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 ID fan sits
- Air filter
- Steam air heater
- Wet cake disperser
- Flash duct
- Cyclone battery
- Exhaust (ID) fan
- Scrubber or stack
The fan sits after the cyclone battery, or after the bag filter or scrubber where one follows, so it handles warm moist air with the starch fines that escape collection. Where it is the only fan, the whole system from air filter to cyclone runs under suction, which keeps starch dust inside but means the fan pressure covers the filter, heater, disperser, flash duct and cyclones, typically 400 to 600 mmWC. That suction also helps lift the cake through the flash duct.
Gas at the ID fan
| Air temperature, normal | 45 to 60°C at the fan inlet |
| Maximum or upset | Up to 100°C or more if the wet cake feed stops while the heater is on |
| Dust at the fan | Very fine starch, low after a bag filter, typically 0.5 to 2 g/m³ after cyclones alone |
| Moisture and dew point | Humid air with a dew point near 40°C; condensation on cold surfaces at shutdown |
| Fire and explosion | Starch dust is a strong combustible dust; deposits gelatinise if wetted and heated |
| Density at the design temperature | About 1.08 kg/m³ at 55°C, 90% 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 starch flash dryers
| Blade form | Backward curved with smooth blades that shed starch; radial where the fan follows cyclones only and handles more dust |
| Material of construction | SS 304 for the impeller and casing, as the fines may return to product and the air is humid |
| Drive and arrangement | Direct coupled or belt driven, with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream with a shaft seal; no cooling needed |
| Flow control | Variable speed drive holding flash duct outlet temperature or system suction |
| Also specified | Earthing, spark-free construction, explosion venting or isolation as per the plant's dust hazard study, and a casing drain |
What this ID fan has to cope with
Starch dust explosion risk
Fine dry starch in air is an explosible dust, and the cyclone and fan are inside the explosion-risk zone. The fan is earthed, impeller-to-casing clearances are set so rubbing cannot occur, and the fan is included in the plant's explosion venting or isolation arrangement. Starch fines in the casing are cleaned regularly.
Gelatinised deposits
Starch that settles on the blades and is then wetted by condensation and warmed again partly gelatinises into a hard, glassy layer. It breaks off unevenly and unbalances the rotor. Dry shutdown, insulation and periodic washing prevent it. The deposit also harbours microbial growth, which matters for food and pharmaceutical grades, so a cleaning routine is part of the plant hygiene plan.
Loss of flash duct velocity
If the fan loses flow, for example as cyclones choke, the cake no longer lifts through the duct and drops to the bottom, causing wet lumps and choking. Sizing the fan for the dirty system and controlling speed on duct suction keeps the conveying velocity.
Energy and control
Starch plants usually run continuously, and the fan is one of the largest motors in the drying section. Running the dryer at a higher outlet humidity, by reducing air flow while holding product moisture, cuts both steam and fan power. A variable speed drive allows that tuning. Better dewatering before the dryer, a point or two of cake moisture, also reduces the air needed.
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 Forced Draft (FD) Fan for starch flash dryers.
Questions buyers ask
Why is a starch flash dryer exhaust fan made of stainless steel?
The air is humid and warm, and fines collected after the fan may be returned to product. Stainless steel resists corrosion from condensate and cleaning and keeps rust out of the starch. Mild steel fans in this duty corrode and shed rust into deposits. They also clean easily.
How much pressure does a starch flash dryer fan need?
Where a single fan pulls the whole system, typically 400 to 600 mmWC, covering the air filter, heater, flash duct and cyclones. With a separate supply fan, the exhaust fan duty is lower. The exact figure depends on the disperser and cyclone design, so the present suction measured at the fan inlet is the best starting point for a replacement selection.
Is starch dust an explosion risk in the fan?
Yes. Starch is a combustible dust, and the dryer, cyclones and fan need a dust explosion assessment. The fan is earthed, built to avoid rubbing and sparks, and included in the venting or isolation design. Housekeeping matters as well: starch layers settled on the fan casing and nearby ducts can fuel a secondary explosion, so external surfaces are kept clean.
Information needed to quote an ID fan for starch 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.
- Fan arrangement. Whether the plant has one exhaust fan only or a supply fan as well, and the present suction at the fan.
- Collection. Number and size of cyclones, and whether a bag filter or scrubber follows.
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.