow and cooling rate needed for forced cooling.
- Burners. Number of burners and their air pressure requirement.
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.