How caustic concentration and fusion furnaces work
Caustic soda from membrane cells at 32% is concentrated to 48 to 50% in steam evaporators, then to 98 to 99% anhydrous caustic for flakes or prills in a final concentrator. Modern plants use falling film concentrators heated by a molten salt, a nitrate and nitrite mixture, at 400 to 430°C. The salt is heated in a fired heater burning natural gas, fuel oil or, in some plants, producer gas or coal, with a flue gas exit at 450 to 550°C. An air preheater recovers much of this heat. Older plants use directly fired cast iron fusion pots, where the flame heats the pot from below. In both, an FD fan supplies combustion air and, where an air preheater or long flue is fitted, an ID fan draws the flue gas to the chimney.
Where the ID fan sits
- Furnace
- Molten salt coils or fusion pot
- Air preheater
- ID fan
- Chimney
The fan sits after the air preheater, which brings the gas down from 450 to 550°C to about 250 to 330°C. The suction covers the furnace, coils, air preheater and flues, typically 100 to 250 mmWC. On fusion pot plants the flue from each pot setting may join a common duct to one ID fan. Gas fired heaters with a short stack sometimes run on natural draft, but the air preheater usually makes an ID fan necessary.
Gas at the ID fan
| Gas temperature, normal | 250 to 330°C after the air preheater |
| Maximum or upset | Up to 500°C or more if the air preheater is bypassed |
| Dust at the fan | Low on gas or oil; fly ash on coal or producer gas |
| Moisture and dew point | Normal combustion moisture |
| Corrosive species | SO₂ with oil or coal; caustic dust from the flaker area if the inlet draws plant air |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 caustic concentration and fusion furnaces
| Blade form | Backward curved, on clean hot gas |
| Material of construction | Mild steel or boiler plate up to about 350°C; heat-resisting steel where the upset exceeds 400°C |
| Drive and arrangement | Direct coupled, arrangement 8, with casing on sliding supports |
| Bearings and cooling | Cooling disc and heat shield; water-cooled bearings for the higher upset duty |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Insulated casing, inlet damper and slow rotation after a hot stop |
What this ID fan has to cope with
High temperature if the preheater is bypassed
A fouled or leaking air preheater may be bypassed, sending gas at 450 to 550°C to the fan. The impeller and shaft must be designed for this, or a dilution damper must limit inlet temperature.
Overheating the salt
If draft is lost, the furnace runs positive and heat transfer to the salt coil becomes uneven. Local overheating can decompose the nitrite salt. A reliable fan with draft interlocks protects the heater.
Shaft bow after a hot stop
With gas near 300°C, a stopped fan bows its shaft from uneven cooling. Slow rotation after a stop, or running at low speed until the gas cools, prevents vibration at restart.
Energy and control
Caustic concentration runs continuously, with output following cell room production. The salt heater duty is steady. Most energy savings come from the air preheater and low excess air; a variable speed drive on furnace draft adds a further saving by avoiding throttling, and helps hold draft as the preheater fouls.
ID fan and FD fan in caustic concentration and fusion furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the air preheater, before the chimney | Before the air preheater, feeding the burners |
| What it handles | Hot flue gas | Ambient air |
| Temperature at the fan | 250 to 330°C, higher on bypass | Ambient |
| Main design concern | High temperature and thermal growth | Clean air and steady burner pressure |
| Control | Furnace draft | Firing rate |
See also the Forced Draft (FD) Fan for caustic concentration and fusion furnaces.
Questions buyers ask
What temperature does the ID fan on a caustic fusion furnace handle?
After the air preheater, typically 250 to 330°C. Without the preheater, or if it is bypassed, the gas can reach 450 to 550°C, which needs heat-resisting construction. The fan should be specified for the upset case as well as the normal one.
Does a molten salt heater for caustic need an ID fan?
Usually yes, because the air preheater adds resistance that the chimney cannot overcome at full firing. Small gas fired heaters without preheaters can run on natural draft. Coal or producer gas firing with a bag filter always needs one.
What is the difference between pot fusion and falling film caustic concentration?
Pot fusion heats caustic in directly fired cast iron pots. Falling film concentration heats caustic through tubes using molten salt, which is heated in a separate fired heater. Both need combustion air and flue gas fans.
Information needed to quote an ID fan for caustic concentration and fusion furnaces
- 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.
- Heater type. Molten salt heater or fusion pots, rating and salt or caustic temperature.
- Air preheater. Flue gas temperature before and after the air preheater, and bypass arrangement.
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.