Abstract:
The invention concerns a device for microwave treatment in an applicator ( 1 ), characterized in that it comprises, arranged in the applicator, impedance adapting dielectric means ( 2, 7, 8 ) having shapes and dimensions adapted to the objects ( 10 ) to be treated and to the characteristics of wave propagation modes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the microwave treatment in particular of items of farm produce or domestic objects or of industrial parts, and relates more particularly to devices of the aforementioned type intended for the microwave treatment of single parts. 
     DESCRIPTION OF THE RELATED ART 
     The microwave heating of single objects or of covered products generally leads to a clear structuring of the spatial distribution of the sources of heat of electromagnetic original and consequently to a structuring of the temperature distribution. 
     This spatial temperature distribution is the end result of the two contributory phenomena:
     a) the reflections of the waves by the interfaces formed by the physical limits of the objects: these reflections are revealed by an interferential phenomenon in the form of an alternation of high-temperature zones and cold spots and concern objects of which the dimensions are of the order of magnitude of the wavelength;   b) the attenuation of the energy density supplied by the waves within the same objects: the attenuation is visible on objects of which the dimensions are great relative to the wavelength or of which the absorptive dielectric characteristic is high (above 1).   

     These two phenomena, reflection of the waves and attenuation, had been observed and studied in the laboratory on specific microwave devices known as applicators, always completely filled with various products or materials, these latter being the products to be transformed by heating or being dielectrics suitable for forming the specific charge of the applicators. 
     These applicators are always characterised by the presence of substantial masses of dielectric materials which serve as input and output interface adapters of the applicators or which permit the choice of suitable global attenuation coefficients. 
     These applicators are for example mould/applicators for the use of composite materials or autoclave/applicators for the particular heating of numerous reagents. 
     The most advanced contribution in terms of potential uses of the results of research in the laboratory is the demonstration and the possibility of evaluation of compensation for the attenuation of the waves. 
     Such compensation is achieved by the use of dielectric materials of variable thickness or permittivity characteristic in the direction of propagation, possibly formed by layers of different materials. 
     SUMMARY OF THE INVENTION 
     The invention concerns an application of the same fundamental concepts to the situation of objects of which the dimensions are in the range between a centimeter and at maximum of the order of a meter, placed in lightly-loaded monomode or multimode applicators. 
     It therefore relates to a device for microwave treatment of objects in an applicator, characterised in that it comprises, disposed in the applicator, dielectric impedance adaptation means having shapes and dimensions adapted to the objects to be processed and to the characteristics of the wave propagation modes. 
     According to other characteristics: 
     
         
         
           
             the dielectric adaptation means are parts made from a dielectric material of which the permittivity is as close as possible to average permittivity of the objects to be heated; 
             the dielectric adaptation means have volumes of prismatic, pyramidal or conical shape of which a base is equal to at least a part of a section of the object to be heated and of which the height is limited to the filling conditions of the applicator; 
             the said dielectric means comprise tips forming interface adapters disposed relative to the object to be treated according to at least one of the three dimension in space; 
             the dielectric adaptation means comprise a tube having a cylindrical internal cavity and an external shape having an intermediate rotational portion and two end portions in the shape of truncated cone tapering away from the rotational portion and, joined to the ends of the tube forming a propagation adapter, solid conical parts which form interface adapters; 
             the intermediate rotational portion has a cylindrical external shape; 
             the intermediate rotational portion is formed by truncated cones joined by the large bases or by their small bases; 
             for the heating of parallelepipedal objects the device comprises at least one waveguide extended by a prismatic antenna disposed on the side of at least one face of at least one object and the dielectric adaptation means comprise a prismatic part accommodated in the said antenna, and the free surface of the prismatic dielectric part is situated in contact with the corresponding face of the parallelepipedal object to be heated; 
             when the said at least one object is to remain accessible for other operations, an air gap is provided between the said adaptation means and the surface of the said object of parallelepipedal shape, the value of the said air gap being a function of the dielectric permittivities of the material of the part to be heated and of the dielectric adaptation means; 
             the value of the air gap is less than or equal to a whole number of half-wavelengths of the propagation mode; 
             the dielectric adaptation means comprise an intermediate portion of parallelepipedal shape in which an impression is provided for the object to be heated, and two prismatic tips which are joined by their bases to the ends of the intermediate portion and of which the ridges opposite the said bases are disposed transversely with respect to the direction of propagation of the waves with respect to the said dielectric adaptation means, the said prismatic tips forming interface adapters; 
             in order to favour the propagation in the object to be heated, several dielectric materials of different permittivities are associated, thus creating a permittivity gradient; 
             when the dielectric parts of the said adaptation means are formed by layers of different materials, it is the permittivity of the central layer which has a value close to that of the objects to be heated; 
             it comprises, disposed in an applicator, a conveyor belt made from a material having a dielectric permittivity adapted to the objects to be treated, the said conveyor belt being displaceable in a gap defined by the interface adapter bases which are placed above and below the belt and have cells to receive objects to be treated by microwaves such as trays for food products. 
           
