How porcelain insulator kilns work
Electrical porcelain insulators, from disc and pin insulators to long rod and station post units, are fired in tunnel kilns or in large shuttle kilns loaded on bogies. The body is a mix of clay, feldspar, quartz and often alumina, glazed before firing and taken to about 1,250 to 1,320°C, usually with a reducing period that develops the glaze and body properties. Large pieces have thick sections, so heating and especially cooling must be slow, and cycles of two to five days are common. Firing is by natural gas, LPG or light oil through high velocity burners. Combustion air comes from an FD fan, and the flue gas is drawn out of the preheat end, or from floor flues on a shuttle kiln, by an ID fan; separate cooling and hot air withdrawal fans complete the kiln.
Where the ID fan sits
- Firing zone burners
- Preheat zone or floor flues
- Dilution air inlet
- Flue duct and damper
- ID fan
- Chimney
On a tunnel kiln the fan stands outside the entrance end and pulls gas through ports in the preheat zone roof or side walls. On a shuttle kiln it draws from flues under the bogie floor, usually through a dilution air inlet that caps the temperature. The gas has to pass through dense settings of insulators and kiln furniture first, and the draft must be held within a few millimetres of water gauge so that the zero pressure point stays where the kiln is set. Suction at the fan inlet is typically 40 to 120 mmWC.
Gas at the ID fan
| Gas temperature, normal | 150 to 250°C at the tunnel kiln preheat exit |
| Maximum or upset | 350 to 400°C from a shuttle kiln near peak firing if dilution air is short |
| Dust at the fan | Low; fine body and glaze dust and fibre from kiln furniture and insulation |
| Moisture and dew point | Combustion water plus residual moisture from heavy greenware; dew point about 50 to 60°C |
| Corrosive species | Traces of SO₂ and fluorides released from the clays; condensate becomes acidic |
| Density at the design temperature | About 0.72 kg/m³ at 220°C, 60% 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 porcelain insulator kilns
| Blade form | Backward curved, for a stable draft at part load on clean gas |
| Material of construction | Mild steel for tunnel kiln duty; heat-resisting steel impeller where a shuttle kiln exhaust can pass about 350°C |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9, with an insulated casing |
| Bearings and cooling | Shaft cooling disc and heat shield, with the bearing pedestal kept clear of the hot casing |
| Flow control | Variable speed drive on kiln pressure, with a manual trim damper |
| Also specified | Casing drain, inspection door and backup power arrangements for cycles lasting days |
What this ID fan has to cope with
Cracking from uneven draft
A heavy station post or long rod insulator heats from the surface inwards. If the draft wanders, hot gas short-circuits around the setting instead of through it, the temperature difference across the piece grows and it cracks in preheat or cooling. A fan with a stable curve at low flow, driven by a speed loop on kiln pressure, keeps the gas flow through the setting even.
Fan trip during the reducing period
If the ID fan stops while the kiln is firing rich, kiln pressure rises and carbon monoxide escapes through car seals, sand seals and doors into the kiln hall. Burner interlocks on fan running and draft, a reliable motor and starter, and a backup supply for the fan are part of the duty, not optional extras.
Acid condensate at cycle start
A shuttle kiln starts each cycle cold, so the fan and duct run below the dew point for hours. Fluorides and SO₂ from the clay body dissolve in the condensate and attack the lower casing and inlet box. External insulation, a drain at the lowest point and coated or stainless parts where attack is heavy extend casing life.
Energy and control
Insulator kilns run long, slow cycles, and on a shuttle kiln the flue gas volume changes widely between early heating, peak firing, reduction and controlled cooling. A fan sized for peak firing spends most of the cycle well below rated flow. Speed control on kiln pressure follows that profile instead of burning power across a nearly closed damper. Holding the correct pressure also limits cold air ingress through seals, which usually saves more fuel than the fan saves in electricity.
ID fan and FD fan in porcelain insulator kilns
| ID fan | FD fan | |
|---|---|---|
| Position | Preheat zone or floor flues, before the chimney | Beside the kiln, feeding the burner air main |
| What it handles | Flue gas with traces of fluoride and SO₂ | Ambient or recovered warm air |
| Temperature at the fan | 150 to 250°C, higher on shuttle kilns | Ambient, or 150 to 250°C recovered air |
| Main design concern | Steady, fine draft control | Stable pressure across wide turndown |
| Control | Kiln pressure | Air main pressure and burner ratio |
See also the Forced Draft (FD) Fan for porcelain insulator kilns.
Questions buyers ask
What draft does a porcelain insulator tunnel kiln need?
The kiln itself runs close to neutral pressure, with a slight suction in the preheat zone. The ID fan must overcome the setting, flue ports, dampers and duct, usually 40 to 120 mmWC at the fan inlet, and it must hold that steadily. Fine control matters more than the absolute figure, because the position of the zero pressure point governs the firing.
Why do large porcelain insulators crack during firing?
Most cracks come from temperature differences across thick sections during preheat or cooling. Uneven gas flow through the setting is a common cause: if draft is unstable, gas bypasses the load and some pieces lag behind. Steady ID fan control on kiln pressure, with correct setting density, removes one of the main sources of this variation.
Can a shuttle kiln ID fan run on a variable speed drive?
Yes, and it suits the duty well. Gas volume changes many times over a shuttle kiln cycle, and a variable speed drive linked to kiln pressure follows it smoothly. The fan should be checked for stable running at the lowest speed used during early heating and for the highest gas temperature it can see at peak firing.
Information needed to quote an ID fan for porcelain insulator 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.
- Kiln type and firing cycle. Tunnel or shuttle kiln, the firing schedule, and the flue gas temperature and volume at each stage of the cycle.
- Reduction firing. Whether the kiln fires a reducing period and how kiln pressure is controlled while it does.
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.