As Brent described, it is possible to boil and freeze water at the same time.
Rapid evaporation by drawing gas molecules away from the surface of a liquid
is the definition of boiling. Water does not have to be heated to boil. The
temperature of 100 C (212 F) is commonly known as the boiling point of water
at sea level (less temperature is needed at higher altitudes). In fact, at
higher altitudes it is not possible for the water to reach 100 C because it
will boil away at a temperture that is lower than 100 C. Boiling, or rapid
evaporation, is a cooling process. The most energetic molecules within the
liquid have the kinetic energy to break free from the surface of the liquid.
When they break away as gas molecules they take energy with them, causing the
remaining liquid to contain less overall kinetic energy (heat) so the liquid
is cooled as it boils. If the gas pressure (normally applied by the weight
and kinetic energy of the sea of air molecules that we are all immersed
within) is mechanically reduced by means of a vacuum pump or by expanding the
volume of an enclosed container (like a syringe or inlet side of a pump or
cylinder) the liquid will boil at a lower temperature. As the gas pressure
is progressively lowered, the liquid will boil more vigorously, further
reducing its temperature. Eventually, the temperature of the water will
reach 0 C (32 F) while it continues to boil. If the process is allowed to
run long enough, and a large enough supply of water was available at the
beginning of the process, the remaining water will start to freeze . . . at
which time the observer will see ice floating in boiling water. Continued
removal of energy through additional boiling will result in the entire volume
of liquid that remains being frozen into ice. Thereafter, if the vacuum pump
continues to remove gas molecules from the container, the ice will sublime
(transform directly from a solid to a gas) until eventually, no water
molecules remain in the container . . . the vacuum pump will have removed
them all.
The answer to the original question . . . "could the water be degassed and
held under vacuum until it freezes?, is NO. Not that it wouldn't freeze, it
would freeze even faster than it normally would without the vacuum pump.
This would not work as a way to eliminate gas bubbles from the ice because
the reduced pressure in the container would actually create MORE gas bubbles.
If you want to eliminate the large gas bubbles, you would have more success
by pressurizing the water (just like we do with urethane casting systems).
Of course, the effects of the removal of the pressure after the water has
solidified is another issue. If the pressure is reduced slowly enough, the
ice may not crack from the internal air pressure that is created by the
gasses (other than any gaseous water molecules) that are trapped in the
interstitial spaces between the frozen water molecules. If the pressure is
removed quickly, the ice is likely to crack and or "explode". Of course, the
actual effects will be dependant on the amount of pressure applied to the
water as it froze as well as the speed with which that external pressure is
released. A similar situation occurs in deep sea divers who rise to the
surface too quickly. Gas molecules that were dissolved in their bloodstream
at the higher pressures of deeper depths in the ocean are suddenly released
as they rise too quickly for their bodies to remove the gas molecules that
"boil" from their liquid blood when the external pressure on their bodies is
suddenly reduced. If they come up slowly, the dissolved gas has time to exit
without destroying their bodies. Same thing should hold true for the ice.
By the way, there are two main reasons vacuum casting systems work well to
eliminate air bubbles from castings:1) the large quantity of gas that is
dissolved in the material (urethanes, etc.) is allowed to boil away when the
pressure in the chamber is reduced. and 2) the reintroduction of atmospheric
pressure on the liquid material that is being cast collapses any additional
bubbles of gas that remained visible in the liquid when it was still under
vacuum. Note what happens to the gas bubbles that remain in a cup of resin
that has been "degassed". The remaining bubbles all collapse under the
weight of the air molecules that have been allowed to reenter the chamber.
One last thought . . . there is no such thing as the "force" we call
"suction". The vacuum pump does not draw the gas molecules into itself. The
pump simply provides an open inlet chamber into which the gas molecule must
find its way. After the molecule enters the inlet chamber, the pump
physically moves the molecule to another location where it is thrown out of
the pump into the sea of atmospheric air. Picture the gas molecule as a
billiard ball bouncing around inside the vacuum chamber where it bounces off
the walls, and other balls, until it finally wanders into the inlet chamber
of the vacuum pump. There is no attractive force called "suction" that pulls
the molecule into the pump. The movement of air or liquid molecules that we
describe when using the term "suction" is caused by a difference in pressures
at various points within the container. Molecules evenly distribute
themselves throughout the enclosed chamber by bouncing off of each other
until they are evenly distributed throughout the container. The opening of
the vacuum pump's inlet chamber (which is mostly empty of molecules) inside
the enclosed chamber introduces a volume of space that has a lower pressure
than the original space within the chamber. The greater concentration of
molecules within the chamber instantly move to redistribute themselves
throughout the newly available space, thereby reducing the pressure
throughout the chamber. Each individual molecule must supply its own power
and find its own way into the pump before it can be removed from the chamber.
That's why it takes so much more time to drop the last 5 Torr (In Hg) than
it does to drop the first 5 Torr or In Hg. At the beginning of the
evacuation of the chamber, there are so many molecules bouncing around inside
the chamber that it is easy for lots of them to find their way into the pump.
Near the end of the evacuation of the chamber it takes a lot longer for the
remaining molecules to find their way into the pump because there aren't as
many other molecules bouncing around in the chamber for them to run into and
be knocked in the direction of the chamber outlet (pump inlet) hole. Note:
the larger the size of the outlet hole leading to the inlet chamber of the
vacuum pump, the easier it is for the gas molecule to find its way out of the
chamber and into the pump . . . and the faster the pump will be able to
reduce the pressure in the chamber by removing the gas molecules.
Sorry, sometimes I get carried away preaching to the choir.
Now, back to work!!!
Ken Miller
Miller Technologies
395 S. 1100 W.
Farmington, UT 84025
(801) 451-7997
foamcaster@aol.com
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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