Technical University of Czestochowa The Faculty of Process & Material Engineering and Applied Physics The Department of Industrial Furnaces and Environmental Protection Marian Kieloch, Agnieszka Klos, Jarosław Boryca, Edyta Warwas Possibilities of measuring the surface temperature of charge in heating furnaces Abstract The knowledge of the precise value of temperature is necessary for the proper control of the heating process. Based on laboratory tests carried out so far, a proposal has been made to utilize digital photopyrometry as a method that could be used for the measurement of temperature in the future. The paper has also compared temperature measurement results obtained using thermovision and digital pyrometry, which could be an alternative to the former. Conclusions The analysis of the test and calculation results has shown that the obtained results of temperature determination using the digital camera little differ from the reference temperatures. The average measuring error within the entire temperature range for the use of digital photopyrometry is larger only by approx. 0.2% compared to thermovision. Above 800  C, on the other hand, the error obtained with the use of photopyrometry does not exceed 1%. Moreover, a prerequisite for the use of thermovision is the correct determination of the emissivity of the material examined. The incorrectly selected value of emissivity causes measuring errors reaching up to several hundred degrees. Thus, digital photopyrometry is an accurate method of measuring steel charge temperature, being, at the same time, much cheaper than thermovision. Digital photopyrometry has a potential to be a major competitor for thermovision among the methods used for industrial temperature measurements. 7. The effect of temperature on the greyscale level of the digital photo The effect of temperature on the greyscale level of the digital photo is illustrated in Figure 5. This dependence was determined for three exposure time settings, namely S=1/60s,S=1/125s and S=1/250s. It has been established that the effect of temperature on the greyscale level can be described with the following equation: 2.The influence of capacity on the temperature distribution in the furnace heating chamber The process of heating charge in industrial heating furnaces relies on the complex phenomena of heat and mass transfer, both within the furnace chamber and in the bulk (cross-section) of the charge being heated. The full understanding of those phenomena is extremely difficult, both in model studies and the industrial operation of furnaces. The performance of heating furnaces is determined by: energy intensity (unit consumption of heat), steel loss for scale, CO 2 emission, scale adhesion to the substrate, and decarburized layer thickness. Very often, the process of heating charge in rolling mills and forges is also decisive to the quality of finished product. The thesis could be put forward that the basis for the determination of the performance a furnace (charge heating) should be the information on the course of thermal phenomena in the furnace chamber, as well as in the charge itself. It is the most favourable course of these phenomena that should govern the process technology, and thus the thermal conditions in the furnace heating chamber. On the basis of theoretical derivations and model study results it can be stated that the performance of heating furnaces is determined by heating technology, and for a given technology – by the capacity of those furnaces. Requirements imposed on the processes of heating charge before plastic working are extremely simple and come down to obtaining the following at the end of the process: the assumed value of charge surface temperature, and the specific temperature difference on the charge cross-section. In industrial furnaces, the continuous measurement of charge surface temperature is very difficult, or even impossible. The furnace temperature is the output parameter in the control system of any fuel-fired furnace. It must, however, precisely inform about the value of the heat flux taken up by the charge, so a strict relationship must exist between the furnace temperature and the charge surface temperature. This relationship is extremely complex, and the furnace temperature depends on the temperature of combustion gas in the chamber, the temperature of the inner furnace wall surface, the charge surface temperature, and on the heat exchange conditions, including: furnace capacity, chamber geometry, combustion gas radiation properties, the radiation properties of the walls and the charge, and combustion gas motion in the furnace chamber. Despite the fact that so measured furnace temperature is little objective and does not directly indicate the temperature of the charge surface, it remains invariably the parameter defining heating process technologies. The surface temperature of charge at the exit from the furnace chamber is equal to the preset temperature and amounts to 1250  C, with the assumed temperature difference in the cross-section of Δt=50 K. After reducing the capacity, for the same technology, the surface temperature values increase. Further reducing the capacity results in a continued increase in the surface temperature. This temperature increase causes directly an unnecessary increase in usable heat and a significant increase in the loss of steel for scale. At the same time, over more than 35% of the furnace length, the charge surface temperature increases to over 1300  C. Reducing the furnace capacity results in a slight increase in wall temperature. As a consequence, the thermal balance items associated with the above temperatures also increase or remain unchanged. The combustion gas temperatures, apart from the equalizing zone, are slightly lower for reduced furnace capacities. The temperature of combustion gas existing the furnace decreases with reducing capacity. The computation results indicate that, for a given heating technology (T1), the heat consumption is determined by furnace capacity (Fig. 1). This is entirely consistent with the results of industrial tests. From the theoretical derivations, computation results and the results of model studies and industrial tests it can be concluded that it is impossible to obtain low heat consumption indices for small furnace capacities. The results of the performed computations show also that the lowest heat consumption will be reached for any heating conditions by adapting the heating technology to the conditions of the ideal process. Fig. 1. The influence of heating technology and furnace capacity on the consumption of heat 3. Methodology of measurements taken using the digital camera Digital photopyrometry is a contact-less method. The measurement and recording of temperature is done using a digital camera [3]. The method consists in making a photograph of the radiating object under examination. The core of this measurement is the recording of the spectral emission density, ecλ. The digital camera has a CCD converter, instead of the traditional photographic plate, and a processor that processes the data and enables them to be stored in the camera’s memory. Data are recorded in the RGB (Red, Green, Blue) mode as a 24-bit colour image. Then, using suitable software, this image can be transformed into an 8-bit multi-grade bitmap to be red out in greyscale. 