Advantages and Disadvantages of Dryers

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Advantages: flexible operation; corresponds to the structure of a working day in small, medium, and large farms or agricultural enterprises; simple operation; low investment costs; suitable for low, medium, and large throughputs; mainly suitable for uncut plants in large layers.

Disadvantages: higher specific energy consumption, danger of uneven drying across the height of layer, higher labor demand for filling and discharging of dryer or high investment costs for automation of product handling, staged drying can be carried out only with high expense in time control of relative humidity of waste air. Basculating-Tier Dryer (Kiln) (Figures 8.2.8, 8.2.9)

Advantages: flexible operation, corresponds to structure of working day in small and medium farms and agricultural enterprises, staged drying possible by additional air conduits with low expense, automatic product turning through gravity dumping leads to an even drying, suitable for cut material that does not stick to the tiers, high drying area combined with low ground area demand, low labor demand for filling and discharging.


Roller-textile woven belt

Waste air recirculating conduct

Waste air recirculating conduct

Textilejwoven tape

Perforated metal sheet floor Air conduit-heated air ted air ted air

Ground plan

Chamber 3

Chamber 2

Chamber 1

Chamber 2

Chamber 1

Woven belt

Movable discharging truck for filling


Air heater i


Air heater i


Air conduit

Air conduit

Temperature probes


Temperature probes

Discharging conveyor band

Movable conveyor band for filling

FIGURE 8.2.6 Static dryer with filling and discharging conveyor bands and air conduit for partial recirculating of air flow [17].

FIGURE 8.2.7 Static dryer for chamomile in Slovakian Republic.
Presse Huile Palme
FIGURE 8.2.8 Basculating tier dryer (kiln) [3, 4].

FIGURE 8.2.9 Basculating tier dryer (kiln) [3, 4].

Disadvantages: medium investment costs; not suitable for whole, uncut plants or material sticking to the tiers.

Advantages: high throughput per drying area, suitable for a small range of different products, staged temperatures and air velocities easy to adapt to the drying curve of all products, even drying through several product turning by gravity dumping, lower energy consumption, corresponds to the structure of a working day in medium and large farms and agricultural enterprises, low labor demand for filling and discharging.

Disadvantages: no correspondence with structure of a working day in small farms and small agricultural enterprises, as a continuous feeding and operation in three shifts per day is necessary for an economic run with good product quality; high investment costs.

The investments for basculating-tier dryers and band dryers can only be justified if medium or higher throughputs in the range above 100-200 kg of raw material per hour will be reached and the period of operation will last up to 6 months per year. That means that other medicinal plants and spices and even vegetables or fruits have to be processed additionally.

In general the investment costs rise with increased degree of automation, causing lower specific energy costs and lower specific labor costs, too.

FIGURE 8.2.10 Five-band dryer, cross section [7].
FIGURE 8.2.11 Five-band dryer [6].
FIGURE 8.2.12 Inclined harp machine for cleaning of dried chamomile flowers [15].

8.2.6 Processing of the Dried Product

The final processing of dried plant material results in a first-class, nearly pure flower product. The dried flowers are cleaned in a so-called inclined harp machine. Long stems are separated from the flowers. Figure 8.2.12 shows such a machine with a throughput of up to 120 kg/h, which was developed in the Slovak Republic [15].

8.2.7 Supplement: Recommendations for Drying

The following are recommendations for drying of chamomile flowers in static and band dryers. Static Dryers

• Quick drying after harvest, storage of harvested material not in high layers and not under the sun, maximum storage time 2 hours, for longer periods cooling with ambient air is necessary.

• Air velocities approximately 0.15-0.20 m/s for static dryers corresponding to a specific air flow of approximately 540-720 m3/(m2*h), for solar-assisted static dryers values are approximately 0.1 m/s or 360 m3/(m2*h).

• Maximum relative humidity of drying air should be below 30% at 60°C for partial recirculating of waste air, for lower air velocities or drying in deep beds, 10% r.H. should not be exceeded, affording a pure fresh air operation without recirculating [14].

• In case of limited heating capacity and a maximum drying temperature of 45°C, fresh air operation is recommended until water content of lot drops under 60% w.w.b.

• Feeding height of layer max. 30 cm corresponding to a raw material charging of approximately 60 kg/m2 (air pressure drop of up to 170 Pa at air velocity of 0.2 m [11]), for solar-assisted dryers with lower air velocity max. 15 cm corresponding to 30 kg/m2 (air pressure drop up to 85 Pa at air velocity of 0.2 m/s) of raw material load.

• Even spreading of material on dryer (bed height, density).

• Drying time at least 16-20 hours at 60°C, at 45°C up to 30 hours, for solar-assisted dryers up to 70 hours.

• Even drying by careful turning of product (manually), for static dryer turning is recommended after 8-10 hours, thus overdrying of bottom layers and recondensing of water vapor in the top layers is avoided and even drying throughout the lot is promoted.

• Reduction of air temperature and air velocity after advanced drying time recommended due to energetic and quality reasons (measuring of relative humidity of waste air as guideline).

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