EP0853866B1

EP0853866B1 - Microwave shielding

Info

Publication number: EP0853866B1
Application number: EP96931354A
Authority: EP
Inventors: Leif Gösta Zettergren
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-09-22
Filing date: 1996-09-20
Publication date: 2002-07-03
Anticipated expiration: 2016-09-20
Also published as: DE69622179T2; AU7005796A; DE69622179D1; ATE220286T1; SE505240C2; SE9503308D0; EP0853866A1; WO1997011579A1; SE9503308L

Abstract

Method of shielding against microwaves for mobile microwave appliances open at the bottom. The radiation path through the small gap between the equipment casing and the surface underneath is lengthened by edge widening. Microwave appliance to be used at the method.

Description

Microwave technologies bring great advantages for heating of different kinds. For that reason it is used to an increasing extent. Microwaves are considered to cause danger for damages to many organisms including humans and other warm-blooded animals. A fundamental condition at all uses of microwave technology is a reliable shielding against leakage of microwaves.
If microwaves are used in closed chambers such as ovens, etc. the shielding is not a large problem. Metals are good reflectors for microwaves and do not release radiation. Even where a certain degree of ventilation is required shielding can with relative ease be arranged by perforated plates or nets of a suitable metal. The apertures in the plate or net must be dimensioned not to let radiation through.
The situation becomes more complicated when microwaves are used for the treatment of objects that cannot be enclosed in an oven or similar construction. Examples of such use can be found in for instance the Swedish patent application no. 9400751-0, PCT/SE95/00218, PCT/FI91/00330 and US 4,765,773. If the equipment is to be mobile, an unavoidable air gap will arise between the treated surface and the equipment casing. In many publications it is said just that this air gap should be small enough not to allow microwave passage. However, information of what is meant is missing. When shielding is proposed, it concerns metal brushes, chains, flexible shields of metallized textiles or plastic sheets etc. It has been found difficult to get sufficiently reliable shielding, without damages of friction against the surface or hooking up at it, in these ways. Further these types of shielding may damage sensitive surfaces. In many applications it is wanted that the shielding does not touch the surfaces treated.
According to US patent no 4,765,733 microwaves are used for high-speed drying of paint applied at road surfaces. The shielding problem is solved by using an undefined microwave absorbing material. This implies a comparatively large distance between the microwave generator and the shielding material. Otherwise, the material would be subjected to unacceptable heating.
According to the Swiss patent no 373491 microwaves are used for the drying of material, for instance wood pieces, placed on a transport chain. The drying equipment is designed as a closed box with openings at each end for in and out passage of chain and material. As shielding at the openings sloping flaps, opening in the movement direction, are used. The arrangement is requiring much space in the longitudinal direction, partly since the flaps need space for the movement, partly since the shielding, to be sufficiently effective, must be completed with comparatively long distances between the microwave source and the opening. The method is useful in an equipment with stationary microwave sources and mobile drying material.
UK patents no 630,738 and 1,034,448 also concern drying of material placed on a transport belt here made of a dielectric material. In the first patent publication a box is proposed. This box is closed at the borders of the belt and has slits in opposite walls for the in and out passage of belt and material. In the second patent application a design is used, which lets the upper and lower part of the enclosure separate for the in and out passage of drying material and close together during the heating and drying phase, i.e. when the microwave apparatus is active. In both inventions one uses an upper and a lower metal plate to prevent microwave leakage. These plates form with the transport belt of dielectric material a capacitive seal for microwaves. Also, these methods seem to require stationary installation of microwave sources and mobile material.
None of the proposals in the patent publications mentioned is applicable at the use of microwaves according to the patent applications SE 9400751-0 and PCT/SE95/00218 mentioned before where microwave equipment is used for the drying of floors, walls, ceilings etc., i.e. a mobile drying equipment, which is automatically moved over a stationary surface. A fundamental condition to make this technique useful is that drying can be done close to walls and other limiting surfaces. Here arranging capacitive microwave traps, such as described in the mentioned patent publications, is impossible, as possibilities to arrange a lower metal plate do not exist.
The inventor has had to test other methods which resulted in the invention as defined in claims 1 and 2. It has been found that very good shielding can be secured with the aid of a metal rim that makes the lower border surface wider. The rim may be angled inwards the apparatus casing or outwards from this casing. The shielding does not touch the surface over which the microwave apparatus is moved. Thus, no danger exists that the apparatus damages the surface treated or is damaged.
The cause of the effect is not completely clarified, but one may assume that it relates to the geometry of the radiation. Common microwave generators deliver electromagnetic radiation in the range around 12 cm. The wave motion propagates itself linearly. Microwave appliances often consist of parallelepipedic plate boxes with one or more magnetrons placed at the top of the box. The radiation source may be considered point-shaped and spreading radiation uniformly in all directions except upwards. The plate material reflects the radiation. The consequence is that one gets a system of standing waves in the enclosed chamber.
In a box open at the bottom and with a gap between the lower edge of the box and the surface underneath part of the reflected waves will hit the gap directly and after that be partly reflected by the surface just outside the box. Part of the waves will have an angle of incidence against the surface that approaches the angle of total reflection and be wholly or partly reflected even if the surface contains dielectric substances and is microwave absorbing. If the reflection occurs near the edge these waves may, after the reflection, have such direction that they can pass through the gap and propagate themselves outside the box. From geometric reasons it is excluded that the waves become completely parallel to the surface. The small gap gives, thanks to the border widening, a longer path for the wave and increases the probability that the spreading will be prevented by reflection against the metal surface and absorption in the surface underneath.
Even a relatively small increase in the border width decreases the radiation considerably but the best effect is obtained if the border width is as large as or larger than the gap width. For wheel provided microwave dryers, such as described in SE9400715-0, standing on rails consisting of plastic pipes of the type use for electric installations this means a border width of 10 to 35 mm.
Fig. 1 shows a sketch of one side of a microwave device, which has been provided with an aluminium brim at the bottom border.
Fig. 2 shows a corresponding sketch of a microwave appliance with a fold pointing inwards with respect to the casing at the lower part of the casing plate.
Fig. 3 shows one side of a microwave appliance where the inner shielding net has been drawn down along the equipment casing and folded to get border widening.
In the figures (1) stands for the outer wall of the equipment casing, (1') for a fold pointing inwards with respect to casing, (2) for a border widening rim, applied to the equipment casing, (3) for a metal net impervious to microwaves and (3') for a drawn in border of this net, and (6) for the surface that the microwave appliance is working on.
In the figure 1 a so-called L-profile and in the figure 3 a corresponding folding has been used. However, nothing prevents that other profile forms (for instance an u-profile with the opening downwards) are used if the aim to lengthen the radiation path through the gap is fulfilled.
The widening can be done by giving the bottom border a rim of a suitable metal for instance aluminium, which is a very good microwave reflector. Alternatively the plate casing may be folded at the lower border to get the widening.
In the figures an inward border widening is shown. This is from practical reasons the usual design. The effect will be the same if the edge widening is made outwards with respect to the equipment casing, but in this case the placing of the wheels becomes problematic. Placing inside the casing is for natural reasons not done. The same applies to the designs with folding of the outside plate.

