Fluegas temperature drop 2

[AndreChE]

Hello all

I have a question related with the heat recovery in the convection section of 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.

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

 

[athomas236]

AndreChE,

Is the convection section you refer to evaporator, superheater or economiser.

What information do you have about the other parts of the process that could be used to calculate heat absortions. For example, if you know say water flow and inlet/outlet water temperatures these could be used to calculate one unknown gat temperature by heat balance.

Regards

ahthomas236

 

[25362]

Discarding any inward air leak, and the heat lost through radiation and convection to the surroundings, one generally recognized problem in basing heat duties only on gas heat content, is the inescapable lack of accuracy of mass and temperature measurements on the flue gas side.

Thus heat absorption calculations as athomas236 indicates are helpful in carrying out enthalpy balances.

 

[AndreChE]

Yes

There is Steam Superheater, feed heater and vaporizer, economizer - steam cracking furnaces.

I understand the lack of accuracy but the heat transfer coefficients are different from design and there is a lot of information that I don't have in this section.

 

[athomas236]

Years ago when I used to be a boiler engineer I often had the problem of analysisng test results with incomplete information.

The approach I used was the create a theoretical model that would calculate gas and steam temperatures with empirical factors to correct the theoretical HT coefficients.

What I usually knew was the gas temperature from the economiser with steam and water temperatures and flows.

I would run the model modifying the correction factors until I got reasonable agreement between measured data and calculated results.

The results were then used to make predictions for other cases.


athomas236