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Posted by Abby Normal on March 28, 2008, 10:45 pm
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
mixture.
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
nitrogen.
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
atmospheric pressure.
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
dewpoint.
I am half pulling this one out of my ass but it sounds plausible to
me.
>
>
>
>
>
> > 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, =A0would be 0.13 x
> > 14.696 =3D 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 have a heated mixture of:
> > > O2: =A012%
> > > H2O: 13%
> > > N2: 70%
> > > CO2: 5%
>
> > > Temperature: is around 200=BAC.
>
> > > 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?
>
> > > Thanks
>
j.6015$sw3.3357@trnddc06...
>
> > > > 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.
> > > > Tony
>
> > > > > 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 diff=
erent
> > > > > gases (for example a mixture of H20, Ar, N2 .......)
>
> > > > > Thanks- Hide quoted text -
>
> > > - Show quoted text -- Hide quoted text -
>
> > - Show quoted text -
>
> Hi Abby.
>
> 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 - waters atomic mass is less?- Hide=
quoted text -
>
> - Show quoted text -
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