GB2254763A - A microwave coupler and method of heating a substance in a vessel using microwaves - Google Patents

Abstract

A microwave coupler for transmitting microwave energy into a vessel 5 through an aperture in the vessel is described which comprises a hollow waveguide 6 having an outer wall 6a for conection at one end thereof to the vessel 5 about the aperture AA', and an inner wall 7 within the outer wall 6a defining a passage 7a, wherein in use an electromagnetic field is confined within the volume between the inner and outer walls 6a and 7, and the inner wall 7 defines a passage from within the vessel 5 and is devoid of the electromagnetic field in use. Viscous material such as honey present in the container is made fluid by the microwave heating and pumped out through the inner passage which may extend into vessel 5 (fig 3, not shown) or a dielectric tube (8, fig 2) that passes through passage 7a. The material is not subject to further microwave heating whilst in the waveguide 6. <IMAGE>

Description

A MICROWAVE COUPLER AND METHOD OF HEATING A SUBSTANCE IN A VESSEL USING MICROWAVES The present invention relates to a microwave coupler and particularly to a microwave coupler and method for transmitting microwave energy into a vessel through an aperture.

There are many substances used in industry that due to their high viscosity are difficult to remove from their shipping containers. The viscosity of such substances can be reduced by heating in order to enable the resultant liquid to be poured or pumped out. However, in some cases the substance e.g. honey is particularly sensitive to temperature and therefore the liquidification usually takes place by storing the containers at an elevated temperature for some time. The energy consumption for such treatment is however high.

Many industrial processes take place in metal vessels which have the potential to be adapted into microwave cavities to permit the heating of a material.

For instance, filter cake could be dried, a resin heated, or fats and oils melted using such a method.

To overcome the disadvantage of a high energy demand when conventional heating techniques are used, apparatus has been designed and disclosed in the specification of US 4778970 which utilises microwave energy to heat the substance. In this document a microwave heating source is disclosed which attaches to the open end of a metal drum.

This apparatus however does not allow for the simultaneous removal of the liquidified substance from the same aperture. If removal of the liquidified substance is desirable simultaneously to the heating operation, then a second aperture must be provided in the container. Such a second aperture is not always available and further the heating apparatus disclosed in this document could not be used for a container such as a standard 45 gallon drum which has a single nominal 2 inch bung hole.

The specification EP 0201947 describes apparatus for the uniform heating using microwaves of a substance within a cavity not specifically designed for microwave use.

However this document is concerned with the uniform heating of the substance within the cavity and not with the provision of a system to allow simultaneous heating and removal of the substance when only one aperture into the cavity is available.

Apparatus is disclosed in the specification of WO 87/03865 which overcomes this limitation by attaching a microwave -waveguide to the aperture of a vessel and is provided with an outlet pipe from the waveguide to allow the liquidified substance to exit the vessel via the waveguide to the outlet pipe. Such operation occurs by the action of gravity and requires that the aperture in the vessel is located near its base.

The problem with such a technique is that the aperture must be of a size large enough to allow the coupling of standard waveguide components. Also, since the liquidified substance is leaving the vessel via the waveguide, the substance may absorb significant microwave energy during transit, this being the region of greatest power density, causing unwanted and unnecessary further heating, and possibly overheating of the substance. This absorption of microwave power would also lead to a reduction in the microwave power being transmitted to the substance in the vessel.

A further problem with such an arrangement is that since the material is passing through the waveguide, the impedance matching of the microwave power source to the interior of the vessel will be influenced and will fluctuate both as a consequence of the container emptying and the heating of the substance in the region of the aperture giving rise to fluctuations in the energy transfer to the substance in the vessel. This impedance mismatch results in the inefficient use of microwave power and possibly as a result of large amounts of reflected power, the damage of the power source.

Yet a further disadvantage of the arrangement described in the specification of WO 87/03865, is that the removal of the liquidified substance is dependent on the action of gravity. This is frequently very slow and further requires that the vessel be positioned with its aperture at a low position. Such a requirement undesirably requires additional handling of the vessels since, for example, metal drums are typically transported on wooden pallets which can be moved by forklift truck and the need to invert or tilt the drum to permit the flow under gravity requires more equipment and manpower.

