EP1415509A1

EP1415509A1 - Device for treating material with the aid of high-frequency electromagnetic radiation

Info

Publication number: EP1415509A1
Application number: EP02741507A
Authority: EP
Inventors: Nicolaas Van Dijk
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-04-20
Filing date: 2002-04-22
Publication date: 2004-05-06
Also published as: WO2002087286A1; NL1017894C1

Abstract

The invention relates to a device for treating material with the aid of high-frequency electromagnetic radiation, comprising a generator (2) for generating the high-frequency electromagnetic radiation, and a treatment cavity. The treatment cavity is formed by a waveguide (1) and also forms the resonator, or tank circuit, of the generator (2). In this way, it is possible to create a very simple device with a very high output (up to more than 1 MW), which is nevertheless not sensitive to changes in the impedance of the material which is to be treated.

Description

Device for treating material with the aid of high-frequency electromagnetic radiation

The invention relates to a device for treating material with the aid of high-frequency electromagnetic radiation, comprising a generator, which comprises a resonator, for supplying high- frequency energy in order to generate the electromagnetic radiation, and a treatment cavity, which comprises at least two parts which lie opposite one another, between which an electric field exists when the device is operating and in which the material can be treated with the radiation which is generated.

In conventional devices, high-frequency energy generated in a separate generator is transported to the treatment chamber where material is to be treated. This is achieved by means of a high- frequency energy-transporting component, which is generally, for example, a (coaxial) cable, waveguide, etc., which, by means of its own impedance is, as it were, to form a bridge between the impedance of the treatment cavity and the impedance of the generator. Since the impedance of the treatment cavity may change with the material to be treated, depending on the material which is to be treated and the state of this material, the impedance has to be matched in each case. For example, when material is being dried or when the quantity changes in the case of a continuous feed, the impedance of the overall treatment cavity may change. This matching of the impedance may be achieved with the aid of tuning units at the treatment cavity or at the generator itself, but it is always possible for high voltages and/or currents to occur in the tuning units, the energy-transporting component and/or the generator. The matching, or the situation in which the impedance is not correctly matched, may cause serious problems. These will become clear from the discussion below.

A general generator for electromagnetic energy comprises a frequency-determining part, a power-amplifying part and a resonator. Obviously, the frequency-determining part determines the frequency of the high-frequency energy which is to be supplied. The power-amplifying part transforms energy from an energy supply into high-frequency energy, which energy is, as it were, temporarily collected/stored in the resonator. In standard generators, in particular powerful generators with an electron tube, the frequency is determined by ensuring that the electron tube conducts or conducts less or does not conduct, at the desired rhythm. In a generator of this type, but also in general, in the first instance a high-frequency signal will be generated, in the form of a square wave or other pulsed signal which comprises numerous frequencies. To supply a defined signal which is required for expedient energy transfer, but also because otherwise the input impedance of the treatment cavity simply cannot be matched to what is required by the generator, some form of filtering is then required. This may be a simple filter, or may be a tank circuit or something similar, which is generally referred to in this context as a resonator. The resonator selects, as it were, the desired frequency from all the frequencies which are generated. The resonator comprises components such as capacitors and coils. In the event of a mismatch of impedances between the various components of the device, for example in the case of load-free treatment cavities, these coils and capacitors, as well as other coils and capacitors which are incorporated in, for example, filters and/or tuning units, will cause (much) higher voltages and/or currents than those which were originally provided, i.e. will act as step-up transformers. For low powers, this is not a problem and it is possible to provide coils and capacitors, and also energy-transporting components, which are able to withstand such conditions. However, this is not readily possible for powers of greater than approx. 150 kW. There is hence a need for devices of the above type and for generators which are suitable for such power levels.

For example, WO 02/05597 describes an appliance for heating or drying material by applying radio-frequency energy to it in a treatment cavity. In this case, the radio-frequency energy is generated in a separate generator and is fed to the treatment cavity via a special supply cable. The known appliance does not always operate satisfactorily. In this case too, generator and treatment cavity are separated from one another by a supply cable. In this case too, it will still always be possible for voltage transformations to occur, as a result of phase shifts and reflected electromagnetic radiation, between various components of the appliance, for example between the treatment cavity and the supply cable if the impedances thereof are not or are no longer tuned to one another. As has been stated, these transformations place high demands on the components, particularly of tuning units, but also on the supply cable etc., which consequently become complex and expensive. The power often has to remain limited for this reason, and consequently many applications remain out of reach. Moreover, in the known appliance the number of components is also greater, for example the distributed inductance, and the increased complexity means that there is also a risk of more failures .

It is an object of the present invention to provide a very simple device of the above type, the power of which is not limited by impedance matching problems, such as step-up transforming of voltages and currents.

This object is achieved by the device according to claim 1. The subclaims prescribe preferred embodiments .

