FR2632660A1 - METHOD FOR FIXING A METAL TUBE ON A METAL SURFACE AND DEVICE THUS OBTAINED - Google Patents

Abstract

The method makes it possible to fix a metal tube 1 on a metal surface 2, the contact between this tube and this surface being intended to effect heat transfer between them at high temperature and with a high temperature gradient. The tube 1 is applied to the surface 2 so that the width of direct contact between the wall 1a of the tube and the surface 2 is at least equal to a quarter of the outer perimeter of the tube 1, that the dihedral AOB formed by the plane of the wall 1c of the tube where it deviates from direct contact and the plane of surface 2 is at least equal to 90degre (s), and the filler metal 3 is projected onto tube 1 and surface 2 so that the projected metal input is mainly located in this dihedral AOB, does not completely cover the tube 1, only covers the surface 2 over a zone of width limited to substantially the width of the direct contact between the tube and the surface , and be relatively thin. Use in particular for making an electric household appliance.

Description

The present invention relates to a method for fixing by "schooping"

  (That is to say by spraying of liquid metal sprayed) heating elements on a metal surface, in order to achieve between these heating elements and this surface a solid mechanical connection and which allows them to heat transfer at high temperature and with

a low temperature gradient.

  The invention also relates to the devices thus obtained. Devices comprising tubes fixed on a surface are known: for example a coil of

  cooling surrounding the wall of a container to be cooled.

  But fixing the tubes on the surface is often not convenient (clamping, points or weld seams, ...), and the contact'du tube on the surface is often not as good

  we would like to have a good heat transfer.

  A method of fixing metal tubes on a metal surface by schooping has already been described, in particular

  by French Patent 1,537,203 (General Electric Co. 1967).

  This method consists in making, by schooping, on the entire wall and on the surface where they are fixed, a coating

relatively thick metal.

  It is applied to fix cooling tubes (o circulates a refrigerant) on the

wall of an enclosure to cool.

  Effectively this process is suitable for applications

  o there is thermal transfer at low or moderate temperature.

  But it is no longer suitable when the heat transfer takes place at high temperature and with a high thermal flux density. In these conditions of transfer, more difficult, it is found that the succession of heating and cooling cycles causes cracking and / or

chipping of the filler metal.

  However, there are many applications in which these conditions are realized. This is particularly the case of electric heating resistors under tubular metal shielding, which are fixed on the metal surface of a room acting as a heat diffuser such as iron soleplate, meat grill, hob, boiler of coffee maker, deep fryer, pressure cooker, electric frying pan, steam generator, etc. In these applications, the contact temperature between the resistance shield and the surface to be heated can commonly reach 200 or 300, or even 350 OC. The heat flow is also high: on the order of 20 to 30 watts / cm 2 of contact surface, which implies a temperature gradient as low as possible to avoid differences in expansion between the various parts of the assembly, and in their mass, so strong constraints

mechanical.

  In these applications, because of the high temperature and the high thermal flux density, it is necessary either to be content with an imperfect fixation of the heating resistor on the surface to be heated, to the detriment of the heat transfer performance. or to use other assembly methods, for example by drowning the heating resistor in the mass of the part to be heated (this part must then be made by

molding), which is more expensive.

  The object of the present invention is to propose a method for fixing a metal tube on a metal surface, the contact between this tube and this surface being intended to achieve between them a heat transfer at high temperature and with a thermal flux density.

high.

  According to the invention, this method is characterized in that the tube is applied to the surface so that the width of direct contact between the wall of the tube and the surface is at least equal to a quarter of the outer perimeter of the tube, that the dihedron formed by the plane of the wall of the tube where it deviates from the direct contact and the plane of the surface is at least 90, and the filler metal is projected onto the tube and the surface of the way that the contribution of metal projected is mainly in this dihedron, and one covers the surface only on a zone of width limited to substantially the width of the direct contact between the

  tube and surface, and be relatively thin.

