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 ID fan sits
- Burner or hot gas generator
- Rotary or fluid bed dryer
- Cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan sits after the bag filter. On a rotary dryer it provides all the draft through the drum, cyclone and filter, typically 180 to 300 mmWC. On a fluid bed dryer the FD fan pushes air through the plate and bed, and the ID fan holds the hood above the bed at a slight suction, overcoming only the hood, cyclone and filter. In both cases the dust load before the filter is large, and the filter is the fan's protection.
Gas at the ID fan
| Exhaust temperature, normal | 90 to 120°C after the bag filter |
| Maximum or upset | Up to 200°C on loss of feed |
| Dust at the fan | Crystalline silica fines; low after a bag filter but extremely abrasive if bags fail |
| Moisture and dew point | Moderate; a dew point of about 45 to 55°C from 4 to 8% feed moisture |
| Health hazard | Respirable crystalline silica; casing and shaft seal leaks must be avoided |
| Density at the design temperature | About 0.93 kg/m³ at 105°C, 78% 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 silica sand dryers
| Blade form | Backward curved with replaceable wear liners; radial in wear plate if the fan follows a cyclone only |
| Material of construction | Mild steel casing with liners at the cut-off and inlet; impeller in wear-resistant plate |
| Drive and arrangement | Direct coupled, arrangement 8, at low tip speed |
| Bearings and cooling | Pedestal bearings outside the gas stream with a shaft cooling disc |
| Flow control | Variable speed drive on drum or hood suction |
| Also specified | Tight shaft seal, gasketed doors, broken-bag detection before the fan |
What this ID fan has to cope with
Rapid wear after a bag failure
Quartz is harder than any steel used in fan wheels. A single torn bag can pass enough sand fines to cut the blade edges in days. Broken-bag detection that alarms quickly, wear-resistant blades and liners, and low tip speed buy time to find the failed bag before the impeller is damaged.
Silica leakage at the fan
Respirable silica escaping at the shaft seal or doors is an occupational health problem. The fan on a positive pressure duct downstream must have a tight shaft seal and gasketed inspection doors, and the discharge duct is kept short so that leaks into the building are minimised.
Hood pressure on fluid bed dryers
If the ID fan suction is too high, it lifts fine sand from the bed and loads the cyclone and filter. If too low, dusty air leaks from the hood. A variable speed drive holding hood pressure close to zero, independent of filter loading, keeps the bed and the plant clean.
Energy and control
Sand dryers in glass and foundry supply often run long hours at a steady output, so a correctly sized fan saves more than any control trick. Many older fans are oversized and throttled. A variable speed drive on drum or hood suction reduces power and holds draft as the filter loads. On rotary dryers, tight drum seals reduce false air and the volume the fan must move.
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 Forced Draft (FD) Fan for silica sand dryers.
Questions buyers ask
Can a backward curved ID fan be used on a silica sand dryer?
Yes, behind a well-maintained bag filter, and fitted with wear liners. It is more efficient than a radial wheel. Where the fan follows only a cyclone, or bag failures are frequent, a radial wheel in wear-resistant plate at low tip speed is the safer choice because quartz fines erode thin blades very quickly.
How is the ID fan different on a fluid bed sand dryer?
On a fluid bed dryer the FD fan pushes air through the distributor plate and the sand bed. The ID fan only holds the hood above the bed at a small suction and draws the exhaust through the cyclone and filter, so its pressure is lower than on a rotary dryer of similar output.
Why does the exhaust fan of my sand dryer wear out so fast?
Usually because sand fines reach it through failed bags, a bypass or a poorly sealed filter. Quartz erodes steel rapidly. Checking the filter for leaks, fitting broken-bag detection, using wear-resistant blades and reducing tip speed usually solve the problem. Repairing the old wheel by hard-facing is a short-term measure only.
Information needed to quote an ID fan for silica sand dryers
- 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.
- Dryer type. Rotary or fluid bed dryer, sand throughput and inlet and outlet moisture.
- Wear history. Life of the present impeller and the condition of the bag filter.
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.