EP0143719B1 - Electric steam generator - Google Patents

Abstract

An electric vapor boiler has a closed vessel (1) containing water, a steam or vapor outlet conduit (4), a conduit (6) supplying liquid and/or returning condensed vapor, and a electric resistance heating device (7) for each high or medium boiler voltage phase immersed in the water. Each heating device (7) includes a bare metallic resistance wire (8) immersed in the water and mounted in such a manner as to vibrate between a pair of vertical open-ended cylinders (13, 14) forming insulating screens. Each wire (8) is directly connected between a phase terminal (11) and ground, that is, the vessel (1) and has a plurality of parallel vertically spaced spires (18) offset symetrically with respect to each other on each side of a generatrix (16) of an oblique surface in the enclosure between the cylinders (13, 14) and extending obliquely to the vertical direction in the vessel (1). The outer cylinder (13) extends above the surface of the water. Alternatively, each wire can be formed as a plurality of parallel strands (38) and be positioned in an open-ended vertically disposed tube (33) having an oblong cross sectional shape.

Description

The present invention relates to a steam generator.

Resistance type electric steam generators used in industry usually include a closed vessel or enclosure containing water, or other liquid, heated by electric resistance heating elements which are immersed in water or other liquid.

The heating elements used up to now consist of sealed insulating sheaths inside which electrical resistors are arranged which communicate their heat to the surrounding water through these sheaths.

These elements are expensive to manufacture, and their rate of thermal transfer is low and their design requires an expensive low-voltage supply for large powers.

FR-A-505.484 describes a steam generator in which a naked helical conductor is immersed in a liquid, being disposed inside an insulating tube having openings through which the liquid penetrates.

The insulating tube is arranged horizontally and the conductor is held in the tube by its ends.

The horizontal arrangement of this tube is not favorable for the release of vapor bubbles. The performance of this device is therefore poor.

The object of the invention is to remedy the drawbacks of known devices by producing an electric steam generator comprising electric heating means which can be supplied with high or medium voltage, having a high thermal transfer rate and in which the conventional heating elements are replaced. by new devices of simple and very economical manufacturing, allowing to realize high power units in a small volume.

The object of the invention is for this purpose an electric steam generator, of the type comprising a closed tank containing a liquid, a steam outlet duct, a liquid supply and / or return duct for condensed steam, electric resistance heating means constituted by bare wires immersed in the liquid, means connecting each of said resistors to a source of alternating electric current through the wall of the tank, and insulating screens between which said resistors are arranged, characterized in that that said bare wire of each resistor is mounted vibrating by being arranged so as to present parallel or roughly concentric strands offset on either side of a generatrix of an oblique surface.

According to one embodiment of the invention, said surface is conical or pyramidal, said wire forming a spiral whose turns are offset on either side of said surface, between two walls of an enclosure formed by said screens.

• La figure 1 est une vue en élévation et en coupe d'un générateur électrique de vapeur suivant l'invention.
• La figure 2 en est une vue en plan de dessus.
• La figure 3 est une vue partielle en coupe à plus grande échelle de l'un des dispositifs chauffants du générateur représenté à la figure 1.
• La figure 4 est une vue en plan de dessus d'une variante du dispositif chauffant de la figure 3.
• La figure 5 est une vue en perspective d'un autre mode de réalisation d'un dispositif chauffant.
• La figure 6 est une vue en élévation et en bout, en coupe, du dispositif représente à la figure 5.
• La figure 7 montre une variante de la fixation du conducteur du dispositif des figures 5 et 6.

The following description, with reference to the accompanying drawings by way of nonlimiting examples, will make it possible to understand clearly how the present invention can be put into practice.

• Figure 1 is an elevational and sectional view of an electric steam generator according to the invention.
• Figure 2 is a top plan view.
• FIG. 3 is a partial view in section on a larger scale of one of the heating devices of the generator shown in FIG. 1.
• FIG. 4 is a top plan view of a variant of the heating device of FIG. 3.
• Figure 5 is a perspective view of another embodiment of a heating device.
• FIG. 6 is an elevation and end view, in section, of the device represented in FIG. 5.
• FIG. 7 shows a variant of the fixing of the conductor of the device of FIGS. 5 and 6.

