WO1992022180A1

WO1992022180A1 - A method and an electrode system for heating media flowing through an insulating pipe

Info

Publication number: WO1992022180A1
Application number: PCT/DK1992/000166
Authority: WO
Inventors: Ib Obel Pedersen
Original assignee: Ib Obel Pedersen
Priority date: 1991-05-28
Filing date: 1992-05-27
Publication date: 1992-12-10
Also published as: BR9206063A; NO934291D0; NO175700C; FI935275A0; NO175700B; JPH06507752A; AU1778792A; PL168000B1; EE9400050A; DK101491D0; CA2110042A1; DK171886B1; FI935275A7; SK133593A3; HUT65950A; HU9303325D0; EP0586469A1; HU214133B; DE69201794T2; DK101491A

Abstract

A method of heating media flowing through an insulating pipe by means of high-frequency electrical power. The medium is fed through a pipe of a substantially electrically non-conducting material and only presenting minor dielectric losses, and around which two ring electrodes are arranged at such a mutual axial distance that the two electrode rings are mutually spaced by a predetermined pipe length connecting said electrodes to the terminals of an HF generator and applying an HF voltage from the HF generator. In this manner a uniform and efficient heating is obtained of the flowing medium or fluid (granulate, liquid or gas) without said medium necessarily getting into direct contact with a heating means and without said medium escaping from its conveying pipe. The latter is very advantageous in connection with many uses, such as when toxic or aggressive media apply, and also when sensitive media are involved which do not endure getting in contact with the atmosphere.

Description

Title: A Method and an Electrode System for Heatin Me¬ dia Flowing Through an Insulating Pipe

Technical Field i

The present invention relates to a method of heating

5 media by means of high-frequency electrical power, where the medium is fed through a pipe of an electrically non¬ conducting material presenting substantially no dielec¬ tric losses.

The method is particularly suited for heating fluid or 10 flowing media, which may be liquid or gaseous as well as solids, especially in the form of powder or granulate.

The present invention relates furthermore to an elec¬ trode system and an assembly for carrying out the inven¬ tion.

15 Background Art

GB-A-2 , 027 , 528 discloses a method and an apparatus for drying a thick wet layer coated on one face of a sheet of board. The sheet of board is carried past a set of electrodes applying an electric high-frequency field 20 thereon in such a manner that the electric field passes through the thick moist layer with the result that said layer is heated.

US-PS- , 839 ,485 discloses an apparatus for heating por¬ tions of articles of food on a conveyor band by means of \ 25 microwaves.

/

GB— —2 , 053 , 629 discloses a process and an device for heat-treatment of filiform elements, where the filaments are passed through an UHF-resonator cavity in which an electric field is parallel to the filaments. From the wood industry and e,g. SE-A-319,129 it is known to use an HF generator and a set of capacitive high-fre¬ quency electrodes for heating wood, e.g. in connection with gluing.

US—PS—3 , 222,491 discloses an apparatus for continuous drying of powdery or granular materials , where the ma¬ terial is conveyed through a rotatable tube mounted to be rotated by a motor. The granulate inside the tube can be heated by high frequency through two elongated plate electrodes mounted diametrically opposite one another outside the tube. The plate electrodes are slightly curved in such a manner that they follow the curvature of the outer surface of the rotating tube at a small distance from said surface. A transverse electric field is formed inside the tube between the two plate elec¬ trodes .

Description of the Invention

The object of the invention is to provide a method of heating materials having dielectric losses, where the method is far simplier than the previously known methods and can be used regardless of the materials being fluid, powdered or solid and without the electrodes necessarily getting in contact with said materials .

The method according to the invention is used for heat— ing media by means of high—frequency electrical power, where the medium is fed through a pipe of an electrical¬ ly non-conducting material presenting substantially no dielectric losses, said method being characterised by

arranging two substantially circumferential electrodes peripherally around the pipe at a mutual axial distance,

connecting said electrodes to the terminals of an HF generator, and applying an HF voltage from the HF generator to the electrodes .

The arrangement of the electrodes according to the in¬ vention results in an electric field varying in time with the HF frequency and extending from one electrode ring to the other electrode ring lengthwise the pipe. The flowing medium must be a lossy medium and conse¬ quently suited for such a heating. If this condition is fulfilled, the longitudinal field is "pulled" into the pipe and through the flowing medium with the effect that said medium is heated.

The above method is so simple that it can be used even on existing pipe systems, where a demand exists for raising the temperature of the flowing medium. The only proviso is that the pipe wall must be made of a rela¬ tively loss—free, insulating material, which is often the case due to the extensive use of plastics for the manufacture of pipes. In addition, a uniform and effi¬ cient heating is obtained of the passing medium or fluid, such as a solid, granulate, fluid or gas, without said medium necessarily coming into contact with a heat¬ ing means and without said medium escaping from the conveying pipe. The latter presents a great advantage in connection with many uses, such as when toxic or aggres- sive media are involved, and also when sensitive media are involved which cannot stand up to coming into con¬ tact with the atmosphere, or when it is desired to use a protective gas during the heating process.

