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 FD fan sits
- Inlet filter
- FD fan
- Combustion air main
- Ratio valves
- High velocity burners
- Kiln
The fan stands beside the kiln and feeds an air main running along the firing zone, or round a shuttle kiln, to every burner. Ratio valves or flow controllers set the air for each burner group in step with the gas. High velocity burners typically need 300 to 600 mmWC at the burner, so with main and valve losses the fan delivers about 400 to 800 mmWC. Some tunnel kilns take warm air from the cooling zone instead of ambient air, which makes the fan larger and hotter.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Warm air option | 150 to 250°C where cooling zone air is used for combustion |
| Dust | Clean air; a filter is needed near glazing and fettling areas |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Turndown | Wide; burners sit at low fire for long periods during slow preheat and cooling |
| 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 porcelain insulator kilns
| Blade form | Backward curved, high pressure single inlet design |
| Material of construction | Mild steel; heat-resisting impeller only for the warm air option |
| Drive and arrangement | Direct coupled at two-pole speed, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; cooling disc only where warm air is handled |
| Flow control | Variable speed drive holding air main pressure |
| Also specified | Inlet filter and silencer, and an air pressure switch wired into the burner management system |
What this FD fan has to cope with
Ratio drift at low fire
During slow cooling and early preheat the burners run at a small fraction of capacity. If the fan is far oversized, main pressure becomes unsteady at this flow and the air-fuel ratio drifts, giving local reduction or excess air on the ware. A fan sized for the real maximum, with a stable curve near its minimum flow and a set minimum drive speed, holds the ratio.
Controlling the reducing period
Reduction is achieved by trimming air at each burner, not by starving the whole fan. If the main pressure falls during reduction, some burners go richer than intended and soot the glaze. Pressure control on the air main keeps every burner on its set ratio while individual valves set the reduction.
Inlet filter loading
Glaze overspray and fettling dust in the kiln hall load the inlet filter, and the fan output falls slowly without any alarm until burners at high fire run short of air. A differential pressure gauge or switch across the filter, a filter sized generously for the air flow and a simple cleaning schedule prevent this, and the intake can often be moved to a cleaner wall.
Energy and control
The combustion air fan runs throughout a cycle that may last several days, mostly at part load. A variable speed drive holding air main pressure, rather than a throttled inlet damper, cuts power substantially during the long low-fire periods. Where cooling zone air is recovered for combustion, fuel use falls further, but the fan must then be sized for the hotter, less dense air.
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 Induced Draft (ID) Fan for porcelain insulator kilns.
Questions buyers ask
Is the FD fan on a porcelain kiln the same as the combustion air blower?
Yes. On a porcelain insulator kiln the forced draft fan is the combustion air blower that feeds every burner through a common air main. It is separate from the cooling air fans and the hot air withdrawal fan, which serve the cooling zone, and from the ID fan, which exhausts flue gas from the preheat end or floor flues.
How much air pressure do high velocity kiln burners need?
Most high velocity burners need roughly 300 to 600 mmWC of air pressure at the burner to reach their rated jet velocity. The fan must add the losses of the air main, ratio valves and fittings, so a combustion air fan delivering 400 to 800 mmWC is typical. The burner maker's data sets the final figure.
Can hot air from the cooling zone be used as combustion air?
Yes, on many tunnel kilns. Recovering cooling zone air saves fuel, but the fan then handles air at 150 to 250°C. It needs a larger impeller or higher speed for the same mass flow because the air is less dense, heat-resisting material where needed, and a shaft cooling disc. The burners must also be rated for hot combustion air.
Information needed to quote an FD fan for porcelain insulator 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 data. Number and type of burners and the air pressure each needs at full firing.
- Air source. Whether combustion air is ambient or warm air recovered from the cooling zone, with its 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.