FR2781112A1 - Radiant heat emitter has space between electrical resistance and refractory matrix produced by sleeve removed by pyrolysis - Google Patents

Abstract

A radiant heat emitter comprises a matrix of electrically resistive refractory material (2), a space (5) around the resistance. The spaces formed after compacting the refractory powder used to make the matrix, or during a subsequent fritting process by enclosing the resistance in a sleeve of a material that can be removed by pyrolysis. The resistance and the space surrounding it have diameters in a ratio of between 0.8 and 1.2:1, and the space has constrictions at intervals formed by projections (7,8).

Description

"Emitter of thermal radiation and its manufacturing process" The

  The present invention relates to a thermal radiation emitter of the type comprising a matrix of refractory material, such as a ceramic material, within which an electrical resistance is arranged; it also relates to a process for manufacturing this transmitter. Given the nature of the materials (refractory material and electrical resistance) generally used in known emitters of this type, there is a large difference in thermal expansion between the electrical resistance and the refractory matrix. This difference is

  the origin of cracking phenomena in the transmitter.

  Indeed, during the manufacture of such emitters, and more precisely during the sintering operation, the refractory material (ceramic) shrinks while the electrical resistance reaches its maximum expansion and is therefore in compression; then, during the cooling phase after sintering, the refractory material (ceramic) continues to withdraw, which leads to blockage of the electrical resistance. It follows that from the first commissioning (heating) of the transmitter, the expansion of the electrical resistance (greater than that of the refractory material) leads to the appearance of cracks which lead to destruction

  often premature of the transmitter.

  Consequently, one of the aims of the present invention is to overcome the problems mentioned above and to do this, there is proposed according to the invention a thermal radiation emitter of the type comprising a matrix of refractory material, such as a ceramic material, within which an electrical resistance is arranged, which is characterized in that at least one space is provided around said electrical resistance on a

  at least part of the length of the latter.

  It will be understood that due to the presence of said space around the electrical resistance, the latter can expand freely both radially and longitudinally; a relative displacement of the resistance with respect to the matrix is therefore possible and the appearance of cracks is then eliminated or at the very least greatly reduced. It will be noted that for reasons of uniformity of radiation from the emitter, said electrical resistance and l0 said space will each advantageously have a circular section of substantially constant diameter all along

resistance or space.

  Furthermore, in order to properly position the electrical resistance within the matrix and to prevent any untimely relative sliding of one with respect to the other, it is preferable according to the invention that said space includes at least one narrowed zone. at which the electrical resistance is in contact with the matrix; said narrowed zone will preferably comprise at least two narrowing segments facing each other and with which the electrical resistance is in contact. Thanks to this contact, the resistance is held in position by blocking at the level of the narrowed zone or of the narrowing segments. In addition, as this zone or these segments are very short in length with respect to the total length of the resistance, advantageously of a length of 0.1-0.5% of the total length, the longitudinal expansion of the resistance to level of this zone or these segments is negligible and therefore not likely to be the cause of cracking; it is the same in the radial direction

  that expansion is also negligible.

  The electrical resistance is advantageously shaped along a sinuous path comprising several successive sections of substantially equal length and connected by respective connection loops, means possibly being provided in the vicinity of these loops to allow free expansion of the electrical resistance when the transmitter is in service; in this case, it is preferable that each section comprises a narrowed zone situated substantially equidistant from its ends; in this way, the electrical resistance will expand substantially symmetrically with respect to the median plane of the transmitter, in which the various narrowed areas are located. According to one embodiment, the abovementioned means consist of a space formed around said connection loops, this space being open to the outside of

the matrix.

  According to another embodiment, said loops

  of connection emerge from the matrix.

  According to yet another embodiment, the aforementioned narrowed zone comprises at least two narrowed parts separated by an enlarged part. In this way, the electrical resistance is clamped by the matrix at the level of said two narrowed parts but released at the level of said widened part. It will be added that the latter will be of very short length, advantageously of a length at

  plus equal to 10% of the total length of the narrowed area.

  It is currently believed that this area structure

  narrowed allows better durability of the transmitter.