Accordingly the present invention provides a microwave coupler for transmitting microwave energy into a vessel through an aperture in said vessel, said microwave coupler comprising a hollow waveguide having an outer wall for connection at one end thereof to said vessel about said aperture, and an inner wall within said outer wall defining a passage, wherein in use an electromagnetic field is confined within a volume between said inner and outer walls, and said inner wall defines a passage from within said vessel, said passage being devoid of the electromagnetic field.

Preferably the inner wall of the hollow waveguide extends from within said end of said hollow waveguide to provide an extension which acts as a monopole antenna when in use and the internal diameter of said inner wall defining said passage can be below a size necessary for the transmission of microwave energy.

The microwave coupler preferably includes a waveguide coupling piece connected to a second end of said outer wall of said hollow waveguide and is adapted to provide good impedance matching to allow the efficient transmission of microwaves through said waveguide coupling piece and said hollow waveguide to said vessel.

In one aspect the waveguide coupling piece is connected to said outer wall of said hollow waveguide at a connection location such that the interior of said waveguide coupling piece is continuous with the volume between said inner and outer walls of said hollow waveguide, said inner wall of said hollow waveguide being extended across the interior of said waveguide coupling piece by a frustoconical portion, and said passage defined by said inner wall of said hollow waveguide being extended through said frustoconical portion and a wall of said waveguide coupling portion opposite the connection location of said hollow waveguide.

The present invention also provides a method of heating a substance in a vessel using microwaves, the method comprising the steps of connecting a hollow waveguide to an aperture in said vessel and transmitting microwaves through said hollow waveguide to heat said substance in said vessel, wherein an electromagnetic field created by the transmission of microwave energy is confined within an outer and inner wall of said hollow waveguide, and said inner wall creates a passage from within said vessel, said passage being devoid of the electromagnetic field and being used to gain access to the interior of said vessel during heating of said substance.

Thus in the arrangement of the present invention the use of a hollow waveguide for the transmission of microwave energy into a vessel leaves a passage free for access into the interior of the vessel. Such a passage can be used to pass a tube through to allow the liquidified substance within the vessel to be pumped out. Thus, in one aspect, the present invention allows for the simultaneous heating and removal of the liquidified substance through a single small aperture.

Examples of the present invention will now be described with reference to the drawings, in which: Figure 1 is a diagrammatic cross-section through the microwave coupler attached to a vessel according to one embodiment of the present invention; Figure 2 illustrates a diagrammatic cross-section through the microwave coupler incorporating a tube of dielectric material that does not absorb microwave energy; Figure 3 illustrates the microwave coupler wherein the inner wall of the hollow waveguide comprises a slideable metal tube of length sufficient to allow contact with the substance within the vessel; Figure 4 illustrates a diagrammatic cross-section of the microwave coupler which includes a microwave window at one end of the hollow waveguide;; Figure 5 illustrates a diagrammatic cross-section of the microwave coupler in which the outer wall of the hollow waveguide is constructed from two pieces; Figure 6 illustrates the collar used to hold together the two parts of the outer wall shown in Figure 5; Figures 7a and 7b illustrate an alternative hollow waveguide with hollow ridges.

Figure 8 illustrates the use of the microwave coupler in an arrangement for heating the contents of a metal drum.

Referring now to the drawings, Figure 1 illustrates a diagrammatic cross-section of the microwave coupler attached to a vessel 5. The microwave coupler comprises a rectangular waveguide coupling piece 1 which has a standard end fitting comprising a fixed flange 2. At the other end there is a metallic short circuit 3, the position of which may be varied along the microwave waveguide. The rectangular waveguide coupling piece 1 is perpendicularly attached to a hollow waveguide 6. The hollow waveguide 6 has an outer wall 6a which at one end attaches to the rectangular waveguide coupling piece 1 and at the other end is adapted to be attached to the aperture of the vessel 5.

Within the outer wall 6a of the hollow waveguide 6 is an inner wall comprising an inner conductor 7 which is tubular and defines a passage 7a from within the interior of the vessel 5 to the outside environment.

