APPARATUS FOR HEATING

Abstract: The present invention is an apparatus that enables a homogenous heating and an efficient measurement of organic as well as a mixture of organic and inorganic materials. The apparatus is characterised in that a load, placed in a cavity with very small dimensions in relation to the wavelength in vacuum at applied frequency, is heated by a generated pulsating near field.

Description: Apparatus for Heating. The needs of homogenous heating of materials which consist of organic or mixture of organic and inorganic materials are large. The materials may be made up of solid, liquid as well as a mixture of solid and liquid components. The needs of heating contain of both large and small volumes. As examples may be mentioned the preparation of food stuff in food industry, the cooking of food in restaurants and homes, the sterilisation of offals, the rotting of wood fibres and the degradation drying and sterilisation of sludge. There are applications where a fast heating without appearance of hot spots or hot areas is a necessity. One example of application is heating of cold blood in connection with blood transfusions in medical care. In industrial processes as the drying of wood there are needs to measure and utilize changes in dielectric properties. Established heating techniques as heating with microwaves, heating with conventional heat radiation and heat convection have in common, that heat absorption in a load is characterised by a little or no depth of penetration. The heating of the inner parts of a load will be done by heat transmission from the heat absorbing surfaces. In the most common dielectric materials heat transmission is a slow process. As a consequence, large volumes of organic materials require a long time for getting a homogenous heat distribution. It is also previously known that dielectric materials can be heated by oscillating high frequency electric fields generated between a pair/pairs of electrodes. This technology has the disadvantage that it is not flexible about differences in load geometry and load composition. It is also known it is possible to heat a dielectric load by emitting electromagnetic radiation from an antenna into a cavity with walls made of electrically conducting material. Heating with microwaves in a resonant cavity is since many years an established technique. A resonant cavity has the advantage that an even distribution of microwaves is possible. A resonant cavity of a microwave oven requires that certain physical conditions are met as. a. The infinite conductivity of the walls. At frequencies above 900 MHz (micro wave frequencies) the currents in a cavity wall are concentrated to the surface. An approximation according to Maxwell's equations valid for the infinite conductivity of walls results in a negligible error. With decreasing frequency the skin depth increases. At frequencies below 300 MHz is the skin depth so considerable that according to known technology a resonant cavity is not regarded as possible. b. The design of the cavity. Besides the cavity walls shall have approximately ώfinite conductivity the cavity shall meet required dimensions for a resonant cavity. For example at least one length of one side of a rectangular cavity shall not be below half a wavelength. To obtain a resonant circular cavity shall the diameter corresponding to 0,76 % of a wavelength. It is possible to obtain a homogenous distribution of microwaves in a cavity. However, microwaves have an insignificant depth of penetration. Thus as to many different applications, particularly if the loads are thick, the result will be a superficial and inhomogeneous heating. Heating in a resonant cavity at frequencies below 300 MHz has theoretically, as to many applications, considerable advantages for getting a fast and homogenous heating of large loads of dielectric materials. Due to reasons mentioned above such cavity has been regarded as an impossibility. This invention makes a resonant cavity possible at frequencies below 300 MHz at the same time as the cavity remains small and compact as well as big and capacious. The invention is characterised in that a pulsating electromagnetic near field is generated in load, placed in a cavity with very small dimensions in relation to the wavelength in vacuum at applied frequency. A near field is defined as a field formed within less than a wavelength from an antenna. Homogenous heating with high power efficiency is achieved for both small and large loads. This is valid if the longest side of the cavity is delimited, the distance between two points in the cavity is less than half a wavelength in vacuum of any wavelength in vacuum conditioned by applied frequency. Favourable heating will especially be obtained if the cavity space is delimited. The cavity space volume shall not exceed 25 % of the cubic root of any wavelength in vacuum conditioned by applied frequency, preferably not exceed 20 % of the cubic root of any wavelength in vacuum conditioned by applied frequency, still better not exceed 15 % of the cubic root of any wavelength in vacuum conditioned by applied frequency Fig.l is an example of an apparatus in accordance to the invention. Between the antenna(2) and the cavity (1) walls there is a phase difference. The magnitude of the phase difference between antenna and the cavity depends on the choice of reference points at the antenna and the cavity. The phase difference ranges from a few degrees to 180 degrees. Measurements show a very good heat distribution in the load (3). Performed computer simulations indicates very complicated field configurations. The electric and magnetic field patterns are not in accordance with the patterns valid for microwave ovens and for traditional dielectric heating. The antenna/antennas enclosed by the cavity, generates in joint action with the cavity walls pulsating electric and magnetic fields in the load. The load is heated with a very good homogeneity and a high power efficiency. Heat concentration to what is called hot spots or to the load surface does not occur. Characteristic of the invention is thus a unique interaction between antenna, cavity walls and the load situated in the cavity. This interaction makes a fast heating process possible. The invention is also characterised in that the cavity, the load, the antenna , the matching and the