I am about to replace our 1960's thorn M gas boiler.
I have been reading books, FAQ's, previous posts
but still have one or two outstanding questions.

This is a really basic question about boilers in general...

It seems that condensation is linked to return temperature.
How does this work..

Lets take a non condensing boiler first...

As I understand it we have a flame that burns,
and is directed at the outside of a heat exchanger.
The heat exchanger is some sort of metal contraption
with a large surface area that the water flows through.
The water is heated to typically 82C goes to CH/HW tank
and returns at typically 71C (in non condensing boiler).
So the water entering the heat exchanger is at 71C.
The heat from the flame passes over this surface and is
then directed to the outside world up the flue
(whatever type that may be).

If the return temperature were to drop below the 55/60C
then condensation forms and this can evidently shorten the
life of the heat exchanger because of condensation.

That's the bit I don't understand. Where does the
condensation come in and why? Does it form on the
outside of the heat exchanger? or on the flue?
Why is it affected by the return temperature?
I just can't reconcile what's going on in the boiler with
what I remember about Dew Point, Boyle's Law etc
that I did at school a hundred years ago !

I have a further question about condensing boilers
efficiency but perhaps it will become clear when
I understand this first question...

Many thanks,
Roy

That's the bit I don't understand. Where does the
condensation come in and why?

Because the flue gases contain a lot of water. Not just atmospheric
humidity, but the byproduct of burning natural gas. When you cool the flue
gases down further (i.e. to 90C-, rather than the 150C+ of a conventional
boiler) due to a better heat exchanger, the gases soon go over 100% relative
humidity and the water condenses.

Of course, there are two efficiency effects for condensing boilers. Firstly,
the lower flue temperature means that more heat is clearly being transferred
to the water. The second effect is that the act of condensing on a surface
causes that surface to heat up, much like evaporation of sweat cools you
down.

As boiler efficiencies are published without reference to this previously
"lost" energy from condensation, condensing boilers can even have official
efficiencies of over 100%.

Christian.

RzB wrote:
Where does the
condensation come in and why? Does it form on the
outside of the heat exchanger? or on the flue?
Why is it affected by the return temperature?

All the fossil fuels (oil, gas and coals) are mostly hydrocarbons,
compounds of hydrogen and carbon. There's other elements mixed in as
well, mostly sulphur. The exact compostion depends on what the fuel is.

Methane is CH4, Butane is C4H10. Paraffin is about 86.3% C, 13.6% H2
and 0.1% S, etc.

If you completely burn any hydrocarbon, you mostly get CO2 and H2O,
carbon dioxide and water vapour. The H2O will appear as water vapour,
if it is at less than 60degC, it condenses, forming water. The heat
excahnger will absorb the latent heat of vaporizatrion which is
released when the vapour condenses, so more energy is available for use
if the water vapour can condense. The coolest bit of the heat exchanger
will be where the cool return water comes in, so this is where the
water usually condenses. You could have a boiler operating at less than
60 degC flow, so then you might get condensation forming over all the
heat exchanger and up the flue. The return connection point is still
coolest so that's where most condensation will form.

The problem occurs mostly with the traces of sulphur in the fuel. These
have been burnt to form sulphur dioxide. This dissolves in the water to
form sulphurous acid. The sulphurous acid will attack the boiler,
UNLESS it is constructed of acid resistant materials, i.e., a
condensing boiler. The damage to the boiler is known as back-end
corrosion (the return usually goes in the back on commercial boilers).
The condensate from condensing boilers is acidic.

There's other stuff involved, dissociation, nitrogen, partial
combustion, etc., but that's the main bit. IANA chemist.

RzB wrote:

Lets take a non condensing boiler first...

As I understand it we have a flame that burns,
and is directed at the outside of a heat exchanger.
The heat exchanger is some sort of metal contraption
with a large surface area that the water flows through.

Sounds right so far...

The water is heated to typically 82C goes to CH/HW tank
and returns at typically 71C (in non condensing boiler).

Note that the water does not get to 82 in one pass - it takes several
trips through the boiler to reach that temperature. The boiler will
usually add about 10 - 12 degrees to it on each pass.

So the water entering the heat exchanger is at 71C.

Once it is up to temperature and has reached equilibrium, yes.

The heat from the flame passes over this surface and is
then directed to the outside world up the flue
(whatever type that may be).

Yup.

If the return temperature were to drop below the 55/60C
then condensation forms and this can evidently shorten the
life of the heat exchanger because of condensation.

