Infrared emitter having a cladding tube and a heating element arranged therein, and method for producing the heating element

Abstract

Known infrared emitters (1; 30) have a cladding tube (2; 31) and a heating element (3a, 3b; 10; 11; 12; 32) arranged in the cladding tube (2; 31) for emitting infrared radiation. In order to specify, proceeding herefrom, an infrared emitter having high irradiation efficiency and having high optical power Popt with simultaneous emission wavelength shift, it is proposed that the heating element (3a; 3b; 10; 11; 12; 32) has a surface (20; 21; 22) with a microstructure (33).

Description

[0001] DESCRIPTION

[0002] Infrared radiator with a sheath tube and a heating element arranged therein and manufacturing method for the heating element

[0003] Technical background

[0004] 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.

[0005] Furthermore, the invention relates to a manufacturing method for a heating element for emitting infrared radiation.

[0006] 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 carbon heating strip 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. Common infrared radiators can be divided into short-wave, medium-wave, and long-wave infrared radiators based on their main emission wavelength. The main emission wavelengths of short-wave infrared emitters are in the range from 0.78pm to 1.4pm (= 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 1.4pm to 3pm (= IR-B, 690°C - 1,800°C) and long-wave infrared emitters in the range above 3pm to 1 mm (= IR-C, < 690°C).

[0007] State of the art

[0008] Known infrared radiators are used to heat 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 10 2011 108 421 B3. Infrared radiators are typically powered by an electrical power Pei and emit radiation with an optical power P op t. In principle, the optical power Popt emitted by an infrared radiator influences the heating rate of the material being heated (measured as heating rate in K / s). The higher the optical power P op t is in the absorbing wavelength range of the heating material, the more energy is absorbed by the heating material and the faster the heating material heats up.

[0009] By increasing the electrical operating power Pei, a higher optical power Pop t. If an infrared heater is operated with a higher electrical power Pei, this is usually accompanied by an increase in the temperature of the infrared heater heating element. The optical power P op The radiative power of an infrared heater increases with the temperature of the heating element. It is directly proportional to the electrical power Pei supplied to the infrared heater and can be described by the Stefan-Boltzmann law:

[0010] Popt = C * £ * A * T 4 , with C: constant, e: emissivity,

[0011] A: Surface of the radiating body and

[0012] T: Temperature of the radiating body.

[0013] Due to the increased temperature of the heating element, the infrared radiator radiates a higher optical power P opt, on the other hand, according to Planck's radiation law, the higher temperature of the heating element also changes the emission spectrum of the infrared radiator, whereby the increase in temperature of the heating element leads to a shift of the peak wavelength into the shorter wavelength range.

[0014] However, heating materials do not absorb equally at all wavelengths. They exhibit an absorption spectrum that reflects the heating material's ability to absorb electromagnetic radiation of specific energies or wavelengths. Therefore, an increase in the temperature of the heating element can have an effect despite an increase in the optical power P op t may prove disadvantageous in terms of heating efficiency and heating time if the temperature-induced emission wavelength shift results in a smaller proportion of the emitted radiation being absorbed by the material to be heated (e.g. water, plastic or glass).

[0015] Therefore, an increase in electrical power does not necessarily lead to an increase in irradiation efficiency or faster heating of the material. In fact, the absorption spectrum of the material typically determines the technically feasible maximum temperature, thus limiting the electrical power Pei required to power the infrared heater to a maximum value.

[0016] Technical task

[0017] The present invention is based on the technical object of providing an infrared radiator with high irradiation efficiency. In particular, the invention is based on the object of providing an infrared radiator that has a high optical power P op t with simultaneous shift of the emission wavelength.

[0018] Furthermore, the present invention is based on the technical object of providing a simple and cost-effective method for producing a heating element for emitting infrared radiation.

[0019] Brief description of the invention

[0020] With regard to the infrared radiator, this object is achieved by a radiator having the features of claim 1. In particular, the object is achieved according to the invention, starting from an infrared radiator of the type mentioned at the outset, in that the heating element has a surface with a microstructure.

