PDA

View Full Version : Vapor Pressure



herefishy
08-03-2002, 05:23 PM
If I plot design room temperatures on the (ASHRAE) psychrometric chart, say room (1) @36degF @60%R.H. and Room (2) @ 28degF @ 80% R.H., whereas the "line" is horizontal straight across the chart, would my vapor pressure be the same?

And as such, would that make the latent load of infiltration between the spaces null?

Prof Sporlan
08-03-2002, 08:42 PM
If I plot design room temperatures on the (ASHRAE) psychrometric chart, say room (1) @36degF @60%R.H. and Room (2) @ 28degF @ 80% R.H., whereas the "line" is horizontal straight across the chart, would my vapor pressure be the same?
Your humidity ratio would stay the same. In this case, it will be about 0.0027 lb water/lb air. And this would also mean the partial pressure of water vapor would be the same for both states. The partial pressure of water vapor at saturation (100 percent RH) would be different for the two states since colder air holds less water. This is why RH increases from 60 to 80 percent.


And as such, would that make the latent load of infiltration between the spaces null?
Interesting observation! Sensible load, however, will be present. Sensible heat energy will move from the 36°F room to the 28°F room at a rate dictated by the insulation or barrier used. But one would not see any infiltration of water at these conditions.

Dan
09-03-2002, 02:35 PM
Ok, let's say there is a large vapor pressure difference. Would an air curtain do anything to impede the flow of moisture? If so, how?

Dan

Prof Sporlan
09-03-2002, 04:42 PM
One would need a vapor barrier to prevent the flow of moisture to an area of lower humidity ratio. It would seem an air curtain would not make an effective vapor barrier.

herefishy
09-03-2002, 05:26 PM
Prof.,

Have you followed the thread that Dan and I have been exchanging in regarding air curtains - "infiltration".

Would any forced air movement (at an entry) decrease infiltration in such the same manner as pressure drop occurs due to "friction of such movement" of gasses or liquids? Would not the same effect of movement/friction, such as say refrigerant in transport tubing, cause a decrease in the "pressure", or difference thereof in a gaseous (air) medium?

Could creating the intrained movement and velocity of (air) cause an opposing force to the force of pressure difference resulting from air densities and pressures?.

Can such effects of pressure drops (or forces) resulting from air movements be determined in relation to the force of infiltration vs. the opposing force of the pressure drop (or opposing force) created by some recirculatory air movement (at an entry)?

Can it be said, that if such air movement can oppose the measureable force of infiltration as demonstrated by Newton's second law, that only the sensible infiltration could be negated, however latent (water) infiltration/exchange would occur through the opposing force of the airstream(s), nonetheless?

To what extent is the water intrained into the air mass which contains it? would it be less than the "heat" which suspends it? Or would OSMOSIS persist and overcome any mechanical forces or venturi type effects?

Would that be REVERSE osmosis?

Dan
13-03-2002, 02:23 AM
Yeah, that's what I have been wondering. Does vapor pressure permit moisture to sneak through an air curtain even when the dry air is impeded. Or is the air to be considered a homogeneous enthalpic mass?

Prof Sporlan
13-03-2002, 05:58 AM
Yeah, that's what I have been wondering. Does vapor pressure permit moisture to sneak through an air curtain even when the dry air is impeded.

Interesting thought. Differential vapor pressure causes moisture to migrate from an area of high humidity ratio to a lower one. The air curtain sets up a pressure differential of its own. Pressure is pressure, so one may conclude that an air curtain could impede the flow moisture, probably in the same manner as garden variety insulation, which is really what an air curtain represents, albeit you can walk thru it.... :)

Dan
14-03-2002, 01:16 PM
Partial pressures is a term that I have never understood. I was wondering if the answer lie in whether air is an enthalpic indivisible sum of how we see air, or.... let me try a different tact:

Can it be that the force of air from warmer toward cooler be simply a reflection of vapor pressure differences? Does a dry bulb temperature exert any force besides conduction on a cooler dry bulb temperature?

Beyond air density, what resistance does a downward flowing air curtain have toward impeding flow between one vapor pressure and another? Surely, just because air is flowing downward there shouldn't be a greater resistance to vapor flow?

We know that a cold surface collects moisture from air in varying degrees relative to dry bulb cooling. I suspect if we had perfectly still air between a warm surface and a cold surface, we would not be able to have still air.

Is it perhaps vapor pressure that moves all the air as an enthalpic mass that creates the breeze?

That we could have no breeze between these surfaces beyond that which would result from the smaller change in dry air density differences?

Clumsy kindergarten wonderings. Perhaps if I understood the concept of partial pressures, I would understand.

But is it wrong to think that differences in vapor temperature/pressures can be independent of differences in dry air temperature/pressures within the same air mass?

herefishy
14-03-2002, 05:00 PM
..... keep in mind..... THE WATER IS MOVING TOO! And it has momentum.

What (vapor) pressure difference would be necessary to overcome the path (momentum), at a given velocity, of the (hypothetically) recirculating vapor?