FR3118903A1 - Electric heating device and method of manufacturing a heating resistor of such a device - Google Patents

Abstract

Electric heating device and method of manufacturing a heating resistor for such a device a liquid circulation chamber, the heating body comprising at least one shielded electrical heating resistor (9) extending in the chamber, said resistor comprising a resistive wire (19) received in a body (21) of generally tubular shape. According to the invention, said body (21) comprises an outer surface (25), on the side opposite the resistive wire (19), having a predefined number of asperities (251). Figure for the abstract: Fig. 4

Description

Electric heating device and method of manufacturing a heating resistor of such a device

The field of the present invention is that of electrical devices for heating and circulating a fluid, in particular a liquid, in particular for a motor vehicle. The invention also relates to a method of manufacturing a shielded electric heating resistor for such a device.

The heating of the air intended for the heat treatment of the passenger compartment of a motor vehicle with a heat engine is ensured by heat exchange between a flow of air and a heat transfer fluid such as a liquid, by means of a heat exchanger. In the case of hybrid or electric vehicles, electric heating devices are known which form a source of calories and in which an electric current circulates to raise the temperature of a heating body extending longitudinally in a casing of this heating device.

The liquid to be heated thus passes through the electric heating device and is brought into contact with the heating body. There is then an exchange of calories between the heating element and the liquid intended to heat the passenger compartment, which then heats up in turn. Traditionally, the chosen liquid is glycol water. The permissible temperature range for a mixture of water and ethylene glycol, in particular at 50%-50%, is generally around -40°C to +130°C.

The heating body usually comprises at least one heating element, for example one or more heating resistors. In order to obtain sufficient heating power for the desired operation, the electric heating resistors can be multiplied in the same electric heating device.

According to a known solution, the electric heating device comprises a casing delimiting at least in part a liquid circulation chamber within the casing. The electric heating device comprises a wall, at least partially delimiting this chamber and carrying the electric heating resistor(s).

A resistance heater is usually made with a smooth cylindrical shape. When the liquid circulates within the chamber, it licks this smooth cylindrical shape. The flow is relatively laminar and the liquid is not or very little subject to disturbances or turbulence, so that the heat transfer is not optimal.

Furthermore, depending on the nature of the liquid to be heated, and in particular if it is a heat-sensitive liquid, the liquid is likely to degrade when it is subjected to excessive temperatures, outside the ranges of usual temperature, for example from -40°C to +110°C. This may be the case if the surface power density of the electric heating resistor is too high and exceeds a certain threshold, in particular beyond 10 W/cm².

It has been observed in particular in the case of glycol water, that depending on the heating power to be delivered, when the electric heating resistance has a power surface density less than or equal to 10W/cm², the mixture is not not degraded. However, if the surface power density of the electric heating resistor is higher, for example if it is greater than 13 W/cm² or 15 W/cm², the mixture can reach at least locally 170° C. or even 190° C. This overheating degrades the glycol, which can generate precipitates, agglomerates which recombine and are no longer mixed with water in a homogeneous manner. These agglomerates can possibly stick together, press against the resistance which can overheat if it can no longer be properly cooled by the water in operation. These precipitates or agglomerates can also obstruct apparatus or devices arranged in the circulation loop of the liquid, for example a circulation pump, heat exchangers or filters.

The object of the invention is to at least partially overcome these problems of the prior art by proposing an electric heating device whose shape and geometry of the electric heating resistance is optimized in order to avoid the degradation of the liquid while improving the heat transfer from the electric heating resistor to the liquid.

To this end, the subject of the invention is a device for electric heating of a liquid such as a mixture of water and ethylene glycol, in particular for a motor vehicle. Said device comprises a heating body and a casing delimiting at least in part a liquid circulation chamber. The heating body comprises at least one heating element extending in the chamber, such as a shielded electric heating resistance comprising a resistive wire received in a body of generally tubular shape. According to the invention, said body comprises an outer surface, on the side opposite the resistive wire, having a predefined number of asperities.

As a result, the outer surface of the generally tubular body of said resistor does not have a smooth circular shape in cross section, unlike the solutions of the prior art. This makes it possible to increase the heat exchange surface, that is to say the peripheral surface of the heating resistor in contact with the liquid to be heated, and therefore to reduce the surface power density of said resistor for a heating power given. Reducing the power surface density avoids the risk of degradation of the liquid, such as glycol water. In addition, this particular non-smooth shape with asperities makes it possible to generate turbulence, the consequence of which is that the heat exchanges between said resistor and the liquid are improved.