[0021] The present invention is based on the idea of ​​increasing the optical power Popt of the infrared radiator not by increasing the heating element temperature, but instead by increasing the emissivity £ and / or the total surface area A of the radiating heating element. Tests have shown that by creating a surface with a microstructure, the emissivity £ and the surface area A of the heating element can be increased in the IR-B and IR-C range, but especially in the technically important wavelength range from 800 nm to 2,500 nm. The term "microstructure" describes the structure of the heating element surface when viewed on a microscopic scale; it is explained in more detail in the "Definitions" section. The surface of the heating element can be fully or partially covered with the microstructure. The microstructure can be periodic, regular, or chaotic.The microstructure can be created by surface processing, for example, by mechanical machining or by laser processing. A microstructure with periodic surface structures can be created, for example, using direct laser interference structuring (DLIP). This is particularly true for the creation of defined, periodic surface structures on the surface of metals. Direct laser interference structuring (DLIP) is particularly suitable for creating a microstructure on a metallic heating element, for example, on a heating element made of tungsten, molybdenum, tantalum, niobium, and / or their alloys, as well as Fe-Cr-Ni alloys.

[0022] An increase in the emissivity £ of the heating element, as well as an increase in the surface area A, is accompanied by a reduction in the heating element temperature. If the emissivity £ and the surface area A of the heating element are increased by microstructuring, at the same electrical power Pei and optical power P op t lowers the temperature of the heating coil, Popt and Pei remain constant. The heating element appears visually less bright, but at a lower temperature, due to its higher emissivity, it emits the same optical power Popt as an identically constructed infrared heater without a microstructured heating element.

[0023] An increase in the emissivity £ and / or the surface A of the heating element consequently has the advantage that a higher electrical power Pei can be coupled into the infrared radiator until the heating element temperature of an identical infrared radiator without microstructure is reached, whereby a higher optical power Popt and a faster heating of the heating material can be achieved.

[0024] By increasing the emissivity £ of the surface A of the heating element and / or the electrical power Pei, the process times for heating materials can be significantly reduced. Since, according to the Stefan-Boltzmann law, the emissivity £ and the surface A of the heating element are directly proportional to the emitted optical power, an increase in the emissivity E, the surface A, and / or the electrical power Pei is associated with a higher optical power of the infrared radiator. The higher the optical power coupled into the substrate, the faster the material heats to the target temperature.

[0025] In a first preferred embodiment of the infrared radiator according to the invention, the microstructure is produced by mechanical processing, chemical processing or by processing the surface with a laser.

[0026] The microstructure can be created mechanically, for example, by sandblasting, emerying, or grinding. Alternatively, the microstructure can also be created chemically, for example, by etching or with a laser.

[0027] In a second, equally preferred embodiment of the infrared radiator according to the invention, the microstructure is created by a surface layer in which quartz glass and / or ceramic particles are embedded, which contribute to the microstructure of the surface.

[0028] The quartz glass and / or ceramic particles are advantageously applied to the heating element as part of a layer. It has proven advantageous for the layer thickness to be in the range of 50 μm to 300 μm. The quartz glass and / or ceramic particles form surface elevations in the layer and thus contribute to the surface microstructure. Quartz glass and ceramic particles are thermally stable in air, regularly at temperatures of up to 1,000°C. Due to the good thermal stability of the quartz glass and ceramic particles in the layer, the surface roughness can be increased and thus a high emissivity E of the layer can be achieved permanently, since the surface roughness and emissivity remain virtually unchanged even after extended operation of the infrared radiator. The quartz glass and / or ceramic particles preferably have an average particle size (Dso) in the range of 50 nm to 50 μm.

[0029] In this context, it has proven advantageous if the layer contains elemental silicon.

[0030] For semiconductor materials such as silicon, the absorption coefficient in the wavelength range from 1 pm to 5 pm is a function of the absorbed wavelength. Preferably, the elemental silicon in the layer is present as particles with an average particle size in the range of 1 pm to 200 pm. Preferably, the silicon contributes to the microstructure. A layer containing particles of elemental silicon leads to an increase in emissivity £. The layer interspersed with silicon particles can be used in air. The silicon particles within it are thermally stable up to 1,000°C.