frequency generator forms a resonant unit. The antenna, the cavity walls and the matching shall from no load to full load in the cavity form an electrical length of half a wavelength or a multiple of half a wavelength in vacuum at applied frequency/frequencies. The favourable conditions remain if the aberration from a multiple of half a wavelength does not exceed +/- 15 % of half a wavelength, preferably does not exceed +/- 10 % of half a wavelength, and still better does not exceed +/- 7 % of half a wavelength. Antenna performance is improved if the antenna surface facing the load space is covered by one or several layers/films of an electrical non conductive material as glass, aluminium oxide, zirconium oxide, ceramics, Teflon, Capton etc. A layer may be constituted in many different ways. It may be an oxide film, a lose lying powder or a lose lying plate. The layer may touch the antenna and it may also adhere to the antenna.. A layer may also be separated from the antenna by vacuum or air. The electrical/physical tliickness of a layer/layers shall not exceed 3 % of the wavelength, preferably not exceed 2 % of the wavelength and still better not exceed 1,5 % of the wavelength conditioned by the applied frequency. The unique interaction between antenna, cavity and the load placed in the cavity makes the invention also very useful for providing parameters for process control of processes in dielectric materials as the drying of wood. The process may be carried out by heating with established technology. The invention offers homogenous heating of dielectric materials without superheating in hotspots or surface zones. Thus it is useful for demanding applications as the heating of cold blood intended for transfusions in medical care. An apparatus in accordance with the invention generates heat in a load when the applied electromagnetic radiation is within the frequency range 50 kHz - 299 MHz, preferably within the frequency range 100 kHz - 299 MHz, still better within the frequency range 300 kHz - 299 MHz, best of all 1 MHz - 299 MHz. Figl is an example of an apparatus in accordance to the invention. An apparatus in accordance to the invention has a cavity (1) with electrically conductive walls and at least one antenna (2) within the cavity. The antenna is connected to one (4) of two outgoing connections from a frequency generator (transmitter), the cavity is connected to the other (5) outgoing connection. The frequency generator (6) feeds through matching units (7) the antenna and the cavity with a current whose frequency is tuned to the antenna, cavity walls, matching units and the load (3). Within the cavity the electromagnetic field generates heat in a load (3) which is placed in the cavity's load space. The load consists of at least one material with a dielectric and/or magnetic lost factor. Characteristic for the invention is that one of the outgoing connections from the frequency generator is connected to the cavity with one or more connections. It is an advantage if the connection (8)/connections to the cavity walls is/are located in such way that the tangent plane of the average antenna surface facing the load space is situated between the load area and the connection (8) to the cavity. If one connection to the cavity wall is used it is advantageous if the connection (5) to the cavity wall is close to the normal (N) through the central point of the combined surface(-es) of the antenna/antennas (7). The intersection point (s) is the point where the normal (N) cut the cavity wall. The distance (a) along the cavity side/sides is the distance between the connection (5)/connections and the intersection (s). The distance (a) shall at applied frequency not exceed 15 % of the corresponding wavelength in vacuum, preferably not exceed 12 % of the corresponding wavelength in vacuum and still better not exceed 10 % of the corresponding wavelength in vacuum. The power efficiency is improved if the antenna(-as) plan surface facing on the cavity's load space is kept below the coincident cavity's cross section plane with 45 %, preferable 40 %, still better 35 % and best of all 30 %. It is also an advantage if the antenna(-as) plane surface facing on the cavity's load space has no sharp corners. Corners tangential to the antenna plane surface shall therefore have a radius exceeding 2 %, preferable 3 % ,still better exceeding 4 % of the largest distance of the coincident cross section area of the cavity. It is a particular advantage if the antenna(-as) plane surface facing on the cavity's load space is circular, oval or a polygon with more than 5 sides. The load space is located on the opposite side of the antenna connection to the frequency generator. The load space is enclosed by the antenna plane surface and the cavity walls. A favourable field distribution in the load requires that the longest distance of the load space between antenna and cavity walls does not exceed 30 % of the wavelength in vacuum of applied wavelength in vacuum conditioned by applied frequency. One test has been performed in an apparatus designed as described in fig.1. A 1500 grams roast beef was placed in the cavity of the apparatus. A RF signal of 136 MHz with a power of 500 Watts was applied for two periods of 15 minutes. In the beginning the meat temperature was 4 °C. After 15 and 30 minutes respectively the average meat temperature had increased 30 and 70 °C respectively. Cross sections of the roast beef shoved perfect red couleur from the steak skin to the centre. Meat pieces in sizes from 100 gram up to several kilos have also been prepared. The invention is also perfect for the measuring chemical changes in a load by establishing changes in resonant conditions. One example is the drying of wood. When wood is drying the dielectric properties will be changed. The system is matched to a perfect resonant circuit. When the moisture content in the wood decreases the resonance conditions are changed and standing waves will arise in the system. By measuring the SWR (Standing Wave Ratio the relationship between generator emitted power and from the system reflected power) the composition for example the humidity of wood can be measured. The composition can also be determined by measuring changes in impedance relationships. One example is that the characteristic impedance will decrease with decreased wood moisture.

