Infrared emitter with a cladding tube and a heating element arranged therein

Abstract

Known infrared emitters (1) have a cladding tube (2) and a heating element (3) arranged therein for emitting infrared radiation. In order to provide, proceeding herefrom, an infrared emitter which is characterised by a particularly long life, it is proposed that the cladding tube (2) has a surface with a microstructure (5).

Description

[0001] DESCRIPTION Infrared radiator with a cladding tube and a heating element arranged therein Technical field The invention relates to an infrared radiator. In particular, the invention relates to an infrared radiator comprising a cladding tube and a heating element arranged in the cladding tube for emitting infrared radiation. Furthermore, the invention relates to the use of a microstructured surface of the cladding tube of an infrared radiator for a new purpose. Infrared radiators are designed to emit radiation in the infrared spectral range. They have a cladding tube in which a heating element is arranged, for example a heating band made of carbon or a heating coil made of tungsten, molybdenum, tantalum, niobium or an alloy of these elements. The cladding tube serves to protect the heating element from mechanical and / or thermal stress.Known infrared emitters can be divided into short-wave, medium-wave, and long-wave infrared emitters based on their primary emission wavelength. The primary emission wavelengths of short-wave infrared emitters are in the range from 0.78 µm to 1.4 µm (= IR-A, rated temperature 1,800°C – 3,450°C, according to IEC 62798:2014, Section 4, Classification of infrared emitters by spectral emission, Table 1), medium-wave infrared emitters in the range above 1.4 µm to 3 µm (= IR-B, 690°C – 1,800°C), and long-wave infrared emitters in the range above 3 µm to 1 mm (= IR-C, < 690°C). State of the art: Known infrared radiators are used for heating and drying materials in a wide variety of industrial manufacturing processes. An infrared radiator with a sheath tube and a heating element arranged therein is known, for example, from DE 102011108421 B3.If an infrared radiator is supplied with an electrical power Pel, it emits radiation with an optical power Popt. During operation of the infrared radiator, the temperature of the heating element increases until it reaches operating temperature. The heating element itself acts as the primary radiator; depending on its chemical composition, it can reach temperatures of up to 2,700K under operating conditions. At the same time, the cladding tube also heats up during operation of the infrared radiator. Cladding tube temperatures above 900K are not uncommon. This means that the cladding tube also acts as a radiator, in this case as a secondary radiator. The ratio of the optical power of the primary radiator Popt, primary, to the optical power of the secondary radiator Popt, secondary, is often more than 80. This means that, all other conditions being equal, over 80 times more energy is emitted by the heating element than by the cladding tube.The radiation emission through the cladding tube is therefore of secondary importance compared to the radiation from the heating element. Nevertheless, a high cladding tube operating temperature impairs the cladding tube's service life. This is because an excessively high cladding tube temperature can damage the cladding tube and reduce the overall service life of the infrared beam. However, with regard to many industrial applications, it is often desirable to be able to operate the infrared radiators used therein for as long as possible. This reduces assembly and downtimes as well as costs. Technical problem The present invention is based on the technical problem of providing an infrared radiator with a particularly long service life. Furthermore, the present invention is based on the technical problem of specifying a use of a microstructured cladding tube surface for a special purpose.Summary of the Invention With regard to the infrared radiator, the above-mentioned object is achieved by an infrared radiator having the features of claim 1. In particular, starting from an infrared radiator of the type mentioned at the outset, the object is achieved according to the invention in that the cladding tube has a surface with a microstructure. Cladding tubes for infrared radiators are made, for example, of quartz glass or ceramic. The present invention is based on the finding that high cladding tube temperatures frequently lead to damage to the cladding tube and can thus contribute to a reduced service life of the infrared radiator. According to the Stefan-Boltzmann law, the optical power Popt of an infrared radiator is directly proportional to the fourth power of the temperature of the radiating body, its emissivity ε, its surface area A, and a constant C that can be neglected in this context. The following applies: P. opt= C * e * A * T 4, with C: constant, ε: emissivity, A: surface area of ​​the radiating body and T: temperature of the radiating body. According to this, for the same optical power Popt, the temperature of the radiating body decreases if the emissivity ε and / or the surface area A of the radiating body are increased. The Stefan-Boltzmann law applies equally to the primary radiator, here the heating element, and to the secondary radiator, namely the cladding tube. Applied to the cladding tube, this means that the temperature of the cladding tube can be reduced by increasing the emissivity ε and / or the surface area A of the cladding tube. The invention is based on the idea of ​​exploiting a targeted increase in the emissivity ε and / or the surface area A of the cladding tube in order to passively cool the cladding tube.Tests have shown that the emissivity ε and the surface area A of the cladding tube can be increased in the IR-B and IR-C range if the cladding tube surface is provided with a microstructure. This applies in particular to the technically important wavelength range from 800 nm to 2,500 nm. The term "microstructure" describes the structure of the cladding tube surface when viewed on a microscopic scale; it is explained in more detail in the "Definitions" section. The outer surface and / or the inner surface can be provided with the microstructure. The surface of the cladding tube can be fully or partially covered with the microstructure. The microstructure can be periodic, regular, or chaotic. The increase in the emissivity ε of the cladding tube, like the increase in the surface area A of the cladding tube, is accompanied by a reduction in the cladding tube temperature.By using a cladding tube with a microstructured surface, the temperature of the cladding tube can be lowered without reducing the electrical power Pel or the optical power Popt or requiring active air or water cooling. This