An electrode system to be used when carrying out the described method is according to the invention charac¬ terised in that it comprises two ring electrodes or rib¬ bon electrodes of an electrically well conducting metal, such as copper, brass or aluminium, said electrodes be¬ ing arranged around the outer side of a pipe of a non- conducting material presenting substantially no dielec- trie losses in such a way that a longitudinal electrical field arises between the two ring electrodes when an HF voltage is applied thereon.

As previously mentioned, the electrode system includes rings, preferably circular rings, but other electrode configurations can be used depending on the shape of the processing pipe, which may be square, rectangular, el¬ liptic, triangular or approximations thereof in cross section.

Such an electrode system is remarkable for its capabili¬ ty of being mounted without problems on the outer side of an insulation pipe and for its capability of estab¬ lishing by way of an HF field a combination of dielec¬ tric losses and resistance losses in the material across the entire cross section of said material inside the processing pipe.

The electrode system is preferably such that the posi¬ tion of the electrode rings or ribbons are adjustable by displacing said electrodes in the longitudinal direction of the pipe in such a manner that the distance, i.e. the pipe length, between the electrodes is optimally adjust¬ ed to the materials to be subjected to an HF processing. In this manner a substantially optimum tuning of the output circuit of the HF generator is ensured.

As a result of the above, it is very simple to establish a high—frequency impedance match between the HF genera¬ tor and the material to be processed.

The system is furthermore remarkable for providing a smooth and uniform heating across the entire cross sec- tion of the material with the result that an extremely fast heating can be obtained because considerably higher amounts of energy can be supplied to the material inside the processing pipe than by other methods. As the electrode system does.not come into contact with the medium to be processing, the system can be used for heat—processing aggressive materials, inflammable ma¬ terials, and materials developing toxic gases which can be removed from the processing pipe through closed suc¬ tion systems .

All materials presenting a dielectric loss factor can be heated. Examples of such materials are water, aqueous solutions, powder, granulates, solids, such as wood, ceramics (in "green" state, i.e. while it is moist), articles of food, medicines, pesticides, and grain.

Brief Description of the Drawing

The invention is described in greater detail below on the basis of embodiments and with reference to the ac- companying drawing, in which

Fig. 1 illustrates an example of an embodiment of a heating system according to the invention, and

Fig. 2 illustrates another embodiment according to the invention.

Best Mode for Carrying Out the Invention

Fig. 1 illustrates an example, in which the heating system comprises a processing pipe 12 of a diameter and length suitable for the processing. The diameter may be e.g. 10 cm, and the length may be e.g. 1 m in one embo- diment, and in another embodiment the diameter may be e.g. 1 m and the length e.g. 3 m. The dimensions depend on the material to be processed, and especially on the amount of the material to be processed.

The material to be processed can be pumped, pressed or conveyed in any suitable manner through the processing pipe. In addition to fluids and gases, the method can also be used for solids advanced for instance on a con¬ veyor band.

The pipe is made of an electrically insulating material, preferably a material presenting minor or none dielec¬ tric losses, such as polyethylene, polypropylene, Teflon (PFTE) , Glass, quartz glass, and aluminiumoxide. The choice of material is determined by the properties of and conditions applying to the material to be heated, such as heating temperature, aggressiveness, emission.

The pipe 12 may be of an arbitrary cross—sectional con¬ figuration, such as round, oval, square or rectangular. Two electrodes 14, 16 are mounted peripherally on the outer side of the pipe, said electrodes being in form of copper rings or collars made of circumferential copper ribbons of an arbitrary width, however with due regard to the cross—section configuration of the pipe and to the electric HF power to be transferred to the medium.

The electrodes are connected by means of high—frequency conductors 18, 20 in form of broad metal ribbons of cop¬ per, brass, aluminium or the like to their respective terminal on an HF generator 22. The HF generator 22 is preferably provided with a balanced, i.e. symmetrical, output, but the output may also be non-symmetrical.

An HF field, cf. the thin dotted lines in Fig. 1, is formed between the two electrode rings. The HF field concentrates particularly in the medium with the dielec¬ tric losses, and the electrical power is converted into heat in said medium.

The positions of the ring or collar electrodes 14, 16 can preferably be adjusted sideways, i.e. they can be displaced along the pipe. In this manner it is possible in a very simple manner to match the load impedance pre- sented to the HF generator 22-

Fig. 2 illustrates another embodiment according to the invention. In this embodiment, flanges 24, 26 are mount¬ ed on two ends of the processing pipe 12, said flanges being made of copper, stainless steel, brass, aluminium, titanium, beryllium copper etc. The metal flanges should preferably, but not necessarily be of the same internal diameter as the insulating processing pipe. Such flanges should be connected to ground. They render it possible to meet special security provisions. In addition, the flanges ensure that the voltages applied do not arise elsewhere on undesired locations in an assembly.