Claims: Claims. 1. Apparatus for the heating and/or measuring of dielectric materials with electromagnetic radiation within the frequency range 50 kHz - 299 MHz with one or more antennas placed in a cavity characterised in that the antenna/antennas in joint action with the cavity walls enclosing the load space generates an electric and/or magnetic field in the load, the longest side of the cavity is delimited in such way as the distance between two points in the cavity is less than half a wavelength in vacuum of any wavelength in vacuum conditioned by applied frequency. 2. Apparatus according to the preceding claim characterised in that the cavity, the load, the antenna and the matching units together with the frequency generator forms a resonant unit. 3. Apparatus according to any of the preceding claims characterised in that the cavity space volume shall not exceed 25 % of the cubic root of any wavelength in vacuum conditioned by applied frequency. 4. Apparatus according to any of the preceding claims characterised in that the antenna, the cavity and the matching units has from no load up to full load an electrical length of half a wavelength or a multiple of half a wavelength at applied frequency/frequencies. 5. Apparatus according to any of the preceding claims characterised in that the largest distance within the load space between antenna/antennas and the cavity do not exceed 30 % of the wavelength in vacuum given by the applied frequency/frequencies . 6. Apparatus according to any of the preceding claims characterised in that the deviation from a multiple do not exceed +/- 15 % from a multiple of half a wavelength 7. Apparatus according to any of the preceding claims characterised in that the antenna surface facing the load space is covered by one or several layers/films of a electrical non conductive material, the mutual electrical/physical thickness of a layer shall not exceed 3 % of the wavelength in vacuum given by the applied frequency/frequencies. 8. Apparatus according to any of the preceding claims characterised in that one of the outgoing connections from the frequency generator is connected to the cavity with one or more connections. . Apparatus according to any of the preceding claims characterised in that the tangent plane of the average antenna surface, facing the load space, is situated between the load space and the connection point/points (8) to the cavity. 10. Apparatus according to any of the preceding claims characterised in that the distance (a) along the side/sides of the cavity between the connection point/points (5) and the intersection (s), at applied frequency does not exceed 20 % of the corresponding wavelength in vacuum. 11.Apparatus according to any of the preceding claims characterised in that the antenna(-as) plane surface facing on the load space of the cavity is below the coincident cross section plane of the cavity with 45 %. 12. Apparatus according to any of the preceding claims characterised in that the antenna(-as) plane surface facing on the load space of the cavity is circular, oval or a polygon with more than 5 sides. 13. Apparatus according to any of the preceding claims characterised in that corners tangential to the antenna plane surface have a radius exceeding 2 % of the longest distance of the coincident cross section area of the cavity. 14. Apparatus according to any of the preceding claims characterised in that chemical parameters are measured by measuring changes in the SWR . 15. Apparatus according to any of the preceding claims characterised in that the changed composition is determined by measuring changes in impedance relationships.

Inventors: EKEMAR LARS, EKEMAR, LARS
Applicants: ANTRAD SYSTEMS AB, EKEMAR LARS, EKEMAR, LARS
Application year: 1998
Main classification: H05B6
IPC codes: H05B6