increases the service life of the infrared radiator. It has proven advantageous if the surface with the microstructure has a surface roughness in the range of 0.1 µm to 50 µm. The term "surface roughness" is explained in more detail in the "Definitions" section. The emissivity ε of the cladding tube depends on the surface roughness of the cladding tube surface with the microstructure. Basically, the smoother the surface of the cladding tube, the lower the emissivity ε and the surface area A of the microstructure. For a surface with a surface roughness of less than 0.1 µm, the emissivity-enhancing effect of the microstructure is small.For a surface with a surface roughness of more than 50 µm, the increased emission can be accompanied by disadvantages, such as reduced mechanical stability. In a first preferred embodiment of the infrared radiator according to the invention, the microstructure is created by mechanically processing the surface, chemically processing the surface, devitrifying the surface, or processing the surface with a laser or sandblasting. The microstructure can be created in a variety of ways. Mechanical surface processing methods include abrasive processes such as sandblasting, sanding, or grinding. However, the microstructure can also be created chemically, for example by etching, in particular by a special surface treatment, for example by Heraeus. ®Special Surface Treatment (SST). Furthermore, the microstructure can also be created with a laser. A microstructure with periodic surface structures can be created, for example, with direct laser interference structuring (DLIP). Alternatively, the microstructure can be created by devitrification of the transparent cladding tube, for example by surface doping or alkali / alkaline earth / Al2O3 application and heating the cladding tube to temperatures above 700°C. In a second, equally preferred embodiment of the infrared radiator according to the invention, the microstructure is created by a surface coating, wherein the surface coating is a quartz glass layer based on SiO2 or a ceramic layer based on a temperature-resistant ceramic, which preferably contains components selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3.The surface coating is preferably produced by thermal spraying of powdered starting material. The surface coating covers the cladding tube surface completely or it covers a partial area thereof, preferably at most a half-shell of the cladding tube surface with a central angle of no more than 180 degrees. The surface coating can, for example, be a quartz glass layer based on SiO2, i.e. the quartz glass layer contains at least 50 wt.% SiO2. In the simplest case, the surface coating is a coating of quartz particles whose particle size (D90 value) is in the range of 50 nm to 50 µm. The quartz particles are sintered mechanically firmly to the surface of the cladding tube at temperatures around 1,200°C. A layer of quartz glass particles has a similar coefficient of expansion to a cladding tube made of quartz glass.It is therefore characterized by good temperature resistance, mechanical resistance, and chemical resistance. Furthermore, it can be applied to a quartz glass cladding tube in a comparatively thick layer of more than 1.5 mm up to 5 mm. The quartz glass layer thickness is preferably in the range of 0.25 mm to 5 mm. However, quartz glass layers with layer thicknesses greater than 5 mm are prone to shrinkage cracks. Alternatively, the surface coating is a ceramic layer based on a temperature-resistant ceramic. This means that the ceramic layer consists of at least 50 wt.% of the temperature-resistant ceramic. The ceramic layer can contain a single component selected from the group consisting of TiO2, Al2O3, SiC, Si3N4, BN, and Y2O3, or it can be a mixed ceramic containing one or more of the aforementioned components. The mixed ceramic makes up at least 50 wt.% of the ceramic layer.It has proven advantageous if quartz glass particles, ceramic particles, and / or particles of elemental silicon are embedded in the surface coating, which contribute to the surface microstructure. The quartz glass, ceramic, and / or silicon particles are advantageously applied to the cladding tube as part of a layer. The quartz glass, ceramic, and / or silicon particles form surface elevations of the layer and thus contribute to the microstructure. They are temperature-stable up to 1,000°C. The quartz glass and / or ceramic particles preferably have an average particle size (D50) in the range of 50 nm to 50 µm. For semiconductor materials such as silicon, the absorption coefficient in the wavelength range from 1 µm to 5 µm is a function of the absorbed wavelength. The elemental silicon is preferably present in the layer as particles with an average particle size (D50) in the range of 1 µm to 200 µm.A layer interspersed with silicon particles can be used in air; it leads to an increase in the emissivity ε of the cladding tube. It has proven advantageous if the surface coating covers a surface area in the range of 1 m. 2 / g up to 50 m 2 / g, preferably in the range of 2 m 2 / g up to 20 m 2 / g. Basically, the larger the surface area of ​​the surface coating, the more pronounced the reduction in the cladding tube temperature. A surface coating with a surface in the range specified above is simple and cost-effective to produce. In addition, it generally has good mechanical stability. It has proven advantageous if the ceramic layer has a layer thickness in the range of 10 µm to 300 µm, preferably in the range of 10 µm to 150 µm. The ceramic layer can be produced by immersion, by application, for example using a brush, and subsequent firing, and particularly advantageously by thermal spraying. Preferably, the ceramic layer is produced by thermal spraying of powdered starting material, for example by thermal spraying of Al2O3 powder with a D50 value in the range of 0.7 µm to 3 µm.The layer thicknesses are advantageously in the range from 100 µm to 300 µm, preferably in the range from 10 µm to 150 µm, particularly preferably in the range from 100 µm to 200 µm. With a layer thickness of less than 10 µm, the service life-extending effect of the ceramic layer is only slightly pronounced. A layer thickness of more than 300 µm is not very stable due to the different expansion coefficients of quartz glass and ceramic and is prone to chipping. The quartz glass layer advantageously has a layer thickness in the range from 50 µm to 5 mm, preferably in the range from 250 µm to 2.5 mm, preferably a layer thickness of less than 1.5 mm. In principle, the greater the layer thickness of the quartz glass layer, the higher the emissivity ε and the lower the cladding tube operating temperature. From a layer thickness of 2.5 mm, the emissivity ε increases only slightly; layer thicknesses of more than 2.5 