Comparison of Drying Methods for Paper Birch SDR Flitches and … · Comparison of drying methods...

United States Department of Agriculture

Forest Service

ForestProductsLaboratory

ResearchPaperFPL 465

Comparison of Drying Methods for Paper Birch SDR Flitches and Studs Timothy D. Larson Robert W. Erickson R. Sidney Boone

Abstract Contents

Several drying methods were evaluated for use with Saw- Dry-Rip (SDR) paper birch flitches and studs, including Introduction .................................................................. high- (£212 °F) and conventional- (£180 °F) temperature Materials and Methods.. .............................................. schedules, dehumidification drying and air drying. The major problem encountered in drying by any of these Log Acquisition and Processing ............................. methods was the presence of darkwood near the pith, Drying Facilities ........................................................ which is more impermeable and therefore slower to dry than whitewood and which had a propensity to check, Steam-Heated Kiln.. .............................................. collapse, and honeycomb. The shortest drying time Dehumidification Kiln.. .......................................... (3 days) and best uniformity of moisture content between darkwood and whitewood were obtained using a high- Air Drying.. ................................................................ temperature schedule with an early period of continuous Drying Procedures ................................................... venting. Dehumidification drying of 7/4 flitches to an average moisture content of 10 to 12 percent required Phase I .................................................................. about 18 days, and air drying of 7/4 flitches to less than Phase II ................................................................. 20 percent moisture content in northeastern Minnesota required 6 months of good air-drying conditions. Fifteen to Treatment Identification ........................................... sixteen days were needed for conventional temperature Collection of Moisture Content Data ...................... schedules to dry 7/4 flitches to 8 to 10 percent moisture content. Thickness shrinkage of flitches dried to Collection of Shrinkage Data. .................................. 10 percent moisture content was 8 to 9 percent. Visual Inspection.. ....................................................

Keywords: Saw-Dry-Rip, SDR, paper birch, drying flitches Results and Discussion.. ............................................. and studs, high-temperature drying, dehumidification High-Temperature Drying ........................................ drying, kiln drying, air drying.

Phase I ..................................................................

Phase II .................................................................

High-Temperature/Conventionally

Sawn Runs (CH). ..................................................

Conventional-Temperature Drying ..........................

Conventional-Temperature/SDR Runs (SC). ......

Conventional-Temperature/

Conventional-Sawing Runs (CC) .........................

SDR/Dehumidification Drying (SD). .........................

SDR/Air Drying (SA). ................................................

Visual Observation of Dried Material.. ....................

Shrinkage.. ................................................................

Summary ......................................................................

Literature Cited ...........................................................

Appendix (Tables A l through A7) ..............................

January 1986

Larson, Timothy D.; Erickson, Robert W.; Boone, R. Sidney. Comparison of drying methods for paper birch SDR flitches and studs. Res. Pap. FPL 465. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory; 1986. 13 p.

A limited number of free copies of this publication are available to the public from the Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53705. Laboratory publications are sent to over 1,000 libraries in the United States and elsewhere.

The Laboratory is maintained in cooperation with the University of Wisconsin.

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Comparison of Drying Methods for Paper Birch SDR Flitches and Studs

Timothy D. Larson, Research Assistant Robert W. Erickson, Professor

Department of Forest Products, College of Forestry University of Minnesota, St. Paul, MN

R. Sidney Boone, Research Forest Products Technologist Forest Products Laboratory, Madison, WI

Introduction

Saw-Dry-Rip (SDR) is a log-to-lumber-processing sequence which has been successful in reducing warp compared to conventional processing of low- to medium-density, small-diameter hardwoods containing high longitudinal growth stresses (Maeglin and Boone 1983). In SDR, logs are live sawn (through and through on one plane) into 7/4 flitches. The flitches are rough edged to produce a compact kiln load and then dried to an average moisture content of about 10 to 12 percent. After drying, the flitches are ripped and surfaced into 2 by 2, 2 by 3, and 2 by 4 studs. A more detailed description of the SDR process is given by Maeglin and Boone (1981).

Paper birch, Betula paperifera, is a medium-density, small-diameter hardwood with significant potential for improved processing by SDR. Proper drying techniques are crucial to the SDR process. The purpose of this research was to develop schedules and evaluate several methods for drying the 7/4 flitches using SDR. This paper summarizes the drying results obtained in the application of SDR to paper birch.

The drying results were collected over two distinct phases of the study. In Phase I, a high-temperature schedule for drying 7/4 paper birch flitches was developed, and dehumidification drying and air drying of flitches were evaluated. The optimal high-temperature schedule derived from Phase I runs was used in Phase II to compare drying of SDR-processed 7/4 flitches and conventionally sawn studs at high- (£212 °F) and conventional-(£180 °F) temperature schedules. Dehumidification drying and air drying were also used in Phase II to generate more material.

Results of drying method experiments in both Phases Iand II are presented in this paper. Studies in Phase II also showed that SDR effectively reduced warp compared to conventional processing (Erickson et al. 1985). Phase Ill has been yielding data on the bending strength of 2 by 4birch studs produced by the various drying methods.

