EP0422979A1 - Heating tube - Google Patents

Abstract

The invention relates to a tube made from transparent refractory material with one or more heating coils inside. The coil or coils are supported by an insulating trough which reflects the calorific rays. Possibly a built-in metal rod allows the electrical connection at one end only. <IMAGE>

Description

The invention relates to heating elements comprising an incandescent metallic resistance inside a tube of refractory material which is more or less transparent to the radiation emitted. In these elements, the metallic resistances are most often helices made of alloys of iron, chromium and aluminum and, the tubes, based on quartz, vitreous silica or vitrified ceramics.

Heating elements of the above type have multiple uses in many branches of industry. They are in particular widely used for drying (papers, fabrics, paints, wood, agglomerated or laminated sheets, various granules). They are often installed in heating boxes, several tubes then being assembled parallel to each other, in front of a reflective metal plate.

It is common to install two resistors parallel to each other in the same tube, for example silica. The electrical separation of the two filaments is ensured either by a partition of the tube itself, it then has an 8-shaped section, or by a second smaller tube, also made of silica which is placed between the two filaments. Among these two traditional systems, the first requires the establishment of an electrical connection outside the tube at both ends. In the second system, on the other hand, it is possible to envisage that the internal tube is slightly set back at the most distant end, which leaves room for an electrical connection between the two filaments. We can even consider provide an electric cable which circulates inside the internal tube thus allowing a parallel connection from a single end.

Often the heating elements constituted by the vitreous silica tubes described above are equipped with a thin layer of gold on their rear face. This reduces the emission of thermal radiation towards the rear. But this layer of gold is mechanically fragile, moreover its temperature of use is limited. Besides, it is expensive.

The two systems described above have certain drawbacks. We have already seen that the first, with its section at 8, required external electrical connections at its two ends. The second system also has specific drawbacks, in particular for the insertion of resistors and because of vibrational phenomena during operation.

The invention aims to overcome the above drawbacks on the one hand by eliminating the gold layer and improving the overall energy efficiency, but also by allowing all types of connection from one end and avoiding the emission of noise in the presence of vibrations.

Radiators whose active element consists of a helical resistor exposed to air have a limited operating temperature. This limit is for example 1000 ° C. Various reasons are at the origin of this limit, the mechanical behavior of the metal, its resistance to oxidation but also because we want the infrared radiation emitted not to be too short wavelength but has at on the contrary, a very broad emission spectrum centered on an average wavelength. It follows from this limit on the temperature that the thermal energy available is also limited, this is why it is desired to associate several parallel filaments, most often two.

Depending on the characteristics of each filament such as diameter, length, temperature, it may be interesting to connect the two twin propellers either series, or in parallel. When using resistors as a support, a tube of transparent or diffusing material whose cross-section has a shape of 8, it is then necessary to mount at the two ends, outside the tube, connection boxes which, depending on their type, allow mounting in series or in parallel. If a tube is used which does not have an internal partition, it is advantageous, in order to be able to immobilize the internal elements easily despite impacts and vibrations, to have a tube of oval section. The filaments are in this case separated from each other by a second smaller tube. But, then, the configuration of the wiring at the furthest end of the tube must, from the outset, be designed differently depending on whether the heating element is intended to be mounted with its two spirals in parallel or in series. It would however be interesting for the fitters who install radiant electric tubes on machines or combine them to make boxes of them to be able to make all the connections on one side, at the same end of the tube and on the other hand, to be able decide, only when connecting, the type of wiring, serial or parallel, which will ultimately be adopted, or even to be able, later, to easily switch from one type of connection to another.

Traditional radiating tubes when they are intended to heat only on one side are generally covered on the other side with a layer of gold deposited for example under vacuum. This material is expensive and its deposit requires an additional production phase. moreover, it requires restrictive precautions for use. As long as the tube is not in place, it is necessary to avoid any contact of the gold layer with a hard surface which could scratch it. In addition, any overheating of the silica / gold interface is prohibited: if the temperature exceeds 800 ° C the gold layer would be practically out of use and would no longer play any role on the radiation.

Devices are known in which means other than a metallic layer are used to reflect and concentrate the radiation emitted by an incandescent filament. The US Pat. No. 4,001,622, for example, presents a linear high temperature tungsten filament placed eccentrically backwards inside a tube of circular quartz section, parallel to its axis, the tube being embedded in a half -coaxial cylinder made of ceramic fibers so as to focus the radiation along a straight line parallel to the axis on the other side of the filament, outside the tube.

This device, which responds to a particular need in the photocopying technique, is not adapted to the problem posed, thus it gives no solution for the electrical isolation of the two incandescent filaments, it would moreover not allow not to position them precisely, moreover, it does not solve the question of connecting the two filaments, either in parallel or in series.

