Boiling point of moisture under vacuum

[Z2TT]

Hello,

I have read that moisture boils out of the system at about 1500 Microns of vacuum

But doesn't this vary depending on ambient temperature? If so how much would it vary from say a colder day (50F) to a warmer day (100F) I'm having trouble finding this out

Thanks.

[Robert Hilton]

Pressure and boiling point go hand in hand. The lower the absolute pressure, the lower the boiling point.
At 1500 microns, water should boil off easily at room temperature. By evacuating to such a low absolute pressure you ensure all water in a system is in vapour form thereby allowing easy removal through the vacuum pump.

[Tesla]

Hi Z2TT
Good question. Firstly I would ask why you are using F instead of °C considering your age and country? There is a corresponding temp press relationship which will change with ambient temps. These charts are available on google, text books (Dossat) and kirkothemers. Not so much the ambient but the internal temp and vacume. Another consideration is the rate of evacuation and heat transfer to the evacuated space with relation to sublimation (considering the temp drops with reducing press). The other consideration for the rate of evacuation is the ability for moisture to be removed from substance like oil and the skin of the metal like osmosis and through the surface of a liquid like oil. To increase the transfer things like IR heat and vibrations help. In short to sort of answer your question it really depends on the temp/press relationship inside the system in which the ambient is a factor but not directly (second or third order).

[Z2TT]

Thanks for the reply, I used F just not to confuse US guys (No rudeness intended).

I found this chart :

http://www.engineeringtoolbox.com/bo...ter-d_926.html

Shows that at 0.5 PSI (Absolute), water boils at 26 degrees C. I'm not sure how to convert 0.5 PSI Absolute to microns or InHG (Gauge) though.

I too was wondering about the fact that the moisture not only has to boil first, but make it's way out and past the oil, even if the micron guage readings show say 600-1000, could there still be a considerable amount of moisture in the system as some would be "stuck" so to say and not picked up by the micron gauge reading? In that case is it a good idea to keep vacuuming for a certain period of time even after you reach the target reading of 400?

Thanks

[Quality]

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have a look at this

[Brian_UK]

A good converter program if wanted is Uconeer
http://www.katmarsoftware.com/uconeer.htm

[mad fridgie]

When comes to vacuuming a system, you should be looking at what is practical and what is theorectically possible.
Water will vapourize close to -100C under a near abosulute vacuum.
The problem comes not how deep a vacuum you can pull, but how much volume it can pull.
For example if you were trying to vacuum in a Blast freezer at -40C, so all being perfect for heat transfer ( water vapourizing point -40C), your density would be inexcess of 8000M3/Kg of water vapour.
lets say you had 10gramms of water (not alot) you would need to pump 80M3, I have a reasonable size service pump, 100L/M, it would take over 13hrs just to remove this.
Best to vacuum when internal temps of the pipework is high, higher boilng point and reduction volume which requires pumping.
Hope that makes sense!!!!!!!!!!!!!?????????

[Tesla]

It is generally not recommended to vac below 400 as if you get to say 250 you would start to boil and deteriorate the oil. Unless you had a dry (no oil) system. To check for a good vac we usually do a holding test where the system is isolated and pump turned off for a period of time. After that time we check the micron reading and allow for a little rise - the times and readings depend on size and type of system, we would not want to use a deep vac for an open drive system as there may be air with moisture sucked in. To get °C or °F it is hold ALT and tap 248 then the C or F.

[Z2TT]

Hi,

I found this chart, Turns out water boils at quite a low amount of vacuum under normal temperatures, 22860 microns at about 76F/24C, so am I right to say if the ambient temp is 24C, and I go well below 22860 Microns (Vac to 1000 microns), I should be confident that almost all the moisture is removed if it's a small system like domestic/automotive