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Originally Posted by thermojohn
OK, brakes applied.

I understand that the dew point is the dew point, no matter what the dry bulb, or RH% the air is. If one is not 'removing' moisture, as in grains, out of the air, and dumping it somewhere via mechanical refrigeration, the dew point will remain the same. The magic in raising or lowering RH will be dependant on DB temperature.

My perspective on the process is that when a certain volume of air with a certain content of moisture (grains) gets heated up, it expands. The grains of moisture still remain, but the volume of air is more since it was heated. The ratio of grains in that air has been reduced, but not removed, thus reducing or lowering the RH%.

Is this the right way to look at it? I picture a balloon with a thimble full of water in it, and the air in it expanding and contracting in size due to differences in temperature. The water in the balloon remains the same volume, but never leaves.

What I wrote in an earlier post was not entirely incorrect. Adding the correction in bold would improve the accuracy, I believe. Correct me if I am wrong.

Let me know if I'm on the right track here.
RH is

(water vapor partial pressure/water vapor saturated pressure)*100

It's completely independent of the air.

2. Originally Posted by thermojohn
OK, brakes applied.

My perspective on the process is that when a certain volume of air with a certain content of moisture (grains) gets heated up, it expands. The grains of moisture still remain, but the volume of air is more since it was heated. The ratio of grains in that air has been reduced, but not removed, thus reducing or lowering the RH%.

bold would improve the accuracy, I believe. Correct me if I am wrong.

Let me know if I'm on the right track here.
Air expands when heated. The grains are measured per lb. of air. There are 12 ft^3 of air in a lb. of 0^F air. There are 13.5 ft.^3 in 70^F air. The heated air is lighter and has slightly less grains of moisture per ft^3 but the grains per lb. of air remains constant. Air at 0^F, 100%RH holds 6 grains/lb, while at 70^F, 100%RH holds +100 grains of air.
Air's ability to hold more moisture has more to do with the higher activity of the water molecules supended in the air. Cooling air slows the speed of the water vapor to the point of condensation. Therefore cold air holds less moisture than warm air.
This is my understanding. We all go through this process as we improve our understanding of the problem. Sometimes not quite perfect. Maybe we will get a PHD in here to give us the "real story".
Regards TB

3. Thank you TB. I am learning every day.

Apologies to ryanl for speed bumping your thread with a side issue. Gotta go to work.

John

4. Originally Posted by teddy bear
Air expands when heated. The grains are measured per lb. of air. There are 12 ft^3 of air in a lb. of 0^F air. There are 13.5 ft.^3 in 70^F air. The heated air is lighter and has slightly less grains of moisture per ft^3 but the grains per lb. of air remains constant. Air at 0^F, 100%RH holds 6 grains/lb, while at 70^F, 100%RH holds +100 grains of air.
Air's ability to hold more moisture has more to do with the higher activity of the water molecules supended in the air. Cooling air slows the speed of the water vapor to the point of condensation. Therefore cold air holds less moisture than warm air.
This is my understanding. We all go through this process as we improve our understanding of the problem. Sometimes not quite perfect. Maybe we will get a PHD in here to give us the "real story".
Regards TB
HAHA, I worked for a guy with a PHD in Physics, He was a brilliant physicist, dumb-as-a-box-of-rocks when it came to EVERYTHING ELSE.

Phd = Piled Higher & Deeper

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Originally Posted by teddy bear
Air expands when heated. The grains are measured per lb. of air. There are 12 ft^3 of air in a lb. of 0^F air. There are 13.5 ft.^3 in 70^F air. The heated air is lighter and has slightly less grains of moisture per ft^3 but the grains per lb. of air remains constant. Air at 0^F, 100%RH holds 6 grains/lb, while at 70^F, 100%RH holds +100 grains of air.
Air's ability to hold more moisture has more to do with the higher activity of the water molecules supended in the air. Cooling air slows the speed of the water vapor to the point of condensation. Therefore cold air holds less moisture than warm air.
This is my understanding. We all go through this process as we improve our understanding of the problem. Sometimes not quite perfect. Maybe we will get a PHD in here to give us the "real story".
Regards TB
I'm no PhD but I've read a lot of their stuff, so I'll give it a shot.

RH can be applied to any two-phase substance, since any liquid substance has an associated vapor pressure. It isn't uncommon to see RH used in the context of refrigerant vapor. For instance, in a closed cylinder of R22 the steady state pressure is the saturated pressure, found on the P/T chart. If at the same temperature the vapor pressure is found to be half of that value, then the RH of the refrigerant vapor would be 50% in that case, and would obviously be in a superheated state. IOW, RH of a refrigerant is an indicator of how close the vapor is to saturation.

Water is also classified as a refrigerant, R718. As with all of the other refrigerants, as the temperature is increased, the saturation pressure increases. The vapor above the liquid surface will not only be at a higher pressure, but also at a higher mass density as well. This is why it is said that "air at a higher temperature can hold more moisture". It really has nothing to do with the air, it has to do only with the temperature in the space, i.e., the temperature of the water vapor. IOW, as the temperature of water vapor is increased, its saturation pressure increases, and thus its maximum density before condensation will begin.

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