mm are complex and expensive.At a layer thickness of less than 50 µm, the emissivity-enhancing effect of the quartz glass layer is minimal. It has proven effective if the quartz glass layer comprises several layers, with each layer having a thickness in the range of 50 µm to 250 µm. In principle, a layer thickness that is as uniform as possible is desirable. In particular, layer thicknesses of more than 1 mm can only be achieved with a uniform layer thickness (with a maximum deviation of ± 5%) by applying a single layer, which is difficult and time-consuming. Applying multiple layers also has the advantage that stresses in the coating can be reduced. The multiple layers are preferably created by spraying. A layer thickness in the range of 50 µm to 250 µm has proven particularly advantageous. Layers with a thickness in the above-mentioned range are particularly easy to produce. A layer thickness of less than 50 µm usually requires the application of several layers.This is complex and cost-intensive. Above a layer thickness of 250 µm, the sprayed-on layers tend to run, so that the cladding tube coating can have regions of greater and lesser layer thickness. In a particularly preferred embodiment of the infrared radiator according to the invention, the heating element is coated with an emission layer. The operating temperature of the cladding tube can be further reduced if the emissivity ε' of the heating element and / or the surface area A' of the heating element are increased by suitable measures. As already explained above, the Stefan-Boltzmann law applies equally to the primary radiator, in this case the heating element. By increasing the emissivity ε' and / or the surface area A', the temperature of the heating element decreases for the same electrical power Pel and optical power Popt.A lower heating element temperature, in turn, is associated with a lower cladding tube temperature and thus a longer service life of the infrared radiator. By coating the heating element with the emission layer, the emissivity ε' and, secondarily, the surface area A' of the heating element can be increased. This is because a heating element with an emission layer exhibits increased emission compared to an identical heating element without an emission layer. The higher the emissivity ε', the more energy the heating element can release into its environment per unit of time. The emission layer preferably has a surface roughness Ra in the range of 1 µm to 30 µm. Advantageously, the emission layer is a lacquer layer and contains an inorganic color pigment with black mineral particles and / or the emission layer contains quartz glass particles and elemental silicon. Advantageously, the emission layer is a black lacquer layer.Color pigments that appear black in the visible wavelength range usually also absorb and emit light in the relevant infrared wavelength range. Inorganic color pigments are thermally stable; they are fixed to the deposition surface during firing. The color pigment can also be formed by thermal decomposition or chemical reaction of a precursor substance during or before firing. The color pigment preferably contains black mineral particles, for example, copper chromite black spinel or manganese ferrite black pigment. The emission layer is advantageously made of a coating material containing an inorganic color pigment with black mineral particles or quartz glass particles and elemental silicon. The coating material is, for example, a paste or a varnish.If the coating material is alkali-free, the cladding tube surface will not be devitrified, i.e., it will not crystallize; it will therefore not lose its optical quality due to the application of the coating material. An emission layer made of quartz glass particles and elemental silicon also exhibits good emissivity and an emissivity-enhancing effect with regard to the heating element. The silicon particles preferably have a particle size in the range of 1 µm to 200 µm and / or the quartz glass particles a particle size in the range of 50 nm to 50 µm. The coating layer preferably contains ceramic particles. It has proven effective if the coating layer has a coating thickness in the range of 50 µm to 300 µm. With a layer thickness of less than 50 µm, the passive cooling effect of the emission layer is low. A layer thickness of more than 300 µm requires increased manufacturing effort; for example by applying it in layers.At the same time, with increasing layer thickness, the risk of the emission layer flaking increases due to the different expansion coefficients when exposed to large temperature differences during operation. A heating element made of molybdenum disilicide (MoSi2) has a particularly high emissivity ε'. With regard to the use, the above-mentioned technical problem is solved according to the invention by microstructuring the surface of a cladding tube for the purpose of cooling the cladding tube during operation of the infrared radiator. The outer surface and / or the inner surface of the cladding tube can be provided with the microstructure. The surface of the cladding tube can be completely or partially provided with the microstructure. The microstructure can be periodic, regularly structured, or chaotic.Increasing the emissivity ε of the cladding tube, like increasing the surface area A of the cladding tube, is accompanied by a reduction in the cladding tube temperature. By using a cladding tube with a microstructured surface, the temperature of the cladding tube can be lowered without reducing the electrical power Pel or the optical power Popt or requiring active air or water cooling. This increases the service life of the infrared emitter. It has proven advantageous for the surface with the microstructure to have a surface roughness in the range of 0.1 µm to 50 µm.It has proven particularly effective if the microstructured surface in the form of a quartz glass layer based on SiO2 or in the form of a ceramic layer based on a temperature-resistant ceramic, preferably selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3, is used to cool the cladding tube during operation of the infrared radiator. Likewise, a surface coating applied to the surface of a cladding tube of an infrared radiator in the form of a quartz glass layer based on SiO2 or in the form of a ceramic layer based on a temperature-resistant ceramic, preferably selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3, can be used to extend the radiator's service life. The service life of an infrared radiator is measured according to IEC 62798:2014. Instead of the previously usual maximum service life of up to 10,000 hours, the use according to the invention allows for lamp service lives of 25,000 to 50,000 hours.000 hours have been achieved. The features discussed above with regard to the infrared radiator also apply