Wood Business

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New SPF Drying Manual

At FPInnovations, one of our main business functions is the presentation and dissemination of technical information for the forest products industry. It is our goal to help strengthen the Canadian forest sector’s global competitiveness through research, knowledge transfer and implementation. It is with regard to the knowledge transfer portion of this goal that this drying article is directed.


Project Background


Lumber drying manuals have been a staple reference in the forest products industry for many years. Internationally, all major lumber producing regions have drying manuals that have been developed to address the species processed and technologies being employed within those regions. Within North America, FPInnovations (and formerly Forintek and the Eastern and Western Forest Products lab) along with the US Forest Products lab have taken the lead role in developing kiln drying manuals. Developing a new or revised kiln manual is a major undertaking and looking back over the history of these publications it is only every 25 to 30 years that significant changes or additions become available.


FPInnovations is pleased to introduce an all new kiln operators manual developed specifically to address the drying of Spruce-Pine-Fir (SFP) lumber, entitled: Drying Spruce-Pine-Fir Lumber (Le séchage de sciages du groupe Épinette-Pin-Sapin). The particular species from within the SPF grouping and processes used to dry this material are unique to Canada (as well as parts of the U.S.) and warrant the development of a specialized manual.


Manual Development


FPInnovations (and its forerunners) have had a very active drying group for many years. Over the past 30 years we have been engaged heavily in conducting research and developing technical services specifically for the SPF lumber drying sector of the industry. As a result we have accumulated a considerable amount of information that, until now, has only been available through our research reports. To a certain extent, our drying articles in Canadian Wood Products have been used to highlight specific research results or technologies that we have developed. As much as possible, our objective in preparing the new manual was to incorporate the knowledge developed through the research we have conducted, and combine that with our interaction with industry.


The new manual is both an educational resource and a working tool for people involved in the drying of SPF lumber. Its contents range from basic drying principles through to the application of techniques specific to the drying of SPF lumber, from commodity construction grades up to higher grades used in high-value engineered wood products (truss flanges, glued-laminated timber, etc.). The authors, from the eastern and western laboratories of FPInnovations’ Forintek Division, have strived to present solutions to cover a wide range of resource and operating conditions.


The manual has 21 chapters covering most of the aspects of drying from the basic principles through to the hands-on procedures for operating a kiln. The following provides some examples of the type of information presented in the manual:


Basic Principles


The first few chapters provide background information important for anyone drying lumber and as well as those who need a better understanding of wood properties and wood moisture relations. Les Jozsa, a world-renowned researcher and educator on wood properties and wood anatomy (retired from our Vancouver lab), has provided much of this material along with some of his famous artistic depictions of wood structure.
The concepts of wood moisture content (MC) and the measurement of moisture in wood are presented in detail. The procedures for conducting an oven-dry determination of MC are described. The principles and practices associated with the use of handheld and in-line moisture meters are covered. FPInnovations has conducted numerous studies on the factors that affect the performance of such meters and much of that information has been summarized here. Both dielectric and DC-resistance meters are covered. As an example, the effect of temperature on meter readings for both meters over a wide range of ambient temperature is presented. Although dielectric meters are affected less, there is still a need to compensate for temperature under some conditions.


Drying Technologies


Brief descriptions are provided of the various technologies used to dry wood. The main focus, however, is on the operation of heat-and-vent kilns, which represents the main class of kilns used to dry SPF lumber in Canada. Several sections deal with the topics of kiln maintenance and trouble-shooting equipment problems. For example, the top figure on page 21 shows how temperature data gathered from around the kiln can be used to evaluate kiln performance. Our recommendation to operators of SPF kilns is to target an entering dry-bulb temperature variation of no more than +/- 5 F (+/-2.8 C) between different regions of the kiln.


SPF Variability


Considerable attention is given to the topic of material variability and its impact on drying productivity and final product quality. Techniques to operate the kiln are presented that help to minimize the impact of the inherent differences of material within this species grouping. The subjects of pre-sorting, to produce more uniform drying groups, or re-drying, to handle slow drying material, are discussed in detail.


Operating the Dry Kiln


Two main tasks associated with operating a dry kiln are monitoring the drying process and selecting and implementing the most appropriate drying schedule. There are chapters dedicated to each of these subjects. Everyone wants the “magic” schedule that will deliver the optimum from their kilns in terms of quality and productivity. The message we try to give in this manual is that there is no “magic bullet” approach to drying lumber. Instead, it is a matter of understanding the principles of drying, the particular material that you are drying, and the capabilities of the equipment you are working with. For that reason we advise the readers to review those sections first and then consult Chapter 15 (Drying Schedules) to select the schedule(s) that are most appropriate to their operation.


Post-Drying Operations


Post-drying operations include topics such as quality assessment of the dry product and quality control practices. Descriptions of drying defects and their causes are presented and ways and means of reducing drying defects are discussed. For example, the proper application of top restraint can reduce degrade losses in the upper 10 to 15 rows of the kiln load by up to 50%. Quality control procedures presented in the manual deal with how to make better use of information that is often routinely collected on final MC. Final MC distribution patterns can be used to diagnose kiln problems or the impact of mixing different material. The graph below shows how the combination of a skewed final MC pattern and large standard deviation can result in large amounts of over-dried material in order to reduce the percentage of “wets” to an acceptable level.

Finally, the manual includes information on heat treatment for phytosanitary purposes, energy requirements (thermal and electrical), and some discussion on economic considerations.


To Order


The manual is available in English and in French under the title Le séchage de sciages du groupe Épinette-Pin-Sapin. For information on how to order copies, please contact the publications groups at either laboratory: Western Region Library (604-222-2743) or Eastern Region Library (418- 659-2647). The e-mail address for both libraries is publications.forintek@fpinnovations.ca. Orders can also be placed through the FPInnovations website (http://www.fpinnovations.ca/index.htm). Log onto the Forintek website, then click on “Publications” and “Special Publications” to access and download an order form for this publication.


December 1, 2011
By Peter Garrahan

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