Thanks for the input. It was a great refresher Chem course - my last
one was nearly 20 years ago at Uni (and it wasn't my best subject...)!
As an ME, I don't use much chemistry. Cheers!
--- Regards, Dan Davis PROTON Rapid Prototyping & Manufacturing Center Hicom Industrial Estate, Batu Tiga, PO Box 7100 Shah Alam, 40918 Selangor MALAYSIA +60 3 515-2380 phone/fax http://www.protoncar.com/rapid/index.htm
> -----Original Message----- > From: Foamcaster@aol.com [SMTP:Foamcaster@aol.com] > Sent: Saturday, April 10, 1999 5:41 AM > To: email@example.com; firstname.lastname@example.org; > DANIEL@proton.com.my; SiderWhite@worldnet.att.net > Cc: email@example.com > Subject: Re:Freeze Casting water that is boiling > > 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 > firstname.lastname@example.org
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