EP0082092B1 - Process and device for heating a dielectric or nearly dielectric liquid, and use of this device for heating, in particular of a calorific fluid - Google Patents

Abstract

1. Process for heating a dielectric liquid consisting in immersing at least one bare resistor, to the terminals of which a potential difference is applied, directly in the said liquid contained in an enclosure, characterized in that for heating a liquid with a resistivity higher than 10**8 OMEGA /cm the liquid is conducted along plate-shaped resistors so that the velocity vectors of the general motion of the liquid under natural or forced flow, in the vicinity of the resistor or resistors are tangent or parallel to the faces of the said plates.

Description

The present invention relates to a method of heating a dielectric or substantially dielectric product using electrical energy. It also relates to a heating appliance implementing said method, and the use of such an appliance.

In the present text the term "product" will mean in the chemical sense of the term both a single chemical and a mixture of two or more chemicals.

The "product" as defined above can be present before heating both in the liquid state and in the solid state, a product in the solid state being able to pass into the liquid state under the effect of heat.

The "product" can of course consist of a single phase or of several phases, it can for example be in the form of a gas / solid, liquid / liquid or even solid / liquid mixture.

The method and apparatus, objects of the present invention, are more particularly intended for the heating of heat transfer fluids.

German Patent No. 2,400,470 describes an instantaneous electric water heater comprising, inside an insulating block, a water circulation channel in which three resistors are placed directly; a sophisticated tension regulation system is provided to avoid scaling. Such a device cannot be used in the presence of a dielectric liquid because it would burn by fusion of the electrical resistances.

French Patent No. 1,600,146 relates to a hot air heating installation using an expanded metal resistance; a liquid can be injected downstream of the resistance in order to humidify the air.

The techniques for heating products using electrical energy by means of the Joule effect, currently used industrially, are identical whether the product to be heated is dielectric or conductive. Thus, for example, liquids are usually heated by means of a heating device comprising at least one electrical resistance located in a metal sheath, an electrical insulation means such as air, magnesia, boron nitride, etc. being provided between the resistance and the inner surface of the sheath, the device being immersed in the sen of the liquid to be heated.

Most often, in order to be able to have sufficient power, the heating device comprises a bundle of sheathed resistances in the shape of a U fixed by their free arms to a plate through which the electrical connections of each U-shaped resistance are ensured. The plate also makes it possible to fix the heating device to a flange surrounding an opening of the enclosure containing the liquid to be heated, this opening also makes it possible to introduce the bundle inside the enclosure.

Although this method of heating is satisfactory, it has numerous drawbacks.

Indeed, as electrical insulation means are necessarily placed between the resistor and the metal sheath, these means also being, generally, thermal insulators, there appears to be a significant difference in temperature between the resistor itself and the surface of the sheath. participating in thermal exchanges with the liquid to be heated.

In addition, the geometry of the resistance bundle is the cause of significant pressure drops, areas of liquid stagnation and therefore overheating and risks of degradation thereof. In addition, this geometry also promotes fouling of the beam.

By their design, the beams are heavy and bulky and moreover, each U-shaped resistor implies being electrically connected at each of its ends. This is a major drawback of this heating method, since there are practically no beams on the market with an available unit power greater than 200 kW, which means providing the enclosure containing the liquid to be heated with a large number of beams to have the necessary power. Thus, when large volumes or flow rates of liquid are to be heated, at temperatures of the order of 300 ° C. for example, in chemical installations, this mode of heating involves bulky and expensive material.

To overcome these drawbacks for the heating of dielectric or practically dielectric products using electrical energy, a heating process has now been developed and an apparatus is implemented implementing said process.

We have now found a method of heating a liquid, dielectric consisting in immersing directly in said liquid contained in an enclosure at least one naked resistance at the terminals of which a difference - of potential is applied, characterized in that for heating of a liquid with a resistivity greater than 10'n / cm the liquid is led along resistors in the form of plates in such a way that the velocity vectors of the general movement of the liquid in natural or forced flow, in the vicinity of the or resistors are tangent or parallel to the faces of said plates.

By "directly" is meant that the resistance is bare, that is to say that between the resistance and the product there are no substantial means of electrical insulation, this does not exclude that in certain cases , and for some applications, the resistance includes a thin coating.

The process, object of the invention, is such that a potential difference greater than 380 V can be applied across the resistance (s).

It is possible in particular to apply across the resistance limits potential differences of up to 10,000 V, potential differences of the order of 5,500 V being commonly available in the factory. Of course, in this case, during the implementation of the method, the skilled person complies will follow the rules in force and the specific specifications for the design and use of high-voltage installations.

