JPH0545873B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Field of Application] This invention provides a method for determining the drying end point in the case of high frequency drying of a material to be dried, such as textiles (rolled cheese, skein yarn, textiles, etc.) to a predetermined moisture content. The present invention relates to a method for determining the drying end point for determining the drying end point.

[Conventional technology]

As a method for determining the drying end point when subjecting a material to be dried to high frequency drying (high frequency dielectric heating), for example, the method disclosed in Japanese Patent Publication No. 59-53392 is known. The method for determining the drying end point is to cool and condense water vapor evaporated by high-frequency drying, measure the amount of water, and set the point when the water amount reaches a predetermined amount as the drying end point.

In addition to this, there is also a method of measuring the weight of the material to be dried, and determining the point at which the drying ends when the decrease in weight from the start of drying reaches a predetermined value.

[Problem that the invention seeks to solve]

The conventional method for determining the drying end point described above detects the volume or weight of water separated from the material to be dried, and determines the drying end point when the volume or weight of this water reaches a predetermined set value. It is a method to
There has been a problem in that the moisture content of the dried material at the end of drying varies greatly due to variations in the initial weight and initial moisture content of the dried material.

An object of the present invention is to provide a method for determining a drying end point that can reduce variations in the moisture content of a material to be dried at the drying end point.

[Means for solving problems]

The drying end point determining method of the present invention is a drying end point determining method for determining the drying end point when a material to be dried is subjected to high frequency dielectric heating for high frequency drying, and the method comprises: Measure the plate current at regular intervals, and when the value falls below a predetermined value, calculate the difference between the previous plate current measurement and the current plate current measurement,
The point in time when the difference becomes smaller than a preset value is defined as the drying end point.

[Effect]

According to this invention, the plate current of the high frequency oscillation tube, which decreases as the moisture content of the material to be dried decreases, is measured at regular intervals, and the difference between the previous plate current measurement value and the current plate current measurement value is determined. The drying end point is defined as the point when this difference becomes smaller than the set value, so when the amount of decrease in the plate current per fixed time becomes less than the predetermined value, that is, when the moisture content of the material to be dried per fixed time High-frequency drying can be stopped when the amount of decrease in the amount of water decreases to a predetermined value or less and the moisture content is close to the equilibrium moisture content in the atmosphere, reducing the variation in the moisture content of the material to be dried at the drying end point. can.

Furthermore, although the equilibrium moisture content of the material to be dried varies depending on its material, the point at which the drying ends is when the amount of decrease in the moisture content of the material to be dried per certain period of time falls below a predetermined value. Regardless of the material of the object to be dried, drying can be terminated when the moisture content reaches a low level close to the equilibrium moisture content of the material.

〔Example〕

An embodiment of a high frequency drying apparatus that determines the drying end point using the drying end point determining method of the present invention will be described with reference to FIGS. 1 to 7. As shown in FIGS. 1 and 2, this high-frequency drying device includes a drying chamber 4 in which a pair of electrode plates 1 and 2 are spaced apart and a material to be dried 3 can be inserted therebetween; 4, a pressure reducing means 5 for reducing the pressure inside the drying chamber 4, and a material to be dried 3 in the drying chamber 4.
and a heating means 6 for applying high frequency voltage to the material to be dried by high frequency dielectric heating.

The drying chamber 4 is provided with an opening/closing door 7 for sealing the interior thereof, and is also provided with a cart 8 on which the material to be dried 3 is placed and inserted into the electrode plates 1 and 2 so as to be transportable therein.

The pressure reducing means 5 includes a vacuum pump 9 for reducing the pressure inside the drying chamber 4 and piping connected to the drying chamber 4 via an automatic valve 10, and a pressure regulator 11 for controlling the pressure inside the drying chamber 4. An automatic drainage valve 12 is provided below the drying chamber 4 for discharging condensate generated by heating the material 3 to be dried and evaporating water.

The heating means 6 includes a high-frequency oscillator 14 provided outside the drying chamber 4, and guides the high-frequency output from the high-frequency oscillator 14 to electrodes 16 and 17 inside the drying chamber 4 via a cable 15. 1
6, 17 to the electrode plates 1, 2 provided on the top and bottom or left and right sides of the object 3 to be dried, and the object 3 to be dried is heated by high frequency dielectric. Thus, the operation of the high frequency oscillator 14 is controlled.

The high frequency oscillator 14, as shown in FIG.
It is composed of a power supply circuit 21, a rectifier circuit 22, a filter circuit 23, an oscillation circuit 24, and a coupling circuit 25.

The power supply circuit 21 maintains the voltage of the three-phase commercial power supply at approximately
It consists of a step-up transformer TR ₁ that boosts the voltage to 10 KV, a power switch SW ₁ provided on the primary side of the step-up transformer TR ₁ , and contacts mc ₁ and mc ₂ of electromagnetic contactors MC ₁ and MC ₂ .

The rectifier circuit 22 is composed of six diodes _D1 to _D6 .

The filter circuit 23 is inserted for the purpose of preventing high frequency waves from the oscillation circuit 24 from flowing out to the power supply circuit 21, and includes a coil L ₁ , capacitors C ₁ , C ₂ ,
It consists of a resistor _R1 .