The hollow waveguide 6 is connected to the rectangular waveguide coupling piece 1 in such a way as to provide good electrical contact between the wall of the rectangular waveguide coupling piece 1 and the outer wall 6a of the hollow waveguide 6. In order to achieve this during construction a circular hole is cut in the broad wall of the rectangular waveguide coupling piece 1 at position BB' to allow the inner conductor 7 to pass into the interior of the rectangular waveguide coupling piece 1. At the boundary BB' between the rectangular waveguide coupling piece 1 and the hollow waveguide 6 a frustoconical portion 4 is positioned about the inner conductor 7 such that the outer diameter thereof increases up to the upper wall of the rectangular waveguide coupling piece 1. The centre of this frustoconical portion is hollowed out to permit the inner conductor 7 to pass therethrough.The tolerance in the diameter of the hole cut in the frustoconical portion 4 must be such that there is adequate electrical contact at microwave frequencies between the frustoconical portion 4 and the inner conductor 7, yet a sufficient gap must be present to allow mechanical movement of the inner conductor 7 through the frustoconical portion 4. It has been found that a so-called "push fit" is adequate.

The hollow waveguide 6 is mechanically attached to the vessel 5 at the aperture AA'. The inner conductor 7 extends into the vessel 5 a distance H to form a monopole antenna which is used to introduce the microwave energy into the vesel 5. The opening CC' at the end of the inner conductor 7 forms the passage 7a into the vessel 5, whilst the region within which the microwave energy is introduced is between the inner conductor 7 and the outer conductor 6a of the hollow waveguide 6. The interaction between the microwaves introduced through the hollow waveguide 6 and the material or devices situated at the passage aperture CC' may be controlled by varying the distance H. As H increases so the interaction decreases. The nature of the monopole antenna used is that the microwave energy spreads out broadly from the region AA' rather than following the direction of the inner conductor 7.

The inner diameter of the inner conductor 7 is chosen to be beyond cut off, i.e. microwave energy cannot propagate through the passage 7a.

The flange 2 is used to connect the microwave coupler to a standard microwave system (not shown) which can comprise a microwave generator, connected to a length of waveguide, portions of which may be flexible for convenience, and a three stub tuner which would connect to the flange 2. If required, a section of the waveguide containing a window transparent to microwave energy but impervious to solids, liquids or gases, may be included within the microwave system. All these components are standard items and their connection and operation are well known. Similarly, with the possible exception of the connection at the aperture AA', which is discussed below, the techniques of fabrication and connection for the microwave coupler are well known.

It would be appreciated by those skilled in the art that to obtain good matching of the system, the following dimensions must be chosen carefully: 1) Dimensions of the frustoconical portion 4.

2) The dimensions of the hollow waveguide 6.

3) Diameter of the inner conductor 7.

4) Length of extension of the inner conductor 7 into the vessel 5.

5) Hole diameter at BB'.

6) Position of the short circuit 3.

In addition, the size of the vessel 5 and the relative volumetric filling of the vessel 5 with material and the electrical properties of the material will influence the matching of the system.

It is considered that for initial tuning purposes it is sufficient to tune the microwave coupler for "free space" conditions, i.e. disconnected from any vessel.

Considering the dimensions of the hollow waveguide 6 and the length and diameter of the inner conductor extending into the vessel 5 and thus forming an antenna, the ratio of the antenna length H to antenna radius R1 is relatively small so that a so-called "thick antenna" is used. Such an antenna offers advantages since its output impedance is relatively insensitive to its length and of such a value that it can be readily matched to the hollow waveguide 6 (See for example J.D. Kraus, "Antennas", 2nd Edition, Mcgraw-Hill, NY, 1988). The practical consequence of this design is that the length of the antenna H is not too critical which means that there is sufficient flexibility to allow the aperture CC' of the passage 7a to be removed well away from the aperture AA' through the vessel 5 if required.

Generally the diameter R0 of the outer wall 6a of the hollow waveguide 6 will be comparable to the diameter of the aperture AA' through the vessel 5, for apertures too small for prior art techniques. The inner diameter Rg may be sized to give a characteristic impedance comparable with the input impedance of the antenna. In turn the hollow waveguide 6 must be matched to the waveguide 1.

This is achieved by using a suitable frustoconical portion 4 and by varying the position of the short circuit 3.

