INFRARED RADIATOR AND COMPONENT EMITTING INFRARED RADIATION

Abstract

Known infrared radiators have a moulded body, which has a radiation surface that emits short-wave or medium-wave infrared radiation with a first peak emission wavelength. In order to provide, proceeding therefrom, an infrared radiator with an emissions spectrum which is well matched to absorption characteristics with a maximum around 2750 nm, and which furthermore can be operated with a high electrical power density, and with which the warming-up time in industrial applications, such as, for example, drying of inks, joining of plastics or bending of glass, can be shortened, according to the invention a radiation converter material is applied to at least a part of the radiation surface and, as a consequence of heating by the infrared radiation of the first peak emission wavelength, emits infrared radiation with a second peak emission wavelength which is of a longer wavelength than the first peak emission wavelength.

Claims

1. An infrared radiator having a molded body which has a radiation surface that emits short-wave or medium-wave infrared radiation with a first peak emission wavelength, wherein a radiation converter material is applied to at least a part of the radiation surface and, as a consequence of heating by the infrared radiation of the first peak emission wavelength, emits infrared radiation with a second peak emission wavelength which is of a longer wavelength than the first peak emission wavelength.

2. The infrared radiator according to claim 1, wherein the radiation converter material comprises a color pigment-containing coating material.

3. The infrared radiator according to claim 2, wherein the color pigment contains black mineral particles and is alkali-free.

4. The infrared radiator according to claim 1, wherein the radiation converter material comprises opaque quartz glass.

5. The infrared radiator according to claim 1, wherein the molded body is formed as a cladding tube made of quartz glass which surrounds a radiation emitter, provided with a power connection, in the form of a heating coil or a heating tape, wherein the radiation surface forms at least part of the tube's lateral surface.

6. The infrared radiator according to claim 5, wherein the radiation surface extends over a circumferential angle between 20 and 360, preferably between 60 and 200, and particularly preferably between 90 and 180 of the tube's lateral surface.

7. The infrared radiator according to claim 2, wherein the radiation converter material comprises a lower layer made of the opaque quartz glass and an upper layer made of the color pigment-containing coating material and applied to the lower layer, and in that at least part of the lateral surface of the cladding tube is covered by the lower layer, and at least one first circumferential section of the lower layer is coated by the upper layer.

8. The infrared radiator according to claim 7, wherein a specular reflector layerpreferably a gold-containing reflector layeris applied in a second circumferential section to the lower layer made of the opaque quartz glass.

9. The infrared radiator according to claim 8, wherein the first circumferential section and the second circumferential section do not overlap and preferably complement one another to form a circumferential angle of 360 degrees.

10. The infrared radiator according to claim 5, wherein at least part of the lateral surface of the cladding tube has a surface roughness, defined as an arithmetic mean roughness R.sub.a, with R.sub.a in the range of 0.5 to 5 m, and preferably in the range of 0.8 to 3.2 m, a first circumferential section of which forms the radiation surface to which the radiation converter material is applied.

11. The infrared radiator according to claim 10, wherein a reflector layer is applied to a second circumferential section of the lateral surface of the cladding tube, wherein the first circumferential section and the second circumferential section of the tube's lateral surface do not overlap, and preferably complement one another to form a circumferential angle of 360.

12. The infrared radiator according to claim 11, wherein the reflector layer is formed by a layer made of opaque quartz glass and/or by a metal-containing layerpreferably a gold-containing layer.

13. The infrared radiator according to claim 1, wherein the molded body is made in the form of a tile from a material emitting infrared radiation during heating, wherein the tile has planar sides positioned opposite each other, one of which planar sides comprises the radiation surface to which the radiation converter material is applied at least partially, and wherein a heating conductor track made of a resistance material and connected to an electrical contact for the supply of a heating current is applied to the other planar side.

14. The infrared radiator according to claim 13, wherein the tile material comprises a ceramicin particular, Al.sub.2O.sub.3 or ZrO.sub.2or in that the tile material comprises a composite materialin particular, a matrix made of quartz glassinto which elemental silicon or carbon is embedded.

15. A component emitting infrared radiation, with a base body made of a base body material, having an absorption surface for absorbing short-wave or medium-wave primary infrared radiation having a first peak emission wavelength, and a radiation surface for emitting secondary infrared radiation having a second peak emission wavelength which is of a longer wavelength than the first peak emission wavelength, wherein a radiation converter material that comprises a color pigment-containing coating material is applied to at least a part of the radiation surface.

