How ceramic tunnel and roller kilns work
Ceramic ware is fired in two main continuous kiln types. In a tunnel kiln, loaded cars move slowly through preheating, firing at about 1,100 to 1,300°C and cooling zones over 12 to 40 hours. In a roller kiln, single layers of tiles or tableware travel on rollers through a much shorter cycle, typically 40 to 90 minutes. Both use counterflow: combustion gas flows from the firing zone back towards the entry, heating the ware, while cooling air enters at the exit end. Fuels include natural gas, LPG, producer gas from coal gasifiers and furnace oil. The flue gas exhaust fan at the entry end acts as the ID fan, and burner combustion air comes from a separate fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Combustion air header
- Burner valves
- Firing zone burners
The fan sits beside the kiln and feeds a header running along the firing zone, with branches to each burner or group of burners. Burners need steady air pressure at all firing rates. On some roller kilns, the combustion air is taken from the cooling zone at 150 to 250°C, which saves fuel but makes the fan a hot-air fan. Fan pressure is typically 300 to 700 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C, or 150 to 250°C where cooling zone air is used |
| Dust | Clean, but glaze and body dust in the plant |
| Pressure at the fan outlet | Typically 300 to 700 mmWC |
| Number of users | Dozens to hundreds of burners along 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 ceramic tunnel and roller kilns
| Blade form | Backward curved, high-pressure, for a stable rising curve |
| Material of construction | Mild steel; heat-resisting design for preheated air |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings; cooling disc for preheated air |
| Flow control | Variable speed drive on header pressure |
| Also specified | Inlet filter, silencer, and standby fan where firing must not stop |
What this FD fan has to cope with
Uneven burner pressure along the kiln
With many burners on one header, pressure at the far end can fall below the near end, especially at high firing rates. This leads to uneven temperatures across the zone. Proper header sizing and a fan with enough pressure and a stable curve ensure all burners receive the air they need.
Hot combustion air
Using cooling zone air for combustion saves fuel but increases air volume and reduces density. The fan must be sized for the hot air volume, and its bearings and seals must cope with the higher temperature. A cooling disc on the shaft and heat-resisting materials are specified for these preheated air duties.
Loss of combustion air
A fan trip stops firing, and on a tunnel kiln the ware in the firing zone can be lost or the kiln can suffer thermal shock. A standby fan with automatic changeover is common practice on continuous ceramic kilns, together with a battery or diesel-backed supply for the most critical fans.
Energy and control
Ceramic kilns use most of their energy as fuel, so air control matters more for fuel saving than for fan power. A variable speed drive on header pressure saves power and stabilises burner air. Using preheated air from the cooling zone, where the kiln design allows, cuts fuel use, and controlling excess air reduces the gas volume handled by the flue fan.
ID fan and FD fan in ceramic tunnel and roller 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 | Flue gas with acid gases | Ambient or preheated air |
| Temperature at the fan | 150 to 250°C | Ambient, or 150 to 250°C preheated |
| Main design concern | Acid dew point and kiln pressure profile | Steady pressure to many burners |
| Control | Preheating zone pressure | Header pressure |
See also the Induced Draft (ID) Fan for ceramic tunnel and roller kilns.
Questions buyers ask
What is the FD fan on a ceramic kiln?
It is the combustion air fan that supplies air to the burners along the firing zone. It usually handles ambient air, although some roller kilns use preheated air from the cooling zone. Cooling air, rapid cooling and hot air recovery fans are separate machines, each with its own duty.
How much pressure does a ceramic kiln combustion air fan need?
Typically 300 to 700 mmWC, depending on burner type and the length of the header. High-velocity burners used for good temperature uniformity need the higher end. The fan is selected on burner data plus header and valve losses. A long header on a large roller kiln adds noticeably to the duty.
Should a ceramic kiln combustion air fan have a standby?
On continuous tunnel and roller kilns, yes, in most cases. Losing combustion air stops firing and can damage the ware and the kiln. A standby fan with automatic changeover, or at least a spare motor ready to fit, is good practice. The changeover is tested at planned stops so it works when needed.
Information needed to quote an FD fan for ceramic tunnel and roller 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.
- Air source. Whether combustion air is ambient or taken from the cooling zone, and 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.