  In the transmitter according to the invention, the space surrounding the electrical resistance is filled with air which is a thermal insulator. It follows that the size of this space will have to be chosen on the one hand, so that in service the electrical resistance can expand freely and on the other hand, so that there is nevertheless a satisfactory thermal transfer of the resistance to the matrix. This size in fact essentially depends on the difference in thermal expansion between the matrix and the resistance and it will be easy for those skilled in the art by some preliminary tests to determine the optimal size of the space according to the nature of the materials constituting the matrix and resistance. It will however be added that with the materials currently used in the manufacture of transmitters of the type of those of the present invention and in the case where the resistance and the space are both of circular section, the ratio of the diameter of the resistance to diameter of

  the space can be in the range of 0.8 to 1.2.

  The present invention further extends to a method of manufacturing the transmitter described above. This process comprises the operations consisting in: - drowning an electrical resistance in a mass of powder of refractory material such as a powder of ceramic material, - in subjecting this mass of powder to a pressing in order to obtain a mass of compacted powder, and - in subjecting this mass of powder compacted to sintering to obtain a matrix of refractory material within which said electrical resistance is embedded; this process is characterized in that it further comprises the operation consisting in forming, after pressing, at least one space around the electrical resistance on a part

  at least the length of the latter.

  According to the invention, the formation of said space is preferably carried out during the sintering operation. To this end, and according to a preferred embodiment of the invention, said electrical resistance is provided with a sheath made of a material capable of being eliminated by pyrolysis and this sheath is eliminated by pyrolysis during the sintering operation, thus giving way to a space

around resistance.

  The aforementioned sheath will advantageously be made of a flexible material (to allow the electrical resistance to be shaped), not liable to degrade said resistance, which resists during pressing against friction with the grains of refractory material and which is capable of being eliminated. by pyrolysis; such a material may by

  example be constituted by flexible polyethylene.

  According to another advantageous characteristic, the method according to the invention can also comprise the operation consisting in eliminating, before the pressing operation, the sheath in at least one zone of the electrical resistance, so that 'after the sintering operation, the matrix is in contact at this zone with said electrical resistance; this contact will maintain the position and block the resistance in

said space.

  This local sheath removal operation preferably comprises the operation of causing said sheath to melt and / or pyrolysis in said zone; this fusion and / or pyrolysis can for example be carried out by applying a hot element to the sheath at a suitable temperature to cause this fusion and / or

this pyrolysis.

  An embodiment of the present invention will be described below by way of nonlimiting example with reference to the accompanying drawing, in which: - Figure 1 is a top view of the transmitter according to the invention, not in service; - Figure 2 is a partial view of the transmitter of Figure 1, in service, with partial cutaway; - Figure 3 is a cross-sectional view along line III - III of Figure 1; - Figure 4 is a partial schematic sectional view of the mixture of refractory powders - electrical wire before pressing; and - Figure 5 is a partial sectional view of

  the whole of FIG. 4 after sintering.

  The thermal radiation emitter which is shown in the drawing comprises, in a manner known per se, a matrix 1 made of a refractory material, for example of generally parallelepiped shape, within which

  arranged an electrical resistance 2.

  This resistance can take the most diverse forms; however, it is preferred that the resistance be constituted by a wire shaped according to a sinuous line in a plane, this line having a general zigzag shape. This layout comprises a certain number of sections of wire 3, these sections being substantially the same length and spaced one

  on the other to avoid contact between them.

  The resistor 2 is arranged in the matrix 1 so that said sections are located in the longitudinal plane of the latter and extend from one end to the other of the matrix. Each section 3 is connected to the next by a connection loop 4, 4 'leading to the outside of the matrix. Thus, as shown in FIGS. 1 and 2, the resistor 2 comprises six sections of wire, the left end of the first section and the left end of the second section being connected by a loop 4 giving on the outside of the left edge of the matrix, the right end of the second section and the right end of the third section being connected by a loop 4 'giving on the outside of the right edge of the matrix and so on, the left end of the front last section and the left end of the last section being connected by a loop 4 leading to the outside of the left edge of the matrix. As for the free right end of the first section and the free right end of the last section, they both emerge from the right edge of the matrix and are intended to be connected to a power source.

electric (not shown).

  The refractory material constituting the matrix 1 will preferably be chosen from those having in particular good resistance to thermal shocks, good mechanical resistance, low cost price and low toxicity,

or even zero.

  The refractory material may in particular be a ceramic material such as for example that described in

French Patent No. 2,711,364.

  For its part, the electrical resistance 2 may be produced from a material compatible with the refractory material, having good resistance to thermal shock of aging, having good resistance to high temperatures and endowed with a good ability to radiate the heat; this material will advantageously be KANTHAL AF

  (trademark of the company KANTHAL).