accordingly to the use of a surface coating applied to the surface of the cladding tube. In particular, the use claims relate to the use of a surface coating applied to a surface of a cladding tube of an infrared radiator according to one of claims 1 to 13.The method for producing an infrared radiator with a cladding tube and a heating element arranged therein for emitting infrared radiation is characterized in that a microstructure is created on a surface of the cladding tube, for example by mechanically processing the surface, chemically processing the surface, devitrifying the surface or by processing the surface with a laser or by sandblasting or by applying a coating precursor material to the cladding tube to produce a quartz glass layer based on SiO2 or a ceramic layer based on a temperature-resistant ceramic, which preferably contains components selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3 and heating the coating precursor material to form the quartz glass layer or the ceramic layer. Definitions Individual terms from the above description are defined additionally below.The definitions form part of the description of the invention. In the event of a contradiction between one of the following definitions and the rest of the description, the statement in the rest of the description shall prevail. If a measurement method is not specified for a parameter, the standard measurement method shall be used for this parameter, and in particular the measurement method laid down in the corresponding ISO standard whose publication date is closest to that of the present application. If measurement conditions are not specified, the standard conditions (SATP conditions) shall apply: temperature 298.15 K (25°C, 77°F) and absolute pressure 100 kPa (14,504 psi, 0.986 atm). Microstructure The term microstructure describes height differences relative to the arithmetic mean of the surface in the size range from 0.05 µm to 250 µm, preferably from 0.1 µm to 25 µm.The microstructure therefore largely belongs to the microscopic range, which can only be detected with a microscope, but also partly to the visible range. One measure of the microstructure is, for example, the surface roughness Ra. Surface roughness Ra The surface roughness Ra is determined in accordance with EN ISO 25178-2. The surface roughness Ra results from the absolute value of the height difference of each point compared to the arithmetic mean of the surface. A white light interferometer is used to determine the surface roughness Ra. White light interferometry (WLI) allows 3D profile measurements of structures with lateral dimensions between a few centimeters up to approx. 0.5 µm and vertical dimensions of a few 100 µm down to 0.1 nm. A Zygo NexView™ NX2 measuring device, which has a height resolution of < 1 nm, was used for the values ​​presented in this application.Specific surface area (BET) The specific surface area is determined based on a sorption measurement according to the method of Brunauer, Emmet and Teller (BET method) based on DIN ISO 9277:2003-05. The measuring instrument is a NOVA-3000 (from Quantachrome), which operates according to the SMART method (Adaptive Rate Dosing Sorption). Aluminum oxide SARM-13 and SARM-214 from Quantachrome are used as reference materials. The saturation vapor pressure of nitrogen (N24.0) is determined, and the sample is dried under vacuum at 200 °C for 1 hour. After cooling, the weight of the sample is determined, and then degassed, evacuating it to an absolute pressure of 200 mbar. In the pressure range in which monolayers and multilayers of adsorbed molecules form, the specific surface area (BET-SSA) is determined from the multilayer adsorption isotherm (BET isotherm) according to Brunauer, Emmett, and Teller.Particle size distribution and mean particle size. Statistical parameters can be derived from a particle size distribution, which shows the cumulative volume of the particles as a function of the particle size. For example, the D10 value indicates the particle size not reached by 10% of the cumulative volume of the particles, and the D50 value indicates the particle size not reached by 50% of the cumulative volume. The D50 value corresponds to the mean particle size. The particle size and particle size distribution are determined using laser diffraction on a dispersed sample in accordance with ISO 13320. The measuring device used is a Malvern Mastersizer 3000. It is equipped with a HeNe laser, a blue LED, and a wet dispersion unit for measurements at ambient temperature (23 °C).The wet dispersion unit is set to an ultrasonic power of 80%, and water serves as the dispersant. The D50 values ​​of the particle size distribution are determined using the 21 CFR device software with a form factor of 1. The D50 value indicates the particle size that is not reached by 50% of the cumulative particle volume (median particle size). Particle sizes greater than 315 µm and corresponding size distributions were determined by sieve analysis using an "Air Jet RHEWUM LPS 200 MC" sieve device (RHEWUM GmbH). Alternatively, the particle size can also be determined using a digital microscope, for example, a digital microscope from the VHX-7000 series from Keyence Deutschland GmbH. Emissivity ε Every body emits heat rays based on its temperature. The emissivity ε indicates how much radiation a body emits compared to a black body.According to Kirchhoff's radiation law, the radiant power emitted by any body is equal to that of a black body at the same temperature multiplied by its emissivity. The following applies: P = ε • Ps; where 0 ≤ ε ≤ 1 P is the radiant power of any body, Ps is the radiant power of a black body at the same temperature, and ε is the emissivity of any body. The emissivity ε is determined as follows: The emissivity ε at room temperature is measured using an integrating sphere. This allows the measurement of the directed hemispheric spectral reflectance Rgh and the directed hemispheric spectral transmittance Tgh, from which the normal spectral emissivity is calculated. The reflection and transmittance can be measured in the wavelength range from 0.78 µm to 2.5 µm, for example, using a Perkin Elmer Lambda 950 grating spectrometer.In the wavelength range from 1.4 µm to 18 µm, for example, a Bruker IFS 66v Fourier transform infrared (FTIR) spectrometer can be used. The measurement of emissivity ε at higher temperatures is carried out in the wavelength range from 0.7 µm to 14 µm using an FTIR spectrometer, for example a Bruker IFS 66v Fourier transform infrared spectrometer (FTIR)), to which a black-body boundary conditions (BBC) sample chamber is coupled via additional optics. The sample chamber has temperature-controlled blackbody environments and a beam exit opening with a detector in the half-spaces in front of and behind the sample