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 ID fan sits
- Firing zone
- Preheat zone
- Exhaust duct
- Lime absorber or scrubber
- ID fan
- Stack
The fan stands at the entrance end of the tunnel kiln. Gas leaves the preheat zone through roof or side ports, passes a cascade lime absorber or dry sorbent filter where fitted, and reaches the fan. The kiln draft is small, but the absorber adds 50 to 150 mmWC, so total suction at the fan is typically 100 to 250 mmWC. On traditional downdraught kilns, an ID fan is sometimes added at the chimney base to speed firing and give more even draft.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C at the preheat exit |
| Maximum or upset | 300 to 350°C during fast firing or kiln stoppage |
| Dust at the fan | Low after an absorber; lime and clay fines if the absorber is bypassed |
| Moisture and dew point | Moisture from tiles that are not fully dry plus combustion water; dew point 50 to 65°C |
| Corrosive species | HF, SO₂ and HCl from the clay; acid dew point well above the water dew point |
| 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 clay roofing tile kilns
| Blade form | Backward curved, on gas cleaned by the absorber |
| Material of construction | Mild steel with corrosion allowance where gas stays hot; stainless steel or coated casing where the fan sees cold starts or wet scrubbing |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9, insulated casing |
| Bearings and cooling | Cooling disc and heat shield |
| Flow control | Variable speed drive on preheat zone pressure |
| Also specified | Casing drain and inspection door; differential pressure monitoring across the absorber |
What this ID fan has to cope with
Fluoride attack below the dew point
Tile clays can release substantial fluorine. HF condenses with water into a strongly corrosive acid that attacks mild steel and even some stainless grades. The fan casing, inlet box and duct must be kept above the acid dew point by insulation, or built from suitable resistant material where condensation cannot be avoided, such as at start-up or after a wet scrubber.
Absorber resistance rising
A cascade lime absorber gains resistance as the limestone granules coat with fluoride and sulphate. The ID fan has to hold kiln draft as the drop rises between maintenance. Selecting the fan for the fouled absorber pressure drop, not the clean figure, with speed control to cover the range, keeps the kiln on its pressure.
Volatile release and tile defects
If the exhaust is weak or unsteady, fluorides and sulphur compounds released in preheat linger around the setting and react with the surface of cooler tiles further back, causing white scumming, discolouration and patchy colour on engobed or glazed tiles. Steady suction from the fan, held on preheat pressure rather than set once by hand, removes the volatiles before they reach the cooler ware.
Energy and control
Tunnel kilns for roofing tiles run continuously, but kiln push rate changes with product mix and market demand. The fan on a variable speed drive follows the firing rate and the slowly rising absorber resistance, rather than running at full speed against a throttle. Avoiding over-draft also stops cold air being drawn in at the entrance door, which keeps fuel consumption per tonne down.
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 Forced Draft (FD) Fan for clay roofing tile kilns.
Questions buyers ask
Why do roofing tile kilns need fluoride control?
Many tile clays contain fluorine, which is released as HF in the preheat zone. HF harms vegetation and health and corrodes steel. Modern tile plants fit a lime absorber or dry sorbent filter after the kiln, and the ID fan must be sized for its resistance and built to resist acid condensate.
What material is best for a clay roof tile kiln ID fan?
Where the gas stays above the acid dew point, mild steel with insulation and a corrosion allowance is used. Where the fan sees cold starts, frequent stops or wet scrubbing, stainless steel or a protective coating on the gas-wetted parts is needed, chosen with the fluoride content in mind.
Can an ID fan be added to a traditional Mangalore tile kiln?
Yes. Fitting an ID fan at the base of the chimney of a downdraught or updraught tile kiln gives stronger, steadier draft, shortens firing time and evens out the temperature. The fan must handle smoky, wet gas from wood or coal firing, so a settling chamber and corrosion protection are usually included.
Information needed to quote an ID fan for clay roofing tile 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 chemistry. Fluorine and sulphur content of the clay, if known, and any emission limits at the site.
- Gas cleaning. Type of absorber or scrubber after the kiln and its clean and fouled pressure drop.
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.