How soda ash calciners work
In the Solvay and dual process, crude sodium bicarbonate from the carbonating towers is calcined at about 160 to 200°C to light soda ash, releasing CO₂, water and ammonia. Large plants use steam-heated rotary calciners whose off-gas is recovered by the CO₂ compressors rather than any fan. Fired calciners, heated by coal, oil or gas burned in a furnace around the shell or in a hot gas generator, remain common on smaller lines and on the dense soda ash route, where light ash is hydrated to monohydrate and dried in a fired rotary or fluid bed dryer at 150 to 250°C. In these fired units an FD fan supplies combustion air, and an ID fan draws the flue gas, or the dryer exhaust carrying soda ash dust, through cyclones and a bag filter.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner or grate
- Hot gas generator
- Dilution mixing chamber
- Calciner or dryer
The fan sits beside the furnace or hot gas generator, drawing ambient air through a filter or screen and delivering it to the burner windbox or undergrate plenum. For coal-fired units a separate fan may supply overfire air, and a dilution air fan or damper mixes ambient air into the hot gas before it enters the dryer. Outlet pressure is moderate, typically 150 to 350 mmWC for grate firing and higher for packaged burners.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Ambient air in a soda ash plant can carry product dust; inlet screen or filter needed |
| Pressure at the fan outlet | Typically 150 to 350 mmWC for grate firing; higher for packaged burners |
| Turndown | Firing follows dryer load, often 40 to 100% |
| Hot gas to the product | Usually limited to 300 to 500°C after dilution, downstream of the fan |
| 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 soda ash calciners
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet vanes on firing rate |
| Also specified | Inlet screen or filter to keep soda ash dust out of the impeller and burner |
What this FD fan has to cope with
Dust drawn into the inlet
Soda ash dust from nearby conveyors and packing reaches the fan inlet and settles on the impeller, especially in humid weather. An inlet filter or a ducted inlet from a clean area prevents imbalance and burner fouling.
Air and dilution balance
If dilution air is taken from the same fan, a change in dilution opening alters combustion air and flame stability. Separate control of combustion and dilution air holds both steady.
Oversized fan at partial load
Fans sized for future capacity run throttled at the actual load, with poor efficiency and unstable control at low firing. Correct sizing and speed control give stable operation.
Energy and control
The combustion air fan runs for as long as the dryer or calciner fires, which is usually continuous. A variable speed drive matched to firing rate, with oxygen trim on the hot gas generator, avoids excess air that would add to the dryer exhaust and to the ID fan load.
ID fan and FD fan in soda ash calciners
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the stack | Before the hot gas generator or burner |
| What it handles | Exhaust gas with water vapour and soda ash dust | Ambient air |
| Temperature at the fan | 120 to 180°C | Ambient |
| Main design concern | Caking of hygroscopic alkaline dust | Steady combustion and dilution air |
| Control | Calciner or dryer suction | Firing rate |
See also the Induced Draft (ID) Fan for soda ash calciners.
Questions buyers ask
Is the FD fan on a soda ash dryer the combustion air fan?
Yes. It supplies combustion air to the burner or grate of the hot gas generator or calciner furnace, and sometimes dilution air. It handles ambient air, not process gas.
Why is dilution air needed for a soda ash dryer?
The combustion gas is far hotter than the product or dryer shell can take. Ambient air is mixed in to bring the gas down to a suitable inlet temperature, which adds to the ID fan volume.
Can one FD fan supply combustion and dilution air?
It can, but control is better with separate fans or at least separate control dampers, because changing dilution air should not disturb combustion air to the burner. Shared fans often trade flame stability for dryer temperature.
Information needed to quote an FD fan for soda ash calciners
- 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.
- Firing system. Fuel, burner or grate type, and combustion air pressure needed at full firing.
- Dilution. Hot gas temperature required at the dryer inlet and whether dilution air comes from this 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.