  Indeed, tests have shown that all of these characteristics provide the connection between the tubes and the surface good mechanical strength and heat transfer even at high temperature and with a density of

high heat flow.

  This result is new and unexpected in many ways. If one refers to the known method for fixing tubes on a surface by schooping, described by the French patent 1,537,203, it is found that, according to this method, the projected filler metal completely covers the tube, and than the surface on which it is fixed, and that it is relatively thick. At first glance, these features would appear to be favorable for good tube attachment and

good thermal transmission.

  But practice has shown that a schooping connection thus realized does not resist the succession of

  heating cycles and cooling at high temperature -

  that is, in the range of 200-3500C - and with a high thermal flux density. This results in cracking of the filler metal, even in the areas where it is thickest (in the dihedral between the surface and the tube, near their contact) and flaking on the tube and on the surface. However, unexpectedly, the tests show that, if the tube is in direct contact with the surface by a sufficient flat, and if the dihedron between tube and surface is at least 90, we obtain better results with a contribution of metal thinner, and which is predominantly

  in the dihedral to their contact and to their immediate neighborhood.

  The question arose as to whether the metal part of the tube in contact with the part to be heated did not create a temperature gradient that significantly reduced heat transfer, and that this part could not be suppressed. The tube is then reduced to a simple U-shaped profile returned to receive on its outer surface projections of sputtered liquid metal. The results were convincing. The tests then focused on the removal of any shielding element around the magnesia, this one

  being simply squeezed around the electrical resistance.

  It can then be considered that the shielding of the resistor is carried out during fixation, by a deposition of formed metal

by schoopage.

  This embodiment is obtained by a method similar to that of fixing a tubular or semi-tubular element, a complementary means for treating magnesia allowing a good cohesion on the surface thereof, and consisting of a coating by one

  silicone resin, followed by polymerization.

  The following description will specify, by

  examples, features and embodiments of

such effective links.

  As the fixing of electric heating resistances under tubular shielding on surfaces to be heated is a case of routine application of the method according to the invention, the appended drawings, which are non-limiting examples, represent the "tube" in the form of such a resistance. But this illustrative example is only indicative and does not limit the scope of the invention,

  which may concern tubes of any kind.

  In the accompanying drawings: - Figure i shows, in section, a shielded heating resistor fixed on a flat surface (for example the sole of an iron); - Figure 2 shows, in section, a shielded resistance of quasi-triangular section fixed on a corrugated surface (for example a meat grill plate); - Figure 3 shows, in section, a shielded resistance of quasi-square section fixed on a flat surface; FIG. 4 represents, in section, a shielded resistance of quasi-trapezoidal section fixed on the flat part of a tubular conduit (for example a coffee-maker boiler duct); FIG. 5 represents, in section, a resistor inserted into a form of magnesia compacted in a U-shaped section, fixed on a flat surface, - Figure 6 shows, in section, a resistance inserted into a form of compacted magnesia directly attached to a flat surface, - Figures 7, 8 and 9 schematically illustrate the steps of producing an electrical resistance that can be used in the process according to the invention; A shielded heating resistor as denoted by i in FIG. 1 consists of a resistive conductor coated with an insulator (conventionally compressed magnesia), all surrounded by a tube (conventionally made of aluminum). This tube, which constitutes the shielding of the resistance, is originally round. But then we can

  stamp it to change the shape of its section.

  In FIG. 1, the section of the tube is substantially

  trapezoidal. Its base is flat, is applied on the surface 2a of the plate 2 forming heat diffuser. The lateral sides lb and lc are almost planar. The top ld can be rounded. As an indication, the resistors used in these tests for implementing the method according to the invention had for shielding a tube of diameter equal to 8 mm at the origin. After forming, the width of the base of the trapezoid

  is about 11 mm; the height of the trapezium is about 8 mm.