Claims

Method for reducing microwave leakage from a mobile microwave appliance open at the bottom, the microwave appliance being used for the drying or heat treatment of floors, walls and ceilings and for being automatically moved over a stationary surface, the microwave appliance being especially used for devices intended for the drying or heat treatment of surfaces limited by walls or boundaries where drying respectively heat treatment must be done close to the walls or boundaries, the method is characterised by using a widened border (2) at the bottom facing the stationary surface (6), to increase the distance that leaking radiation has to travel when passing through a small gap between the widened border and the stationary surface underneath, the widened border extending inwards towards the interior of the microwave casing of the microwave appliance, the widened border comprises:

a) a metallic rim applied at the inside of metal casing plates (1) of the microwave appliance, or

b) a fold (1') pointing inwards, the fold being folded in the lower part of the metal casing plates, or

c) an inwards pointing fold (3') which is provided in an inner shielding net (3), the net is located in the metal casing, the net is extending downwards towards the open bottom of the metal casing, and the shielding net is folded inwards in order to obtain the inwards pointing fold (3') in the shielding net.
A mobile microwave appliance with reduced microwave leakage, the microwave appliance is used for the drying or heat treatment of floors, walls, ceilings etc. and automatically moved over a stationary surface (6), where drying respectively treatment must be done close to bounderies of the stationary surface, the microwave appliance is characterised by a widened border at its bottom facing the stationary surface, the widened border extending inwards towards the interior of the microwave casing of the microwave appliance, the widened border is provided as

a) a metallic rim at the inside of metal casing plates (1) of the microwave appliance, or

b) a fold (1'), pointing inwards, in the metal plates, or

c) an inwards pointing fold (3') in a shielding net (3) located in the metal casing, the net extending downwards towards the open bottom of the metal casing.