7 replies to this topic

[raj1]

Hello professionals. V have balanced draft furnace contain combustion air preheater to recover heat from flue gas leaving convection section. Earlier it was operated with more negative draft pressure, less air flow (say 37 KLBH), excess O2 v were getting ~ 4% after load variation ,again furnace was brought back to same load and same process condition (conversion, ethane feed flow, feed composition) with little bit high draft pressure (more positive side than earlier case) with same almost excess O2, more air flow than earlier (say 50 KLBH), almost same fuel gas flow as comparing earlier case. However, my coil temperatures and last pass temperature were higher than earlier.can anyone explain me the possible cause? How coil temperature increase with   excess air..seems contradictory..

[Bobby Strain]

Maybe you didn't get the data correct. You would need to look at the preheated air temperature to complete the data. Maybe you can also tell us what you mean by coil temperatures and last pass temperature.

 

Bobby

[raj1]

coil temperature is nothing but actual temperature of coil inside radiant box. lass pass temp is the temperature of the coil at last pass before leaving radiant box.

[raj1]

There is no temp indication for preheated  combustion air. Does my coil temperature increase when i operate the furnace with excess O2 than earlier for similar process condition?

[sgkim]

Hi raj,

For a fixed fuel gas rate, the increase of excess air will surely drop the flue gas temperature. To investigate your plant's abnormality, first check the furnace system more in detail--(1) air flow meter (2) temperature gauges (3) coil leakage, if any, and then check the heat balance in the heater. 

 

~Stefano

[Pilesar]

Your control system is probably operated to achieve a specified process duty in the radiant section. More excess air means more fuel burned to achieve the same process duty. If the radiant section duty is the same, the result is more hot gas to the convection section. 

[raj1]

S pilesar. To maintain the fixed radiation duty my fuel gas flow will increase. Which will increase my convection section duty and the same heat will be recovered in feed preheater and combustion air preheater. Unfortunately i dont have any feed temperature before going into radiant box and air temperature before entering into burners. So i cant able to conclude what cause the rise in temperature for the same excess O2?

Edited by raj1, 02 February 2017 - 03:43 AM.

[Pilesar]

Heat transfer is not a function of temperature only. With more excess air, you also have more mass of gas through the convection section. The excess oxygen measurement is on a percentage basis and indicates composition which is not directly relevant to convection section heat transfer. Calculate the gas mass rate based on combustion equations and fuel flow.