  The three faces lb, lc and ld have been classically etched by blasting (projection of abrasive by compressed air). For this purpose was used corundum grains of 0.5 mm. The base was simply defatted, but no

sanded, so left smooth.

  Plate 2, made of aluminum, was also etched by

sandblasting on its surface 2a.

  The shielded resistor i has been temporarily maintained on the face 2a of the plate 2, by a conventional clamping. Sprayed aluminum in the liquid state has been projected according to the known technique of schooping on the surfaces 1b and 1c of the resistor 1 and on the surface 2a of the plate 2, so that the maximum deposit is formed.

  in the junction zone of the resistor 1 and the plate 2.

  This metal projection was carried out using an electric arc gun, according to the usual techniques in this field. Only the projection nozzle is modified so that the projection angle is more closed than in the case of a schoopage intended for coatings of

  metal parts over their entire surface.

  The characteristics of the projection jet were as follows: Projection angle: 40 to 55 Projection distance: 55 to 65 mm Movement speed: 150 mmY / s for a flow rate of 0.65 g / s of aluminum Jet width : width 20 to 25 mm (flat jet nozzle) Wire diameter

of aluminum used: 1.6 mm.

  Given the heat fluxes to be transferred from the armored resistor 1 to the plate 2, temperature gradients and the expansion differentials that they cause, the projected metal which, while solidifying, retains a certain porosity, must nonetheless have mechanical characteristics. minimum, and the shape of the deposit must obey certain rules, if one wants to: - achieve assembly at a competitive cost compared to other techniques, - withstand a number of thermal cycles

  compatible with the estimated life of the finished product.

  Thus: - the tensile strength of the projected aluminum must be at least 9 kg / mm2; - - resistance to tearing parallel to the

  surface area must be at least 1.2 kg / mm2.

  These resistances can only be achieved if the thickness of the filler metal per pass is very small: less than 0.1 mm (the values given above for the speed of movement and the flow rate of the jet satisfy this condition), and that these passes are done simultaneously on both sides of the armored resistance 1 by two pistols

schooping, alternatively.

  Since, even in spite of these precautions, the resistance of the filler metal to tearing remains relatively low, the shape of the deposit must comply with the following conditions: - The direct contact width of the shielded resistor 1 with the plate 2 must be at least 1/4 of the perimeter of the cross-section of the armored resistance, and

  preferentially close to 2/5.

  In the case of the embodiment shown in FIG. 1, this width is approximately 10 or 11 mm, on a

  original perimeter of 25 mm, or 40%.

  - The dihedral AOB formed by the plane OA following the lateral surface lb or lc of the shielded resistor 1 and the plane OB following the surface of the plate 2 adjacent to the resistor must be open by at least 90 so that a deposit of metal 3 can be formed at the top O of the dihedral without leaving any empty space. This angle can even be greater than

    in some embodiments (see Figure 2).

  - The contribution of metal must be mainly

in the dihedral AOB.

  - The thickness of the filler metal 3 must be relatively small. A thickness of 0.5 to 0.8 mm (in any case less than 3 mm) is sufficient at the top of the dihedral AOB. On the side wall 1b, 1c of the armored resistor 1, the thickness decreases towards the apex opposite to the base 1a. There may be no filler metal on the top of the section of

resistance.

  - The attachment zone of the metal deposit 3 on each side of the resistor 1 on the plate 2 should not be too wide but substantially equal to the width of

  direct contact of the resistor 1 on the plate 2.

  FIG. 2 illustrates the implementation of the method according to the invention in the case of a shielded electrical resistance 10 of substantially triangular section, fixed by schooping to a griddle cooking plate 11 having corrugations whose profile is adapted to receive the substantially triangular section of the

resistance 10.

  In this embodiment, the width of direct contact between the resistor 10 and the plate 11 is greater than 50% of the initial perimeter of the tube from which the

resistance 10 was obtained.

