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Old August 26th 05, 08:11 PM
Rick Matthews
 
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wrote on 8/25/2005 8:37 AM:
rick1matthews wrote:


Pour water on the floor, and it will evaporate without addition of heat...



It's more accurate to say that the heat required to evaporate the water
(about 1000 Btu/lb) comes from the surroundings, ie this evaporation
cools the floor, the air, and so on.


Some of the heat comes from the surroundings in
this example. Some comes from within the water itself.

Key to this discussion is that no external source of
heat is needed to evaporate water.

Were a 70 degree F drop of water suspended (perhaps in a weightless
environment) in a hard vacuum, which in turn is completely
surrounded by shiny surfaces cooled to 100 milliKelvin, the drop
would still evaporate.

So where did the heat come to evaporate it?

Statements were made earlier that water only boils when
heat is added and that water only evaporates when heat
is added. Neither statement is true.

Water evaporates whenever the partial pressure of water
in the surrounding atmosphere is so small that the
capture of the water molecules from the air is less
than the escape of water molecules from the surface
of the water. The rate of escape is determined by
the water temperature. The rate of capture is determined
by the partial pressure of water. The water temperature
at which the two are in equilibrium is called the dew point.
If the water is cooler than the dew point, you get net
condensation. If the water is warmer than this dew point,
you get net evaporation.

When air pressure is very low, the partial pressure
of water can only be lower. Therefore the dew point must
be quite low, so that 72 degree F water (or 35 degree F
water, for that matter), is warmer that its dew point.
Water molecules are always escaping, and there are few
water molecules in the environment to balance the process.

Hence, the water evaporates.

The cooler it is, the slower it evaporates, but it does
still evaporate. In a perfect vacuum, even a 1 Kelvin ice
cube would eventually sublime way, though not in billion
years.

--
Rick Matthews


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Old August 30th 05, 12:46 AM
Richard J Kinch
 
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Rick Matthews writes:

Statements were made earlier that water only boils when
heat is added and that water only evaporates when heat
is added. Neither statement is true.


It is true in the sense that the heat content of the water itself can
only evaporate a small portion of it. If you squirted warm water out of
the Space Shuttle in orbit, some would flash to vapor but most of it would
just wind up floating around frozen. The point is simply that vacuum
itself does not evaporate water, it is heat, contrary to the popular
notion.
  #63   Report Post  
Old August 30th 05, 01:50 AM
Rick
 
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True, most of the water will freeze before evaporating. But
you also said:

It *will not* sublime at all if there is no heat added, even in a perfect vacuum.


which is not true.

The ice will sublime in a perfect vacuum if it is above absolute zero,
as long as there is any ice left.

Change "at all" to "quickly" and I will agree with you.

Rick

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Old August 30th 05, 09:06 AM
Richard J Kinch
 
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Rick writes:

The ice will sublime in a perfect vacuum if it is above absolute zero,
as long as there is any ice left.


If it is above absolute zero, then it must have had some heat added.
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Old August 30th 05, 09:21 AM
Richard J Kinch
 
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The thing you are missing is gravity. Ice is not going to "float
around" in that pipe. It will lay on the pipe (compressor, valves etc)
and heat from the ambient air on the other side of the pipe will
evaporate the water.


Some of it will gain heat by this process, although much more slowly
than one might expect or hope. Remember it takes a huge amount of heat
transfer to vaporize water, and the vacuum is an excellent insulator.
Parts of a
glob of water in contact with the system will freeze and then tend to
sublimate a thin layer of vapor to separate themselves from the heat
source, instead of staying in intimate contact. And remember this
doesn't work at all for water entrained in or bound to something else.

Another practical problem in a AC system is that the water may trapped
on the other side of 50 feet of of 3/8 inch tubing from the vacuum
source, such as if the service connection is at the compressor on a
split system with the water stuck up in the evaporator. The back
pressure of a long thin line will spoil the vacuum on the other end.

My point is simply to correct the common misconception among AC techs
that if they pull a good vacuum on a system that it is guaranteed to be
dry, because they believe "water boils in a vacuum" from seeing a high
school science demonstration. The purpose of vacuuming an AC system is
to remove non-condensibles, not to clean out contaminants like water.



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Old August 30th 05, 03:02 PM
Rick
 
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That does not follow.

Rick

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Old August 31st 17, 02:33 PM posted to alt.home.repair
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Default 500 microns? - then you are a HACK!!

Summary judgment God. Glad to know asking means finished. You should run Nasa. You know everything based on one question. And you are polite. Never insulting, never profabe. What a guy
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Old July 30th 20, 04:52 PM posted to alt.home.repair
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Default 500 microns? - then you are a HACK!!

Well put. Old school gentlemen who is an excellent teacher. I can tell you didnt read that in a book , and if applied and refined. Amen. Like the no name calling, shows confidence. 35 yrs tech., installer, engineer.


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