How sanitaryware tunnel kilns work
Vitreous china washbasins, closets and cisterns are fired once, body and glaze together, at about 1,200 to 1,250°C in a gas or LPG-fired tunnel kiln with a cycle of 12 to 20 hours. Kiln cars carry lightweight refractory setting with large, thick-walled pieces that crack if heating or cooling is uneven. High-velocity burners fire into the gaps between the setting to drive circulation and even out the temperature from top to bottom of the car. Flue gas moves back towards the entry through the preheating zone, where the remaining moisture and organic binders leave the ware, and is drawn off by the flue gas exhaust fan. Rapid cooling, final cooling and hot air recovery fans serve the exit end.
Where the ID fan sits
- Firing zone burners
- Preheating zone
- Roof and side offtakes
- Flue duct with dilution damper
- Flue gas exhaust (ID) fan
- Stack
The fan is at the entry end, drawing gas through offtakes distributed along the preheating zone so that gas can be pulled from low or high positions to balance temperatures in the setting. The kiln resistance is small, typically 30 to 100 mmWC, but the fan must hold the preheating zone at a precise negative pressure while the entry door opens to admit each car, every 30 to 60 minutes.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C at the offtakes |
| Maximum or upset | Up to 350°C if the car push is delayed |
| Dust at the fan | Very light; glaze dust and refractory fines from the setting |
| Moisture and dew point | Residual water from the ware and combustion water from gas firing; dew point about 50 to 60°C |
| Corrosive species | Small amounts of SO₂, fluorides and chlorides from the body; organics from binders |
| 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 sanitaryware tunnel kilns
| Blade form | Backward curved, on clean gas, with a steady pressure characteristic |
| Material of construction | Mild steel with an insulated casing; SS 304 in the first metres of duct where condensate forms |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on preheating zone pressure |
| Also specified | Dilution air damper, casing drain and a standby motor or fan |
What this ID fan has to cope with
Cracking from pressure swings
Each time the entry door opens, cold air rushes in and the preheating zone pressure changes. If the flue fan responds slowly, the cold air strikes the lower setting, and thick pieces such as closet bowls can crack. A fast speed loop on preheating zone pressure, and an air curtain at the door, keep the zone steady through each push.
Temperature spread in the setting
Sanitaryware cars are tall and dense, and gas tends to flow over the top. Offtakes at different heights with individual dampers, all served by one flue fan with enough pressure margin, let the operator draw gas from the bottom and reduce the spread from top to bottom of the car.
Condensate from binders and moisture
Ware entering the kiln still releases water and organic binder vapour. In a cold duct this condenses with acid gases into a sticky, corrosive film. An insulated duct and fan casing, a drain and stainless steel at the coldest points prevent corrosion and keep the fan wheel clean.
Energy and control
Sanitaryware kilns run at a fixed push rate for long periods, so the flue fan duty is steady. Many fans are oversized and throttled to hold low suction. A correctly sized fan with a variable speed drive saves power and gives the fine control the preheating zone needs. Reducing false air at the entry door and car seals cuts the gas volume and fuel use.
ID fan and FD fan in sanitaryware tunnel 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 | Humid flue gas with binder vapour | Clean ambient air |
| Temperature at the fan | 180 to 250°C | Ambient |
| Main design concern | Steady preheating zone pressure | High, steady burner air pressure |
| Control | Preheating zone pressure | Header pressure |
See also the Forced Draft (FD) Fan for sanitaryware tunnel kilns.
Questions buyers ask
Why is the flue fan so important on a sanitaryware tunnel kiln?
It sets the pressure and gas flow in the preheating zone, where large sanitaryware pieces are most at risk of cracking. Unsteady suction lets cold air reach the setting or upsets the temperature balance between top and bottom. Steady, fast flue fan control reduces cracking losses and improves first-quality yield.
What temperature does the exhaust fan of a sanitaryware kiln handle?
Typically 180 to 250°C at the offtakes in the preheating zone, with short excursions to about 350°C if car pushing is delayed. Dilution air is used to limit the temperature. The fan casing is insulated to keep the humid gas above its dew point during low-load periods.
Can I fit a VFD to an existing sanitaryware kiln flue fan?
Yes, and it is one of the most useful upgrades for kilns with damper control. A variable speed drive on preheating zone pressure saves energy and holds the pressure steadier. Check that the motor suits inverter duty and that the fan does not pass through a resonant speed in its range.
Information needed to quote an ID fan for sanitaryware tunnel 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 data. Kiln length, car size, push interval and firing temperature.
- Offtakes. Number and position of flue offtakes and present preheating zone pressure set point.
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.