Process for drying large pieces of wood at subatmospheric pressure or in vacuo, particularly for drying delicate wood and/or wood which is easily split

A process of drying large pieces of wood comprising a step of heating a drying gas and a pile of said pieces of wood in a sealed container to accumulate heat in the wood under inherently increasing pressure of said gas, followed by a step of connecting the container to a vacuum source to produce evaporation of moisture from the wood by the heat accumulated in the latter,

• wherein the heating step comprises:
• circulating within said sealed container the said gas in humid condition in a closed circuit such that at every turn the gas is compelled to pass through the pile and then return along a distinct path which does not pass through the pile and is in heat-exchange relation with the walls of the container,
• heating the container as a whole from the outside to heat said gas on its return path by inflow of heat from the walls of the container to the gas, whereby the thermal content of the gas increases at each subsequent turn by a diminishing amount until the gas reaches a substantially steady state at which there is no overall increase of its thermal content at each subsequent turn while at the same time the absolute humidity of the gas remains substantially constant, both on passage through the pile and on return, and
• continuing the circulating and heating of the humid gas under said steady state conditions until the temperature drop of the gas between its entrance to and exit from the pile is less than 3.degree. C.