holder. The sample is heated to a specified temperature in a separate oven and placed into the beam path of the sample chamber with the blackbody environments set to a specified temperature for measurement.The intensity detected by the detector is made up of an emission component, a reflection component, and a transmission component. Namely, the intensity emitted by the sample itself, the intensity falling from the front half-space onto the sample and being reflected by it, and the intensity falling from the rear half-space onto the sample and being transmitted by it. To determine the individual quantities of emissivity, reflectance, and transmittance, three measurements must be carried out. Exemplary embodiment The invention is explained in more detail below using exemplary embodiments and drawings. In detail, the following schematic representation shows: Figure 1 a diagram in which the cladding tube temperatures T. s of an infrared radiator with a gold layer and an infrared radiator coated with quartz glass particles as a function of the layer thickness d of the quartz glass layer, Figure 2 shows a diagram in which the cladding tube temperature Ts of the infrared radiator coated with quartz glass particles from Figure 1 as a function of the surface A of the quartz glass layer, Figures 3, 4 emissivity-temperature diagrams showing the dependence of the cladding tube temperature T s of an infrared radiator from the emissivity ε of a surface coating applied to the cladding tube of the infrared radiator, Figure 5 is a power-time diagram showing the electrical power Pel of an infrared radiator according to the invention over a period of 25,000 hours, Figure 6 is a temperature-time diagram in which the cladding tube surface temperature T swhen an infrared radiator is switched on with and without an emission layer, Figure 7 is a light microscopic image, magnified 1,000 times, of the surface of a heating coil coated with a layer of lacquer in a new state, and Figure 8 is a sectional view of an embodiment of an infrared radiator according to the invention. Figure 8 shows a sectional view of an embodiment of an infrared radiator according to the invention, which is assigned the reference numeral 1 overall. The illustration in Figure 8 is not to scale; in particular, components and layers may be shown larger for reasons of better visibility. The infrared radiator 1 is a round tube radiator; it has a cylindrical cladding tube 2 made of quartz glass. The cladding tube 2 has an outer diameter of 13.7 mm, an inner diameter of 12.5 mm and a length of 300 mm.A tungsten heating element 3 with a length of 280 mm, a width of 10 mm, and a thickness of 1 mm is arranged in the cladding tube. A black lacquer layer 4 is applied and baked onto the surface of the tungsten heating element 3, for example, using an Ulfalux lacquer. ® - Thermal coating 1590ST. Alternatively, the black paint is a layer of black thermal dispersion paint with the following composition: Aluminosilicate solution 15 wt.% Copper chromite black spinel 30 wt.% Water 40 wt.% Volatile organic components 15 wt. The paint layer 3 has a layer thickness of 100 µm. The paint layer 3 lowers the temperature of the tungsten heating element 3 via radiative cooling. This increases the service life of the infrared radiator, as it also lowers the cladding tube surface temperature T sis lowered. A microstructure 5 with a surface roughness of 5µm is produced on the cladding tube 2. Production of the microstructure The microstructure 5 on the cladding tube 2 can be produced in a variety of ways: a) Mechanical processing In particular by grinding the cladding tube using a rotating grinding wheel or a diamond tool in two steps, coarse grinding with 15µm grain and fine grinding with 5µm grain b) Chemical processing c) Etching of the outer surface using Heraeus ®Special Surface Treatment (SST) using 10% hydrofluoric acid (etching time: 30 minutes at 25°C). Devitrification d) Creating a suspension of demineralized water 95% (wt%) and Al2O3 powder (5 wt%), immersing the cladding tube in the suspension, and sintering for one hour at 1000°C. Laser processing: Raster-based dotting of the outer surface of the cladding tube using an O2 laser with an output power of 100 watts and a spot diameter of 10 µm on the surface. e) Sandblasting f) Blasting the outer surface of the cladding tube with a sandblasting device using quartz glass beads with a diameter of around 100 µm at 3 bar and spaced 10 mm apart. Coating (A) Quartz glass layer Preparation of the slip Amorphous quartz glass grains with grain sizes in the range of 250 µm to 650 µm are wet-ground with deionized water to form a homogeneous base slip with a solids content of 78%.Subsequently, 2.56 wt.% silicon (Si) powder (D50 value: approx. 5 µm with particle sizes in the range of 1 µm to 15 µm) is added to the base slip, and this mixture is homogenized for several days. The slip can then be applied to a heating element. Coating the cladding tube The coating is sprayed onto the cladding tube in three layers using a spray gun (nozzle diameter: 1.0 mm). Alternatively, the cladding tube can be immersed in the slip for 10 seconds, during which excess slip is allowed to drip off. Baking the slip The slip is then baked in an oven at … °C for … hours (???). The quartz glass layer has a surface area of ​​5m. 2 / g, based on the mass of the sintered coating. The quartz glass layer is created by spraying four layers. Each layer has a thickness of 300 µm. (B) Ceramic layer. These layers are created by thermal spraying using starting powders and then fired at a high temperature of, for example, 1200°C for two hours. Table 1 below shows various compositions of surface coatings according to the invention. The surface coating has a layer thickness of at least 10 µm and a surface area of ​​5 m². 2 / g. Table 1 Coating Percentage of Total Composition Layers Layer Thickness (in wt.