The electric heating device may also comprise one or more of the following characteristics described below, taken separately or in combination.

Said resistance is preferably arranged in the chamber so that the external surface of the generally tubular body is in direct contact with the liquid when it circulates in the chamber.

The asperities are preferably made over the entire outer surface of the generally tubular body intended to be arranged in direct contact with the liquid.

In general, the asperities can form protuberances in the chamber.

According to one embodiment, at least some asperities are produced by ridges forming protrusions on the outer surface of said body, the protrusions extending in the direction of the chamber. Such ridges extend along the generally tubular body, and in particular over the entire length of the generally tubular body.

The generally tubular body has for example a section in the general shape of a star.

As an alternative or in addition, at least certain asperities are produced by a groove of generally helical shape around the body of generally tubular shape. This groove or helical rib gives movement to the liquid while increasing the exchange surface.

The resistive wire may be arranged at the center, or substantially at the center, of the generally tubular body, and an electrical insulating material may be disposed between the resistive wire and an internal surface of the generally tubular body.

• au moins une étape d’extrusion de manière à obtenir un corps de forme générale tubulaire présentant un nombre prédéfini d’aspérités sur sa surface externe, et
• une étape d’insertion d’un fil résistif dans le corps de forme générale tubulaire.

The invention also relates to a method of manufacturing a shielded electric heating resistor of an electric heating device as defined above. Said method comprises:

• at least one extrusion step so as to obtain a body of generally tubular shape having a predefined number of asperities on its outer surface, and
• a step of inserting a resistive wire into the generally tubular body.

The generally tubular body can be made from an extruded wall of constant thickness. The wall of constant thickness can be shaped so as to achieve the predefined number of asperities.

The predefined number of asperities is advantageously produced by said extrusion. Such an extrusion makes it possible to obtain in a simple way the particular shape of the body of generally tubular shape with the asperities on the external surface making it possible both to reduce the surface density of power of said resistance to avoid degrading the liquid, and to increase turbulence to improve heat transfer from the resistive wire to the liquid.

The particular shape of the generally tubular body is preferably made before inserting the resistive wire inside said body. Subsequently, the assembly forming the electrical resistor can be shaped so as to define at least in part a helical shape.

The invention also relates to an electric heating resistor thus obtained, as well as a heating body comprising such a resistor.

The invention also relates to an installation for heating and/or ventilation and/or air conditioning of an air flow comprising at least one electric heating device as defined previously.

Other advantages and characteristics of the invention will appear more clearly on reading the following description given by way of illustrative and non-limiting example, and the appended drawings, among which:

shows an electric heater.

is a cross-sectional view of the electric heater of the .

is a representation of a heating body of the electric heating device comprising an electric heating resistor according to a first embodiment.

shows an electric heating resistor according to a second embodiment.

In these figures, identical elements bear the same reference numbers.

The following achievements are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference is to the same embodiment, or that the features apply only to a single embodiment. Simple features of different embodiments can also be combined or interchanged to provide other embodiments.

With reference to the , the invention relates to an electric heating device 1, in particular for a motor vehicle such as an electric or hybrid vehicle. This electric heating device 1 is intended in particular to cooperate with an installation for heating and/or ventilation and/or air conditioning of an air flow for such a motor vehicle. This is an electric heating device 1 for heating a fluid, in particular a liquid such as a mixture of water and ethylene glycol generally called glycol water.

The electric heating device 1 comprises a casing 3. The casing 3 can have a generally cylindrical or conical shape or else a generally parallelepipedic shape.

The box 3 extends for example longitudinally along an axis of extension of the electric heating device 1, called the longitudinal axis A.

According to the illustrated embodiment, the electric heating device 1 comprises a partition 5 or closing wall, on which the casing 3 is fixed. The partition 5 is arranged at the level of a longitudinal end of the casing 3.

Also referring to the , the electric heating device 1 comprises at least one heating body 7, for example two heating bodies 7 in the example shown.

Each heating body 7 comprises one or more heating elements. These are in particular electric heating resistors 9, exemplary embodiments of which are detailed below.