16. The component according to claim 15, wherein the color pigment contains black mineral particles and is alkali-free.

17. The component according to claim 15, wherein the radiation converter material comprises opaque quartz glass.

18. The component according to claim 15, wherein the base body material is quartz glass.

19. The infrared radiator according to claim 5, wherein the radiation converter material comprises a lower layer made of the opaque quartz glass and an upper layer made of the color pigment-containing coating material and applied to the lower layer, and in that at least part of the lateral surface of the cladding tube is covered by the lower layer, and at least one first circumferential section of the lower layer is coated by the upper layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0059] The invention is explained in more detail below with reference to an exemplary embodiment and a drawing. The following are shown in detail:

[0060] FIG. 1 a cross-sectional and schematic view of an embodiment of a short-wave quartz tube radiator, a radiation converter material being applied to the cladding tube on the outside;

[0061] FIG. 2a a schematic view of a further embodiment of a short-wave quartz tube radiator based upon the base form shown in FIG. 1;

[0062] FIG. 2b a photograph of the embodiment of the short-wave quartz tube radiator according to FIG. 2a;

[0063] FIGS. 3 through 5 further embodiments of a short-wave quartz tube radiator based upon the base form shown in FIG. 1.

[0064] FIGS. 6 and 7 a cross-sectional and schematic view of further embodiments of a short-wave quartz tube radiator, a radiation converter material being applied to the cladding tube on the outside;

[0065] FIG. 8 a cross-sectional and schematic view of an embodiment of a tile-shaped infrared radiator, a radiation converter material being applied to the radiation surface;

[0066] FIG. 9 a diagram of the radial emission of a short-wave quartz tube radiator according to FIG. 7;

[0067] FIG. 10 a diagram of the diffuse and direct transmission of a short-wave quartz tube radiator according to FIG. 1; and

[0068] FIG. 11 a diagram with the result of measurements, over time, of the irradiance in the case of a short-wave quartz tube radiator according to FIG. 1 as compared to a short-wave quartz tube radiator according to FIG. 4.

[0069] FIG. 1 shows schematically a first basic variant of the infrared radiator according to the invention. It is a short-wave infrared radiator with a lamp tube 1 made of quartz glass. The lamp tube 1 is closed on both sides and surrounds a tungsten heating wire (not shown), which is provided with an electrical connection and can be heated to temperatures up to 2,300 C. The lateral surface of the lamp tube is coated completely (360 degrees) with a QRC layer 2 made of opaque quartz glass, which acts as radiation converter material.

[0070] The QRC layer 2 on the outer lateral surface of the lamp tube 1 is produced in accordance with the known slip method described in DE 10 2004 051 846 A1. Here, the pourable, aqueous SiO.sub.2 slurry is applied as a slurry layer onto the lamp tube 1, and the slurry layer is then dried and vitrified to form the QRC layer 2. It consists of porous opaque quartz glass. It has a density of about 2.15 g/cm.sup.3 and a mean layer thickness in the range of 0.5 to 2 mm. Its surface area is open-pored, as shown by a color penetration test.

[0071] The QRC layer 2 converts the short-wave primary radiation of the infrared radiator into longer wave, secondary radiation with a peak emission wavelength of approximately 2,750 nm. As a result, it is possible to operate the short-wave infrared radiator with an emission spectrum that is well matched to the heating application at 700 to 800 C. and to enable rapid temperature changes and nevertheless achieve a high electrical power density of more than 50 W/cmfor example, of at least 120 W/cm.

[0072] The electrical power density in the unit, electrical power per heated length (W/cm), is almost 100% converted to optical power (W/m.sup.2). A power density of a short-wave infrared radiator with, for example, 120 W/cm is converted to primary radiation with a first peak emission wavelength and, by the use of the radiation converter material such as, for example, the QRC layer 2, to medium-wave infrared radiation with a peak emission wavelength which is of a longer wavelength; for example, at a distance of 200 mm (from the heating filament), approximately 1.2 kW/m.sup.2 in total arrives at the detector.

[0073] The entire lateral surface of the cladding tube acts as a radiation surface, i.e., the infrared radiator has a three-dimensional emission characteristic.

[0074] FIGS. 2 through 5 show modifications of the basic variant of FIG. 1 with additional layers. The representations are not to scale; in particular, the thicknesses of the additional layers can be shown thicker to improve recognizability.

