How vertical shaft lime kilns work
A vertical shaft lime kiln burns limestone in a vertical refractory shaft, with stone fed at the top and lime drawn from the bottom. In the common mixed-feed kiln, coke or anthracite is charged with the stone and burns within the bed; gas- or oil-fired shaft kilns use burners or lances at the burning zone. Air enters at the bottom, cools the lime, and rises through the burning zone and the preheating stone, leaving the top at 100 to 300°C. Because the fuel burns in the bed with little excess air, the kiln gas is rich in CO₂, typically 30 to 40% on mixed-feed kilns, which is why sugar, precipitated calcium carbonate and soda plants use these kilns to supply CO₂. The top gas is cleaned in a cyclone and bag filter or a scrubber.
Where the FD fan sits
- Inlet filter
- FD fan (air blower)
- Air ring or bottom cone
- Lime cooling zone
- Burning zone
The blower stands near the kiln base and delivers air through a distribution ring or cone below the cooling zone. The air must pass the lime column, the burning zone and the preheating stone, typically 300 to 800 mmWC at the blower outlet. The lime discharge mechanism must be sealed so that the air does not short-circuit out of the bottom. On gas- or oil-fired shaft kilns, a separate combustion air fan feeds the burners.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Lime dust near the discharge area; filtered intake advisable |
| Pressure at the fan outlet | Typically 300 to 800 mmWC |
| Variation | Changes with stone size, fines content and draw rate |
| Air after the bed | Heated by the lime before reaching the burning zone, downstream of the fan |
| 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 vertical shaft lime kilns
| Blade form | Backward curved, with a steep curve for changing bed resistance |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive or inlet damper on air flow |
| Also specified | Inlet filter, flow measurement and non-return damper |
What this FD fan has to cope with
Fines raising bed resistance
Undersized stone and fines fill the gaps in the column and raise resistance, reducing air flow and moving the burning zone. Screening the stone helps the kiln; a blower with pressure margin and a steep curve keeps the air flowing meanwhile.
Air channelling at the wall
Air takes the easier path along the shaft wall, leaving the centre underburnt. A well-designed distribution cone and enough blower pressure to force air into the centre give even calcination.
Leakage at the discharge
Air escaping through the lime discharge gate wastes blower power and carries dust into the shed. Sealed discharge gates or rotary valves keep the air in the kiln.
Energy and control
The bottom air blower runs continuously and its air is preheated by the lime, so the quantity matters for fuel use as well as power. A blower matched to the actual bed resistance with speed control, rather than throttled, saves energy, and screening fines from the stone reduces the pressure it needs.
ID fan and FD fan in vertical shaft lime kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and filter or washer | At the kiln base, below the lime column |
| What it handles | CO₂-rich kiln top gas | Ambient air |
| Temperature at the fan | 120 to 200°C dry; 40 to 60°C washed | Ambient |
| Main design concern | Dense gas, CO and lime dust | Pressure through the stone column |
| Control | Kiln top pressure | Air flow |
See also the Induced Draft (ID) Fan for vertical shaft lime kilns.
Questions buyers ask
What pressure does a vertical lime kiln blower need?
Typically 300 to 800 mmWC, depending on shaft height, stone size and fines content. Kilns fed with small or poorly screened stone need more pressure.
Is a positive displacement blower used on lime shaft kilns?
Some small kilns use positive displacement blowers, but a centrifugal fan with a steep curve and speed control usually serves well and is quieter and easier to control for air flow.
Does the FD fan on a shaft lime kiln handle hot air?
No. It blows ambient air into the base of the vertical lime kiln. The air is heated by the burnt lime above, after it leaves the fan.
Information needed to quote an FD fan for vertical shaft 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.
- Kiln and stone. Shaft dimensions, stone size range and output.
- Air distribution. Arrangement of the air inlet at the kiln base and the discharge seal.
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.