The Vacutherm, VacuPress dryer is capable of drying lumber and dimension stock up to 15 times faster than conventional methods, by using the benefits of a vacuum to create a pressure gradient between the shell and the core of the wood. Conventional drying methods rely on moisture differential or gradient to move the water from the wetter core toward the dryer shell. This can be slow; in that the operator must be careful not to over dry the shell or degrade will result. Vacuum drying, using the Vacutherm VacuPress Technology, can achieve these fast drying rates by maintaining a small differential between the shell and the core moisture contents throughout the drying period. Typically, the core of the wood is essentially at atmospheric pressure, while the shell of the wood is at a much reduced pressure, causing the water to flow toward the shell more rapidly.

For the purpose of drying lumber it is necessary to increase the temperature while maintaining a vacuum (see Diagram 1). This causes the environment within the dryer to become superheated and therefore the equilibrium moisture content is reduced causing the moisture from the wood to convert to water vapor at the reduced pressure. The moisture from the wood can then be condensed and removed from the pressure vessel and drained via the vacuum pump.

An additional and important benefit of the VacuPress dryer is the rubber membrane which is placed on top of the lumber pile, which is allowed to collapse on top of the lumber during the drying process (see Diagram 2 - number 2 or Diagram - 3).

The pressure applied to the top of the lumber can be as high as 1,200 pounds per square foot and is maintained even while the lumber is cooling after the final moisture content has been achieved. The result is very flat and straight lumber, free from side bend, cup twist and bow.

Heating the lumber is necessary to circulate water (see Diagram - 3) within the wood and bring it to the surface so that it can be evaporated as steam, which either condenses on the cool walls of the pressure chamber and runs to drain or is drawn out of the pressure chamber by the vacuum pump as steam. VacuPress kilns use aluminum heating plates, which are heated by hot water that is circulated within the plate (not unlike a radiator).

This heating system is very effective at heating the wood without damaging the wood. Due to the low pressure (vacuum) environment the direct heating of the wood will not darken the wood, in fact the low oxygen environment actually contributes to the very bright colors that result from the vacuum drying process. Hot water heating plates are also a very economical method of heating the wood, because the water can be heated by almost any boiler system (i.e. Wood Waste, Gas, Propane, Oil or Electric).

As water is removed from the wood by both the forces of heating and vacuum, the water either remains on the surface of the wood to humidify and condition the surface against over drying or it is evaporated as steam, which occurs during the drying phases of the schedule. A schedule of drying and conditioning the wood is used to relieve internal and external stress in the wood through the process. This is especially important for woods such as Red Oak or other difficult to dry species.