GB1583462A

GB1583462A - Apparatus for continuous dielectric heating by means of microwave energy

Info

Publication number: GB1583462A
Application number: GB38381/77A
Authority: GB
Original assignee: Hermann Berstorff Maschinenbau GmbH
Current assignee: KraussMaffei Extrusion GmbH
Priority date: 1976-09-21
Filing date: 1977-09-14
Publication date: 1981-01-28
Also published as: DE2642335C2; FR2365269B1; DE2642335B1; SE7710547L; FR2365269A1; SE424131B

Description

(54) APPARATUS FOR CONTINUOUS DIELECTRIC HEATING BY MEANS OF MICROWAVE ENERGY (71) We, HERMANN BERSTORFF MASCHINENBAU GmbH, a body corporate organised and existing under the laws of the Federal Republic of Germany, of 3, Hannover Kleefeld, An der Breiten Wiese 3/5 Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to apparatus for con tinuous dielectric heating by means of microwave energy.

Such apparatus may have a treatment chamber, particularly a treatment chamber which is constructed as a tunnel oven, for the continuous dielectric heating or rod-shaped material, particularly made from synthetic plastics or rubber.

The microwave generator, an energy supply waveguide, the treatment chamber and the load in most cases exhibit different impe dances in microwave systems. If the impe dance of the microwave generator, the energy supply waveguide and the treatment chamber on the one hand and the impedance of the load on the other are different, then it is necessary to incorporate between the two a matching transformer, the purpose of which is to adapt the impedance of the load to the impedance of the generator, the energy supply waveguide, and the treatment 'chamber.

Optimum utilisation of energy is guaranteed only if all the energy generated in the mic rowave generator is converted into heat. in the load. This means that there must be no reflections from the load.

In order to ensure low-reflection from m the load, it would be possible to dispose one or a plurality of tuning elements, e.g. rods or screws, so that they are displaceable and can protrude into the microwave energy supply waveguide. With such a tuning element or elements, which acts or act as a matching transformer or transformers, it is in theory possible to adapt any desired load impedance to any desired generator impedance.

In the event of quite substantial differences in impedance of microwave generator and load, high reactive powers occur between the matching transformer and load and act on the supply waveguide, and can represent many times the effective power. The excessively high field strength values which occur as a result of the high reactive powers, in the gap between a tuning rod and a rod guide bore or between a tuning screw and a threaded aperture in which it is mounted, produce arc discharges which lead to erosion and thus to gradual destruction of the contact point between the tuning element and the wall of the energy supply waveguide.

According to the invention, there is provided apparatus for continuous dielectric heating by means of microwave energy comprising a treatment chamber constructed as a tunnel oven, for the continuous dielectric heating of rod-shaped material, and at least one tuning element so disposed that it can be moved and caused to protrude into an energy supply waveguide extending from a microwave generator to the treatment chamber, wherein the tuning element is guided for axial movement in a bore in the wall of the waveguide and, for length X/4 measured along the bore from the plane of the inner face of the wall of the waveguide, there is an insulating gap between the surface defining said bore and the tuning element, at a distance of X/4 from said plane the crosssection of the bore widens and remains wider for an intermediate portion of the bore of length A/4 and an outer portion of said bore is of reduced cross-section compared with said intermediate portion.

Thus ready matching between the microwave generator and the load can be made possible without the risk of flashover and arcing in the gap between the tuning element and its guide bore in the wall of the energy supply waveguide.

The necessary electrical contact between the tuning element and the wall of the waveguide is established without the requirement for a solid conductor. Thus the variations in cross-section of the bore act to produce an effective electric short-circuit between the tuning element and the wall of the waveguide in said plane.

The widening out of the cross-section of the bore after the inner portion of length A/4 means that the energy density in the intermediatc portion of enlarged cross-section is very much less than in the inner portion with the X/4 long insulation gap, so that at the cross-sectional narrowing after the intermediate portion of length A/4, a substantially lower current loading occurs than at the plane of the wall of the waveguide. Therefore, the problem of contact at the outer portion of narrowed cross-section is not so serious.

The tuning element is disposed in the bore in insulated fashion in said inner portion of the bore of length A/4. In the simplest case, this insulation can be achieved by a narrow air gap. If required however the tuning element and/or the bore may be provided with an insulating coating.

It is an important advantage of the invention that an adjustment of the matching transformer can be possible while the microwave plant is operating, without harmful arc discharges and flashovers occurring in the gap between the tuning element and its guide in the wall of the energy supply waveguide.

The invention is diagrammatically illustrated by way of example with reference to the accompanying drawings, in which: Figure 1 shows apparatus for continuous dielectric heating by means of microwave energy according to the invention, in the form of a mic rowave plant with a treatment chamber constructed as a tunnel oven; and Figure 2 shows on an enlarged scale a detail of the apparatus of Figure 1.

