High-temperature kiln-drying of 4/4 lumber from 12 hardwood species

Transcription

1 High-temperature kiln-drying of 4/4 lumber from 12 hardwood species R. Sidney Boone Abstract One-inch lumber of 12 hardwood species was kilndried by two schedules: 1) 230 F on stock green from the saw to 6 to 8 percent moisture content (MC) in 50 to 55 hours and 2) combination of conventional temperature (~ 180 F) from the green condition to about 20 percent MC, then dried to 6 to 8 percent at 230 F requiring a total of 100 to 250 hours. Fifteen thousand board feet (BF) of lumber representing about 1,200 BF for each of the 12 species were dried in 15 kiln charges. The amount of drying degrade present in standardized blanks sawn from 8-foot lumber varied with species, When dried by the first schedule, basswood, yellow-poplar, white ash, sweetgum, and soft maple dried with only minor amounts of drying defects. Black cherry, hard maple, beech, and pecan-hickory had sufficient defects to be considered unsuitable for this treatment. American elm, cottonwood, and blackgum showed intermediate amounts of drying defects. The combination schedule decreased the drying degrade, but increased the kiln residence time over high temperature alone. However, the combination schedule reduced kiln residence for most species over completely drying at conventional temperatures. The most common drying defect was honeycomb or combinations of honeycomb, with other defects such as warp or collapse. Drying defects were commonly associated with knots, pith, or mineral streak. Interest in high-temperature (HT) kiln-drying for hardwood lumber has been encouraged by success with this process on southern yellow pine. High-temperature kiln-drying is accomplished at dry-bulb temperatures of 212 F or higher, usually from 230 to 250 F. Experience with southern pine has shown much shorter drying times with little decrease in quality compared to conventional kiln-drying. Hardwoods are generally more difficult to dry than softwoods at conventional (=s 180 F) kiln temperatures. Hardwoods are usually dried to lower moisture contents (MC) than softwoods (6% to 8% vs. 15% to 19%), and the uses of hardwoods require less residual drying stress and degrade in the lumber. Can we dry some or all of our hardwood species using HT kiln schedules and expect results similar to those of southern pine, thus offering savings in energy and lumber inventory? This study was conducted to determine the type and amount of drying defects in l-inch lumber of 12 hardwood species kiln-dried at high temperatures greenfrom-the-saw or by a schedule combining conventional temperatures and high temperatures. Literature reviews on HT drying of hardwoods have been published by Wengert (9) and Boone (l). Much of the research on HT drying of hardwoods reported in the literature was done in Europe and Canada. Results were species dependent and show that low- to mediumdensity species, such as yellow-poplar, mahogany, and birch, dry rather successfully starting with green material. Higher density species, e.g., beech and oak, show appreciable drying defect when dried green-from-thesaw at high temperature. Partial drying at temperatures below 212 F before subjecting the material to high temperatures decreased the drying degrade with beech, walnut, ramin, and mahogany (3). In U.S. research, Richards (7) dried 18-inch-long pieces of seven species in an oven at 230 F with no The author is a Research Forest Products Technologist, USDA Forest Serv., Forest Prod.,Lab., P.O. Box 5130, Madison, WI The following organizations assisted in obtaining logs for this study: Algoma Lumber Co., Algoma, Wis., Buchanan Lumber Co., Montgomery, Ala., Coastal Lumber Co., Dailey, W. Va., Hinchcliff Lumber Co., Parsons, W. Va., International Paper Co., Beirne, Ark., Kretz Lumber Co., Anti go, Wis., Monongahela National Forest, USDA Forest Service, Parsons, W. Va., Northeastern Forest Experiment Station, USDA Forest Service, Parsons, W. Va., Owen Smith, Inc., Richland Center, Wis., Pine River Lumber Co., Long Lake, Wis., Southeastern Forest Experiment Station, USDA Forest Service, Asheville, N. C., Taylor-Ramsey Corp., Lynchburg, Va., and the Tennessee Valley Authority, Norris, Tenn. This paper was received for publication in April Forest Products Research Society Forest Prod. J. 34(3): MARCH 1984

