How silica sand dryers work
Washed silica sand for glass, foundry, resin coating and construction chemicals leaves the washing plant with about 4 to 8% moisture and must be dried to below 0.1 to 0.5%. Two dryer types are common. A rotary dryer receives hot gas from a burner or hot gas generator at 500 to 700°C. A fluid bed dryer blows heated air up through a perforated plate so the sand bed fluidises, usually followed by a fluid bed or rotary cooler. Sand is nearly pure quartz, the most abrasive mineral that ordinary dryers handle, and the fines are a respirable crystalline silica hazard. The exhaust passes a cyclone and bag filter to the ID fan. The FD fan supplies burner or fluidising air.
Where the FD fan sits
- Inlet filter
- FD fan
- Burner or air heater
- Plenum below the plate
- Fluidised sand bed
On a rotary dryer the FD fan supplies the burner and works against 150 to 300 mmWC. On a fluid bed dryer the fan takes ambient air, pushes it through a direct-fired combustion chamber or air heater into the plenum, then through the distributor plate and the bed. The plate and a bed of dense sand together need typically 500 to 900 mmWC, so the fluidising fan is a high-pressure machine. It is placed before the heater so that it handles cold air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the heater | 250 to 500°C entering the plenum, downstream of the fan |
| Dust | Sand dust in the plant; a filtered inlet is essential |
| Pressure at the fan outlet | 150 to 300 mmWC for a burner; 500 to 900 mmWC for fluidising air |
| 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 silica sand dryers
| Blade form | Backward curved for burner air; high-pressure backward curved or radial for fluidising air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive on air flow or bed pressure |
| Also specified | Inlet filter and silencer; non-return damper so hot sand cannot flow back on trip |
What this FD fan has to cope with
Defluidisation at reduced flow
A fluid bed needs a minimum air velocity to stay fluidised. If the fan cannot hold flow as bed depth or plate resistance rises, parts of the bed slump, sand dries unevenly and lumps form. A fan with a steep pressure curve and flow measurement on the fan inlet holds the air rate against these changes.
Sand back-flow on fan trip
If the fluidising fan stops, sand can fall through the plate into the plenum and hot gas can flow back towards the fan. A non-return damper at the fan outlet and a plate designed to prevent sifting protect the fan and simplify restart. The restart sequence starts the fan before the burner, so the bed is fluidised before hot air reaches it.
Silica dust drawn into the inlet
Sand plants are dusty, and a fan inlet near conveyors draws fine silica into the impeller, which wears it and the bearing seals. A filtered inlet from a clean area protects the fan and also keeps the distributor plate holes from wearing open, which would otherwise upset the air distribution across the bed over time.
Energy and control
The fluidising air fan is often the largest motor on a fluid bed sand dryer because of its high pressure. Operating it at the minimum flow that keeps the bed well fluidised, rather than at a fixed high setting, saves energy. A variable speed drive with flow measurement does this reliably. On rotary dryers the burner fan is small and savings come from lower excess air.
ID fan and FD fan in silica sand dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | Before the burner or air heater |
| What it handles | Exhaust with crystalline silica fines | Clean ambient air |
| Temperature at the fan | 90 to 120°C | Ambient |
| Main design concern | Quartz erosion and leak-tightness | Bed pressure on fluid bed dryers |
| Control | Drum or hood suction | Fluidising air flow or burner rate |
See also the Induced Draft (ID) Fan for silica sand dryers.
Questions buyers ask
What is the FD fan on a fluid bed sand dryer?
It is the fluidising air fan. It pushes ambient air through the burner or air heater, the distributor plate and the sand bed. Because the plate and bed together need a high pressure, it is usually a high-pressure fan, unlike the low-pressure combustion air fan on a rotary dryer.
How much pressure does a fluidising air fan need for silica sand?
Typically 500 to 900 mmWC, made up of the distributor plate, the sand bed, which is roughly its weight per unit area, the heater and the ducts. The exact figure depends on bed depth and plate design, so the fan is sized from the dryer data with margin for bed variation.
Does the fluidising fan handle hot air?
Usually not. It is placed before the burner or air heater so that it handles cold ambient air, which keeps it smaller and simpler. Fans that recirculate hot air, used on some designs to save fuel, are a separate duty and are specified at the higher temperature.
Information needed to quote an FD fan for silica sand 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.
- Dryer type. Rotary or fluid bed dryer, and for fluid beds the bed area, depth and plate pressure drop.
- Heating. Direct-fired burner or indirect air heater, and the air temperature required at the plenum.
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.