%) (in µm) 1 SiC / Si3N4 40 / 60; 35 / 65 2-4 50-100 2 MoSi2 / Al2O3 / SiO2 20 / 30 / 50 3-5 100-200 3 SiO2 100 4-6 1200-1500 4 SiC / TiO2 60 / 40 2-4 10-50 5 BN / SiO2 5 / 95 4-5 600 6 Al2O3 / SiO2 3 / 97 3-5 800 Quartz glass particles with an average particle size D50 of 5µm, ceramic particles with an average particle size D50 of 3µm and silicon particles with an average particle size D50 of 30µm can be embedded in the respective coatings. Tests have shown that the greater the coating thickness, the greater the temperature-reducing effect of a microstructure applied to the surface of a cladding tube. Successively applied layers accumulate. This means: the more spray layers applied to a cladding tube, the greater the coating thickness and the higher the emissivity ε.Figure 1 shows a diagram illustrating the dependence of the cladding tube surface temperature Ts under operating conditions on the layer thickness d of the quartz glass layer (curve B). For this purpose, the cladding tubes of identical infrared radiators were each coated with quartz glass layers of different thicknesses d, and the cladding tube surface temperature T was then measured. s measured with a pyrometer. The quartz glass layers have a surface roughness of 5µm. The identical infrared radiators have a cylindrical quartz glass cladding tube with an outer diameter of 19mm, an inner diameter of 16mm, and a length of 500mm. A tungsten heating coil with a length of 450mm and a width of 2mm is arranged in the cladding tube. The infrared radiators are designed for a nominal power Pel of 1000 watts at a nominal voltage of 230V. The diagram in Figure 1 shows that the surface temperature T sof the cladding tube decreases with increasing layer thickness d of the quartz glass layer, even though the thermal mass of the quartz glass layer increases. With increasing layer thickness, the surface temperature asymptotically approaches a minimum value not specified here (indicated in Figure 1 by extrapolation (curve C)). For comparison, curve A shows the surface temperature of an infrared radiator with a gold-coated cladding tube. Figure 2 shows a diagram in which the cladding tube surface temperature Ts under operating conditions is plotted as a function of the surface area A of the quartz glass layer. For this purpose, identical infrared radiators (as described in Figure 1) were coated with layers of quartz glass nanoparticles with a specific BET surface area of ​​5 m 2 / g, resulting in different surface sizes depending on the mass of the quartz glass nanoparticles. The surface size can be determined by the mass of the applied quartz glass nanoparticles and their BET surface area (5m 2 / g). Subsequently, the cladding tube surface temperature was determined for each infrared radiator under operating conditions (curve D). Curve E shows the further course of curve D in an approximation. This shows that the cladding tube surface temperature Ts decreases when the surface area A of the quartz glass layer increases. The unit m used in the diagram in Figure 2 2 per m duct, can be converted into the unit m 2per g of applied quartz glass nanoparticles from the weight and BET surface area of ​​the quartz glass nanoparticles. The emissivity ε depends fundamentally on the penetration depth a of the radiation into the cladding tube. This, in turn, depends on the wavelength λ and the extinction coefficient k of the cladding tube material. The following applies: with a: penetration depth into the cladding tube material, ε: emissivity of the cladding tube material, k: extinction coefficient of the cladding tube material Figures 3 and 4 show emissivity-temperature diagrams that show the relationship between the emissivity ε of a cladding tube surface coating of an infrared radiator and its cladding tube temperature T sclarify. As Figures 3 and 4 show, the cladding tube temperature decreases with increasing emissivity ε of the cladding tube surface coating. This can be explained by the fact that with increasing emissivity of the cladding tube surface coating, more energy is radiated, which is accompanied by a radiative cooling effect. In general, the lower the surface temperature Ts of the cladding tube, the longer the service life of the infrared radiator. This is particularly demonstrated by the power-time diagram in Figure 5, which shows the electrical power Pel of an uncooled infrared radiator according to the invention over a period of 25,000 hours. The power-time diagram is derived from an infrared radiator, namely a twin-tube radiator, which has a cladding tube in the form of a double tube made of two quartz tubes arranged side by side and fused together. The cladding tube has a width of 23mm, a height of 11mm and a length of 400mm.A tungsten heating element is arranged in the cladding tube. A half-shell-shaped microstructure (with a center angle of 180°) is created on the cladding tube by applying and firing a layer of quartz glass beads with a particle size (D50) of 10 µm. The microstructure has a surface roughness of 30 µm. As the power-time diagram shows, the electrical power Pel of the infrared radiator remains virtually constant over a period of 25,000 hours. Even after a period of 25,000 hours, the infrared radiator still exhibits an electrical power Pel of 97.9% of the original electrical power at the start of operation (time: 0 h) (measurement accuracy of ±3%). In contrast, conventional infrared radiators often have a service life of only 3,000 to 10,000 hours.Figure 6 shows two temperature curves in a temperature-time diagram: a first temperature curve 60 for an infrared radiator whose heating element has no emission layer applied to it, and a second temperature curve 61 for an infrared radiator whose heating element is provided with an emission layer. Application of the emission layer: A black layer of thermal paint is sprayed onto a medium-wave (MW) heating coil of an infrared radiator. The infrared radiator is a round-tube radiator with a cladding tube with an outer diameter of 10 mm; it has a nominal power of 300 W at a nominal voltage of 115 V. To apply the thermal paint, approximately half of the heating coil is manually pulled out of the cladding tube and fixed to a cardboard box with adhesive tape. At a distance of 20 cm, the thermal paint is sprayed onto the heating coil using a spray device. The coil is rotated 180°, and the process is repeated. The thermal paint is alkali-free.It contains an aluminosilicate solution (10 to 20 wt.%), copper chromite black spinel as a mineral color pigment (25 to 35 wt.%) and water (40 to 60 wt.%). Suitable thermal paints are commercially available as oven paints, for example from the companies ULFALUX Lackfabrikation GmbH (e.g. Ulfalux. ®-Thermo coating 1590ST) and Aremco Products Inc., with the following organic ingredients listed: xylene, ethyl acetate, butyl acetate, ethylbenzene. Multiple coats ensure a completely sealed layer. After spraying, the thermal paint is baked on the coil in a lamp, forming a layer of paint. This is done in stages at 50V, 70V, 90V, and 115V, each for 10 minutes. This leaves one half of the coil uncoated, while the emission layer is applied to the other half. The layer thickness of the paint layer is 100µm. Figure 7 shows a 1,000x magnified light micrograph of the surface 20 of a coated heating coil section 10. A Type K thermocouple is welded to the underside of two blackened copper plates. Both copper plates are positioned with their upper sides on the cladding tube, with one copper plate facing the uncoated half of the coil and the other copper plate facing the coil with the emissive layer. The infrared heater is operated at 230V and 1,250W for 5 minutes. The temperature of the plates is recorded with a data logger.