Materials and Methods

Log Acquisition and Processing

Logs for Phase I from the yard of a cooperating sawmill were sawed into flitches to a rough target thickness of 1-13/16 inches and rough edged. The edging procedure made the logs approximately 1/2 inch wider than the width dictated by the National Grading Rules for study/ grade lumber. After edging, the flitches were double-end- trimmed to 97 inches. Approximately 5,000 board feet (fbm) of flitches were produced for Phase I.

For Phase II, trees 8 to 12 inches in diameter at breast height (dbh) were harvested as part of a Forest Service sale located about 40 miles north of Grand Marais, MN. These trees were bucked into 216 logs 100 inches long, and each log was given a tree number plus a log number indicating its position in the tree. Logs bucked for Phase II had a minimum small-end diameter inside the bark (dib) of 4.5 inches.

The logs were randomly divided into 12 groups. Each group represented one of two replications of six treatments. Prior to treatment, the logs were divided into butt logs and nonbutt logs because studs from butt logs of some softwood species have greater warp than studs from nonbutt logs (Blake and Voorhis 1981). Butt and nonbutt logs were then sorted into l-inch-diameter classes, with the largest and smallest classes being 10 inches (9.5 to 10.4 inches) and 5 inches (4.5 to 5.4 inches). The treatments combined sawing and drying methods as follows:

Saw&method Drying Method

SDR (live) High temperature SDR (live) Conventional temperature SDR (live) Dehumidification SDR (live) Air Conventional High temperature Conventional Conventional temperature

All logs were sawn into flitches at the cooperating sawmill in northeastern Minnesota in the manner described for Phase I. The green flitches allocated to conventional processing treatments were subsequently gang ripped into studs at a sawmill near St. Paul, MN, just prior to drying.

Drying Facilities

Steam-Heated Kiln All the high-temperature and conventional schedule runs were conducted in an experimental steam-heated kiln with a capacity of 500 fbm. Each run in Phases I and II consisted of approximately 500 fbm, with each charge measuring 4 feet wide by 8 feet long. The air velocity on the leaving side of the charge was 850 feet per minute with fan reversal every 6 hours. The charges were built with 3/4-inch stickers placed 2 feet on center. The first high-temperature run in Phase I had a top load of 12.5 pounds per square foot (Ib/ft2) and the remaining runs a top load of 60 Ib/ft2. Set points for initial kiln conditions were reached in 2 to 4 hours for all runs.

Dehumidification Kiln The dehumidification kiln is an experimental unit with amaximum capacity of about 1,250 fbm. Each run with paper birch flitches consisted of a charge that was 8 feet long, 4 feet wide, and about 4 feet high, with l-inch stickers placed 4 feet on center. Air velocity on the leaving side of the charge was 700 feet per minute (ft/min.) There was a unidirectional airflow for both Phases I and II.

The conditions within the kiln are controlled by manual operation of the compressor. The dry-bulb temperature (dbt) setting was maintained by heat dissipation at a heat exchanger. No top loading was employed in Phase I, but an evenly distributed load of 60 Ib/ft2 was used in Phase II.

Air Drying

Lumber was air dried in the air-drying yard of the cooperating mill in Grand Marais, MN. The lumber was stickered with 3/4-inch stickers placed 2 feet on center. In Phase I, three units were built, each 4 feet wide and containing approximately 640 fbm. The two units in Phase II were also 4 feet wide and contained approximately 500 fbm each. In both phases, the units were placed one on top of another in a single stack. No top loading was employed other than a coverboard.

Drying Procedures

Phase I Table 1 describes the conditions for the high-temperature runs in Phase I. Technically, the third run was not performed at a high temperature since the dbt was less than 212 °F (205 °F). An equilibrium moisture content of about 3.5 percent was maintained during the initial stage of drying for each of the runs. Drying continued at the initially set conditions until the driest sample board had amoisture content of 10 percent. At that point, an equalizing treatment was used until the wettest sample board had a moisture content of 15 percent. The equilibrium moisture content used for equalizing in all three runs was 10 percent. The three high-temperature runs in Phase I will be referred to as Run 1, Run 2, and Run 3 throughout the rest of this paper.

Table 1.—Drying conditions for high-temperature Runs 1, 2, and 3 in Phase I

High temperature Equalizing

Run Drybulb

Wetbulb

Equilibriummoisturecontent

Time Drybulb

Wetbulb

Equilibriummoisturecontent

TimeTotal kiln residence

time

°F °F Pct Hr °F °F Pct Hr Hr1 240 190 3.5 25 190 178 10 43 682 220 175 3.5 48 190 178 10 96 144

1 3 205 160 3.5 66 190 178 10 96 162

1Considered as a high-temperature run in this study even though the dry-bulb temperature was less than 212 °F.

The compressor was operated continuously for the runs in the dehumidification kiln, and the three fan motors and the compressor were used as the only heat sources. At the onset of drying, the dbt was 70 °F with an equilibrium moisture content of about 10 percent, which are the conditions that exist within the laboratory. On the second day of drying, the dbt was 90 °F with an equilibrium moisture content of 11.5 percent. By the tenth day, the dbt had risen to a maximum of 125 °F with a 4 percent equilibrium moisture content. This maximum dbt was maintained throughout the rest of the drying process, while the equilibrium moisture content dropped and was about 3 percent at the end of drying.