2 humidity control and reported that 4/4 yellow-poplar, beech, blackgum, hickory, and sweetgum sapwood could be dried without visible defect. Kozlik (4) found 4/4 and 5/4 red alder had more defects when dried at 220 F than at 210 F or 200 F. Espenas (2) observed that red alder dried at high temperatures showed higher shrinkage values. Results of this past research indicate that the four most commonly reported defects in HT drying are honeycomb, collapse, checking, and discoloration (darkening). Drying defects occurred more often in heartwood than in sapwood. Exploratory work at the Forest Products Laboratory (FPL) showed that green 8-foot-long 4/4 basswood, American elm, sweetgum, and soft maple could be HT dried with little or no degrade while red and white oak would not tolerate high temperatures when green. However, the oaks were successfully finished at temperatures of 230 F if first air-dried or kiln-dried at conventional temperature to an MC of 20 percent or less (10). Encouraging results in the literature and exploratory work done at FPL suggested the need for a larger scale study to determine the feasibility of using high temperature alone, or in combination with conventional kiln schedules to dry several of our important commercial hardwood species. This paper reports our findings. Materials and methods Species selection Species selected for this study are commercially important in manufacturing exposed and upholstered furniture parts. Both red and white oak were omitted because previous research established the need for oaks to be dried to 20 percent or lower MC before being subjected to high drying temperatures. The species selected were as follows: 1. Ash, white Fraxinus americana 2. Basswood Tilia americana 3. Beech Fagus grandifolia 4. Black cherry Prunus serotina 5. Blackgum Nyssa sylvatica 1 6. Cottonwood Populus deltoides 7. Elm, American Ulmus americana 8. Maple, hard Acer saccharum 9. Maple, soft Acer rubrum 10. Pecan-hickory Carya sp. l 11. Sweetgum Liquidambar styraciflua 12. Yellow-poplar Liriodendron tulipifera Log acquisition, storage, and processing All material was acquired in log form to assure control over initial lumber MC and to help insure species identity. These 12 species were obtained from loggers or sawmills in Wisconsin, West Virginia, Virginia, North Carolina, Tennessee, Alabama, and Arkansas. Logs of each species were obtained from three different logging sites and in most cases in at least two states. The logs (nominal 8 to 10 ft. long and minimum 10- to 12-in. scaling diameter) were brought to Madison and stored under water spray until processed. We processed the logs for HT drying in species groups of three. For each species in a group approximately 1,000 board feet (BF) log scale (International l/4-in. log rule) were sawn to l-l/8-inch green thickness. All boards were edged and coded to identify species, geographical location of source, and log. Boards were end trimmed as needed to produce lengths between 8-1/2 and 9 feet. Only lumber graded No. 2 Common and Better was used for the study. 2 Each kiln load contained about 1,000 BF, consisting of 300 to 350 BF of each of the three species. Three kiln charges of each species group of green lumber were dried by HT schedules. Sawn lumber awaiting kiln-drying was stored solid piled, and covered with a plastic wrap in a room at 36 F and 82 percent relative humidity (RH). Later, one kiln charge of each species group (also containing 1,000 BF) was sawn and dried on conventional schedules from green to about 20 percent MC, then finished at a high temperature to a final MC of 6 to 8 percent. Basswood was not included in this combination schedule as a sufficient number of logs were not available. Drying techniques Kiln charges pile configuration. The random width lumber was stacked on 3/4-inch-thick stickers spaced 18 inches apart, forming a pile about 6 feet wide and 20 to 23 courses high. A top load of iron and/or concrete weights was used to minimize warp in the top courses. Ten to twenty pounds per square foot (psf) were used in the earlier runs, but this was increased to 40 to 50 psf in later runs to improve warp control. Air speed through the load averaged about 800 feet per minute (fpm) and fans were reversed every 6 hours. All loads were dried in an aluminum, steam-heated, track-loaded kiln of 1,500 BF capacity. Kiln charges monitoring moisture content. Three sample boards per species were used in each kiln charge to monitor MC. The kiln was cooled for about 5 minutes to safe temperatures before entry to remove sample boards. To obtain MC of sample boards without interrupting drying during the first 18 hours, additional matched sample boards were prepared and retrieved, with a wire, from the kiln load while the-kiln fans were shut down for 1 to 2 minutes. These boards were weighed and their MC computed, and were not returned to the kiln. High-temperature drying regime for green lumber. The drying regime consisted of five steps: 1) warmup, 2) HT period, 3) cooling, 4) equalizing, and 5) conditioning. For the first six kiln runs (Table 1), warmup was achieved with steam spray and sufficient heat radiation to maintain a small wet-bulb depression. Wet-bulb temperatures of 190 F and dry-bulb temperatures of 200 F were reached in 2 to 3 hours. At these temperatures, full radiation was turned on, steam spray turned 1 Varieties or species that grow in lowland or on flooded sites were excluded. 2 National Hardwood Lumber Association Grade Rules. FOREST PRODUCTS JOURNAL Vol. 34, No. 3 11

