Possible Energy Saving through Partial Recirculating of Air Flow [13

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Drying temperature

Relative humidity due to partial recirculating of air flow

Prolongation of drying time/reduction of dryer capacity

Energy saving in comparison to pure fresh air operation

Drying time in min

FIGURE 8.2.5 Microwave-assisted warm air drying of chamomile flowers [8], influence of microwave energy application on drying time.

Drying time in min

FIGURE 8.2.5 Microwave-assisted warm air drying of chamomile flowers [8], influence of microwave energy application on drying time.

curves for different microwave energy concentrations, calculated in kWh per kg of dried product with a final water content of 10%. All experiments were carried out at an air temperature of 60°C, a relative humidity of 10-14%, and an air velocity of 0.2 m/s. A high microwave energy input of 10.8 kWh/kg results in a reduction of drying time of 60% in comparison with nonmicrowave warm air drying [8].

Focusing the interest on the results of the experiments of K4 and K5, a shorter drying time in the case of K5, it can be stated despite equal microwave energy concentration. In K4 microwave energy is applied in the water content range of 85 to 65% w.w.b. (5.67 kg/kg to 1.86 kg/kg bone dry basis, or b.d.b.), whereas in the case of K5 the water content ranges from 81 to 50% w.w.b. (4.26 kg/kg to 1.00 kg/kg b.d.b.). Obviously there is a higher efficiency in microwave energy absorption in this lower water content range. The microwave energy input in these experiments is rather high; further trials are necessary to optimize the quantity and the time of application of microwave energy.

The analysis in Table 8.2.2 shows higher total contents of the essential oil for the microwave-treated samples and supports the application of microwave energy. K0 is a reference sample naturally dried in a shaded place at ambient temperature [8].

Microwave energy application also affects the optical quality of dried chamomile flowers. A better color preservation and bigger flower heads were stated for microwave-treated samples in comparison with air-dried products [10].

8.2.5 Drying Equipment

The following systems and dryers can be applied for the drying of chamomile flowers or mixtures of herbs and flowers: drying on a covered floor, drying in trays, discontinuous static dryers with or without solar preheating of air, half-continuous basculating tier dryers (kilns), and continuous-band dryers. Table 8.2.3 gives a summary on dryers for chamomile.

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