Table 2 summarizes the monthly weather data for air drying during Phase I. Monthly average temperatures in early spring were 4.6 to 1.1 °F above normal, and precipitation was about 3/4 inch above normal. Monthly average temperatures in late spring were slightly below normal, and precipitation was about 1 inch above normal. For the entire summer. average monthly temperatures were above normal and precipitation was approximately 1 inch below normal.

Phase II One of the major problems encountered in drying paper birch was the presence of darkwood. Darkwood refers to discolored streaks of wood that are centered primarily around the pith of the tree and whose origin can often be traced to branch sites. Approximately three out of four 2 by 4’s from Phase I contained darkwood. On the average, one-third (volume) of each piece of wood was darkwood. This type of wood is quite impermeable and has a higher initial moisture content than whitewood (sapwood and normal heartwood) (Erickson et al. 1984).

Due to the difficulty of drying darkwood in Phase I we decided to initiate equalizing in the steam-heated kiln when the driest sample board had reached a moisture content of 8 percent and to continue until the wettest sample board had a moisture content of 12 percent.

From the results in Phase I we decided to utilize the high-temperature conditions of Run 1. This decision was based on three conclusions: (1) Run 1 had a faster drying rate than Runs 2 and 3; (2) a statistical analysis of the crook data from Phase I showed no significant differences among the three runs: and (3) visual inspection indicated that the runs were similar in degrade.

The drying schedule (T8-C3) recommended by the Forest Products Laboratory (FPL) for 8/4 paper birch (Rasmussen 1961) was used for treatments using aconventional temperature schedule. Again. equalizing began and terminated according to the high-temperature schedule described for Phase I.

The procedure for the dehumidification kiln in Phase II was the same as that described for Phase I.

The monthly weather conditions during the air drying in Phase II are presented in Table 2. During early spring, the temperatures were slightly below normal, and precipitation was below normal during April. The summer months were also cooler than normal, particularly July and August which were 3.0 and 2.9 °F below the normal average monthly temperatures. Precipitation was 1 inch above normal in July and about 1 inch below normal in August and September. In October. precipitation was 2.45 inches above the norm.

Treatment ldentificationAbbreviations used to designate the various treatments for Phase II are listed in table 3. The first letter in each abbreviation represents the sawing method (S, SDR; C, conventional), and the second letter represents the drying treatment (H, high temperature: C. conventional temperature; D, dehumidification; A, air). The subscript letters refer to one of two replications of each treatment. In the case of SH, wbtu (wet-bulb temperature uncontrolled) means the wet-bulb reservoir went dry and the kiln was vented continuously for 15 hours early in the drying cycle. The subscript wbtc (wet-bulb temperature controlled) refers to the replication where the wet bulb functioned normally.

Table 2.–Summary of weather data for Grand Marais, MN; site of air drying in Phases I and II. (Source: National Weather Service, Minneapolis-St. Paul)

Phase I Phase II

Deviation Deviation Deviation DeviationMonth Average from Average from Average from Average from

temper- normal precipi- normal temper- normal precipi- normalature tempera- tation precipi- ature tempera- tation precipi-

ture tation ture tation

°F In. °F In.March 30.1 +4.6 0.84 -0.46 25.2 0.3 2.10 +0.80April 39.1 +1.1 3.43 +1.24 35.2 - 2 . 8 .95 -1 .24MayJune

45.352.1

- 1 . 3- 1 . 6

1.956.02

-1.11+2.2

47.452.1

+ .8 - 1 . 6

4.731.57

+1.67-1.95

July 61.2 + .8 2.93 - . 0 7 57.4 - 3 . 0 1.47 +1.09August 64.7 + 3.3 .67 -2.41 58.4 - 2 . 9 2.08 -1.00September 56.6 + 2.7 2.84 - .45 53.5 - .4 1.97 -1.32October 41.4 + 3.5 3.23 +1.01 46.2 +1.3 4.67 +2.45

Table J.--Description of treatments and summary of kiln conditions for Phase II

Treatment Treatmentabbrevi- descrip- Drying conditions1 Equalizing*

ation tion

SHwbtu Saw-Dry-Rip (SDR)/high-temperaturedrying/wet-bulb temperature (wbt)uncontrolled

SHwbtc SDR/high-temperature drying/wbt controlled

CH Conventional sawing/high-temperature drying

SC SDR/conventional-temperature drying

Conventional sawing/conventional-temperature drying

SD SDR/dehumidification drying

SA SDR/air drying

Dry-bulb temperature (dbt) of 240 °F. Kiln vents wide open for 15 hours followed by 24 hours at wb-temperature of 190 °F

dbt of 240 °F and wbt of 190 °F for 26 hours


Dried by Forest Products Laboratory (FPL) schedule T8-C3. Average drying time of 373 hours for each of the two runs.

Dried by FPL schedule T8-C3. Average drying time of 290 hours

1 horsepower unit. Average drying time of 460 hours for each of the two runs.

