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I currently working with naphtha hydrotreater plant we have splitter column which split only light and heavy naphtha.in our reboiler (fired heater) of that column convection inlet temperature is 155C and convection outlet is 181C further it goes in radiation inlet where temperature same as a connection outlet but outlet of radiation is 161C. I don't understand that why temperature in radiation zone decrease compare to convection outlet.
 
Answers
20/04/2017 A: S Sivasankaran SABANAYAGAM, CPCL, cpclsiva@yahoo.com
It looks like an unlikely scenario and thermowell at radiant outlet may be giving a wrong indication.
To check the correctness of temp measurement, thermal imaging can be carried out at Convection I/L, cross over and at radiant outlet after removal of insulation at thermowell locations. By this way average metal temp at these locations will be known with which the flow medium temperature can be correlated.
You may also Check whether any steam connection is provided at the cross over tube (normally not given for the service indicated ) which may cool down the stream
20/04/2017 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
It temperature decreases in a furnace normally occur because the cooling effect of vaporization is happening faster than heat input from the heater. This phenomena is seen quite commonly in vacuum heaters where the peak temperature occurs before the furnace outlet. The rapid increase in velocity that occurs as vaporization starts quickly starts increasing the pressure drop. The lower pressure promotes more vaporization. The heat sensible heat increase is lower than the latent heat cooling effect so the temperature drops, even though there is still heat being added to the process.
20/04/2017 A: Mike Watson, Tube Tech International Ltd, mike.watson@tubetech.com
This maybe because the finned convection banks are fouled. Solution is to use the latest robotic cleaning technology which is the only system to guarantee to clean every row of your tube bank and thereby reduce your stack temperature back to design parameters.