At the Research Center for Non-Ferrous Metals (FNE) in Freiberg a microwave drying system with a power output of 12 KW was built. For control, regulation and monitoring of the microwave power the fiber-optic thermometer getTemp-4 was used. The getTemp-4 contains 4 measuring probes. Therefore the temperature can be measured at 4 different positions in the oven.
In order to avoid local overheating or firing due to the coupling of the material to be dried, a temperature regulation is coercively necessary.
Front view of microwave drying system
Measurement setup:
The fiber-optic probe of the thermometer was positioned inside the oven (right onto the sample) by means of a lead-through in the housing of the plant. Due to the fiber-optic signal transfer microwaves cannot influence the signal. The thermometer is applicable for temperatures up to 350°C.
The temperature can either be read off the display or the temperature is evaluated by the software included in the delivery. Furthermore it is possible to use 4-20 mA output signal like in this application case.
Performance and controlling of the plant:
The conditions for the exact compliance of the actual value presettings for the drying experiment were unfavorable concerning the material quantity / power ratio. It was approx. at 2 kg Si, approx. 1,5 l water and a heating power supply of von 6 kW.
Graphic process of a microwave drying cycle, temporal course of actual and set-point temperature and microwave power
The curve progressions in figure 2 show that the process regulation via the getTemp temperature sensor of the fiber-optic system works markedly well. The set-point presetting for the heating-up curve is traced by the actual value with slight and technically arguable deviations.
The same applies to the holding circuit range at 80°C. Furthermore the effect of the beginning water evaporation along with the upturning actual value temperature of 100°C +/- 2°C at a regulator presetting of 108°C. After complete evaporation of the water the temperature rises erraticly, yet the regulator is able to react immediately which is shown by the control factor of the microwave power returning to “0”. After the cool-down and approach to the set-point presetting of 110 °C the power switches in, however without water evaporation with an accordant lower power.
Perspective:
It is expected that with increasing material quantity and therefore a better material quantity / power ratio the curves will be maintained with higher accuracy. Moreover the overheating after complete evaporation should turn out lower.
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