In the present text, "resistance" means a conductor in which all the electrical energy received is transformed into heat by the Joule effect.

A device has also been found for heating a dielectric liquid with a resistivity greater than 10 ⁸ 52 / cm which implements the heating method, object of the invention.

Such a heating device is characterized in that it comprises an enclosure containing said liquid in which is immersed, at least one naked resistance in the form of a plate, said one or said resistors in the form of a plate being placed in such a way that the speed vectors of the general movement of the liquid in natural or forced flow, in the vicinity of the resistor (s) being tangent or parallel to the faces of said plates, and means for connecting the resistor (s) to an electrical distribution network.

Advantageously, the heating device, object of the invention, comprises several resistors electrically connected in series and / or in parallel.

According to one embodiment, the device according to the invention can comprise at least three resistors electrically connected in a triangle or in a star. This embodiment makes it possible to connect the resistors to a three-phase electrical distribution network.

The term "resistance" in the present text designates both a single resistance and a resistance formed by several elementary resistances, identical or not, connected in series and / or in parallel.

Those skilled in the art will determine the electrical connection diagram of the resistors, according to the heating power required, the equipment available, the construction requirements ...

The apparatus for heating a dielectric or substantially dielectric product, object of the invention, can be such that the enclosure containing the product is swept by the product to be heated.

The enclosure of the appliance can be a section of pipe swept by the product to be heated. Such an apparatus is more particularly intended for maintaining the temperature of a product in motion by compensating for the thermal losses which have occurred, for example during the transport of the hot product over long distances.

The enclosure, swept by the product to be heated, of such an apparatus can also be for example the reaction zone of a chemical reactor, the heat necessary for the reaction being directly supplied to the reaction mixture by one or more resistors immersed in this one. Thus the enclosure swept by the product can be the reaction zone of a fluidized bed reactor, the resistors being immersed directly in the gas / solid mixture.

By "enclosure swept by the product" is meant above that the flow rate of the product by current the enclosure is relatively large compared to the volume of product present in the enclosure.

On the contrary, if after when the enclosure is a "container comprising inlet means and outlet means" it is understood that the latter contains a certain volume of product and that the flow of product passing through the enclosure is low. -to the product present therein.

According to another embodiment, the enclosure of a heating device according to the invention can be constituted by a container comprising means for entering the product to be heated and means for leaving the hot product. Such an apparatus may include a container of sufficient volume to constitute a storage of hot product.

The boilers are heating devices according to this embodiment. Such heaters are more particularly intended for heating organic, natural or synthetic heat transfer fluids, used in industrial installations. It is also possible to use as heat transfer fluid products or mixtures of substantially dielectric molten mineral products.

According to another embodiment, the enclosure can be constituted by a closed container which comprises means for filling and draining the product, at least one resistance being immersed in the product. Such an apparatus makes it possible to receive heat, to store it and to restore it.

According to yet another embodiment, the enclosure of a heating device object of the invention can be constituted by a container comprising means for entering the product to be heated and means for leaving the vapor of said product. Preferably an apparatus according to this embodiment is intended for heating a liquid, the liquid being heated to its boiling point in the container.

The boilers are heating devices according to this other embodiment.

The general movement of the product is the movement of the product corresponding to the movement thereof, without taking into account local movements, such as vortices, in the vicinity of the faces of the plates.

The purpose of this arrangement is to reduce as much as possible the pressure losses due to the resistances and thus to promote thermal exchanges with the product while maintaining high scanning speeds of the faces of the resistances by the product.

Although the term "plate" more precisely designates a sheet of a rigid, flat and not very thick material, the meaning of this term will be extended in the present text to thin sheets which are not planar, for example cylindrical or left.

Resistors in the form of left plates will be placed in such a way that the velocity vectors of the general movement of the product, in natural or forced flow, in the vicinity of the resistances, are tangent to the faces of said plates.

Resistors in the form of flat plates will be placed in such a way that the velocity vectors of the general movement of the product in natural or forced flow, in the vicinity of said surfaces, are parallel to the faces of the plates.

In heating devices according to the invention, having a natural or forced flow of the product, of general direction parallel to a given direction, it is possible to use resistors in the form of a substantially flat plate placed parallel to the general direction of movement of the product.

Resistors in the form of an elongated rectangular flat plate can be folded, for example in a zigzag, according to their length and placed in such a way that their width is parallel to the general direction of movement of the product.