256 greyscale levels (from 0-Black to 255-White) are recorded. Such transformation enables the temperature of the examined object to be represented as a function of the absolute greyscale level. The greyscale level is determined by the computerized analysis of digital photos using a suitable graphical program [4]. Fig. 2. The digital photography of sample for two temperature: a) 700ºC and b) 1100ºC The greyscale level determination is a basis for developing a temperature characteristic. Such a characteristic will define the dependence of the surface temperature of the heat radiation-emitting object being photographed on the average greyscale level of the photograph showing the object under examination. The Fig.2 is presented the digital pictures example. 4. The effect of camera settings on the form of the temperature characteristic The effect of the ISO sensitivity of the CCD matrix on the behaviour of the relationship t = f(r) is illustrated in Fig. 3. This relationship is rectilinear in a certain section of the temperature range. A large increase in ISO sensitivity produces an effect in the form of a shift of the rectilinear section of the characteristic curve toward lower temperatures. Fig. 3. Influence of affection ISO on course of dependence t = f(r), S = 1/60 s, A = 4 [12] 5. Methodology of measurements performed using a thermovision camera A thermovision camera converts the temperature radiation coming from the object being observed into an electronic signal. Thermovision apparatus makes it possible to conduct the scientific observation and examination of thermostatic and thermokinetic variations and enables the contact-less measurement of temperature. Thermovision finds application in the assessment of the construction of furnaces and heaters, the monitoring of charge heating and cooling processes, the determination of heat losses and the examination of heating element operation [13]. However, in order to make a correct temperature measurement with a thermovision camera, the emissivity, ε, of the material being examined will have to be determined. This method is expensive, which substantially limits its application. Nevertheless, it will be irreplaceable in the observation of both static and dynamic surface temperature distributions.The termographs example is illustrated in Fig. 4. Fig.4. The termographs of sample in two temperatures: a) 700ºC and b) 1100ºC 6. The measuring stand Tests were carried out on a round steel specimen. The specimen was heated up directly in the chamber of an electric-gas oven. The reference specimen temperature was measured using an Ni-CrNi thermocouple and red out in stationary heat flow conditions, with the simultaneous photographic recording of the image of the specimen being examined. The distance of the camera from the test specimen was approx. 1.5m. A reflex camera by Olympus® was used for the tests. The technical specification of the camera is as follows: ISO 80, 160, 320; S= 1/60 s, 1/125 s, 1/250 s; A=4 [12]. For comparison, the recording of test specimen temperature was also done using a thermovision camera. The distance of the thermovision camera from the test specimen was approx. 1.5m, similarly as for the photographic camera. The emissivity, on the other hand, was measured by adjusting the temperature measured with the camera to the temperature value indicated by the thermocouple. The measured ε value was ε=0.9. Fig.5. Influence of temperature on the degree greyss of digital photo for carbon steel, ISO 80, A=4 where: a, b, c, d – constant value, r – degree greyss, t – temperature, °C. On the basis of measurement results, e.g. for S=1/60s, the following relationship is obtained: The calculation of the value of temperature from the developed relationship is difficult. Therefore, relationship (1) is represented in the following form: The effect of the greyscale level of the object examined on the value of temperature is represented graphically in Fig. 6. Fig.6. The effect of time of time exposure on the course of dependence t = f(r), ISO =80, A =4 In the identical manner, the relationship under consideration can be developed for other conditions of measurements carried out. 8. Comparison of measurement results Measurement results obtained by three different methods are shown in Fig. 7. The temperature values obtained using the digital camera differ from the reference temperatures to a considerably lesser extent compared with the temperature values measured with the thermovision camera. In the case of the digital camera, a larger error is observable at lower temperatures. By contrast, for the thermovision camera, the error increases with the increase in specimen temperature. Fig.7. Results of temperature measurements with utilization of three method 9. Assessment of measurement accuracy It can be found from the performed assessment of accuracy that the results obtained using digital photopyrometry exhibit relatively small deviations from the reference temperatures. Differences between the reference temperatures and the measured temperatures for photopyrometry are even smaller than for the thermovision camera. Digital photopyrometry is distinguished by higher measuring accuracy, particularly in temperature ranges above 750  C. Comparison of temperature measurement results obtained using the thermovision camera and the digital camera against the reference temperature values is given in Table 1. Table 1. Comparison of the result of temperature measurement