The oscillation circuit 24 is a high frequency oscillation tube whose plate is connected to the rectifier circuit 22 via the filter circuit 23.
TU, an LC tuning circuit TK connected to the plate of this high frequency oscillation tube TU via a DC blocking capacitor _C3 , a grid circuit GR connected to the grid of the high frequency oscillation tube TU, and a high frequency oscillation tube TU.
filament transformer that heats the filament of
The contact mc ₂ of the electromagnetic contactor MC ₂ is turned on to heat _the filament, and the contact mc ₁ of the magnetic contactor MC ₁ is turned on to apply a plate voltage to the high frequency oscillation tube TU. When it is done,
High-frequency oscillation is performed at a frequency of 3 to 30 MHz, and this high-frequency oscillation force is applied between the electrode plates 1 and 2 via the coupling circuit 25 to heat the object 3 to be dried between the electrode plates 1 and 2 by high-frequency dielectric heating.

The coupling circuit 25 consists of fixed capacitors C ₄ and C ₅ and variable capacitor VC.
The capacity of the VC can be changed by a motor (not shown), etc., and the plate current decreases due to the decrease in dielectric loss due to the decrease in the moisture content of the material to be dried 3 due to high frequency drying, which increases the drying efficiency. In order to prevent the plate current I _P from decreasing, the capacitance of the variable capacitor VC is increased using a motor etc. so that the plate current I _P is kept constant.
The coupling between the oscillation circuit 24 and the electrode plates 1 and 2 is gradually made tighter. Note that once the moisture content has decreased to a certain level and the coupling degree of the coupling circuit 25 reaches its maximum, if the moisture content decreases thereafter, the plate current is out of the controllable region, so the moisture content of the material to be dried 3 cannot be controlled. As the dielectric loss decreases, the plate current I _P decreases.

The curve in Figure 4 is the high frequency oscillator tube TU at this time.
This is a time chart showing changes in the plate current I _P , and the section from A to B is the plate current controllable region, and its value is controlled by I _PA . The section after B is the plate current uncontrollable region, where the plate current I _P decreases as the water content decreases.

As shown in FIGS. 3 and 5, the control device 18 includes a resistor R ₂ that detects the plate current _IP of the high-frequency oscillation tube TU, and a voltage across the resistor R ₂ through a filter 26 for removing high-frequency waves. an amplifier AP to amplify;
A sample-and-hold circuit SH that samples and holds the output of the amplifier AP, an analog-to-digital converter AD that converts the output of the sample-and-hold circuit SH from analog to digital, and a timer TM.
It consists of an alarm HC and a central processing unit CPU. Note that in FIG. 5, illustration of the filter 26 is omitted.

This control device 18 is for determining the drying end point when performing high-frequency dielectric heating on the drying object 3 and high-frequency drying the drying object 3, and controls the plate of the high-frequency oscillation tube TU for high-frequency dielectric heating. The current I _P is measured at regular intervals, and the value is set as a predetermined value.
When the value falls below I _PC , the difference ΔI _P between the previous plate current measurement value I _PY and the current plate current measurement value I _PX
The drying end point is determined when the difference ΔI _P becomes smaller than the set value ΔI _PR , and the high frequency oscillation is stopped and a drying end notification is issued.

Here, the central processing unit in the control device 18
The operation of the CPU will be explained in detail with reference to the flowcharts of FIGS. 6 and 7.

First, as shown in Fig. 6, the previous plate current measurement value I _PY is initialized, and the plate current set value I _PC and the difference set value ΔI _PR at which the determination of the difference ΔI _P should be started are set. Set (step x ₁ ).

Next, the electromagnetic contactor MC ₂ is excited to heat the filament of the high frequency oscillation tube TU (step X ₂ ),
After waiting for _a certain period _of time ( _step Start.

Next, start the timer TM (step
X ₅ ), the timer TM generates a pulse at fixed time intervals T.

Then, the timer TM starts and the timer TM
The interrupt process shown in the flowchart of FIG. 7 is performed every time a pulse is input from the CPU to the central processing unit.

In this interrupt processing, the central processing unit CPU issues commands to the sample-and-hold circuit SH and the analog-to-digital converter AD to operate them.
As a result, the current measured value I _PX of the plate current output from the analog-to-digital converter AD is input (step Y ₁ ).

Next, it is determined whether the current measured value I _PX of the plate current is smaller than the set value I _PC of the plate current (step Y ₂ ), and if the determination result is NO, the process ends.

On the other hand, if the judgment result is YES, calculate ΔI _P = I _PY − I _PX and calculate the previous plate current measurement value.
Find the difference ΔI _P between I _PY and the current measured value I _PX of the plate current (step Y ₃ ).

Next, it is determined whether the difference ΔI _P is smaller than the set value ΔI _PR (step Y ₄ ), and if the determination result is NO, I _PY = I _PX and the current measured value I _PX of the plate current is set to the previous value. Let the measured value of the plate current be I _PY (step Y ₅ ),
finish.