  All useful information on this resistance can be found in the above-mentioned French patent No. 2,711,364. In accordance with the present invention, the transmitter comprises a space 5 formed around the resistor 2 over the entire length of the latter (the space 5 is visible in FIGS. 3 and 5, but has not been shown in the

  Figures 1 and 2 for better clarity of the latter).

  Note that at the connection loops 4, 4 ', this space 5 is open to the outside to allow free expansion of the electrical resistance 2 when

the transmitter is in service.

  As shown in Figures 3 and 5, the space 5 has in the example chosen a circular section and the same is true of the wire constituting the resistor 2; in addition, the diameter of the wire and that of the space are substantially constant over the entire length of the wire and the spaces. Of course, any other form of section remains

  in the context of the present invention.

  According to another characteristic of the invention, the space 5 has narrowed zones 6 of very short length (for example at most 1 cm) which, as shown more clearly in FIG. 5, each comprise two narrowing segments 7, 8 facing each other and enclosing the wire 2. In addition, each section 3 comprises a narrowed zone 6, the latter being located substantially equidistant from the ends of the section considered; in other words, the various narrowed zones 6 are substantially situated in the median transverse plane of the transmitter. In this way, each section 3 of wire is immobilized in the space 5, which prevents any untimely movement of the electrical resistance in the matrix 1. Furthermore, the narrowed zones 6 do not prevent a free expansion of the resistance when the transmitter is in service. Indeed, the parts of the wire sections located to the right of the narrowed zones 6 can freely expand towards the right side of the transmitter, the loops 4 ′ then emerging from the right side of the latter; it is the same for the parts of the sections of wire situated to the left of the narrowed zones 6, which undergo an expansion of substantially the same amplitude, the corresponding loops 4 then emerging from the left side of

the transmitter (see Figure 2).

  Advantageously, each narrowed zone 6 consists of two narrowed parts 6a, 6b separated by an enlarged part 6c, as shown in the figure, the length of this enlarged part 6c not exceeding

  preferably 1 mm in the example chosen.

  It will be noted that the emitter as just described is preferably supplemented by two protective sleeves 9, 10 also made of refractory material such as a sintered product made of cordierite-mullite. It will be added that these two sockets are shaped to come to be fitted respectively on the left side and the right side of the matrix 1; these same sockets each further comprise a recess 11, 12 the shape and dimensions of which are chosen to receive the loops 4, 4 'respectively when these emerge from the matrix as a result of the expansion of the electrical resistance The emitter which just described can preferably

  be manufactured by the following process.

  In a pressing mold (not shown) dimensioned to obtain a matrix of substantially parallelepiped shape, a mixture 13 of powders of ceramic material is introduced, such as that described in the aforementioned French patent, and comprising for example approximately 70% by weight of cordierite, approximately 20% by weight of mullite, 10% by weight of ARTAL 23 (trademark of the French company ARGILES ET MINERAUX AGS), as well as a binder (for example: polyvinyl alcohol) and a plasticizer

  amounts leading to 100% by weight.

  Next, an electric wire made of KANTHAL AF (Fe: 72.2%, Cr: 22% and Al: 5.8%) and provided with a sheath 14 of flexible and pyrolyzable polyethylene or any other equivalent material is brought under the desired shape, namely a

  zigzag shape as described above.

  For example, the wire has a diameter of 0.5 mm and its length is 6.2 m; under these conditions, it makes it possible to reach, under an alternative electrical supply of 230V, a temperature of the order of 1000K and to generate a power of approximately 1200W. As for the sheath, it will advantageously have a thickness of the order of 0.01 to 0.05 mm,

for example 0.01 mm.

  Furthermore, each section of wire 3 of the zigzag has a

  length substantially equal to that of the pressing mold.

  The wire thus obtained is then subjected to an operation consisting in locally eliminating the sheath substantially in the middle of each section of wire and at the level of two very close parts (for example spaced no more than 1 mm in the example chosen). To do this, a hot element is applied twice and slightly offset (not more than 1 mm) transversely to the upper then lower middle part of the sections of the wire, the temperature of this element being sufficient to cause melting and preferably the pyrolysis of the sheath at the points of impact of said element with the sheath. This hot element can in particular be an electric resistance carried

  by Joule effect at a temperature of 400 ° C or more.