In the present invention, the treatment cavity in which material can be treated is made into the resonator of the generator. By thus enabling the treatment cavity to form part of the generator, i.e. of the generator circuit, it is possible to eliminate the separate high-frequency energy transporting component and the separate resonator of the generator, or more accurately the conventional tank circuit, filter, etc. in the form of coils, capacitors and the like, of the standard generator. The treatment cavity in the form of a waveguide acts as a resonator for the generator and is not sensitive to high voltages or currents, and nor does this resonator provide voltages or currents which are too high for the power-amplifying part of the generator. Therefore, the device no longer comprises components which may supply excessively high voltages as a result of step-up transforming or reflections in the event of mismatched impedances . Even if the resonant frequency of the treatment cavity does not correspond to the frequency which is generated by the frequency-determining part of the generator, only the load of the power-amplifying part will vary, without high voltages being generated. There is no transporting of electromagnetic radiation which may lead to step-up transforming by reflections at impedance mismatching. Therefore, there is no need to impose particular demands on in particular the high- voltage strength of components. A varying load does not per se limit the generator, in particular the power-amplifying component, in any particular way.

Within the context of the present invention, the term treatment of material with electromagnetic radiation is intended to mean that electromagnetic radiation is supplied to this material, so that a certain effect can be achieved by dielectric interaction.

This will be used in particular through the action of the electric field of this radiation, which has a much higher energy density than the associated magnetic field, for heating in the material through dielectric losses, i.e. absorption of radiation in the material. This heating may be an objective in itself, but it is often desired to dry the product. In this context, consideration may be given to a very wide range of products, such as foodstuffs, especially rice, herbs, chemicals, cotton, wood, etc .

Particularly for inherently inexpensive bulk materials, such as wood, cotton and the like, there is a need for drying devices which have a very considerable power and do not have the drawbacks of standard drying techniques, such as vacuum drying or drying using burners or hot surfaces. With these techniques, the drying is often too superficial and too uneven. Electromagnetic radiation has the advantage that it can dry from the inside out, but this requires a relatively great penetration depth. The frequency of the electromagnetic radiation must therefore not be too high. For example, microwave radiation (from "microwave ovens" and the like) is in fact too short- waved, and moreover the associated generators often have a too low output .

In the context of the present invention, the term "high- frequency" is understood as meaning: having a frequency of between 5 MHz and 1 GHz, although in principle the invention is not restricted to this range. This frequency range is limited primarily by the dimensions of the treatment cavity, which is a measure of the associated wavelength. Therefore, for a frequency of 1 GHz, it is possible to build a small device, for example for use in the pharmaceutical industry, which generally treats small quantities of material. The power of the device will then generally be limited to a few kilowatts. Frequencies of around 10⁸ Hz are suitable for treatment cavities with dimensions of the order of magnitude of 2 m and can be used, for example, for relatively expensive or sensitive materials, such as herbs and chemicals . The power of these devices will generally lie between 1 kilowatt and 10 kilowatts, although other powers are not ruled out. Low frequencies, such as the industrially important frequencies 40.68 MHz, 27.12 MHz and 13.56 MHz, can be used for large quantities of material to be treated. The powers may in this case even amount to more than 1 megawatt. Applications of the said installations will be explained in more detail below.

In principle, the treatment cavity is a waveguide of any desired shape. It comprises at least two conductors lying opposite one another, the (application) electrodes. In the space formed between them, the electric field which can be used to treat material is formed when the device is operating. The space which is formed is generally closed off by side walls, which are usually, though not necessarily, also conductors. However, conductive side walls are preferred, since they prevent problems with emission of radiation. A standard waveguide therefore usually comprises a hollow, electrically conductive body with a substantially constant cross-sectional profile.

The waveguide is preferably short-circuited at both ends, for example by closing off the ends using a conductor, although this is not imperative. Although one can also refer to it as a resonant cavity, for the sake of simplicity we shall continue to refer to it as to a waveguide. The waveguide advantageously has an electrical length which corresponds to an integer multiple of half the wavelength of the high-frequency electromagnetic radiation, more advantageously an electrical length which is equal to half the wavelength of the high-frequency electromagnetic radiation. The intention of this measure is to ensure that the radiation precisely "fits" into the waveguide and can produce a standing wave therein. In fact, it is not only the length of the waveguide which is important, but rather a combination of the length, height and width. However, height and width are often selected more or less independently, in order to select a lowest frequency range (cut-off frequency). The length of the waveguide then has to be adjusted accordingly.

The energy transfer is achieved particularly effectively if the radiation precisely "fits" into the waveguide in terms of its wavelength. In the waveguide, at the level of a maximum of the standing wave, a high impedance, equivalent to a parallel circuit of a capacitor and a coil, will exist between those parts between which the electric field prevails.

