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 ID fan sits
- Furnace chamber
- Roof or sidewall flue
- Dilution air inlet
- Recuperator (if fitted)
- ID fan
- Stack
The fan sits at the end of the flue, after any recuperator, and usually beside the furnace. Gas leaving the furnace ranges from a few hundred degrees early in the cycle to over 1,000°C at soak, so dilution air is added to keep the fan at about 300 to 400°C. During forced cooling the fan exhausts large volumes of air blown through the chamber, at decreasing temperature. Suction is modest, typically 50 to 150 mmWC.
Gas at the ID fan
| Gas temperature, normal | 250 to 400°C after dilution, varying with the cycle |
| Maximum or upset | 500 to 600°C if dilution air is short at soak |
| Dust at the fan | Low; scale and refractory dust from the bogie seals |
| Cycle pattern | Ramp, soak and cooling over many hours, with cold starts |
| Cooling phase | Large volume of hot air from forced cooling |
| Density at the design temperature | About 0.57 kg/m³ at 350°C, 47% 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 bogie hearth furnaces
| Blade form | Backward inclined plate blades, robust for thermal cycling |
| Material of construction | Heat-resisting steel impeller where gas exceeds about 400°C; mild steel casing with insulation |
| Drive and arrangement | Direct coupled, with sliding casing supports for repeated thermal cycles |
| Bearings and cooling | Cooling disc and heat shield |
| Flow control | Variable speed drive on furnace pressure through all cycle phases |
| Also specified | Dilution air damper on fan inlet temperature; slow turning after shutdown |
What this ID fan has to cope with
Thermal cycling fatigue
A bogie hearth fan heats and cools with every batch, sometimes daily. Repeated expansion and contraction cracks welds at the impeller hub and blade roots and loosens fits. Impeller design for cyclic thermal stress, generous fillet welds, and a shaft fit that stays tight at temperature extend fan life.
Changing flow through the cycle
Flue gas flow is low at soak and high during heat-up and forced cooling. A fan sized for the peak cooling flow runs throttled most of the cycle. Variable speed control on furnace pressure handles the range and reduces power. The control must also be stable at the low soak flows, where a fan with a flat curve tends to hunt and upset furnace pressure.
Air ingress at the bogie seals
The seals between bogie and furnace walls wear and leak. If the fan pulls too much suction, cold air enters around the load and affects temperature uniformity, which matters for heat treatment quality. Holding a small positive pressure protects uniformity. The seals should also be inspected and replaced at regular intervals, since no fan control can make up for a badly worn seal.
Energy and control
A bogie hearth furnace spends most of its cycle at soak, when firing and flue gas flow are low. A variable speed ID fan holding furnace pressure cuts power during the long soak and avoids drawing cold air through the bogie seals, which improves uniformity as well as fuel use.
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 Forced Draft (FD) Fan for bogie hearth furnaces.
Questions buyers ask
Does a bogie hearth furnace need an ID fan?
Many bogie hearth furnaces work with a natural draft flue and damper. An induced draft fan is used when a recuperator is fitted, when the flue is long, or when tight furnace pressure control is needed for temperature uniformity in heat treatment. The fan also handles the forced cooling air on many designs.
What temperature does a bogie hearth furnace ID fan see?
It follows the cycle. Flue gas at soak can exceed 1,000°C, so dilution air brings it to about 300 to 400°C at the fan. The impeller and bearings must handle the highest temperature reached if dilution is short. During forced cooling the fan handles a much larger volume of air at falling temperature, which sets the maximum flow.
How does pressure control affect heat treatment uniformity?
If the furnace runs under suction, cold air enters through bogie seals and door gaps and creates cold spots around the load. Holding a small positive pressure with ID fan control keeps the furnace atmosphere even and helps meet uniformity requirements. It also reduces fuel use and scale on the load.
Information needed to quote an ID fan for bogie hearth 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.
- Heat treatment cycle. Maximum temperature, ramp rates, soak time and forced cooling method.
- Flue arrangement. Dilution air, recuperator and the flue gas temperature at furnace outlet.
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.