The or each heating body 7 also comprises a base 11. One or more electrical heating resistors 9 can be fixed in a sealed manner to this base 11.

A seal can generally be placed between the base 11 and the internal wall of the box 3 to ensure the seal between the base 11 and the box 3.

The base 11 extends transversely with respect to the longitudinal axis A. It may be an internal wall extending inside the box 3. This internal wall may or may not be defined by the box 3.

The base 11 and at least one electric heating resistance 9 therefore form a heating body 7. More precisely in the example illustrated, each heating body 7 comprises a single electric heating resistance 9 fixed on a base 11. Other configurations can be considered. Each heating body 7 could comprise more than one electric heating resistance 9, for example two or even more than two electric heating resistances. Different arrangements of the electric heating resistors 9 between them can be provided, for example by arranging them concentrically and/or by aligning them axially. Such arrangements are not described in further detail.

The heating element(s) 7 are respectively received in an associated housing 13 defined in the housing 3. The housing 13 defines a shape complementary to that of the associated heating element 7. In the example illustrated, the housing 13 of a heating body 7 defines a generally conical shape. Other shapes can be envisaged, for example cylindrical, parallelepipedal.

The housing or housings 13 can respectively be closed in a sealed manner by the base 11 of the associated heating body 7.

The electric heating device 1 further comprises at least one liquid circulation chamber 15 delimited at least in part by the housing 3. In the example illustrated the electric heating device 1 comprises two liquid circulation chambers 15 . Each chamber 15 is delimited inside the box 3, and more precisely in a housing 13 closed by a base 11.

The or each chamber 15 may extend parallel to the longitudinal axis A. The chamber 15 is defined with a generally cylindrical or conical shape. Other alternatives can be considered, for example with a parallelepiped shape.

The electric heating resistors 9 extend respectively longitudinally in an associated chamber 15. In operation, the liquid to be heated can circulate within a chamber 15 and rise in temperature in contact with the electric heating resistor 9, by heat exchange. The heated liquid can then be evacuated from the electric heating device 1, so as to be directed to other components of the motor vehicle.

The electrical heating resistors 9 are provided with electrical terminals 91. The electrical terminals 91 extend for example in a direction substantially parallel to the longitudinal axis A. The electrical terminals 91 extend from the side of the base 11 opposite to the chamber 15, so as to be able to be connected to a power supply, for example via an electric or electronic circuit.

The bases 11 make it possible to create a separation between the chambers 15 and a cavity 17 of the electric heating device 1, for example of the box 3, within which the electric circuit or at least one element for connecting the electric terminals 91 can be mounted. to the electrical or electronic circuit for example.

In general, the electric heating resistors 9 can be at least partly wound, so as to define a general hollow shape, such as a cylindrical shape and/or at least partly helical. In this case, the or each heating body 7 is advantageously arranged in the housing 13, so that, in a plane orthogonal to the longitudinal axis A, all the points on a wound portion of an electric heating resistor 9 are equidistant , or substantially equidistant from an internal surface of the housing 3 defining the housing 13.

The electrical heating resistors 9 are shielded. Referring to Figures 2 to 4, they respectively comprise a resistive wire 19 received in a body 21 of generally tubular shape, designated hereinafter tubular body 21, of central axis B. More specifically, the resistive wire 19 is arranged in the center of the tubular body 21. The diagrams in figures 3 and 4 are not to scale but are simplified to facilitate their understanding.

The resistive wire 19 is electrically conductive and is intended to dissipate heat by Joule effect. The resistive wire 19 may have a diameter less than or equal to 1mm.

The tubular body 21 forms a shielded sheath around the resistive wire 19. It comprises an internal surface 23 and an external surface 25 opposite each other. The internal surface 23 is arranged on the side of the resistive wire 19 and the external surface 25 is arranged on the side opposite the resistive wire. This outer surface 25 is arranged to come into contact with the liquid to be heated.

The tubular body 21 extends longitudinally along the axis B before the shaping of the electric heating resistor 9, for example by rolling it up to define the helical shape shown in FIGS. 2 and 3. The tubular body 21 may have a maximum outer diameter of the order of 4mm to 6mm.

The tubular body 21 is for example made from a wall of constant thickness. This is in particular a metal wall.