The two preceding reflector devices, a layer of gold on the tube or an external fiber-based reflector do not escape the disadvantage of causing unnecessary heating of the rear wall of the tube which is crossed twice by the radiation which reduces its rigidity and lowers the thermal efficiency of the system.

The invention relates to a tubular heating element comprising, inside the refractory tube, at least one incandescent filament housed inside the groove of a heat-reflecting, thermally insulating refractory material. In a variant of the invention, the filaments are two in number, parallel to the axis of the tube, they are electrically insulated from one another by the heat-reflecting material.

This material has a thermal conductivity at 1000 ° C less than 0.35 Wm⁻¹K⁻¹ and preferably less than 0.25 Wm⁻¹K⁻¹. It can advantageously be based on silica obtained from a slip according to the so-called "slip-cast" process or ceramic fibers based on silica and / or alumina.

The invention also provides for the installation of a metal rod inside the tube. This can be used to bring the electric current up to the farthest end of the tube.

• La figure 1 représente un tube chauffant traditionnel équipée de ses deux filaments,
• La figure 2 est une variante du même dispositif,
• La figure 3 présente une réalisation conforme à l'in­vention,
• Quant à la figure 4, elle montre schématiquement un branchement électrique selon l'invention.

The operation of the invention as well as the advantages which it presents compared to the prior techniques will appear in the following description illustrated by the figures.

• FIG. 1 represents a traditional heating tube equipped with its two filaments,
• FIG. 2 is a variant of the same device,
• FIG. 3 shows an embodiment in accordance with the invention,
• As for Figure 4, it schematically shows an electrical connection according to the invention.

Heating tubes of the type of those of the invention are intended to equip household or industrial appliances either singly or in combination with several in a box. This involves heating to dry, cook, polymerize, calcine, etc. the most diverse materials. The required characteristics are both physical and practical. In the foreground, it is a question of having the widest possible radiation spectrum. Ideally, it should extend from 1.5 to 10 µ so as to be able to meet the widest range of uses. This characteristic is often achieved by the use of re-emitters: a single source of radiation such as the metallic filament: for example of iron, chromium and aluminum alloy 1 of FIG. 1 is brought to a given temperature, for example 800 ° C. At this temperature, the maximum radiation emitted is 2.7 µ. The radiation emitted by the filament is transmitted in part by the envelope 2 most often made of vitreous silica. But a certain part is absorbed by this material which heats up - to a temperature lower than that of the filament - and re-emits energy in the spectral domain where it absorbs, i.e. in the domain of the largest wavelengths. In most applications, the thermal radiation emitted is used only on one side of the tube, we therefore seek to eliminate it or at least to reduce it on the other side. This is why the rear face of the tube 3 is covered with a layer of gold 4. This reflects the direct radiation emitted by the filament 1 and, thanks to the very low emissivity of the gold, prevents the heated silica 3 from emitting radiation towards the rear. However, the thermal radiation passed through the rear wall of the tube twice and helped to heat it up further and increase convection loss.

FIG. 1 also shows how the partition 5 makes it possible to electrically isolate the two filaments 1 and 6.

Figure 2 shows a variant of Figure 1 where the oval outer tube 7 does not have a partition. The two filaments 8 and 9 are separated here by a generally circular cross-section tube 10. Here, too, it is possible to put on the rear face of the tube 7 a reflective and non-emissive layer 11, for example made of gold.

Compared to the previous system, this has the advantage of making it possible to use the internal channel of the tube 10 to pass an electrical conductor there (not shown). It is thus possible to completely close the tube 7 at one of its ends, to make, thanks to a tube 10 slightly shorter than the tube 7, electrical connections at this end inside the tube 7 and to be able to thus make all the electrical connections from the other end of the same tube 7. However, such a system has the disadvantage of a delicate assembly when it is necessary to introduce the two spirals 8 and 9 and the tube 10 simultaneously into the tube 7. Furthermore, the clearance required between the tubes 10 and 7 allows the tube 10 to vibrate noisily, causing shocks to the tube 7. This phenomenon is sometimes annoying.

In summary, with the prior art of FIGS. 1 and 2, there are expensive systems, fragile and limited in temperature because of the gold layer 4 or 11. In addition, there is a compulsory connection at both ends in the case of Figure 1 since the central partition 5 requires leaving the tube 2 to electrically connect the filaments 1 and 6, and, in the case of Figure 2, delicate assembly and noisy operation. In addition the rear wall of the tube unnecessarily heats up which decreases the thermal efficiency.

