Investigation, comparison and improvement of technical infrared radiators

Abstract

We investigated and compared relevant properties of technical infrared radiators, for example: foil radiators, quartz-tube and ceramic radiators. As typical representatives of the various radiator types we tested radiators by “Krelus Infrared”, Hirschthal, Switzerland, “Matherm”, Villars-les-Dombes, France, “Heraeus”, Kleinostheim F.R.G. and “Infrared International”, Ballydehob, Ireland. Special attention was paid to the determination of the spectral directional emissivities of the effective radiating areas of the infrared radiators. Our measurements were performed with an infrared spectrometer converted and adapted to spectral emissivity measurements. With this spectroscopic system we succeeded in measuring spectral directional emissivities of hot surfaces at temperatures from 973 to 1273 K between 1 and 16 μm wavelength with an accuracy of 2%. In order to perform the emissivity measurements, we constructed various models of blackbody-reference radiators. The quality of our final reference radiators was tested by comparing their normal spectral emissivity with the emissivity of two commercial blackbody-reference radiators. From comparative measurements and theoretical analysis of the spectral emissivity of blackbody cavities, we conclude that our reference radiators achieve a spectral emissivity better than 0.98 for wavelengths between 1 and 2 μm and better than 0.99 between 2 and 16 μm. In order to improve foil-type radiators, we developed ceramic-enamel layers for the protection of the foils against corrosive atmospheres without reducing their spectra emission. We succeeded with two different types of ceramic-enamel layers which can be applied to the “Krelus Infrared” foils, one of which is optically black. For the foils coated with each of the two enamels we found a long life time in the standard atmosphere. We assume also a good protection by these ceramic enamels against corrosive media. The “Krelus Infrared” foils coated with the optically black enamel exhibit similar spectral properties as the uncoated oxidized foils. An estimation of the quality of different infrared radiators is only possible with regard to a specific application and has to be performed, therefore, by the user of an infrared radiator. In addition to the determination of the fundamental radiator properties, this requires the consideration of the energy transfer within the entire radiator-sample system. We start by discussing its theoretical aspects. Subsequently, we present approximative solutions of particular energy-transport effects between radiator and sample for simple radiator-sample configurations. Special efforts have also been made to provide data for the energy transfer through air which is of importance for most radiator-sample systems. Of particular interest is the infrared transmittance along a path through the air between the radiator and the sample surface. Consequently, the transmittance of dry air was determined experimentally for pathlengths between 0.75 and 20.25 m. For these short pathlengths, comparisons of our measurements with “Lowtran” and “line-by-line“ calculations show a good agreement for spectral regions of strong CO 2 absorption.