How Hoffmann kilns work
A Hoffmann kiln is a continuous annular kiln of 14 to 24 or more chambers built round a central smoke main. Bricks stay in place while the fire moves from chamber to chamber, typically a few metres a day. Coal, lignite, petcoke or sawdust is fed through small holes in the roof, and much of the fuel may already be mixed into the clay. Air enters through open wickets on the cooling side, is heated by the fired bricks, burns the fuel at about 950 to 1,050°C, then dries and preheats the green bricks before leaving through a flue damper into the smoke main. The classic Hoffmann kiln relies on a tall chimney; many have been converted to an ID fan, which also allows warm air to be drawn from cooling chambers to the dryers.
Where the ID fan sits
- Cooling chambers (air inlet)
- Firing chambers
- Drying chambers
- Flue damper
- Central smoke main
- ID fan
- Stack
The fan stands at the base of the old chimney or beside the kiln, taking gas from the end of the central smoke main. Each chamber connects to the main through a bell or mushroom damper, and only the chamber at the end of the drying section is open; the closed dampers still leak. Suction at the fan is typically 60 to 150 mmWC, higher on long kilns with heavy settings. On converted kilns the chimney often stays as the outlet stack.
Gas at the ID fan
| Gas temperature, normal | 80 to 150°C at the smoke main outlet |
| Maximum or upset | 200 to 300°C when the fire runs close to the open damper |
| Dust at the fan | Fly ash, char and clay dust; moderate, settled partly in the smoke main |
| Moisture and dew point | High from drying bricks; dew point 50 to 65°C |
| Corrosive species | SO₂ from coal or petcoke and fluorides from clay; acidic condensate in the main and fan |
| Leakage air | Cold air through closed dampers and wickets dilutes the gas and adds volume |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 Hoffmann kilns
| Blade form | Backward inclined flat blade, tolerant of wet ash |
| Material of construction | Mild steel with wear plate on blade inlets; coated or stainless lower casing where condensate stands |
| Drive and arrangement | Belt driven, arrangement 9, or direct coupled with a drive |
| Bearings and cooling | Standard bearings, cooling disc where gas can pass 200°C |
| Flow control | Variable speed drive, with the chamber dampers setting the gas path |
| Also specified | Casing drain, inspection door and a branch for drawing hot air to dryers where used |
What this ID fan has to cope with
Damper leakage inflating the duty
Bell dampers in a Hoffmann kiln seal poorly, and every closed chamber lets some cold air into the smoke main. The fan then handles far more volume than the fire needs, at a lower temperature. Measuring the actual flow before replacing a fan, and repairing damper seats, often allows a smaller, more efficient fan.
Moving resistance
As the fire advances, the number of chambers between the air inlet and the open damper varies, and so does the setting resistance. A fan selected for one point either starves the fire on long paths or over-draws on short ones. A fan with a stable curve over the full range, run on a variable speed drive, follows the cycle.
Acid corrosion in the smoke main and fan
Gas cools along the long brick smoke main and condenses. Coal sulphur and clay fluorides make the condensate acidic, attacking the fan casing bottom and inlet duct. A drain, insulated steel duct where it is exposed, and coated or stainless lower casing sections extend life.
Energy and control
A Hoffmann kiln fires all year with only short stops, so the fan runs continuously. The fire speed and so the output is set by the draft, which operators often adjust with a damper on a fan running at full speed. A variable speed drive does the same job with less power, and reducing over-draft cuts cold air leakage through dampers and wickets, which improves fuel use per brick.
Questions buyers ask
Can a Hoffmann kiln chimney be replaced by an ID fan?
Yes, and many have been converted. The fan gives draft that does not depend on weather or chimney condition, allows the fire speed to be controlled precisely, and makes it practical to add a scrubber. The old chimney is often kept as the stack, with the fan installed at its base on the smoke main.
Why is the Hoffmann kiln fan bigger than expected?
Much of the gas the fan handles is cold air leaking in through closed bell dampers and wickets, not combustion gas from the fire. This leakage can be a large share of the total. Repairing damper seats and measuring the real flow before selecting a replacement fan usually reduces the fan size and power.
What temperature does a Hoffmann kiln ID fan see?
Usually 80 to 150°C, because the gas gives its heat to green bricks in the drying chambers and is diluted by leakage air. It can rise to 200 to 300°C if the fire approaches the open damper, so the impeller and bearings are checked for that short-term upset temperature.
Information needed to quote an ID fan for Hoffmann 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 layout. Number of chambers, ring length, smoke main size and damper type.
- Hot air recovery. Whether hot air is drawn from cooling chambers to dryers through the same or a separate 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.