WO1994017647A1

WO1994017647A1 - Arrangement for continuous heat treatment with microwave energy

Info

Publication number: WO1994017647A1
Application number: PCT/SE1994/000031
Authority: WO
Inventors: Tommy NORÉN; Petter Söderhjelm; Bo Hansson; Anders Ekstrand
Original assignee: Alfa Laval Ab
Priority date: 1993-01-21
Filing date: 1994-01-18
Publication date: 1994-08-04
Also published as: EP0680684A1; AU5981594A; SE9300167L; JPH08505733A; SE470570B; SE9300167D0

Abstract

An arrangement for continuous heat treatment with microwave energy of product units comprises a treatment chamber (6) in the form of a tunnel. It also comprises means (2, 8, 9) to generate and distribute microwave energy in the chamber and a conveyor belt (3) which is transparent to microwaves for transportation of the product units (7) through the chamber. In the treatment chamber (6) there are means (10) which shield parts of the product units from microwave energy during the transportation through the treatment chamber.

Description

Arrangement for continuous heat treatment with microwave energy

The present invention relates to an arrangement for continuous heat treatment of product units with micro¬ wave energy. The arrangement comprises a treatment chamber in the form of a tunnel, means for generation and distribution of microwave energy in the chamber and a conveyor belt which is transparent to microwaves for transportation of the product units through the chamber.

Such arrangements are known for example in DE 38 06 816, US 4 246 464 and US 4 889 966.

A problem with microwave heating of product units which by means of a transport device are conveyed through a microwave field is the tendency that the energy is concentrated to the edges and corners of the product units. In order to protect these for a local overheating one has tried to pack the product units in packages with built in protection zones or tried to distribute the microwave energy with sophisticated control methods. Hovever both these methods involve considerably increased costs per treated product unit.

According to the invention there is now proposed a new solution of the problem of distributing microwave energy in the product units which are to be treated in the arrangement.

The arrangement according to the invention is mainly characterized in that means which shield parts of the product units from microwave energy during the trans¬ portation through the treatment chamber are arranged in the same. The product units which are treated in the arrangement according to the invention may be packaged or unpackaged. They may consist of food or some other product which demands some kind of heat treatment. The heat treatment may aim at a thermal stabilization, a pasteurization or a sterilization. The product units which are to be treated in the arrangement have in common that they are relatively thin, 5-20 mm. Also products with a larger thickness may be treated in the arrangement.

The arrangement according to the invention is with advantage provided with a second movable belt which is arranged above the conveyor belt on a distance from the same which mainly corresponds to the thickness of the product units. With such an arrangement there is ensured an undisturbed movement through the treatment chamber simultaneously as the product units are given a mechanical support during the heat treatment.

The means which shield the product units from microwave energy suitably consist of exchangeable, longitudinal inserts arranged between the means which supply the microwave energy coming from the microwave generators - the applicators - and the product units on the conveyor belt. The shielding is arranged above the conveyor belt when the applicators are arranged above the conveyor belt. If one for some reason should choose to supply the microwave energy from below the shielding shall of course be arranged under the conveyor belt. By using exchangeable inserts the shielding may rapidly and simply be adapted to product units with varying form and size. According to another embodiment of the invention the shielding consists of sections which are untransparent to microwave energy inserted or interwoven into the second mentioned movable belt and/or the conveyor belt.

The extension of the mentioned inserts perpendicular to the direction of motion of the conveyor belt may vary along the length of the treatment chamber. With such an arrangement the microwave energy which is supplied to the product units may be closely controlled and adapted to the demands of the specific product. The shielding may consequently cover a larger part of the area of the product unit either immediately after the entrance into the treatment chamber or during a later phase, alter- natively be changed stepwise in a manner which is suitable for the product.

By varying the amount of added microwave energy by supplying a larger or a smaller amount of microwave energy to the treatment chamber by way of the appli¬ cators connected to the microwave generators, there is according to the invention an additional possibility to control the distribution of the microwave energy in a simple manner. With advantage microwave generators are used, which give a larger amount of microwave energy for heating in the first part of the treatment chamber seen in the direction of transportation while generators with a lower effect are used in the rear part of the treat¬ ment chamber.

The means which distribute the microwave energy in the treatment chamber (applicators) are with advantage arranged in such a way that they give a microwave field which is at an angle to the direction of movement of the conveyor belt. With such an arrangement it is avoided that microwave energy which is reflected against the bottom of the treatment chamber is radiated into the applicator.

The applicators which are used work with two vertical TM-modes, at which the dominant mode is close to the limit frequency. The dominant mode has such a low impedance that it heats without reflection but is simultaneously mismatched to the free space.

The arrangement according to the invention is described further with reference to the attached drawings, which show an embodiment chosen as an example only of the arrangement.

In fig 1 there is shown schematically an arrangement comprising treatment chamber and conveyor belt, in fig 2 there is shown schematically a cross-section through the treatment chamber and in fig 3 and 4 there are shown examples on the design of shielding inserts.