[0002] Measuring the temperature of the heating coil. The temperature of the heating coil is measured with a pyrometer. Result: Applying a coating of paint lowers the cladding tube surface temperature. This increases the service life of the infrared heater. The following Table 2 summarizes the results of the heating coil temperature measurement: Table 2 Case I Case II Coil Coil Uncoated Coated Uncoated Coated ε 0.7 0.93 0.7 0.93 U[V] 140 140 160 160I [A] 3.34 3.34 3.78 3.78P [W] 468 (100%) 468 (100%) 605 (129%) 605 (129%)T [°C] 883 800 970 885Case I: Recording the heating coil temperatures To quantify the increase in performance due to the heating coil coating, the radiator was operated at its rated power and the heating coil temperatures with and without the emissive coating were recorded with a pyrometer. The temperature of the uncoated coil at a rated power of 468 W (=100%) is 883°C (standard heating coil temperature), while the coated coil shows a significantly lower temperature of 800°C. Case II: Increasing the power until the standard heating coil temperature is reached In Case II, the power of the infrared radiator was adjusted so that its heating coil with emission layer had the same temperature as the uncoated heating coil at nominal power.In other words, the temperature of the coated heating coil (here: 885 °C) was set approximately to the standard heating coil temperature (here: 883 °C). This necessitated an increase in the electrical power Pel to 605 watts. For comparison, Table 2 also shows the measurement results for the uncoated heating coil, which becomes significantly hotter when operated at an electrical power of 605 watts and, with a filament temperature of 970 °C, reaches the limits of its usability. The ratio of electrical power converted to optical power at 160 V is 1.29 (605 / 468 = 1.29), compared to the previous value (at 140 V).