It is also possible to use plate-shaped resistors of generally cylindrical shape, the cylinder being generated by a generator moving parallel to the general direction of movement of the product by resting on a director, open or closed, of curved shape, by circular example, or polygonal, several cylindrical resistors can be placed concentrically.

The resistors can be in the form of a plate of continuous structure or of discontinuous structure.

By “continuous structure” of plate-shaped resistors is meant that the structure of the resistors does not have a hole, thus resistors of continuous structure are similar to sheets.

By “discontinuous structure” of resistors in the form of a plate is meant that the structure of the resistors has holes, thus resistors of discontinuous structure can be made of a fabric obtained by weaving, knitting, braiding or twisting. They can also consist of a perforated sheet obtained by cutting with removal of material, for example by punching.

Preferably plate-shaped resistors of discontinuous structure are obtained by cutting a plurality of slots in a sheet and stretching in a direction perpendicular to the slots.

For the discontinuous structure resistors formed of a fabric or obtained by cutting and stretching, it is considered that the face of the plates is the surface which envelops the reliefs of the plates.

The resistors are made of materials, resistant to corrosion. Thus, the resistors can be made of stainless steels of nuances usually used for this type of material, in refractory stainless steels, special steels can also be used after they have undergone an anticorrosion treatment.

The surfaces of the resistors can be of substantially smooth appearance, preferably they are of rough appearance. This rough appearance can be obtained for example by sanding.

Resistors whose surfaces are rough in appearance are more particularly intended for the production of heating devices objects of the invention which comprise means for entering a liquid to be heated and means for leaving the vapor of said liquid . In fact the roughness of the resistors promotes the nucleated boiling of the liquid.

Optionally, the surfaces of the resistors may be provided with a coating, this coating may be porous, in order to promote nucleated boiling, it may also be a thin anti-corrosion coating, for example made of silicone enamel or thermostable.

The apparatus for heating a dielectric or practically dielectric product, according to the invention, can be used in particular for heating chemicals with little or no conductivity of electricity, used in industrial processes.

The invention will be described below with particular reference to a device for heating a liquid product.

This heater is particularly intended for heating heat transfer fluids used in chemical installations.

• La figure 1 est u'ne vue en coupe par un plan axial d'un mode de réalisation d'un appareil de chauffage selon l'invention.
• La figure 2 est une vue partielle d'un mode de réalisation d'une résistance de structure discontinue.
• La figure 3 est une vue partielle en coupe par le plan III-III de la résistance selon la figure 2.

The understanding of the invention will be facilitated by the attached figures, which illustrate by way of example, schematically and without a determined scale, an embodiment of a heating appliance according to the present invention and an embodiment of the resistance structure.

• Figure 1 is a sectional view through an axial plane of an embodiment of a heating apparatus according to the invention.
• Figure 2 is a partial view of an embodiment of a discontinuous structural resistance.
• FIG. 3 is a partial view in section through the plane III-III of the resistor according to FIG. 2.

The heater (1), object of the invention, shown schematically in Figure 1, is more particularly a heater for heat transfer fluid used in an industrial installation.

This heating device (1) comprises an enclosure (2) containing the heat transfer fluid (3), in the heat transfer fluid (3) are immersed six resistors (4) and means (5) for connecting the resistors (4) to a electrical distribution network (6).

The enclosure (2) is, according to the present embodiment, a container formed by a body (7) substantially cylindrical, closed at its ends by a cover (8) and a bottom (9).

The enclosure (2) comprises at its lower part inlet means (10) of the heat transfer fluid to be heated and in the vicinity of its upper part outlet means (11) of hot heat transfer fluid, these means consist of pipes .

The inlet pipe (10) is preferably provided with a three-way valve (12) allowing the emptying of the heating appliance (1

Resistors (4), according to the embodiment shown in Figure 1, are in the form of substantially rectangular flat plates, their faces are placed parallel to the general direction of movement of the fluid in the heater, upward movement represented by the arrow F. They are held in place using insulating supports (14) fixed to the body (7) of the enclosure (2).

The six resistors (4) are connected to each other in series by conductive bars (15), the two extreme resistors include the means (5) for connecting the assembly to the electrical distribution network (6). The connections between the resistors (4) and the electrical network (6) pass through the bottom (9) of the enclosure (2) through an opening (16) provided with means of electrical insulation (17) and sealing.

The resistors (4) and their mode of electrical connection have only been described above by way of example and it is not going beyond the ambit of the invention to replace at least one resistance (4) with several elementary resistances and / or by establishing the electrical connections between them differently.

Advantageously, the enclosure (2) is not completely filled with heat transfer fluid (3), in the free space (18) between the surface of the heat transfer fluid (3) and the cover (8) of the enclosure, a nitrogen pressure, the cover (8) being provided with a tube (19) for this purpose.

