# Dew Point Calculation in a Gas Mixture

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• posted on March 19, 2008, 5:11 pm
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
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• posted on March 20, 2008, 5:06 am
Marc O'Brien wrote:

And I don't want to be the poo pooer, but who the f*** cares? I only care about the dew point as it relates to the air I live in. And quite frankly Betty, I don't give a sh** about *your* air. LOL
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Zyp

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• posted on March 20, 2008, 8:57 am
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 me:
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.
wrote:

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.
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• posted on March 20, 2008, 5:13 pm
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

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• posted on March 21, 2008, 9:57 am
I have a heated mixture of: O2: 12% H2O: 13% N2: 70% CO2: 5%
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?
Thanks

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• posted on March 21, 2008, 4:33 pm

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.
-zero
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• posted on March 21, 2008, 4:40 pm
-zero wrote:

Well there ya go... see, I didn't see the writing on the wall :)
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Zyp

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• posted on March 21, 2008, 10:37 pm

Well,,, sniffing too much flue gas will do that to ya. ;o)

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• posted on March 22, 2008, 7:29 pm
-zero wrote:

ROFL
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Zyp

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<%-name%>
• posted on March 21, 2008, 6:15 pm
Do you know if there is any standar to calculate this. The composition will not remain always the same, so the dew point should change
Thanks

dew
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• posted on March 21, 2008, 10:38 pm

You may want to start here; http://www.jehar.com/dewpnt.htm
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 burner 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?
-zero
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• posted on March 21, 2008, 11:12 pm

He wants you to do his homework for him :-)
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• posted on March 22, 2008, 4:11 am

True. That's never ruled out of the realm of possibility on alt.hvac. ;-)

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• posted on March 22, 2008, 4:09 am
I think you get low dewpoints with the sulfur, you get acid consensing out not so much water condensing out
some acids need water, but some can be a liquid unto themselves
wrote:

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• posted on March 22, 2008, 4:03 am
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 saturation temperature.
I just pulled that out of my ass but would probably work.
Otherwise tell then it is a 140F

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• posted on March 22, 2008, 4:11 am

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• posted on March 22, 2008, 4:24 am

if all else fails then the ASHRAE HOF has a chapter on combustion, has dewpoint charts to use
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• posted on March 27, 2008, 12:25 pm
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.

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• posted on March 29, 2008, 2:45 am
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.

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• posted on March 29, 2008, 6:33 pm

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 - """""WHAT"""""waters atomic mass is less?