Background of the invention

The invention relates to a method for the heating of food¬ stuffs such as meat products, dairy products and vegetable products, as described in the preamble to claim 1, and an apparatus as disclosed in the preamble to claim 8.

It is known in several different connections to heat products by means of microwaves. The different techniques are used for different purposes, ranging as widely as from the drying of wood to the boiling of meat. Probably the best-known technique is in connection with microwave ovens, which have become a consumer commodity.

However, the hitherto-known techniques suffer serious dis¬ advantages, and these are amplified and particularly pro- nounced in connection with large-scale operations.

From DK patent application no. 4718/86 it is known continuously to lead meat products through feeding ele¬ ments, for example a tube, on which electrodes are mounted on diametrically-opposite sides. Before the meat products are fed to the active zone, a vacuum pump is used to remove the air contents.

From DK patent application no. 1014/91 it is known to lead the products through a feeding element on which annular electrodes are mounted in two places around the tube.

Both of the above-mentioned methods are encumbered with certain disadvantages which become particularly pronounced during large-scale operations. The depth of penetration and herewith the tube diameter for the systems is thus limited.

Moreover, a problem arises in connection with the uniform¬ ity of the electromagnetic field as a function of the dielectric properties of the product.

Furthermore, on each side of the electrodes there will also arise a field which must be conducted away by means of an earth or frame connection. The frame or earth connection must necessarily be placed at a certain distance from the annular electrodes, the reason being that these will other- wise be short-circuited to earth or frame.

The result is that a large part of the generator's power is dissipated between the electrodes and frame, which contributes towards a particularly disparate heating or boiling zone, which in principle consists of three different zones, i.e. the primary heating zone between the electrodes, and the two secondary heating zones between the electrodes and frame.

Consequently, there arises a location-dependent temperature gradient which varies over three areas. This results in problems in connection with the heating or boiling of pro¬ ducts or materials which require a uniform heating, since in practice it is difficult to compensate for this multi- zone heating during continuous operation. This is similarly problematic in connection with the heating of products which have a temperature-dependent conductivity, the reason being that the depth of penetration is herewith strongly diminished.

The object of the invention is thus to achieve a uniform and herewith a controllable heating or boiling of a pro¬ duct, and also to achieve a greater depth of penetration in the product.

Advantages of the invention

As disclosed in claim 1, by controlling the current through the secondary paths of conductivity, either completely or partly by means of one or more reactances which are coupled in series and/or parallel with the secondary path of con¬ ductivity, it is achieved that the current which flows between electrode and frame can be controlled in a simple manner. The current in the secondary path or paths of con¬ ductivity can thus be completely controlled.

As disclosed in claim 2, by letting the electrical impedance between the electrode and frame have at least one pole at the frequency of the frame current which is to be eliminated or minimized, it is achieved that the frame current is minimized or eliminated at the same time that the frame connection is maintained. This results in an increased depth of penetration, an improved heating in the primary patch of conductivity. The invention thus combines a more uniform field with a field optimization.

As disclosed in claim 3, by letting the feeding element consist of a tubular body with a cross-sectional shape suitable for the product to be processed, the resulting embodiment is one with which it is possible to quickly feed and discharge the product to and from the heating zone.

As disclosed in claim 4, by letting the electrode system consist of two electrodes which are connected to an HF generator, which are cylindrical or annular and extend around the primary feeding element, it is possible to apply the electromagnetic field to the feeding elements in a manner which is particularly advantageous in connection with the application of fields in a tubular feeding ele¬ ment.

As disclosed in claim 5, by preheating the product before

it is supplied to the feeding elements, or before the heat¬ ing takes place, it is achieved that the heating according to the invention is made easier, in that the conductivity of the product is improved before or at the same time as it is heated, which results in a greater depth of penetration. Consequently, this makes it possible for the feeding ele¬ ments and herewith the heated product to be of greater dimensions transversely to the feeding axis.

As disclosed in claim 6, by carrying out the preheating on¬ ly in or around that area of the product which in relation to the electrodes has the greatest depth of penetration, the effect achieved is more or less the same as that in claim 2, but with the use of less power during the preheat- ing.

As disclosed in claim 7, the result of having substantially the whole of the heating system encapsulated in a conductive structure is that the electromagnetic radiation is more or less eliminated. Moreover, the Kelvin effect which arises with the use of HF frequencies ensures that the current will flow only on the inner side of the con¬ ductive structure. This is of significance for the working environment around the machine, but it also contributes towards ensuring that the various measuring instruments which are associated with the operation of the machine are not disturbed or require special screening.

