How clay roofing tile kilns work
Clay roofing tiles, from traditional Mangalore pattern tiles to modern interlocking and ridge tiles, are fired at about 950 to 1,100°C. Traditional plants use intermittent downdraught or updraught kilns fired with wood, coal or biomass, drawing on a chimney. Modern plants fire tiles standing in H-cassettes or on refractory setters in tunnel kilns, and fast roller kilns are used for some profiles. Fuel is natural gas, LPG, producer gas or fuel oil. Roofing tile clays often carry fluorine, sulphur and chlorine, released as HF, SO₂ and HCl in the preheat zone between about 500 and 900°C, so newer kilns pass the flue gas through a dry lime absorber. The ID fan draws gas out of the preheat end and through any absorber; the FD fan supplies combustion air.
Where the FD fan sits
- Inlet filter
- FD fan
- Air main
- Burner valves
- Side or roof burners
- Kiln
The fan sits beside the firing zone and feeds an air main serving burners along both side walls, or in the roof on some kilns. High velocity burners need about 300 to 500 mmWC at the burner, so the fan typically delivers 400 to 700 mmWC. On traditional downdraught kilns firing wood or coal there is usually no FD fan; air enters by natural draft through the fire boxes, and a small blower is added only where burners or underfiring are fitted.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clay and lime dust around tile plants; filtered intake needed |
| Pressure at the fan outlet | Typically 400 to 700 mmWC |
| Turndown | Moderate; push rate changes with product and demand |
| Recovered air | Some kilns use warm cooling air at 100 to 200°C for combustion |
| 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 clay roofing tile kilns
| Blade form | Backward curved, high pressure |
| Material of construction | Mild steel; heat-resisting impeller for recovered warm air |
| Drive and arrangement | Direct coupled, two-pole speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter, silencer and low pressure interlock to the burner system |
What this FD fan has to cope with
Uneven firing across cassettes
Tiles at the centre of a cassette stack fire cooler than those at the outside if the burner jets are weak. When the air main pressure is low, the outer tiles over-fire while inner ones stay porous. Selecting the fan for the burners' full rated pressure, with margin for the main, keeps jet momentum high.
Dusty intake air
Tile plants handle dry clay, glaze, engobe and limestone granules for the absorber, and the air around the kiln carries fine abrasive dust. Unfiltered, it wears burner nozzles and ratio valves and builds up on the fan impeller, upsetting balance. A filtered intake drawing from a clean area, with a pressure gauge across the filter, avoids both problems at low cost.
Burner air loss in power dips
A short dip in the supply slows the fan, and the burners lose air before the flame safeguard reacts, giving smoky, rich combustion and soot on the tiles. In rural tile clusters such dips are common. A variable speed drive with ride-through capability, correctly set low air pressure interlocks and a fan with some inertia handle this without nuisance trips.
Energy and control
The combustion air fan runs with the kiln around the clock. Using a variable speed drive on air main pressure instead of a throttled damper reduces power when the kiln push rate is lowered for slower-selling products. Keeping the air-fuel ratio correct at every burner, which depends on stable main pressure, has a larger effect on fuel cost than the fan's own power.
ID fan and FD fan in clay roofing tile kilns
| ID fan | FD fan | |
|---|---|---|
| Position | Kiln entrance, after any lime absorber | Beside the firing zone, feeding the burners |
| What it handles | Flue gas with HF, SO₂ and moisture | Ambient filtered air |
| Temperature at the fan | 150 to 250°C | Ambient |
| Main design concern | Acid corrosion and absorber resistance | Even burner jet momentum |
| Control | Preheat zone pressure | Air main pressure |
See also the Induced Draft (ID) Fan for clay roofing tile kilns.
Questions buyers ask
Does a tile kiln need a combustion air fan?
A modern gas or oil fired tunnel or roller kiln does: the FD fan supplies air to every burner. Traditional wood or coal fired downdraught kilns usually do not, as air enters naturally through the fire boxes. A blower is added there only if burners are installed.
What pressure does a roofing tile kiln burner fan need?
High velocity burners on tile kilns usually need around 300 to 500 mmWC at the burner. With the air main and valve losses, the fan delivers about 400 to 700 mmWC. The burner manufacturer's data at full firing sets the final figure, and a margin is kept for future burners.
Why are high velocity burners used on tile kilns?
Tiles stand close together in cassettes, and hot gas must be driven into the setting to fire the middle tiles evenly. High velocity burner jets stir the kiln atmosphere and drive heat through the cassettes. They depend on the combustion air fan to supply full pressure.
Information needed to quote an FD fan for clay roofing tile 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 arrangement. Number, position and type of burners and their air pressure at full firing.
- Intake location. Where the fan will draw air from and the dust conditions there.
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.