In a particular embodiment, the treatment cavity determines the frequency of the radiation which is generated. In this case, the treatment cavity serves as a frequency-determining component of the generator of the device. At least partially feeding back a signal from the treatment cavity to the power-amplifying part of the generator automatically results in power being supplied only at the resonant frequency of the treatment cavity. This means that the treatment cavity and the remaining parts of the generator are automatically ideally tuned to one another. In principle, therefore, the transfer of energy to the treatment cavity and therefore to the material to be treated will always be optimum. In this case, the wavelength automatically fits precisely simply because the cavity itself determines the frequency as a result of only the resonant frequency/frequencies being optimally accepted.

In another preferred embodiment, the generator comprises an external oscillator which determines the frequency of the radiation which is generated. In this case, the frequency of the radiation which is generated is therefore determined independently of the resonant frequency of the treatment cavity. This is important, for example, if the frequency of the 5 electromagnetic radiation has to be fixed in connection with statutory radiation requirements. In this case, it is possible that the frequency of the radiation which is generated may not completely match the resonant frequency of the treatment cavity, i.e. the wavelength may not precisely "fit" in the treatment 10 cavity. As has been discussed, this only affects the efficiency of energy transfer. However, there is no generation of (excessively) high voltages.

In a preferred embodiment, the external oscillator has an

15 adjustable frequency. In this way, the external oscillator can easily be tuned to the resonant frequency of the treatment cavity. After all, this resonant frequency may change depending on the material which is to be treated. If the frequency of the external oscillator and the resonant frequency of the treatment

20 cavity correspond, the wavelength will still be made to fit, and the transfer of energy to the material will be at its maximum level. The device advantageously comprises frequency-control means which are designed to measure the resonant frequency of the treatment cavity and are also designed to adjust the

25 adjustable frequency of the external oscillator on the basis of the measured resonant frequency of the treatment cavity. The result is a device which operates completely automatically and always ensures the maximum degree of energy transfer. Another way of ensuring energy transfer is by using frequency-adjustment

30. means in the treatment cavity. These means must then be adjustable, in such a manner that they can match the resonant frequency of the treatment cavity to the frequency of the external oscillator. This will be dealt with in more detail below.

35

In a preferred embodiment of the device according to the invention, in the treatment cavity a treatment compartment is separated therefrom with the aid of electrically insulating material, in which treatment compartment the material can be treated. The said electrically insulating material expediently defines a section of the treatment cavity as the treatment compartment, the treatment compartment being separated from, in particular, the remaining parts of the generator. This provides, 5 inter alia, an increased electrical safety. The electrically insulating material which can be used preferably comprises polypropylene, polyethylene or polytetrafluorethylene, although other materials are not ruled out.

10 The treatment compartment is preferably located in a region inside the treatment cavity where, when the device is operating, the strongest electric field, i.e. the highest electric field strength, prevails. The highest transfer of energy to the material which is to be treated will be able to take place at

15 the location of the highest electric field strength, for example in the case of dielectric heating. In the case of a waveguide of electrical length which is equal to half the wavelength of the electromagnetic radiation which is generated and which is short- circuited at both ends, the field strength will vary over the

20 length of the waveguide, approximately as half a sine wave. In that case, the highest electric field strength will exist halfway along the length of the waveguide. If the waveguide has a different length, the electric field strength will be a different function of the position over the length of the

25 waveguide, but even in this case it is often possible to designate an area where the field strength is highest.

At least at the location of the two parts which lie opposite one another and between which, when the device is operating, an

30. electric field exists, the treatment cavity advantageously comprises a narrowed section, by means of which the electric field strength is increased. The said parts are also known as the (applicator) electrodes. The narrowed section is such that the distance between the said two opposite parts between which

35 an electric field exists is reduced. This has the advantage that, on account of the increased electric field strength, the power density supplied is increased. This opens up the possibility of more intensive treatment, for example more rapid heating. Obviously, a narrowed section is to the detriment of the space in which material can be treated, but it is generally the case that this space is large enough for industrial applications carried out at a high power even with the narrowed section. This is the case in particular at relatively low 5 frequencies of the electromagnetic radiation, since in this case the distances are often great. This will be explained in more detail in the description of the figures with reference to an example. Another advantage of providing a narrowed section is that other parts of the generator can more easily be connected

10 to the treatment cavity, i.e. in particular to the (applicator) electrodes, which generally form part of the treatment cavity. In many cases, an electron tube will be used as the power- amplifying part of the generator, and this tube can be fitted more easily at a narrowed section, i.e. a reduced distance

15 between two (applicator) electrodes.

The narrowed section preferably comprises at least one indentation of the treatment cavity. In this context, the term indentation is understood as meaning that, at that location, an

20 (applicator) electrode is at least partially bent inwards with respect to the cross-sectional profile of the treatment cavity, in such a manner that the distance between this (applicator) electrode and another (applicator) electrode is shortened. It should be noted that in the present context the term bent does

25 not mean that the surface of the (applicator) electrode has to have an undulating surface or any such feature. By way of example, and indeed preferably, the section which is moved inwards is inherently flat.