[0075] In the modification of the basic variant shown in FIG. 2a, half (front side) of the QRC layer 2 (180 degrees) is blackened by being coated with a lacquer layer 3 made of a temperature-stable black lacquer. The radiation surface corresponds here to the lamp tube's lateral surface coated with the blackened lacquer layer 3. The lacquer layer retains its black colorand thus also its emission spectrumeven during heating to 800 C. and beyond.

[0076] The lacquer layer 3 is produced by spraying or brushing on a thermal paint. The thermal paint is alkali-free. It contains an alumino-silicate solution (10 to 20 wt %), copper chromite black spinel as mineral color pigment (25 to 35 wt %), and water (40 to 60 wt %). Suitable thermal paints are offered as oven paints by, for example, the companies ULFALUX Lackfabrikation GmbH and Aremco Products, Inc., wherein the following are indicated as further organic ingredients: xylene, ethyl acetate, butyl acetate, ethyl benzene.

[0077] Multiple painting ensures a completely closed layer. After spraying, the thermal paint is dried at 250 C. and is then touch-resistant. The final state is achieved by heating the lacquer layer 3 to 1,200 C. Such heating can take place when putting the infrared radiator into operation. Ceramic components are sintered onto the lamp tube's surface or onto the surface of the QRC layer to produce a solid substance-to-substance bond so that the lacquer layer 3 is largely scratch-resistant. The lacquer layer 3 is approximately 40 m thick. The manufacturer has specified the emissivity of the lacquer layer 3 to be above 90% at 800 C.

[0078] The QRC layer 2 lying underneath the lacquer layer 3 generally shows open porosity and acts as an adhesion promoter. By flame polishing the surface of the QRC layer 2, the lacquer can be prevented from penetrating into the porous surface structure, as a result of which a visually more appealing surface structuring is achieved. The fire polishing takes place by heating the QRC layer 2 with an oxyhydrogen burner. As a result, very high temperatures around 1,800 C. are produced locally, which enables the production of a glass film as thin as possible within a few seconds, which seals the porous surface.

[0079] During operation, the thin black lacquer layer 3 heats up to 700-750 C. within a few seconds and thus emits infrared radiation in a medium-wave range (preferably in the wavelength range of 2,500 to 3,500 nm). The following applies: absorption=emission; i.e., the short-wave radiation emitted on by the lamp tube 1 and rapidly absorbed in the lacquer layer 3 releases the virtually identical energy with high intensity just as quickly, but at a lower temperature (i.e., in the medium-wave range). The black lacquer layer 3 acts as a radiation converter in that it converts high-energy short-wave radiation to medium-wave radiation of high intensity. Rapid energy supply response times in the seconds range are possible as a result of the short-wave tungsten heating filament.

[0080] The photograph of FIG. 2b shows the embodiment of the infrared radiator of FIG. 2a in a three-dimensional view. In addition, the electrical connections la led out at one end of the lamp tube 1 can be seen here.

[0081] In the modification of the basic variant shown in FIG. 3, half of the QRC layer 2 (180 degrees) is coated with a rear-side reflector layer in the form of a gold layer 4.

[0082] The gold layer 4 is produced by applying, with a brush, a gold-containing emulsion (gold resinate) onto the surface of the QRC layer 2, which is open-pored or which has been sealed by thermal treatment. The emulsion is subsequently baked by heating. During baking, the gold resinate resolves into metallic gold and resin acid which, like the other components of the paste, are volatilized by the high baking temperature. What remains is a closed, specular gold layer 4 that acts as a reflector and whose thickness is preferably in the range of 50 to 300 nm, depending upon the reflectance requirement. The thicker the layer, the higher the reflectance. Here, the radiation surface corresponds to half (180 degrees) of the lamp tube's lateral surface coated by the QRC layer 2 but not by the gold layer 4.

[0083] The gold layer 4 reduces the emissivity in the region of the lamp tube's rear side and effects a very good reflection of the radiation, which is reflected forwards onto the lacquer layer 3 and absorbed there. This amount of radiation contributes considerably to the rapid heating of the black lacquer layer 3.

[0084] In the modification of the basic variant shown in FIG. 4, the entire QRC layer 2 (360 degrees) is coated with a 0.04 mm thick lacquer layer 3 made of thermal paint (production and properties are explained with reference to FIG. 2). The entire lateral surface of the cladding tube acts as a radiation surface, i.e., the infrared radiator has a three-dimensional emission characteristic. In the modification of the basic variant shown in FIG. 5, one half of the surface of the QRC layer 2 (180 degrees) is coated with a 0.04 mm thick lacquer layer 3 made of thermal paint (production and properties are explained with reference to FIG. 2), and the non-congruent half of the surface (180 degrees) is coated with a 0.1 mm thick gold layer 4 (production and properties are explained with reference to FIG. 3). Here, the radiation surface corresponds to half (180 degrees) of the lamp tube's lateral surface, which is coated by the lacquer layer 3 but not by the gold layer 4.

