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 ID fan sits
- Firing zone
- Preheating zone
- Flue offtakes
- Flue duct
- Flue gas exhaust (ID) fan
- Stack or treatment
The fan is at the entry end, drawing gas through the preheating zone, where the dense setting offers more resistance than in lighter ceramic kilns, typically 80 to 200 mmWC. It holds the preheating zone under suction and fixes the neutral point in the kiln. The gas has given up most of its heat to the bricks, but carries water, SO₂ and, for some products, tar vapours.
Gas at the ID fan
| Gas temperature, normal | 200 to 300°C at the kiln entry |
| Maximum or upset | Up to 400°C if cars are delayed or the setting is light |
| Dust at the fan | Light; refractory fines and soot |
| Moisture and dew point | Water from bricks and combustion; acid dew point raised by SO₂ |
| Corrosive species | SO₂ from lignosulphonate binder and fuel; tar and pitch fumes on basic brick kilns |
| Density at the design temperature | About 0.68 kg/m³ at 250°C, 57% 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 refractory brick tunnel kilns
| Blade form | Backward curved, or backward inclined flat blades where tar deposits are expected |
| Material of construction | Mild steel with corrosion allowance; SS 304 or 316 in cold sections |
| 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 | Insulated casing, drain, access door for cleaning and temperature trip |
What this ID fan has to cope with
Sulphur corrosion
Lignosulphonate binders release SO₂ during the early heating of the bricks, and fuel oil adds more. Where the flue gas or casing cools below the acid dew point, sulphuric acid condenses and corrodes the fan and duct. Keeping the gas warm, insulating the fan and using stainless steel at cold points give a long service life.
Tar deposits on basic brick kilns
Pitch-bonded and tar-impregnated basic bricks release tar fumes in the preheating zone. These condense on the fan blades and casing, building up and causing unbalance, and they are a fire risk. Flat blades that shed deposits, an access door for cleaning and a gas temperature above the tar dew point reduce the problem.
Dense setting and draft
Heavy refractory settings resist gas flow more than lighter ceramics, and resistance changes with each product. The fan must have enough pressure margin to hold preheating zone suction with the densest setting, while running efficiently on lighter ones. A variable speed drive covers this range without heavy throttling.
Energy and control
Refractory tunnel kilns run continuously with long cycles, so the flue fan runs almost all the time. A variable speed drive on preheating zone pressure saves energy and gives better control as settings change. Recovering heat from the cooling zone for dryers, and limiting false air at the entry, reduce fuel and the gas volume the fan must handle.
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 Forced Draft (FD) Fan for refractory brick tunnel kilns.
Questions buyers ask
What is the ID fan on a refractory tunnel kiln?
It is the flue gas exhaust fan at the entry end of the kiln. It draws the combustion gas through the preheating zone, where the gas heats the incoming bricks, and sends it to the stack. It also sets the pressure profile in the kiln, which affects temperature uniformity.
Why does the flue fan on a refractory kiln corrode?
Binders such as lignosulphonate release SO₂ during heating, and fuel oil adds more. When the flue gas cools below the acid dew point, sulphuric acid condenses on the fan casing and impeller. Insulating the fan and duct and using stainless steel at cold points prevent this.
Does tar from basic bricks affect the flue fan?
Yes. Pitch-bonded and tar-impregnated basic bricks release tar fumes in the preheating zone, which condense on the fan and build up as sticky deposits. The fan needs blades that shed deposits, cleaning access and a gas temperature high enough to keep the tar in vapour form.
Information needed to quote an ID fan for refractory brick 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.
- Products. Brick types fired, binders used and firing temperature.
- Kiln data. Kiln length, car size, push interval and the present flue gas 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.