Does anybody knows where to find information to calculate the Dew Point
temperature of water in a gas mixture.
In air it is stated in ASHRAE, but what happens with water in different
gases (for example a mixture of H20, Ar, N2 .......)
Thanks for your reply. However I think (but not sure) that theoretically, it
not should be the same. I will try to explain with an example familiar to
Natural gas, is a mixture of gases (C1 to C6+). C6+ is the heavier and so is
the first that condensate when you cool down the mixture. However, this
temperature is not always the same, it depends on the exact composition of
the mixture. So I understand it should be similar for water in different
gases that ambient air.
As I understand, as long as the others don't have dew points close to
waters then the dew point of water does not change.
It is a question I have often asked myself but have never properly
investigated, though I have investigated it - if you know what I mean.
The dew point of water is the same for an atmosphere made up purely of
water as it is for our earthly atomosphere. The mixture of air and
water only introduces a wet bulb condition.
I don't know what are you trying to get or do
water contents in gases are measured in parts per million
and the dew point temperature varies with temperature
of particulars gas/refrigerant lower the Temp. the lower the dew
point, example while 10 parts per million maybe ok for R-134A
it is not good for R-508B which it needs to be 7 parts per million
or lower, there for dew point will be much lower.
if you are looking for formulas sorry can't help you.
In another process you can dry air what you are breathing
to 100 below zero dew point or lower using chemical reactivation.
which I am sure you are not looking for that.
I have a heated mixture of:
Temperature: is around 200ēC.
Somebody is asking me about the dew point. I know how to calculate the dew
point if it were H2O in air (using psicometric ASHRAE diagrams) but I am not
able to calculate it in this mixture.
Does anybody knows any standar o procedure to calculate this?
That would be a flue-gas mixture, not a fuel-gas mixture. It will
condense according to the environment the flue is discharged into.
To keep it from condensing inside the pipe, insulate the pipe
to keep it above 250° Fahrenheit.
You may want to start here;
This will calculate the fuel dewpoint based upon the sulfur content, and
will very according to burner, fuel type and combustion air used.
However, your numbers look more like sampling from a Kiln or Process
Dryer. That involves variable moisture from the process interacting with
firing rate. It can be VERY complicated to calculate in advance.
You can monitor that in real time if you must, but why you would want
to, I have no idea.
Why DO you need this information?
looks like the products of combustion with way too much excess air
try daltons law of partial pressures, work out the pressure of the
water vapour then go look in a staurated steam table for the
I just pulled that out of my ass but would probably work.
Otherwise tell then it is a 140F
I think the Dalton's law approach would would work
the water vapour pressure will make up 13% of the pressure in the flue
gas. The total pressure in the flue should be close to atmospheric so
the vapour pressure of water, assuming sea level, would be 0.13 x
14.696 = 1.9 psia
So looking in a saturated steam table on hand here at home the
stauration temperature and therefore the dewpoint will be in between
120F (1.69 psia) and 130F (2.22 psia), maybe around 124F. This is low
for natural gas, but you have a lot of excess air which lowers the
'pounds moisture per pounds dry air' if you will.
Usually 140F is an assumed dewpoint for natural gas, I am rusty but it
is for lower excess air amounts maybe around 50%.
I used to work for a custom build OEM, on our modulating gas burners,
we used to ramp up the combustion air blowers on low fire to get the
dewpoint down and reduce the risk of condensation.
Some one mentioned sulfur in fuel oil, sulfuric acid will condenses
out at higher temperatures than water will.
I think the most basic pyschrometric equations are really just based
on Daltons Law of partial pressure and the ideal gas law.
Air has the oxygen, nitrogen ,trace gases and water vapour
Daltons law says when you have a mixture of gas, the total pressure is
the sum of the partial pressures exerted by all the gas in the
So if you assumed air was 20 percent oxygen andd 80 percent nitrogen,
then 20% of the pressure came from oxygen and 80 percent from the
So what really limits how much mositure can be held in the air, is how
well the vapour pressure of the water vapour can compete against
I think if you were in a high altitude place and atmospheric pressure
was lower, water vapour pressure can more easily compete. So you can
get a higher humidity ratio in terms of pounds of moisture per pound
of dry air in high altitudes compared to sea level.
When you heat up water until it starts boiling, its vapour pressure is
equalling atmospheric, there is nothing holding it back and it can
change state 'at will' as long as there is enough heat being added.Go
up in altitude and water boils at a lower pressure.
So anyways that wet flue gas the original poster was talking about was
13% water vapour, so I figure it is making 13% of the pressure in the
flue gas. Outside of the effect of draft, that flue gas is under
atmospheric pressure. 13% of atmospheric is the vapour pressure of the
water vapour, and then you just look in a saturated steam table to get
the corresponding saturation temperature for that pressure as your
I am half pulling this one out of my ass but it sounds plausible to
Surely the water vapour will assume its own vapour pressure. The
higher its vapour pressure then just the less of the other gases?
Surely the water vapour pressure is constant, regardless of the rising
total pressure on account of the pressure of each additional gas?
With or without air is the water vapour dew point not the same? This
is why wet air is lighter than dry air - """""WHAT"""""waters atomic mass is
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