         
       
    
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood by reading the following description which is given by way of example and with reference to the accompanying drawings, and in which: 
         FIG. 1  is an exploded schematic perspective view of a first embodiment of an applicator with impedance adapter according to the invention; 
         FIG. 2  is a schematic perspective view of another embodiment of the applicator with impedance adapter according to the invention; 
         FIG. 3  is a schematic perspective view of another embodiment of the applicator with impedance adapter according to the invention; and 
         FIG. 4  is a schematic perspective view of yet another embodiment of the applicator with impedance adapter according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An applicator for the microwave heating of a cylindrical object is shown schematically in exploded perspective in  FIG. 1 . 
     The applicator comprises a metal tube  1  forming a waveguide in which is disposed an adapter having a tube  2  of a diameter smaller than that of the applicator and comprising an intermediate portion  3  of cylindrical external shape and two end portions  4  with the external shape of a truncated cone tapering away from the intermediate portion  3 . 
     The two ends  5 ,  6  of the portions in the shape of truncated cones  4  opposite the intermediate portion  3  are extended by solid cones  7 ,  8 . 
     The tube  2  has a cylindrical cavity  9  in which a product  10  to be heated is disposed. 
     This product  10  which extends practically over the length of the tube  1  has a cross-section smaller than the cross-section of the cavity  9  of the tube  2 . 
     It includes an end portion  12  for example of hemispherical shape, or of any shape. 
     The parts  2 ,  7  and  8  are joined side by side. 
     The solid cones  7 ,  8  form interface adapters, whilst the tube  2  forms a propagation adapter. 
     Although in the embodiment according to  FIG. 1  the intermediate rotational portion  3  has an external surface of cylindrical shape, this external surface may be formed by truncated cones joined by their large bases or by their small bases. 
     The contours of the external surface of the tube which are thus obtained make it possible to adapt the propagation of the waves with a view to obtaining a uniformity of heating of the treated part over all of its length. 
     The adapter  3 ,  7 ,  8  is placed in the applicator  1  on supports (not shown) disposed in such a way that their interfaces are parallel to the electric field of the propagating wave in the applicator. 
     The adaptation means are produced from materials of which the permittivity is as close as possible to the average permittivity of the parts to be heated at the very least in the central part of these adaptation means according to the characteristics of the chosen propagation mode or modes. 
     The materials from which these adaptation means are made are essentially composite materials of which the matrix can be easily used by moulding or by machining. 
     These materials are for example silicones, PTFE, polyolefins. 
     The charge of the matrix, principally mineral, can be easily adjusted and has interesting dielectric properties. 
     The charge materials are for example mica, alumina, titanium oxide, glass, quartz. 
     The directions of propagation of the waves are indicated by the arrows F 1  and F 2 . 
     It is possible to choose to use waves which propagate in one or the other direction indicated by the arrows F 1  and F 2  or even in the two directions simultaneously. 
     It is equally possible to choose that the waves propagate alternately in the directions of the arrows F 1  and F 2  if the attenuation of the waves is low. 
     According to the embodiment shown in  FIG. 2 , the products to be heated  20  are products in parallelepipedal blocks of great thickness which can be displaced on or under the microwave heating device or even in it. 
     This heating device comprises an applicator  22  formed by a rectangular waveguide  24  terminated by a hollow metal prismatic antenna  26 . 
     In the present example, the rectangular end  28  of the antenna  26  turned towards the product  20  to be heated is separated from the product by an air gap  29  which contributes to the adaptation of the interface of the product and which is necessary in order to render the product accessible to other treatments such as surface cooling by circulation of a fluid or some other. 
     In the hollow prismatic antenna  26  there is accommodated an interface adapter in the shape of a solid prism  30  of which the end faces  31 ,  32  are joined by a ridge  34  and of which the base  35  is extended in a parallelepipedal portion  36 . This parallelepipedal portion is flush with the rectangular end  28  of the antenna  26 . 
     The interface adapter surface  30  which is turned towards the product and which is flush with the end  28  of the antenna  26  can also be in contact with the surface of the parallelepipedal object  20  to be heated. 