  In this example the dihedral AOB has an angle greater than 180. As in the example of FIG. 1, the filler metal 3 is located mainly in the aforementioned dihedron and only partly covers the resistor 10. Moreover, this filler 3 only covers the surface of the plate 11 over a width substantially limited to the contact width

  direct between the resistor 10 and the plate 11.

  The embodiment of FIG. 3 illustrates the case of a shielded resistance 20 of square section fixed by schooping.

on a plate 2.

  In this example, the dihedral AOB formed between the wall of the resistor 20 and the plate 2 is equal to 90. In this embodiment, the contact width 20a between the resistor 20 and the plate 2 is equal to a quarter of the outer perimeter of the tube from which the resistor 20 has been made. FIG. 4 represents a shielded resistor 30 of substantially trapezoidal section whose large base is in direct contact with the flat face of a tube 31 intended for the passage of a hot fluid such as hot water or steam and intended to to undergo large temperature variations. The dihedral formed between the flat face of the tube 31 and the adjacent wall of the resistor 30 is clearly

greater than 90.

  The width of the contact between the resistor 30 and the tube 31 is greater than a quarter of the perimeter of this resistor. - The intake metal 3 is located mainly in the aforementioned dihedron and does not completely cover the

resistance 30.

  The above tests have shown that bonds made in accordance with the foregoing, withstand very well a succession of heating and cooling cycles, without any cracking or spalling of the filler metal. In particular, it is noted that during solidification, the withdrawal of the filler metal causes a

  energetic tightening of the shielded resistor 1 on the plate 2.

  The asperities created by sanding on the surface 2a of the plate 2 then penetrate into the smooth surface of the face of the armored resistance. It follows that resistance

  between shielded resistance 1 and plate 2 is low: -

less than 6-10-2 C / W.

  The tests seem to show that the flow streams -

  thermal, emitted by the armored resistor, is evacuated in the plate 2 by the direct contact surface of the resistor,

  the remainder being evacuated by the mass of filler metal 3.

  FIG. 5 shows a resistor 40 under magnesia 41 whose shielding is reduced to only parts 41b, 41c, 41d. The portion 41a in contact with the plate 42 does not include a metal wall. For this purpose the resistor 40 is inserted into the magnesia not in a tube, but in an open metallic form. This resistance is achieved as follows: in a steel die 60 (see Figure 7) is formed a groove 61 at the bottom of which is placed an aluminum profile 62 whose shape, substantially inverted U, recalls the upper part of Shielded resistors 1c, 1d and 1b of FIG. 1. A quantity of powdered magnesia is poured into the groove 61. A punch 63 is lowered into the groove and a force is applied thereto. The punch 63 has a footprint 63a of such

  way that a groove 64 is arranged in the magnesia 65.

  In this groove 64 is deposited the resistant wire 66 wound helically to non-contiguous turns (see Figure 8). A certain amount of magnesia is again poured into the groove 61 and another punch 67 is lowered and applied with a force substantially identical to that of the first operation. This force is such that magnesia, after compaction, reaches a density of about 3.2. Figure 9 shows the completed element. Ejection bars 68

  allow to get out of the throat 61.

  The fixing method of this heating element is the same as that previously described for fixing

of tubular element.

  FIG. 6 shows a resistor 50 under magnesia 51, the shielding of which has been made directly on the plate 52, and consists of projected metal. The method of manufacturing the bar magnesia 51 is the same as that

  described above, the aluminum profile 62 does not exist.

  In the preceding case, magnesia adheres strongly to aluminum foil 62 because the grains of magnesia penetrate superficially into the metal. In the present case, it is obvious that if it is desired to unmold an intact bar 51, it is necessary for the groove 61 to be made of a hard, polished steel whose surface resistance is greater than

  the punching force of the grains of magnesia.

  If one proceeds to a projection of molten metal sprayed directly on a bar thus manufactured, one notices that, considering the withdrawal of the metal, the first projected layers deform and tend to curl on themselves, with the image of birch bark. To avoid this major drawback, two ways are possible: 1) Make very fine projections,

  simultaneously on both sides and the top of the bar.

