How regenerative burner reheating furnaces work
A regenerative burner reheating furnace uses burners in pairs, each with a compact bed of ceramic balls or honeycomb. While one burner fires, the flue gas is drawn back through its partner, heating that bed; every 30 to 60 seconds reversing valves swap the roles, so the combustion air passes through a hot bed and reaches 900 to 1,100°C. Most of the flue gas, typically 70 to 90%, leaves through the beds at only 150 to 250°C, drawn by an exhaust fan. The remainder escapes through a small conventional flue or the doors. The result is high fuel efficiency on natural gas, coke oven gas or mixed gas, with a low exhaust temperature but a pulsing, reversing flow pattern in both fan systems.
Where the FD fan sits
- Inlet filter and silencer
- Combustion air (FD) fan
- Air main
- Reversing valve
- Ceramic regenerator bed
- Firing burner
The fan supplies a common air main feeding the reversing valves of every burner pair. The air passes the valve, the hot bed and the burner throat, so the fan pressure is higher than on a recuperative furnace, typically 600 to 1,000 mmWC. Because only half the burners fire at any moment, the air flow per burner is high, and the main must be sized so that reversal does not starve the burners furthest from the fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the bed | 900 to 1,100°C, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 600 to 1,000 mmWC |
| Flow pattern | Pulses at each reversal |
| 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 regenerative burner reheating furnaces
| Blade form | Backward curved, high pressure, with a steep stable curve |
| Material of construction | Mild steel; high strength plate for high tip speed |
| Drive and arrangement | Direct coupled at two-pole speed |
| Bearings and cooling | Standard bearings, no cooling |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter and silencer, non-return damper and standby fan on continuous mills |
What this FD fan has to cope with
Pressure dip at each reversal
When the reversing valves move, air flow drops briefly and then surges. A fan with a flat curve lets the main pressure swing, which upsets the air to fuel ratio for the first seconds of each firing period. A steep fan curve and a fast speed loop keep pressure steady through the swap.
Hot gas backflow on air loss
If combustion air fails while the furnace is hot, gas can flow backwards through the bed and valves into the air main. Fast-acting interlocks to close the valves and stop fuel, a non-return damper at the fan outlet, and a standby fan protect the air system.
Filter and bed fouling
Dust entering with the combustion air settles in the ceramic beds, which already collect scale from the exhaust side, raising pressure drop over time. A good inlet filter on the fan protects the beds, and the fan needs a pressure margin to cover gradual fouling between bed cleanings.
Energy and control
On a regenerative furnace the combustion air fan works at higher pressure than on a recuperative furnace, so its power is significant. A variable speed drive matched to firing rate saves power during mill delays and low output. Keeping the beds clean and the air to fuel ratio correct gives larger savings than any change to the fan itself.
ID fan and FD fan in regenerative burner reheating furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the reversing valves on the exhaust main | Before the reversing valves on the air main |
| What it handles | Flue gas drawn back through the beds | Ambient air |
| Temperature at the fan | 150 to 250°C, spikes on valve faults | Ambient |
| Main design concern | Stability through reversals and temperature spikes | High, steady pressure through reversals |
| Control | Exhaust ratio and furnace pressure | Air main pressure |
See also the Induced Draft (ID) Fan for regenerative burner reheating furnaces.
Questions buyers ask
How is the combustion air fan on a regenerative furnace different?
It must deliver higher pressure, typically 600 to 1,000 mmWC, to push air through the reversing valves and ceramic beds, and it must stay stable while the air path switches every 30 to 60 seconds. The forced draft fan still handles ambient air; the beds heat it after the fan.
Does the regenerative burner air fan handle hot air?
No. The combustion air fan handles ambient air. The air is preheated to 900 to 1,100°C in the bed of the firing burner, downstream of the fan, so the fan itself is an ordinary cold air machine, built for high pressure rather than high temperature.
Can we convert a recuperative furnace to regenerative burners with the same FD fan?
Usually not without checking. Regenerative burners need higher air pressure, and the air flow distribution changes. The existing fan is often short of pressure. A revised duty from the burner supplier, compared with the fan curve, shows whether it can be uprated or needs replacing.
Information needed to quote an FD fan for regenerative burner 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 pairs. Number of burner pairs, firing capacity of each and the reversal interval.
- Air side losses. Pressure drop through the reversing valves, beds and burners at full firing.
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.