30. A treatment cavity which is often used is the waveguide described above, which in turn often has a rectangular cross section. A preferred embodiment of such a treatment cavity with an indentation is what is known as a single-ridged waveguide. This has a flat elevation in a wall, with the electric field

35 existing between this wall and the opposite wall, hence in this case the (applicator) electrodes. The elevation in this wall preferably runs from one side wall, which adjoins this wall, of the treatment cavity to the other, opposite side wall, and in particular over the length of the waveguide. Not only is the electric field strength increased in this way, but also a very even electric field exists between the elevated section of one (applicator) electrode and the opposite section of the opposite (applicator) electrode, which has a beneficial effect on the treatment of material, for example very uniform heating thereof. Obviously, it is also possible to provide both opposite parts, between which an electric field exists when the device is operating, with an indentation, in which case the waveguide is known as a double-ridged waveguide. In this context, it is necessary to draw a distinction with respect to what are known as semi-ridged waveguides, which have one or more crosswise ribs . These are intended to make the waveguide flexible.

Particularly in the case of a ridged waveguide, it is possible to draw a clear analogy to a parallel LC circuit as resonator of the generator. The two opposite parts between which the electric field prevails then function, as it were, as capacitor C, while that section of the treatment cavity which is encompassed by these two parts, optionally with material present therein, forms an inductance L. The two parts of the capacitor, in this case in principle two electrically conductive plates opposite one another, are, in the case of the waveguide, extended on both sides by a section of a waveguide which is closed off at the end by a short circuit, all this in such a manner that the overall electrical length of the entire waveguide is an integer multiple of half the wavelength of the electromagnetic radiation. In this way, it is easy to create what is in principle a completely closed box, or treatment cavity, within which the remaining parts of the generator can be arranged. This closed box as a whole will not emit any radiation, and consequently there will also be no signals which interfere with external equipment.

In another embodiment of the device according to the invention, the narrowed section comprises an electrically conductive component which is electrically insulated from the treatment cavity. When the device is operating, voltages and/or currents which are such that the electrically conductive component will effectively act as a wall part of the treatment cavity, i.e. as an (applicator) electrode, will be induced in the said electrically conductive component. In this way, therefore, it is very easy to set the desired electric field strength, for example by selecting the dimensions, in particular the thickness, of the electrically conductive component and/or its position.

The electrically conductive component advantageously has a position which can be adjusted at least in a direction of the electric field. This makes it even easier to set the desired electric field strength of the electric field. For this purpose, it is merely necessary to adjust the distance from the electrically conductive component, for example a plate or the like, to the opposite wall, as seen in the direction of the electric field lines. The adjustment may be effected, for example, with the aid of electrically insulating adjustment means which can move through the outer wall of the treatment cavit .

In a preferred embodiment of the device according to the invention, the treatment cavity is provided with air-conveying means which can convey air along at least part of the material which is to be treated. In many cases, the treatment of the material which is to be treated will involve drying this material. In that case, but also in other possible cases, it may be advantageous if air is conveyed along the material which is to be treated in order to be able to entrain evaporating moisture .

The treatment cavity preferably comprises an air inlet opening and an air outlet opening, and the air transporting means are designed to transport air from outside the treatment cavity along at least a section of the material which is to be treated. In this context, it is advantageous that it is possible to use outside air, since otherwise there is a risk of air which is present inside the treatment cavity becoming saturated with evaporating moisture. Obviously, it would also be possible to select a different gas, which is guided through an optionally closed circuit. The air inlet opening and the air outlet opening may, for example, comprise openings which are protected by grates. These grates are used to prevent radiation from escaping from the treatment cavity. The person skilled in the art will know how to select a suitable size for the openings in the grate. The air transporting means may, for example, comprise a simple fan, but it is also conceivable, for example, to use passive air transporting means, in particular updrafts effected by heat which is generated. In the case of a single-ridged or double-ridged waveguide, the air transporting means could expediently be positioned in (one of) the indentations, on the outer side of the treatment cavity.

The air transporting means are advantageously designed to guide the air which is to be transported along a power-amplifying part of the generator or a cooling device of that part, for example a heat exchanger. In this way, the air which is to be transported can also be used to cool the said power-amplifying part of the generator, for example an electron tube (triode). The air which is heated in this way is advantageously supplied to the material which is to be treated. This preheated air is then even more effective at removing moisture from the material.

