Using steam during the warm-up to improve the drying quality of sawn timber

The use of steam for humidification of air during the warm-up phase of kiln drying is investigated both experimentally and theoretically. An existing drying simulation model is expanded to take into account the limited heating and steaming capacity of an industrial kiln. Drying experiments are performed in a laboratory scale to investigate the effect of the warm-up period on the final checking of sawn timber to be observed at the end of the drying process. The results show that the most important single factor causing drying checking of sawn timber is the initiation of so-called micro-checks during the warm-up phase of drying. With steam the micro-checks can be avoided and drying checking thus reduced by 50%. Also the time needed for the warm-up phase can be diminished by 50-80% and thus the drying capacity be increased as much as by 10%.