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Faculty of Process & Materials Engineering and Applied Physics of The Czestochowa University of Technology. The Department of Industrial Furnaces and Environmental Protection. Influence of changes objects temperature on the factor emissivity value. Mgr inż. Krystian Nowak*
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Faculty of Process & Materials Engineering and Applied Physics of The Czestochowa University of Technology The Department of Industrial Furnaces and Environmental Protection Influence of changes objects temperature on the factor emissivity value Mgr inż. Krystian Nowak* Dr hab. inż .Lech Szecówka, Prof. PCz Dr inż. Henryk Radomiak *Author is a participant of a program called „ Plan Rozwoju Politechniki Częstochowskiej” which is sponsored by European Social Found, European Union
Emissivity is one of basic bright property of matter. With temperature it characterizes radiation heat transfer, and it plays very important role in radiation-convectional, radiation-conductional and radiation-convection-conductional heat transfer.
Thermovisions technique Each body which temperature is superior than absolute zero emits heat energy in the form of thermal radiation energy in seen as well as invisible ( infrared ) range. It strictly depends on objects temperature. Thermo vision camera taking advantage of this dependence and registering this radiation appoints researched surface temperature.
Thermovisions technique • To main adwantages of thermo vision belong: • very wide temperature range, • capability appointment temperature • of flame and gasses, • big accuracy of measurement • received results give information about • holistic temperature arrangement on researched • surface.
Emissivity Perfectly black body is an ideal source of thermal radiation ( temperature ). Quantity defining defection of radiation between perfectly black body( theoretical ) and real body we call emissivity. This quantity depends (among other) from temperature. It is possible to appoint emissivity with the aid of ration equivalent bright quantities of real and perfectly black bodies - stream of emissivity A - field of emitting surface. C - constans of radiation which value is 5,6693 W/(m2K4) T - temperature Emissivity:
Exemplary results of industrial thermovision measurements The presented exemplary thermogram comes from second heating zone of unclogging furnaces. Temperature measured in this zone with the aid of thermocouples was 1100oC. Exactly this temperature is accepted as parameter which defining closest surroundings. On thermogram there are singled out two fields: „ Ar1” and „ Ar2 ”, which are subject to detailed analysis.
Measuring position and software used to analyses Asample (dimension: 30x30x50 mm) has been placed in heating chamber of electric furnace (dimension 295x105x120). It has been put on isolating pads about 10 mm high, what was suppose to minimize uneven heating by participation of heat transfer on conductive way. Sample was preliminary oxidized, however, thin coat of oxidized material strictly adhere to the remainings.
Measuring position and software used to analyses Under the surface of the researched sample two thermocouples connected to measuring card DaqLab2000 have been placed, which were a comparative fix-up for thermovision camera. To determinate surrounding temperature in heating chamber of furnace thermocouples from furnace measuring systems were taken. Heating chamber was fullfiled by nitrogen, to obtain indifferent atmosphere. It has allowed to eliminate oxidizing of farthest materials. Such phenomenon could cause to rise of considerable unsimilarity on researched surface , and what goes behind it, differences in emissivity and registered temperatures. Possible range of measuring temperature is between - 40oC +2000oC, at accuracy of measurement to 2%. Basic element of this camera is uncooled mikrobolometric detectors matrix. Analysis has been executed in the form of tables basing on standard software -ThermaCAM Reporter 7.0.
Experiment Thepurposeof the experiment was to determine a change emissivity coefficient influenced by temperature, for sample with oxidized surface. Additional element is the verification of how erroneous introduction of emissivity coefficient effect accuracy on measurement executable at utilization of thermovision camera. For the purpose of the realization comparative method was done. Measurement of temperature was executed with pin method and lecture from thermovision camera with alternate emissivity coefficient, introduce in period of processing and analyses thermogram.
Result andanalyses Checked off field “Ar1” has been chosen as part subjected analysis, where for presentation from this area was chosen average value of temperature. Conditionality was accommodation and depth that thermocouples have been fixed. Reflected temperature was accepted on base of lecture executed from regulating match of furnace. For each measurement two emissivity coefficient was adopted, such values of temperatures was closest from average pin measurements. T T
Result andanalyses It was executable during farthest attempt of charge heating. Distinct, visible differences in colors on surface of sample are an effect of progressing from 800oC temperature of separate oxidize coat from the rest of researched material. Reason for this phenomenon was thermal expansibility differences between external oxidized coat and internal not oxidized. Error in designation emissivity at the level of 0,01 can cause inaccuracy in lecture of temperature (in showed case even 1,7oC).
Resume Usefulness of thermo vision measurement in steel industry in big degree depends on measuring techniques. In order to the result was to be reliable, it is required from the person carrying measurement elementary knowledge from range of heat transfer, acquaintances of objects, researched material and certain dependences. Often indispensable condition is the execution of preliminary research allowing the assignment of basic factor effecting the measurement. It is important to pay attention to such elements: - emissivity - surroundings temperature - state (condition) researched surface. Despite certain troubles in practice of thermo vision measurements, continuous perfecting of measurement technique and obtained new practical experiences can cause generalization of these investigative methods, giving full information about temperature and schedule on researched surface of the material ( object ).