The treatment cavity preferably comprises a material supply opening and a material discharge opening, as well as material transporting means for transporting the material which is to be treated through the treatment cavity. Particularly for continuous processes, this represents an advantageous embodiment in which large quantities of materials can be passed through the treatment cavity continuously. The material transporting means comprise, for example, an endless conveyor belt or chain. The material which is to be treated may be arranged on this conveyor belt or chain, etc. The material can be introduced into the treatment cavity via the material supply opening and can be removed again from the treatment cavity via the material discharge opening, which may incidentally be identical to the material supply opening.

In an advantageous embodiment, the material supply opening and the material discharge opening are surrounded by a screening waveguide, the lowest pass frequency of which is higher than the frequency of the radiation which is generated. This is because in principle the material supply opening and the material discharge opening will allow electromagnetic radiation which is generated, often in particular harmonics of the radiation of the fundamental frequency, to escape from the treatment cavity. Surrounding the said material supply and discharge openings with a screening waveguide of the abovementioned type, at least substantially prevents such radiation from escaping. The said screening waveguide has a lowest pass frequency which is dependent on its dimensions and damping which is dependent on the length of the tunnel. The person skilled in the art will find it easy to select the dimensions used for the screening waveguide so that the electromagnetic radiation which occurs will be effectively damped. If desired, use is made of what are so-called chokes, which are known per se, on the screening waveguide.

The abovementioned problems of radiation escaping only occur if there is at least one opening in the treatment cavity. This is the case, for example, with the continuous processes which have been described. On the other hand, it is also possible to treat material in batches. In that case, the treatment cavity is in principle completely closed, apart from air passage openings, when the device is operating. It is then possible, for example, to provide a door, flap or the like for loading the treatment cavity.

If electromagnetic radiation is still able to escape from the treatment cavity, it is expedient if the device according to the invention also comprises frequency-stabilizing means. This makes it possible in particular to satisfy an additional requirement, namely that radiation which is emitted is frequency-stabilized, preferably within the statutory limits therefor. If an external oscillator is being used, the frequency is obviously already stabilized within the limits of the external oscillator itself, but in this case the said frequency-stabilizing means can be used to tune the resonant frequency of the treatment cavity to the frequency of the external oscillator. If the treatment cavity is used as a frequency-determining element, frequency- stabilizing means are desired if, for example, radiation can leak to the outside. The frequency-stabilizing means comprise, for example, a variable capacitor. This is arranged inside the waveguide. In a preferred embodiment, the variable capacitor is designed as a moveable capacitor plate. The capacitor plate is then electrically connected to one of the two waveguide walls between which the electric field exists and can be moved towards the other, opposite wall. The capacitor plate is preferably secured to a wall of the waveguide which lies opposite a ridge and is designed to move towards the ridge. In fact, the frequency-stabilizing means, if the treatment cavity is the frequency-determining component, may also be used as frequency- adjusting means, since, after all, they are able to change the resonant frequency of the treatment cavity.

If the treatment chamber is not being used, but rather the energy is tapped from the whole of the described device in a customary way, using any means which is suitable for this purpose and is known from the prior art, the invention in fact creates a robust high-frequency generator. This generator can in principle be combined with any measure which has been described for the treatment device, to form a preferred embodiment.

The invention will be explained in more detail with reference to the appended drawing, in which:

Figure 1 shows a diagrammatic illustration, partially cut away and in perspective, of an embodiment of the device according to the invention;

Figure 2 shows another embodiment of the device according to the invention, with screened material supply and discharge openings;

Figure 3 shows a diagrammatic cross-sectional view of a device according to the invention, with frequency-stabilizing means; and

Figure 4 shows yet another part of a different embodiment of the device according to the invention, with a differently connected power-amplifying component.

Figure 1 shows a single-ridged waveguide 1, having an indentation or ridge which is diagrammatically denoted by la and two waveguide parts lb. A power-amplifying component, in this case an electron tube, is denoted by 2 and has the grid inside it, shown by dashed lines. A high-voltage feed-through capacitor is denoted by 3 and a grid capacitor by 4a, a coil by 4b and a limiting resistor by 4c. 5a and 5b represent the connections to the high-voltage power supply (not shown) of the electron tube 2, while a cathode voltage is applied between 5b and connection point 6.

An air inlet grate and an air outlet grate are denoted by 7 and 8, respectively, while two insulating plates are denoted by 9. Grates 7 and 8 and plates 9 delimit a treatment compartment 10, which in the figure is filled with material to be treated. Air is passed through the grates 7, 8 and the treatment compartment 10 in the direction indicated by arrows A.

Together with a front and back wall (not shown), the waveguide 1 forms a box which is in principle closed to radiation. The box is generally made from a metal, for example aluminium or steel. Other metals with a good conductivity, such as silver or copper, could also be used, although in many cases the price and/or chemical properties are less favourable.