The connections between the resistors (4) and the electrical network (6) can also be carried out through an opening of the cover (8), in the area of the cover (8) not bathed by the heat transfer fluid.

The inlet pipes (10) of the heat transfer fluid to be heated and the outlet (11) of hot heat transfer fluid can be connected directly to the circuit for implementing the hot fluid, that is to say to the circuit for heating the industrial installation.

The inlet pipes (10) of the heat transfer fluid to be heated and of the outlet (11) of the hot heat transfer fluid can be connected to a heat exchanger in which another heat transfer fluid is heated, this being used in the circuit for heating the industrial installation. Such an embodiment makes it possible to better safeguard the cleanliness, and consequently, the dielectric properties of the heat transfer fluid heated in the heating apparatus object of the present invention.

The resistors (4) can be produced in the form of substantially flat plates of discontinuous structure as shown in partial views in FIGS. 2 and 3.

The plate-shaped resistor of discontinuous structure, a fragment of which is represented in FIG. 2, comprises holes (19) substantially in the form of a diamond obtained by cutting from a sheet of metal a plurality of slots aligned along their length and placed in staggered rows, then stretching in a direction perpendicular to the slits, that is to say parallel to the small diagonals of the diamonds. This stretching causes a rotation of the ribbons (20) separating the diamonds (cf. FIG. 3). According to this embodiment of the resistors, it is considered that the faces of the plates are the surfaces, materialized by the two broken lines (21, 22), which envelop all the reliefs of the plates.

FIG. 3 shows the speed vectors V of the general movement of the flowing heat transfer fluid, the speed vectors V are here parallel to the faces of the plate, this general movement does not take account of local movements, such as the vortices T, created by the reliefs of the discontinuous structure plate. These T vortices favor the transfer of heat between the resistance and the heat transfer fluid.

The method and the apparatus, objects of the invention, have numerous advantages over the apparatuses according to the prior art.

Indeed, the electrical resistance (s) being immersed directly in the dielectric or practically dielectric product allow better heat exchange because, with the heating method according to the invention, the surface which participates in the heat exchange with the product to be heated is the very surface through which the Joule effect born in resistance dissipates. Thus, the entire surface of the resistor participates in the exchange with the product and the temperature difference between the resistor and the product is small, which limits the risks of thermal degradation of the product which is directly in contact with the resistor.

The method and the heating device according to the invention also have the advantage of allowing the construction of compact heating devices, in particular boilers. In fact, according to the invention, it is possible to have, for boilers, an electrical power of at least 1 kW per liter of space in the boiler exchange zone.

The reduction in the number and size of the electrical connections, compared to heating devices comprising a bundle of electrical resistors provided with sheaths, also promotes the compactness of the heating devices according to the invention.

Another advantage of the heating appliances which are the subject of the invention is that, by the shape of the resistors and by their positioning in such a way that the velocity vectors of the general movement of the product in the vicinity of the resistors are tangent or parallel to the faces of the plates, the pressure drops of the product in the heater are relatively low. These relatively low pressure drops for an appliance intended for heating a liquid product often allow thermosyphon operation of the appliance or, failing this, assisted circulation of the liquid, without requiring the use of powerful pumping means. In addition, when such a heater is used as a boiler, it allows a free passage section for large bubbles, unlike the boilers according to the prior art.

In addition, the heaters according to the invention are easy to build and maintain, the sealing problems at the resistance sheaths being very limited.

The advantages of the method and of the heating apparatus according to the invention are particularly advantageous to use for the heating of dielectric liquids such as heat transfer fluids.

The examples below demonstrate the advantages of the method and of the heating apparatus implementing the method, objects of the present invention, during various uses of the apparatus.

Example 1

One has produced a substantially parallelepipedal boiler. It consists of a container surmounted by an air exchanger. The container is made of partially insulated sheet metal to ensure, taking into account the regulation system, almost permanent heating operation. It has three resistors, connected in series, in the form of an elongated rectangular plate, zigzag folded along their length, and located at different levels parallel to the bottom of the boiler.

The resistors are plates of discontinuous structure as shown in Figure 2, they dissipate a power of 2 kW.

The boiler is equipped with temperature measurement probes placed above and below the resistors.

The boiler contains 30 liters of heat transfer fluid consisting of partially hydrogenated polyphenyls, marketed under the name of GILOTHERM TH by the company RHONE-POULENC SPECIALITES CHIMIQUES.