Northeastern Minnesota air drying yard from May through November. Stickered units were then covered with plastic and stored outdoors in St. Paul, MN, during December and January.

dot of 200 °F and wbt of 178 °F for 27 hours


dbt of 200 °F and wbt of 178 °F. About 44 hours for each of the two runs

dbt of 180 °F and wbt of 160 °F. Approximately 24 hours for each of the two runs

dbt of 180 °F and wbt of 160 °F for about 90 hours for each run

None

None

1With the exception of treatment SHwbtu, equalizing began when the driest sample board reached 7 to 8 pct moisture content and continued until the wettest reached a moisture content of about 12 pct. In treatment SHwbtu the equalizing commenced when the wettest kiln sample was about 10 pct and continued until the average of all 6 sample boards was approximately 6 pct. There was a top-load restraint of 60 Ib/ft2 for all treatments except SA. 2Total kiln residence equaled drying plus equalizing hours.

4

CC

Results and Discussion

Collection of Moisture Content Data

The standard procedure (Rasmussen 1961) for cutting moisture content sections, using sample boards, and estimating ovendry weights was followed throughout the study. A heavy application of black plastic roof cement was used for end coating the sample boards. The sample boards were weighed at intervals suited to the graphing of drying curves. At the end of drying, four l-inch sections were cut from each sample board to evaluate average moisture content, darkwood versus whitewood moisture content (if applicable), moisture gradient (shell and core), and stress.

After the dried material reached room temperature, aresistance-type moisture meter was used to take moisture content readings on about 30 percent of the flitches or the conventionally sawn studs. Two readings were taken at three locations along the length: about 18 inches from each end and at mid-length. For 2- by 2- and 2- by 3-inch studs only one reading was taken at three locations along the length. The insulated needles were driven to a depth of 3/8 inch.

Collection of Shrinkage Data

The shrinkage data reported in this paper were collected on the sample boards employed in Phase II. A vernier caliper and dial gauge were used to measure width and thickness respectively to the nearest 0.001 inch. Shrinkage values are expressed as a percent of green dimensions.

Visual Inspection

Dried materials were qualitatively observed for degrade, honeycombing, casehardening, collapse, end checking, and color change.

Drying curves of sample boards in both Phases I and II are shown in figure 1. Data showing percent moisture content of shell, core, whitewood and darkwood of sample boards in both phases as well as average moisture content are presented in tables A1-A7 in the Appendix.

High-Temperature Drying

Phase I For the first high-temperature run of Phase I, the average initial moisture content of the six sample boards was 76.5 percent, varying from a high of 86.2 percent (approximately 60 percent darkwood) to a low of 64.2 percent (all whitewood) (fig. 1).

After 24 hours of drying, the average moisture content of the sample boards was 12.6 percent with a low and high of 5.4 percent and 22.0 percent, respectively. Equalizing was started at this time, and after 42 hours the average moisture content was 13.8 percent, with a low and high moisture content of 8.1 percent and 21.2 percent, respectively. The average moisture content was higher after equalizing than at the onset of equalizing because the whitewood gained moisture more rapidly than the darkwood lost moisture. Table Al (Appendix) illustrates the slow-drying nature of the darkwood and its failure to respond to the equalizing treatment. The high moisture content of the darkwood at the end of drying led to postdrying and excessive shrinkage and warp. Shell and core moisture contents, which represent the moisture gradients, were relatively large at the end of drying. Note that sample board 5 has a shell-core moisture content difference of 12.7 percent, but this large moisture gradient can be attributed to the presence of darkwood and its impermeability. Sample boards 2, 3, and 6 did not contain darkwood and show very low gradients going from shell to core.

The dry-bulb temperatures used in Runs 2 and 3 of Phase I were lower than that of Run 1, and the total kiln residence for Runs 2 and 3 was more than twice as long as that for Run 1 (table 1). Table 4 shows both the average and range of moisture content for the various drying methods used in Phase I. In spite of the 96-hour equalizing period used in Runs 2 and 3, moisture content remained high in the darkwood. In fact, the range of moisture content for the darkwood of Runs 2 and 3 was actually greater than that for the darkwood of Run 1. Even with the extended kiln residence of Runs 2 and 3compared to Run 1, the lower dry-bulb temperatures were apparently not able to lower the level of moisture in the type of darkwood most resistant to dehydration.

5

Figure 1.–Drying curves, (Phase I, SH) High-temperature drying of sample boards in Run 1 of Phase I; (Phase II, SH) high-temperature drying (with 15 hr of continuous venting) of SDR f/itches in Phase II; (CH) high-temperature drying of conventional/y sawn studs in Phase II; (SC) conventional schedule drying of SDR f/itches in Phase II; (CC) conventional schedule drying of conventionally sawn studs in Phase II; (SD) dehumidification drying of SDR f/itches in Phase II. (ML85 5369)

