Why does a condensing boiler create a plume

[bernardgreen]

The plume from a "condensing" boiler contains water vapour.

Doesn't this suggest that the boiler is not condensing all the water vapour internally.

Could this mean that when producing a visible plume a "condensing" boiler is not achieving the best efficiency ?

 

[Lectrician]

You are never going to produce flu gases that are cold. They will always have some latent heat that will quickly condense and plume. You can't expect a condensing boiler to recover all the heat from the flu gas.

 

[boringoldcodger]

I can't help feeling that this'll get more accurate answers from "Plumbing and Central Heating", but ...

You're right that not all the water is condensed internally, that's because the temperature of the flue gases
is their dew-point, i.e. the air is holding as much water vapour as it possibly can at that temperature. When these gases hit the cooler air outside they immediately cool down and can hold less water vapour and so you get visible mist.

If a condensing boiler isn't running in condensing mode then you don't get a plume (just like you don't from a conventional boiler) and it's then not running as efficiently as it should.

Cheers, Graham

 

[Jackrae]

Steam is invisible, water vapour (condensed steam) is not. The latent heat is recovered in the conversion of steam into water vapour. Plumes confirm the conversion has taken place even if it is a visible nuisance.

 

[bernardgreen]

I can't help feeling that this'll get more accurate answers from "Plumbing and Central Heating", but ...

Maybe, but electricians seem ( in generally ) to be more open minded than plumbers

But yes it was my error posting in the wrong forum.

 

[DaveHerns]

BG, were you were hoping to involve BAS in a long debate? Seems he's left the forum.

 

[bernardgreen]

BG, were you were hoping to involve BAS in a long debate?

No. I was interested in the answers that would come out of the "professionals." The question was actually raised by a 9 year old about the plume from his granny's new boiler ( he has physics as a hobby )

 

[Jackrae]

Then please ensure he is aware that latent heat recovery in the boiler exhaust comes from the conversion of steam to condensate, not from the further cooling of condensate. During the transfer from (invisible) steam to (visible) vapour the temperature remains constant. The cooling of water vapour to extract heat requires a change in temperature.

 

[ericmark]

I will admit I also wondered why if all the latent heat has been extracted you still have water vapour visible from the flue, a small amount yes, but it does seem too much is visible. Some boilers today have Sterling engines which extract even more energy and convert it to electric power and feed it into your house in same way as solar panels. So there must be quite a large amount of energy still wasted by standard condensing boilers.

I would say off hand the problem is size, we look at the old steam railway engine and much of the energy was unused, but with a power station, with condense the steam and form a part vacuum where it exists the turbine so extract much more energy, same was done with ships. With the experimental solid fuel fire using condensing it used a plastic flue pipe and there was no need to keep a fall with the pipe, the heat was removed plus all the nasty stuff in wood burning fires by using high pressure sprays of water.

Other more simple wood burners allow water to dribble out of the flue.

But these are massive units compared with the gas boiler. One of the advantages of gas is it is so easily controlled, but it is all well and good in theroy, as each thermostatic radiator valve closes the return temperature of the water increases so the flame hight has to alter to match, until it can't burn any lower when it goes out. But in real terms there must be a delay in the water temperature of the return water rising and the sensors telling the boiler to reduce flame hight.

However with my mothers boiler plumes of steam points to the sediment trap needing emptying. So it may be as simple as that with all boilers, will be interesting to see what the heating and ventilating chaps say? Is there a name for heating and ventilating guys? Clearly not Plumbers as they don't work with lead, and I don't like calling anyone without level 5 or better qualifications an engineer, even if in USA a train driver is called an engineer.

 

[Jackrae]

Water vapour cooling is via extraction of "sensible" heat and involves a temperature change. Since it is the returning CH water that does the cooling to extract latent and sensible heat then the coolest to be expected in the exhaust is probably around 70 degrees based on a CH return of 55 degrees. There must always be a delta-T for heat to transfer from one surface to another. As long as there is a plume then it is reasonable to presume latent heat extraction has taken place. Sensible heat extraction wll depend on how hot the CH water circuit return is but that implies a cooler return extracts more sensible heat at the 'cost' of cooler running radiators

 

[Scarum]

I can't think of a method of thermal energy transfer which is > 99% efficient. Hopefully this young child will work it out for the next generation.

 

[ericmark]

Once steam is converted to water vapour it then tends to travel with the gases around it rather than settle out and be drained away. But that would be a continuous vapour output rather than plumes of steam. Be it a TRV closing or a storage tank empting something has to cause the plumes of steam which is simply not just not being less than 100% condensing. I know the condensate is stored and released in batches to reduce the chance of freezing. That could cause the plume, but to date no real answer.

 

[bernardgreen]

Steam is invisible, water vapour (condensed steam) is not.

http://www.weatherquestions.com/What_is_water_vapor.htm

Water vapor is water in its gaseous state-instead of liquid or solid (ice). Water vapor is totally invisible. If you see a cloud, fog, or mist, these are all liquid water, not water vapor.

The plumes on my boiler and my neighbour's boiler become visible an inch or so from the terminal. This suggests that invisible water vapour is condensing into water droplets at it meets cooler air.

If the concentric flue was made of heat conducting metal instead of plastic then possibly some of the heat in the flue gases could be transfered to warm the incoming air. The water vapour could also condense on the metal transfering the latent heat of condensation into the incoming air.

With a conventional boiler ( non condensing ) the temperature of the flue gases and the water vapour is high and the vapour travels further away from the terminal before it has been cooled enough to form water droplets. The droplets form and are dispersed over a large area and thus do not appear as mist or steam or a plume.

A condensing boiler produces a steam from it's flue. Call it steam in the marketing literature and people will think "wasted heat" but call it a plume and the obvious connection with wasted heat is no longer there.

 

[Jackrae]

It depends what the OP meant by "plume"
As defined a plume is "a long cloud of smoke or vapour resembling a feather as it spreads from its point of origin"
So was the OP referring to a more-or-less continuous stream of vapour (normal situation)
or "puffs" of vapour somewhat like is expelled by a steam engine (seriously abnormal situation)

 

[Jackrae]

I stand corrected but at least I try to be polite
No-one's mentioned dew point yet, which may well be relevant to the OPs observations of the exhaust plume.