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Promising research results using IR technology
Work to improve the drying process is constantly ongoing and is conducted not only out at sawmills, but also within academia. Last year Luleå University of Technology (LTU) launched a project using non-contact IR thermometers to measure the surface temperature of drying timber during simultaneous tomography and thus obtain direct indications of changes.
“This means that we do not need to interrupt the process and cut up a piece of the wood to measure the moisture content. There is potential in this. We have performed five runs to date with constant and variable climate and it is really encouraging that when we make changes in the kiln the measuring method seems to pick it up accurately. That’s where we’re at right now,” says associate professor Margot Sehlstedt-Persson at LTU in Skellefteå, Sweden. The three-year project “Utveckling av industriell virkestorkning” (Development of industrial wood drying), which began in 2016, is a continuation of the pilot study “Robust processtyrning av vandringstorkar för jämnare torkningskvalitet” (Robust process control of kilns for more even drying quality) that was conducted in collaboration with SP, which is now part of RISE, an acronym for Research Institutes Of Sweden which brings together SP, Swedish ICT and Innventia.
The pilot study brought together a number of kiln managers with sound experience of working with continuous kilns.
“We wanted to know what sort of different problems those working with the kilns encounter. The objective is a highly-automated system controlled by an adaptive self-tuning process,” says Margot Sehlstedt-Persson.
The development work has been carried out with support from Valutec, often in the form of tests in the special kiln combined with x-ray tomography in LTU’s lab environment.
“This enables us to conduct very controlled studies and the results look promising.”
Different types of timber, varying moisture ratios and also scarcity of raw materials affect both the drying process and the decisions a drying technician responsible for continuous kilns has to make. One challenge has been that the technician needed to wait a long time before the effect of decisions made earlier in the drying process became visible – sometimes resulting in damage and downgrading of the timber.
Thanks to IR technology, Margot Sehlstedt-Persson has been able to measure the environmental temperature of the timber and thus get a good indication of when the transition from capillary to diffusion-controlled drying takes place.
“With the non-contact sensors we can see how the surface temperature of the timber immediately changes when we have a variable climate; for example, with operations in a continuous kiln when fans stop and doors open. With the aid of the temperature signals and the differences that arise this gives early indications of the moisture status of the timber.”
When the free capillary water in sapwood is almost gone, the surface temperature of the timber is approaching the dry temperature in the kiln.
“The greater our understanding of the process, the better we are able to spot usable adaptive signals and methods for interpreting the drying process.”