The length, height and width directions of the waveguide shown are denoted by arrows h, b and 1, respectively, in Figure 1. The height and width of the waveguide parts have an influence on the impedance of the overall waveguide 1 and the pass frequencies thereof, and together with the length have an influence on the effective wavelength of radiation which is generated in the waveguide. The length of the waveguide must be matched to the effective wavelength of the radiation, i.e. must be equal to an integer multiple of half the effective wavelength of the radiation which is generated. In fact the resonant wavelength, and therefore the resonant frequency, are partly determined by the three dimensions length, width and height of the waveguide. In the case of a box-like treatment cavity (waveguide), it is easy to calculate the relationship which is required between the dimensions of the treatment cavity and the frequency (and wavelength) of the desired radiation. The condition is that the radiation which is generated should be resonant in the treatment cavity. If the treatment cavity itself is the frequency- determining component, this condition will automatically be satisfied, generally for one or more modes of (standing) waves. On account of decreasing amplification properties for higher modes, the lowest mode will generally predominate.

If the shape is not precisely a box, as for example in the case of the single-ridged waveguide with a narrowed section and in the case of the double-ridged waveguide with two narrowed sections lying opposite one another, there will be a more complicated relationship between dimensions of waveguide and ridge(s) and the desired resonant frequency. This can still be calculated, but if desired may also be determined by carrying out tests . It is expedient to select at least one wall of the treatment cavity, and in the presence of material transporting means preferably a side wall, which is located symmetrically with respect thereto, of the treatment cavity to be displaceable, for example telescopic. In this way, it is possible to obtain a tuneable treatment cavity.

The electron tube 2 may comprise any suitable electron tube which is able to supply sufficient power. This does not even have to be a problem if powers of 1 MW or even more are required. If desired, a plurality of tubes may be connected in parallel. The tubes are connected to the waveguide 1 via metal plates, which simultaneously serve as current conductors. The points at which they are connected to the walls of the treatment cavity preferably lie close to the ridge and therefore to the (applicator) electrodes. This is easier on account of the shorter distances which have to be spanned. However, this is sometimes disadvantageous in connection with interference fields from the metal plates and/or the power-amplifying component. By way of example, when cheeses and the like are being heated as uniformly as possible, there must be as little interference as possible in the electric field. It is then better for the connecting plates and the power-amplifying component to be arranged at a distance from the material which is to be treated. It is also expedient to move the connections, specifically towards the end wall of the treatment cavity, if the material which is to be treated has very poor radiation adsorption properties, i.e. has a low loss factor. In this case, an electric field-amplifying effect may occur, as a result of the voltage which is generated in the power-amplifying part and supplied to the connecting points of the wall of the treatment cavity no longer coinciding with the position of the maximum of this field strength. This maximum still lies in the centre of the treatment cavity (or at least at a peak of the standing wave) . The maximum field strength will therefore increase, and therefore so will the amount of energy absorbed in the material.

In the circuit which is shown, the capacitor 4a, coil 4b are responsible for correct feedback, i.e. for the correct phase shift, of high-frequency energy which is generated using the grid of the tube 2. Limiting resistor 4c ensures that the feedback current does not become too great. The feedback circuit is not limited to the circuit shown. For example, Fig. 4 shows a second example. This will be explained in more detail below. As an alternative to a feedback circuit, it is also possible to effect coupling to an external oscillator, so that the latter in principle determines the frequency.

Although in principle the entire waveguide 1 can be used as the treatment cavity, in practice a treatment compartment 10 is often separated off with the aid of insulation plates 9. These plates comprise, for example, polypropylene plates. They are used, inter alia, to screen against high voltage prevailing at the tube 2. Moreover, in the ridge la they define a region with an elevated and more constant electric field strength, where, therefore, more intensive treatment can take place.

Air supply and discharge grates 7 and 8 are not required but are often desirable. These are conductive grates and in this case basically form the applicator electrodes. They have small holes with dimensions which prevent radiation from escaping. However, this is not generally a problem, since most frequencies which are used have associated wavelengths which are well below most air-hole dimensions. Air displacement means (not shown), usually a fan, displace air, or another gas if required by the product to be treated, in the direction indicated by arrows A.

Figure 2 shows another embodiment of the device according to the invention, with screened material supply and discharge openings. 11 denotes a material transporting means in the form of an endless belt which runs through screening waveguides 12 which are provided with chokes 13. It should be noted that identical reference numerals denote identical or similar components throughout the entire drawing.

The endless belt 11 can guide material through the waveguide 1 via openings (not shown in more detail). These openings are surrounded by suitable screening waveguides 12, in order to prevent radiation from leaking out of the treatment cavity. Not only could this reduce the output, but also leaks of radiation could cause interference or even physical harm in the surrounding area. In many cases, therefore, there are statutory requirements regulating the level and frequency (bands) of leaks of radiation.