When the electric heating resistor 9 is arranged in an associated chamber 15, the outer surface 25 of the tubular body 21 is intended to be in direct contact with the liquid when it circulates in the chamber 15.

The intermediate space between the resistive wire 19 and the internal surface 23 of the tubular body 21 is filled with an electrically insulating material 27. This electrically insulating material 27 is thermally conductive. It transfers the heat from the resistive wire 19 to the outer surface of the tubular body 21 . It may be a resin. Alternatively, the electrical insulating material 27 can be produced in powder form or even in granular form. Advantageously, the electrically insulating material 27 is compressed in the tubular body 21 .

The electrical heating resistor 9 may comprise, at its terminal sections for the electrical connection of the resistive wire 19, an electrically insulating sleeve 29 arranged at the longitudinal end of the tubular body 21 and traversed by an electrical terminal 91 ( ). This sleeve 29 can be received inside the base 11 carrying the electric heating resistor 9.

Furthermore, the outer surface 25 of the tubular body 21 has a predefined number of asperities 251 ( ), 253 ( ). The asperities 251, 253 are made over the entire outer surface 25. The asperities 251, 253 are made over the entire length of the tubular body 21. They can be shaped around the entire circumference of the tubular body 21 . The asperities 251, 253 form protuberances in the chamber 15.

A first embodiment is shown in the . According to this first embodiment, at least some asperities are produced by edges 251 forming protrusions on the outer surface 25 of the body 21. When the heating body 7 is arranged in the associated liquid circulation chamber, the edges 251 s extend towards this chamber.

The edges 251 extend along the tubular body 21. The edges 251 are for example longitudinal before the shaping of the electric heating resistor 9, for example by rolling it up to define the helical shape represented in FIGS. 2 and 3. According to the example illustrated, these edges are straight before the shaping of the electric heating resistor 9, for example by winding it to define the helical shape represented in FIGS. 2 and 3. Once the electric heating resistor has been shaped, such edges 251 extend according to the given shape to the tubular body 21. These ridges 251 are preferably of the same length as the tubular body 21 . They are regularly distributed around the tubular body 21 .

The outer surface 25 of the tubular body 21 has an alternation of hollows 252 and such edges 251 forming sawtooth ridges. The tubular body 21 then has a section in the general shape of a star. This is the cross section with respect to the central axis B of the tubular body 21 forming a longitudinal axis before the shaping of the electric heating resistor 9, for example in a helical shape. The vertices of the star shape are made by the edges 251. This section can be constant along the tubular body 21.

With such ridges 251, the developed surface of the outer surface 25 of the tubular body 21 is greater than with respect to a cylinder with a smooth outer surface without asperities.

There shows a second embodiment of the electric heating resistor 9. According to this second embodiment, at least some asperities are produced by a groove 253.

The spline 253 is in this example made with a generally helical shape around the tubular body 21 . This helical groove 253 (or rib) fits, meanders, around the tubular body 21 by winding a predetermined number of times along the tubular body 21.

The pitch of this 253 helical spline, i.e. the distance between two crests, can be adjusted according to the heat exchange coefficient and the tolerated pressure drop.

Such an external helical groove 253 along the tubular body 21 makes it possible to generate a helix movement of the liquid such as glycol water when it circulates in the chamber receiving the electric heating resistor 9. Such a movement makes it possible to increase all the more turbulence compared to the first embodiment, which makes it possible to accelerate the flow rate while increasing the heat exchange surface.

The asperities 251 and 253 according to the first embodiment and according to the second embodiment can be combined to form a new embodiment.

Other variants of asperities, not shown, can be made on the outer surface 25 of the tubular body 21 receiving the resistive wire, for example in the form of barbs extending towards the chamber 15, or even ridges, or grooves or ribs according to other configurations than the previous embodiments.

According to an advantageous embodiment, the tubular body 21 can be extruded and the asperities 251, 253 can be shaped by extrusion.

With reference to FIGS. 3 and 4, the process for manufacturing a shielded electric heating resistor 9 as described above is described below in more detail.

The tubular body 21 can be made from a wall of constant thickness. This wall of constant thickness is shaped into a straight cylindrical liner and is shaped so as to produce the defined asperities on the outer side of the cylindrical liner. These may be the ridges 251 conferring a star-shaped section, or the external helical groove 253 or even a combination of the two, and/or any other asperities.

