Challenge
The rice flour entered at 38% moisture and had to leave at 13%. The first trial stalled with the outlet moisture stuck around 20%: in the middle section the moisture was still above 30%, and once the material temperature rose the rice starch gelatinised, the particles turned sticky, lumped and built up on the wall, fouling the heat exchange surface and making the discharge worse.
Solution
The temperature curve was split into three stages - hold the temperature below 30% moisture, release it step by step from 30% to 20%, and raise it to finish below 20%, with air volume made up; a high-speed rotary disintegrator was added in the feed section; a cooling section brought the discharge to ambient temperature before bagging; and the dust collector was selected for combustible dust with venting, earthing and cross-bonding.
Result
On the re-run the outlet moisture held steady at 13% and 1.5 tonnes per hour of dry flour was reached; after cooling there was no moisture regain or caking; the line of about 54 kW passed factory audit and safety inspection.
Equipment in this case
Flash Airflow Dryer
Capacity varies with operating conditions and is confirmed by a trial run; parameters fixed by machine size, such as air volume, can be stated precisely.
Related questions
How is dryer capacity stated?
A few points need to be clear about capacity figures:
1. Capacity is not a fixed value. The same machine will produce different figures for a different material, a different initial moisture content or a different target final moisture.
2. Figures must state whether they are on a dry basis or a wet basis. Dry basis means the mass of bone-dry material; wet basis includes the moisture. The two differ substantially and the ambiguity causes problems if it is not stated.
3. Official data comes from a trial run. Until the operating parameters are fixed, any capacity figure is only an order of magnitude.
4. Air volume can be stated precisely. It is fixed by the machine size, does not change with the material and can be given at the selection stage.
Providing the material name, the initial moisture content and the target final moisture is what makes a recommendation usable.
Flash airflow dryer or rotary dryer: which one?
The two have different structures and suit different duties:
Flash airflow dryer
It dewaters through brief contact between high-temperature air and the material. Drying is fast, which suits inorganic salts and chemical powders that need rapid dewatering. Sizes are Φ220, Φ280, Φ325, Φ400, Φ500 and Φ600.
Rotary dryer
The rotating drum tumbles the material for more even heating. It suits sawdust, plastic pellets and flake materials that do not break up while tumbling.
How to decide
Look at the material form and whether it is sticky. Powders and fast-dewatering duties point to flash airflow; granular or flake materials needing even heating point to rotary. The moisture content and target final moisture still have to be considered.
Electric heating or steam heating?
The choice mainly follows the site energy supply:
Steam heating
Suits plants that already have a boiler or steam piping. Running cost is relatively lower and it fits long continuous production.
Electric / electromagnetic heating
Suits sites with no steam supply, requiring no piping, with flexible deployment and fast start-up. The electromagnetic series handles roughly 200–5000 kg/h, provided the electrical capacity matches.
How to decide
Check whether a boiler exists, whether electrical capacity is sufficient, and which running cost is lower over time. With adequate power and no steam, electric heating is the simpler choice.
Can the equipment be customised, and what is the lead time?
Equipment is built to suit the operating conditions. Lead time depends mainly on the machine size and how complex the configuration is, and scheduling is arranged once the requirement is clear.
The parameters to provide in advance are: material name and properties, initial moisture content, target final moisture, daily throughput, and the available space and energy supply (electrical capacity or steam parameters).
The clearer the parameters, the more accurate the selection and quotation, and the easier the schedule. If they are not yet clear, a small trial run is the usual way to establish them.
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