X-Source: The Hotrod Mailing list
Subject: Boiling water in a vacuum    
Date: Wednesday, Jul 27 1994 11:22:03
From: ijames@helix.nih.gov (Carl F. Ijames)     

John responds to someone's question:
>>  What's the boiling point of H2O @ 30"Hg, 29"Hg, 28"Hg?
>
>I don't have my table handy either cuz it's packed for the Great Move
>but I know that it takes a much better vacuum than the average
>refrigeration pump is capable of to boil water at room temperature.
>The typical pump will make water evaporate faster but it won't
>cause it to boil out of places where it might be absorbed.
>What I do is pull as good a vacuum as I can and then heat the system with
>a heat gun.  Seems to work.

Vapor pressure of water at 20 C is 17.5 torr or 29.2 inches of mercury.
Boiling a container of water by pulling a vacuum is difficult because the
water cools by evaporation which lowers the vapor pressure (and
substantially lowers the evaporation rate).  Its easy to boil water (and
make ice simultaneously :-)) with a two-stage rotary vacuum pump (which
bases out at say 50 millitorr or less), but I imagine that a refrigeration
pump (exhausting to atmospheric pressure) would have a hard time.  The
evaporative cooling is also one reason why you should evacuate an AC system
for at least 30 minutes, even though by the gauge the water looks like it
is all gone.  Another is that the equilibrium vapor pressure of water
adsorbed in a molecular sieve (the drier) is greatly reduced (to less than
1 millitorr at 20 C), so it takes a long time to pump it dry (which is why
John recommended heating if possible).  So just where is the drier on my
'86 Buick Regal?  Is it in the accumulator?

Regards,

Carl Ijames     ijames@helix.nih.gov

X-Source: The Hotrod Mailing list
Subject: Re: A/C thoughts (fwd)
Date: Monday, Jul 25 1994 09:22:03
From: John De Armond

>  What's the boiling point of H2O @ 30"Hg, 29"Hg, 28"Hg?

I don't have my table handy either cuz it's packed for the Great Move
but I know that it takes a much better vacuum than the average
refrigeration pump is capable of to boil water at room temperature.
The typical pump will make water evaporate faster but it won't
cause it to boil out of places where it might be absorbed.
What I do is pull as good a vacuum as I can and then heat the system with
a heat gun.  Seems to work.

>  I wonder exactly what those recycling units contain in order
>to neutralize all the crap found in used R-12.  Anyone ever had
>the chance to use or inspect one?

Nothing other than a tank, a pump, maybe an oil separator and maybe
a dryer.  The law says that recaptured freon is not to be re-installed
in any other system and must be returned to a reprocessing station.
No service tech I've ever talked to does it that way because he would
then have to buy his freon back from the reprocessor.  As long as water
doesn't drip out the hoses, most all of them just tank it and reuse
it.

John