The benefit of vacuum drying is the low drying temperatures and fast drying speeds. The energy consumption is relatively small. No substances that evaporate from wood are released in the air because the temperature of the steam inside the drier is lowered so that it turns in to a fluid.
Vacuum drying is based on the decreasing boiling points (done by decreasing air pressure) and excess pressure increasing inside the wood. Due to the excess pressure inside the wood, the water passes through much faster from the inside to the surface. This means that there are no moisture differences created by the tension between the inside and the surface of the wood. Water can boil at a temperature as low as 35-40°, though normally a boiling point of 50-80°C is used.
The water is compressed into water. This happens when steam moves through the vacuum pump, into the condenser. Some of the steam is compressed on the walls inside the chamber during winter.
The problem with vacuum driers is the heat transferring on the wood. Drying chambers don’t have air and not a lot of steam to push heat from the radiators on to the wood. This is why the wood is usually heated before its put in the vacuum drier.
Other heating methods are for example high-frequency heating and the use of metal boards in between the sawed wood, inside the drying chamber.
In high-frequency heating, the wood that has the most water heats up faster and some overheating may occur. High-frequency energy turns into heat inside the wood. The pressure inside the wood increases, this is when water moves efficiently towards the surface of the wood. The surfaces of wood aren’t, also never have to be, as dry as the inside of the wood, this means that no drying tension is born. The method is suitable for drying so called air dried wood.
High-frequency heating (RFV)
Vacuum drying is regularly used for drying expensive special wood and also wood that dries harder. The quality of the dried wood is usually good; there aren’t a lot of cracks or tension in the wood. Drying air dried wood doesn’t change the color of the wood. However drying fresh wood can cause problems. The drying method is expecially suitable for thicker wood due to the speed of the drying process.
- Short drying times
- Very small drying defects.
- No color shifting
- Suitable for thick pieces of wood
- The method is fairly new, therefore there aren’t diagrams for every situation.
- The short drying does not necessarily equalize moisture variations