It seems there is a wee bit of misinformation here. Yes!
As I see it, the vacuum is to accelerate the drying process without negatively affecting the quality of the wood. Absolutely. I have no idea whether the vacuum kiln will distort the wood or cause checking. Both of these problems are caused at least in part by unequal drying in the wood. The vacuum process should equalize evaporation, since unlike heat and fan processes, it is equally distributed at all points inside the kiln. If you discover that the vacuum process causes warping, then you will wish to adjust your vacuum switch upward to apply less vacuum.
ANY pressure differential will greatly affect the process. Where does the heat come in? You got that right! By creating a vacuum kiln, you have made an air conditioner of sorts, with the wood acting as the "evaporator". This is the part of your car that gets cold when you turn on the AC.
Water has what is called a "vapor pressure" at every temperature. At 212° F, that vapor pressure exceeds the atmospheric pressure, and the water boils. When you reduce the atmospheric pressure with vacuum, the vapor pressure of water for any given temperature does not change, and the water can boil off at room temperature; it can even boil at temperatures below 32° F. As the water boils off (or just evaporates), the heat contained in the water goes into the air, and the water containing vessel gets colder. Effectively, you are pumping heat out along with the water vapor.
Example: In a laboratory, I have put a flask of water under high vacuum, and the water begins boiling. It rapidly cools off so much that the boiling stops entirely, and frost begins to form on the outside of the flask, caused by the cooling that has occurred on the inside due to evaporation. If I then put just my hand on the bottom of the flask, the water begins to boil again! This a particularly fun thing to do, and most kids in High School chemistry have done it.
In the note I posted above, a small electric heater is suggested so as to reverse the evaporative cooling caused by the vacuum. Just putting your vacuu-ln in the sun would have the same effect.
As I recall, and I'm not googling it, it is a 3 - 4 week process of increasing heat to kiln dry wood. Culminating with 180 or so degrees which is the bug kill.
My ancient memory, and I'm not googling it, is that we are 13 - 15 pounds of pressure now. Yep. So even taking the pressure down 2 pounds should really pump the water out. Again, you got that right!
My objective is to achieve this 2 pound (or whatever is easy) differential quickly, and when the internal pressure of the board equalizes, then allow to return to atmospheric. This is a logical flaw in your process: the pressure in the wood will be equalized almost instantly, since the wood does not really act like a pressure vessel that can hold air pressure in. What will occur is that the water evaporates out of the wood at a faster rate than normal. When you turn off the vacuum, the rapid evaporation stops, but the wood will have been cooled off by the accelerated evaporation. If you just do that on a short-cycle basis, the cooler wood will cause condensation, and then at least some of the water vapor will condense back inside the wood.
There is another serious flaw with this plan: you will be expending energy ($$$) to create the vacuum. If you simply vent the vacuum, you will be wasting that energy. Just let the water in the wood out-gas until the pressure comes back up...then hit it again. An adjustable vacuum switch would do that automatically at relatively low expense.
[/B]
It should 'fog' immediately with a pressure drop. No fog will form until you release the vacuum. Your pressure vessel will be warmer than the internal atmosphere (that evaporative cooling gets around), and it will not condense on the higher temperature material. When you release the vacuum, the vessel will have been cooled just a little bit, and then it will fog on the inside.
Two Assumptions:
Above Freezing
Even if it is well below freezing, water ALWAYS has a vapor pressure, and that causes the H2O molecules to jump into the air. When this occurs from liquid water, we call it evaporation. When H2O leaps into the air from it's solid form (ice), we call it sublimation. Ever had freeze dried food? That is how they make it. Freeze it...Vacuum the water out. Do you also remember that freeze dried food has mostly the same shape as the original food? I would think freeze-drying the wood might reduce warping.
Either way, your wood will dry faster under continuous vacuum.
The moisture ladened air is removed from the environment via the vacuum.
True. Under a continuous vacuum, all of the the air is removed and your vacuu-ln is filled with 100% water vapor. Effectively, it is pure steam at a reduced pressure. I suppose that there will be some volatile elements of the wood that might jump into the atmosphere also.