Referring to the drawings and more particularly to Figures 1 and 2, the steam generator according to the invention comprises in known manner a tank 1 carried by feet 2, coated with a thermal insulator 3 and comprising at its upper part an opening 4 access, a steam outlet duct 5 and, at its lower part, a duct 6 for the return of condensed steam.

According to the example shown, the generator operates on a three-phase current and comprises three identical heating devices 7, one for each phase of the supply current. Since they are identical only one of them will be described.

The heating device 7 comprises a non-insulated continuous conductor 8 immersed directly in the water contained in the tank 1.

The conductor 8 forms a spiral, the turns of which are arranged vertically behind a screen forming an insulating enclosure.

The upper end of the conductor 8 is connected by a conductor 9 to a supply terminal through the tank 1 by means of an insulator 11.

The lower end of the conductor 8 is connected by a conductor 12 to the ground, that is to say to the tank 1.

The spiral formed by the conductor 8 is disposed inside a double insulating enclosure which is formed, according to the example shown, by two concentric tubular screens 13 and 14 made of an electrically insulating material, arranged vertically, supported by a cross-member horizontal 15 and open at their ends.

Reference will now be made to FIG. 3 which is a partial schematic view on a larger scale showing the arrangement of the conductor 8 in the insulating enclosure.

The upper end of the outer tube 13 extends above the level of the water, or other liquid contained in the tank, while the upper end of the inner tube 14 extends only a certain distance below from the surface of the water.

In the circular enclosure delimited between them by the tubular screens 13 and 14 the turns formed by the naked conductor are offset on either side of a generator 16 of a frustoconical surface whose small base is located upwards, below the surface of the water, approximately at the level of the upper end of the inner tube 14 as shown in 17, at the connection of the upper end of the spiral with the phase conductor 9.

The turns 18 are held in position by means of elastic members 19 acting in traction and fixed alternately to the outside and inside tubes.

The lower end 20 of the spiral is connected to the ground as indicated above, that is to say to the tank 1.

Thanks to this arrangement of the turns 18 on either side of the generator 16, when the current flows in these turns, electromagnetic forces arise in each turn, and each of them is thus attracted towards the nearest neighboring turn close as represented by arrows and this attraction acting on each in opposite directions generates a result indicated by the arrows 21 and which are oriented roughly in opposite directions on the alternating turns. This results in a continuous vibration of these turns.

Consequently, the vapor bubbles which form on the turns during the passage of the current in the conductor 8 detach and rise towards the surface of the water as indicated by the vertical arrows at the right part of FIG. 3, causing thus the water heated up. This rise in water and steam is compensated by a water supply indicated by an arrow 22 while the hot water passes over the upper end of the central tube 14 and is recycled as indicated by the arrow 23. There is thus a continuous circulation of water by which the heat transfer rate is improved.

Referring again to Figure 1 we see that the fact that the outer insulating tube 13 rises above the surface of the water for each heating device connected to a phase of the supply, the leakage lines between the conductors 9 and between them and the earth are too long for a leakage current to be established.

The insulating chambers of each heating device being open at their lower end, a general circulation of water is established in the tank between the various devices, circulation which accelerates the rise in temperature of the liquid.

FIG. 4 shows a variant of the fixing of the turns 18 of the conductor 8, in which the elastic members 19 are fixed on rigid uprights 24 made of an insulating material, for example ceramic, disposed between the concentric screens 13 and 14 .

FIGS. 5 and 6 show an embodiment of the heating devices in which the conductor 28 is arranged in an insulating enclosure constituted by a tube of oblong section, for example rectangular, the two large parallel walls 33 of which delimit a space in which the conductor 28 is arranged so as to have parallel strands 38.

