Abstract: A timber drying plant has a drying chamber through which air may be circulated by a fan in a circuit which includes a refrigerating unit. The refrigerant fluid circuit of the unit includes a dehumidification evaporator for outlet air and a condenser in the recycle air flow for returning to the latter least part of the heat extracted by the evaporator. The refrigerant fluid circuit also includes an auxiliary evaporator with control valve means whereby the auxiliary evaporator can be inserted in said fluid circuit so that the refrigerating unit operates as a heat pump absorbing heat from the external atmosphere through the auxiliary evaporator. A supplementary evaporator may also be included in the refrigerating unit and is connectable in parallel with the auxiliary evaporator to increase the heat pump effect. In another position of the control valve means the auxiliary evaporator acts as a condenser in circuit with the dehumidification evaporator to reduce the heating of the recycled air.

Abstract: A process for kiln drying timber in which heated air is passed over the timber, subsequently cooled to dehumidify it, and then reheated and recirculated, wherein times are substantially shortened by separating the process into two stages, a first stage where the temperature of the air passing over the wood is held relatively low, that is between 20.degree.C and 30.degree.C, and a second stage in which the temperature is raised to between 34.degree.C and 38.degree.C, the changeover from the first stage to the second being effected when the moisture content of the wood being dried has fallen to between 16 and 25 percent.