I have found that the surface temperature of steel sheet in the centre of our gas fired radiant tube continue furnace is higher than its surrounding air temperature. What is the reason for this? I would be very grateful if anyone can explain this thermodynamically or practically.
Please clarify your question, you mean tube metal temperature in a process fired heater? if so what is the process fluid? If you are talking about a steel melting furnace please specify
The size of the steel sheet is 3000mm by 2000mm and the thickness range from 2 mm to 6 mm.
The steel sheets are hanged up vertically and pass through the furnace slowly for heat treatment process where the temperature of the furnace is about 800 to 850C.
The gas fired radiant tube in our continue furnace is about 500 mm away from the steel sheet.
We have measure the surface temperature of the steel sheet and the air temperature around the steel sheet about 20 mm away from the steel surface use k type thermocouple. And we found that the temperature of steel surface is higher than of the temperature of air 20 mm away from the steel surface. For example temperature of steel surface is 820C. temperature of air 20 mm away from the steel surface is about 800C. This happened mostly in the bottom of the sheet.
How do you attach the thermocouple to steel surface? please specify in detail, for example what type of welding (if there is any) has been used? does the termocouple has a thermowell? If so does the thermowell see the radiating tube? Faulty temperature readings can happen because of direct radiation and also type of connection and attachment to surface. Tell me the details or show me a photo of actual measurement maybe I can help.
Is it not the case that metallic surfaces have a higher conductivity than air? And measuring the radiant thermal energy of the surface of metals at a variable distance from the air measured will yield a reasonable temperature difference in view that air can provide insulation properties to an environment. It is not surprising to my understanding that a delta-T of approximately 2% is given by your parameters. This is also evident from your description of a variable temperature gradient with the furnace, which is to be expected from most fired or radiant heaters.
Based on your explanation about your system, I believe radiation is the main heat transfer to the plat and hence, the plate would have higher temperature than the surrounding air due to higher conductivity as explain by Don Hargis and also has higher heat capacity. You can do heat transfer calculation at the plate and the surrounding air to confirm this.
If your plate enter the furnace vertically from the bottom then the bottom side of your plate will got more heat since it is expose to the heat longer than the upper surface. You may need to apply holding time when all of the plate (rather than slow down further the plate entering the furnace) inside the furnace to ensure thermally equilibrium.
Heat transfer occurs whenever there is temperature difference. There are two ways heat enters the steel sheet in your case: convection and radiation. If surface T > air T, the heat is escaping from the sheet to the air. Meanwhile, electromagnetic waves coming from the surrounding surfaces strike at the sheet - radiation heat trransfers to the sheet. If possible, measuring T of the surrounding surface may help.
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