To do this, the method may comprise at least one extrusion step making it possible to obtain the tubular body 21 having the predefined number of asperities 251, 253 on its outer surface 25. The extrusion makes it possible to shape the wall of constant thickness to achieve the asperities 251, 253 defined. At this stage, that is to say before the shaping of the electric heating resistor 9, the edges 251 can extend along the central axis B of the tubular body 21 which is then a longitudinal axis, or the groove 253 can form a helix around this central longitudinal axis B.

A possible step may follow to stretch the tubular body 21 in a die so as to reduce the diameter.

The method includes a step of inserting a resistive wire 19 into the tubular body 21. This insertion of the resistive wire 19 inside the body is carried out after the particular shaping of the tubular body 21 with the asperities 251, 253, for example by extrusion.

The electrically insulating material 27 and thermal conductor can be introduced within the tubular body 21 around the resistive wire 19. Then, the assembly can be compacted so as to guarantee electrical insulation. An electrically insulating sleeve may possibly be fixed at the end of the tubular body 21 and be crossed by an electrical terminal 91 connected to the resistive wire 19.

A shielded electric heating resistor 9 is thus obtained. It can be shaped to at least partially define a helical shape as in the example shown. Two or more shielded electric heating resistors 9 obtained according to this process can also be arranged by winding resistors around each other, and/or by aligning resistors axially.

The shielded electrical heating resistor(s) 9 obtained and possibly shaped to define a helical shape, can be fixed by any appropriate means to a base 11 so as to form a heating body 7.

Thus, for a fixed heating power, the asperities 251, 253 made on the outer surface 25 of the tubular body 21 provide a double effect.

On the one hand, this makes it possible to guarantee the durability and to preserve the physicochemical properties of the liquid such as glycol water by increasing the peripheral surface of heat exchange having the consequence of reducing the surface density of power and therefore reduce the risk of glycol degradation.

On the other hand, this particular shaping makes it possible to create zones of turbulence and thus to increase the heat exchange coefficient, which promotes heat transfer from the resistive wire 19 to the glycol water.

Claims (10)

Device (1) for electric heating of a liquid, in particular for a motor vehicle, comprising a heating body (7) and a box (3) delimiting at least in part a chamber (15) for circulating the liquid, the heating body (7) comprising at least one shielded electric heating resistor (9) extending in the chamber (15), said resistor comprising a resistive wire (19) received in a body (21) of generally tubular shape, characterized in that said body (21) comprises an outer surface (25), on the side opposite the resistive wire (19), having a predefined number of asperities (251, 253). Device (1) according to the preceding claim, wherein said resistor (9) is arranged in the chamber (15) so that the outer surface (25) of the body (21) of generally tubular shape is in direct contact with the liquid when he circulates in the room (15). Device (1) according to the preceding claim, in which the asperities (251, 253) are made over the entire outer surface (25) of the body of generally tubular shape intended to be arranged in direct contact with the liquid. Device (1) according to one of the preceding claims, in which at least certain asperities are produced by ridges (251) forming protuberances on the external surface (25) of said body (21), the protuberances extending in the direction of the bedroom (15). Device (1) according to the preceding claim, in which the edges (251) extend along the body (21) of generally tubular shape. Device (1) according to one of Claims 4 or 5, in which the body (21) of generally tubular shape has a section in the general shape of a star. Device (1) according to one of Claims 4 to 6, in which at least certain asperities are produced by a groove (253) of generally helical shape around the body (21) of generally tubular shape. Device (1) according to one of the preceding claims, in which the resistive wire (19) is arranged in the center of the body (21) of generally tubular shape, and in which an electrically insulating material (27) is placed between the resistive wire (19) and an internal surface (23) of the body (21) of generally tubular shape. Method of manufacturing a shielded electric heating resistor (9) of an electric heating device (1) according to one of the preceding claims, said method comprising:

• at least one extrusion step so as to obtain a body (21) of generally tubular shape having a predefined number of asperities (251, 253) on its outer surface (25), and
• a step of inserting a resistive wire (19) into the body (21) of generally tubular shape.

Method according to the preceding claim, in which the body (21) of generally tubular shape is made from a wall of constant thickness shaped so as to produce the predefined number of asperities (251, 253).