How refractory brick tunnel kilns work
Fireclay, high alumina, silica and basic bricks such as magnesia and magnesia-chrome are fired in long tunnel kilns at about 1,350 to 1,750°C, depending on the product, with cycles of two to five days. Cars carry dense, heavy settings, often several tonnes per car. Natural gas, furnace oil or producer gas burners fire from the side walls, and the cooling zone air flows forward through the ware as secondary combustion air. Flue gas leaves at the entry end through the preheating zone, carrying moisture and binder residues such as lignosulphonate, which releases SO₂, or tar and pitch from bonded basic bricks. The flue gas exhaust fan is the ID fan, and burner combustion air comes from a separate fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Air header
- Burner valves
- Side-wall burners
The fan feeds a header along both sides of the firing zone, supplying burners through control valves. The cooling air blown in at the exit end flows forward through the ware, is preheated, and becomes secondary combustion air. The burner air from this fan provides flame momentum and mixing, which must penetrate the dense, tall settings. Fan pressure is typically 400 to 800 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Refractory dust in the plant; filtered inlet needed |
| Pressure at the fan outlet | Typically 400 to 800 mmWC |
| Firing temperature | 1,350 to 1,750°C depending on product |
| 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 refractory brick tunnel kilns
| Blade form | Backward curved high-pressure, with a stable curve |
| 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 and silencer; standby fan with automatic changeover |
What this FD fan has to cope with
Flame penetration into dense settings
Refractory settings are tall and dense, and the burner flame must reach into them to fire the inner bricks evenly. Low burner air pressure gives short, lazy flames and uneven firing. A fan with enough pressure for high-velocity burners, held steady by speed control, ensures good flame penetration.
Refractory dust at the inlet
Refractory plants are dusty from pressing, grinding and handling. Abrasive refractory dust drawn into the fan wears the impeller and bearings. A filtered inlet from a clean area protects the fan and keeps the burner air passages clear. Refractory grog is hard and wears the impeller quickly if it is drawn in.
Loss of combustion air
A fan trip stops firing in a kiln with days of production inside. Uneven cooling can crack bricks and damage the kiln. A standby fan with automatic changeover and a reliable power supply are standard practice, with the changeover tested during planned stops so that it can be trusted when a trip occurs.
Energy and control
Refractory kilns fire at very high temperatures and fuel is the dominant cost. A variable speed drive on header pressure saves fan power, but most savings come from good air-fuel ratio control and from using preheated cooling air as secondary air. Keeping burner air at the minimum needed for flame shape reduces cold air entering the firing zone.
ID fan and FD fan in refractory brick 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 | Flue gas with SO₂ and tar fumes | Clean ambient air |
| Temperature at the fan | 200 to 300°C | Ambient |
| Main design concern | Acid and tar deposits, dense settings | Flame penetration into dense settings |
| Control | Preheating zone pressure | Header pressure |
See also the Induced Draft (ID) Fan for refractory brick tunnel kilns.
Questions buyers ask
What is the FD fan on a refractory tunnel kiln?
It is the combustion air fan that supplies air to the burners along the firing zone. Most of the combustion air comes from the cooling zone, flowing forward through the ware, so the FD fan supplies burner primary air for flame shape and mixing. Separate fans handle cooling air and hot air recovery.
What pressure does a refractory kiln burner fan need?
Typically 400 to 800 mmWC, higher than for most ceramic kilns, because high-velocity burners must drive the flame into dense refractory settings. The duty is taken from the burner maker's data, with header and valve losses and margin for the full firing rate at the highest product temperature.
Should a refractory tunnel kiln have a standby combustion air fan?
Yes. A trip stops firing in a kiln holding days of production and can crack bricks and damage the kiln lining. A standby fan with automatic changeover protects both. The cost of a standby is small compared with a lost kiln load or a damaged lining, and a spare motor alone is not enough because changing it takes too long.
Information needed to quote an FD fan for refractory brick tunnel 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.
- Products. Brick types fired and firing 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.