I believe there are too many variables to provide a single answer. Wood dries faster or slower depending on species, thickness, temperature and pressure. I think these temperatures for the boiling point of water at different pressures are appropriate to this discussion.
psi = °C
14.6 = 100.
5.00 = 72.1
1.00 = 38.1
0.70 = 32.0
0.50 = 26.7
0.35 = 20.5
0.25 = 15.5
0.20 = 11.6
0.15 = 7.3
0.10 = 2.2
I air dry the timber I mill, so I do not need a vacuum chamber/kiln for special species projects with critical delivery times. I got the prototype system working for a bush engineer friend and his mate. I later adapted it for vacuum impregnation of preservative. It seemed happy to dry timber at 1 psi, maybe the vacuum gauge was unreliable and it was running at slightly lower pressures (0.2 psi) when colder.
For an unheated vacuum drying chamber, the operating pressure will depend on the temperature of the charge. The temperature will fall as the vacuum is pulled. There will then be a problem with supplying the energy needed to evaporate the water.
Wood holds onto water so the boiling point has little effect on tightly bound water. But once the water is free, if it condenses onto surfaces, it will be in equilibrium and prevent further drying. At a fixed temperature, removing a volume of steam will boil more water and stabilise the pressure again. The chamber will then have a steam atmosphere. That steam must be continuously removed economically to keep the drying progressing.
The liquid water in the timber gradually evaporates to a much greater volume of steam. That volume is increased again 14 times at 1 psi abs. The pump used to pull the initial vacuum needs a high capacity. The pump that removes the water vapour towards the end of the process may be a different pump, it needs to operate efficiently and economically for several days.
The low-density steam being extracted would normally condense in the exhaust port of the extractor pump. But if it could be condensed in the chamber it would help to maintain the low operating chamber pressure. One experiment I have contemplated would be to have a cooler and drip tray in the chamber, (insulated from below). Steam at chamber temperature would condense into the tray, then flow as liquid to a cool external port where the water could be extracted more efficiently. The volume pumped multiplied by the pressure difference would effect a huge reduction in pumping costs compared with the low-density steam extraction. Let me know if anyone tries it.
I believe there are also chemical changes that take place over time while air drying timber. I expect the vacuum is helping to dry the timber, while only the heat of a kiln can accelerate the chemical changes. Near room temperature, for each increase by 10°C, the rate of a chemical reaction approximately doubles. A kiln increases the temperature, so it increases the rate of chemical changes. Going from 20°C to 90°C is a 70°C increase, 2^7 = 128. So a one year reaction would take 3 days. To boil at 1 psi requires warming to 38.1°C, which is easy to do from 20°C. But the chemical reactions will only speed up by a factor of 4. So a one year reaction would take 3 months. It would be interesting to know what chemistry takes place, and when it happens if the material was vacuum dried quickly.