[0031] Advantageously, the surface has a surface roughness in the range of 0.1 pm to 50 pm. It has proven particularly advantageous if the surface has a surface roughness in the range of 1 pm to 30 pm.

[0032] The term "surface roughness" is explained in more detail in the "Definitions" section. The emissivity £ depends on the surface roughness of the heating element. Generally speaking, the smoother the surface of the heating element, the lower the emissivity £. For a surface with a surface roughness of less than 0.1 μm, the emissivity-enhancing effect of the microstructure is lost. For a surface layer with a surface roughness of more than 50 μm, its increased emission is outweighed by other disadvantages, such as its low mechanical stability.

[0033] In a preferred modification of the infrared radiator, the surface with the microstructure is formed on a heating element in the form of an elongated wire, a twisted wire, a metal foil and / or several wires twisted together.

[0034] The heating element can be a single wire or a foil, or it can consist of several interconnected, in particular twisted, individual wires. The microstructure is advantageously applied only to the outer surface of the heating element. This is especially true if the heating element is made of several interconnected or twisted individual wires. In this case, only the outer surface of the heating element, but not the individual wires themselves, is provided with the microstructure. The reason for this is that when the wires are connected or twisted, forces regularly act on the individual wire that could damage the coating of a previously coated individual wire.

[0035] In this context, it has proven advantageous if the elongated wire with the microstructure is twisted along the wire's longitudinal axis and / or the metal foil with the microstructure is twisted along the foil's longitudinal axis, or compressed or folded into a spring. Twisting, compressing, or folding shortens the length of a heating element. This makes it possible to install heating elements in the cladding tube that are intrinsically longer than the cladding tube when extended. At the same time, twisting, compressing, or folding increases the surface area of ​​the heating element relative to the length of a cladding tube section, thus increasing the optical output of the infrared emitter.

[0036] In a preferred modification of the infrared radiator according to the invention, it is provided that the cladding tube has a front side for emitting the infrared radiation and a rear side opposite the front side, and that the front side is provided with a black lacquer or a layer of quartz glass, ceramic and silicon particles.

[0037] The emissivity £ of the cladding tube influences the temperature of the cladding tube and the infrared radiator as a whole. The higher the emissivity £ of a body, the more energy it can emit back to its surroundings per unit of time. If the front of the cladding tube is coated with a layer of high emissivity, the overall radiation of the infrared radiator increases, which passively cools the surface of the infrared radiator. This allows either the infrared radiator to be fed with a greater electrical power Pei until the same temperature is reached, or the temperature of the infrared radiator can be lowered so that the cooling that would otherwise be necessary can be reduced.

[0038] Preferably, the reflector layer is coated with a black emissive layer in the form of a black lacquer or a layer of quartz glass, ceramic, and silicon particles. The layer of quartz glass, ceramic, and silicon particles can have the same chemical composition as the layer applied to the heating element. Such layers exhibit high emissivity and provide good passive cooling. At the same time, this increases the service life of the infrared radiator.

[0039] If an infrared radiator is not intended to emit infrared radiation evenly in all spatial directions, the cladding tube can be equipped with a reflector. This reduces the radiation emission in certain spatial directions and increases it in others. An infrared radiator with a reflector layer applied to the cladding tube has a particularly compact design. A reflector layer is preferably applied to the back of the cladding tube.

[0040] Even though the reflector layer and the high emissivity layer each cover a partial area of ​​the cladding tube surface, the reflector layer and the high emissivity layer do not overlap.

[0041] With regard to the size of the surface with the microstructure, it has proven particularly advantageous if the surface with the microstructure has a specific BET surface area in the range of 1 m 2 / g up to 50 m 2 / g.

[0042] A microstructure with a large specific surface is rugged and contributes to a high surface area A and thus to a good optical performance P op t of the infrared radiator. The BET measurement is used to characterize solid surfaces using gas adsorption.

[0043] With regard to the manufacturing method for a heating element, the above-mentioned object is achieved by a method having the features of claim 9. In particular, the object is achieved according to the invention, starting from a method of the type mentioned at the outset, in that the method is designed to produce a microstructure on a heating element and comprises the method steps:

[0044] (a) applying a slurry containing quartz glass and / or ceramic particles to the heating element, and

[0045] (b) Sintering of the slip on the heating element to form the microstructure.

