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 FD fan sits
- Inlet screen
- FD (combustion air) fan
- Undergrate air duct
- Furnace grate
- Heat exchanger (flue side)
- Furnace ID fan
- Chimney
The combustion air fan sits beside the furnace and blows ambient air under the grate and, on some furnaces, through overfire air nozzles. The flue gas passes the heat exchanger tubes and is pulled out by the furnace ID fan. The drying air on the other side of the tubes is moved by the dryer's exhaust fan, so the FD fan sees only the combustion duty. The pressure is low, typically 50 to 150 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Unheated, straight under the grate |
| Dust | Fuel dust and husk in the yard air can enter without an inlet screen |
| Humidity | No significant effect on the duty |
| Pressure at the fan outlet | Typically 50 to 150 mmWC through the grate and fuel bed |
| 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 tapioca and sago flash dryers
| Blade form | Backward curved, or radial on small units for simplicity |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor or belt driven |
| Bearings and cooling | Standard bearings |
| Flow control | Inlet damper or variable speed drive on drying air temperature |
| Also specified | Inlet screen and a simple damper per air zone |
What this FD fan has to cope with
Variable fuel
Units burn whatever wood, husk, tapioca stems or coconut shell is available, each needing different air. A fan with margin and a damper or speed control adjusted by the operator keeps the drying air temperature steady. Where a single fuel is used most of the season, the fan is selected for it, with margin for the alternatives.
Overheating the exchanger
Too much air with a dry fuel can overheat the heat exchanger tubes, and too little leaves soot. Controlling FD air on the drying air temperature, rather than leaving it fixed, protects the exchanger. A temperature sensor on the drying air, linked to the FD fan speed or damper, gives the operator a simple and dependable loop.
Fuel dust in the fan
Husk and fine fuel dust drawn into the fan builds up on the blades. An inlet screen and an intake away from the fuel yard keep the fan clean. Ash and fuel dust are also kept clear of the motor fins, since an overheated motor is a common cause of mid-season stoppage on small furnaces.
Energy and control
The combustion air fan is small compared with the dryer exhaust fan, but it runs through the whole season. Speed control linked to the drying air temperature saves fuel more than fan power, because it stops excess air carrying heat up the chimney. Insulating the furnace and exchanger and cleaning the tubes give the largest fuel savings.
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 Induced Draft (ID) Fan for tapioca and sago flash dryers.
Questions buyers ask
What is the FD fan on a tapioca starch dryer?
It is the combustion air fan for the furnace that heats the drying air through a heat exchanger. It supplies air under the grate or to the burner, and handles only ambient air. The furnace also has an ID fan on the flue side, which works together with the FD fan to hold a slight suction in the furnace.
Can the same fan supply combustion air and drying air?
Not on an indirect dryer. The combustion air and the drying air are kept separate by the heat exchanger, so the product never meets flue gas. Each side has its own fan. A single fan would push smoke into the drying air. The furnace fans therefore stay on the flue gas side, and the exhaust fan handles the starch air.
What fuels are used for tapioca flash dryer furnaces?
Wood, coal, rice husk, coconut shell and other agri-waste, depending on local availability. The combustion air fan needs enough margin to handle the fuel that needs the most air. Fuels with a high fines content, such as husk, also need an inlet screen and a furnace grate that suits them.
Information needed to quote an FD fan for tapioca and sago flash 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.
- Furnace type. Grate size, fuel and whether overfire air is used.
- Heat exchanger. Flue gas side pressure drop and the furnace ID fan details, if a replacement is also needed.
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.