Summary

In the Netherlands kilns are especially designed to dry stacked lumber. The air circulates through the lumber by a number of fans. Usually the drying air is heated by direct-fired natural gas heaters to maintain the air temperature at 60-85 °C.
High temperature drying aims at shorter drying cycles and lower energy consumption. This method, also called HT-drying, uses superheated steam instead of air and operates at temperatures till 125 °C. Due to this critical drying process an advanced drying control system is required.
In co-operation with contract-research organisation TNO TPD, HB Koeltechniek Engineering bv, located in Almelo, the Netherlands, developed a computer control system for lumber drying. This control system determines the desired drying condition based on the drying behaviour of the product. This enables individual settings of every drying duty cycle.
The standard drying schedule comprises of seven phases. During the drying phase at 110-120 °C the wood temperature is monitored. The derived wood moisture content enables the system to calculate the optimum drying time using a wood-drying model.
HT-drying may reduce the drying time to 50% of the conventional process when uniform drying conditions are maintained. Based on experience at a Dutch wood processing company, HT-drying lead to total energy savings of 9,6 m³ of natural gas per m³ lumber.
Compared with conventional drying the drying results are better reducing deformations, 'twist' and 'spring'.
The control system is mainly applicable in wood processing companies.