By way of example, when designing a microwave coupler for operation at 2,450 MHz to inject microwaves into a metal vessel with an aperture of approximately two inches diameter, the optimum shape of the frustoconical portion was found by trial and error. The following dimensions were used, D = 48 mm R0 = 51.6 mm RI = 28 mm H = 30 mm or larger The position of the short circuit 3 and the optimum antenna length H for free space conditions using the above arrangement were found experimentally by connecting the microwave coupler to a standard network analyser system and using well known methods to measure the voltage standing wave ratio (VSWR) which is a measure of the matching of the microwave coupler to the antenna.It was found that once the position of the short circuit 3 was determined to give a minimum VSWR value then fine tuning to further reduce the VSWR towards the optimum value of 1 was achieved by varying the length H of the antenna by sliding the inner conductor 7 through the frustoconical portion 4.

Prior art teaches that for a thick antenna, the antenna impedance is readily matched to the hollow waveguide 6 when the length H of the antenna is an odd multiple of a quarter wavelength at the operating frequency. Further, as the length H of the antenna is increased, the change in impedance of the antenna diminishes.

When the material to be heated in the vessel 5 is situated close to the aperture AA' then the material within this region has a strong influence on the matching between the antenna and the hollow waveguide 6. The larger the dielectric constant of the material, the greater the effect. In practical terms, water with a dielectric constant of around 80 represents a worst case. Typically, it was found by experiment that provided the level of the water was further than about 50 mm from the aperture AA' matching was achieved by the simple methods described. For closer contact additional matching aids may be used as will be described hereinafter. It can be appreciated that the length H chosen will depend upon the particular application.

For safety reasons the vessel 5 must be opaque to microwaves in order to contain the electromagnetic field.

The use of the microwave coupler is therefore primarily aimed at small apertures in metal drums which do not allow for prior art microwave heating techniques. The structure of the microwave coupler provides for the separation of the region in which microwave energy is being transmitted and the region allowing access into the interior of the vessel 5. The passage 7a allows for the insertion of material or mechanical appliances such as pumps or stirrers simultaneously to the application of microwaves to heat the substance within the vessel 5. In this way, disturbances in the microwave transmission into the cavity are minimised and unnecessary or unwanted heating of the substance can be avoided.

Figure 2 illustrates the situation when the substance 9, which is a liquid for example, in the vessel 5 is sufficiently removed from the aperture region AA' such that further matching steps need not be taken. In this arrangement a tube 8 constructed of dielectric material which does not absorb microwave energy is inserted through the passage 7a formed by the inner conductor 7. The tube 8 could be part of a pump, or could be used to inject material into the liquid 9, for example crystallised material to be dissolved in the liquid 9. As shown here the inner conductor 7 is of short length so that the antenna formed by the inner conductor 7 remains above the liquid level. Alternatively, the antenna may be extended into the liquid and holes drilled into it so that a tube 8 connected to a pump may progressively remove the liquid 9 from the vessel 5 as it descends down the inner conductor 7.The tube 8 is slideable within the inner conductor 7 and can thus be removed when desired.

Figure 3 illustrates another embodiment in which the inner conductor 7 and the tube 8 of Figure 2 have been replaced by a similar metal tube 10 that forms part of the conventional pump or other device. The tube 10 is free to slide through the frustoconical portion 4 and the insertion of the tube may be controlled in many ways. Figure 3 depicts the situation wherein a solid material 13 is melted to form a pool of liquid 14 which is pumped out via the tube 10. The descent of the tube 10 is controlled by a float 12 attached to the end of the tube 10 with holes 15 and 11 in both the float and end of the tube 10 to allow removal of the liquid 14 from the vessel 5. Again, many variations of this basic concept of using the metal tube 10 from an existing piece of equipment to form the inner conductor 7 of the hollow waveguide 6 and the antenna are possible.

Figure 4 illustrates the situation when the substance 9 to be heated is close enough to the aperture region AA' to require additional matching techniques. This is achieved by using a dielectric plug 16 which acts as a transformer to match the antenna to the hollow waveguide 6. The practical embodiment of such a plug is well known to those skilled in the art and may consist, for example, of a so-called quarter wavelength matching plug of appropriate dielectric constant (see for example "Microwave Engineering", P.S. Rizzi, Prentice Hall, Englewood Cliffs, NJ, 1988). In addition the dielectric plug 16 may also serve as a seal between the vessel 5 and the hollow waveguide 6, preventing the substance 9 being heated from entering the rectangular waveguide coupling piece 1.

Consequently, such an arrangement can be used for apertures situated at the base of vessels.