Figure 3 shows a heating element according to the invention. We see an oval tube 12 in transparent refractory material such as for example vitreous silica. Its section has the largest external dimension 18 mm and the smallest 9 mm, the walls are 1.5 mm thick, we also see filaments 13 which may be single or two in number or more, installed parallel to each other. others, they are made of an alloy based on iron, chromium and aluminum of helical shape, their diameter is from 2 to 10 mm for a diameter of the unitary wires from 0.2 to 1 mm. These filaments are flexible, they are associated in pairs in the figure and are supported by a gutter 14 (which is double in the figure). This gutter which therefore serves as a sheath for filaments such as 13 is made of a refractory material which has two essential characteristics, good thermal insulation and good reflection for the radiation emitted by the filament 13. Two materials have been successfully tested for constitute this gutter, a first based on ceramic fibers, the second based on porous silica. The latter material is silica implemented by a slip technique which is evaporated and then baked at high temperature (the so-called "slip-cast silica" technique). A molded product is obtained with good tolerances and, depending on the conditions of use, a controlled micro-porosity. The micro-porosity for producing the gutter according to the invention must be such that it leads to a conductivity of less than 0.35 Wm⁻¹K⁻¹. The silica in question has a particularly high hemispherical spectral reflectance, of the order of 85% for wavelengths greater than 0.8 μ. When the gutter is made with refractory fiber, it is advantageous to use ceramic fiber based on alumina and silica such as that of the brand KERLANE-Pyronappe 50. To obtain the shape of the gutter we will use preferably a mineral binder based on silicates.

The geometry chosen for the section of the gutter shown in Figure 3 is only an example. The shape according to the invention must be such that it allows easy introduction of the gutter-filament assembly inside the tube 12 and sliding over its entire length.

This shape must also allow precise positioning of the filaments 13. The other important criterion is the residual thickness between the rear of the filament 13 and the rear wall of the tube 12. This thickness must be sufficiently large given the conductivity of the material constituting the gutter 14 so that the heating of the rear face of the tube remains limited.

With a fiber of the type described above and whose conductivity at 1000 ° C is of the order of 0.2 Wm⁻¹.K⁻¹, a thickness of the order of 4 mm at the place thinner has proven satisfactory.

In FIG. 3, on the coolest side of the tube 12, towards the rear, there is an empty space 15. This is intended to receive a metal rod, not shown, which plays on the one hand a role of mechanical resistance for the 'introduction of the gutter into the tube 12 in the case of the gutter of fibrous material and also the role of electrical conductor in all cases. The section of this rod made of a temperature-resistant metal such as stainless steel for example has a suitable section, for example in the shape of a T, which provides it with good inertia in all directions.

In Figure 4, we see the type of connection which can advantageously be made in the case of two helical filaments. The metal rod shown schematically by a line 16 extends over the entire length of the tube not shown, parallel to the filaments 17, 18. The isolation of the three elements from each other is provided by the not shown gutter. The three conductors are electrically connected to each other at their distal end, that is to say the most distant. At the end where the electrical connection is made, the three conductors 16, 17 and 18 terminate in respective pins 19, 20 and 21 which allow a connection of the two filaments either in series or in parallel: in series the current is plugged between 20 and 21, in parallel, the pins 20 and 21 are electrically connected and the current is established between these pins and the pin 19.

The above description has made it possible to show that the invention presents a cheaper and more practical technique, in particular for the electrical connection, than the existing techniques.

Claims (10)

1. Tubular heating element comprising inside a refractory tube at least one incandescent filament housed inside the groove of a heat-reflecting refractory material, characterized in that the heat-reflecting material is thermally insulating. 2. Heating element according to claim 1, characterized in that it comprises two filaments parallel to the axis of the tube separated by the heat reflecting material. 3. Heating element according to claim 1, characterized in that the heat reflecting material has a conductivity at 1000 ° C less than 0.35 W m⁻¹ K⁻¹. 4. Heating element according to claim 3, characterized in that the conductivity is less than 0.25 W m⁻¹ K⁻¹. 5. Heating element according to one of the preceding claims, characterized in that the insulating material is based on ceramic fibers of silica and / or alumina. 6. Heating element according to claim 4, characterized in that the insulating material is based on silica obtained from a slip (cast-slip). 7. Heating element according to claim 1, characterized in that it comprises a metal rod parallel to the axis of the tube and electrically isolated from the filament (s). 8. Heating element according to claim 7, characterized in that an electrical connection is made at one end of the tube between the metal rod and the filament (s). 9. Heating element according to claim 2 and claim 8, characterized in that the electric current is established either between the free ends of the two filaments or between the metal rod on the one hand and the two filaments also joined by their free ends d 'somewhere else. 10. Heating element according to claim 8 or claim 9, characterized in that the refractory tube is hermetically sealed at the end where the electrical connection is not made.

Images