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 FD fan sits
- Inlet filter and silencer
- Combustion air FD fan
- Reversing valves
- Top of burning shaft
- Stone bed
- Crossover channel
The combustion air blower stands in a blower house near the kiln and delivers through reversing valves to whichever shaft is burning. The air passes down the burning shaft, through the crossover channel and up the regenerating shaft, so the pressure is high, often 2,000 to 4,500 mmWC depending on shaft height and stone size. During reversal the blower is unloaded through a vent valve and then reloaded on the other shaft.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | High; often 2,000 to 4,500 mmWC |
| Flow pattern | Constant during burning, unloaded at each reversal |
| Other air supplies | Cooling air blower at the shaft bottoms, lance cooling air blower |
| 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 parallel flow regenerative lime kilns
| Blade form | High pressure centrifugal, radial or backward curved at high tip speed; positive displacement blowers are common at the top of the range |
| Material of construction | High-tensile steel impeller for the tip speed |
| Drive and arrangement | Direct coupled, two-pole speed, with standby |
| Bearings and cooling | Bearings with temperature and vibration monitoring |
| Flow control | Constant flow by speed control, with a blow-off valve for reversal |
| Also specified | Inlet silencer, discharge silencer and acoustic enclosure |
What this FD fan has to cope with
Constant air against varying pressure
As stone size and fines change, the bed resistance rises and falls. A centrifugal blower with a flat curve lets the air flow drift and spoils the air-fuel ratio. A steep-curved high pressure fan with flow control, or a positive displacement blower, holds the air steady.
Unloading at reversal
The blower must not dead-head when the reversing valves close. A blow-off valve opens before the valves move and closes after, so the blower runs to atmosphere briefly instead of surging.
Noise
High pressure blowers at two-pole speed are loud, and the blow-off at every reversal adds a repeated noise burst. Silencers on the inlet and the blow-off line and an enclosure are usually specified.
Energy and control
The combustion and cooling air blowers together are the main electrical load of a PFR kiln, running at high pressure all the time except during the brief reversals. Matching blower flow to the kiln output by speed control rather than venting saves power. Keeping stone fines out of the kiln lowers the pressure needed, which reduces blower power directly.
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 Induced Draft (ID) Fan for parallel flow regenerative lime kilns.
Questions buyers ask
What pressure does a PFR lime kiln combustion air blower need?
Typically 2,000 to 4,500 mmWC, because the air passes both stone columns in series. The exact figure depends on shaft height, stone size and fines, and the kiln supplier's design.
Can a centrifugal fan replace a Roots blower on a PFR kiln?
In many cases, yes, where the pressure is in the range a high pressure centrifugal fan can deliver efficiently. A centrifugal fan needs flow control to hold constant air against changing bed resistance, and a blow-off arrangement for reversals.
Why does a PFR kiln have several blowers?
Combustion air, cooling air and lance cooling air have different flows and pressures, and each must keep running correctly through the reversal cycle. Separate blowers for each duty make control simpler.
Information needed to quote an FD fan for parallel flow regenerative lime kilns
- 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.
- Blower duties. Flow and pressure for combustion, cooling and lance cooling air, and whether replacing existing positive displacement blowers.
- Reversal sequence. How the blowers are unloaded during reversal and the cycle timing.
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.