How ceramic tableware kilns work
Ceramic tableware, whether vitrified hotelware, hard porcelain, bone china or stoneware, usually goes through two or three firings. A bisque firing hardens the body, a glost firing matures the glaze, and a decoration firing fuses decals or on-glaze colours at about 750 to 850°C. Fast roller kilns fire single layers of plates in a few hours; tunnel kilns with refractory setters handle mixed ware. Glost temperatures run from about 1,050 to 1,150°C for bone china up to 1,350 to 1,400°C for hard porcelain, which is fired with a reducing period. Kilns are gas or LPG fired. An ID fan exhausts flue gas from the preheat end, an FD fan supplies combustion air, and decoration kilns add a fume exhaust for the organic media burnt off decals.
Where the FD fan sits
- Inlet filter
- FD fan
- Air main
- Burner ratio valves
- Burners above and below rollers
- Kiln
The fan sits beside the firing zone and supplies an air main that branches to dozens of small burners, often arranged above and below the roller plane in several control zones. Each zone sets its own firing rate, so total air demand changes continuously. Burner pressure needs are moderate, and the fan typically delivers 250 to 500 mmWC. Some roller kilns recover warm air from the rapid cooling section for combustion; then the fan sees 100 to 200°C.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Recovered air option | 100 to 200°C from the cooling section, where used |
| Dust | Clean air; filtered where glaze dust is present |
| Pressure at the fan outlet | Typically 250 to 500 mmWC |
| Load pattern | Continuous, with dips during product changes and gaps in loading |
| 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 ceramic tableware kilns
| Blade form | Backward curved, for a stable pressure curve |
| Material of construction | Mild steel; heat-resisting impeller for recovered hot air |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; cooling disc for the recovered air option |
| Flow control | Variable speed drive on air main pressure |
| Also specified | Inlet filter, silencer and low pressure interlock |
What this FD fan has to cope with
Zone interaction
When upper and lower burner zones change firing together, the air main pressure dips and every other zone sees a ratio change. On fast firing cycles this shows up as colour variation between plates. A fan with a steep curve, a correctly sized main and a fast speed loop on main pressure keep the zones independent.
Excess air from an oversized fan
An oversized fan running against nearly closed burner valves tends to leak air through valves at low fire, so the kiln runs with more excess air than set. The extra air cools the zone and wastes fuel. Sizing the fan for the real burner demand avoids it.
Noise in the kiln hall
Combustion air fans feeding many small burners run at two-pole speed and often stand close to the kiln walkway where operators work all shift. Inlet noise is the main source. An inlet silencer, acoustic lagging on the casing and outlet duct, and placing the fan away from the loading and unloading stations keep noise in the kiln hall within acceptable limits.
Energy and control
Tableware kilns change product often, and the burner load falls during gaps. A variable speed drive holding the air main pressure tracks these changes and avoids running the fan at full speed against closed valves. The larger saving usually comes from correct air-fuel ratio across all zones, which a steady main pressure makes possible.
ID fan and FD fan in ceramic tableware kilns
| ID fan | FD fan | |
|---|---|---|
| Position | Kiln entrance end, after the preheat zone | Beside the firing zone, feeding the burner main |
| What it handles | Flue gas with moisture and volatiles | Ambient or recovered warm air |
| Temperature at the fan | 180 to 280°C | Ambient, or 100 to 200°C recovered |
| Main design concern | Removing volatiles before they spoil the glaze | Steady main pressure across many zones |
| Control | Preheat zone pressure | Air main pressure |
See also the Induced Draft (ID) Fan for ceramic tableware kilns.
Questions buyers ask
What does the combustion air fan on a roller kiln do?
It supplies air through a common main to all the burners above and below the roller plane. Each zone has valves that set the air in proportion to the gas. The fan's job is to keep the main pressure steady so that every burner gets the right air whatever the other zones are doing.
What pressure does a tableware kiln burner air fan need?
Typically 250 to 500 mmWC at the fan outlet, depending on the burner type, the length of the air main and the losses in the ratio valves. High velocity burners on tunnel kilns need more than the small nozzle-mix burners used on many roller kilns. The burner supplier's pressure requirement at full firing is the starting point for the fan selection.
Is the FD fan separate from the cooling air fan on a tableware kiln?
Yes. The FD or combustion air fan supplies only the burners. Rapid cooling and final cooling air are delivered by separate fans, and the hot air from the cooling section is removed by its own exhaust fan, often for use in the ware dryers. Each fan has a different duty, so they are selected separately rather than as one large unit.
Information needed to quote an FD fan for ceramic tableware 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 zones. Number of zones and burners, burner type and air pressure needed at the burner.
- Recovered air. Whether combustion air is taken from the cooling section and at what temperature.
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.