6

- - - - - - - - - - -

Phase II At the conclusion of the three high-temperature runs in Phase I, we decided to use the schedule from Run 1 in Phase II because of its faster drying rate and the more uniform final moisture content obtained, and because collapse, honeycomb and discoloration were apparently no more severe in Run 1 than in Runs 2 and 3. Figure 1shows the drying curves of the first high-temperature run of Phase II. After 4 hours, the wet-bulb reservoir went dry; this led to continuous venting for the next 15 hours and some unexpected, but favorable, results. Average initial moisture content for the six sample boards was 64.9 percent, with a high and low of 70.4 and 61.1 percent, respectively At 19 hours the average moisture content was only 11.4 percent and the highest moisture content, 17.1 percent. After 39 hours the charge was actually overdried, with an average moisture content of only 4 percent and a high of 5percent. Equalizing was started at 39 hours to raise the moisture content to the targeted 8 to 10 percent. After 26 hours of equalizing, the kiln was shut down and the average sample board moisture content was 6.0 percent, with a high of 6.6 percent and a low of 5.6 percent. The drying curves shown in figure 1 and tabular data (table A2, Appendix) both demonstrate continuous venting effectively removed moisture from the darkwood. Table A2 also shows that there was essentially no difference in the final moisture content of darkwood and whitewood or shell and core. Further investigation of this type of schedule has confirmed its effectiveness in reducing moisture content levels in darkwood (Erickson et al. 1984).

In the second high-temperature run of Phase II the wet-bulb temperature was controlled and the equalization period was 144 hours. The conditions were identical to those used in Run 1 of Phase I, with the exception of lengthening the equalization period, which resulted in afinal average moisture content of 10.0 percent, with a high of 13.1 percent and a low of 7.1 percent (table 5). Also note that the final average darkwood moisture content in SHwbtc was 14.4 percent compared to 28.3 percent for Run 1 of Phase I (see table Al, Appendix). Extending the equalizing period by 100 hours apparently caused the lower final moisture content for the darkwood.

High-Temperature/ConventionallySawn Runs (CH) The results from the first run for conventionally sawn studs dried at high temperature are shown in figure 1. The results represent both conventional sawing/high-temperature (CH) drying runs. The sample boards had an initial average moisture content of 69.8 percent, with ahigh of 82.1 percent (55 pct darkwood) and a low of 63.0 percent (all whitewood). After 23.5 hours of drying the average moisture content was 7.7 percent, with a low of 5.2 percent. This average moisture content was considerably less than the moisture content of the SDR material dried by high temperature (SH) at 24 hours. Although this discrepancy may be partially attributed to natural variability, the reduced dimension of the rough green studs compared to the dimension of the flitches probably played an important role in accelerating the drying.

Table 4.–Final moisture content data obtained with aresistance meter on a 30 percent sample of the SDR flitches dried by high temperature, dehumidification, or air in Phase I

Moisture content Drying Whitewood DarkwoodMethod’

Average Range Average Range

Pct - - - - - - - - - - -

HT Run 1 7.8 7.0 to 13.0 13.4 7.0 to 21.0HT Run 2 9.7 8.0 to 13.0 17.4 7.0 to 26.0HT Run 3 10.3 7.0 to 13.0 13.8 7.0 to 30.0DH Run 1 8.7 7.0 to 14.0 10.3 7.0 to 25.0DH Run 2 10.9 7.0 to 22.0 12.4 7.0 to 21.0Air dried 11.0 9.0 to 14.0 10.5 8.0 to 15.01HT = high temperature; DH = dehumidification.

Equalizing began after 23.5 hours and continued for 44.5 hours; at this time the average moisture content was 7.3 percent, with a high of 11.7 percent and a low of 6.0 percent. The final average darkwood moisture content of 8.4 percent (table A3, Appendix) suggests that the darkwood of conventionally sawn studs dries faster compared to that of SDR flitches. Faster drying was probably due to the larger surface-to-volume ratio of the conventionally sawn material compared to the SDR flitches. Also note that the larger surface-to-volume ratio in this drying method cut down on the severity of moisture gradients when compared to the first high-temperature run of Phase Iwhich was discussed earlier. Only sample board 5exhibited a large shell-core moisture content difference of 9 percent. Nevertheless, the moisture content of the wettest sample board (55 pct darkwood) dropped only 3 percent during 44.5 hours of equalizing

Conventional-Temperature Drying

Conventional-Temperature/SDR Runs (SC) Figure 1 and table A4 (Appendix) give representative results for the drying of SDR flitches by the conventional FPL schedule. (See table 3 for schedule details.)

The initial average moisture content was 81.9 percent, with a high of 90.0 percent and a low of 66.1 percent. After 48 hours the average moisture content was 62.5 percent. By comparison, the moisture content of the high-temperature SDR runs was in the range of 13 to 15 percent after 48 hours of drying, indicating that high temperature accelerated drying.

7

- - - --

Table 5.–Moisture content of all sample boards by treatment and replication in Phase II

Average Final average Range of

Treatment1 initialmoisture

moisture content final average moisture

Dryingtime

content Overall Whitewood Darkwood content

Pct - - - - - - - - - Pct - - - - - - - - - Pct Hrs

SHwbtu 65.4 6.0 5.8 6.0 5.6 - 6.2 65SHwbtc 78.8 10.0 8.3 14.4 7.1 - 13.1 167SC 81.9 9.9 8.9 13.7 7.8 - 12.6 360

69.9 3.3 8.6 10.6 6.7 - 9.9 434SD 55.1 8.9 8.4 11.0 6.2 - 11.1 395

68.4 12.3 11.2 16.7 8.7 - 16.0 524SA

CH

71.874.469.8

19.4 22.7 22.022.3 22.2 21.3

7.3 6.4 8.4

12.5 - 22.4 19.8 - 25.6 6.0 - 11.7

(2)(2)68

64.4 6.2 6.2 6.6 5.2 - 7.0 6981.2 8.4 8.4 10.0 6.8 - 9.7 38274.3 8.1 7.7 9.4 7.0 - 10.2 379

1See table 3 for definitions. 2ln air-drying yard at Grand Marais, MN, from May through most of November 1981, and then outdoor storage at St. Paul, MN, on stickers and covered with plastic film during December and January.