The arrangement 1 comprises microwave generators 2 ( fig 2 ) arranged above a conveyor belt 3. Above the conveyor belt 3 there is a second movable endless belt 4. Between leakage traps 5, which hinder the microwave energy to leak out from the arrangement there is a treatment chamber 6, where product units 7, which are to be heat treated, are heated by microwave energy.

As is shown more clearly in fig 2 the product units are fed in between two conveyor belts 3 and 4 transparent to microwaves. The microwave energy emitted from the micro¬ wave generator is led to an applicator 9 by way of a wave guide 8, which distributes the microwave energy towards the product units on the conveyor belt. As has been mentioned earlier the field is directed in an acute angle to the direction of movement of the conveyor belt in order to avoid that energy is reflected against the bottom of the treatment chamber up towards the appli- cator. The reflection is about 30 % of supplied effect, which is used to heat the product units from the under¬ side.

Between the applicator and the product units there are shieldings 10 which hinder the microwave energy to enter into sections of the product units.

The shieldings 10 may be shaped as an exchangeable rack ( fig 3 ) which is arranged in the treatment chamber after the leakage trap or as longitudinal inserts (fig 4) with an extension which varies along the length of the treat¬ ment chamber. The inserts may consist of metallic material including aluminium which effectively shields the microwaves and at that hinders these from entering into parts of the product units. As may be seen the inserts may be shaped in a large number of ways depen¬ ding on the form and thickness of the product units, their dielectric properties, structure and homogneous- ness. The distribution of temperature in the product units is controlled by the combination: time, microwave effect and degree of shielding.

The number of generators with accompanying wave guides and applicators depends on the capacity of the arrange- ment, preferably between 100-1000 kg/hour.

At the entrance to the arrangement there is a stacking device for "shingled" products alternatively a handling device for stacked, cylindrical, square or rectangular packages in order to obtain a constant feeding of product units into the arrangement. This is of impor¬ tance in order that the amount of product that the microwaves should heat per time unit must be kept constant in order to obtain an even temperature of the product units. The stacking principle depends on the fact that the feeding belt runs with a suitably higher speed in order to achieve an overlapping. The speed of the stacking belt is mechanically adjusted to the speed of the conveyor belt at which the same overlapping is obtained if the rate of the conveyor belt i.e. the treatment time is changed.

For other products there are equipment which packs them together and if so is desired, depending on the shape of the product or on the package, turns every second product unit round in order to achieve that the micro¬ waves consider the product units as a homogenous "treatment train".

Suitable products within the food field may consist of sliced products as cooked ham, German sausage, prepared products as hot dogs, patties, soft cheese, hamburgers and meat balls.

As examples of products which are not food there may be mentioned packed infusion solutions and contact lenses and products which shall be dried or demand some kind of heat stabilization.

The treatment time for the product units, i.e. the rate for the conveyor belt is usually chosen such that the holding time is 60-120 seconds, for products which are more difficult to treat the holding time may be as long as 4 minutes.

Claims

1. Arrangement ( 1 ) for continuous heat treatment with microwave energy of product units comprising a treatment chamber (6) in the form of a tunnel, means (2, 8, 9) to generate and distribute microwave energy into the chamber and a microwave transparent conveyor belt (3 ) for transportation of product units (7) through the chamber, c h a r a c t e r i z e d i n that means (10) which shield parts of the product units from micro¬ wave energy during the transportation through the treat¬ ment channel are arranged in the same.

2. Arrangement according to claim 1, c h a r a c - t e r i z e d i n that a second movable belt (4) is arranged above the conveyor belt on a distance from the same which corresponds to the thickness of the product units.

3. Arrangement according to claim 1 or 2, c h a ¬ r a c t e r i z e d i n that the means which shield the product units from microwave energy consist of exchangeable longitudinal inserts (10) arranged between the means ( 9 ) which feed the microwave energy emitted from the microwave generators - the applicators - and the product units on the conveyor belt (3 ) .

4. Arrangement according to claim 1-2, c h a r a c ¬ t e r i z e d i n that the means which shield the product units from microwave energy consist of the sections which are untransparent to microwave energy in the mentioned second belt (4) .

5. Arrangement according to claim 3, c h a r a c - t e r i z e d i n that the extension of the inserts (10) perpendicular to the movement direction of the conveyor belt varies along the length of the treatment chamber.

6. Arrangement according to claim 5, c h a r a c ¬ t e r i z e d i n that the extension of the inserts (10) is adapted to the amount of supplied microwave energy, which latter varies along the length of the treatment chamber.

7. Arrangement according to claim 6, c h a r a c ¬ t e r i z e d i n that the amount of supplied microwave energy is at the greatest when the product units first enter the treatment chamber and diminishes along the length of the treatment chamber.

8. Arrangement according to any of the preceding claims, c h a r a c t e r i z e d i n that the means ( 9 ) which distribute the microwave energy (applicators) give a field which is at an angle in relation to the direction of movement of the conveyor belt (3).