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 ID fan sits
- Burners in side walls
- Kiln chamber and setting
- Car deck flues
- Underground or side flue
- Dilution air damper
- Flue (ID) fan
- Stack
The fan is at the end of the flue from under the kiln, before the stack. Upstream, a dilution air damper admits cold air in increasing amounts as the flue gas gets hotter, so that the fan inlet stays within its limit. Without a recuperator, gas leaving the setting at peak firing is close to kiln temperature. The fan holds the kiln at a slight positive pressure, often 0 to 2 mmWC, against low resistance.
Gas at the ID fan
| Gas temperature, normal | Rises over the cycle; held at 250 to 350°C by dilution at peak firing |
| Maximum or upset | 500°C or more if the dilution damper fails at peak firing |
| Dust at the fan | Little; setting and refractory dust, more during early firing |
| Moisture and dew point | High early in the cycle as water and binders leave the ware |
| Corrosive species | SO₂ and fluorides from the body; organics during binder burn-out |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 shuttle kilns
| Blade form | Backward curved or radial, selected for a wide range of gas temperature and density |
| Material of construction | Heat-resisting steel impeller; mild steel casing with insulation where the inlet stays below about 350°C |
| Drive and arrangement | Direct coupled, arrangement 8, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc and heat shield |
| Flow control | Variable speed drive on kiln pressure, with dilution damper on fan inlet temperature |
| Also specified | Temperature trip, casing drain and access door |
What this ID fan has to cope with
Changing gas density over the cycle
At the start of firing the gas is cold and dense; at peak it is hot and light, even after dilution. A fan sized for the hot condition draws much more power when cold, while one sized cold runs short at peak. A variable speed drive and a motor rated for the cold, dense condition handle both ends of the cycle.
Dilution damper failure
If the dilution damper sticks closed at peak firing, the fan sees gas at several hundred degrees and can be damaged. A temperature trip that reduces firing and opens an emergency bleed, and an impeller in heat-resisting steel, protect the fan. The dilution damper is also checked at every planned stop, because it works hardest at the moment the fan most needs it.
Condensation early in the cycle
In the early stages, water and binder vapour from the ware travel through a cold flue and fan. Acidic condensate forms and corrodes the casing. Warming the flue and casing before firing, insulating the casing and keeping its drain open reduce this damage. Stainless steel at the casing bottom helps on kilns that fire binder-rich ware.
Energy and control
Shuttle kilns have a changing duty throughout the cycle and stand idle between firings. A variable speed drive controlled on kiln pressure follows the cycle and saves much of the power that a fixed-speed fan with a damper wastes. Reducing dilution air, by fitting a recuperator or controlling the damper closely, reduces fan volume and power.
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 Forced Draft (FD) Fan for shuttle kilns.
Questions buyers ask
What does the ID fan do on a shuttle kiln?
It draws the flue gas from the kiln chamber through the setting and the car deck flues to the stack. It also controls kiln pressure, which affects temperature uniformity. Some small kilns use natural draft, but larger kilns and those with tight schedules usually need a flue fan.
How is the shuttle kiln flue fan protected from high temperatures?
A dilution air damper admits cold air ahead of the fan as the flue gas gets hotter, keeping the inlet below the design limit. A temperature trip reduces firing and opens an emergency bleed if the damper fails, and the impeller is made in heat-resisting steel.
Why does my shuttle kiln flue fan draw more current at the start of firing?
At the start of the cycle the flue gas is cold and dense, so the fan moves more mass and draws more power than at peak firing. A motor sized for the cold condition and a variable speed drive that reduces speed early in the cycle solve the problem.
Information needed to quote an ID fan for shuttle 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 schedule. Peak temperature, cycle time and the flue gas temperature profile over the cycle.
- Dilution. How dilution air is controlled and the maximum temperature allowed at the fan.
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.