Condensing exhaust temperature

[John D v2.0]

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?

 

[Dan Robinson]

Why don't you ask in the CC?

 

[Dan Robinson]

Oh that's right .... You got one of the most knowledgeable people on this subject banned.

 

[Dan Robinson]

Oh, and I read that article too. It was painful .

 

[jj4091]

I don't know about a figure of 56 degrees, but perhaps this explains how you get steam at a lower temp.. https://www.scientificamerican.com/article/how-does-bathwater-give-off-steam/

 

[stem]

I cycled yesterday and my breath was visible, normal body temperature is only 37 degrees C. Might have been one or two more though, I was peddling uphill.

 

[nwgs]

Got to be a wind up ?

 

[John D v2.0]

Oh that's right .... You got one of the most knowledgeable people on this subject banned.

Are you sure you're not mixing me up with another John d? I've never been in the CC and I've certainly never got anyone banned from there!

 

[John D v2.0]

Thanks for the replies, i agree that when humid air drops below its dew point you get visible water vapour, eg warm breath or hot bath, but as far as i know there's not a lot of latent heat in that condensation. I thought the big bonus is from turning from steam into water. But do correct me if I'm wrong as my physics is a bit rusty.

 

[Dan Robinson]

Are you sure you're not mixing me up with another John d? I've never been in the CC and I've certainly never got anyone banned from there!

My apologies yes....

 

[boringoldcodger]

But do correct me if I'm wrong as my physics is a bit rusty.

I do believe your physics is a bit rusty. If you add heat to some water at 100C and turn it into steam then you've added a certain amount of heat. If you turn some water vapour at, say, 50C into liquid water (still at 50C) and (hey presto) you find you've got the same amount of water as you previously turned to steam then you'll have got back the same amount of heat.

 

[John D v2.0]

thanks @boringoldcodger so what you're saying is the flue gas isn't actually steam (from a physics point of view) it's just evaporated water, and in fact the dew point of the flue gas is lower than 100c, so cooling it below 100 makes no difference, it's only cooling it much lower that causes it to start to condense?

But even so, there's still no magic point where all the water condenses - from the dew point and downwards, further water would condense from the gases all the way down to outside air temperature and beyond. So is 55c just a realistic target for the quoted efficiency and nothing special or is that the dew point of gas burnt with the perfect air:gas ratio?

 

[Gasguru]

Put a jumper on and save 10%..the boiler manufacturer figures are all made up. (they devise ever clever ways to fool you).

 

[John D v2.0]

the boiler manufacturer figures are all made up. (they devise ever clever ways to fool you).

Yes I'm sure they do, I'm just trying to understand what kind of trick. Were condensing boilers all a scam? They should continuously measure flue gas temperature and have a display saying you whether you're really getting your "A rated" or whether it's more like "F rated" and flash it up on those smart meter screens or something.

Anyway it does seem indeed that the dew point is around 55c for a boiler with a premix burner
https://www.engineeringtoolbox.com/flue-gases-average-dew-point-typical-fuels-d_1584.html

And it looks like the water capacity of air at 55c is 1.2kg/kg, wheras at 40c is only 0.5kg/kg, therefore if the dew point is 55 then cooling to 40 would get the majority, but not all of the latent heat back.
see https://www.engineeringtoolbox.com/moisture-holding-capacity-air-d_281.html

I suppose after all that the simplest way is just look at the condensate:gas ratio and be aware of diminishing returns below about 40c.

 

[ScottishGasMan]

56*c is a rough number they use for this, the lower the better. ien a full underfloor heating system you could have the return temperature below 30*c so achieving a higher amount of condensing.

Yea, water boilers around 100*c, but there is humidity in room temperature air. So the energy gained when the water condenses in the boiler is what were looking for, so at 70*c 10kw of gas burned in a perfect system would get 10kw of heat back out (assuming actual perfect system with no losses) but the make up of the gas is such that water vapour is produced which also has energy in it which would normally be lost through the flue gas. In a condensing boiler at say 40*c that would condense giving about 11kw.

Thats how you see strange efficiencys quoted in boiler marketing of 106% efficient ect, as their using the CV of gas as the input and the CV of gas plus condensed water vapour energy as output.