How zigzag brick kilns work
A zigzag kiln is a continuous brick kiln in which the fire travels along a zigzag path through the green brick setting, instead of straight down a trench as in a Bull's trench kiln. Coal, lignite or biomass is fed through holes in the roof cover. The longer gas path improves heat transfer and burnout, and cuts black smoke and fuel use compared with older straight-line kilns. Air enters through fired bricks that are cooling, passes the fire at about 950 to 1,050°C and leaves through bricks still drying. Natural draft zigzag kilns use a tall chimney; induced draft kilns use a fan with a short stack, often with a gravity settling chamber or a simple wet scrubber before it.
Where the ID fan sits
- Cooled bricks (air inlet)
- Zigzag fire path
- Drying bricks
- Main flue
- Settling chamber
- ID fan
- Short stack
The fan stands outside the kiln at the end of the main flue, usually after a gravity settling chamber or a water spray chamber that catches coarse ash. The flue connects through dampers to whichever chamber is at the end of the drying section, so the gas path length changes as the fire moves. Suction at the fan is typically 50 to 100 mmWC. Because the gas has given its heat to the green bricks, it arrives at the fan cool and close to saturation.
Gas at the ID fan
| Gas temperature, normal | 60 to 120°C at the fan |
| Maximum or upset | Up to 200 to 250°C if the fire runs too close to the flue |
| Dust at the fan | Fly ash, unburnt char and clay dust; reduced by a settling chamber |
| Moisture and dew point | Near saturation from drying bricks; dew point 50 to 70°C, at or above the gas temperature at times |
| Corrosive species | SO₂ from coal and fluorides from the clay, both acidic in condensate |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 zigzag brick kilns
| Blade form | Radial tip or backward inclined flat blade, to shed wet ash |
| Material of construction | Mild steel with wear plate on blades; stainless or coated lower casing where condensate collects |
| Drive and arrangement | Belt driven, arrangement 9, simple to service on site |
| Bearings and cooling | Standard bearings with good sealing against water and dust |
| Flow control | Variable speed drive or outlet damper |
| Also specified | Casing drain, large inspection door and a diesel generator or engine drive option for power cuts |
What this ID fan has to cope with
Mud on the impeller
When saturated gas carrying clay dust and ash cools in the fan, the dust turns to wet mud on the blade backs and casing. The deposit grows unevenly and breaks away in pieces, causing sudden vibration. A radial tip blade, an inspection door for washing, and keeping the flue insulated to delay condensation reduce the frequency of cleaning.
Corrosion of the casing bottom
Acid condensate from SO₂ and fluorides collects in the lowest part of the casing and inlet box and eats through mild steel within a few seasons. A drain point, a stainless or coated lower half, and a sloping inlet duct so water does not stand in the fan all help.
Fire stops in a power cut
Without the fan there is no draft, the fire stops moving and fuel smoulders in the roof feed holes. Zigzag kilns in rural areas see frequent supply interruptions. The fan should be planned with a diesel generator or an engine drive so that the kiln keeps firing.
Energy and control
A zigzag kiln fires continuously through the brick season, with the fire speed set by draft. Many kilns run the fan flat out and throttle with a damper, or change pulleys when output changes. A variable speed drive lets the operator set the fire speed directly and saves power when the kiln is fired more slowly. Holding just enough draft also limits cold air leakage through the kiln walls and wicket gates, which saves coal.
Questions buyers ask
What is the difference between natural draft and induced draft zigzag kilns?
A natural draft zigzag kiln relies on a tall chimney to pull air through the brick setting. An induced draft zigzag kiln uses an ID fan with a short stack. The fan gives controllable draft, allows a settling chamber or scrubber to be added, and does not depend on stack height or weather.
What size ID fan does a zigzag brick kiln need?
It depends on the number of bricks fired per day, the fuel and the resistance of the setting and flues. Suction is typically 50 to 100 mmWC. The flow is best estimated from the fuel rate and excess air in the kiln, then checked against the flue cross-section and any settling chamber.
Why does the zigzag kiln fan corrode so quickly?
The gas leaves the drying bricks wet and cool, so it condenses in the fan. SO₂ from coal and fluorides from the clay make the water acidic. The lower casing and inlet box rust through first. A drain, stainless or coated lower parts and an insulated flue reduce the damage.
Information needed to quote an ID fan for zigzag brick 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 size and fuel. Bricks per day, number of chambers, and coal, lignite or biomass used.
- Power supply. Grid reliability at site and whether a diesel generator or engine drive is needed.
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.