This project was initiated to provide technical assistance to the Alberta wood drying industry. The specific objective was to identify opportunities to improve product quality through modification of the drying schedules. Seven mills representing almost 50% of the solid, softwood lumber production in the province were selected for the project. All mills provided a great deal of cooperation and commitment to the project was excellent. In general, lumber drying operations in Alberta are in good physical condition and operating personnel have a sound knowledge of basic drying concepts.
In general, drying schedules were found to be quite harsh. the specific concerns at most mills related to too rapid a heat-up rate and extremely low relative humidity at the end of the drying cycle. Most of the schedule modification called for more gradual and controlled heat-up rates with higher wet-bulb temperatures. The objective of this modification is to avoid setting up conditions othat promote variability in moisture content from board to board. Higher relative humidity is required at the end of the drying cycle to avoid over-drying faster drying boards. Achieving a reduction in final moisture content variability and a higher overall average moisture content should be the objective of drying schedule modifications.
Mill visits were used not only to review drying schedules but also to conduct a brief inspection of drying practices and equipment. It would be unproductive to identify schedule modifications if there were obvious shortcomings in other areas of the operation that would make it difficult to implement or over-shadow the effect. The primary concern with drying equipment is the leakiness of the structures. A common recommendation to mills was to tighten up kiln doors and walls in order to retain more moisture in the kiln environment. Another area of concern was related to lumber handling operations. Most problems in this area could be addressed through educating and training staff working at stackers, handling material in the yard, or preparing loads for the kiln.
Logging and log storage practices at all of the mills visited has a serious and detrimental impact on the drying operations. At most times of the year, operators are having to deal with a wood supply that has a mix of initial moisture content conditions. Most mills seem to manage the small percentage of balsam fir in their mix effectively. Some future gains may be achieved through refined presorting techniques that take into account initial MC variability as well as differing drying characteristics between species.
This manual is intended to serve as an educational resource and working tool for people actively involved in the drying of Spruce-Pine-Fir (SPF) lumber. The manual covers subject matter from the basic principles of drying through to the application of techniques specific to the drying of this species group. The range and depth of information presented has been selected to meet these objectives.
Disponible en français: https://library.fpinnovations.ca/en/permalink/fpipub7507
Laboratory and industrial testing was conducted on a commercial, dielectric moisture meter (Wagner model L612) and an accompanying stack sampling probe (Wagner model L712). The purpose of the testing was to identify and quantify the various factors affecting the accuracy of moisture content (MC) estimates obtained when using this equipment. Laboratory tests were conducted to determine which wood-related properties and environmental factors had an influence on meter readings. Testing of black spruce over a range of MC’s from 9 to 18% identified the need for a new correction factor for this species. Moisture gradient, wood temperature, surface roughness, and board width were found to have an effect on meter reading. Effect of wood temperature is significant but is roughly half the effect previously documented on DC-resistance. The presence of knots or compression wood in the sampling area also influenced meter reading. Temperature that the wood was dried at was not found to have an effect.
Simulated and actual industrial tests were conducted to test the applicability of various corrections identified in the laboratory testing phase. For black spruce, improved estimates of MC were obtainable by applying the newly developed species correction and, depending on the specific tests, other corrections for wood temperature, surface quality, etc. Tests were also conducted using the stack sampling probe. Even under well controlled laboratory conditions, there was not a good correlation between readings taken with the stack sampling probe and either readings with the handheld meter or oven-dry moisture content. Readings taken with the stack sampling probe typically underestimate the actual MC. More accurate and consistent readings were obtained when sampling wood stacks at a uniform wood temperature.
Readings from both dielectric and DC-resistance meters were compared against oven-dry MC for laboratory and industrially dried lumber. The DC-resistance meter performed marginally better when evaluated on the basis of proportion of readings within given error limits. MC estimates from the dielectric meter can be correlated with DC-resistance MC estimates regardless of which correction factors are applied. Dielectric and DC-resistance meters are not used totally interchangeably in the wood industry as each has specific applications where they are advantageous to use. As a result, any small difference in accuracy is not always the determining factor.