How expanded clay aggregate kilns work
Expanded clay aggregate is made by forming a bloating clay into pellets, drying and preheating them, then firing them in a rotary kiln at about 1,100 to 1,200°C, where gas released inside the softening clay expands each pellet into a light, cellular granule. Many plants use two drums: a drying and preheating drum followed by a shorter expansion drum, with the burner at the discharge end firing gas, oil, coal or pet coke. The kiln gas travels counter to the material and leaves the feed end at about 250 to 450°C with clay dust. It is cooled by dilution air or water spray, cleaned in cyclones and a bag filter and drawn out by the ID fan. The hot product is cooled in a rotary, grate or fluid bed cooler whose air partly returns to the kiln as secondary air.
Where the ID fan sits
- Expansion drum
- Preheating drum
- Dust chamber
- Gas cooling
- Cyclones
- Bag filter
- ID fan
- Stack
The fan follows the bag filter, so the bulk of the clay dust is removed before it, though a share of fines bypasses during filter faults. Its suction covers the kiln hood draft, both drums, the dust chamber, the cooler or quench, the cyclones and the filter, typically 200 to 350 mmWC. Because the kiln draft also decides the flame shape and the temperature profile, the fan is the operator's main tool for holding the expansion zone where it should be.
Gas at the ID fan
| Gas temperature, normal | 170 to 230°C after gas cooling and the bag filter |
| Maximum or upset | Up to 300°C if the spray or dilution fails |
| Dust at the fan | Fine clay and shale dust; low after the bag filter, several g/m³ if the filter is bypassed |
| Moisture and dew point | Moisture from pellet drying and spray cooling; acid dew point raised by SO₂ |
| Corrosive species | SO₂ from pyrite in the clay and from fuel sulphur, especially with pet coke |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 expanded clay aggregate kilns
| Blade form | Backward curved plate blades, with a wear allowance at the leading edge |
| Material of construction | Mild steel with wear-resistant liners at the blade inlet and casing scroll |
| Drive and arrangement | Arrangement 3 or 8, coupled, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a cooling disc; oil bath bearings on larger units |
| Flow control | Variable speed drive on kiln hood draft |
| Also specified | Insulation, inspection and clean-out doors, and a high temperature trip |
What this ID fan has to cope with
Erosion by clay fines
Expanded clay plants generate fine dust from pellet abrasion in both drums. If the bag filter is bypassed for temperature protection, the fan sees heavy dust for hours. Wear liners at the leading edges and casing, and a conservative tip speed, keep erosion within an overhaul cycle.
Sulphur and corrosion
Pyritic clays and pet coke firing release SO₂, and the gas can fall below its acid dew point during start-up or after over-spraying in the cooling stage. Holding the fan inlet above the dew point and draining the casing reduce corrosion.
Draft swings and product quality
Bloating depends on a narrow temperature window. Unstable kiln draft moves the flame and the expansion zone, giving dense or overburnt pellets. A speed-controlled fan with fast, stable response holds the hood draft within a few mmWC.
Energy and control
The kiln runs continuously, but throughput and fuel change with clay quality and target density. A variable speed drive on the ID fan follows these changes without damper losses. Reducing air leakage at the kiln seals and making better use of cooler air as secondary air lower both the fuel rate and the gas volume the fan must handle.
ID fan and FD fan in expanded clay aggregate kilns
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | At the burner and product cooler |
| What it handles | Kiln gas with clay dust and SO₂ | Ambient combustion and cooling air |
| Temperature at the fan | 170 to 230°C | Ambient |
| Main design concern | Clay dust erosion and kiln draft | Stable flame and air to fuel ratio |
| Control | Kiln hood draft | Burner firing rate |
See also the Forced Draft (FD) Fan for expanded clay aggregate kilns.
Questions buyers ask
What does the ID fan do on a LECA kiln?
It draws the gas from the expansion drum back through the preheating drum and gas cleaning, and holds the kiln hood at a small suction. The draft it sets controls how heat reaches the drying and preheating zones and where the expansion zone sits.
Does an expanded clay kiln ID fan need wear protection?
Usually yes. Clay pellets abrade as they tumble and the fines are carried with the gas. Even with a bag filter, filter bypass during upsets sends dust to the fan, so wear liners at the blade inlet and casing are a sensible specification.
How is the kiln gas cooled before the bag filter?
By dilution with ambient air, by a water spray in a conditioning tower, or by an air-to-air cooler. Spray cooling keeps the gas volume lower but adds moisture and raises the dew point, which the fan design must allow for.
Information needed to quote an ID fan for expanded clay aggregate 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.
- Clay and fuel. Sulphur content of the clay and the fuel, and whether pet coke or coal is fired.
- Gas cooling method. Dilution air, water spray or heat exchanger, and the gas temperature at the filter inlet.
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.