boiler exhaust gases analysis to efficiency evaluation

ppiotr · December 17

for study reasons I am trying to understand the meaning of some regulatory requirements on the analysis of exhaust gases from home boilers (30kW) for the purposes of evaluating their efficiency:

η= 100-Q = 100-(A1/(21-O2)+B)*(Texh-Tamb) )

(A1,B costants): what I am presenting to you is a conceptual question that I cannot be able to solve: I read in the UNI_10389 standard that the socket for the analysis of methane combustion gases, for the purposes of calculating the efficiency, of a B11 boiler with natural draft with integrated antirefouleur device, must be made by drilling the exhaust duct downstream of the antirefouleur; it is also recommended to avoid re-entry of ambient air into the hole where the probe is inserted.

I would like to kindly ask: downstream of the antirefouleur the exhaust gases are mixed with ambient air by approximately 25%: does the analysis of these gases so “diluted” (in terms of O2 and Temperature), still keep it significant for the purposes of calculating the efficiency by means of the above formula? is there perhaps a “compensation” between the 2 effects of lowering Texh and increasing O2, i.e. the combined effect of the 2 factors cancels out? or, is this contribution negligible? what am I missing?

If there was an exhaust gases analysis socket placed by the manufacturer directly on the body of the boiler, before the antirefouleur, couldn’t it result more significant?

I thank you in advance for any information or ideas you may wish to provide me

jesmed1 · December 17

jesmed1 · December 17

Apparently you're asking about the Siegert formula commonly used in Europe for calculating boiler efficiency.

Yes, you are correct. Adding dilution air should affect the combustion efficiency measurements using the Siegert formula. The measured oxygen content is used by the formula as an indirect way of measuring CO2 (carbon dioxide), because for a given (known) fuel, the heat of combustion can be calculated by knowing how much carbon dioxide is in the exhaust products.

So adding dilution air (with oxygen) will mess up the CO2 calculation in the Siegert formula, and also will change the flue gas temperature. Although these two changes have somewhat opposite results in the Siegert formula, they are not self-cancelling. So the result will be wrong.

You'll note that the UNI standard you quoted even says to "avoid entry of ambient air into the hole where the probe is inserted." Which tells you that the addition of dilution air is going to change the results.

Here in the US, the common digital combustion analyzers are always inserted into the exhaust gases before any air dilution of the flue gas, as close to the flue gas exit from the boiler as possible, but downstream of the heat exchanger.

So I would think that you'd want your probe port in that location: just at the flue gas exit from the boiler. It can't be in the body of the boiler itself, because you can't get an accurate efficiency measurement until the exhaust gas stream is completely past the boiler heat exchanger.

As an example, here's a photo of my boiler with a pencil pointing to the hole in the flue where the boiler technician inserts the combustion analyzer probe. You'll notice it is BEFORE the barometric damper which admits dilution air.

ppiotr · December 18

thank you very much for your reply; I also imagine that measuring downstream the antirefouleur it may result difficult to implement some kind of automatic or manual “correction” of the analyzer so that it takes into account the additional ambient air: the section of the antirefouleur may vary with model; also the draft and gas velocity of the specific installation could modify via venturi effect the amount of additional air

thank you again for your time and cooperation

best regards

jesmed1 · December 18

https://forum.heatinghelp.com/discussion/comment/1827589#Comment_1827589

That is also correct.

boiler exhaust gases analysis to efficiency evaluation

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