Continuous drying is still in its relatively early stages and mills are currently dealing with process adjustments to obtain desired throughput and quality of the final product. Field measurement carried out in 2015-16 illustrated a number of opportunities for process optimization involving each of the three main stages of current continuous kilns. Simulations of industrial continuous drying at laboratory level performed in 2016-17 were successful and allowed the evaluation of each of the drying stages to be fully characterized (lumber temperatures, drying schedule conditions of dry and wet bulb temperatures). Thus, different drying schedules provided an excellent opportunity to examine the impact of schedule conditions on drying defects, drying rates and kiln residence times.
The main objectives of the project for 2017-18 were to simulate continuous drying in laboratory conditions for different products, products mix, species and green sort groups. In addition, a detailed evaluation of potential technologies was carried out to explore the concept of dynamically adjusting speed (push rates), based on drying rates and moisture content.
Piecewise regression was used to identify the optimum push rate and suggest design modifications of continuous kilns. This method proved to be efficient in identifying potential reductions in drying time for different sorts of sprue/pine (SP) lumber without compromising the quality of the final product. Simulations also allowed identifying the push rate of 2 feet/h to satisfactorily dry green hem-fir 2-inch lumber.
Initial tests showed that mid-sort sub-alpine (moisture content below approximately 70%) could not be mixed with wet sort SP in a continuous kiln operating at push rate of 4.2 feet/hr because only 73% of the sub-alpine sort dried below 21%. Decreases in push rate will reduce the percentage of sub-alpine fir wets but will also increase the amount of over-dried lumber. Changes in kiln configuration may reduce the drying time but increase the percentage of over-dried lumber.
The results indicated that additional laboratory tests are required to develop drying schedules and temperature profiles in the main drying zone of continuous kilns, drying times and final moisture content distribution.
This project evaluated a number of opportunities to coastal producers related to kiln drying issues such as drying practices related to high-value products, drying with superheated steam vacuum and internal core temperature monitoring for large timbers during the heat-up phase. In summary, this project included several laboratory studies to evaluate the using superheated steam/vacuum (SS/V) for drying 7/8”x 6, green western red cedar lumber, and 8x8 and 5x(5,6,7,8,9,10,12) Douglas-fir timbers. SS/V drying yielded faster drying schedules when compared to the results obtained in industrial conventional kilns. The results obtained from the SS/V drying of WRC indicated the potential benefits of technology for drying specialty products especially when compared to drying times obtained with conventional drying (longer than 7 days). However, the results obtained also emphasize the importance of green sorting that is, sorting prior to drying to optimize drying times and reduce the variation of final moisture content.
For large cross section Douglas-firs the drying times were between 3 and 14 days depending on the severity of the drying schedule and initial moisture content distribution. The influence of moisture content and cross section during the early and late stages of the heating process were evaluated on 5x5, 6x6 and 8x8 Douglas fir timbers. Thermodynamic equilibrium was reached after 20 hours regardless of moisture content or cross section size. The knowledge is intended to be used to design conventional drying schedules for large cross section timbers.
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
In British Columbia, due to the decline of lodgepole pine, mills should expect higher volumes of sub-alpine fir in their species mix. The impact on drying is significant. For example, drying times for green SPF (spruce, pine, sub-alpine fir) vary from 24 to 36 hours whereas drying times for sub-alpine fir can easily exceed 70 hours. In addition to longer drying times, the drying of species such as sub-alpine fir using current procedures often results in wet lumber and value loss can be higher than $100 per Mfbm. The potential annual impact for a typical BC mill is estimated to be in the range $1,000,000 to $1,500,000.
Along the years, sawmills have invested millions of dollars in drying technology (conventional drying and green sorting systems) which, for the most part are efficient and relatively low cost. Thus, under the circumstances outlined above, sawmills urgently need to find ways to minimize the problems associated with the drying of sub-alpine fir that is, new procedures or combination of methods, to ensure maximum grade recovery at the end of drying and reduce drying times (increase productivity and lower processing costs). In addition, the pressure exerted by typical longer drying times for sub-alpine fir will impact the drying of spruce and pine. Thus, strategies to speed the drying for those two species are needed to maintain annual production targets.
The main objective of this project is to evaluate several strategies using existing technology so that sawmills can readily implement them throughout their drying operations dealing with larger volumes of sub-alpine fir and for mills with kiln capacity constraints which could compromise their production targets.
BC Coastal mills will need to diversify the drying technologies currently used and consider new approaches in order to improve productivity, reduce drying costs, regain competiveness and continue to play a significant role in the increasingly stringent quality market for forest products. New demands for drying are related to energy efficiency, low environmental impact and of course, quality of the final product.
