Abstract
This paper presents an experimental evaluation of the first commercial scale dry-sort-redry (DSRD) strategy for drying of 2x4 Pacific coast hemlock (PCH) lumber. The DSRD strategy is a methodology designed to reduce final moisture content variability in kiln dried lumber by complementing conventional drying with radio frequency vacuum (RFV) drying technology. The strategy’s objective is to avoid producing over-dried lumber in conventional drying by setting the target moisture content to a value much higher than those usually used in industry. Then, RFV drying technology is implemented to quickly and efficiently re-dry the lumber that remains wet (under-dried) after the first conventional pass. Six experimental tests were performed in an industrial sawmill with the intention of studying the effect of target moisture content on the properties and quality of the dried lumber. In all cases, the first drying pass was performed in a 260m 3 industrial heat-and-vent conventional kiln, and the re-drying of wets was performed in a 75m 3 RFV kiln. Additionally, a mathematical model developed for prediction of data dispersion in lumber drying was calibrated with experimental data, and used to simulate the DSRD strategy under other hypothetical conditions. The results of the study demonstrate that the DSRD strategy reduces drying time, shrinkage and kiln drying degrade in comparison with a single conventional pass.
Highlights
In freshly cut softwood lumber there is always considerable variation in green moisture content (Mg) among pieces of lumber of the same charge, mainly due to the moisture differences between sapwood and heartwood
The DSRD strategy for drying Pacific coast hemlock (PCH) lumber was tested at an industrial level with a major US lumber producer that operates with eight 260m3 conventional kilns and a 75m3-300kW radio frequency vacuum (RFV) kiln (Figure 1)
The results of the industrial-scale evaluation showed that the SDRD strategy reduces drying time, increases lumber grade of quality, and reduces the planed lumber area
Summary
In freshly cut softwood lumber there is always considerable variation in green moisture content (Mg) among pieces of lumber of the same charge, mainly due to the moisture differences between sapwood and heartwood. Once the lumber is in the kiln, random differences in Mg and wood properties induce individual boards of the same charge to dry at a different rates. Lumber moisture content (M) after drying fluctuates within a wide range of values. In industrial drying, this problem is further aggravated by non-uniform kiln drying conditions due mainly to hardware calibration and malfunction issues. Typical sources of non-uniformity in an industrial kiln involve poor lumber package arrangement; unbalanced hot air distribution from the heaters; unbalanced fresh air distribution from the vents; incorrect air flow distribution from the kiln fans; excessive temperature drop across the load and excessive cold air leakage, among others (Esping 1982)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have