As disclosed in claim 8, by connecting the secondary path of conductivity in series and/or in parallel with one or more reactances which are dimensioned in such a manner that the secondary path of conductivity between electrode and frame is an oscillatory circuit with one or more poles, the possibility is provided of controlling the current which flows between electrode and frame. The apparatus thus achieved is one wherein the power consumed in the primary

and secondary zones can be regulated without any noteworthy loss of effect. By regulating the oscillatory circuit, it is thus possible to achieve the desired field in the heat¬ ing or the boiling zone.

As disclosed in claim 9, by letting at least one of the poles in the oscillatory circuit be tuned more or less to resonance at the frequency of the undesired frame current, a minimization or elimination of the frame current is achieved. Moreover, the greatest possible dissipation of power is achieved in the heating zone without any significant loss of effect outside the primary boiling and heating zone.

As disclosed in claim 10, by letting the secondary path of conductivity consist of an inductive conducting part, the secondary inductance, between electrode and frame, and a capacitance substantially coupled in parallel, the secondary capacitance, said secondary capacitance being substantially tuned so that the parallel oscillatory circuit is more or less in resonance at the frequency of the HF generator, a simple and inexpensive embodiment according to the invention is achieved, in that this requires a minimum of external components and adjustment of said components.

As disclosed in claim 11, by letting the secondary path of conductivity to frame consist of a tubular electrode which is made of an electrically-conductive material, there is achieved a well-defined path of conductivity to frame which has well-defined inductive characteristics. Moreover, the tubular electrode screens the electromagnetic field.

As disclosed in claim 12, by coupling the secondary path of conductivity in parallel wholly or partly with a condenser, so that the secondary path of conductivity constitutes a

parallel oscillatory circuit, a simple embodiment is achieved by means of which it is easy to adjust or eliminate the frame current.

As disclosed in claim 13, the insertion between the electrodes and the oscillatory circuits of a trimming capacitance which introduces a capacitance between electrode and product, and a capacitance between electrode and the oscillatory circuit, the possibility is provided by means of this trimming capacitance not only to carry out an adjustment of the depth of penetration, but also an adjust¬ ment of the power dissipated between the electrodes.

As disclosed in claim 14, by letting the trimmer capacitance consist of a space between the electrode and the secondary tube part which is suitably filled with a dielectric, for example air, a practical embodiment of a trimming capacitance is achieved, while at the same time a greater depth of product penetration is achieved for the electromagnetic field from the end parts or end plates of the electrodes.

As disclosed in claim 15, by inserting external trimming capacitances, a simple and well-defined possibility of adjustment is achieved. There are thus considerable advantages with regard to the dimensioning of the trimming capacitances, in that these are particularly well-defined due to the fact that the temperature of the capacitances when these are placed externally in relation to the electrodes is relatively low and constant, in that their temperature will not vary with the temperature of the electrodes. This provides further operational advantages, in that the risk of breakdown is kept at a minimum. More¬ over, it is advantageous that the capacitances, in the event of a possible breakdown, do not give rise to a disin¬ tegration of the heating or boiling wall.

As disclosed in claim 16, by coupling the HF generator in parallel with a compensation coil, which is substantially tuned so that the dispersion capacitances from the terminals of the generator to frame are in resonance with the compensation coil, a good efficiency of the generator is achieved.

As disclosed in claim 17, by letting an extended electrically-conductive structure extend from the connection points of the leads to the electrodes in over the primary tube part and surround a smaller part of the length of same, it is achieved that the field which extends into the primary tube part is symmetrical. The result is that cold zones do not arise in the primary tube. A discreet capacitance across the secondary tube part will namely give rise to a disparate charge density on the tubular electrode, said charge density being greatest in the area around the terminals of the capacitance. However, the extended tube part in towards the primary tube part will result in the charge density distributing itself homogeneously over the extended tube part, and hereby give rise to a uniform symmetrical field inside the primary tube part.

As disclosed in claim 18, by letting the capacitive reactance be a tubular condenser which surrounds the whole of the tube part, a uniform and homogeneous charge density is achieved at the terminals of the condenser. This will in turn give rise to a uniform symmetrical field inside the primary tube part, and thus no cold zones in said tube, without the use of an extended conductive structure.

As disclosed in claim 19, by providing a preheating mechanism, for products having dielectric characteristics which are temperature-dependent, it is achieved that the current path can be "predetermined" by means of the hot

zone or zones which arise in the product.