Table 6.–Final moisture content data obtained with aresistance meter on a 30 percent sample of flitches or studs dried by a given method in Phase II

Moisture content Drying method Average Range

Pct - - - - -SDR flitches:

SHwbtu1 (2) 7.0 to 7.0

SHwbtc (2) 7.0 to 8.0SC1 9.8 7.0 to 23.0SC2 7.7 7.0 to 14.0SD1 8.4 7.0 to 11.0SD2 9.9 7.5 to 14.0

Conventionally sawn studs: SA 16.1 11.0 to 23.0CH1 8.0 7.0 to 18.0CH2 8.3 7.0 to 22.0CC1 9.1 7.0 to 15.0CC2 8.3 7.0 to 12.5

1Subscript represents replication. See table 3 for definitions of the abbreviations. 2Meter readings ware taken but the majority were less than 7 pct moisture content and did not register.

After 334.5 hours of drying the average moisture content was 12.5 percent, with a low of 8.5 percent, and equalizing was commenced. At the end of drying the average moisture content was 9.9 percent, with a high of 12.6 percent and a low of 7.8 percent. Despite the total kiln residence of 359.5 hours, one board had a final moisture content of 23.0 percent (table 6) and one board had a darkwood final moisture content of 18.5 percent (table A4, Appendix). Low moisture gradients existed in all sample boards with the exception of number 6 which had a shell-core moisture content difference of 8.8 percent. Again, the sample boards with the larger moisture gradients at the end of drying contained darkwood.

Conventional-Temperature/Conventional-Sawing Runs (CC) Figure 1 shows the results of drying conventionally sawn studs by FPL schedule T8-C3. The initial average moisture content was 81.2 percent, with a high of 100.4 percent and a low of 66.1 percent. At 314.5 hours, the average moisture content was 9.9 percent and the lowest 7.3 percent, and equalizing was then begun. In the conventional as well as the high-temperature schedules, edge drying shortened the drying time of the conventionally sawn stud compared to flitches. After 67 hours of equalizing, the average moisture content was 8.4 percent, with a high of 9.7 percent and a low of 6.8 percent. The differences in moisture content between shell and core or whitewood and darkwood were very small: this was mostly caused by edge drying and long kiln residence.

8

CC

SDR/Dehumidification Drying (SD)

Figure 1 presents the drying results from the first of two dehumidification runs completed in Phase II. A maximum dbt of 125 °F was reached in 5 days and was maintained until the end of drying. The wet-bulb depression was 17 °F on the second day of drying, was increased to 40 °F by the fifth day, and was continued at that level to the end of drying.

The average starting moisture content was low, due to drying that had occurred during several months storage in a cold room (≈33 °F). The initial average moisture content was 55.1 percent with a high of 70.2 percent (ail whitewood) and a low of 39.8 percent (all whitewood). After 200 hours of drying, the average moisture content was 16.9 percent, with a high of 21.6 percent and a low of 12.9 percent.

The run was terminated after 395 hours, at which time the average moisture content was 8.9 percent, with a high of 11.1 percent (20 pct darkwood) and a low of 6.2 percent (table A6, Appendix). Maximum moisture content of darkwood at the end of drying was 16.2 percent, and the largest moisture gradient was 8.8 percent. Both conditions were found in sample board 1. These results are similar to those obtained by using the FPL schedule in the steam kiln However, dehumidification drying took about 3 days longer than conventional-temperature drying, to arrive at similar final levels of moisture content. This was no doubt due to the lower dry-bulb temperature.

SDR/Air Drying (SA)

Figure 2 and table A7 (Appendix) show the drying curves and moisture content data for the samples air dried in Phase II. Because the rainfall was below average when the stacks were built, a good drying rate was consequently obtained. The initial average moisture content was 73.1 percent, with a high of 87.3 percent and a low of 60.3 percent.

After only 40 days of drying the average moisture content was 20.9 percent, with a high of 24.9 percent. At the conclusion of 200 days of air drying the average moisture content was 13.4 percent, with a high of 16.1 percent and a low of 9.5 percent. These results were better than expected for March to October since the total precipitation was 2.75 inches below normal. The high levels of final moisture content were due to wet conditions in December and January while the stickered units were located outside Kaufert Laboratory in St. Paul, MN. Although the units were protected by a plastic cover from direct rain and snow, high-humidity air entered from the uncovered bottoms of the stacks and was contained within the plastic “tent.” A surprisingly high level of moisture was adsorbed, resulting in a final average moisture content of the sample boards of about 22.4 percent. Unfortunately this large increase in moisture content at the end of drying masked any moisture content differences that may have existed between shell and core or whitewood and darkwood.