• 1 000 heures à 300°C
• 500 heures à 340°C
• 640 heures à 350°C.

The tests were carried out by heating the GILOTHERM TH successively:

• 1000 hours at 300 ° C
• 500 hours at 340 ° C
• 640 hours at 350 ° C.

At 300 ° C the boiler operated in thermosyphon, in natural convection.

At 350 ° C, the gradual formation of light products was noted and the convection became more energetic: the boiler operated as a thermosyphon boiler. The average exchange coefficient was 700 W / m2 ° C and the temperature difference between the surface of the resistor and the GILOTHERM TH in boiling range of around 24 ° C.

Between each test sequence, the resistances were removed and observed: they show no trace of fouling.

Analyzes and measurements of the properties of the heat transfer fluid used in the boiler show that it has not undergone any degradation.

Example II

In the same boiler, using a resistance of 2 kW dissipating 40 kW / m ² , was heated for 288 hours at 255 ° C, a heat transfer fluid consisting of 26.5% by weight of diphenyl and 73.5% by weight of phenyl oxide, marketed under the name of GILOTHERM DO by the company RHONE-POULENC SPECIALITES CHIMIQUES. The heat transfer coefficient was 2000 kW / m ² ° C, the temperature difference between the resistance and the boiling fluid was 20 ° C.

After this test the resistance was dismantled and examined, no trace of fouling was observed.

Example III

A parallelepipedal boiler was produced having a resistance capable of dissipating 250 W. A mixture of terphenyls consisting of 12% orthoterphenyl, 60% metaterphenyl, 28% paraterphenyl, was introduced into the boiler container. final melting point of 150 ° C., marketed under the name of OMP terphenyls by the company RHONE-POULENC SPECIALITES CHEMIQUES.

After solidification and cooling to room temperature, the mixture of OMP terphenyls was heated several times to 200 ° C.

The melting of the mixture of terphenyls was carried out without difficulty, the resistance was not fouled.

Example IV

In the same device, a mixture of viscous polyisobutenes with an average molar mass of around 900 was easily warmed from ambient temperature to 240 ° C., sold by the company NAPHTACHIMIE under the name of NAP-VtS 10.

Claims (13)

1. Process for heating a dielectric liquid consisting in immersing at least one bare resistor, to the terminals of which a potential difference is applied, directly in the said liquid contained in an enclosure, characterized in that for heating a liquid with a resistivity higher than 10° Ωlcm the liquid is conducted along plate-shaped resistors so that the velocity vectors of the general motion of the liquid under natural or forced flow, in the vicinity of the resistor or resistors are tangent or parallel to the faces of the said plates.

2. Process according to Claim 1, characterized in that a potential difference higher than 380 V is applied to the terminals of the resistor or resistors.

3. Apparatus for heating a dielectric liquid with a resistivity higher than 10⁸ 0/cm, with the aid of electrical energy, characterized in that it makes use of the heating process according to either of Claims 1 and 2.

4. Apparatus according to Claim 3, characterized in that it comprises an enclosure (2) containing the said liquid (3) in which at least one bare plate-shaped resistor (4) is immersed, the said plate-shaped resistor or resistors being placed so that the velocity vectors of the general motion of the liquid under natural or forced flow, in the vicinity of the resistor or resistors are tangent or parallel to the faces of the said plates, and means (5) for connecting the said resistor or resistors to an electrical supply system (6).

5. Apparatus according to Claim 4, characterized in that it comprises several resistors (4) connected electrically (15) in series and/or in parallel.

6. Apparatus according to Claim 4, characterized in that it comprises at least three resistors connected electrically in a triangle or star-fashion.

7. Apparatus according to any of Claims 4 to 6, chracterized in that the said enclosure is a length of conduit swept by the liquid to be heated.

8. Apparatus according to any one of Claims 4 to 6, characterized in that the said enclosure (2) consists of a receptacle (7, 8, 9) comprising means for the entry (10) of the liquid to be heated and means for the exit (11) of the hot liquid.

9. Apparatus according to any one of Claims 4 to 6, characterized in that the said enclosure consists of a receptacle comprising means for the entry of the liquid to be heated and means for the exit of the vapour of the said liquid.

10. Apparatus according to any one of Claims 4 to 9, characterized in that the resistor or resistors are in the shape of a substantially plane plate or of a substantially cylindrical plate.

11. Apparatus according to any one of Claims 4 to 10, characterized in that the resistor or resistors, plate-shaped, are continuous in structure.

12. Apparatus according to any one of Claims 4 to 10, characterized in that the resistor or resistors, plate-shaped, are noncontinuous in structure.

Images