On the other hand, if the above judgment result is YES, the excitation of the electromagnetic contactors MC ₁ and MC ₂ is stopped, the high frequency oscillation is stopped, and the high frequency drying is completed (step
_Y6 ).

Next, the alarm HC is used to notify that drying has been completed (step _Y7 ).

Then, the timer TM is stopped and the process ends (step _Y8 ).

With this configuration, as shown in Figure 4,
Every time T after the start of high-frequency drying, the plate current I _P
is measured, this measured value I _PX is compared with the set value I _PC , and only when the set value I _PX becomes smaller than the set value I _PC , for example at points C to H in FIG. _P is calculated, this difference ΔI _P is compared with the set value ΔI _PR , and when the difference ΔI _P becomes smaller than the set value ΔI _PR , for example, the magnetic contactors MC ₁ and MC ₂ are excited at point H in FIG. is stopped, high-frequency oscillation is stopped, high-frequency drying of the object to be dried 3 is completed, and the notification circuit HC notifies that the drying has ended.

In addition, before the high frequency drying operation described above, the object to be dried 3 is placed on a trolley 8 and carried into the drying chamber 4, inserted between the electrode plates 1 and 2, and the door 7 is closed to seal the drying chamber 4. Next, it is necessary to operate the vacuum pump 9 to reduce the pressure inside the drying chamber 4.

Further, after the drying is completed, the automatic valve 13 is operated to introduce air into the drying chamber 4, the door 7 is opened, and the material to be dried 3 is taken out.

According to this embodiment, the plate current I _P of the high frequency oscillation tube TU, which decreases as the moisture content of the material to be dried 3 decreases, is measured at regular intervals, and the previous plate current measurement value I _PY and the current plate current are calculated. Find the difference ΔI _P from the measured current value I _PX , and this difference ΔI _P becomes the set value ΔI _PR
The drying end point is the point when the size becomes smaller.
When the amount of decrease in the plate current I _P per constant time T becomes less than a predetermined value, that is, the amount of decrease in the moisture content of the material to be dried 3 per constant time becomes less than the predetermined value, and the equilibrium moisture content in the atmosphere is reached. The high frequency drying can be stopped when the moisture content of the material to be dried 3 reaches a low value close to The variation in the moisture content of the material to be dried 3 at the drying end point can be reduced.

Moreover, although the equilibrium moisture content of the dried material 3 varies depending on the material, the
Since the drying end point is when the amount of decrease in the moisture content of the material becomes less than a predetermined value, regardless of the material of the material to be dried 3, drying is completed when the moisture content reaches a low moisture content close to the equilibrium moisture content of the material. can be terminated.

In the above embodiment, the drying is performed under reduced pressure. However, depending on the type of the material to be dried 3, it may be dried under atmospheric pressure.
It is not limited to internal pressure.

Further, in the above embodiment, fibers were explained as an example of the material to be dried 3, but the material to be dried 3 is not limited to this, and may be a ceramic product such as ceramics or wood such as a board or a veneer.

〔Effect of the invention〕

According to the method for determining the drying end point of the present invention, the plate current of the high frequency oscillation tube, which decreases as the moisture content of the material to be dried decreases, is measured at regular intervals,
The difference between the previous plate current measurement value and the current plate current measurement value is calculated, and the point at which this difference becomes smaller than the set value is considered the drying end point, so the amount of decrease in plate current per fixed time is determined by the predetermined amount. High-frequency drying is stopped when the moisture content of the material to be dried falls below a predetermined value, that is, when the amount of decrease in the moisture content of the material to be dried per certain period of time falls below a predetermined value and the moisture content becomes low, close to the equilibrium moisture content in the atmosphere. This makes it possible to reduce variations in the moisture content of the material to be dried at the drying end point.

Furthermore, although the equilibrium moisture content of the material to be dried varies depending on its material, the point at which the drying ends is when the amount of decrease in the moisture content of the material to be dried per certain period of time falls below a predetermined value. Regardless of the material of the object to be dried, drying can be terminated when the moisture content reaches a low level close to the equilibrium moisture content of the material.

[Brief explanation of the drawing]

Fig. 1 is a schematic side view showing the configuration of a high-frequency drying device used in carrying out the present invention, Fig. 2 is a schematic front view of Fig. 1 with the door open, and Fig. 3 is a circuit diagram of the high-frequency oscillation device. , Figure 4 is a time chart showing the time change of the plate current of the high frequency oscillator tube.
FIG. 5 is a block diagram of the control device, and FIGS. 6 and 7 are flowcharts showing the operation of the control device. 3... Material to be dried, TV... High frequency oscillation tube.

Claims (1)

[Claims] 1. A method for determining a drying end point when a material to be dried is subjected to high frequency dielectric heating for high frequency drying, the drying end point being determined by keeping the plate current of a high frequency oscillation tube for high frequency dielectric heating constant. Measure every hour, and when the value falls below a predetermined value, calculate the difference between the previous plate current measurement and the current plate current measurement,
A method for determining a drying end point in which the drying end point is determined as the point in time when the difference becomes smaller than a preset value.