Claims

PATENT CLAIMS 1. An infrared radiator (1) comprising a cladding tube (2) and a heating element (3) arranged therein for emitting infrared radiation, characterized in that the cladding tube (2) has a surface with a microstructure (5).

2. An infrared radiator (1) according to claim 1, characterized in that the surface with the microstructure (5) has a surface roughness in the range from 0.1 µm to 50 µm.

3. An infrared radiator (1) according to claim 1 or 2, characterized in that the microstructure (5) is produced by mechanically processing the surface, chemically processing the surface, devitrifying the surface, processing the surface with a laser, or sandblasting. 4.Infrared radiator (1) according to claim 1 or 2, characterized in that the microstructure (5) is produced by a surface coating, wherein the surface coating is a quartz glass layer based on SiO2 or a ceramic layer based on a temperature-resistant ceramic, which preferably contains components selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3.

5. Infrared radiator (1) according to claim 4, characterized in that quartz glass particles, ceramic particles and / or particles of elemental silicon are embedded in the surface coating, which contribute to the microstructure (5) of the surface.

6. Infrared radiator (1) according to one of the preceding claims 4 or 5, characterized in that the surface coating has a surface in the range of 1m. 2 / g up to 50m 2 / g, preferably 2m 2 / g up to 20m 2 / g, has.

7. Infrared radiator (1) according to one of the preceding claims 4 to 6, characterized in that the ceramic layer has a layer thickness in the range from 10 µm to 300 µm, preferably in the range from 10 µm to 150 µm.

8. Infrared radiator (1) according to one of the preceding claims 4 to 6, characterized in that the quartz glass layer has a layer thickness in the range from 50 µm to 5 mm, preferably in the range from 250 µm to 2.5 mm, preferably a layer thickness of less than 1.5 mm.

9. Infrared radiator (1) according to one of the preceding claims 4 to 6 or 8, characterized in that the quartz glass layer comprises several layers, each layer having a layer thickness in the range from 250 µm to 700 µm, preferably less than 500 µm.

10. Infrared radiator (1) according to one of the preceding claims, characterized in that the heating element (3) is coated with an emission layer. 11.Infrared radiator (1) according to claim 10, characterized in that the emission layer is a lacquer layer (4) and contains an inorganic color pigment with black mineral particles and / or that the emission layer contains quartz glass particles and elemental silicon.

12. Infrared radiator (1) according to claim 11, characterized in that the lacquer layer (4) has a lacquer layer thickness in the range from 50 µm to 300 µm.

13. Infrared radiator (1) according to one of the preceding claims, characterized in that the heating element (3) is made of molybdenum disilicide (MoSi2).

14. Use of a surface coating in the form of a quartz glass layer applied to a surface of a cladding tube (2) of an infrared radiator (1). based on SiO2 or in the form of a ceramic layer based on a temperature-resistant ceramic, preferably selected from the group TiO2, Al2O3, SiC, Si3N4, BN, Y2O3, for cooling the cladding tube (2) during operation of the infrared radiator (1).

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