3 off, and vents kept closed. Steam spray use is an effective heat transfer mechanism during warmup, and also minimizes surface drying and therefore surface checking. The technique was reported by Ladell (5). Kiln runs 8 and 9 did not use this warmup technique. Instead dry-bulb and wet-bulb set points were set at the time of kiln startup and were achieved in 2 to 4 hours. No difference in the amount of surface checking could be detected between the two techniques, and subsequent kiln runs did not use the steam spray warmup (Table 1). An HT drying period at 230 F dry-bulb temperature (DBT) followed warmup and was held until the lumber reached an MC of 10 percent or less (20 to 26 hr.) and was ready for equalizing. During the first four kiln runs, the wet-bulb temperature (WBT) was allowed to fall, reaching 130 to 125 F by the end of the HT period. In the remaining runs, WBT was held to 180 F by adding steam spray. Exploratory work at FPL and by Rosen (8) suggests better quality of the dried lumber when WBT is kept under control. A lumber cooling period is necessary to obtain the proper conditions for equalizing and conditioning. The charge of lumber is cooled to a point where the WBT (about 180 F for our kiln) can be precisely controlled. We achieved this cooling in 1/4 to 1/2 hour with radiation and spray off, vents open, and fans running, Commercial kilns may require 2 to 3 hours or more for this cooling. An equalizing period is necessary since the very rapid drying can create rather large variations in MC between and within boards. We equalized at 4 percent equilibrium moisture content (EMC) with the wet bulb at the highest temperature we could control (6). This was usually 180 F (DBT 220 F), but some kiln runs were equalized at about 160 F WBT, and DBT of 203 F. Equalizing conditions were maintained for about 16 hours, varying somewhat with species. Since this lumber was to be used for furniture stock, it was necessary to relieve the drying stresses, i.e., condition the lumber. This conditioning treatment was done in the conventional manner using the highest temperature at which we could maintain WBT control. To agree with our final target MC of 6 percent, a conditioning EMC of 10 percent was set using a 180 F WBT (6). Early runs were only conditioned for 3 to 4 hours. Since stress tests showed this to be inadequate, later loads were conditioned 10 to 12 hours, which provided adequate stress relief. Drying regime for combination conventional temperature/high-temperature schedule. For kiln runs and 15 (Table 1). a combination of conventional and high temperatures were used. In the first combination schedule (kiln run 7), about 200 BF each of American elm, black cherry, hard maple, soft maple, and white ash was loaded in the same fashion as lumber HT dried from the green condition. The lumber was dried by a conventional temperature black cherry schedule (6) until the average MC of the sample boards was 15 to 17 percent (112 hr. drying time). We then raised the DBT to 230 F and the WBT to 180 F for 7-1/2 hours until the average MC was between 7 and 8 percent. The usual equalizing and conditioning treatments followed. Total kiln residence time was 148 hours. In the second group dried by the combination schedule (kiln run 11, blackgum, cottonwood, yellowpoplar), about 300 BF of each species was dried in 71 hours by a conventional temperature schedule for yellow-poplar (6) until the average of the sample boards was 18.5 percent, and no sample board was above 24 percent. Kiln conditions were then changed to 230 F DBT/180 F WBT and held for 9 hours, when the average MC of the sample boards was 6.5 percent. An equalizing and conditioning treatment followed, giving a total kiln residence time of 98 hours. 12 MARCH 1984

4 Kiln run 15 contained about 300 BF each of beech, pecan-hickory, and sweetgum. A beech schedule (6) was used until the average MC of the sample boards was down to about 19 percent (176 hr. drying time). We then used 230 F DBT and 180 F WBT for 10 hours to reduce the average MC to 6.9 percent. Conventional equalizing and conditioning treatments also followed this run. Total kiln time was 234 hours. Preparation of stock for evaluation and drying defects noted Drying degrade is difficult to evaluate because the definition of acceptable degrade depends on the end use or product. Since the intended use of the study lumber was furniture stock, we considered furniture blanks the end product. The rough dry boards were end trimmed and crosscut to two specified lengths: 30 inches