     The direction of propagation of the waves is indicated by the arrow F 3 . 
     During the displacement of the applicator  22  in the direction of the arrow f along the products  20  to be heated, the said products are heated by successive zones delimited by the dimension of the antenna  26 . 
     Although the device of  FIG. 2  comprises a single waveguide applicator disposed above the objects  20  to be treated, it is possible to provide a plurality of applicators disposed side by side along the displacement path of the objects. It is equally possible to dispose one or several applicators of this type below the objects  20 . 
     The applicator shown in  FIG. 3  comprises a rectangular waveguide  40  in which is disposed an adapter  41  having an intermediate portion of parallelepipedal shape  42  and two prismatic tips  43 ,  44  which are joined by their bases to the ends of the intermediate portion  42  and of which the ridges  45 ,  46  opposite the said bases are disposed transversely with respect to the direction of propagation of the waves in the wave guide  40 . 
     As in the example of  FIG. 1 , the waves can propagate in the guide  40  in the two directions indicated by the arrows F 4  and F 5  alternately or simultaneously. 
     A cavity is provided in the central portion  42  of the adapter and in this cavity an object to be heated  50  is placed which is of a shape complementary to that of the cavity. 
     The prismatic tips  43  and  44  form interface adapters. 
     The choice of the arrangement of the said prismatic tips forming interface adapters is guided by the interfaces producing the most intense reflection. 
     Thus prismatic adapters placed in a higher or lower position relative to the objects are entirely conceivable. 
     In the lower position, the adapters can form part of the means for supporting the objects to be heated. 
     In other cases, these adapters will be an integral part of conveyor means such as belts, belt conveyors, removable bases or others. 
       FIG. 4  shows schematically in perspective an embodiment of a device for microwave treatment of objects according to the invention which includes, mounted in an applicator  51 , a belt conveyor  52  in which cells  54  are provided for objects to be treated by microwaves such as trays  56  for food products. 
     The conveyor, of which only the portion inside the applicator is shown, is made from a material having a dielectric permittivity adapted to the heat treatment of the contents of the trays  56 . 
     Interface adapters  58 ,  60 , placed above and below the belt  52  are disposed on the path of the conveyor  52 . These interface adapters are formed by pyramids each having a tip in the form of a ridge  62 ,  64  and facing one another with their bases  66 ,  68 . 
     The belt  52  is moved in the gap  70  provided in the interface adapters  58  and  60 . 
     It will be seen in this embodiment that the interface adapters  58 ,  60  are oriented in a general normal direction to the direction of movement of the objects  56  on the conveyor belt  52 . 
     This orientation of the interface adapters is in this case, bearing in mind the shape and the arrangement of the objects  56 , the optimum orientation in which the interfaces  58 ,  60  produced the most intense reflection. 
     The arrangement of the tips forming adapters can therefore be chosen with respect to the objects to be treated in at least one of the three dimensions in space. 
     In the devices which have just be described with reference to the drawings, in order to favour the propagation in the object to be heated it is possible to produce at least certain of the dielectric adaptation means by association of several dielectric materials of different permittivities, thus creating a multilayer structure which makes it possible to ensure control of the attenuation of the waves. 
     Such a multilayer structure makes it possible in particular to improve the performances of the propagation adapters. 
     In the case of the use of such multilayer structures, it is the central layer which has a dielectric permittivity close to the average permittivity of the object to be heated. 
     The fields of use of the dielectric equipment according to the invention are those where the raising of the temperature of the objects in a uniform manner in space is essential. 
     These are first and foremost products of the farm food industries for which food safety requires that the range of temperature necessary for defrosting, pasteurisation, even sterilisation should be uniform in space. 
     They may also be technical products. 
     It is possible to envisage the curing of objects made from thermosetting resin when it is not necessary to use a mould with application of a mechanical pressure. 
     Thus the invention is advantageously used for heating glues, coating products, reactive products stored at low temperature (−70° C. to −20° C.) and up to ambient temperature or the temperature of use.