How sanitaryware tunnel kilns work
Vitreous china washbasins, closets and cisterns are fired once, body and glaze together, at about 1,200 to 1,250°C in a gas or LPG-fired tunnel kiln with a cycle of 12 to 20 hours. Kiln cars carry lightweight refractory setting with large, thick-walled pieces that crack if heating or cooling is uneven. High-velocity burners fire into the gaps between the setting to drive circulation and even out the temperature from top to bottom of the car. Flue gas moves back towards the entry through the preheating zone, where the remaining moisture and organic binders leave the ware, and is drawn off by the flue gas exhaust fan. Rapid cooling, final cooling and hot air recovery fans serve the exit end.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Zone control valves
- High-velocity burners
The fan feeds a header along both sides of the firing zone, with control valves for each zone and a ratio valve at each burner. High-velocity burners need air at high pressure so the exit velocity is enough to stir the kiln atmosphere and reach between the pieces, typically 400 to 700 mmWC at the fan. The fan sits beside the kiln, away from the hot exit end.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean; glaze spray and fettling dust in the factory |
| Pressure at the fan outlet | Typically 400 to 700 mmWC |
| Burner count | Several dozen burners in the firing zone |
| 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 sanitaryware tunnel kilns
| Blade form | Backward curved high-pressure, with a stable rising curve |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on header pressure |
| Also specified | Inlet filter and silencer; standby fan with automatic changeover |
What this FD fan has to cope with
Falling burner velocity at low fire
High-velocity burners depend on air pressure for jet velocity. If the header pressure falls when firing drops, the burners lose their stirring effect and the temperature spread in the setting widens. Holding header pressure constant with speed control, while the burner valves set each burner's flow, keeps the jets effective at all firing rates.
Glaze dust and overspray at the inlet
Sanitaryware plants spray glaze, and fine overspray drawn into the fan coats the blades and burner air passages. An inlet filter and a duct taken from outside the glazing area avoid deposits and unbalance. The filter is easy to change, and its pressure drop is allowed for in the fan duty so the burners do not run short as it loads.
Loss of air during firing
A combustion air fan trip stops firing in the middle of a long cycle, and the large pieces in the firing zone can crack as the kiln cools unevenly. A standby fan with automatic changeover and a reliable power supply protect the ware in the kiln.
Energy and control
The combustion air fan runs continuously at a high pressure, so it consumes noticeable power. A variable speed drive on header pressure avoids running at full speed against nearly closed burner valves. Fuel is the larger cost, and accurate air-fuel ratio control at each burner, with excess air kept low, saves more than any fan change.
ID fan and FD fan in sanitaryware tunnel kilns
| ID fan | FD fan | |
|---|---|---|
| Position | At the kiln entry, after the preheating zone | Beside the firing zone, feeding the burner header |
| What it handles | Humid flue gas with binder vapour | Clean ambient air |
| Temperature at the fan | 180 to 250°C | Ambient |
| Main design concern | Steady preheating zone pressure | High, steady burner air pressure |
| Control | Preheating zone pressure | Header pressure |
See also the Induced Draft (ID) Fan for sanitaryware tunnel kilns.
Questions buyers ask
Why do sanitaryware kiln burners need high air pressure?
High-velocity burners use the momentum of the flame jet to stir the kiln atmosphere and drive hot gas between the large pieces. This needs air at a high pressure, typically 400 to 700 mmWC at the fan, so the combustion air fan is a high-pressure machine rather than a simple low-pressure blower.
Is the FD fan on a sanitaryware kiln the same as the cooling fan?
No. The FD fan supplies combustion air to the burners in the firing zone. Cooling air at the exit end comes from separate rapid cooling and final cooling fans, and hot air from the cooling zone is often recovered for the dryers by yet another fan.
Should a sanitaryware tunnel kiln have a standby combustion air fan?
Yes in most cases. A trip during firing stops the burners and can crack large pieces in the firing zone. A standby fan with automatic changeover, tested at planned stops, protects both the ware and the kiln lining, and is cheap compared with a lost kiln load.
Information needed to quote an FD fan for sanitaryware tunnel 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.
- Burners. Number and type of burners and the air pressure required at each.
- Standby. Whether a standby fan or automatic changeover is required.
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.