If the substance 9 shown in Figure 4 was to be melted and removed via a tube inserted through the innerconductor 7, as previously described, then practice has shown that large differences in matching conditions may occur between the initial situation when the substance is closed to the aperture region AA' and subsequently when the level of the substance is sufficiently removed from the aperture. In this case it may be expedient to remove some of the unmelted substance from the aperture region prior to connecting the microwave system to the aperture by, for example, using a scoop, or alternatively matching the microwave system for conditions when the material is well away from the aperture and tolerating the initial mismatch, provided it is not too severe.In the latter case, since initially the microwave energy is likely to be concentrated in a small region close to the aperture, melting in this region will occur rapidly, creating the necessary cavity in the material to provide good matching.

An important feature of any microwave system is the safe operation of the equipment. If the microwave coupler is to be permanently connected to an aperture then standard connecting techniques may be used and normal safety precautions can be followed. However, if it is intended that the microwave system is to be frequently connected and disconnected from the vessel 5, then additional precautions must be made so that the microwave power supply cannot be turned on when the microwave coupler is disconnected from the vessel 5.

In essence safety interlocks which are activated by proximity switches connected to the microwave coupler are used to prevent the operation of the microwave power supply when the coupler is disconnected from the aperture to the vessel 5. The requirements for an interlock system may be discussed in general terms with reference to Figure 1. It may be appreciated that it is not enough to position the proximity sensor so that it senses the presence of the surface of the vessel 5 when the microwave coupler is connected since in a similar manner if the disconnected coupler were inadvertently placed on the vessel 5 the proximity switch would still be activated to allow the operation of the power supply.Instead the proximity switch must only be activated by a part of the structure of the vessel 5 which is only accessible to the proximity sensor when the microwave coupler is correctly connected to the vessel. This may be achieved by using the pipework of the actual aperture and in particular the tapped bung hole as shown in Figures 5 and 6. The general principle is applicable to other vessels provided the connection to the aperture is constructed in a similar way.

Referring to Figure 5, the vessel 5 has a threaded aperture 17 with a metallic lip 18 on the outer side forming a flange. There is a spacing DD' between the metallic lip 18 and the vessel 5.

The microwave coupler shown in Figure 5 is essentially that of Figure 1 but with some additional details. The outer wall 6a of the hollow waveguide 6 is now constructed from two parts 36 and 36'. The first part 36' has concentric flanges 22 and 20 on the outer surface thereof and a threaded portion 19 at a portion below the lower flange 20. In use the first part 36' is screwed into the aperture 17 such that the screw threaded portion 19 fits the screw thread in the aperture 17. The length of the threaded portion 19 is such that the flange 20 which is of the same diameter as the lip 18, finishes in contact with the lip 18, provided that the first part 36' has been screwed in fully.Once the first part 36' is connected, the remainder of the microwave coupler is attached with the inner conductor 7 passing through the first portion 36' and the second or upper portion 36 mating with the first or lower portion 36' via the two flanges 21 and 22 which are of the same diameter and which finish in contact with one another provided that a proper contact has been made.

In order to keep the two halves of the microwave coupler together and to provide the required safety interlocks, a cylindrical collar is required. Figure 6 shows a cross-section through such a collar which is split into two similar halves 23 and 23', which when used correctly are placed over the hollow waveguide portions 6 and 6' and held together by toggle clips (not shown for clarity).

Grooves 24, 24', 25 and 25' are cut in the cylindrical collar. In use one half 23' of the cylindrical collar is first inserted over the connected hollow waveguide portions 36 and 36' with the bottom portion 31 of the cylindrical collar fitting within the space DD', and then the other half 23 is added and clamped to the first half 23'. The groove 25, 25' is designed so that it holds the flanges 18 and 20 and similarly the groove 24, 24' is designed to hold the flanges 21 and 22. The dimensions and positions of the grooves are such that if the first or lower portion 36' has not been fully screwed into the aperture 17 and the two halves 36 and 36' of the outer conductor 6a of the hollow waveguide 6 have not been correctly mated then the two halves 23 and 23' of the collar cannot be correctly positioned over the hollow waveguide and clamped together.Provided that the microwave coupler has been correctly connected, then in each half of the cylindrical collar, a pin 26, 26' pushes against the lip 18 and is forced outwards as the collar is placed in position. This movement of the pins 26, 26' in turn pushes intermediate lugs 28 and 28' pivoted at 27 and 27' which in turn pushes the pins 29 and 29' of standard interlock switches 30 and 30', which are connected in series to an interlock in the microwave generator in such a way that unless both switches 30 and 30' are activated no microwave power is available. Two switches are provided to allow for possible malfunction. In the event that the cylindrical collar is not properly positioned over the microwave coupler then the pins 26 and 26' are not pushed outwards and the interlocks 30 and 30' are not activated.

