How sodium silicate furnaces work
Sodium silicate, or water glass, is made by melting silica sand with soda ash at about 1,300 to 1,450°C in a regenerative or recuperative tank furnace, typically 50 to 300 tonnes per day. The molten glass is tapped and cooled into lumps, then dissolved in water under pressure to make liquid silicate. Many plants fire furnace oil, coal gas or producer gas, others natural gas. The soda ash releases a large volume of CO₂ as it reacts, which adds to the flue gas, and sodium vapour and fine carryover leave with it as sodium carbonate and sulphate fume. Waste gas leaves the regenerators or recuperator at about 450 to 700°C. Older plants run on chimney draft; newer ones add a waste heat boiler or gas cooler, a bag filter or ESP, and an ID fan.
Where the FD fan sits
- Inlet filter
- FD fan
- Reversing valve
- Regenerator or recuperator
- Ports and burners
- Melting tank
The fan stands near the regenerator base or the recuperator. On regenerative furnaces it feeds the reversing valve, which sends air to whichever regenerator is on the air cycle, where it is preheated to about 900 to 1,100°C. On recuperative furnaces the air reaches 500 to 700°C. Fan pressure is typically 150 to 400 mmWC, rising as the checkers or recuperator foul with alkali.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after heat recovery | 900 to 1,100°C after regenerators; 500 to 700°C after a recuperator |
| Dust | Soda ash and sand dust from the batch area; filtered intake |
| Pressure at the fan outlet | Typically 150 to 400 mmWC, rising with checker fouling |
| Duty pattern | Continuous, with reversal every 20 to 30 minutes on regenerative furnaces |
| 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 sodium silicate furnaces
| Blade form | Backward curved, with pressure margin for checker fouling |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, with a standby fan on larger furnaces |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air flow ratioed to fuel |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Checker plugging raising resistance
Sodium fume and batch carryover deposit in the regenerator checkers, gradually raising the air side resistance over the campaign. A fan selected for clean checkers runs short of air later, and the furnace runs with less excess air than needed. Selecting with pressure margin for the fouled condition, on speed control, keeps air flow up.
Air leakage across regenerator walls
Worn or cracked regenerator walls let combustion air leak from the air side to the waste gas side, so not all the fan's air reaches the flame and the ID fan handles the extra. The furnace then runs short of air although the fan appears to deliver enough. Comparing flue oxygen with measured fan output identifies leakage before it is mistaken for a fan problem.
Soda ash dust in the intake
Soda ash dust is fine and hygroscopic. Drawn into the intake from the batch area, it cakes on filters and fan blades in humid weather and can unbalance the impeller. Taking air from a location away from the batch house and unloading points, through a filter with a pressure gauge, avoids this and keeps the regenerators cleaner too.
Energy and control
The combustion air fan runs continuously, often for years. Speed control on measured air flow saves power when pull is reduced and avoids throttling losses as checker resistance changes over the campaign. Accurate air-fuel ratio saves fuel and also limits alkali volatilisation, which rises with excess air and flame temperature, so it slows checker plugging as well.
ID fan and FD fan in sodium silicate furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the filter or at the chimney base | Before the regenerators or recuperator |
| What it handles | CO₂-rich waste gas with sodium fume | Ambient filtered air |
| Temperature at the fan | 180 to 250°C cleaned; 400 to 550°C uncleaned | Ambient |
| Main design concern | Alkali deposits and gas volume | Pressure margin for checker fouling |
| Control | Furnace pressure | Air flow ratioed to fuel |
See also the Induced Draft (ID) Fan for sodium silicate furnaces.
Questions buyers ask
What is the FD fan on a sodium silicate furnace?
It is the combustion air fan. On regenerative sodium silicate furnaces it blows ambient air through the reversing valve into the regenerator on the air cycle, where the hot checkers preheat it before it reaches the port. On recuperative furnaces it pushes air through the recuperator tubes and an insulated main to the burners.
Why does the combustion air fan run short of pressure over time?
Sodium fume and batch carryover gradually plug the regenerator checkers, raising their resistance through the campaign. A fan selected for clean checkers cannot supply full air later, and the furnace runs rich or loses pull. Selecting with pressure margin for the fouled condition, and controlling speed, allows the fan to compensate as fouling builds.
Does a sodium silicate furnace need a standby combustion air fan?
On larger continuous furnaces, yes. Losing combustion air stops firing, chills the melt and can damage the regenerators or recuperator, which is particularly costly on a recuperative furnace. A standby fan with automatic changeover keeps the furnace firing during a failure and allows maintenance on either fan without a hot hold.
Information needed to quote an FD fan for sodium silicate 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.
- Heat recovery type. Regenerative or recuperative, with air side pressure drop clean and fouled.
- Fuel and burners. Fuel type, burner arrangement and air pressure needed at the ports.
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.