Water Vapor/Humidity in Air
Water Vapor/Humidity in Air
(OP)
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
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





RE: Water Vapor/Humidity in Air
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RE: Water Vapor/Humidity in Air
RE: Water Vapor/Humidity in Air
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
RE: Water Vapor/Humidity in Air
Are there equations or methodologies that anyone can refer me to?
RE: Water Vapor/Humidity in Air
Describe your project in detail, someone here may have dealt with it before, and you'll get good advice.
Good on ya,
Goober Dave
RE: Water Vapor/Humidity in Air
RE: Water Vapor/Humidity in Air
What are the gases? That's a complex problem.
Goober Dave
RE: Water Vapor/Humidity in Air
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?
RE: Water Vapor/Humidity in Air
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RE: Water Vapor/Humidity in Air
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.
RE: Water Vapor/Humidity in Air
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RE: Water Vapor/Humidity in Air
RE: Water Vapor/Humidity in Air
RE: Water Vapor/Humidity in Air
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install
RE: Water Vapor/Humidity in Air
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
RE: Water Vapor/Humidity in Air
The way we build has a far greater impact on our comfort, energy consumption and IAQ, than any HVAC system we install