Definition of Heat Outputs

[Mechwill]

Hi everyone,

I have posted this thread recently, but I guess I have put it in the wrong section and I hope I can get some answers here.

http://www.eng-tips.com/viewthread.cfm?qid=378320

I understand this is very fundamental. I am working on a small low pressure boiler design. To find out what the BTU output is, I am familiar with Q=m*cp*delta_T. The water inlet and outlet temperatures have been measured as well as the water flow rate. This will give me a general ideal of the heat exchange rate of this boiler.

However, there is another way to determine the heat output ant that is by analyzing the exhaust emission, which provides the data of efficiency, flue temperature, net temperature, and etc. Using these data, the heat input and output can also be calculated. In other word, this heat calculation is based on fuel consumption

The question I have is that is there a direct relationship between these two heat outputs?

Does the efficiency that shows from the emission analyzer mean the efficiency of the boiler?

 

[IRstuff]

I don't see how your exhaust emission alone can tell you efficiency, but your fuel consumption rate is much more useful than looking at the flue output, I would think.

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[Mechwill]

IRstuff

When we test a boiler, we have an emission analyzer which measures the O2 percentage, CO level, excess air, flue and net temperature, and efficiency.

You are right that about the importance of fuel consumption. The CSA provides formulas and information about heat input and output which includes the measured fuel consumption and measured efficiency. However, in my opinion, these information represent the emission in the combustion process, not the heat exchange process.

 

[IRstuff]

No, the efficiency that an emission analyzer would be measuring is only combustion efficiency, i.e., how close to stoichiometrically perfect was the combustion process. It says nothing about the overall quantity of fuel that you consumed nor how much of the heat generated was transferred into the boiler

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[ione]

The flue gas analyzer gives, among other values, a measure of the combustion efficiency, which represents a part of the boiler efficiency (or steam to fuel efficiency). Besides the combustion efficiency it is necessary to keep into account the energy losses due to radiation and convection phenomena in order to get an evaluation of the boiler efficiency.

 

[Mechwill]

The amount of energy input provided by any type of fuel can be determined by the following equation:

Qin = HHVfuel * mf,actual

which this equation is based on based on fuel consumption and energy value.

Efficiency by definition can be generally expressed as this:

Eff (%) = (Qout / Qin) * 100%

The amount of energy output is the result of the amount of energy input that goes through a system (in this case a boiler). If I understand what you guys are saying, does this define how efficient the burning system is in the combustion process, instead of the overall boiler efficiency?

Or, another way to look at this. Does the "True BTU Output" is measured and calculated by the water temperatures and flow rate?

 

[ione]

The combustion effciency takes into account only the energy lost through the stack as flue gases go into the atmosphere with their sensible heat. But this is just a part of the total enery lost one has to account for when erforming an energy balance as during the heat transfer process, part of the enrgy supplied is lost through the shell because of radiation and convection as well.

 

[IRstuff]

"True BTU Output"

You've used this term twice, but whose output are you referring to?

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[Mechwill]

IRstuff

That was the same question I have when I kept hearing people saying this phrase. I guess in this application, the heated water is used from the boiler to provide both hot air and hot water. So, my guess is that true BTU output of this boiler is the BTU outlet from the water side instead of the emission side.


Ione,

I finally get what you mean by that now. It depends on what I am looking at here. Combustion efficiency explains how efficient a burner is designed where the overall boiler efficiency is based on how well the heat exchanging system is.

Thanks for all the helps and info on this topic, truly appreciate