Six trials were conducted at the Department of Wood Science, UBC on the drying of "redry" and green SPF veneer using a 0.3 m3 laboratory Radio Frequency/Vacuum (RF/V) apparatus. A larger scale trial was conducted at Canfor's Eburne mill, on the drying of "redry" SPF veneer using a 60 m3 pilot plant apparatus. Results from all these trials showed there was a wide variation in the final moisture content (m.c.) of the veneers with many veneers showing low m.c. regions. This m.c. variation could pose problems for bonding with moisture tolerant phenolic adhesives. Results from the Eburne RF/V trial showed there was a species effect in drying rates and subalpine fir veneer dried at a faster rate than spruce veneer using radio frequency heating.
Steam press drying of green SPF veneers was evaluated on a 4 x 8 foot prototype steam press set up at a local plywood mill. A used industrial press has been modified into a single opening oil heated hot press with grooves machined into the upper platen. Standard green nominal 4 x 8 foot 3.2 mm (1/8 inch) thick mill veneer was dried with super-heated steam injected through 6 mm (1/4 inch) holes in the bottom platen. All veneers were incised with Forintek's patented veneer incisor. The project demonstrated that steam injection can dry green veneers three to four times faster than conventional veneer driers. From the results of this project, a study on the press drying of redry veneer was initiated and showed good potential for industrial implementation.
A literature review was made of publications on veneer drying. A discussion on the three types of air-circulation dryers, longitudinal, cross-flow and jet dryers is presented as well as other drying methods including platen, steam-press, radiofrequency (RF) and RF/vacuum drying. Good temperature control in veneer dryers such as in jet dryers using high temperatures in the green end and lower temperatures in the dry end will result in higher quality veneer with less surface inactivation. By maintaining a high humidity in veneer dryers, the following benefits result: faster drying rates, lower energy costs, less chance of surface inactivation and less chance of dryer fires. The following veneer drying benefits result by incising veneer on the lathe: faster drying rates and flatter veneer for easier handling on automated lay-up lines. Other important benefits include: fewer spin-outs at the lathe, less curl-up for the veneer near the core, especially spruce, higher veneer yields, reduced "blows" in plywood during pressing and improved preservative treatability of plywood. A drying strategy involving drying incised veneer to a uniform high moisture content and pressing this face incised veneer (15% m.c.) with dry core incised veneer (3% m.c.) using a moisture tolerant phenolic adhesive, could allow up to 30% reduction in pressing time.
This project was placed top priority because veneer drying is the bottleneck and the biggest item for energy consumption in plywood mills. During the past three years, the Forintek veneer drying manual was first upgraded. Our research capabilities in drying were significantly improved by the establishment of a mini-dryer and the installation of pilot scale veneer dryer. While the former allows for in-situ monitoring of moisture content change in veneer during drying, the latter can be used to simulate industrial dryers such as jet dryers or longitudinal dryers. Both are capable of testing the effects of such parameters as drying temperature, humidity and air flow. Based on the improved understanding from the experimental tests and theoretical analyses, initial computer models (VDRY-L and VDRY-J) have been developed to simulate the drying processes for longitudinal dryers and jet dryers, respectively.
The combination of laboratory studies and computer simulation led to effective mill studies to evaluate the existing drying technologies, to optimize the existing dryers without capital cost and develop future opportunities.
The key findings from these studies were:
Lab tests showed that temperature and airflow rates are dominant factors affecting drying rate for the whole drying process. While veneer drying increases as drying temperature and airflow rates are increased, higher temperatures and airflow rates both have a greater influence on drying rate at the early drying stages compared to the final stages.
Lab tests also showed that veneer can be dried at high rates under high humidity and temperature conditions. A combination of high temperature and humidity is a good drying mode to save energy and increase output.
Mill case studies showed that the sorting of green veneer prior to drying was poorly done. Lab tests showed that a probable reason for this was due to the inaccuracy of RF sensors used for measuring moisture content of green veneers. The RF accuracy significantly dropped when veneer moisture content exceeded 30%.
Mill case studies showed that a sensitivity analysis of drying parameters is a very useful method for determining effective measures for optimizing dryer performance.