[0046] The present invention is based on the idea of ​​increasing the optical power Popt of the infrared radiator by increasing the emissivity £ and / or the total surface area A of the radiating heating element. For this purpose, a microstructure is created on the surface of the heating element. The surface of the heating element can be completely or partially provided with the microstructure.

[0047] A slurry containing quartz glass and / or ceramic particles can be easily applied to the surface of a heating element. It has proven advantageous for the layer thickness to be in the range of 50 μm to 300 μm. The quartz glass and / or ceramic particles preferably have an average particle size (D 50 ) in the range of 50 nm to 50 μm. The slurry preferably contains elemental silicon, preferably with an average particle size in the range of 1 μm to 200 μm.

[0048] Sintering the slip onto the heating element can be achieved, for example, by firing it in a furnace or by operating the heating element. After firing or sintering, the resulting surface layer has good thermal and sufficient mechanical stability. The quartz glass and / or ceramic particles contained in the surface layer protrude from the central surface as surface elevations. They contribute to the surface microstructure.

[0049] Advantageously, the slip is applied according to process step (a) by immersing the heating element in a slip mass or by spraying the slip mass.

[0050] Immersing the heating element in the slip is particularly easy. Spraying the slip has the advantage of producing a sprayed layer of the desired thickness. This is advantageous when the slip cannot be applied in one step due to the final thickness of the layer, but must be applied in several layers.

[0051] Definitions

[0052] Individual terms from the above description are defined below for additional information. These definitions are part of the description of the invention. In the event of a conflict between one of the following definitions and the rest of the description, the remaining definitions in the description shall prevail.

[0053] If a measurement method is not specified for a parameter, the standard measurement method shall be used for that parameter, and in particular the measurement method specified in the corresponding ISO standard whose publication date is closest to that of this application. If measurement conditions are not specified, the standard conditions (SATP conditions) shall be 298.15 K (25°C, 77°F) for temperature and 100 kPa (14,504 psi, 0.986 atm) for absolute pressure. Microstructure

[0054] The term microstructure describes height differences relative to the arithmetic mean of the surface in the range of 0.05 pm to 250 pm, preferably 0.1 pm to 25 pm. The microstructure therefore largely belongs to the microscopic range, which can only be detected with a microscope, but also partially to the visible range. One measure of the microstructure is, for example, the surface roughness Ra.

[0055] Surface roughness Ra

[0056] The surface roughness Ra is determined in accordance with EN ISO 25178-2. The surface roughness Ra is determined by the height difference of each point compared to the arithmetic mean of the surface.

[0057] 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 ranging from a few centimeters to approximately 0.5 pm and vertical dimensions from a few hundred pm 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.

[0058] The specific surface area is determined based on a sorption measurement according to the method of Brunauer, Emmet and Teller (BET method) in accordance with DIN ISO 9277:2003-05. The measuring device is a NOVA-3000 (from Quantachrome), which operates according to the SMART method (Adaptive Rate Adsorption). Aluminum oxide SARM-13 and SARM-214 from Quantachrome are used as reference materials. The saturation vapor pressure of nitrogen (N2 4.0) 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 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.

[0059] From a particle size distribution, which shows the cumulative volume of the particles as a function of the particle size, statistical parameters can be derived, such as the Dio, Dso, or Dgo value. The Dio value, for example, indicates the particle size not reached by 10% of the cumulative volume of the particles, and accordingly the Dso value indicates the particle size not reached by 50% of the cumulative volume. The Dso 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, which 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 Dso values ​​of the particle size distribution are determined using the device software 21 CFR with a form factor of 1. The Dso value indicates the particle size that is not reached by 50% of the cumulative particle volume (median value of the particle size).

[0060] Particle sizes of more than 315 pm and corresponding size distributions were determined by sieve analysis using a sieving device “Air Jet RHEWUM LPS 200 MC” (RHEWUM GmbH).

[0061] Alternatively, the particle size can also be determined using a digital microscope, for example a digital microscope of the VHX-7000 series from Keyence Deutschland GmbH.

