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 FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Hot air duct
- Burners
- Furnace
The fan stands beside the heater and delivers ambient air through the air preheater, where it is heated to about 250 to 350°C, then to the burner windbox. Outlet pressure covers the preheater, duct and burner, typically 300 to 600 mmWC. The inlet should be ducted from a clean area, away from the flaker and packing section. The fan is usually paired with a standby on continuous plants.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 250 to 350°C, downstream of the fan |
| Dust | Caustic dust near flakers and packing; inlet filter needed |
| Pressure at the fan outlet | Typically 300 to 600 mmWC |
| Turndown | Steady firing with moderate turndown |
| 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 caustic concentration and fusion furnaces
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with a coating resistant to alkaline dust |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on firing rate |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Caustic dust at the inlet
Caustic dust is strongly alkaline and hygroscopic. It sticks to the impeller and casing, corrodes them and fouls the burner. A filtered inlet from a clean area keeps it out.
Air preheater leakage
Leaks in the preheater short-circuit air to the flue side, increasing FD and ID fan load. Oxygen measurement before and after the preheater shows the leak. Repairing leaks at each shutdown keeps both fans on their curves.
Burner air pressure
If the fan cannot hold burner air pressure at full firing, the air to fuel ratio drifts and the salt coil heats unevenly. A correctly sized fan with speed control holds the pressure.
Energy and control
The combustion air fan runs continuously at a steady load. Speed control on firing rate, with oxygen trim, avoids excess air, which raises flue gas loss in a heater whose flue gas already leaves hot. The fuel saving from low excess air is larger than the fan power saving. Air preheater leaks should be checked during each shutdown.
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 Induced Draft (ID) Fan for caustic concentration and fusion furnaces.
Questions buyers ask
Is the FD fan on a caustic fusion furnace the combustion air fan?
Yes. It supplies combustion air to the burners of the fusion pots or molten salt heater, usually through an air preheater. It never sees the hot flue gas, so it is a simple mild steel fan.
Why is filtered air important for the caustic furnace FD fan?
Caustic dust from flakers and packing is hygroscopic and alkaline. Unfiltered, it builds up on the impeller and burner and causes corrosion and imbalance. The intake is best ducted from outside the building, away from the flaker.
What air temperature reaches the burner on a molten salt heater?
With an air preheater, typically 250 to 350°C. The fan itself handles ambient air before the preheater. The hot air duct and burner windbox after the preheater are insulated, but the fan stays cold.
Information needed to quote an FD fan for caustic concentration and fusion furnaces
- 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.
- Burners. Number of burners, fuel and air pressure at full firing.
- Air preheater. Air side pressure drop and hot air temperature.
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.