Mill case studies showed that by using higher temperatures and humidity levels in the early drying stages and lower temperatures in the final stages, veneer production can be increased and drying energy can be reduced.
An impact analysis showed that mills can potentially save up to 10% in energy costs and increase production by 10% by optimizing dryer performance.
It is recommended that mills use the results and methodologies from this project as guidelines for optimizing dryer performance. Further research should be undertaken to improve green veneer moisture sorting. The current computer simulation models of drying should be calibrated and used as a quality control tool for determining optimum dryer parameter settings.
On March 31, 2003, Forintek Canada Corp completed the project “Optimization of Veneer Drying Processes”. Forest Industry Investment and Forintek members funded this one-year project. The purpose was to develop practical methodology to transfer new Forintek knowledge about veneer drying and quantify the benefits of optimizing existing dryers. Currently in BC there are 15 softwood plywood mills producing approximately 1.7 billion square feet of plywood (3/8” basis) per year and employ over 2800 people.
In plywood production, veneer drying is one of the key manufacturing processes. Currently, dryers create a bottleneck in the mill, restricting plywood production. Dryers are expensive to purchase and operate. The results show that by optimizing existing dryer operations the potential for increased production and efficiency savings is significant. This provides BC manufacturers with the opportunity to reduce costs without investing capital for costly new equipment or modifications.
At the beginning of the project, five BC mills were visited to identify and quantify key variables affecting dryer operation. Discussions with mill personnel were important for determining specific limitations that could be made to the drying process for optimization and to understand the main problems and potential for improvement from a mill perspective.
At Forintek’s laboratory in Vancouver, small-scale drying tests were conducted to further develop the fundamental knowledge of commercial drying. For this purpose, a laboratory dryer was specially designed to simulate industrial drying processes. Testing focused on determining the drying rate relationships involving drying air temperature, airflow speed and humidity. Results showed consistently that the best set-up for maximizing drying rate was by operating the dryer at high temperature with high humidity and airflow speed. The lab study also showed that these conditions are most importantly applied at the early stage of drying. Near the end of the drying process, the effects of high temperature were diminished, suggesting that temperature can be reduced at this stage to economize on energy consumption without significantly affecting drying rate. As well, the lower drying temperature near the end of the drying process reduces the risk of surface inactivation (loss of bonding sites) in the veneer. Additional testing at Forintek was conducted to determine the accuracy of radio frequency moisture sensors currently used for sorting green veneer prior to drying. Results showed that moisture content measurement error increased significantly with moisture content above 30%.
Case studies were conducted at two BC mills to quantify the relationships of key drying parameters and to test methods for optimization. In both mills, prior to drying, measurements of green veneer showed that sorting was very inefficient with a large proportion of veneers incorrectly directed into the heart, light-sap and sap bins. Dryer control modifications demonstrated that dryer temperatures could be increased by the same order of magnitude as in the laboratory tests by restricting damper openings to raise humidity. This not only increased veneer feed speed, but also reduced steam consumption used to heat the dryer. At one mill, changes to temperature and humidity conditions resulted in an increase of the veneer feed speed from 9.3 to 10.8 ft/min. for light sap veneer, representing an increase of 16% in productivity through the dryer. At the same time, it was estimated that 10% of the annual energy use for the dryer could be saved based on reduced steam consumption.
Based on the case study results, it was conservatively estimated that the 15 plywood mills currently operating in BC could potentially increase plywood production by 5% or 85 million square feet per year. At today’s prices, this translates into approximately $31 million per year by implementing, at no cost, the methodology presented here to improve dryer performance. In addition, results also showed that dryer energy consumption could be potentially reduced by 10% A practical 5% reduction would amount to a savings of $3.0 million for the BC plywood industry. No additional implementation cost was required to achieve this energy savings. In addition, Forintek estimated that better green veneer sorting by mills could significantly reduce the moisture content variability of dry veneer, further improving dryer productivity. However, this would require more accurate moisture sensing technology than is now commercially implemented. It is recommended that new technology be developed.