[0062] Emissivity e

[0063] Every body emits heat radiation based on its temperature. Emissivity £ 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 of the same temperature multiplied by the emissivity of the body. The following applies:

[0064] P = £ • P s ; with 0 < £ < 1 P radiation power of any body,

[0065] Ps Radiation power of the black body equal

[0066] Temperature, and s emissivity of any body.

[0067] The emissivity £ is determined as follows:

[0068] The emissivity £ 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 measurement of the reflectance and transmittance in the wavelength range from 0.78 pm to 2.5 pm can be performed, for example, using a Perkin Elmer Lambda 950 grating spectrometer. In the wavelength range from 1.4 pm to 18 pm, a Bruker IFS 66v Fourier transform infrared (FTIR) spectrometer can be used.

[0069] The measurement of emissivity £ at higher temperatures is carried out in the wavelength range from 0.7 to 14 pm 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 black-body environments in the half-spaces in front of and behind the sample holder, as well as a beam exit aperture with a detector. The sample is heated to a specified temperature in a separate oven and placed into the beam path of the sample chamber with the black-body environments set to a specified temperature for measurement.The intensity detected by the detector consists of an emission, a reflection, and a transmission component: namely, the intensity emitted by the sample itself, the intensity incident on the sample from the front half-space and reflected by it, and the intensity incident on the sample from the rear half-space and transmitted by it. To determine the individual quantities of emissivity, reflectance, and transmittance, three measurements must be performed.

[0070] Detailed description of the invention The invention is explained in more detail below using exemplary embodiments and drawings. The following schematic representation shows:

[0071] Figure 1 shows an emission spectrum of an infrared radiator without a microstructured heating coil compared to an emission spectrum of an infrared radiator with a microstructured heating coil,

[0072] Figure 2 is a light microscopic image, magnified 200 times, of the surface of an uncoated heating coil made of Kanthai® D (Kanthai® is a registered trademark of Kanthai AB, Sweden),

[0073] Figure 3 is a light microscopic image, magnified 200 times, of the surface of a non-fired heating coil made of Kanthai® D (Kanthai® is a registered trademark of Kanthai AB, Sweden) coated with a triple layer of quartz glass slurry.

[0074] Figure 4 is a light microscopic image, magnified 1,000 times, of the surface of the heating coil from Figure 3 after firing,

[0075] Figure 5 shows a first embodiment of an infrared radiator according to the invention in perspective view,

[0076] Figure 6 shows three embodiments of heating elements according to the invention, and

[0077] Figure 7 shows a second embodiment of an infrared radiator according to the invention in a sectional view.

[0078] The illustrations in Figures 5 to 7 are not to scale; in particular, components and layers may be shown larger for reasons of better recognition.

[0079] Figure 5 shows a schematic perspective view of a first embodiment of an infrared radiator 1 according to the invention in the form of a twin-tube radiator; it has a cladding tube 2 in the form of a double tube made of two quartz tubes arranged side by side and fused together. The quartz tubes together have a width of 23 mm, a height of 11 mm, and a length of 200 mm. The quartz tubes are closed at both tube ends. A black lacquer 5 is applied to the cladding tube 2, for example, by spraying or brushing on and baking a thermal paint. 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 ULFALUX Lackfabrikation GmbH (e.g. Ulfalux® thermal coating 1590ST) and Aremco Products Inc.offered, with the following additional organic ingredients listed: xylene, ethyl acetate, butyl acetate, ethylbenzene. Multiple coats of paint ensure a completely sealed layer. After spraying, the thermal paint is dried at 250°C and is then touch-resistant. The paint layer 5 achieves its final finish by heating to 1,200°C. This heating can be performed when the infrared heater is commissioned. Ceramic components are sintered onto the lamp tube surface, creating a solid, integral bond, making the paint layer largely scratch-resistant.