By way of example, in one practical situation radiation at 27 MHz is supplied. If tunnels of 1.3 x 1 m are then fitted (cf. the example which will be discussed below) , the lowest pass frequency will be approx. 120 MHz. Only the 4^th and higher harmonics could then still escape, with an amplitude which is dependent on the length of the tunnel . This may be selected in a suitable way, all this, of course, in practice also being dependent on the length of the endless belt. If the amplitude of the leaking radiation is still above the limit value, chokes 13, which are known from the prior art, may be fitted to the screening waveguides 12 in order to achieve further damping of the relevant frequencies.

In another advantageous embodiment, damping means, in the form of resonator elements, are arranged in the treatment cavity. The purpose of the damping means is to damp the harmonics and generally undesired electromagnetic radiation, in order to prevent it from being emitted. It should be noted that this applies not only to the device according to the invention, but also in principle to all resonant cavities from which electromagnetic radiation could escape.

Resonator elements are elements which resonate at the (frequency of the) undesired radiation which is to be damped. They are, for example, metal so-called quarter-lambda rods of a suitably selected length which are arranged at suitably selected locations in the resonant cavity (treatment cavity) . This length and these positions can be determined by means of tests. In this case, it is advantageous if the length and/or position of the resonator elements can be adjusted. In the device according to the invention, the damping means can be used to specifically damp remaining higher harmonics. For example, if, in the example, the 4^th harmonic is the strongest radiation still present, it is possible to fit suitable damping means for this radiation.

The mechanism of the damping means is currently not entirely understood. However, it is assumed, without the applicant wishing to be tied to this assumption, that the resonator elements impart a phase shift to the prevailing electromagnetic field at the location of possible openings through which radiation could escape which is such that, at the location of these openings, energy can no longer escape, or at least is - damped to a greater or lesser extent.

Figure 3 shows a diagrammatic cross-sectional view of a device according to the invention having frequency-stabilizing means. The frequency-stabilizing means are formed by a moveable capacitor plate 14, a suspension means 15, an arm 16 and a motor 17. A measurement circuit 18 with an amplifier 19 are optionally fitted.

The capacitor plate 14 is intended to change the resonant frequency of the waveguide 1, in order in this way either to match the frequency to the frequency applied by an external oscillator or, if necessary, to additionally stabilize the prevailing frequency in order to satisfy statutory requirements. For this purpose, the frequency matching has to take place sufficiently quickly with respect to the frequency changes resulting from processes in the treatment cavity, such as changes in the quantity of material or intrinsic properties of the material caused by heating, drying, etc. However, most processes take place relatively slowly, so that this will not be a problem.

The said capacitor plate 14 is connected to waveguide 1 pivotably, or at least moveably, by means of suspension means 15 and arm 16. The arm 16 is operated by a motor 17. The position of the capacitor plate 14 has to be varied with respect to the waveguide, and in particular with respect to the ridge, if present. It also holds for the capacitor plate that, if the minimum possible interference with the electric field in the treatment compartment is desired, the distance between plate 15 and treatment compartment has to be sufficiently great.

To determine whether the frequency of the waveguide has been well tuned, it is possible to consider the efficiency of energy transfer to this waveguide. If necessary, the plate 15 can be moved. It is also possible, and this arrangement is easier to automate, for the frequency in the waveguide to be measured directly with the aid of measurement circuit 18. After processing via amplifier 19, the measured signal can be fed to the motor 17, so that the capacitor plate 15 can automatically be correctly positioned.

By selecting the dimensions, in particular the width, of the capacitor plate 15 to be sufficiently large, this design is able to process very high currents without problems, and the distance to the ridge can remain sufficiently great to prevent sparkovers .

Figure 4 shows part of another embodiment of the device according to the invention, having a differently connected power-amplifying component, specifically what is known as a grounded-grid circuit. The part which is shown corresponds to the right-hand part of the device shown in Figure 1.

20 denotes an electron tube, 20a the anode thereof and 20b the cathode, in this case the filament. 21 denotes the grid, which is grounded via capacitor 4a and also at one high-voltage power supply connection 5b. 22 denotes a coil, 23 denotes a heater transformer and 24 denotes capacitors.

Heater transformer 23 supplies current to the cathode 20b via coil 22. High-frequency currents which are generated are short- circuited to ground via capacitors 24, so that they cannot reach the heater transformer.

Some of the high-frequency anode alternating voltage which is generated is returned to the cathode by the internal (not shown) capacitance of the electron tube between anode and cathode. If this internal capacitance is insufficient, a capacitor may be added.

Coil 22 and the internal capacitance (plus optionally added capacitor) of the electron tube between grid 21 and cathode 20b are tuned to a frequency which is slightly lower, for example approximately 10% lower, than the resonant frequency determined by the waveguide. As a result, the high-frequency voltage which is returned at the cathode will be largely in phase with the high-frequency anode voltage, and oscillation occurs if the ratio between anode voltage and cathode voltage is lower than the amplification.

