How does a Flue Gas Analyser calculate efficiency?

[Ivan]

I've got the basic idea - it measures ambient air temperature and exhaust temperature together with free oxygen and it's told what type of fuel is being burned...but how does it know how fast the fuel is burnt?

Does a Flue Gas Analyser give an accurate measure of efficiency or is it an inaccurate estimation?

[Justme]

Why does speed of burn affect it?

I would think that these are the things it needs

flue gas temp
oxygen levels
hydrocarbons (IE unburnt fuel)

If all the fuel is being burnt & the flue heat is low then the efficiency is high.

Or is it just measuring the efficiency of the burn & not how much of the created energy goes in to useful purposes?

[Ivan]

Good question! Anyone know the answer?

[Baz]

A gas analyser will as stated just measure elements of the burned gas perhaps checking co & co2 aswell which will give  a scientific view of the efficiency. If rate of fuelling is given too it enables more user friendly or gimmiky calculations like number of trees saved etc.

[dhaslam]

Efficiency would just  be calculated from what is left over after combustion.

The actual measurement of the   gases seems to be quite a complicated  process

www.ferret.com.au/c/Testo/Exhaust-gas-analysis-system-available-from-Testo-n776729

[Alan]

Quote “Does a Flue Gas Analyser give an accurate measure of efficiency or is it an inaccurate estimation”

It will give an accurate reading of the types of unburned fuel remaining if readings are taken from the correct place.
As previously posted. Oxygen readings are of no use if air is introduced down stream of the combustion process.
The important readings are  Carbon Monoxide. Nitrogen Monoxide Nitrogen Dioxide and Sulphur Dioxide.

If say the burner was a duel fuel burner, having been run on oil for a period of time then switched over to gas. The Carbon Monoxide readings at the boiler flue exit would not be correct. They would indicate the unburned fuel ( Carbon Monoxide ) being picked up by the air flow over various passes of the boiler heat exchanger.

On a large burner during the design stages the readings would be taken with a water cooled probe at the flame tips while the probe is rotated through 360 degrees. The main cause for concern being flame temperature which relates to production of various oxides of Nitrogen, Nitrogen Monoxide and Nitrogen Dioxide.

The measurement of air temperature at the inlet of the combustion process and at the final exit after the last heat exchanger will give an indication of the combustion efficiency only.

Regards

Alan

[johnrae]

For bedtime reading, and an explanation of the numerous equation, try the following :

http://www.habmigern2003.info/

jack

[Eleanor]

Hi Ivan, is this the info you were looking for? Usually O₂ and CO concentration along with ambient and flue gas temperatures are measured. The other values including CO₂ concentration and efficiency are then calculated. Most combustion analysers allow selection of the fuel type to account for the stoichiometry so all the fuel related factors are lumped together in the software. As has been stated by others it's location, location, location of the probe when measuring. It will only give the efficiency of the burner and as has been said previously any ingress of air post combustion will give incorrect readings. You could use the CO and O₂ readings at various points and do your own calculations. Depends on what you are trying to achieve  
Handheld analysers tend to use electrochemical cells which usually give reliable readings but do need replacing every few years and can be poisoned in weird and whacky applications. There can be interferences but again this doesn't happen in normal eg gas/heating oil boilers and the analysers can correct for it.

[Ivan]

Thanks Eleanor,

Those equations confirmed the quick calculation I did in my head.........

[Eleanor]

It brought back fond memories of a job I had years ago with Testo where one of the tasks was to hang about in plumbing trade counters and convince the punters that what they really wanted to do was part with around £1000 on such an item 

[Ivan]

So is there a way to measure appliance efficiency from flue gas temperatures? I remember doing some simple calculations a while ago - assuming that the combustion temperature was say 1000C, and assuming that the flue gas is 200C when it leaves the appliance, you must have approximately 80% appliance efficiency x combustion efficiency - is there a meter to do this with a combustion temperature sensor?

Of course, it's more complicated than that - need to measure air supply temperature and also need to take into account any air being sucked from the room by the heating appliance.....

[Lars]

I'm none the wiser after reading this. I would like to know how to calculate or measure the efficiency of a gas appliance. Now I think there are 2 issues here.
A- efficient combustion
B- efficient extraction of heat

Say e.g. an open fire might be very efficient at burning a particular type of fuel, but if I use it in conjunction with a big open flue most of the heat will disappear out of the roof. I would assume that the overall efficiency is determined by how much flue gas is leaving the building and at what temperature. But how do I measure that effectively???

I'm specifically interested in gas and wood stoves in conjunction with a traditional chimney.

Anybody has got any answers on this?


thanks