[0080] The two quartz tubes of the cladding tube 2 each surround a heating element 3a, 3b, namely a tungsten heating band with a length of 180 mm, a width of 20 mm, and a thickness of 12 μm. The tungsten heating bands are connected in series, and the terminals 4a, 4b serve to electrically contact the tungsten heating bands. The entire surface of the two tungsten heating bands has a microstructure. The microstructure is created by applying a slurry and firing the slurry. The process for creating a microstructure on a heating element is explained in more detail below:

[0081] Preparation of the slip

[0082] Amorphous quartz glass grains with grain sizes ranging from 250 μm to 650 μm are wet-milled with deionized water to form a homogeneous base slurry with a solids content of 78%. 2.56 wt.% silicon (Si) powder (D50 value: approximately 5 μm with particle sizes ranging from 1 μm to 15 μm) is then added to the base slurry, and this mixture is homogenized for several days. The slurry is then ready to be applied to a heating element.

[0083] Coating of the heating element

[0084] The coating is sprayed onto the heating element in three layers using a spray gun (nozzle diameter: 1.0 mm). Alternatively, the heating element can be immersed in the slurry for 10 seconds, allowing excess slurry to drip off.

[0085] Burning the slip

[0086] The slip is then fired in a furnace at 1,200°C for 2 hours. Alternatively, the slip can also be fired in the presence of infrared heater 1.

[0087] Such a microstructure can be produced on many other heating element types in the manner described above.

[0088] Example 1 - Medium wave heating coil made of Kanthai® D

[0089] A medium-wave heating coil made of an iron, chromium, and aluminum-containing heating conductor alloy (Kanthai® D; registered trademark of Kanthai AB, Sweden) with a nominal voltage of 115 V and a nominal power of 300 W is coated three times with the previously described slurry. Figure 2 shows a light microscopic image, magnified 200 times, of surface 20 of the uncoated Kanthai® D heating coil. Figure 3 shows a light microscopic image, magnified 200 times, of surface 21 of the Kanthai® D heating coil coated three times with the previously described quartz glass slurry, but not yet fired.

[0090] The slurry is baked using a radiant heater, in stages at 50 V, 70 V, 90 V, 115 V, 130 V, and 150 V for 5 minutes each, and finally at 180 V for 45 minutes. Figure 4 shows a 1,000x magnified light micrograph of the surface 22 of the heating coil from Figure 3 after baking.

[0091] Subsequently, the parameters E, U, I, P and Tf were determined for two radiators, namely the radiator with the coated heating coil made of Kanthai® D as described above and for an identical radiator with an uncoated heating coil made of Kanthai® D (see Table 1 below).

[0092] Table 1

[0093] Case I: Recording the heating coil temperatures

[0094] To quantify the effects of the microstructure on the heating element temperature, both heaters were operated at their rated power levels, and the heating coil temperatures of both heaters were recorded with a pyrometer. The temperature of the uncoated coil at a rated power of 472 W (=100%) was 863°C (standard heating coil temperature), while the coated coil showed a significantly lower temperature of 824°C.

[0095] Case II: Increasing the power until the standard heating coil temperature is reached In Case II, the power of the radiator with the coated heating coil was adjusted so that its heating coil had the same temperature as the uncoated radiator at nominal power. In other words: The temperature of the coated heating coil (here: 865°C) was set approximately to the standard heating coil temperature (here: 863°C). This required an increase in the electrical power Pei to 534 watts. For comparison, Table 1 also shows the measurement results for the uncoated radiator, which becomes considerably hotter when operated at an electrical power of 534 watts and reaches the limit of its usability with a filament temperature of 930°C. The ratio of electrical power converted into optical power at 150 V is 1.13 (534 / 472 = 1.13) compared to before (at 140 V).

[0096] Example 2 - Water drying

[0097] To dry the water from a heating material, two identical shortwave (SW) infrared radiators are used, differing only in their heating elements. The first SW infrared radiator has a conventional tungsten heating element without a microstructure, while the second SW infrared radiator, according to the invention, has a tungsten heating element provided with a microstructure. The first and second SW infrared radiators are operated with an electrical power Pei of 2,000 W, and the optical power P op t of both lamps is also 2,000 W. The wavelength spectrum suitable for water drying is considered in the technically relevant range NIR to MIR, i.e. from 1,400 nm to 5,000 nm. Table 2 shows the measured values.

[0098] Table 2

[0099] 1 Tt: Heating coil temperature, also: filament temperature.

