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David PS
02-05-2003, 06:53 PM
I am trying to establish various pieces of information concerning an enquiry I have had.

The enquiry is for a -40'C freezer room with 5% RH.

I think I can achieve the RH with a wheel dehumidifier however before I can go go this far I need some assistance in working out the dew point etc.

Any assistance would be greatly appreciated.

Prof Sporlan
02-05-2003, 09:26 PM
The enquiry is for a -40'C freezer room with 5% RH.

The dew point will be in the area of -64°C (-83°F). The Prof will have to dig out some low temp psych charts to confirm, but this should be close.

David PS
02-05-2003, 10:49 PM
Thanks Prof

Could you be so kind as to explain how you came to this answer and what is the relation between the dewpoint and wet bulb temperature.

Thanks

David

aruiloba
02-05-2003, 11:19 PM
David:

You may download a free, very useful software HCON from www.geinet.com regarding properties of air: RH, dew point etc.
Look at Tools Area

You may enter -40 deg C and 5% RH and obtain Wet Bulb -40 deg C and Dew point -61 deg C


Augusto

Andy
03-05-2003, 10:53 AM
Hi David,
the dehum people we use are munters.http://www.munters.co.uk
Regards, Andy:)

Prof Sporlan
03-05-2003, 03:29 PM
Nice little program that HCON, aruiloba :)

If the Prof may be so bold, you can try his Java psychrometric calculator: http://members.socket.net/~aschoen/psychcalc.html He fogot he still had his calculator on the web... :)

Hcon gives us a -61.0°C dew point. The Prof's calculator give a frost point of -63.8°C and a dew point of -65.6°C. The latter numbers are probably more accurate, in his humble opinion... :). BTW, many programs that calculate dew points below 0°C are actually calculating frost point.

For a given state, dew point is where you reach 100 percent RH by sensible cooling only. Wet bulb is actually a thermodynamic condition that you cannot measure (only approximated by a psychrometer).

Gary
04-05-2003, 12:32 AM
Hmmmmm... I'm trying to picture how there could possibly be a dewpoint below freezing temperature. For that matter, how can water vapor remain in vapor form in below freezing air? Does mixing water with air change its properties? Kinda like mixing water with glycol? I must be missing something. :(

Prof Sporlan
05-05-2003, 01:48 AM
The Prof was also amused by this anomaly when he discovered the difference between frost and dew point. He'll provide an explanation when he gets a chance to review his notes on this subject.

Dan
07-05-2003, 02:33 AM
The Prof was also amused by this anomaly when he discovered the difference between frost and dew point. He'll provide an explanation when he gets a chance to review his notes on this subject.

This should be good.

I have a question or two. :)

Prof Sporlan
07-05-2003, 04:30 AM
Let us define a few psychrometric terms:

Dew point: the temperature at which air (or any gas) must be sensibly cooled in order to reach its saturation point with respect to water. At the dew point, relative humidity will be 100 percent. Note that it is improper to refer to the dew point as the temperature at which condensation starts to occur, because condensation at the dew point requires removal of latent heat from the vapor to induce condensation, and this can only occur if the air is cooled below the dew point.

Frost point: the temperature at which air (or any gas) must be sensibly cooled in order to reach its saturation point with respect to ice. It is the temperature at which visible frost begins to form on the surface being chilled.

Relative humidity: is the ratio of actual vapor pressure to the saturation pressure with respect to water at the prevailing dry bulb temperature.

So to properly figure 100 percent relative humidity at less than 32°F, one must calculate the saturation pressure of water, and not ice!

So what does it exactly mean to calculate saturation pressure of water below 32°F, you ask? Well, the Prof isn’t absolutely sure either, but then, he happens to be enjoying one of his better homemade stouts as he writes, and at least he can point out that this information exists in the literature to do such a thing.

And the bottom line is the saturation pressure of water below 32°F is greater than of ice, which shouldn’t surprise anyone, assuming one accepts such a thing, which means dew point must have a lower temperature than frost point.

You might find this link interesting here (http://www.humiditycalibration.com/softwarepg/software.html)

Thunder Scientific Corp’s calculator also differentiates frost and dew points below 32°F. It agrees with the Prof’s calculator of a –63.8°C frost point for a –40°C dry bulb and 5 percent RH, but it calculates a –68.5°C dew point compared to the Prof’s -65.6°C. Since the Prof recognizes he is taking some short cuts with his saturation pressure of water calculations below 32°F, he will defer to Thunder Scientific’s value here… :)