Condensing exhaust temperature

[John D v2.0]

1.46l/cu m of methane is the condensate. Measure it and you'll know how much condensate there is, I have done it but you need to separate the flue from the hex.

Interesting and that's more than i expected and a huge amount in potential for a boiler using a cubic metre an hour on average

Three times an hour means a reduction in fuel saved over running cooler looking for the latent heat over an hour

Correct and that's a great point in the case where the boiler is hardly condensing at all during the cont running. At least you will get some time condensing. I wonder if the controls could take that into account!

Low modulation also means the boiler runs continuously but to achieve that greater quantities of excess air are use to keep the burner cool reducing flue temp and efficiency.

That's not true, the aim isn't too keep the burner cool by dumping heat to outdoors, that would be truly perverse! Condensing boilers are specifically designed to keep the gas and air in similar proportion throughout the range and simply increasing the air is designed to cause more gas to be burnt. The coolness of the hex is caused by the heat being disappated by the radiators internally.

 

[Grumpy Gasman]

That's not true, the aim isn't too keep the burner cool by dumping heat to outdoors, that would be truly perverse! Condensing boilers are specifically designed to keep the gas and air in similar proportion throughout the range and simply increasing the air is designed to cause more gas to be burnt. The coolness of the hex is caused by the heat being disappated by the radiators internally.

Low modulation will have the flame sit closer to the burner, Xs Air is increased in order to lower the flame temp @ surface area of burner (reduced CO2%).
Why you feel the need to question Vulcancontinental on the subject of combustion is laughable.

 

[vulcancontinental]

You overestimate my knowledge Grumpy Gasman but there are studies that show this and I glean info from them. Then again studies are often published by those with a vested interest in making a point.

Some boilers with extreme rates of turndown actually adjust the turndown ratio down in windy weather so fan speed to fuel increases to maintain flame stability as well as lift it from the burner surface. Cooler flame is also necessary to reduce NOX which is looked at more carefully now.

Some designs suit operation one way better than others, just look at the differences between cast ali, drawn tube ali, radial coil ali, stainless tubular, stainless coil, some exchangers achieving reverse flow across the exchanger, some with a retarder some without, some with turbulators in the waterways, some high mass, some low and all within a percentage point of two of each other when certified.

All this not including different burners, turn-down ratios and emissions plus how the system controls interact with the boiler.

The more I learn the less I know; R&D know but then the product has to be pared down to sell for a few hundred pounds and be fitted by someone who may or may not care a jot about what he or she is doing or where it's going to spend its working life.

 

[John D v2.0]

Low modulation will have the flame sit closer to the burner, Xs Air is increased in order to lower the flame temp @ surface area of burner (reduced CO2%).
Why you feel the need to question Vulcancontinental on the subject of combustion is laughable.

Sorry, my aim is not to offend but I'll question anyone on anything, especially if they have far more knowledge than me and I'd expect the same in return. But i realise that attitude is only helpful in certain industries. I should say thanks for all the helpful replies and I'm questioning to understand because I'm interested although i admit i could improve my manner (blame the mobile)

Furthermore having reread i realised he actually started keep the burner cool, and i read it in haste as hex, so that means it was entirely my mistake! So if it came across badly then sorry @vulcancontinental

 

[vulcancontinental]

Didn't even register on my radar. The amount of stuff I've read and incorrectly interpreted is massive and continuing.

 

[Paul Goff]

Hi all,
Apparently the magic temperature for the steam in flue gases to condense is 56c. But every school child knows the boiling point of pure water is 100c at mean atmospheric pressure. Obviously flue gas has carbonic acid dissolved in it which could explain.
But is there a common explanation why and what is in the flue gases that changes the boiling point to 56c?

The basic explanation lies in the Latent Heat Of Evaporation, in a nutshell it requires 540 times the heat energy to produce one gram of water vapour at 100 degrees as it does to heat one gram of water from 99 to 100! This is why when boiling an electric kettle, you hear a noticeable "sag" in the audible activity of the water within just before it boils properly.

As the main products of the combustion of hydro carbon are water vapour and CO2 there is a large amount of energy going up the flue, a lot of it in the form of water vapour.

If you condense the water vapour back to water this energy is released into the medium used for the condensation, the cooler the condensing medium is, the better the energy recovery. in the case of a condensing boiler the medium is the return water flow from the system.

The reason that the figure of 56 degrees is used is probably that the designers have found that the energy recovery efficiency falls off above this flow temperature, but there is still a need to remember the initial mission, which is to heat the building.

The Latent Heat Of Evaporation is the principle that refrigeration and air conditioning works, albeit using a different fluid.

An interesting aside is that there is also a Latent Heat Of Freezing, similar in principle to that of evaporation, but the ratio is only about four times the amount of heat is required to turn one gram of ice into one gram of water

 

[vulcancontinental]

Just a couple of points, the latent heat of evaporation isn't really the point. The water in flue products is the result of a chemical reaction not evapouration. It's the enthalpy of condensation (which is equal to the enthalpy of evapouration).

The vapour is not turned to liquid in the system water but by coming into contact with a cooler surface. It could be the heat exchanger but more likely to be the flue tube in a concentric flue.

As the dewpoint changes in relation to the excess air present during combustion 56 degrees is often quoted as a sort of mid point between 50 and 59 degrees.

 

[Paul Goff]

Direct quote from Wikipedia
"One of the hot gases produced in the combustion process is water vapour (steam), which arises from burning the Hydrogen content of the fuel. A condensing boiler extracts additional heat from the waste gases by condensing this water vapour into liquid water, thus recovering it's latent heat of evaporation."

The cooler the cooling medium that does the condensing, in this case the return water flow, the more energy is recovered. However there has to be a compromise, somewhere in the mid 50s degrees is no more than the compromise between condensing efficiency and actually having hot enough water to heat the building.

 

[bernardgreen]

by condensing this water vapour into liquid water, thus recovering it's latent heat of evaporation."

As VulcanContinental said there is no evaporation of water into water vapour, the water vapour is created in the exothermic reaction of the oxidisation of hydrogen.

Also steam is not water vapour, water vapour is invisible, steam is visible due to the small drops of water that have condensed from the water vapour.

 

[Paul Goff]

So Bernard and and Vulcan, is it your contention that the water vapour in boiler flue gas does not have the latent heat of evaporation within it?

I think that Wikipedia chose to qualify water vapour as "(steam)" merely to make the concept more familiar to the layman. After all engines and turbines are referred to as steam powered but they are actually driven by super heated water vapour, more commonly called super heated steam.

My original post was made without having googled the subject, but having been challenged I did so, and the most succinct definition was the wiki one I quoted, there are many others, they all say the same.

 

[vulcancontinental]

Suit yourself

 

[D_Hailsham]

The attached pdf from Viessmann has some useful information.

 

[John D v2.0]

I'm surprised this thread has come back to life but anyway just for @Paul Goff benefit: since the post you quoted I realised it's not the steam turning to water, it's simply very humid flue gases returning some of their evaporated water to liquid water and giving up energy in the process. The significance of the dew point is no water condenses unless you get below it.

 

[vulcancontinental]

The water isn't evaporated, it's a vapour formes as a result of chemical reaction. The dewpoint changes in relation to excess air.

 

[John D v2.0]

That wasn't the point of my post but thanks for the information, you're right that the water would never have been in liquid state in this case.

Regarding your suggestion that the chemical reaction necessarily produces vapour it was actually the temperature that caused it to be vapour. There are plenty of oxidising reactions that produce water in liquid state e.g. metabolism of glucose in your cells.

We're really getting into nit picking now though, I think hopefully Paul is clear that my original query no longer stands.