and 32 inches. Three pieces or cuttings, all either 30 or 32 inches, were cut from each nominal 8-foot board (range 100 to 108 in. long, mostly 102 to 103 in.). The cuttings from the ends of each rough board were marked so we could later determine if the end of a given processed blank coincided with the end of a board. All cuttings were then ripped to random widths of 2-1/2 to 6 inches to minimize cup, and surfaced to a standard thickness of 25/32 inch. The processed cuttings or blanks were visually examined on all surfaces and the following drying defects noted: a) honeycomb, b) surface checking, c) collapse and warp, the severity of which were judged by failure to surface clean to 25/32 inch and reported as did not dress clean, and d) combinations of these drying defects. For the early kiln runs, every blank was examined and evaluated. After completing four to five groups, we determined that a sampling of one-third to one-half the blanks would give the required accuracy. This sampling intensity was followed for the remaining runs. Results and discussion Rate of drying Drying curves for three species are shown in Figure 1. The curve for each species is based on the data of a typical sample board. Drying curves for the other species followed this general pattern. Moisture contents, ranging from 50 to 185 percent in the green condition, dropped rapidly in the first 24 hours, averaging 10 percent or less in all species at the end of the HT period (Fig. 1 and Table 2). We were able to achieve our target of 6 to 8 percent MC in 50 to 55 hours. Several species sample boards showed final MC in the range of 5 to 6 percent. The bar graphs in Figure 2 provide a drying time comparison of the HT technique with conventional temperature (s 180 F) kiln-drying for selected species. Equalizing and conditioning treatments With HT drying, the MC differences between boards appear to be greater than when stock is dried by conventional schedules (Table 2). Moisture gradients within a piece are higher with lumber dried at high temperature than at conventional temperatures. The effect of our 16-hour equalizing at 4 percent EMC can be seen in Figure 1 Figure 1. Drying curves for the species group American elm, and soft and hard maple are typical for HT drying. and Table 2. Average MC values of sample boards and standard deviations are lower after the equalizing treatment in all cases. The effect of the falling WBT during the first four runs resulted in three sample boards each of soft maple, hard maple, and basswood reaching an MC of 3.0 percent or less during the HT period. The three basswood samples ranged from 1.0 to 2.1 percent for minimum MC. In kiln runs with WBT maintained at 180 F, no sample boards fell below an MC of 3.1 percent. Our conditioning treatment at 10 percent EMC for 10 to 12 hours provided adequate stress relief in all species. Average MC of sample boards showed higher values than at the end of the equalizing period, as expected. Standard deviations were less, which indicated more uniformity of MC after conditioning (Table 2). Final MC values were within our target range. Evaluating drying degrade Since usable material can be so variable between manufacturers, we decided that to be judged acceptable, a blank would have to be completely free of drying defects. The unacceptable category varied from having the slightest visible surface check, end check, or honeycomb to being riddled with checks or honeycomb. FOREST PRODUCTS JOURNAL Vol. 34, No 3 13

5 Figure 2. Comparative drying times for HT kiln schedules and conventional kiln schedules. (Conventional kiln schedules from McMillen and Wengert (6). Times do not include equalizing and conditioning.) When inspecting surfaced stock, it is frequently difficult to distinguish between surface checks and honeycomb. Honeycomb is usually defined as internal checking in contrast to surface checking which is a separation of wood fibers, usually adjacent to a wood ray, starting on the surface and extending into the interior. In surfacing our cuttings from a rough dry thickness of 1 inch plus to 25/32 inch, the removal of the material served to expose honeycomb. In many cases it was difficult to distinguish between wood failures that were generated internally and subsequently exposed by surfacing, or the remains of a surface check or extended surface check. This was particularly difficult to differentiate on the wide face of the blank, and in these cases, I opted to use the term honeycomb rather than surface check. For the unacceptable or reject group, an estimate of severity in relation to area affected (slight = l% to 25% of area; moderate = 25% to 50%; severe = 50% to 100%) was recorded; but, all recorded degrees of area of defect severity are pooled in reporting percent rejects. However, in all 12 species, 