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 ID fan sits
- Furnace zones
- Exhausting burner
- Ceramic regenerator bed
- Reversing valve
- Exhaust gas main
- Exhaust (ID) fan
- Stack
The fan sits after the reversing valves on the common exhaust main and draws gas through whichever bank of burners is exhausting. It works against the burner throat, the bed and the valves, typically 300 to 600 mmWC, higher than on a recuperative furnace because the beds are dense. A furnace pressure controller trims the exhaust flow so that the remaining gas leaving by the conventional flue keeps the hearth at a slight positive pressure.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C after the regenerator beds |
| Maximum or upset | 350 to 450°C if a reversing valve sticks or a bed is undersized or clogged |
| Dust at the fan | Low but persistent: ceramic fines, scale and oxide carried through the beds |
| Moisture and dew point | High with natural gas or coke oven gas; risk of condensation at the low end of the temperature range |
| Flow pattern | Pressure and flow pulse at each reversal, every 30 to 60 seconds |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 regenerative burner reheating furnaces
| Blade form | Backward curved, selected well clear of stall to ride through reversal transients |
| Material of construction | Mild steel or high strength plate; heat-resisting steel where upset exceeds about 400°C |
| Drive and arrangement | Direct coupled, double bearing pedestal with a stiff base |
| Bearings and cooling | Heat shield and cooling disc; oil-lubricated bearings with temperature monitoring |
| Flow control | Variable speed drive on exhaust ratio and furnace pressure |
| Also specified | Inlet temperature trip, casing drain, and a dilution air inlet for high temperature protection |
What this ID fan has to cope with
Reversal transients
Every reversal briefly closes one path and opens another, so the fan sees a sharp change in resistance many times an hour. A fan selected near the peak of its curve can stall and surge during each swap. Choosing the operating point well to the right of the peak, with a stable rising characteristic, keeps the fan steady through the switching.
Temperature spikes from a saturated bed
If a reversing valve sticks or a bed is partly blocked, hot gas passes through without giving up its heat and the fan inlet temperature can double. An inlet temperature trip, an automatic dilution air damper and an impeller designed for the upset temperature protect the fan until the valve is cleared.
Ceramic and oxide fines
Ceramic balls abrade and break, and scale and iron oxide are carried into the beds and through them. The fines are hard and settle in the casing. A blade form tolerant of light dust, a casing drain and inspection access, and periodic bed screening or replacement keep wear and unbalance under control.
Energy and control
Regenerative furnaces already save a great deal of fuel; the exhaust fan is the price paid for drawing gas back through dense beds. Running it on a variable speed drive that follows furnace firing, rather than a fixed speed with a damper, cuts power at the low rates during mill delays. Setting the exhaust ratio correctly keeps both fuel saving and fan power where they should be.
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 Forced Draft (FD) Fan for regenerative burner reheating furnaces.
Questions buyers ask
Why does a regenerative burner furnace need an exhaust fan?
The flue gas must be drawn back through the burner and the ceramic regenerator bed, which have a high resistance. A chimney cannot provide that suction, so an exhaust fan acting as an induced draft fan pulls most of the gas through the beds and discharges it to the stack at about 150 to 250°C.
What temperature does the regenerative furnace exhaust fan handle?
Normally about 150 to 250°C, much cooler than a recuperative furnace fan, because the ceramic beds recover most of the heat. If a reversing valve fails or a bed is clogged, the temperature can rise sharply, so the fan needs an inlet temperature trip and an impeller suited to the upset case.
Why does the ID fan on our regenerative furnace surge?
Each burner reversal changes the system resistance suddenly. A fan selected too close to the peak of its pressure curve stalls briefly at every swap and the pressure hunts. Reselecting the fan, or modifying the impeller, so it runs on the stable part of its curve usually cures the problem.
Information needed to quote an ID fan for regenerative burner reheating 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.
- Regenerator system. Number of burner pairs, reversal interval, bed pressure drop and design exhaust ratio.
- Exhaust temperature. Normal exhaust gas temperature and the highest temperature seen during valve or bed faults.
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.