How shuttle kilns work
A shuttle kiln is a batch kiln. One or two cars of ware are pushed in, the door is closed, and the kiln is fired through a programmed schedule from ambient to peak, typically 1,000 to 1,400°C for ceramics or higher for technical ceramics and refractories, then cooled. Cycles run from about 12 hours for lightweight kilns to several days for large pieces. Gas or LPG burners fire through the side walls, and the flue gas leaves through openings in the car deck or the lower walls to a flue under the kiln. Some kilns use natural draft; most use a flue fan with dilution air. The combustion air fan feeds the burners and often blows cooling air after firing.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Burner ratio valves
- Burners
The fan sits beside the kiln and feeds a header that supplies all the burners through ratio or control valves. At the start of the cycle, firing is very low and the fan delivers a small flow; at peak firing it delivers its full duty. After firing, many kilns use the same fan to blow cool air through the burners for controlled cooling. The fan works against the burners and valves, typically 300 to 600 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clean; plant dust from loading and unloading |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | Very wide, from low firing at the start to full firing at peak, then cooling air |
| 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 shuttle kilns
| Blade form | Backward curved high-pressure, with a stable curve over a wide range |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on header pressure |
| Also specified | Inlet filter, silencer and low-pressure interlock |
What this FD fan has to cope with
Poor control at low firing
At the start of the schedule, firing is a small fraction of full load. A fan throttled hard by a damper gives unsteady pressure, and the burners struggle to hold a low, steady rate. A variable speed drive keeps the fan in its stable range and lets the burners follow a slow, controlled heating ramp.
Cooling air demand
When the fan also supplies cooling air, the flow after firing can be larger than the combustion air flow. The fan must be sized for the larger duty, and the controls must switch smoothly from combustion to cooling without thermal shock to the ware. A ramped change in speed, set in the kiln controller, avoids a sudden blast of cold air.
Burner balance
Uneven air at burners around the kiln leads to temperature differences in the setting. Shuttle kilns have burners on both side walls and sometimes at several levels. A header sized for even pressure, fed by a fan with margin, ensures each burner receives the air it needs at every stage of the schedule, so the setting heats evenly from front to back.
Energy and control
Shuttle kiln combustion air demand changes over every cycle, so a fixed-speed fan with a damper wastes a lot of power. A variable speed drive on header pressure saves energy and gives better control. Fuel is the larger cost, and accurate air-fuel ratio control saves more than any fan change.
ID fan and FD fan in shuttle kilns
| ID fan | FD fan | |
|---|---|---|
| Position | End of the flue, after dilution damper | Beside the kiln, feeding the burner header |
| What it handles | Flue gas rising in temperature over the cycle | Clean ambient air |
| Temperature at the fan | 250 to 350°C at peak with dilution | Ambient |
| Main design concern | Wide range of gas temperature and density | Wide turndown and cooling duty |
| Control | Kiln pressure and fan inlet temperature | Header pressure |
See also the Induced Draft (ID) Fan for shuttle kilns.
Questions buyers ask
Does the shuttle kiln combustion air fan also provide cooling air?
On many kilns, yes. After firing, the burners are shut off and the fan blows cool air through them to cool the kiln and ware at a controlled rate. The fan must then be sized for the larger of the combustion and cooling duties, and the control must switch smoothly between them.
What turndown does a shuttle kiln combustion air fan need?
Very wide, because the schedule starts with very low firing and ends with full firing at peak. A variable speed drive is the best way to provide stable air at low flows. A fan with a stable, rising curve over the whole range is selected.
What pressure does a shuttle kiln burner fan need?
Typically 300 to 600 mmWC, depending on the burner type and the air header. High-velocity burners, which stir the kiln atmosphere for even temperature in a batch setting, need the higher end. If the same fan supplies cooling air, the cooling flow at lower pressure is checked on the same curve.
Information needed to quote an FD fan for shuttle 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.
- Firing schedule. Firing rate at the start and peak of the cycle, and the cooling air requirement.
- Burners. Number and type of burners and the air pressure required at each.
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.