  This solution adds to investment and turnaround times. 2) Impregnate superficially the bar magnesia a coating that can be destroyed after fixing the bar. Tests have shown that a silicone coating, polymerized by heating in an oven, provided a permissible adhesion during the schooping. This coating is destroyed during the first commissioning of the heating element, the residues combining with magnesia without altering its properties.

dielectric qualities.

  The fixing method of these bars of magnesia is the same as that previously described. It is obvious that the thickness of the metal must be greater than that required on a metal shield. This extra thickness must also be substantially equal to the thickness of the shielding

that the resistance could have had.

  One can think that according to the same process, it is possible to fix heating rods of other kinds than compacted magnesia. Experience shows that heating rods in stearate, industrial porcelain, alumina, or other ceramics, in that the expansion differentials are very important, can not be fixed by schoopage (for large thermal flux transfers). Indeed, there is tearing of the projected metal in the connection zone of the bar with the plate. Unlike these ceramics, the grains of magnesia keep a certain freedom of movement and allow the bar to follow the dilations of the plate

and shielding.

  Of course, the invention is not limited to the embodiments described above, and can be made to them many modifications without departing from the scope of the invention. Thus, for example, the process can be used regardless of the constituent metals of the tube or the

  resistance and the surface on which one wants to fix it.

  They may be the same or different, and the filler metal may still be different. For example: it is possible to fix heating elements protected by an aluminum U on a steel surface by schooping with brass like

filler metal.

Claims (9)

  A method for fixing a heating element (1, 10,, 30, 40, 50) on a metal surface (2, 11, 31, 42, 52), the contact between said heating element and said surface being intended to realize between they are thermally transferable at high temperature and with a high thermal flux density, characterized in that the heating element (1, 10, 20, 30, 40-41, 50-51) is applied to the surface (2, 11, 31, 42, 52) so that the contact width is at least equal to a quarter of the outer perimeter of the heating element, the dihedral (AOB) formed by the plane of the wall (1b or 1c) of the heating element where it deviates from the direct contact and the plane of the surface is at least 90, and the filler metal (3) is projected onto the heating element and the surface so that the contribution of projected metal is mainly in this dihedral (AOB), and is relatively thin. 2. Method according to claim 1, characterized in that the projected metal supply covers the surface (2, 11, 31, 42, 52) over an area of width limited to substantially the width of direct contact between the element heating and the surface.   Method according to claim 1, characterized in that the heating element (1, 10, 20, 30) is a shielded electric heating element fixed on a   plate acting as a heat diffuser.   4. Method according to claim 1, characterized in that the heating element (40-41) is a resistive wire embedded in compacted magnesia adhering to a metal form of U section. 5. Process according to claim 4, characterized in that the compacted magnesia (41) is in direct contact with the plate (42).   Process according to Claim 1, characterized in that the heating element (50-51) is a wire   resistive embedded in compacted magnesia.   7. Process according to claim 6, characterized in that the compacted magnesia bar constituting the heating element is superficially coated. of polymerized silicone resin.   8. Process according to one of claims 1, 4   or 6, characterized in that the projection of filler metal (3) is carried out in successive passes, each of which   Deposit no more than 0.1 mm of metal thickness.   9. Device comprising a heating element (1, 10, 20, 30, 40, 50) fixed on a metal surface (2, 11, 31, 42, 52), characterized in that the contact width between the heating element (1, 10, 20, 30, 40, 50) and the surface (2, 11, 31, 42, 52) is at least equal to a quarter of the outer perimeter of the heating element, that the dihedral (AOB) formed by the plane of the wall of the heating element at the point where it deviates from the direct contact and the plane of the surface is at least 90 and in that the heating element is connected to the surface by a metal of contribution (3) projected by schoopage, this filler metal being located mainly in the dihedron (AOB) and forming a layer relatively thin.