How bentonite rotary dryers work
Raw bentonite is mined with 25 to 35% moisture and is often mixed with soda ash and aged to convert calcium bentonite to the sodium form. It is then dried in a co-current rotary dryer to about 8 to 12% moisture, not bone dry, because over-dried bentonite loses swelling and gel strength. Hot gas from a lignite, coal, biomass or oil-fired hot gas generator enters at 450 to 700°C and leaves at a relatively low 80 to 110°C. The dried clay is very fine and hygroscopic, and the humid exhaust carries it to a cyclone and bag filter or wet scrubber, then to the ID fan. The FD fan supplies combustion air to the generator.
Where the FD fan sits
- Inlet screen
- FD fan
- Grate or burner
- Hot gas generator
- Mixing chamber
- Rotary dryer
The fan is at the hot gas generator and pushes ambient air under the grate or to the burner. On lignite and coal grates it is often split into undergrate and overfire air, sometimes with a separate fan for each. Tempering air reaches the mixing chamber through a damper, drawn by the ID fan. The FD fan works against the fuel bed and generator, typically 100 to 200 mmWC on a grate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Clay and fuel dust around the generator and feed area |
| Pressure at the fan outlet | Typically 100 to 200 mmWC on a grate generator |
| Fuel | Lignite, coal, biomass or furnace oil, which sets air quantity and pressure |
| 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 bentonite rotary dryers
| Blade form | Backward curved, on clean ambient air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive linked to fuel feed and drum outlet temperature |
| Also specified | Inlet screen and filter; zone dampers on undergrate air for larger grates |
What this FD fan has to cope with
Product moisture drifting
Bentonite must be dried to a narrow moisture band. If combustion air does not follow fuel feed, the generator temperature swings, and with it the outlet temperature and product moisture. Linking the fan speed to the fuel feed, with trim from drum outlet temperature, keeps moisture within its limits.
Lignite fines and clay at the inlet
Generators fired on lignite or coal sit beside dusty fuel and clay handling. Fines drawn into the fan wear the bearing seals and coat the impeller. An inlet screen and a duct taken from a clean area protect it, and they are easy to inspect during routine checks.
Excess air adding to exhaust humidity
Excess combustion air does not raise the dew point, but it does increase the gas volume through the dryer and the ID fan. Keeping excess air low at the generator saves fuel and lets the ID fan run slower, with less risk of carrying clay fines.
Energy and control
Bentonite dryers run with a heat demand that varies with feed moisture and the season, and the combustion air fan follows it. A variable speed drive tied to the firing control avoids the losses of a throttled damper. Lower excess air saves fuel and reduces the gas the exhaust fan must move.
ID fan and FD fan in bentonite rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter or scrubber | At the hot gas generator |
| What it handles | Very humid exhaust with clay fines | Clean ambient air |
| Temperature at the fan | 80 to 110°C | Ambient |
| Main design concern | Dew point margin and clay paste | Steady firing for product moisture |
| Control | Drum suction | Fuel feed and outlet temperature |
See also the Induced Draft (ID) Fan for bentonite rotary dryers.
Questions buyers ask
What does the FD fan supply on a bentonite dryer?
It supplies combustion air to the hot gas generator, often lignite or coal fired on a grate. In some plants it also supplies the overfire or tempering air. It handles clean ambient air, while the separate ID fan handles the humid exhaust from the drum after the collector.
How is product moisture controlled on a bentonite rotary dryer?
Mainly by the drum outlet temperature, which is held by adjusting the generator firing and the combustion air together. The ID fan holds drum suction. Because bentonite must not be over-dried, steady combustion air control is more important here than on most mineral dryers, where some over-drying does no harm.
What pressure does a lignite-fired hot gas generator air fan need?
Typically 100 to 200 mmWC for a grate generator, depending on the fuel bed depth and air distribution. The fan is selected on the generator design data plus duct losses. Plants that switch between lignite and coal need margin for the fuel that requires more air.
Information needed to quote an FD fan for bentonite rotary dryers
- 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.
- Fuel. Fuel fired in the generator and its typical calorific value and moisture.
- Air split. Whether undergrate, overfire and tempering air come from one fan or several.
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.