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 FD fan sits
- Air intake
- FD (primary air) fan
- Burner
- Expansion drum
- Cooler air fan
- Product cooler
- Secondary air to kiln
The primary air fan feeds the burner at the discharge end of the expansion drum; it carries coal or pet coke where a solid fuel is fired. The cooler air fan pushes ambient air through the hot product in the cooler, and much of that heated air is drawn into the kiln hood by the ID fan as secondary combustion air. Neither fan sees kiln gas or dust under normal conditions.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Secondary air | Heated by the product in the cooler to several hundred °C, downstream of the cooler fan |
| Dust | Clean ambient air; the cooler fan intake is often near dusty product handling and needs a filter |
| Pressure | Moderate for the cooler fan; higher for a burner primary air fan with swirl and axial air |
| Fuel conveying | With pulverised coal or pet coke, the primary air also conveys the fuel |
| 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 expanded clay aggregate kilns
| Blade form | Backward curved for burner air on clean intake; radial or paddle wheel where the fan also conveys fuel |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven, with a variable speed drive |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive linked to the burner firing rate |
| Also specified | Intake filter and silencer, and flow measurement for air to fuel ratio |
What this FD fan has to cope with
Flame shape and bloating
The bloating window is narrow, so a flame that lengthens or wanders changes the expansion. Burner primary air must be steady and measured, and the fan selected so its operating point stays stable at low firing rates.
Dusty intake air
Cooler and burner fans often sit beside screening and conveying of the product. Unfiltered intake air carries dust into the fan and burner. An intake filter and a fan placed away from product handling avoid it.
Cooler air balance
If the cooler fan supplies more air than the kiln can take as secondary air, the excess must be vented. The cooler fan duty is matched to the kiln draft so that hot air is not wasted.
Energy and control
The burner and cooler fans run with the kiln, at loads set by the firing rate. Variable speed control lets the air follow the fuel accurately, which keeps excess air low and fuel use down. Using as much cooler air as possible as hot secondary air recovers heat from the product and reduces the fuel the burner must fire.
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 Induced Draft (ID) Fan for expanded clay aggregate kilns.
Questions buyers ask
Which fan is the FD fan on a LECA kiln?
Usually the burner primary or combustion air fan. A separate cooler air fan supplies air to cool the product, and that heated air returns to the kiln as secondary air, so both fans together form the forced draft side of the kiln.
Does the cooler air fan handle hot air?
No. It takes ambient air and blows it through the hot product; the air heats up downstream of the fan. Only a fan that recirculates or exhausts cooler air sees hot, dusty air.
Why must the combustion air be steady on an expanded clay kiln?
Clay bloats only within a narrow temperature range. Unsteady air changes the flame length and the temperature profile, which moves the expansion zone and gives pellets of the wrong density.
Information needed to quote an FD fan for expanded clay aggregate 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.
- Burner type. Burner make and type, fuel, and required primary air pressure and share of total air.
- Cooler. Cooler type, cooling air flow and the share returned to the kiln.
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.