Evaluation of three semi-empirical models for superheated steam vacuum drying of timbers

Superheated steam drying at sub-atmospheric pressure (SSV) has been successfully employed in Europe and Asia for drying some types of timbers, showing that drying time could be reduced by 50% with respect to conventional drying without significant losses in the quality of the final product. This reduction is the consequence of a different heat and mass transfer control mechanism. Since SSV drying is carried out in absence of gaseous air, diffusion of the generated vapor is not a limiting factor and drying rate becomes more dependent on heat transference. Therefore, classical interpretation of timber drying as a process based on moisture migration control is not applicable to SSV. This work is targeting the development and validation of a simplified semi-empirical model for SSV drying of timbers. Mathematical representation of the proposed model is uncomplicated and straightforward to apply, and the comparison between model predicted and experimental data showed a high degree of agreement under variable drying conditions.