[0100] 2 Intensity in the wavelength range 1,400 nm to 5,000 nm.

[0101] 3 The increase in surface area A of the heating element is mathematically taken into account in the doubling of e.

[0102] Result: The microstructure causes an intensity increase Popt in the wavelength range from 1,400 nm to 5,000 nm of 53% (468 / 305 = 1.53) for the same electrical power Pei.

[0103] Example 3 - Increasing Tf The inventive infrared radiator with microstructure from Example 2 is fed with a higher electrical power Pei such that the filament temperature Tf is again 2,200°C (see Table 3).

[0104] Table 3

[0105] Result:

[0106] The intensity Popt is almost doubled when the emissivity £ is doubled (603 / 305 = 1 .98).

[0107] In practice, doubling the intensity Popt means halving the heating time.

[0108] Example 4 - Medium-wave (MW) infrared emitter (state of the art)

[0109] As an alternative to the shortwave (SW) infrared heater without a microstructure shown in Example 2, it is also common to use mediumwave (MW) infrared heaters to generate infrared radiation in the wavelength range from 1,400 nm to 5,000 nm. These typically exhibit very high emissivity with £ values ​​of up to 95%. However, with MW heaters, the possible heating element temperature is usually limited, usually to approximately 950°C. Table 4 below shows the key figures for an MW heater with a heating coil made of Kanthai® D.

[0110] Table 4

[0111] Result:

[0112] The conventional Kanthal® MW emitter without microstructure from Table 4 shows in the relevant wavelength range from 1,400 nm to 5,000 nm only an intensity Popt of 81,900 W / m 2 This corresponds to a relative intensity of only 17.5% (81.9 / 468 = 0.175) compared to the infrared radiator with microstructure from Table 2. This means that a heating material can be heated to the target temperature more than five times faster with the infrared radiator according to the invention from Table 2 - compared to the Kanthal® MW radiator without microstructure.

[0113] Figure 6 shows three embodiments of heating elements 10, 11, 12 according to the invention, the surface of which is provided with a microstructure, for example—as described above—with a baked-on slip layer or with a microstructure produced in some other way, for example, by laser or mechanical processing. In the latter case, the average surface roughness is preferably in the range of 5 μm to 50 μm; in the exemplary embodiment, it is 25 μm.

[0114] Figure 6B shows a heating element 11 in the form of an elongated wire. The microstructure increases the surface area of ​​the heating element. To further increase the surface area of ​​the heating element relative to a cladding tube section, the heating element can be compressed, folded, or twisted. Figure 6A shows a heating element 10 in the form of a foil that is alternately folded to increase the overall surface area of ​​the heating element. Figure 6C shows a compressed and thus corrugated heating element 12.

[0115] Figure 7 shows a sectional view of a second embodiment of an infrared radiator according to the invention, which is assigned the reference numeral 30 overall. The infrared radiator 30 has a cladding tube 31 made of quartz glass. The cladding tube 31 is cylindrical and has a length of 500 mm, an outer diameter of 13.7 mm, and an inner diameter of 12.5 mm. It surrounds a heating wire 32 made of Kanthai® A (Kanthai® is a registered trademark of Kanthai AB, Sweden), which is provided with an electrical connection and can be heated to temperatures up to 950 °C. A microstructure 33 is created on the surface of the Kanthai heating wire 32 by applying and firing a slip layer.

[0116] The outer surface of the cladding tube 31 is semi-tubular (180°) coated with a gold reflector layer 34. The reflector layer 34 has a layer thickness of 0.2 μm. It provides very good reflection of incident radiation, so that the radiation emitted by the heating wire 32 is essentially directed toward the outer surface not coated with the reflector layer 34. The outer surface of the cladding tube 31 not coated with the reflector layer 34 forms the actual radiating surface of the infrared radiator 30. The slip described above is applied as layer 35 and baked onto the outer surface not coated with the reflector layer 34. The thickness of layer 35 is approximately 40 μm. The layer 35 lowers the temperature of the cladding tube via radiative cooling. This increases the service life.Alternatively, the lateral surface not provided with the reflector layer 34 can also be coated with a black lacquer, for example, with the aforementioned Ulfalux® thermal coating 1590ST. Alternatively, the black lacquer can be a layer of black thermal dispersion paint with the following composition:

[0117] Aluminosilicate solution 15 wt.%

[0118] Copper chromium itblack spinel 30% by weight

[0119] Water 40 wt%

[0120] Volatile organic components 15 wt%.

