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.