Convection section in a fired heater 1

[AndreChE]

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

 

[jalvarez]

Yes, AndreChE, you are correct.
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.[Δ]T assumption is OK.
Regards

 

[AndreChE]

Yes I am considering all of that.

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

 

[furnacebengineer]

1.5 - 2% is typical for overall heat loss - as a percentage of total heat into the firebox. If you've got 2.8% in a convection section you've got problems that should be addressed.

 

[AndreChE]

This furnace casing will be replaced in December 2004.

A revamping for this furnace is ongoing, including radiant coils replacement.

AndreChE

 

[sailoday28]

"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)

 

[25362]

AndreChE, you can obtain graphic as well as tabulated heat losses Btu/(ft².hr) from heater surfaces as published in chapter 3, titled Efficiency and Fuel Conservation, of Furnace Operations by Robert D. Reed (Gulf Publishing Company).