[0085] FIG. 6 shows schematically a first basic variant of the infrared radiator according to the invention. This too is a short-wave infrared radiator with a lamp tube 1 made of quartz glass.

[0086] The lamp tube 1 is closed on both sides and surrounds a tungsten heating wire (not shown), which is provided with an electrical connection and can be heated to temperatures up to 2,300 C.in the case of halogen radiators up to 3,000 C.and predominantly emits in the short-wave range.

[0087] Typical infrared radiators with a quartz tube have a gold reflector or a diffuse reflector made of QRC or ceramic on their rear side in order to bring the radiation energy forwards into the heating material via the radiation surface of the transparent quartz glass lamp tube 1.

[0088] In this embodiment of the infrared radiator, one half of the lateral surface of the lamp tube 1 (180 degrees) is blackened by being coated with a lacquer layer 3 made of a temperature-stable black lacquer (production and properties are explained with reference to FIG. 2). The radiation surface corresponds here to the lamp tube's lateral surface coated with the blackened lacquer layer 3. In another embodiment (not shown) of the infrared radiator, the entire lateral surface of the lamp tube 1 (360 degrees) is blackened.

[0089] On a smooth lateral surface of the lamp tube, the black lacquer layer 3 may, under certain circumstances, flake off at high temperature over a few hundred hours. To improve the adhesion of the lacquer layer 3, the lamp tube's surface is roughened. The region of the roughening 6 is symbolized by a dashed line.

[0090] Roughening is carried out mechanically by sandblasting or grinding, or chemically by treatment with an etching solution. A suitable etching solution (NH.sub.4+HF+acetic acid) and its application for roughening a quartz glass surface are described in DE 197 13 014 C2. The mean roughness depth R.sub.a is preferably in the range of 0.8-3.2 m; in the exemplary embodiment, it is 3 m. Roughening 6 not only brings about a better bond between the lacquer layer 3 and the lamp tube's surface, but also an even more homogeneous distribution of the medium-wave radiation by scattering the radiation at the roughened surface. The radial distribution of the converted radiation is distributed very uniformly to the front and over half the circumference of the quartz tube (see radial distribution according to FIG. 9).

[0091] The black lacquer layer 3 acts as a radiation converter and emits in the medium-wave range at temperatures in the range of 700 to 750 C. Endurance tests have shown that the lacquer layer or the infrared radiator can achieve a service life of up to 10,000 hours in the absence of visual or functional impairments.

[0092] The heating of the lacquer layer 3 to about 700 C., i.e., up to medium-wave emission at approximately 3 m, takes approximately 10 s. In comparison, standard medium-wave infrared radiators require approximately 5 min to reach thermal equilibrium.

[0093] In the modification shown in FIG. 7 of the second basic variant according to FIG. 6, the half of the lamp tube's surface (180 degrees) that is not coated by the lacquer layer 3 is with a 0.1 mm thick gold layer 4 (production and properties are explained with reference to FIG. 3). Here, the radiation surface corresponds to half (180 degrees) of the lamp tube's lateral surface, which is coated by the lacquer layer 3 but not by the gold layer 4.

[0094] FIG. 8 shows schematically a planar, tile-shaped infrared radiator 8 made of a composite material of quartz glass and elemental silicon embedded therein, as described in DE 10 2015 119 763 A1. A heating conductor track (not shown) is applied to the tile-shaped base body 9 of the infrared surface radiator 8 and heats the base body during current flow, so that the latter emits infrared radiation. Such infrared radiators 8 reach an emissivity of approximately 0.82 at a wavelength of 2.75 m at a temperature of 1,000 C. This wavelength represents the peak wavelength at this temperature. The composite material loses its high emissivity as the temperature drops. To counteract this, a layer 10 made of a radiation converter material is applied to the radiation surface.