Not only the heat exchanger, but any metalwork in the boiler including
the burner tray etc. (which historically would be situated under the
exchanger)

That's the bit I don't understand. Where does the
condensation come in and why? Does it form on the
outside of the heat exchanger? or on the flue?

Yes, and yes... primarily the first, but if due to the low exchanger
temperature the flue gasses are cooled too much, then you will also see
them condensing all along the exit path after the exchanger.

Why is it affected by the return temperature?

The lower the temperature of the heat exchanger, the greater the heat
flow into it for a given flame temperature, and hence the lower the
temperature of the flue gasses.

(there is an implication here that even when correctly configured a
"normal" boiler will still condense a little when running up from cold -
since it will take a while to get the system to normal operating
temperature. I am not sure if that is actually a problem, or if it is a
case of it being so short lived that any moisture generated will just be
boiled off again in short order)

I just can't reconcile what's going on in the boiler with
what I remember about Dew Point, Boyle's Law etc
that I did at school a hundred years ago !

Well if you start from the premise that the flue gasses contain
significant volumes of water (resulting from the oxidisation of a hydro
carbon with oxygen - giving heat + co2 + water) this will remain in
vapour form until the temperature of the gas falls below the dew point.

The dew point is not a fixed temperature, but will vary depending on the
saturation of the gas. The more water vapour in there, the higher the
dew point.

So in this case the vapour content percentage is set by the combustion
process. Since this is plenty hot enough, the water stays in vapour form
- until it cools down to about 56 deg C IIUC. If the water vapour
content percentage was different, the temperature of condensation would
also be different.


--
Cheers,

John.

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Christian/Aidan/John,
Many thanks for your excellent answers. I guess my
mind was sot of linking condensation with cold surfaces.
Anyway - I understand now.

I have a further linked question... I'll raise another post...
Many thanks,
Roy

On Wed, 7 Sep 2005 13:55:53 +0100, "RzB"
wrote:

I am about to replace our 1960's thorn M gas boiler.
I have been reading books, FAQ's, previous posts
but still have one or two outstanding questions.

This is a really basic question about boilers in general...

It seems that condensation is linked to return temperature.
How does this work..

Lets take a non condensing boiler first...

As I understand it we have a flame that burns,
and is directed at the outside of a heat exchanger.
The heat exchanger is some sort of metal contraption
with a large surface area that the water flows through.
The water is heated to typically 82C goes to CH/HW tank
and returns at typically 71C (in non condensing boiler).
So the water entering the heat exchanger is at 71C.
The heat from the flame passes over this surface and is
then directed to the outside world up the flue
(whatever type that may be).

If the return temperature were to drop below the 55/60C
then condensation forms and this can evidently shorten the
life of the heat exchanger because of condensation.


This would be true if a non condensing boiler were run in this mode.
However, that is difficult to achieve because the thermostats tend to
prevent it happening. A condensing boiler of modern design has
materials suitable for coping with the mild carbonic and sulphurous
acid mix that results.


That's the bit I don't understand. Where does the
condensation come in and why? Does it form on the
outside of the heat exchanger? or on the flue?

The physics of it are that the latent heat of condensation from the
gaseous phase of water (steam) to the liquid phase (water or water
vapour) is released because of the state of change. It is exactly
the same principle as a fridge. Inside the fridge, the refrigerant
evaporates from liquid to gaseous phase and requires the latent heat
to do that. Outside it is the reverse and the latent heat is
released.

The condensing boiler achieves this by having surfaces at a
temperature below the dew point. If that is the heat exchanger, then
the latent heat is released and to a considerable extent will be added
to the heat produced by combustion. Thus the return temperature is
important. The lower it is, the more surface area of heat exchanger
is used for condensing and the better the outcome.

If condensation happens within the flue system within the house or the
boiler, then the heat is released within the building but through the
boiler case and to some lesser extent, the heat exchanger.

Bear in mind that even if the flue gives a plume, this is actually
water droplets, not steam, and the condensation has already taken
place. In older non condensing boilers, the flue gases are actually
steam on leaving the flue and condense to water droplets on cold days.



Why is it affected by the return temperature?
I just can't reconcile what's going on in the boiler with
what I remember about Dew Point, Boyle's Law etc
that I did at school a hundred years ago !

I have a further question about condensing boilers
efficiency but perhaps it will become clear when
I understand this first question...

Many thanks,
Roy





--

..andy

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