50 percent or more of the defects were classified as slight. In 8 (yellow-poplar, basswood, sweetgum, blackgum, white ash, beech, cottonwood, and pecan-hickory) of the 12 species, 75 percent or more of the defects were judged to be slight. In yellow-poplar and sweetgum, 90 percent of the defects were judged slight. An indication of severity by species is covered later. Nonacceptable blanks with only slight defects would probably be used in many commercial operations. General response of species. Species have been grouped into three response categories according to the percent of acceptable (defect free) pieces including both 30- and 32-inch blanks (Table 3). Grouping into these categories is somewhat arbitrary, but serves to emphasize which species have the best and poorest chance of success. The reason for rejection by cause is also shown for unacceptable (reject) blanks. In 10 of 12 species (exceptions were American elm and cottonwood) the greatest cause for rejection was honeycomb. Of these 10, honeycomb alone was the most frequent cause for rejection in all but pecan-hickory. For pecan-hickory, combination of drying defects was a close second. Honeycomb was also one of the contributors to combination of drying defects in all species. For American elm and cottonwood, combination of drying defects accounted for the most rejects. In both of these species it was a combination of honeycomb and did not dress clean (failure to surface clean to 25/32 in.) that caused rejection. For American elm the failure to surface clean was due mainly to warp, and for cottonwood it was due to collapse. In all species, except cottonwood and pecan- 14 MARCH 1984

6 hickory, warp occurred in more blanks than did collapse. Surface checks and did not dress clean as individual classifications accounted for a very low percentage of rejected blanks. In the first six kiln runs (Table 1) only 32-inch blanks were cut. A high proportion of the observed honeycomb occurred in ends of the blanks that coincided with the ends of the boards. This is consistent with experience that honeycomb near the end of boards frequently is the result of end checks extending inward. Since our log lengths varied from about 8-1/2 to about 10-1/2 feet, differing amounts of end trim (from none to 1-1/2 to 2 ft.) were removed to achieve the range of 8-1/2 to 9 feet for the lumber going into the kiln. When three 32-inch cuttings were made from a board after drying, some end trim variance occurred. When boards were cut into three 30-inch cuttings, more of the total board length went into end trim with the apparent result being fewer instances of honeycomb in the ends of the blanks produced from the cuttings. This observation was substantiated by selecting several 32-inch blanks from the ends of the boards where end honeycomb was evident. Cutting the blanks back in l-inch increments until 2 inches were removed resulted in a substantial increase in defect-free ends for all species except black cherry (Table 4). Figure 3 illustrates the appearance of defects in the cut ends. Data in the first three columns of Table 5 suggest that for several of the species, the percent of acceptable (defect free) pieces can be doubled by cutting three 30-inch rather than three 32-inch blanks. Using all data in Table 5, we can assume that for most other species (e.g., basswood, soft maple, American elm) the percentage of acceptable pieces could be expected to increase noticeably if all cuttings had been cut to 30 inches. It appears that the length of the log(s), and the subsequent variance in amount of end trim of both logs and lumber had an influence on the amount of honeycomb which occurred in the ends of cuttings and subsequent blanks in this study. Those species in which all inspected blanks were 32 inches long would be expected to have a lower percent of defect-free blanks than those species with both 30- and 32-inch lengths included. This expectation should be considered when evaluating the honeycomb related data in Table 3 and other tables that separate length of blanks. General observations of material dried in this study suggest that rapid drying achieved at high temperatures contributes to increased drying degrade in the ends of boards. Wolfe (11) indicates that more cutback on the ends due to end splitting is not uncommon in HT-dried material. Location of drying defects in relation to natural defects, sapwood and heartwood. Incidence of drying defects, especially honeycomb, was closely associated with knots (even pin knots in cherry and yellow-poplar), discolored wood, wet streaks, mineral streak (common in the maples), and pith. The severity of defects in reject blanks for all species is based on area affected (slight equals l% to 25%). The associations, by species, are found in Table 6. FOREST PRODUCTS JOURNAL Vol. 34, No 3 15