The specific objectives of the project were: (1) to improve the conventional drying of 4.5” x 4.5” Douglas-fir lumber, (2) to evaluate the superheated steam/vacuum (SS/V) drying of 4.5” x 4.5” Douglas-fir lumber, (3) to develop a green sorting strategy for hem-fir lumber and (4) to determine the time required to reach 56°C in the core of 5¼ x 5¼ lumber using the requirements of CFIA PI-07 Heat Treatment schedule Option D - Generic Phytosanitary Heat Treatment Schedule, Heat Treatment with Moisture Reduction.
The results showed that the drying time in conventional drying of 4.5” x 4.5” Douglas-fir lumber can be reduced by up to 25% without compromising the quality of the lumber. This can be achieved by increasing the temperature in the final drying stages and using lower relative humidity at the beginning of the drying process. In addition, final moisture content (MC) variation was reduced from 6.2% to 3.9%.
Reductions of drying times from 26% to 41% were observed when drying 4.5” x 4.5” Douglas-fir under SS/V drying. Quality of the lumber at the end of drying was better when compared to the quality of the lumber at the end of conventional drying. In addition, specimens exhibited less final MC variation.
Based on drying rate measurements of green hem-fir lumber dried to 9.0% MC, a new database was developed which in turn was incorporated into OASiS 2.0 software to evaluate different pre-sorting scenarios. Pre-sorting simulations allow end users to estimate the impact of kiln productivity, final MC distribution and drying degrade. The results showed that different correlations between the time to reach 19.0% MC and initial weight or initial MC could be established. The best correlation with an R square of 0.77 was made between initial weight and MC. After performing several simulations with the new database an optimum cut-off point of 65% yielded the best results in terms of potential increase of productivity and quality of the final product.
Wood heating rate test results showed that CFIA Option D may be extended for 5¼ x 5¼ lumber as long as the dry-bulb = 71°C (= 160°F) for 36 hours at the end of the heat treatment. Total heat treatment time required, including the time required to reach 71°C (160°F), is 72 hours.
English version available: https://library.fpinnovations.ca/en/permalink/fpipub7315
Le présent manuel se veut une ressource didactique et un instrument de travail pour ceux et celles qui participent activement au séchage de sciages du groupe Épinette-Pin-Sapin (EPS). Il couvre un vaste éventail de sujets, depuis les principes de base du séchage jusqu'à l'application de techniques de séchage propres à ce groupe d'essences. La portée du contenu et la profondeur de son traitement ont été déterminées à la lumière de ces objectifs.
La diversité de la matière première au sein du groue EPS et ses incidences sur les décisions concernant le séchage ont fait l'objet d'une attention particulière. Les auteurs ont tenté de formuler des solutions englobant un vaste éventail de conditions touchant la ressource et les opérations de séchage. Les premiers chapitres du manuel traitent des connaissances préalables pour l'analyse des solutions éventuelles que le lecteur pourra retenir à la lumière de sa situation particulière.
Des études menées dans nos laboratoires constituent une importante toile de fond pour l'information présentée dans ce manuel. Les rapports techniques énumérés à la section Suggestions de lecture contiennent les résultats détaillés d'essais effectués chez FPInnovations - Division Forintek sur le séchage des sciages d'EPS.
Le séchage du bois d'oeuvre ne se limite pas aux seules actions qui se déroulent dans les séchoirs ou dans le parc de séchage à l'air. Un des objectifs du présent manuel est de démontrer que l'opérateur de séchoir doit tenir compte d'un grand nombre de facteurs lors du processus de séchage. Selon Joseph M. Juran, pionner dans le domaine de la gestion de la qualité, un processus est "... une série systématique d'action axées sur l'atteinte d'un but. La performance du processus et des variations excessives auront des effets directs sur les résultats financiers d'une entreprise". [traduction] Il ne s'agit pas d'éliminer toute variation, mais d'en comprendre la nature et l'origine afin de minimiser ses incidences sur les résultats de l'opération.
Softwood lumber producers have been using conventional drying systems (batch drying) for many years. The original premise of those systems was to design kilns that could dry large quantities of lumber at relatively low costs. Based on the evidence throughout the industry across Canada, those
requirements have been historically met and the industry has greatly benefitted from the existing system for decades. Currently, however, due to environmental pressures, increased processing costs, more stringent quality and moisture content requirements and the need to improve productivity, softwood lumber producers must consider alternative ways to dry lumber to ensure their competiveness in traditional markets and to explore opportunities for entering new markets. Drying faster without compromising the quality of the final product will position Canadian lumber producers to achieve the
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According to the last forecasts released by BC Hydro, in 20 years the demand for electricity in B.C. will increase about 40%. A typical sawmill in Canada has between 4 and 8 kilns which operate on a constant basis throughout the year. Each kiln dries on average about 16 to 20 kiln charges per month and every kiln charge is on average 250 Mfbm of lumber (based on 2-inch thickness). A typical crossshaft kiln is equipped with fifteen 25 hp motors (approximately 18 kW) so the total installed power per
kiln is about 270 kW. Kilns operate an average of 660 hours per month. Thus, mills with drying operations such as in the example above will consume a significant amount of electricity to dry their
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