Water Vapor/Humidity in Air 1

[mielke]

I am familiar with an equation that gives me the kilograms of water vapor in air, which is...

water vapor(kg)=air(kg)*relative humidity/100*Sat. Pressure of water/Operating Pressure*MoleWeight of water/MoleWeight of air.

I am pretty confident in this equation but lets say for example I have 66.6% RH at 25C, with an operating pressure of 0.101MPa and a Sat Pressure of 0.00317MPa and if I have 100kg of air then I should have 1.3kg of water vapor in my air (according to the above equation).

But when I look at the properties for water at 0.101MPa and 25C obviously water is in the liquid state. And if the air is at 25C then shouldn't it condense any water in the air?

I am just hoping for any insight into the appearant confusion I have.

Thank You

 

[IRstuff]

Is this for school?

TTFN

FAQ731-376

 

[mielke]

No, I am sizing an air cooled heat exchanger. I found the above example and equation from the internet 'Air water vapor mixturessychrometrics' by Leon R. Glicksman c1996

 

[DRWeig]

Mielke,

0.101 MPA is atmospheric pressure, and 25C is a warm room temperature. Water still evaporates in these conditions, eh?

Your answer is right.

The dew point at those conditions is 18C (±) so water will condense on a surface that's below 18C. As it is, the air is not saturated so unless you are adding more water to the air by evaporation or taking it away by condensation with a cold surface, it will remain 66.6% relative humidity.

Good on ya,

Goober Dave

 

[mielke]

I know that there are psychrometric charts to find the dew point and what not for water vapor in air, but what do you do when you have other kinds of vapor that can condense other than water in other kinds of gases than air.

Are there equations or methodologies that anyone can refer me to?

 

[DRWeig]

Mielke, as I answered in your other post -- it might be best if you enlighten us. This isn't the place to learn about partial pressures and fundamentals of physics.

Describe your project in detail, someone here may have dealt with it before, and you'll get good advice.

Good on ya,

Goober Dave

 

[mielke]

I have a mixture of gases, based on the pressure and temperature of the gas mixture, I want to know if and to what extent any of the gas mixture constituents condenses.

 

[DRWeig]

Mielke,

What are the gases? That's a complex problem.

Goober Dave

 

[mielke]

It is a complex problem. The gases are all ideal gases (ie Methane, C02, hydrogen, ethane, nitrogen...etc).

Ive seen it done for water and air (see original post) I just wondered if it is safe to extend that equation to other ideal gases or if anyone had any other suggestions?

 

[IRstuff]

Well, certainly, if you exceed the saturation pressure, the gas will condense. If you lower the temperature, then a previously non-condensing saturated gas will condense. This is all pretty much high school gas chemistry.

TTFN

FAQ731-376

 

[mielke]

IRstuff,

If you refer to my example, air at 14.7 psia at 70 F has a Sat pressure of 0.36336 psia (obviously we exceded sat pressure so we should have condensed air). but IRstuff if your reasoning is correct than we would never have any water vapor in the air at 14.7 psia at 70 F, contrary to the example shown above and to my weatherman.

 

[IRstuff]

I didn't say it would ALL condense.

TTFN

FAQ731-376

 

[MintJulep]

I wouldn't worry too much about methane, CO2, hydrogen and nitrogen spontaneously condensing out of an air mixture anywhere near 25C.

 

[mielke]

These are just examples of gases, I am looking for a systematic, quantitative method for determining condensing of any gas mixture for any temperature and pressure

 

[AbbyNormal]

review daltons law of partial pressure and review temperature/pressure tables of the gases in your mixture



The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

an analysis of wet flue gas by volume from an atmospheric vent as an example

if you know the percent volume of water vapour by taking a measurement.

Neglecting a little bit of draft, the flue gases are at atmospheric pressure.

the fraction by volume of water vapour times the atmospheric pressure will be the vapour pressure of the 'steam'

Use this pressure and a saturated steam table to get the saturation temperature-- there is your dewpoint

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install

 

[AbbyNormal]

the pychrometric chart is basically daltons law and ideal gas law combined

The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install