[0121] Figure 1 shows two emission spectra 50, 51. The first emission spectrum 50 is that of a first infrared radiator without a microstructured heating coil. The second emission spectrum 51 comes from a second infrared radiator with a microstructured heating coil.

[0122] The first and second infrared emitters are two identical shortwave (SW) infrared emitters with a tungsten heating element designed for heating element temperatures Tf of up to 2,400°C. The first and second infrared emitters differ in that the surface of the heating coil of the second infrared emitter has been provided with a microstructure by sandblasting and has a higher average roughness of 10 μm than the surface of the first infrared emitter with 1 μm. The higher roughness of the heating element leads to a higher emissivity Σ and surface area A of the heating element of the second infrared emitter. With the same Pei and P opt, this lowers the temperature of the heating coil. The heating element of the second infrared radiator appears less bright visually, but at a lower temperature Tf of 2,273 K, due to its higher emissivity £, it radiates the same amount of energy as the heating element of the first infrared radiator at a Tf of 2,473 K. Due to the lower heating element temperature, the peak wavelength shifts to the medium wave range according to Planck's radiation law. This means that the second infrared radiator radiates more energy in the medium wave range than in the short wave range. The result is a shift in intensity from the short wave range to the medium wave range, without increasing the electrical power.

Claims

PATENT CLAIMS 1. Infrared radiator (1; 30), comprising a cladding tube (2; 31) and a heating element (3a, 3b; 10; 11; 12; 32) arranged in the cladding tube (2; 31) for emitting infrared radiation, characterized in that the heating element (3a; 3b; 10; 11; 12; 32) has a surface (20; 21; 22) with a microstructure (33).

2. Infrared radiator (1; 30) according to claim 1, characterized in that the microstructure (33) is produced by mechanical processing, chemical processing or by processing the surface with a laser.

3. Infrared radiator (1; 30) according to claim 1, characterized in that the microstructure is produced by a surface layer in which quartz glass and / or ceramic particles are embedded, which contribute to the microstructure of the surface.

4. Infrared radiator (1; 30) according to claim 3, characterized in that the layer (35) contains elemental silicon.

5. Infrared radiator (1; 30) according to one of the preceding claims, characterized in that the surface (20; 21; 22) has a surface roughness in the range of 0.1 to 50 pm.

6. Infrared radiator (1; 30) according to one of the preceding claims, characterized in that the surface (20; 21; 22) with the microstructure (33) is formed on a heating element (3a; 3b) in the form of an elongated wire, a twisted wire, a metal foil and / or a plurality of wires twisted together.

7. Infrared radiator (1; 30) according to claim 6, characterized in that the elongated wire with the microstructure (33) is twisted in the wire longitudinal axis and / or the metal foil with the microstructure (33) is twisted in the foil longitudinal axis or compressed or folded to form a spring.

8. Infrared radiator (1; 30) according to one of the preceding claims, characterized in that the cladding tube (2; 31) has a front side for emitting the infrared radiation and a rear side opposite the front side, and in that the front side is provided with a black lacquer (5) or a layer (35) of quartz glass, ceramic and silicon particles.

9. Infrared radiator (1; 30) according to one of the preceding claims, characterized in that the surface with the microstructure (33) has a specific BET surface area in the range of 1 m 2 / g up to 50m 2 / g.

10. A method for producing a microstructure (33) on a heating element (3a; 3b; 10; 11; 12; 32) for emitting infrared radiation, comprising the method steps: (a) applying a slurry comprising quartz glass and / or ceramic particles to the heating element (3a; 3b; 10; 11; 12; 32), and (b) sintering the slip on the heating element (3a; 3b; 10; 11 ; 12; 32) to form the microstructure (33).

11. Method according to claim 10, characterized in that the slip is applied according to method step (a) by immersing the heating element (3a; 3b; 10; 11; 12; 32) into the slip mass or by spraying.

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