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## Main Question or Discussion Point

Does the humidity in the air affect the density???

and how does it affect?

thanks

and how does it affect?

thanks

- Thread starter khaichou
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- #1

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Does the humidity in the air affect the density???

and how does it affect?

thanks

and how does it affect?

thanks

- #2

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The density of standard air is 0.075lb/cu.ft where as the density of saturated steam(or water vapor) at 60F is mere 0.0008292lb/cu.ft.

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mathman

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FredGarvin

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Temperature (T) 15.56 C 60.00 F

Pressure (P) 0.0177 bar 0.256 psi

Density Saturated Liquid (f) 998.97 kg/m3 62.364 lb/ft3

Specific Volume Saturated Liquid (vf) 0.0010010 m3/kg 0.016035 ft3/lb

Saturated Vapor (vg) 75.344 1206.9

Internal Energy Saturated Liquid (uf) 65.31 kJ/kg 28.08 Btu/lb

Evaporated (ufg) 2331.6 1002.4

Saturated Vapor (ug) 2396.7 1030.4

Enthalpy Saturated Liquid (hf) 65.31 kJ/kg 28.08 Btu/lb

Evaporated (hfg) 2464.6 1059.6

Saturated Vapor (hg) 2530.0 1087.7

Entropy Saturated Liquid (sf) 0.23258 kJ/kg-K

(mayer) 0.05555 Btu/lb-R

Evaporated (sfg) 8.5360 2.0388

Saturated Vapor (sg) 8.7684 2.0943

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To be frank, I was indeed puzzled by the hint of your post, initially, for quite sometime. Good steam properties calculator like steamtab which is based on IAPWS formulae should never lie, so it became my responsibility to check for a possible solution. This is what comes to my mind.

The specific volume of any gas, at STP, can easily be calculated by Avagadro's law if we know the molecular weight of the gas. Molecular weight of water is 18, as you rightly said, and average molecular weight of air is 29.

So, one lb mole of water vapor occupies 379/18 = 21.05 cu.ft at STP. So the density is about 1/21.05 = 0.0475lb/cu.ft. One lb mole of air occupies 379/29 = 13.07 cu.ft at STP. So the density is about 1/13.07 = 0.0765lb/cu.ft. So what you said should be right.

But the key word here is STP(14.696psia and 60F - a general standard, though there are many variations to the definition of STP). The saturation pressure of water vapor, at 60F, is 0.256398psia. So, you have to expand the water vapor from 14.696psia to 0.256398psia(isothermally) and the ratio is about 57.31. So the density should be 0.0475/57.31 = 0.0008288lb/cu.ft. This is close to the value I specified above and I think my procedure is fair if not accurate.

Thanks for pointing out.

Regards,

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mathman

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- #7

Astronuc

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quark said:

The density of standard air is 0.075lb/cu.ft where as the density of saturated steam(or water vapor) at 60F is mere 0.0008292lb/cu.ft.

In a sense, you are both right.mathman said:I think the data you are discussiing is for steam, which is not the same as for 100% humidity air.

The key is 'saturated' steam, or water vapor. At 60F the sat pressure is 0.2563 psia, compared to 14.7 psia, or 1 atm.

If one were to look at the property of 'saturated' steam at 1 atm (14.7 psia), which would have a temperature of 212F, then the density is 0.037 lbm/ft

The partial pressure of water vapor in air is very low - near the sat. pressure IIRC. It's been so long since I've looked at this stuff - my memory is a bit 'foggy'.

Here's a basic discussion on humidity - Getting a handle on humidity

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