Figure P.-Drying curves of air-dried SDR flitches. (ML85 5370)

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Visual Observation of Dried Material

Honeycomb was present in some of the darkwood regardless of the drying method. Visual observations indicated that the frequency and severity of honeycomb were related to the dbt, being greatest in the high-temperature runs, less in the dehumidification and conventional-temperature runs, and least in the air-dried material (fig. 3). With all drying methods it was observed that the darker the darkwood, the more severe the honeycombing. It should be noted, however, that in all treatments there was unhoneycombed darkwood, which suggests variable permeability in this type of wood.

SH

SC

SD

SA

Figure 3.—Examples of honeycomb in darkwood of finished stud by treatment. SH = SDR f/itch, high-temperature dried; SC =SDR flitch, conventional- temperature dried; SD = SDR f/itch, dehumidification dried; SA = SDR flitch, air-dried. (M85 0242)

The color of whitewood darkened and lost brightness at higher drying temperatures compared to air drying. Material dried at high temperatures darkened the most, taking on a dull light-beige color. Material dried by dehumidification and the FPL schedule fell between air-dried and high-temperature-dried material in the color spectrum, but the studs could not be distinguished from each other by color.

End checking was most prevalent in the air-dried material and was found to have an average length of about 3inches. Minimal end checking occurred with the other types of drying.

White rot was found in all material but appeared to be most severe in the air-dried flitches. Also, most of the air-dried flitches exhibited the presence of blue stain.

Stress sections cut at the end of drying indicated the presence of case-hardening in all cases except the following. First, when the wet-bulb reservoir went dry and the average moisture content increased during equalizing, the stress sections cut at the end of drying showed no casehardening. Secondly, stresses were absent at the end of drying in the air-dried material from Phase II which increased in moisture content during December and January. Thirdly, the conventional-sawing/conventional-temperature treatment produced stress-free material. The most severe casehardening was found in flitches dried by dehumidification.

Shrinkage

The shrinkage data from Phase II are summarized in table 7. For SDR processing, the thickness shrinkage is important as it will help determine the necessary rough green target size. The data indicate that a maximum thickness shrinkage of 8 to 9 percent should be used in determining rough green size when drying paper birch flitches to an average moisture content of 10 percent.

Table 7.—Shrinkage values for sample boards from Phase II

Treatment’Average shrinkage2

Width ThicknessFinal average

moisture content

- - - -Pct- - - - - - - P c t - - -

SHwbtu 4.68 9.88 6.0SHwbtc 3.27 9.46 10.0SC 5.65 7.62 9.1SD 4.71 6.85 10.6SA 3.44 3.79 20.9CH 4.85 13.68 6.8CC 7.34 7.04 8.31See table 3 for definitions. 2For the SH treatment the replications are listed individually due to the different drying environments that resulted for each replication. The value for each of the other kiln-drying treatments is the average for 2 runs. For air drying (SA), the values are derived from the total of 8 sample boards contained in the 2 units.

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Summary Literature Cited

SDR is a successful processing technique for reducing the amount of warp in studs manufactured from low- to medium-density hardwoods. Proper drying techniques are an important aspect of SDR. In this study several drying methods, including high temperature, conventional temperature, dehumidification, and air drying, were evaluated for paper birch SDR 7/4 flitches and conventionally sawn studs. Evaluation of warp in these materials is discussed in another paper (Erickson et al. 1985).

The results of the different drying techniques were as follows:

• Of the treatments used for drying 7/4 SDR birch flitches, the high-temperature schedule with an early period of continuous venting led to the shortest drying time (65 hr to 6 pct MC) and excellent uniformity of moisture content between darkwood and whitewood. We estimate that industrial kilns may need to increase time in the kiln by 20 to 25 percent over the time required in our small experimental kiln.

• Conventional temperature drying of 7/4 SDR flitches required 15 to 16 days to reach an average moisture content of 8 to 10 percent, and dehumidification drying required 17 to 19 days to reach an average moisture content of 10 to 12 percent.

• In northeastern Minnesota, green 7/4 SDR birch flitches air dried to an average moisture content of 12 to 15 percent during the months of April through September.

• A thickness shrinkage of 8 to 9 percent can be anticipated in drying paper birch flitches to an average moisture content of 10 percent.

The major problem encountered in drying paper birch was the presence of darkwood, which had a propensity to check, collapse, and honeycomb. Approximately three out of four 2 by 4’s produced in Phase I of the research contained darkwood. The birch flitches which contained both darkwood and whitewood usually had large within-board moisture content variation at the end of drying. In general, shorter drying times resulted in greater within-board variation of moisture content, because of the impermeability of the darkwood.

Blake, B. R.; Voorhies, G. Studies of kiln drying young growth ponderosa pine studs. Southern Lumberman. 241(3002): 7-9; 1981.

Erickson, Robert W.; Petersen, Harlan D.; Larson, Timothy D. Obtaining uniform final moisture content in the high temperature drying paper birch flitches. Forest Products Journal. 34(2): 27-32; 1984.

Erickson, Robert W.; Petersen, Harlan D.; Larson, Timothy D.; Maeglin, Robert R. Producing studs from paper birch by saw-dry-rip (SDR). Submitted to the Northern Journal of Applied Forestry; 1985.