  The wire thus obtained is then positioned horizontally within the above-mentioned mixture of powders 13 so that the layers of mixture situated respectively above and below the wire have substantially the same thickness. Note that at the parts 6a, 6b where the sheath 14 has been eliminated, the metal of the wire is exposed so that said mixture 13 is in contact there.

direct with metal.

  Then pressing the said mixture at room temperature for example according to the cycle of

  pressing described in French Patent No. 2,711,364.

  This results in a compact mass which is removed from the pressing mold before subjecting it to sintering, for example according to the cooking cycle also described in the aforementioned French patent. During sintering, the compact mass 13 is transformed into a solid and dense matrix 1 and the sheath 14 of the wire is removed by pyrolysis so as to

  leave a space 5 empty of any material around the wire 2.

  This space 5 extends over the entire length of the wire 2 except at the areas where the sheath 14 had been removed before pressing and where the sintered powder defines narrowed portions 6a, 6b each comprising two shrinkage segments 7.8 which are face and enclose the wire 2 so as to ensure the maintenance in position and the blocking of said wire which at this stage is none other than the resistance

electric 2 mentioned above.

  The emitter described above finds applications in the most diverse fields, for example for drying, in particular for drying the paints arranged.

on a support.

Claims (16)

  1. Emitter of thermal radiation of the type comprising a matrix (1) of refractory material, such as a ceramic material, within which an electrical resistance (2) is arranged, characterized in that at least one space (5) is formed around said electrical resistance over at least part of the length of the latter. 2. Transmitter according to claim 1, characterized in that said electrical resistance (2) and said space (5) each have a circular section of substantially constant diameter throughout the resistance or space. 3. Transmitter according to claim 1 or 2, characterized in that said space comprises at least one narrowed zone (6) at the level of which the electrical resistance is in contact with the matrix to ensure the maintenance in position and blocking it.   4. Transmitter according to claim 3, characterized in that said narrowed zone comprises at least two narrowing segments (7,8) facing each other and with which the   electrical resistance is in contact.   5. Transmitter according to one of claims 1 to 4, characterized in that the electrical resistance is shaped according to a sinuous path comprising several successive sections (3) of substantially equal length and connected by connection loops (4,4 ') respective, means being possibly provided in the vicinity of these loops to allow free expansion of the electrical resistance when the transmitter is in service.   6. Transmitter according to claim 5, characterized in that said means consist of a space (5) formed around said loops, this space being open towards the outside of the matrix (1).   7. Transmitter according to claim 5, characterized in   that said loops (4,4 ') emerge from the matrix (1).   8. Transmitter according to claim 5, 6 or 7, characterized in that each section comprises a narrowed zone situated substantially equidistant from the ends of said section.   9. Transmitter according to any one of the claims   3 to 8, considered in that the narrowed zone (6) comprises at least two narrowed parts (6a, 6b) separated by a enlarged part (6c).   10. Method of manufacturing the transmitter according to one   any of the preceding claims, including   operations consisting in: - drowning an electrical resistance in a mass of powder of refractory material such as a powder of ceramic material, in subjecting this mass of powder to a pressing in order to obtain a mass of compacted powder, and - in subjecting this mass of compacted powder to a sintering in order to obtain a matrix of refractory material within which said electrical resistance is disposed, characterized in that it further comprises the operation consisting in forming, after pressing, at least one space around the electrical resistance on at least part the length of the latter.   11. Method according to claim 10, characterized in that the operation consisting in forming said space is   performed during the sintering operation.   12. The method of claim 11, characterized in that for the operation of forming said space, said electrical resistance is provided with a sheath of a material capable of being removed by pyrolysis, and in that this sheath is eliminated by pyrolysis during the sintering operation.   13. Method according to claim 12, characterized in that the material constituting the sheath is constituted by a flexible polyethylene.   14. The method of claim 12 or 13, characterized in that it further comprises the operation of eliminating, before the pressing operation, the sheath at at least one area of the electrical resistance, so that after the sintering operation the matrix is in contact in this area with said electrical resistance.   15. Method according to claim 14, characterized in that the operation consisting in eliminating the sheath comprises the operation consisting in causing the fusion and / or the   pyrolysis of said sheath at said zone.   16. The method of claim 15, characterized in that the melting and / or pyrolysis is carried out by applying to the sheath a hot element at a temperature   suitable for causing this fusion and / or pyrolysis.