Convection section in a fired heater
Convection section in a fired heater
(OP)
Hello everyone
To calculate the Heat Absorbed in each part of the convection I can use Q=U.A.DeltaT(log)
But in normal operation I cannot know the outlet temperatures in each section.
Can I use Q=M.cp.DT for the fluegas? The heat absorbed by all sections in the convection is the same as the heat given by the fluegas to the process.
I can calculate the fluegas flowrate and cp. I have the bridgewall and stack temperatures.
Am I correct?
AndreChE
To calculate the Heat Absorbed in each part of the convection I can use Q=U.A.DeltaT(log)
But in normal operation I cannot know the outlet temperatures in each section.
Can I use Q=M.cp.DT for the fluegas? The heat absorbed by all sections in the convection is the same as the heat given by the fluegas to the process.
I can calculate the fluegas flowrate and cp. I have the bridgewall and stack temperatures.
Am I correct?
AndreChE





RE: Convection section in a fired heater
I recommend to consider some heat losses for the fluegas due to radiation losses. Let's say 2%, it will depend on the furnace condition (maintenance of thermal insulation), the wind (yes!), ambient temperature, and other heat leaks.
If you are trying to simulate or balance the furnace, you will see that you need to consider all this to close it.
From other thread I suppose you are dealing with a cracking furnace. If I'm correct, and if in the future you will try to balnce heats for both sides (process and flue gas) take into account the low amount of cracking reaction of some feedstocks occurring in the convection section. The supplier must inform this (normally). But again, your m.Cp.ΔT assumption is OK.
Regards
RE: Convection section in a fired heater
The losses are exactly calculated by the Wimpress method (considering wind velocity and everything)
Some furnaces are higher than 2% (2,8% the worst)
Thank you.
AndreChE
RE: Convection section in a fired heater
RE: Convection section in a fired heater
A revamping for this furnace is ongoing, including radiant coils replacement.
AndreChE
RE: Convection section in a fired heater
"To calculate the Heat Absorbed in each part of the convection I can use Q=U.A.DeltaT(log)
But in normal operation I cannot know the outlet temperatures in each section."
I believe that DeltaT(log) is used when one of the temperatures is constant.
Otherwise, it should be DeltaT(mean)
RE: Convection section in a fired heater