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 . And on which condition this can be happened.
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.
I agree with Vinicius answer that the steel sheet absorbs heat energy more efficiently than the surrounding air. It is due to the fact that the heat capacity of steel sheet higher than surrounding air so that the heat from surrounding air and irradiant heat are absorb effectively within the steel sheet and make the temperature of the steel sheet higher. The temperature different between the steel plate and surrounding air is happen mostly at the bottom of the steel plate due to the fact that hot air will be lighter than cold air. The hot air will be moved to the top of the furnace while the colder air will be at the bottom of the furnace, hence, make the temperature difference between the surrounding air and steel sheet is greater at the bottom.
However, above conditions happen only the furnace is not fully sealed (there is a leakage air) so that the conditions inside the furnace (the steel sheet and surrounding air) never reach an equilibrium state (almost no temperature difference).
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