Air Preheater Economics 1

[d8210]

I am a fairly new ChE grad and work at a petroleum refinery and have been tasked with finding out the economics of running a heater in one of my units without the air preheater. I know the design duty of both the air preheater and heater. For instance, if the air preheater had a design duty of 8 MMBTU/h and we decided not to use it, I don't believe that it means we would now be using 8 MMBTU/h more in fuel for the heater to make up for the lost duty of not running the air preheater. To my understanding, airpreheaters only give about a 5-7% increase in heater efficiency when used. Is there a method to calculate the lost duty that is more accurate? Is maybe taking a ratio of the airprheater and heater duties versus a ratio of the heater and air preheater efficiencies a more accurate method? Any ideas, and/or help are appreciated.

 

[zdas04]

The basic equation is Q=mc_pΔT. It is usually better to do the arithmetic than to rely on rumors.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[d8210]

I know the equation. My only problem being limited data available. Thank you for the response

 

[zdas04]

Sorry for trying to help. By the way, you will find if your career develops that the data available is ALWAYS limited.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Latexman]

A former boss/Assistant Plant Manager once told me, "In God we trust, all others bring data."

Good luck,
Latexman

 

[TD2K]

What is the process duty of the heater relative to the preheater's 8 MMBTU/hr?

I would suggest doing a combustion calculation with and without air preheat so you can see how the numbers work out. As David suggested, there's nothing like working through the numbers yourself rather than just finding some chart off the Internet.

You are burning your fuel gas with air which goes into heating the various streams in the fired heater (process, maybe steam generation), some casing losses and stack losses ultimately. As a first pass to looking at the effect of air preheat, assume that your stack temperature is constant (it won't be exactly) and see the efficiency with preheated and ambient air.

 

[rutherford703]

It shouldn't be that difficult.
First as TD2K said, do a combustion calculation: Fuel flowrate, fuel composition, excess air used to get flue gas composition.

You may find flue gas outlet temperature using two ways: either through a thermal couple at the exit of the furnace or using a heat balance for the furnace. For big furnace, consider 1% heat loss.

Then you can assume a flue gas outlet temperature from air preheater. Using the formula David provided to calculate the heat recovery with an air preheater. With an air preheater, possibly an ID fan is required to get the flue gas through the stack. The energy required for running the ID fan

According to the price of energy $xx/KWh, you can find how much $ you saved with the air preheater.

Considering the cost of the air preheater, you can figure out your RI (return of investment)

 

[buelowj]

Do you plan to switch the heater to natural draft mode when you bypass the APH? If not, you better check the design temperature of your ID fan. Frequently the ID fan design temperature assumes cooling from the APH and may not be able to handle the hot flue gas.

 

[25362]

In addition to what has been said it wouldn't harm to also check what would happen when shutting down the preheater:

a. to the heat fluxes in the radiant and convecton zones.
b. to the combustion efficiency if the preheating surpassed 400°C.
c. to the duties of the radiant and convection sections if/when used for different processes.

Useful reading:

http://en.citizendium.org/wiki/Air_preheater

http://www.flox.com/documents/07_TP.pdf