JPS6146955B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-frequency dielectric heating device for drying, for example, wood by high-frequency dielectric heating.

Conventionally, high-frequency dielectric heating devices have been used to properly dry wood in a short period of time.

When drying wood using a high-frequency dielectric heating device, it is desirable to input the optimum power according to the moisture content of the wood as a drying schedule for the wood. Therefore, with conventional high-frequency dielectric heating equipment, wood is placed in a sealed vacuum container, high-frequency waves are applied to the wood to perform high-frequency dielectric heating, and the moisture content is calculated by measuring the amount of moisture evaporated from the wood. death,
Electricity was input in stages according to the water content.

However, measuring the amount of moisture evaporated from the wood, calculating the moisture content, and setting the power input step by step is complicated and impractical.

The present invention was made in view of the above circumstances, and
First, the high-frequency power is kept constant, and then the high-frequency power is controlled so that the voltage between the electrodes that apply the high-frequency power to the object to be heated is constant, and high-frequency dielectric heating is performed. It is an object of the present invention to provide a high frequency dielectric heating device that allows easy input of electric power.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows one embodiment of the present invention, in which high-frequency power generated from a high-frequency generator 1 is applied to an electrode 4 that applies high-frequency power to an object to be heated, such as wood 3, through a matching circuit 2. The electrode 4 and the wood 3 are placed in a sealed vacuum container. The inter-electrode voltage of the electrode 4 is detected by an inter-electrode voltage detection device 5 and sent to a control circuit 6.
added to. This control circuit 6 adjusts the output of the control circuit 6 so that when the inter-electrode voltage detected by the inter-electrode voltage detection device 5 reaches a predetermined voltage, the inter-electrode voltage of the electrode 4 becomes approximately constant at that voltage. This is added to device 2. For example, by adjusting the matching device 2 with the output of the control circuit 6, the high frequency generator 1
The output of the control circuit 6 controls the DC input current of the electron tube constituting the oscillator.

That is, in high frequency dielectric heating, the high frequency power absorbed by the wood inside the electrode 4 is expressed as W=KfE ² εtanδ. Here, W is the high-frequency power absorbed by the wood, K is the proportionality constant, f is the high-frequency power frequency, E is the voltage between the electrodes, ε is the dielectric constant of the wood, and tanδ
is the dielectric loss of wood.

In the above equation, since the high frequency power frequency f is constant, if the high frequency power W absorbed by the wood is constant, the dielectric constant ε and dielectric loss tan δ of the wood will decrease as the moisture content of the wood decreases. , the interelectrode voltage E increases. This means that the interelectrode voltage E
If detected, the dielectric constant ε and dielectric loss tan δ of the wood
By knowing this, it is possible to estimate the moisture content of wood.

Therefore, while applying high frequency power to the wood 3, the inter-electrode voltage of the electrode 4 is constantly measured, the moisture content of the wood 3 is estimated, and when the inter-electrode voltage of the electrode 4 corresponds to a predetermined moisture content, The high frequency power absorbed by the wood 3 is controlled so that the voltage is kept substantially constant.

In this way, in so-called low-temperature drying using high-frequency dielectric heating of wood in a sealed vacuum container, the moisture content of the wood is high in the early stages of drying, and the high-frequency generator can operate at close to 100% output. As the water content decreases as the process progresses, the impedance as a load increases and the voltage between the electrodes also increases. In such a state, dielectric breakdown or corona discharge may occur, making continuous safe operation impossible. Therefore, when the interelectrode voltage reaches a predetermined voltage,
From constant power control of high frequency power absorbed by wood,
By switching to constant voltage control of the inter-electrode voltage, heating and drying can be carried out safely. In particular, when the load is wood, drying proceeds at high efficiency until the predetermined moisture content is reached, and after that the high frequency voltage of the electrode is suppressed to a voltage that does not cause discharge, which reduces the power density per unit volume. This also eliminates the so-called cracking phenomenon in which the wood forms in a row at the end of drying, and allows the drying to be completed safely without damaging the wood due to electrical discharge.

In addition, in order to control the high frequency power absorbed by the wood, the DC input may be controlled by directly applying the output of the control circuit to the high frequency generator 1.

Also, switching from constant power control to constant voltage control is as follows:
The moisture content of the wood may be estimated in advance and the switching may be performed based on time, or the moisture content of the wood may be detected and controlled.

FIG. 2 is a concrete example. Here, beech board material is used as the wood to be dried, the filling amount of wood is about 15 m ³ , the initial moisture content of the wood is about 65%, the finished moisture content is about 10%, the internal pressure of the sealed vacuum container is constant at 45 Torr (the saturated steam temperature is about
36℃), oscillation frequency _4MHZ as a high frequency generator
Equipped with automatic alignment adjustment device. In FIG. 2, E is the voltage between the electrodes (kV), W is the high frequency power absorbed by the wood (kV), and B is the moisture content (%) of the wood. That is, judging from the resin and shape of the wood to be dried, the moisture content is continuously heated and dried under constant power control regardless of the interelectrode voltage E until the moisture content reaches approximately 22 to 25%, and after that, the moisture content is reduced to 10% from 22%.
Until then, constant voltage control was used. As shown in Figure 2, the interelectrode voltage E changes as the water content changes, and A ₃
Up to this point, constant power control is performed regardless of the interelectrode voltage E, and a water content of about 22% is detected at ₃ points A, and from then on, constant voltage control is performed. Like this A ₃
By setting the points, it is possible to measure the moisture content of the sample material during drying, or to automatically predict the moisture content without depending on the amount of evaporated water and control the input power accordingly. As a result, a drying schedule can be automatically set according to an arbitrary program from the start to the end of drying without complicated drying.

As described above, according to the present invention, heating and drying can be performed efficiently and safely by first performing high-frequency dielectric heating under constant power control and then performing high-frequency dielectric heating under constant voltage control. A high frequency dielectric heating device suitable for long-term heating and drying can be provided.

[Brief explanation of the drawing]

FIG. 1 is a configuration explanatory diagram showing an embodiment of the present invention;
FIG. 2 is a curve diagram showing an example of electrical control characteristics according to the present invention. DESCRIPTION OF SYMBOLS 1... High frequency generator, 2... Matching circuit, 3... Wood, 4... Electrode, 5... Inter-electrode voltage detection device, 6... Control circuit.

Claims (1)

[Claims] 1. An electrode that supplies high-frequency power to be absorbed by the object to be heated, a high-frequency generator that applies high-frequency power to this electrode, an inter-electrode voltage detection device that detects the voltage of the electrode, and an inter-electrode voltage detection device that detects the voltage of the electrode. 1. A high-frequency dielectric heating device, comprising: a control circuit that controls high-frequency power applied to the electrode so that the voltage applied to the electrode is substantially constant at a predetermined voltage.