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 ID fan sits
- Burning zones
- Upper shaft and stone preheating
- Kiln top offtake
- Recuperator
- Bag filter
- ID fan
- Stack
The fan sits after the bag filter. Upstream are the lime cooling zone, the burning zones, the stone column, the recuperator and the filter, so the suction at the fan is high, typically 500 to 900 mmWC. All the cooling air drawn through the base and the combustion products pass through it, and the kiln top must be held at a set suction so that the ejector circulation and burner chambers work as designed.
Gas at the ID fan
| Gas temperature, normal | 120 to 180°C after the recuperator and filter |
| Maximum or upset | Up to 250°C when the recuperator is bypassed or fouled |
| Dust at the fan | Low after the filter; fine lime dust if bags fail |
| Moisture and dew point | Moderate; dew point about 50 to 60°C |
| Corrosive species | SO₂ from fuel oil or coal-derived gas, mostly absorbed by lime |
| Density at the design temperature | About 0.82 kg/m³ at 160°C, 68% 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 annular shaft lime kilns
| Blade form | Backward curved, for efficiency at high suction on clean gas |
| Material of construction | Mild steel or boiler quality plate |
| Drive and arrangement | Direct coupled, with variable speed drive |
| Bearings and cooling | Shaft cooling disc where temperature can exceed about 200°C; pedestal bearings |
| Flow control | Variable speed drive on kiln top suction |
| Also specified | Casing drain, inspection door and vibration monitoring |
What this ID fan has to cope with
Suction governs calcination
Because the cooling air and part of the combustion air are drawn in by the ID fan, any drift in fan speed changes the air flow through the lime. The burning zone moves and the lime quality varies. Close speed control on kiln top suction keeps the kiln stable.
Recuperator fouling
Dust in the waste gas fouls the recuperator tubes, raising pressure drop and cutting drive air temperature. The fan must have pressure margin for a fouled recuperator, and the duty is checked at both clean and fouled conditions.
High suction and casing leakage
At 500 to 900 mmWC suction, small leaks at the fan shaft seal, inspection door and duct joints draw in noticeable air, raising volume and power. Welded casings, good seals and gasketed doors matter on this duty.
Energy and control
The ID fan is the largest drive on an annular shaft kiln and runs at high suction continuously. A variable speed drive matched to kiln output gives substantial savings at reduced production, and reducing leakage at the kiln top, recuperator and filter keeps the gas volume down. A clean recuperator also lowers the pressure the fan must develop.
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 Forced Draft (FD) Fan for annular shaft lime kilns.
Questions buyers ask
Does an annular shaft kiln run under pressure or suction?
Under suction. The ID fan draws the waste gas and the cooling air through the annular shaft kiln, while the ejectors circulate hot gas inside it. This is different from a PFR kiln, which runs under pressure from its blowers.
What suction does an annular lime kiln ID fan develop?
Typically 500 to 900 mmWC, covering the lime and stone column, the recuperator and the bag filter. The fan needs margin for a fouled recuperator and for dense, fine stone.
Why does lime quality change when the ID fan speed changes?
The fan draws the cooling and combustion air through the kiln. A change in speed changes the air flow, which moves the burning zone and alters calcination. Steady speed control on kiln suction keeps lime quality consistent.
Information needed to quote an ID fan for annular shaft 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.
- Recuperator. Gas pressure drop across the recuperator, clean and fouled, and gas temperature after it.
- Kiln suction. Required suction at the kiln top and the cooling air quantity.
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.