As disclosed in claim 20, by mounting the external components on the primary feeding tube, a particularly advantageous embodiment is achieved, in that for different products it will be necessary to have different forms of feeding tubes which are adjusted to suit precisely the relevant product. This in turn will mean that the feeding tube is given changed characteristics, and that the system must be re-calibrated. By mounting pre-dimensioned components which suit the feeding tube to be used, it is achieved that the apparatus can very quickly be switched from one production to another.

The drawing

An example of the known technique and two example embodi¬ ments of the invention will be described in more detail in the following section with reference to the drawing, where

fig- 1 shows a known heating apparatus, and

fig. 2 shows an embodiment according to the invention.

Description of the example embodiments In fig. 1 is seen a known heating tube where the voltage generator 1 via leads 7, 8 is connected to electrodes 2, 3. The electrodes 2, 3 are placed as annular electrodes around a tube 4 which is not electrically conductive. Two frame electrodes 5, 6 are connected to earth or frame via frame connections 11, 12. The product which is to be heated is led through the tube in the direction of the arrow.

During operation of the apparatus, the mode of operation is that the generator 1 applies an electromagnetic field across the electrodes 2, 3. The electromagnetic field will in principle extend in two ways, i.e. in a primary zone A

between the two electrodes, and in two secondary zones B between electrodes and frame electrodes. As will be seen, this means that there are three heating zones which, as mentioned above, give rise to some disadvantages in several connections. A part of the current which is applied to the product in the tube will thus flow to frame or earth across the secondary zones.

In fig. 2 is seen an embodiment according to the invention, where the field and the current in the secondary path of conductivity are to all intents and purposes eliminated, or at least adjusted to a level which is suitable for the given arrangement.

The shown example embodiment is intended especially for the heating and boiling of meat products, but there will be nothing to prevent other foodstuffs such as dairy and vegetable products from being heat treated according to the invention in a corresponding manner.

The meat product is fed in the direction of the arrow through an electrically-insulating tube 4. The tube is secured by means of retaining elements 9, 10.

Via leads 7,8, a generator 1 is connected to tubular electrodes 20, 22 on the outside of the tube via trimming capacitances 24, 25 and connection points 27, 28 on the front edge of the tubular electrodes. Across the generator terminals there is connected a compensation coil 26 which compensates for parasite capacitances between the terminals and frame. The tubular electrodes 20, 22 are connected directly to frame via frame connections 11, 12.

Two capacitances 21, 23 are each coupled in parallel across one of the electrodes 20, 22. Between the supply connection and frame, each electrode works as a path of conductivity

which is primarily inductive. The capacitances 21, 23 are therefore tuned so that the two oscillatory circuits, con¬ sisting respectively of the capacitance 21 and the tubular electrode 20 and the capacitance 23 and the tubular electrode 22, are both in resonance or in the area of resonance. Consequently, no current of any significant value will flow between the connection point 28 and frame 12, nor between the connection point 27 and frame 11.

This means that the field inside the tube is no longer in several zones, but is concentrated in the primary zone A. The heating in the primary zone A is herewith more effective, the reason being that the generator can emit more power in the primary zone, since no power is lost in the secondary zones. Moreover, this is connected with the advantage that the product is not "preheated" in the secondary zone, before it comes into the primary zone. This is important in connection with the boiling or heating of products having dielectric characteristics which are temperature-dependent, in that the preheating in the secondary zone according to fig. 1 is not particularly concentrated and is capable only of heating the border layer up against the tube. This means that the product, when it reaches the primary zone, has a heated surface.

The current which flows in the meat as a function of the applied field will hereby prefer to flow in the surface of the product, the reason being that the surface has the least impedance. This thus results in a kind of negative feedback which is overcome in the embodiment shown in fig. 2, in that the product is first exposed to a field when it is fed into the primary zone.

Two extended electrically-conductive structures 29, 30 extend at the front edges 20, 22 of the tubular electrodes.

These are electrically connected directly to the tubular

electrodes 20, 22.

The result is that the charge density, and herewith the field inside the primary tube, is homogeneous, in that the charges distribute themselves uniformly the whole way around the feeding tube 4 on these electrically-conductive structures 29, 30. If cylindrical capacitances 21, 23 are used, and these are placed symmetrically around the tubular electrodes 20, 22, the extended electrically-conducting structures can be omitted, in that such capacitances in themselves will give rise to a homogeneous charge density the whole way around the tube 4.