Referring to FIG. 6, it can be seen that the parallel strands 38 of the conductor 28 are generally arranged offset with respect to a generator 26 of an oblique surface, thus also producing vibrations of the strands, 38, which have the effect of detaching the Steam bubbles rising to the surface, carrying hot water inside the enclosure and at the same time inducing a call for cold water as indicated by the curved arrows in Figure 6. The upper end of the conductor 28 is connected to the phase conductor 29 and its lower end is connected by a conductor 32 to the ground.

In this example, the strands 38 of the conductor can also be held in position by elastic members 29 in a manner similar to that described with reference to FIG. 3.

In a variant shown in FIG. 7, the strands 38 of the conductor are carried by rigid supports 30 by means of elastic members 29.

In each heating device there is established in water, which is slightly conductive, an electric current between elements of the metallic conductor. with a potential difference. This current, parallel to the main current, also participates in the transmission of thermal energy to water.

The geometry of the devices, the cross sections of the current in the water, the distances between elements of conductors at different potentials, make it possible to predetermine, for a given supply voltage, the value of this current. This value is limited so that, depending on the nature and geometry of the conductor chosen, it does not cause, by hydrolysis, a hydrogen and oxygen content incompatible with use.

The high heat transfer rates that are obtained allow the generator to be supplied with high and medium voltage, which constitutes an economical solution for high power devices.

The nature and cross-sectional shape of the metallic conductor are chosen so that it withstands the high temperatures to which it is subjected in the generator, that it does not undergo oxidation or chemical attack at this temperature and has a resistivity sufficient electrical power so that it is not necessary to give it too long a length. For example, use a Ni-Cr steel with 70% nickel and 30% chromium having a resistivity of the order of 1.2 ohms mm ² / m.

Furthermore, the electrical insulating screens between phases will be made of a material that does not absorb water under pressure, to preserve their insulating properties in permanent immersion, for example porcelain or a material plastic of the polytetrafluoroethylene type, or other. Finally, it will be noted that the generator according to the invention makes it possible to use non-demineralized softened water, which constitutes yet another important advantage.

Claims (10)

1. Electric steam generator of the kind comprising a closed vessel containing a liquid, a steam outlet conduit, a conduit for liquid supply and/or for condensed steam return, electric resistance heating means comprising bare wires immersed in the liquid, means connecting each of the resistances to an alternating electric current source through the wall of the vessel and insulating screens (13, 14, 33) between which the resistances are located, characterised in that the bare wire (18, 28) of each resistance is vibrantly mounted by being disposed so as to form parallel or substantially concentric lengths spaced on one side or the other of the generatrix of an oblique surface.

2. Steam generator according to claim 1, characterised in that the surface is conical or pyramidal, the wire (18) forming a spiral, the turns of which are spaced on one side or the other of the surface betwen two walls of an enclosure formed by the screens (13, 14, 33).

3. Steam generator according to claim 2, characterised in that the enclosure is formed by two open-ended concentric tubular screens (13, 14) disposed vertically one inside the other, the outer tubular screen having a greater length than that of the interior tubular screen and extending above the surface of the liquid.

4. Steam generator according to claim 1, characterised in that the surface is a plane, the parallel lengths of the wire (28) being spaced on one side and the other of this plane in an enclosure comprising a vertical insulating tubular screen (33) open at its ends and extending above the surface of the liquid.

5. Steam generator according to claim 3, characterised in that the upper end of the interior tubular screen (14) is located below the level of the liquid.

6. Steam generator according to claim 4, characterised in that the insulating tubular screen (33) has an oblong shape in transverse section.

7. Steam generator according to any of claims 1 to 6, characterised in that the bare wire (18, 28) is suspended upon rigid insulating supports by way of elastic means (19, 29).

8. Steam generator according to claim 7, characterised in that the supports are formed by the enclosures (13, 14, 33).

9. Steam generator according to claim 7, characterised in that the supports are formed by mounting members (24) provided inside the enclosures (13, 33).

10. Steam generator according to either of claims 2 or 4, characterised in that each of the vibrantly-mounted bare wires (18, 28) is connected directly between one phase of the source and the material of the vessel, which is itself connected to earth.

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