[0095] In one embodiment, this is a QRC layer 2. The production and properties of which are explained with reference to FIG. 1. Another embodiment is a lacquer layer 3. The production and properties of which are explained with reference to FIG. 3. Or, in a third embodiment, it is a combination of a lower layer that is a QRC layer 2, and an upper layer that is a lacquer layer 3. In FIG. 8, these possible combination are symbolized by the combined reference number 2/3. The coating of the radiation surface with a layer made of radiation converter material makes it possible for a high emissivity to be maintained, regardless of the temperature of the base body 9. In this way, the already high efficiency of the tile-shaped infrared radiator is further increased, since the emissivity is high even at lower temperatures, and energy transmission can thus be carried out in the best possible manner.

[0096] The tile 9 has plate sizes of up to 400400 mm.sup.2, with a thickness of up to 2 mm. Alternatively, the tile 9 consists of ceramic material, such as aluminum oxide or zirconium oxide. The thermal excitation of the ceramic is made possible by means of resistance heaters. A lacquer layer 3 is applied to the radiation surface of the tile 9 (production and properties are explained with reference to FIG. 3). The lacquer layer 3 emits the majority of the absorbed energy by means of radiation. The temperature of the ceramic tile 8 determines the peak emission wavelength. Temperatures up to 1,100 C. can be reached. With ceramic tiles, even larger dimensions than the above-mentioned, as well as curved geometries, are particularly easy to realize.

[0097] A radial measurement was carried out to determine the effect that the radiation converter material has on the radial distribution of the emitted optical power density (irradiance). Radial measurement is carried out in the usual manner using an infrared radiator mounted onto a rotatable support, which rotates by 360 degrees in 5-degree increments. A thermopile sensor mounted at a distance of 25 cm detects the radiation emitted by the infrared radiator. In the diagram of FIG. 9, on the circle radius, the normalized irradiance (rel. unit) is plotted against the circumferential angle position (in degrees) of the measuring points. The measurement curve shows the result of the radial measurement for an infrared radiator according to FIG. 7 with a lacquer layer 3 on the front side (radiation surface) and a specular gold layer 4 on the rear side.

[0098] In the rear radiator chamber 90, the measurement curve A shows a small proportion of irradiation intensity. This is composed of transmitted primary radiation and of secondary radiation, which is due to the heating of the gold layer 4. In the actual irradiation field 91, however, the measurement curve shows a high irradiance and a homogeneous distribution of the medium-wave radiation. The radial distribution of the converted radiation is distributed uniformly to the front and over half the circumference of the quartz tube.

[0099] In the diagram of FIG. 10 relating to the transmission of a short-wave quartz tube radiator according to FIG. 1, the total transmission T (in %) determined per Ulbricht sphere is plotted against the wavelength A (in nm). The Ulbricht sphere allows the measurement of the direct hemispherical spectral transmittance that comprises diffuse and direct transmission.

[0100] It is found that, after the infrared radiator has been switched on, a significant amount of the primary radiation is emitted by transmission through the QRC layer 2 due to multiple reflections. The non-transmitted radiation heats the quartz glass casing tube 1 together with the QRC layer 2 over time, and thus additionally generates secondary radiation in the medium-wave range. Thermal equilibrium is reached after a few minutes, and the infrared radiator emits a broadband spectrum consisting of short-wave primary and medium-wave secondary radiation.

[0101] The diagram of FIG. 11 shows measurements, over time, for an infrared radiator according to FIG. 1 in comparison to an infrared radiator according to FIG. 4. The optical power P (in W/m.sup.2) is plotted on the y-axis against the power-on time t (in s) which is plotted on the x-axis. A measurement, over time, of the irradiance shows that the infrared radiator (FIG. 1) coated only with a QRC layer 2 generates approximately 50% of the maximum irradiance directly after being switched on. It then takes approximately 4 minutes until full optical power is reached. In the case of the infrared radiator (FIG. 4), which is additionally completely blackened by means of the lacquer layer 3, the irradiance rises more slowly, but reaches the maximum power earlier, after about 3 min., due to the higher absorption. Above all, the rapid availability of some of the total optical power is advantageous for the application in the printing industry because the use of shutter systems for shading the paper web against the infrared radiators, which are still hot despite having been turned off, can be dispensed with.

[0102] In the case of the tile-shaped infrared radiators of the invention, which are explained with reference to FIG. 8, a heating conductor track is provided on one of the plate sides of the tile, said heating conductor track generating heat in the event of a current flow and delivering said heat to the tile by heat conduction, thereby heating the tile. The described tiles without the heating conductor track can be used as passivecurrent-freeheating elements if they are heated by an external heating source emitting medium-wave or short-wave infrared radiation instead of by the heating conductor track. The coatings with the radiation converter material may have the same effect, as explained abovefor example, with reference to FIG. 8.