Example

An example of an installation for drying cotton from a moisture content of 17% to 6% will be considered. An industrial frequency of 27 MHz is selected for this purpose. In practice, it has been found that, at a moisture content of 12% and a field strength of 15 kV/m, approximately 250 kW of power are dissipated per 100 kg of cotton. In loose form, cotton of this type weighs approximately 100 kg/m³. Therefore, to evaporate 12 kg of water per 100 kg of material, energy of approx. 27.6 MJ is required. If it is desired to maintain a drying time of 2 minutes, a generated power of 1 MW can therefore dry approximately 400 kg of cotton.

The surface of the treatment compartment where the electric field prevails is, for example, approximately 3 metres long. Over this length, a 1 MW tube will, for example, generate a high-frequency voltage of on average 15 kV. If the thickness of the layer of cotton is 1 m, the field strength will be 15 kV/m and the ridge has to be approximately 1.3 metres wide.

Other suitable uses of high-frequency energy are, for example, the even thawing, heating or melting of frozen products or products which are to be melted, such as for example butter, cocoa or, for reuse, asphalt. Sensitive products are advantageously protected against overheating, burning, while numerous quality aspects, such as odour and taste, are optimally retained.

It is also possible to apply high-frequency energy in order to sterilize or pasteurize material or to bring about or maintain chemical processes.

Claims

1. Device for treating material with the aid of high-frequency electromagnetic radiation, comprising a generator, which comprises a resonator, for supplying high-frequency energy in order to generate the electromagnetic radiation, and a treatment cavity, which comprises at least two parts which lie opposite one another, between which an electric field exists when the device is operating and in which the material can be treated with the radiation which is generated, characterized in that the treatment cavity is the resonator of the generator, which treatment cavity comprises a waveguide ( 1 ) .

2. Device according to claim 1, characterized in that the waveguide (1) has an electrical length which corresponds to an integer multiple of half the wavelength of the high-frequency electromagnetic radiation.

3. Device according to claim 1 or 2 , characterized in that the waveguide (1) has an electrical length which corresponds to half the wavelength of the high-frequency electromagnetic radiation.

4. Device according to one of claims 1-3, characterized in that the treatment cavity determines the frequency of the radiation which is generated.

5. Device according to one of claims 1-3, characterized in that the generator comprises an external oscillator which determines the frequency of the radiation which is generated. -

6. Device according to claim 5, characterized in that the external oscillator has an adjustable frequency.

7. Device according to one of the preceding claims, characterized in that in the treatment cavity (1) a treatment compartment (10) is separated therefrom with the aid of electrically insulating material (9), in which treatment compartment (10) the material can be treated.

8. Device according to claim 7, characterized in that the treatment compartment (10) is situated in an area inside the treatment cavity (10) where, when the device is operating, the strongest electric field prevails.

9. Device according to one of the preceding claims, characterized in that the treatment cavity ( 1 ) has a narrowed section at least at the level of the two parts ( 8 , 9 ) which lie opposite one another and between which an electric field exists when the device is operating, as a result of which narrowed section the electric field strength is increased.

10. Device according to claim 9, characterized in that the narrowed section comprises at least one indentation (la) in the treatment cavity (1).

11. Device according to claim 9 or 10, characterized in that the narrowed section comprises an electrically conductive component which is electrically insulated from the treatment cavity (1).

12. Device according to claim 11, characterized in that the electrically conductive component has a position which can be adjusted at least in a direction of the electric field.

13. Device according to one of the preceding claims, characterized in that the treatment cavity ( 1 ) is provided with air-conveying means which can convey air along at least part of the material which is to be treated.

14. Device according to claim 13, characterized in that the treatment cavity ( 1 ) comprises an air supply opening and an air discharge opening, and in that the air-conveying means are designed to convey air from outside the treatment cavity (1) along at least part of the material which is to be treated.

15. Device according to claim 14, characterized in that the air-conveying means are designed to guide the air which is to be conveyed along a power-amplif ing part ( 2 ) of the generator or a cooling device for this part.

16. Device according to one of the preceding claims, characterized in that the treatment cavity ( 1 ) comprises a material supply opening and a material discharge opening, as well as material conveying means (11) for conveying material which is to be treated through the treatment cavity (1).

17. Device according to claim 16, characterized in that the material supply opening and the material discharge opening are surrounded by a screening waveguide ( 12 ) , the lowest pass frequency of which is higher than the frequency of the radiation which is generated.

18. Device according to one of the preceding claims, characterized in that damping means, in the form of resonator elements, are arranged in the treatment cavity.

19. Device according to one of the preceding claims, characterized in that it also comprises frequency-stabilizing means .

20. Device according to claim 19, characterized in that the frequency-stabilizing means comprise a variable capacitor.

21. Device according to claim 20, characterized in that the variable capacitor is designed as a moveable capacitor plate (14).