low temp. psychrometric questions

[jistre]

I'm trying to figure out what happens to psychrometry when air gets really cold. I'm trying to get a handle on the TR required to bring air down to -65, but I have two problems. One, the ASHRAE charts only go down to -40. Two, it makes no sense to me to talk of relative humidity once air gets below 32F. The ASHRAE low temp charts follow this, as the chart I'm looking at does not have RH lines below 32F.

So, I guess my questions for you guys are

1) Is there a such thing as a psychro chart for -65F or equations of state that can be used at those low temps?

2) Is my thinking about air and RH below 32F on the mark? To me, it seems like you can chill air until 32F saturated, then the water begins precipitating out as a solid, then the air continues chilling as dry air to final temps. My gut tells me that to calculate loads, you'd start at the initial air state, then find the enthalpy difference between there and 32F saturated. Then, the absolute humidity would be used to calculate the amount of water in the air at 32F saturated, and the heat of fusion for that amount of water would be added to the load. Finally, the air would be treated as dry from 32F to -65F to calculate the third component. Does this seem reasonable?

 

[jistre]

Clarification. My chart does not show RH lines below 0F, not 32F.

Still, RH below the freezing point of water is confusing me.

 

[IRstuff]

everything being "relative" ;-), I think that you can still tell the difference between 0%RH and 90%RH at colder temperatures.

don't forget that "freezing" does not necessarily mean that ALL molecules are frozen. Thermo is a physics of amassed particles, so there is a statistic distribution of particle energies that allow some subset of the molecules to be at an elevated energy. Otherwise, triple-points wouldn't even be possible.

TTFN

FAQ731-376

 

[jistre]

Okay, the statistical nature of thermo makes the RH question make sense to me. Temperature is an average measure, so there have to be some molecules above freezing, even though the bulk temperature is below 32.

Now that I have the nugget of information that lets my head accept dewpoints and humidities below 32F, do you know of any resources for low temperature psychrometrics below those -40F ASHRAE charts?

 

[DRWeig]

It helps if you start calling it frost point instead of dew point when you fall below 32°F... But it's still confusing.

http://www.aktonassoc.com/

was the first link when googling "cryogenic psychrometric" -- it appears they have some calc software for very cold conditions.

Good on ya,

Goober Dave

 

[willard3]

I have an Enthalex (Wing Co) psych chart that goes to -30F. There must be others.

 

[DRWeig]

jistre's need is -70°F.

Since last post, I checked out the Akton software, it goes down to -100°F. They're worth a call. It's not expensive.

Good on ya,

Weig

 

[jistre]

Thanks, guys. I'm checking out the Akron software. Only thing is that the enthalpy of the air at the summer design temperature reported by the Akron software is about 33% higher than the enthalpies reported by two other software packages, which agree with each other closely.

 

[ione]

Why not give a shot to this free calculator

http://www.gpengineeringsoft.com/pages/pdtpsychrocalc.html

They claim humid air properties are computed via ASHRAE formualtion

 

[Compositepro]

H20 has a vapor pressure at any temperature down to absolute zero. Ice has a vapor pressure and will sublime slowly in your freezer. Relative humidity is simply the ratio of the partial pressure of H20 in the air (i.e., concentration) to the vapor pressure of H2O at that particular temperature.

Perry's Handbook has thermodynamic properties of moist air down to -160F. CRC Handbook has vapor pressure date down to -100C.