Maeglin, Robert R.; Boone, R. Sidney. Manufacture of quality yellow-poplar studs using the saw-dry-rip (S-D-R) concept. Forest Products Journal. 33(3): 10-18; 1983.

Maeglin, Robert R.; Boone, R. Sidney. Manufacturingquality structural lumber from hardwoods using the saw-dry-rip process. Proc. of the Ninth Annual Hardwood Symp. of the Hardwood Research Council. Pipestem, WV; May 25-28, 1981.

Rasmussen, Edmund F. Dry kiln operator’s manual. Agricultural Handbook. No. 188. U.S. Department of Agriculture, Forest Service; 1961.

Acknowledgment

The authors wish to acknowledge the cooperation of Hedstrom Lumber Company, Grand Marais, MN, and Lake Elmo Hardwoods, Inc., Lake Elmo, MN, in conducting this study.

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Appendix (Tables Al through A7)

Percent moisture content of shell, core, whitewood and darkwood and average moisture content of sample boards in Phase I and Phase II are shown in the following tables.

Table Al.-Moisture content of sample SDR flitches dried by high temperature in Phase I

Board Average Shell Core Whitewood1 Darkwood

Pct moisture content* - - - - - -

AFTER 44 HOURS OF DRYING

13.7 8.0 18.1 8.2 22.96.8 6.5 6.68.4 7.0 8.9

21.3 10.4 29.8 7.7 30.921.3 9.9 34.4 8.7 34.1

6.9 6.8 7.0

– x 13.1 8.1 17.5 8.2 29.3

AFTER 67 HOURS OF DRYING (FINAL)

14.9 9.6 20.3 9.1 24.98.1 7.3 7.9

10.3 8.7 11.219.9 11.0 29.5 9.5 27.521.2 11.1 33.8 9.8 32.5

8.3 7.6 8.6

– x 13.8 9.2 18.6 9.5 28.3 1X-section divided into whitewood and darkwood for boards containing both types of wood. 2Individual X-sections used for average, shell/core, whitewood/darkwood

Table A2.—Final moisture content of sample SDR flitches dried by high temperature in Phase II

Board Average Shell Core Whitewood1 Darkwood

Pct moisture content2 - - - - - -

6.0 5.7 5.7 5.7 5.96.2 5.7 5.9 5.9 6.05.7 5.4 5.05.7 5.6 5.45.6 5.5 5.3 5.6 5.46.6 5.8 6.2 6.0 6.6

– x 6.0 5.6 5.6 5.8 6.0 1X-section divided into whitewood and darkwood for boards containing both types of wood. 2lndividual X-sections used for average, shell/core, whitewood/darkwood.

Table A3.—Final moisture content of sample conventionally sawn studs dried by high temperature in Phase II

Board Average Shell Core Whitewood 1 Darkwood


6.5 5.6 7.16.0 5.0 5.46.6 5.2 7.06.8 5.3 8.2 6.8 7.2

11.7 6.9 15.9 6.5 11.86.0 5.3 6.2 5.9 6.2

– x 7.3 5.5 8.3 6.4 8.4 1X-section divided into whitewood and darkwood for boards containing both types of wood. 2lndividual X-sections used for average, shell/core, whitewood/darkwood.

Table A4.—Final moisture content of sample SDR flitches dried by a conventional schedule in Phase II


- - - - - - Pct moisture content 2 - - - - - -

11.2 8.8 14.3 10.6 13.09.7 8.2 12.1 9.1 8.48.7 7.5 10.77.8 7.0 9.49.4 8.2 13.4 7.8 14.7

12.6 8.4 17.2 8.7 18.5


Table A5.—Final moisture content of sample conventionally sawn studs dried by a conventional schedule in Phase II



7.9 6.8 9.49.2 7.3 10.9 8.5 11.38.0 5.7 9.36.8 6.8 8.49.7 8.3 12.0 9.4 9.98.8 7.1 9.7 7.4 8.8


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Table A6.—Final moisture content of sample SDR flitches dried by dehumidification in Phase II



11.1 6.5 15.3 9.4 16.210.9 6.8 14.3 10.0 13.38.4 5.4 11.18.2 5.9 11.2 8.0 8.66.2 3.9 8 .2 6.0 6.08.8 5 .7 11.7

– x 8.9 5.7 12.0 8.4 11.0

1X-section divided into whitewood and darkwood for boards containing both types of wood. 2Individual X-sections used for average, shell/core, whitewood/darkwood.

2 . 5 - 1 2 / 8 5

Table A7.—Final moisture content of sample air-dried SDR flitches in Phase II



21.3 21.3 20.6 21.4 21.3 21.3 20.8 20.5 22.1 21.4 24.7 26.2 21.8 22.4 25.4 22.4 22.7 21.4 19.8 19.0 19.7 19.8 19.7 19.8 18.0 20.3 19.3 19.7 25.6 24.6 23.9 24.4 25.6 23.9 23.9 23.4 21.6 24.0

– x 22.4 22.1 21.5 21.6 22.4 1X-section divided into whitewood and darkwood for boards containing both types of wood. 2Individual X-sections used for average, shell/core, whitewood/darkwood.

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