7 Figure 3. Appearance of the ends of 32-inch cherry blanks at time of inspection, after 1 inch trimmed and after 2 inches trimmed. Color was not measured with instruments, but general observations indicated some darkening of wood occurred in all species dried completely at high temperature. This darkening was most noticeable in beech and the maples. Planed stock was lighter than rough, but was somewhat darker than stock dried at conventional temperatures. Combination of conventional temperature schedules followed by a high-temperature schedule. The yield of defect-free blanks for the combination schedule is dramatically higher than those dried at the HT schedule for all species (except basswood which was not dried by the combination schedule). A substantial increase in defectfree blanks in both the 30- and 32-inch lengths was observed when using this schedule rather than the HT schedule (Table 5). Table 7, combining both lengths, shows a comparison of the percentage of acceptable blanks for the two techniques, and causes for rejection. Even with the combination schedule, honeycomb was the biggest cause for rejects. The relative ranking of the other three reject categories changed in the combination schedule run, with surface checks becoming a distant second-most-common in many species. Collapse and warp, respectively, were a close second in cottonwood and blackgum. The improvement in blank quality for the combined schedule was achieved at the expense of longer kiln residence times. For the five species dried in kiln run 7 (Table 1), residence time was about three times as long as the HT schedule alone. In run 11, the kiln time was doubled; and in kiln run 15, the time required was more than four times as long as the HT schedule alone. However, drying by the combined schedules offered time reductions of 25 to 60 percent of that for drying completely by conventional temperature schedules; except for beech and pecan-hickory which showed no reduction (6). In commercial situations, drying species separately would permit use of optimum schedules for each and reduce kiln residence time. For several of these species, particularly the intermediate response group (Table 3), some combinations of conventional and HT schedules appear preferable to the HT schedule alone. We plan further studies to test this hypothesis. Comparisons with other results reported in the literature. This study confirms literature findings that the most common HT drying defects are honeycomb, collapse, checking, and darkening of the wood. It also demonstrated that response to HT drying is very species dependent. In the area of species response, there are some differences; for example, our results do not support Wengert s (10) findings that green cherry and pecan can be kiln-dried successfully at high temperature. But we 16 MARCH 1984

8 agree with his conclusions that green soft maple, sweetgum, basswood, and elm can be dried successfully on an HT schedule. Richards (7) used end-coated specimens, 18 inches long, dried in a forced circulation oven at 230 F with no humidity control. Relative humidities in such ovens are quite low (< 10%). He reported that 4/4 yellow-poplar, beech, blackgum, hickory, and sweetgum sapwood could be dried without excessive visible defect. Our results support Richards findings for yellow-poplar, blackgum, and sweetgum sapwood, but not for beech and hickory. Summary and conclusions Quality of blanks sawn from lumber that has been HT dried (230 F) green-from-the-saw is species dependent. Basswood, yellow-poplar, white ash, sweetgum, and soft maple dried with only minor defects. Black cherry, hard maple, beech, and pecan-hickory showed sufficient drying degrade to be considered unsuitable. American elm, cottonwood, and blackgum showed an amount of degrade that was considered intermediate. The most common drying defect was honeycomb. The next most common was the combination of drying defects involving honeycomb and warp or collapse followed by the combination of honeycomb and surface checking. Drying defects were commonly associated with knots, pith, mineral streak, or other discolored wood. A combination schedule in which lumber was dried at conventional temperatures (S 180 F) from green to 18 to 20 percent MC, then finished at 230 F, decreased the drying degrade but increased kiln residence time over the HT schedule alone. However, depending on species, this combination schedule offered a reduction in kiln residence time of 25 to 60 percent over drying at completely conventional temperatures. Beech and pecan-hickory were exceptions, offering no reductions in kiln residence time. Further studies using a combination schedule are planned. FOREST PRODUCTS JOURNAL Vol. 34, No. 3 17

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