How annular shaft lime kilns work
An annular shaft lime kiln has an outer shell and a concentric inner cylinder, with the limestone moving down the ring between them. Burners fire into combustion chambers around the shell at two levels. Hot gas is drawn from the lower burning zone through the inner cylinder and returned by ejectors driven by preheated drive air, which creates a parallel-flow burning zone at the bottom and a counter-current zone above. Cooling air is drawn up from the base through the lime. The waste gas leaves at the kiln top at about 150 to 250°C, passes a recuperator that heats the ejector drive air, and then a bag filter. Unlike the PFR kiln, the annular kiln runs under suction, so the exhaust fan draws all the gas and cooling air through the kiln.
Where the FD fan sits
- Inlet filter
- Drive air FD fan
- Recuperator
- Hot drive air main
- Ejectors
- Kiln
The drive air fan is on the cold side of the recuperator, typically at ground level near the kiln. The air it delivers is heated in the recuperator and distributed to ejectors around the kiln, where its momentum draws hot gas from the inner cylinder and returns it to the lower burning zone. The fan pressure covers the recuperator, the hot main and the ejector nozzles, and is high compared with the cooling air drawn in by the ID fan. A separate fan supplies burner combustion air where needed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the recuperator | About 400 to 500°C, downstream of the fan |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | High; set by the ejector nozzle design |
| Variation | Changes with kiln output and recuperator condition |
| 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 annular shaft lime kilns
| Blade form | Backward curved, at a tip speed suited to the pressure |
| Material of construction | Mild steel; high-tensile impeller at high tip speed |
| Drive and arrangement | Direct coupled, often with a standby fan |
| Bearings and cooling | Standard bearings with monitoring |
| Flow control | Variable speed drive on drive air flow |
| Also specified | Inlet filter, silencer and low flow trip linked to the recuperator protection |
What this FD fan has to cope with
Recuperator overheating on loss of air
The drive air also cools the recuperator tubes, which sit in hot waste gas. If the fan trips, the tubes overheat quickly. A standby fan or an automatic waste gas bypass protects the recuperator.
Ejector performance
The recirculation in the kiln depends on the drive air momentum at the ejectors. If the fan cannot hold pressure as the recuperator fouls, recirculation falls and the lower burning zone runs too hot. A fan with pressure margin and flow control keeps the ejectors working.
Noise
A drive air fan at the kiln base runs at high pressure and tip speed, close to where operators work on the burner platforms and discharge. Inlet silencers and casing insulation are needed in most plants to bring the noise within limits.
Energy and control
The drive air fan runs continuously with the kiln. Its air carries heat recovered from the waste gas back into the kiln, so the right quantity improves fuel use. Speed control on drive air flow, matched to kiln output, saves power over damper control, and a clean recuperator lowers the pressure required.
ID fan and FD fan in annular shaft lime kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the recuperator and bag filter | Before the recuperator, feeding the ejectors |
| What it handles | Waste gas and cooling air from the whole kiln | Ambient drive air |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | High suction and stable draft | Pressure for the ejectors and recuperator cooling |
| Control | Kiln top suction | Drive air flow |
See also the Induced Draft (ID) Fan for annular shaft lime kilns.
Questions buyers ask
What is the drive air fan on an annular shaft kiln?
It is the forced draft fan of the annular shaft kiln. It supplies air through the recuperator to the ejectors, which use the preheated air to recirculate hot gas from the inner cylinder into the lower burning zone.
Does the drive air fan handle hot air?
No. It sits on the cold side of the recuperator and handles ambient air. Placing it after the recuperator would mean a larger fan running at 400 to 500°C, which is avoided.
Why is a standby fan used for drive air?
The recuperator tubes rely on the drive air for cooling. A fan trip with hot waste gas flowing can damage the recuperator, so a standby fan or a fast gas bypass is usual.
Information needed to quote an FD fan for annular 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.
- Ejector duty. Drive air flow and pressure required at the ejectors and the recuperator air side pressure drop.
- Protection. Present arrangement for recuperator protection on a fan trip.
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.