Since the pins 26 and 26' are positioned within the cylindrical collar they cannot be accidentally activated.

It will be appreciated that other designs are possible in which the first and second portions 36 and 36' of the hollow waveguide 6 are not separate and that the principle of the safety collar described is generally applicable to other pipework which may form the entry port to the aperture, provided provision is made for a flange on the pipe comparable to the lip 18 as shown in Figure 5 with which to activiate the-pins within the safety collar.

As a further safety precaution a shroud made from a commercially available fabric coated with microwave absorbing material may be wrapped around the collar and a microwave leakage monitor constructed from an appropriate diode detector may be positioned close to the coupler to provide an alarm signal should there be any significant leakage of radiation. The signal from such a microwave detector may be used to activate a safety interlock which cuts off the microwave power at a predetermined level.

Figures 7a and 7b illustrate an alternative arrangement for the microwave coupler. Figure 7a illustrates the hollow waveguide 41 in cross section. The hollow waveguide 41 is a ridge waveguide with hollow ridges 42 extending within the outer wall of the hollow waveguide. Each of the hollow ridges 42 is connected to an outlet 43.

Figure 7b illustrates the hollow waveguide 41 connected to a rectangular waveguide 40 in an arrangement that provides good impedance matching. The rectangular waveguide is similar to that described in relation to Figures 1 to 3 in that it has a flange 44 at one end and a short circuit 45 at the other end thereof. However, the short circuit is arranged at an angle to ensure a good impedance match between the hollow waveguide 41 and the rectangular waveguide 40. The hollow waveguide 41 is connected to the rectangular waveguide at right angles such that the interior of the rectangular waveguide 40 communicates with the interior 46 of the hollow waveguide.

The end of the hollow waveguide 41 distal from the rectangular waveguide 40 is provided with a threaded portion 47 for attachment to a vessel. Thus, when the microwave coupler is connected to a vessel, microwaves can be transmitted to the interior of the vessel to heat a substance within the vessel. The hollow ridges 42 and the outlets 43 can provide a passage into the vessel to allow the simultaneous heating and access to a vessel via a single aperture. For instance, a pipe could be fed through the outlets 43 and hollow ridges 42 to remove a heated liquid from the vessel.

Figure 8 shows a possible arrangement for heating material held within a metallic barrel. In an industrial environment such barrels may have bent surfaces due to rough handling so that the aperture may not lie in a horizontal plane. Consequently any practical microwave system must be flexible enough to allow connection at other than the strictly vertical position. This may be accomplished by the arrangement shown in Figure 8. The microwave coupler 40 is permanently attached to the microwave power supply 32 which is suspended by a metal chain from a hook 37 attached to a jack 33, connected to a slide 38 mounted on a trolley 34 with wheels 35 and 35'.

In use the trolley 34 may be roughly positioned around the barrel 39. Fine horizontal and vertical movements of the microwave system are obtained with the slide 38 and jack 33 respectively while the chain connected to the hook 37 allows the power supply to be pivoted in any direction.

Using a two-piece coupler 40 and collar 23, 23' as described in Figures 5 and 6, the first portion 36 of the coupler 40 is first attached to the barrel 39 and then the microwave system with the remainder of the coupler 40 is positioned to allow correct mating of the two halves of the coupler 40 so that the collar can be attached. For operational safety the wheels 35 and 35' are lockable and the barrel 39 is clamped within the framework of the trolley 34 to prevent it from moving or being moved by accidental contact from other equipment, for example, a passing forklift truck.

Thus a microwave coupler is provided for an aperture of small size in a vessel such as a barrel, which allows for the simultaneous heating of a substance in the vessel using microwaves and for access to the interior of the barrel. In the coupler described hereinabove the passage of material or insertion of a device into the barrel during heating does not hinder the propagation of the microwave energy.

