How tapioca and sago flash dryers work
Tapioca roots are washed, rasped and the starch is separated and settled, then dewatered in basket centrifuges or on drying yards to a wet starch of about 35 to 45% moisture. In a flash dryer the wet starch is fed into hot air at typically 150 to 200°C, dried in seconds to about 12 to 13% moisture and collected in a cyclone. The hot air usually comes from a wood, husk, coal or agri-waste fired furnace through a tubular heat exchanger, so the product never meets flue gas. The furnace has its own draft fan. Sago pearls are a separate line: the starch is globulated, gelatinised by roasting and then dried in tray, belt or fluid bed dryers, or in the sun on smaller units.
Where the ID fan sits
- Furnace heat exchanger (air side)
- Wet starch feeder
- Flash duct
- Product cyclone
- Exhaust (ID) fan
- Wet scrubber or stack
The exhaust fan sits after the product cyclone, and on some plants after a second cyclone or a small wet scrubber that recovers fines. It pulls air through the heat exchanger, the flash duct and the cyclone, typically against 300 to 500 mmWC. The furnace ID fan, a separate and smaller fan, sits after the heat exchanger tubes on the flue gas side and handles hot flue gas with ash at about 180 to 280°C, before a small cyclone and chimney.
Gas at the ID fan
| Air temperature, normal | 50 to 65°C at the exhaust fan inlet |
| Maximum or upset | Above 120°C if the starch feed stops while the furnace is firing |
| Dust at the fan | Fine tapioca starch, typically 1 to 3 g/m³ after a single cyclone |
| Moisture and dew point | Humid air, dew point about 40 to 45°C |
| Heat exchanger leakage | A cracked exchanger tube lets smoke and ash into the drying air, discolouring the starch |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 tapioca and sago flash dryers
| Blade form | Backward curved with smooth blades, or radial where cyclone efficiency is poor |
| Material of construction | SS 304 for the exhaust fan; mild steel for the furnace ID fan |
| Drive and arrangement | Belt driven on most units, allowing speed adjustment between seasons |
| Bearings and cooling | Standard bearings for the exhaust fan; cooling disc on the furnace ID fan |
| Flow control | Variable speed drive or inlet damper on the exhaust fan |
| Also specified | Casing drain, inspection door and earthing |
What this ID fan has to cope with
Starch loss through the fan
With a single cyclone, a few percent of the fine starch can pass the fan to the stack, which is lost product and visible emission. A second cyclone or a wet scrubber after the fan recovers it, and the fan pressure is selected to include it.
Deposits after idle periods
Many units run in the harvest season and stand idle for months. Starch left in the casing absorbs moisture, sours and hardens. Cleaning the fan at season end and a casing drain prevent a difficult restart. Bearings are greased and the rotor turned by hand before the new season.
Smoke in the product
A leaking heat exchanger tube lets flue gas into the drying air under the exhaust fan's suction. Keeping the flue side at a stronger suction than the air side, through the furnace ID fan, makes leaks run from air to flue gas instead of the other way.
Energy and control
Tapioca starch units run hard during the harvest season and slow down when roots are scarce. A variable speed drive or a belt drive with changeable pulleys matches the fan to the season's throughput. Better centrifuge dewatering lowers the wet starch moisture and the air and fuel needed. On the furnace side, keeping the exchanger tubes clean of soot lowers both fuel and furnace ID fan power.
ID fan and FD fan in tapioca and sago flash dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the product cyclone | At the furnace, under the grate |
| What it handles | Humid air with tapioca starch fines | Ambient combustion air |
| Temperature at the fan | 50 to 65°C | Ambient |
| Main design concern | Starch loss, deposits and smoke ingress | Steady firing on variable fuels |
| Control | Flash duct suction | Drying air temperature |
See also the Forced Draft (FD) Fan for tapioca and sago flash dryers.
Questions buyers ask
How many fans does a tapioca starch flash dryer need?
Usually two: the main exhaust fan that pulls air and starch through the flash duct and cyclone, and the furnace ID fan for the indirect heater. Larger units add a supply fan before the heater. Sago pearl dryers are a different arrangement, usually tray, belt or fluid bed, with circulation and exhaust fans rather than a flash duct.
Why does smoke smell come into the tapioca starch?
Usually a cracked or leaking heat exchanger tube. The exhaust fan's suction draws flue gas into the drying air. Repairing the exchanger and keeping the flue gas side at a lower pressure than the air side with the furnace ID fan solves it. Smoke in the product is also a food safety issue, so it is worth fixing before the season starts.
What material should the starch flash dryer fan be made from?
Stainless steel for the exhaust fan, because the air is humid and the fines are food product. The furnace ID fan handles dry flue gas and can be mild steel. The dryer exhaust also passes fines to atmosphere, so a second cyclone or a small scrubber after the stainless fan is worth considering where product loss or neighbours are a concern.
Information needed to quote an ID fan for tapioca and sago flash 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.
- Furnace and fuel. Fuel fired in the furnace and the heat exchanger type, with the hot air temperature.
- Season and output. Peak and lean season output in tonnes of starch per day.
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.