How starch flash dryers work
Native maize, wheat or potato starch arrives from the peeler centrifuge or vacuum filter as a wet cake of about 35 to 40% moisture. A disperser or feeder throws the cake into a stream of hot air, typically 140 to 170°C, in a vertical flash duct, where it dries in a few seconds to about 12 to 13% moisture. The air, cooled to about 45 to 60°C, carries the dry starch to a battery of cyclones, sometimes followed by a bag filter or wet scrubber for the fines. The drying air is filtered and heated in steam coils or by an indirect heater. On smaller plants a single exhaust fan after the cyclones pulls air through the whole system; larger plants add a supply fan before the heaters.
Where the FD fan sits
- Weather louvre
- Air filter
- FD (supply air) fan
- Steam coil heater
- Wet cake disperser
- Flash duct
The supply fan sits after the air filter and before the steam coil heaters, so it handles cool, clean air and pushes it through the heaters to the disperser. On plants with only an exhaust fan, there is no FD fan and the heaters run under suction. A balanced arrangement, with the supply fan and exhaust fan each covering about half the losses, is used on larger dryers where a single fan would need very high pressure.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heaters | 140 to 170°C, downstream of the fan |
| Dust | Filtered air; the air contacts food starch |
| Humidity | Seasonal humidity changes the drying capacity of the air |
| Pressure at the fan outlet | Set by the filter, heaters and disperser, typically 150 to 300 mmWC |
| 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 starch flash dryers
| Blade form | Backward curved or aerofoil for efficiency on clean air |
| Material of construction | Mild steel with food-grade coating, or stainless steel where the plant specifies it |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on air flow, coordinated with the exhaust fan |
| Also specified | Flexible connections and an access door for inspection |
What this FD fan has to cope with
Balancing two fans
With both fans, the pressure at the disperser depends on their relative speeds. If the supply fan pushes too hard, starch blows out at the feed point; too little, and the heaters run under suction. The control system sets the exhaust fan on duct pressure and the supply fan on air flow.
Filter loading
Air filters load with seasonal dust, and a fixed-speed supply fan loses flow as they do. A differential pressure alarm and speed control hold the drying air flow. Filter changes are then scheduled on the measured pressure drop rather than by the calendar, which keeps the drying conditions steady through dusty months.
Heater coil fouling
Steam coils foul with dust when filtration is poor, raising resistance and lowering heat transfer. The fan selection includes a fouling margin, and filtration protects the coil. Condensate drainage from the steam side also affects air temperature, so a coil running partly flooded looks like a fan problem when it is not. Checking both sides of the coil avoids a wrong diagnosis.
Energy and control
The supply fan moves all the drying air through the heaters, so its efficiency matters across a continuous run. A backward curved or aerofoil wheel on a variable speed drive, and keeping the filters and coils clean, are the main measures. Recovering heat from the exhaust air to preheat supply air adds a coil pressure drop to the fan duty but saves steam.
ID fan and FD fan in starch flash dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclones or bag filter | After the air filter, before the steam heater |
| What it handles | Moist air with starch fines | Filtered ambient air |
| Temperature at the fan | 45 to 60°C | Ambient |
| Main design concern | Dust explosion, deposits and condensation | Pressure balance with the exhaust fan |
| Control | Flash duct suction or outlet temperature | Drying air flow |
See also the Induced Draft (ID) Fan for starch flash dryers.
Questions buyers ask
Does every starch flash dryer have an FD fan?
No. Many smaller dryers use one exhaust fan that pulls air through the filter, heater and flash duct. A separate supply fan is added on larger dryers to share the pressure and keep the system near neutral at the feed point. The arrangement also makes it easier to start up and shut down without dust escaping.
Should the supply fan be before or after the steam heater?
Before. The fan then handles cool, dense air, which keeps it smaller and avoids hot air on the bearings and casing. The steam coils are then on the pressure side, so any coil leak blows outward. A filter ahead of the fan protects both the fan and the coil from dust, and the fan curve must cover the coil at its fouled condition.
How are the supply and exhaust fans controlled together?
The exhaust fan usually controls pressure or suction in the flash duct, and the supply fan controls the air flow. Both on variable speed drives give stable drying and avoid starch blowing out at the feed point. The two drives are interlocked so that the supply fan cannot run without the exhaust fan.
Information needed to quote an FD fan for starch 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.
- Heater. Steam coil or indirect heater type, its air side pressure drop and the air outlet temperature.
- Filtration. Filter grade and its clean and dirty pressure drop.
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.