How parallel flow regenerative lime kilns work
A parallel flow regenerative lime kiln has two or three connected shafts. One shaft burns while the other acts as the regenerator; every 10 to 15 minutes the flows reverse. Fuel is injected through lances into the burning shaft, and combustion air enters at the top, preheated by the hot stone. The gas flows down through the stone, crosses to the other shaft through a channel and flows up, preheating the stone there before leaving the top at a low temperature, typically 70 to 130°C. Cooling air blown in at the bottom of both shafts cools the lime. This regenerative layout gives the lowest fuel use of any lime kiln, and the kiln works under positive pressure, so all air is supplied by blowers and an exhaust fan draws the waste gas through the filter.
Where the ID fan sits
- Burning shaft
- Crossover channel
- Regenerating shaft
- Exhaust gas duct
- Bag filter
- Exhaust (ID) fan
- Stack
The fan sits after the bag filter. The kiln itself runs under pressure from the combustion and cooling air blowers, so the exhaust fan works mainly against the filter and ducts, typically 150 to 300 mmWC, while keeping the gas at the shaft tops at a small set pressure. During each reversal the air supply is stopped and the kiln depressurised, and the fan sees a sharp drop and recovery in flow.
Gas at the ID fan
| Gas temperature, normal | 70 to 130°C at the fan |
| Maximum or upset | Up to 200°C with long reversal cycles or burning close to the top |
| Dust at the fan | Low after the filter; fine lime and stone dust if bags fail |
| Moisture and dew point | Stone moisture and combustion water give a dew point of about 50 to 60°C |
| Flow pattern | Cyclic, dropping during each reversal every 10 to 15 minutes |
| Density at the design temperature | About 0.93 kg/m³ at 110°C, 77% 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 parallel flow regenerative lime kilns
| Blade form | Backward curved, for clean, cool gas after the filter |
| Material of construction | Mild steel with corrosion allowance; coating on casing interior where condensation is frequent |
| Drive and arrangement | Direct coupled, with variable speed drive |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on shaft top pressure, with fast response at reversal |
| Also specified | Casing drain, insulated casing and inlet duct, inspection door |
What this ID fan has to cope with
Condensation at low gas temperature
Exit gas at 70 to 100°C is only a few tens of degrees above its dew point. Cold ducts, cold mornings and stops bring it below, and lime dust in the condensate forms hard scale. Insulation, drains and avoiding long cold stops with gas in the system keep the fan dry.
Reversal transients
At each reversal the blowers stop or vent and the kiln pressure collapses, then recovers. A fan control loop that is too slow lets the shaft top swing between pressure and suction, pulling air through seals or pushing dust out. The control is tuned for the reversal cycle, often with a feed-forward signal from the kiln sequence.
Fatigue from cyclic load
Flow and pressure changes every 10 to 15 minutes add up to tens of thousands of load cycles a year. Welds and shaft design are checked for fatigue, and the rotor is balanced closely to keep stresses low.
Energy and control
The PFR kiln has the lowest fuel use of lime kilns, and its exhaust is cool and moderate in volume, so the exhaust fan is smaller than on a rotary kiln of the same output. A variable speed drive following kiln output and the reversal cycle keeps power down. Air leaks into the filter and ducts raise the gas volume and cool the gas closer to its dew point, so sealing pays twice.
ID fan and FD fan in parallel flow regenerative lime kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Blower house, feeding the burning shaft top |
| What it handles | Cool kiln exhaust near its dew point | Ambient air |
| Temperature at the fan | 70 to 130°C | Ambient |
| Main design concern | Condensation and reversal transients | High pressure and constant flow |
| Control | Shaft top pressure | Air flow, unloading at reversal |
See also the Forced Draft (FD) Fan for parallel flow regenerative lime kilns.
Questions buyers ask
What temperature is the exhaust gas of a PFR lime kiln?
Typically 70 to 130°C, because the gas gives up most of its heat to the stone in the regenerating shaft. This low temperature is why the PFR kiln is efficient, and also why condensation in the filter and exhaust fan is a concern.
Does a PFR lime kiln need an ID fan?
The kiln runs under pressure from its combustion and cooling air blowers, but an exhaust fan is usually fitted after the bag filter to overcome the filter resistance and hold the shaft top pressure steady.
How does the reversal cycle affect the exhaust fan?
At each reversal the air supply stops briefly and the kiln depressurises, so the gas flow to the fan falls and then recovers. The fan control must follow this quickly, and the fan structure must tolerate the repeated load changes.
Information needed to quote an ID fan for parallel flow regenerative lime kilns
- 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.
- Reversal cycle. Reversal interval, its duration and how the blowers are unloaded during it.
- Gas condition. Exhaust temperature range over the cycle and the moisture content of the stone.
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.