Electric heating – Capacitive dielectric heating – With power supply system
Patent
1992-12-09
1994-04-05
Leung, Philip H.
Electric heating
Capacitive dielectric heating
With power supply system
219773, H05B 654
Patent
active
053007492
DESCRIPTION:
BRIEF SUMMARY
The present invention is a method and an apparatus for reducing distance-dependent voltage increase of electrodes occurring in an apparatus comprising parallel, rod-like electrodes connected to a high-frequency voltage source.
Electrode structures of this type are used, for example, in heating and/or drying of various material webs, sheets or layers with high-frequency energy. The material to be treated in said machines is passed close to the rod-like electrodes, several of which are placed in parallel essentially transversely with respect to the travel direction of the material.
The parallel electrodes are alternately connected to the high-frequency power supply for the purpose of forming an electromagnetic field between the electrodes to be directed to primarily influence the material to be treated. If the material to be treated contains moisture or otherwise possesses similar dielectric properties, the high-frequency electromagnetic field that is formed between two electrodes mainly is directed to the material to be treated. If the material also has a high dielectric loss factor, the high-frequency field generates a heating effect in the material, which, in turn, results in the desired heating and/or drying of the material.
Examples of applications for apparatuses of this type are heating of paper web, wood veneer or textile materials with the aim of drying the material or equalizing their moisture content, heating of layers with which the material are coated or which are absorbed in it, after-treatment of bakery products proceeding on a transport conveyor, heating of a powdery or grainy stuff layer proceeding on a conveyor, etc.
In most applications, the material to be treated passes as a wide web or mat, across which the electrodes must reach in order to bring about the desired effect. Long electrodes are involved with the well known problem of standing waves increasing the voltage with increasing distance from the voltage supply point. In many applications, size restrictions of the apparatus and similar structural factors enable voltage supply to the electrodes only from one end, at the most from both ends of the electrode, thus limiting the possibilities to eliminate the voltage increase.
As is well-known, the above structures have provided a solution for the reduction of voltage increase, in which inductive coils are connected between adjacent electrodes at fixed intervals. This arrangement provides a serviceable solution for the reduction of voltage increases, in case it is applicable as far as the other apparatus structure is concerned. In paper and wood veneer dryer, for instance, in which some 5-meter electrodes and an alternating voltage frequency of 13.56 MHz are used, the solution is capable of keeping the voltage within the range of .+-.5% of the initial voltage (1.5 kV) along the entire electrode. The solution requires two inductive coil connections along the rods.
The above known technology, based on the use of so-called centralized compensating coils, suffers, however, from certain problems in a system containing several parallel electrodes. The structure is unfavourable in terms of space need. It is also prone to getting dirty, and is difficult to clean. Inaccuracy in dimensioning of the apparatus, caused by the mutual inductances of the compensating coils further presents a functional problem. A solution for the elimination of the voltage increase occurring in the electrodes, based on a different principle, has been introduced in the Finnish patent specification 55922. In this solution, the electrodes are combined into groups of two electrodes, and a common supply is used by having a supply conductor located between the electrodes at an equal distance from them. Supply points are favourably chosen at several points between the ends of the electrodes. By taking the frequency of the alternating voltage properly into account when choosing the supply points, a compensating effect to the voltage in the electrodes can be achieved. Compensating of the electrode voltage is also influenced by t
REFERENCES:
patent: 2871332 (1959-01-01), Northmore et al.
patent: 3329797 (1967-07-01), McDougall Clark
patent: 3461263 (1969-08-01), Manwaring
patent: 3701875 (1972-10-01), Witsey et al.
patent: 4670634 (1987-06-01), Bridges et al.
Imatran Voima Oy
Leung Philip H.
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