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 ID fan sits
- Firing zone
- Preheat zone
- Exhaust ports
- Afterburner (decoration kilns)
- ID fan
- Chimney
The fan stands at the entrance end of the kiln and draws gas counter-current through the preheat zone, where it warms the incoming ware. On roller kilns the exhaust is taken through ports above and below the roller plane; on tunnel kilns through side flues. Decoration kilns burn off the organic decal media between about 250 and 450°C, and their exhaust often passes through a thermal afterburner before the fan. Suction at the fan is modest, typically 30 to 80 mmWC, but it must stay steady.
Gas at the ID fan
| Gas temperature, normal | 180 to 280°C at the preheat exit |
| Maximum or upset | Up to 350°C during fast firing or after a roller kiln stoppage |
| Dust at the fan | Low; glaze dust, setter flakes and roller coating |
| Moisture and dew point | Body moisture from bisque ware and combustion water; dew point about 50 to 60°C |
| Corrosive and condensable species | Fluorides and sulphur from bodies and glazes; tarry organic residues from decal media on decoration kilns |
| Density at the design temperature | About 0.69 kg/m³ at 240°C, 58% 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 ceramic tableware kilns
| Blade form | Backward curved, on clean gas; a radial tip impeller on decoration kilns where residues build up |
| Material of construction | Mild steel for glost and bisque kilns; stainless steel for decoration kiln exhaust with condensable organics |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9, insulated casing |
| Bearings and cooling | Shaft cooling disc and heat shield |
| Flow control | Variable speed drive on preheat zone pressure |
| Also specified | Cleaning doors and casing drain; afterburner interlock on decoration kilns |
What this ID fan has to cope with
Glaze defects from poor exhaust
If the preheat exhaust is weak, water vapour and sulphur compounds released from the body linger and condense on cooler glazed ware, causing pinholes, dull spots or sulphur scumming. A fan with enough margin to hold the designed preheat suction at full throughput, and controlled on pressure rather than set once by hand, removes these volatiles steadily.
Organic residue on decoration kilns
Decal media burn off as smoke and vapour that condense to sticky tar in a cool duct or fan casing. The deposits unbalance the impeller and can catch fire. Keeping the exhaust above its condensation point, using an afterburner ahead of the fan and providing cleaning access are the practical answers.
Hot spots on roller kiln restart
When a roller kiln stops with ware inside, the exhaust temperature climbs as the firing zone heat moves towards the entrance. The fan impeller and bearings must be rated for that short upset, and the control should not raise fan speed blindly on rising pressure.
Energy and control
Roller kilns for tableware run continuously, but the load varies between full belts and gaps during shape changes. Exhaust demand follows the firing rate, so variable speed control on preheat pressure saves power during gaps and weekend low-fire holding. A kiln that over-exhausts pulls in cold air at the entrance, which raises fuel use, so the energy case covers both fan power and kiln fuel.
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 Forced Draft (FD) Fan for ceramic tableware kilns.
Questions buyers ask
Why does a tableware kiln need an exhaust fan?
A roller or tunnel kiln for tableware is open at both ends and runs close to neutral pressure. The ID fan draws the combustion gas counter-current through the preheat zone, where it heats the incoming ware, and removes moisture and volatiles before they can spoil the glaze. Natural draft cannot hold the zone pressure steadily enough for consistent glaze quality.
What causes sulphur scumming on glazed tableware?
Sulphur compounds released from the body or present in the fuel react with the glaze surface in preheat, especially when water vapour is not removed promptly. Adequate, steady exhaust from the preheat zone, clean fuel and correct firing curves all help. The ID fan is one part of the answer, but a weak or unstable fan often contributes.
What fan is used on a decal firing kiln?
A decoration kiln needs an exhaust fan that handles smoke and organic vapour from the decal media. Stainless steel construction, a radial tip or open impeller that tolerates residues, cleaning access and an afterburner ahead of the fan are typical. The fan must also keep the duct warm enough to avoid tar condensation.
Information needed to quote an ID fan for ceramic tableware 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.
- Firing stage. Whether the kiln is for bisque, glost or decoration firing, with peak temperature and cycle time.
- Exhaust treatment. Any afterburner or scrubber before the fan and the gas temperature leaving it.
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.