Further, the substance in the vessel can be physically separated from the region adjacent to the aperture and within the interior of the microwave cavity within which the microwaves are propagating between the aperture and the associated microwave equipment. The substance is thereby shielded from the microwave energy as the material passes to or from the vessel. Alternatively controlled contact between the material and microwaves in a region adjacent the aperture may be achieved to allow heating in this region.

It should be apparent that the techniques to be described are equally applicable to apertures without size restrictions and to custom designed cavities, although it is recognised that the need for such a device in these cases may not be so great. Similarly, the technique can be scaled up to work at the other recognised microwave frequencies available for industrial heating.

Claims (18)

1. A microwave coupler for transmitting microwave energy into a vessel through an aperture in said vessel, said microwave coupler comprising a hollow waveguide having an outer wall for connection at one end thereof to said vessel about said aperture, and an inner wall within said outer wall defining a passage, wherein in use an electromagnetic field is confined within a volume between said inner and outer walls, and said inner wall defines a passage from within said vessel, said passage being devoid of the electromagnetic field.

2. A microwave coupler as claimed in Claim 1, wherein said inner wall extends from within said end of said hollow waveguide to provide an extension.

3. A microwave coupler as claimed in Claim 1 or Claim 2, wherein the internal diameter of said inner wall defining said passage is below a size necessary for the transmission of microwave energy.

4. A microwave coupler as claimed in any of Claims 1 to 3 further including a waveguide coupling piece connected to a second end of said outer wall of said hollow waveguide and adapted to provide good impedance matching to allow the efficient transmission of microwaves through the waveguide coupling piece and said hollow waveguide to said vessel.

5. A microwave coupler as claimed in Claim 4, wherein said waveguide coupling piece is connected to said outer wall of said hollow waveguide at a connection location such that the interior of said waveguide coupling piece is continuous with the volume between said inner and outer walls of said hollow waveguide; said inner wall of said hollow waveguide being extended across the interior of said waveguide coupling piece by a frustoconical portion; and said passage defined by said inner wall of said hollow waveguide being extended through said frustoconical portion and a wall of said waveguide coupling portion opposite the connection location of said hollow waveguide.

6. A microwave coupler as claimed in Claim 4 or Claim 5, wherein one end of said waveguide coupling portion is provided with a short circuit.

7. A microwave coupler as claimed in Claim 6, wherein the position of said short circuit within said waveguide coupling piece can be varied.

8. A microwave coupler as claimed in any of Claims 5 to 7, wherein said inner wall of said hollow waveguide comprises a metal tube extending through and slideable within said frustoconical portion and the length of extension of said inner wall from within said end of said outer wall is adjustable by sliding said inner wall within said frustoconical portion.

9. A microwave coupler as claimed in any preceding claim further including a microwave transparent window fitted between said inner and outer walls of said hollow waveguide at a position adjacent said one end thereof.

10. A microwave coupler as claimed in Claim 9, wherein said window is adapted to provide an impedance match between said hollow waveguide and the interior of said vessel.

11. A microwave coupler as claimed in any of Claims 4 to 10 including a microwave transparent window fitted to a second end of said waveguide coupling piece.

12. A microwave coupler as claimed in any preceding claim wherein the outer wall of said hollow waveguide is formed from a first portion for attachment to said vessel about said aperture and a second portion for connection to said first portion.

13. A microwave coupler as claimed in any preceding claim further including a collar for connecting said hollow waveguide to said vessel.

14. A microwave coupler as claimed in Claim 12 and Claim 13, wherein said collar is adapted to also connect together said first and second portions of said outer wall.

15. A microwave coupler as claimed in Claim 13 or Claim 14, wherein said collar includes at least one switch to control a microwave power supply in response to the correct connection of said collar to said hollow waveguide and said vessel.

16. A microwave coupler as claimed in any preceding claim further including a tube of dielectric material that does not absorb microwave energy, extending through and slideable within said passage.

17. A method of heating a substance in a vessel using microwaves, the method comprising the steps of connecting a hollow waveguide to an aperture in said vessel and transmitting microwaves through said hollow waveguide to heat said substance in said vessel, wherein an electromagnetic field created by the transmission of microwave energy is confined within an outer and inner wall of said hollow waveguide, and said inner wall creates a passage from within said vessel, said passage being devoid of the electromagnetic field and being used to gain access to the interior of said vessel during heating of said substance.

18. A microwave coupler as hereinbefore described with reference to the drawings.