The work presented in this report addresses the FRBC value-added research priority area and shows good potential for creating jobs in the forest products sector and leading to new technology for manufacturing a value-added product. Experiments were conducted to evaluate steam-injection pressing of 7-ply SPF plywood and 13-ply SPF laminated veneer lumber (LVL) on a laboratory scale at Forintek and a pilot plant scale at Alberta Research Council (ARC). The laboratory experiments provided a basis for the pressing schedules used in the pilot plant trials at ARC. The following parameters were necessary for the steam-injection pilot plant trials: Top and bottom screens (0.6 mm stainless steel 30-mesh screen), incised veneer and a phenolic glue (Borden 2020) resistant to washout. Using steam-injection technology in pilot plant trials, substantial reductions in pressing time were achieved both for 7-ply SPF plywood and 13-ply SPF LVL. Specifically, a steam-injection time of one minute using saturated steam at 80 psi reduced the pressing time of 7-ply SPF plywood by 27% compared to conventional platen pressing. A steam-injection time of eight minutes using saturated steam at 80 psi reduced the pressing time of 13-ply SPF LVL by 32% compared to conventional platen pressing. Both the control and steam-injected 7-ply SPF plywood and 13-ply SPF LVL panels exhibited excellent bond quality and the average percentage wood failure was much greater than 80% in all cases thereby meeting the average percentage wood failure requirement in the CSA 0151 plywood standard. There was no significant difference in shear strength between control and steam-injected shear specimens. There was no statistical difference in average modulus of elasticity or modulus of rupture under flatwise and edgewise bending for the steam-injected LVL compared to the control LVL made using conventional platen pressing. A previous economic analysis of return on investment for thick plywood products and LVL showed that a 30% reduction in pressing time for a medium-size manufacturing plant would generate an additional profit of $3,000,000 per year.
The purpose of this study was to determine if press-drying of redry veneer is a practical option for the plywood industry. Two press-drying schedules were tested - one minute and two minutes at 340 degrees F with 10 psi press pressure. Drying two redry sheets per opening for two minutes gave the best results. Moisture pockets in the veneer were reduced to an acceptable level and moisture equalization occurred between the two sheets in the press and during the veneer load cool-down time. Press drying redry veneer may provide a relative low cost processing option for mills to complement more capital-intensive alternatives such as radio-frequency (RF) drying.
The existing softwood kilns are vastly oversized and ill equipped to dry the volume of hardwood lumber identified for the study. A dehumidification kiln with a twenty thousand board foot (20 Mbf) capacity would be the most cost effective kiln drying technology. It would be well matched to the annual volume to be dried and does not require specialized skills to operate. Such an operation would require a price differential between green and dry lumber of approximately $120 per Mbf to be viable. Within the study area, there exists a small volume of veneer quality hardwood resource. A longitudinal veneer slicing system would be well suited to process the volume available. Such a system would process green lumber to produce high quality, sliced face veneer. White birch, however, has little figure or grain when sliced and hence, little advantage over rotary cut birch veneer. To be viable, markets for the sliced veneer would have to be developed at the current market price for rotary cut face veneer. This should not be too difficult as the volume is relatively small. The fact that suitable technology exists for the kiln drying and veneering operations is encouraging. If markets in these two areas can be developed, it is recommended that consideration be given to the concept of a wood products industrial park. Initially, the core enterprise of the park would be a lumber concentration yard with dry kilns and possibly dressing facilities. It could be expanded at a later date to include a small sawmill and the veneer operation. As markets develop, consideration could be given to other complementary industries which build on the expertise and resources of the industrial park. This concept is further detailed in the report.
Experiments were conducted to evaluate steam-injection pressing of plywood using saturated and superheated steam conditions and three steam-injection times of 1, 2 and 3 minutes. The results showed faster pressing times were achieved with the lower steam-injection times of 1 and 2 minutes. All the panels prepared under a variety of steam conditions exhibited excellent bond quality and the average % wood failure was greater than 80% in all cases. Using steam-injection times of 1 and 2 minutes for both superheated and saturated steam conditions, the pressing time was reduced by over 30% compared to conventional platen heating. An economic analysis of return on investment for thick plywood products shows the pay-back period for retrofitting an existing plywood press for steam injection is less than 2 months.