Dehumification 1

[pavlik]

If you cool air, you will reduce its ability to carry moisture- everyone who took Thermo knows that.
But what does that mean on molecular level?
I am trying to figure out a relationship (equations) that describes that phenomenon.

Thanks

 

[ChrisConley]

Well, a good place to start would be the psychrometric tables. Which outline the ability of air to hold moisture from -40 deg C to 70 deg C, at various altitudes.

There are already numerous formula which detail the relationship between dry bulb air temperature and its maximum moisture carrying capacity. Chapter 6 of ASHRAE Fundamentals, or any textbook on psychrometrics will give you all the formula that you need.

 

[friartuck]

for psychrometric charts go to handsdownsoftware.com

drapes

 

[Istre]

O.K., I'll bite on this one. Consider completely dry air. Since temperature is simply a measure of the average kinetic energy of each molecule, when you cool the air from say 80F to 40F, what you are actually doing is slowing down the motion of each individual air molecule.

Now, consider a mass of dry air and a mass of water, equal in temperature, separated by an impermeable membrane, and enclosed in a perfectly insulated container. Rupture the membrane to allow the air to contact the water. In time, the air will achieve 100% relative humidity and will reach equilibrium. Some of the water has obviously vaporized. The water initially didn't contain enough energy to vaporize, as it was a liquid. Since the container was insulated perfectly, the energy to vaporize had to come from inside. The only source of energy to vaporize the water was the kinetic energy of the gas molecules being transferred through collision with the water molecules. Since temperature is a measure of the average kinetic energy of the molecules, as you raise the temperature of the air, it has more energy available to give to the water for vaporization. Since there is more energy available, the equilibrium point is going to be higher in water content, and hence, the air will carry more water vapor.

Of course, I could be completely wrong, but at least it sounds good.

 

[imok2]

Istre is correct, I liken it to an evaporative cooler which is essentially a large fan with water-moistened pads in front of it. The fan draws warm outside air through the pads and blows the now-cooled air throughout the house.Ths is an exchange of kenetic energy amounting to about 20*F and each lb. of water vaporized= ~ 970 btus

 

[25362]

In addition to Istre's example, it may be interesting to note that -contrary to common sense- a moist gas can be dehumidified by spraying cold water into it, as long as the water temperature is lower than the entering gas dew point.

In this particular case heat and mass are transferred in the same direction, from the gas to the water. The sensible heat rise of the water equals the sensible heat drop of the gas plus the latent heat of the condensing humidity.