Referring to the drawings a rubber rod 4 extruded from an extruder 3 is fed by means of an endless conveyor belt 6, driven by a motor 5, continuously through an inlet aperture 7 of a treatment chamber 8 constructed as a tunnel oven of approximately square cross-section. After passing through the entire length of the treatment chamber 8,.

and now dielectrically heated rubber rod 4 emerges through an outlet orifice 9 and is fed to further treatment stages, e.g. conventional heating apparatus.

The microwave energy is introduced into the treatment chamber 8 at two coupling positions 11 and 12. The first coupling position 11 for microwave energy is disposed onequarter of the length of the treatment chamber from the inlet aperture 7. A rectan gular hollow waveguide 13, connected to a first microwave generator (not shown), is provided horizontally on one side of the passage and forms a first energy supply means.

The second coupling position 12 for microwave energy is disposed threequarters of the length of the treatment chamber from the inlet aperture 7. A hollow waveguide 14 constituting a second energy supply means and of rectangular cross-section like the first waveguide 13 is provided to extend in a direction at right-angles to the first waveguide 13 and into the top of the treatment chamber and links a second microwave generator 15 with the interior of the treatment chamber 8. By virtue of the fact that the two microwave supply means are offset by 90" from each other, two Hw waves are generated which are rotated through 90 with respect to one another. For different rod profiles of the rod of material 4 which is to be heated, therefore, there is a favourable energy absorption angle between profile lip and electrical field line.

Provided on the waveguide 14 which forms the second energy supply means is a matching transformer 16. The matching transformer 16 has a bush 17 inserted into a bore 18 provided in a wall 21 of the waveguide 14. A collar 19 provided on the outer face of the wall 21 reinforces the bush 17. The bore of the bush 17, is designed and dimensioned to form a narrow insulation gap 23, for a zone which extends a distance equal to one-quarter of the free wave length, i.e.

X/4, from the plane of the inner face 24 of the wall 21, around a tuning element 25 which projects through the bore of the bush 17 and is displaceable into the waveguide 14.

After said distance equal to one-quarter of the free wave length, the diameter of the bore in the bush 17 is increased for a zone 26.

Viewed in the axial direction of the bush 17, this increased diameter zone 26 likewise extends for a distance A/4. Then, for a final outer zone 28 the bore is of the same size as it is in the zone in which the insulation gap 23 is provided, thereby to form an air gap 22 in the zone 28.

An adjusting means (not shown) engaged with a lug 27 on the tuning rod 25, can effect adjustment of the position of the tuning rod 25.

The fact that the cross-section of the bore in the bush 17- widens out in the zone 26, after the first portion of axial length A/4, produces a substantially diminished energy density in the zone 26 of the bore in the bush which is of enlarged cross-section. Therefore, in the zone 28 of the bore which is remote from the waveguide 14, there is a considerably lower current loading than at the inner face 24 of the wall 21. In the zone 28 which is remote from the waveguide 14 the bush 17 need not be insulated from the tuning rod 25.

WHAT WE CLAIM IS: 1. Apparatus for continuous dielectric heating by means of microwave energy comprising a treatment chamber constructed as a tunnel oven, for the continuous dielectric heating of rod-shaped material, and at least one tuning element so disposed that it can be moved and caused to protrude into an energy supply waveguide extending from a microwave generator to the treatment chamber, wherein the tuning element is guided for axial movement in a bore in the wall of the waveguide and, for length A/4 measured along the bore from the plane of the inner face of the wall of the waveguide, there is an insulating gap between the surface defining said bore and the tuning element, at a distance of )i/4 from said plane the crosssection of the bore widens and remains wider for an intermediate portion of the bore of length A/4 and an outer portion of said bore is of reduced cross-section compared with said intermediate portion.

2. Apparatus for continuous dielectric heating by means of microwave energy substantially as hereinbefore described and illustrated with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

28 which is remote from the waveguide 14 the bush 17 need not be insulated from the tuning rod 25.

WHAT WE CLAIM IS: 1. Apparatus for continuous dielectric heating by means of microwave energy comprising a treatment chamber constructed as a tunnel oven, for the continuous dielectric heating of rod-shaped material, and at least one tuning element so disposed that it can be moved and caused to protrude into an energy supply waveguide extending from a microwave generator to the treatment chamber, wherein the tuning element is guided for axial movement in a bore in the wall of the waveguide and, for length A/4 measured along the bore from the plane of the inner face of the wall of the waveguide, there is an insulating gap between the surface defining said bore and the tuning element, at a distance of )i/4 from said plane the crosssection of the bore widens and remains wider for an intermediate portion of the bore of length A/4 and an outer portion of said bore is of reduced cross-section compared with said intermediate portion.
2. Apparatus for continuous dielectric heating by means of microwave energy substantially as hereinbefore described and illustrated with reference to the accompanying drawings.