Temperature sensors and/or moisture sensors can be mounted in connection with the pipe 12, said sensors monitoring the temperature and/or the moisture of the medium and optionally being coupled to a control unit controlling the output power supplied by the generator 22.

On the inner side the electrode rings can be lined with a heat-resistant material, such as Teflon, i.e. poly- tetrafluoroethylene , presenting minor or predominantly none dielectric losses in order to protect the heat-pro¬ cessing pipe against possible heat arisen in the elec¬ trodes due to copper loss.

Thus the system is also advantageous in providing an automatic regulation of the output power transferred to the materials inside the processing pipe by means of sensors built into the system, and of the temperature, the moisture etc. Accordingly, it is possible by such a method to carry out processings, such as pasteurisation, sterilisation etc, requiring an exact control of various parameters .

The operational frequences most suited for the method turned out to be 13,56 MHz and 27,12 MHz, but both con¬ siderably lower and higher HF frequences can be used where the frequency is determined by the actual applica¬ tion.

The output power transferred to the medium or the fluid is measured according to the following formula

Output power in kW - 0.07 x Δ T x M

where M — amount of 1 per min and Δ T — temperature dif¬ ference in ^βC.

As the medium is simultaneously heated across its entire cross section, a several times higher output can be transferred without damages to a medium compared to the conventional heating methods , where the heating is car¬ ried out by way of heat conduction and where the heat— transmitting properties of the medium are decisive for the heating rate. In this manner the damages of the pro¬ cessed material resulting from the conventional heating methods have been avoided, said conventional heating methods involving a heating by way of heat conduction which results in a superheating of the surface and a too poor heating in the core of the material.

Claims

Claims .

1. A method of heating media by means of high-frequency electrical power, where the medium is fed through a pipe (12) of an electrically non-conducting material present- ing substantially no dielectric losses, c ti a r a c - t e r i s e d by

arranging two substantially circumferential electrodes (14, 16) peripherally around the pipe at a mutual axial distance ,

connecting said electrodes to the terminals of an HF generator (22), and

applying an HF voltage from the HF generator (22) to the electrodes (14, 16).

2. A method as claimed in claim 1, c h a r a c t e r — i s e d by the medium being a fluid flowing through an insulation pipe.

3. A method as claimed in claim l or 2, c h a r a c ¬ t e r i s e d by the output power of the HF generator being adjusted in response to' temperature sensors and/or moisture sensors.

4. A method as claimed in claim 1, 2 or 3, c h r ¬ a c t e r i s e d by the output power of the HF genera¬ tor being adjusted in response to the flow rate of the medium or the fluid through the pipe.

5. A method as claimed in claim 1, 2, 3 or 4, c h a r- a c t e r i s e d by the operating frequency of the HF generator being 13.56 MHz or a frequency suited for the medium in question, which means that the frequency of the output of the generator is chosen such that a major portion of the energy contained in said output of the HF generator is transferred to .the medium.

6. An electrode system to be used by a method as claimed in claims 1, 2, 3 or 4, c h a r a c t e r ¬ i s e d in that it comprises two ring electrodes or ribbon electrodes (14, 16) of an electrically well con¬ ducting metal, such as copper, brass or aluminium, said electrodes being arranged around the outer side of a pipe (12) of a non-conducting material presenting sub¬ stantially no dielectric losses in such a way that a longitudinal electrical field arises between the two ring electrodes when an HF voltage is applied thereon.

7. An electrode system as claimed in claim 6, c h a r— a c t e r i s e d in that the positions of the elec¬ trode rings or ribbons (14, 16) are adjustable by dis- placing said electrodes in the longitudinal direction of the pipe in such a manner that the distance, i.e. the pipe length, between the electrodes is optimally adjust¬ ed to the materials to be subjected to an HF processing.

8. An electrode system as claimed in claim 6 or 7, c h a r a c t e r i s e d in that the inner diameter of the ring electrodes corresponds to the outer diameter of the pipe, i.e. said inner diameter is substantially equal to said outer diameter of the pipe.

9. An electrode system as claimed in one or more of the preceding claims 6 to 8, c h a r a c t e r i s e d in that the pipe at both ends is terminated by flanges (24, 26) of metal, preferably an electrically well conducting metal, and that the choice of material depends on said material being capable of standing up to contact with the medium to be heated.

10. An assembly for carrying out the method as claimed in one or more of the preceding claims 1 to 4, c h a r¬ a c t e r i s e d in that it comprises an insulating pipe (12) forming part of a- conveying system for a me¬ dium to be heated, and that two substantially circumfe¬ rential electrodes (14, 16) are arranged to surround the pipe at a mutual axial distance, each of the electrodes being connected to a terminal on an HF generator (22).