For kiln operators in northern regions of Canada, the impact of winter conditions on operations is always a critical issue. If drying times tend to be longer during the cold season, this is frequently attributed to the need to thaw out the lumber. But is this really so? What is the effect of the preheating phase on final results? Very little research has been done on these issues because they are specific to cold climate, and this was the reason for the present study.
As part of this research, Forintek developed a monitoring and analysis system to provide operators of industrial kilns with a better understanding of the preheating phase. This portable and autonomous data acquisition system will also help members study their own operations. The tests conducted so far have allowed us to observe and analyze the processes involved in the drying cycle.
Our observations led us to understand the major role played by condensation during the thawing-out phase, and the need to consider the thermodynamics of air during the process. Frozen wood was found to thaw out rapidly (in one to four hours in our tests) with no temperature gradient between the core and the shell of the pieces at the end of the preheating phase. By the end of the preheating phase, all boards in a kiln load reached the same temperature. In fact the main limitation on preheating speed does not appear related to the lumber, but rather to the kiln’s heating capacity (i.e.,basic thermal capacity, air leakage and inadequate insulation).
Forintek members are welcome to request the data acquisition system to investigate their kilns with our scientists’ assistance. With the data they will collect, and the information provided by this report, they will be in a position to determine what adjustments may be required to optimize their drying operations.