Jensen, C. E.

Kotok, E. S.

Lowery, David P.

Western Dry Kiln Clubs

Western Dry Kiln Clubs. Meeting (20th : 1969 : Redding, Calif.)

Undetermined

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SURFACE TEMPERATURE AS AN INDICATOR OF WOOD MOISTURE CONTENT DURING DRYINGDr. David P. Lowery.Wood TechnologistU. S. Forest ServiceIntermountain Forest and RangeExperiment StationMissoula, MontanaMr. E. S. ICotok, PrincipalWood TechnologistU. S. Forest ServiceIntermountain Forest and RangeExperiment StationMissoula, MontanaMr. C. E. Jensen, PrincipalStatisticianIntermountain Forest andRange Experiment StationOgden, UtahABSTRACT Average thickness, specific gravity, initial moisture content, elapsed time in oven, and surface tempera-ture data were obtained for short sections of ponderosa pine board dried in a forced-air circulation oven. Thesedata were subjected to a multiple regression analysis to determine their value as predictors of average woodmoisture content. Results of the analysis showed that a model including surface temperature and elapsed timehad an R2 value of 0.89. From this, it appears that surface temperature and time could be used effectively formonitoring the moisture content of wood during drying. Infrared sensing devices promise easy measurement ofsurface temperature in such processes.INTRODUCTION The drying of wood from the green condition has been categorized into two drying periods, exclusive of theinitial warmup interval. In the first period--the constant-rate period--the drying rate per unit of drying surfaceis constant. This is succeeded by a falling-rate period in which the water removal rate is gradually deceased.The wood moisture content at the point of change from a constant-rate to a falling-rate period has been termedthe critical moisture content. During the constant-rate period, free water is evaporated. During the falling-rateperiod, bound water and water vapor are moved from the interior of the wood to the surface.When the water evaporates from a wood surface, the heat, of vaporization cools the surface. The coolingeffect is relatively uniform during the constant-rate period but tends to decrease during the falling-rate period.Because of this predictable change in cooling, it has been hypothesized that a relation exists between the moisturecontent of a piece of wood and its surface temperature (fig. 1).If wood surface temperature can be used as the control factor in the forced-air drying of individual piecesof lumber, prospects for the development of automatic, continuous drying processes will be enhanced..The development of such a drying system is the objective of research being conducted by the IntermountainForest and Range Experiment Station scientists stationed at the Forestry Sciences Laboratory in Missoula, Montana.Reported here are the results of one phase of this broad research effort--in which the strength of wood moistureprediction models was explored. Independent variables included: surface temperature, initial moisture content,specific gravity, wood thickness, and elapsed time.PROCEDURE Two select grade 1-inch by 6-inch by 18-foot rough green ponderosa pine boards were chosen at the greenchain of a Missoula, Montana, sawmill. Both boards were flat-grained sapwood and were selected sufficientlyfar enough apart in the pile to insure that they had been cut from different logs.The boards were cut into sections 3 inches long; any sections containing defects were discarded. All sectionswere identified as to board and maintained in a green condition until used by immersing them in water. At thetime of testing, five sections from each board were removed from the tank, numbered consecutively, end-coatedwith an asphalt paint, and weighed. Seven replications were made using material from each board. Thus, atotal of 70 sections was dried.-6-Test specimens obtained from the first board had a mean thickness of 0. 941 inch, range 0. 138 inch; meanspecific gravity (green volume-ovendry weight) of 0. 398, range 0. 058; and mean initial moisture content of 161percent, range 44 percent. For the second board the mean thickness of the test specimens was 0. 964, range0.095 inch; mean specific gravity (green volume-ovendry weight) 0. 347, range 0.041; and mean initial moisturecontent 202 percent, range 52 percent.A 4-mil copper-constantan thermocouple was inserted at the approximate center of the tangential face ofeach section as close to the surface as possible. The ideal procedure would have been to have the thermocouplehalf embedded and half exposed on the surface. But this was impossible; so a small cut, about 1/8-inch deep by1/4-inch long, was made in each section surface using a scalpel, and the thermocouple was carefully inserted inthe cut.The sections were then placed on two wood strips alined crosswise in a forced-air circulation drying oven,which had been preheated to 180° F. The sections were thus positioned on edge at right angles to the airflowso that the air flowed over the specimen surfaces. The airspeed on the specimen surfaces was about 150 feet perminute.The surface temperature of the sections and the oven temperature were recorded at 10-minute intervalsduring the drying period. As each section attained its assigned temperature it was removed from the oven andweighed. Sections were removed at surface temperatures of 120°, 130°, 140°, 150°, and 160° F.After weighing, the sections were ovendried in a second oven in which the temperature was maintainedat about 220° F. The initial as well as the percent moisture content at the time the sections were removed fromthe first oven were then calculated. End-coating had a negligible effect on the percent moisture content values.The average thickness of each section then was determined by averaging three measurements made with amicrometer caliper.A 1-inch cube was next cut from the approximate center of each test section and used for specific gravitydetermination. All cubes were impregnated with water to return them to a green condition so that volume couldbe determined by water immersion. The cubes were then ovendried and the specific gravity calculated.Finally, the data for all sections--average thickness, specific gravity, initial moisture content, elapsed timein the oven, surface temperature, and the dependent variable (moisture content at time of removal from oven)were subjected to multiple regression analysis.The algebraic form of the input variables was determined using two- and three-dimensional trends thatappeared to be present in the data. Moisture content was expressed as a function of every combination of theindependent variables and their transforms. The strength of each of these regression components then was evalv-ated on the basis of its contribution to R2 in the full array of models thus assembled. Statistical tests of com-R2ponent contributions to R were not attempted in this complete-screening analytical approach, but increases of0.05 in R2 were tentatively regarded as real. Expected and finally adopted forms for the moisture content (M)and the independent variables were as follows:1. Moisture content (M) vs. elapsed time (E) and board thickness (). --Expected relations are shown infigure 2 but neither a curvilinear main effect of T nor an interaction T-effect appeared to be present in thedata--probably because of the very small range of T in the wood sections tested (range = 0. 16 inch, with mean =0. 94 inch). As a result, T was admitted to analysis only as an independent linear effect. Values for E ranged upto 330 minutes. The E effect was well characterized by (5-0.01E) 1 ' 5 and this form was used throughout theanalysis.2. M vs. E and specific gravity (G). --Expected relations are shown in figure 3 but, again, neither acurvilinear main effect of G nor the expected interaction was apparent in the data. The range of G was 0. 11 witha mean of 0.37. So G also was admitted to analysis only as an independent linear effect.-7-3. M vs. E and initial moisture content (I ). --An interaction of the two variables was readily apparent inthe data. The I-effect was linear--which possibly can be attributed to the fairly narrow range of I encountered(range = 74 percent with a mean = 161 percent). The surface was well characterized by the form (5 -0. 01E) 1. 5(I). The expected relation between moisture content and elapsed time for three different initial moisture contentlevels is similar to that shown in figure 2.4. M vs. E and surface temperature (S).  --A curvilinear main effect of S appeared to be present in the data,but an interaction was not. S, maximum 180° F. , was admitted to the analysis only as an independent lineareffect. The expected relation between moisture content and elapsed time for three surface temperature levels issimilar to that shown in figure 3.RESULTS AND DISCUSSION In combination with elapsed time, surface temperature and specific gravity proves to be, strong contributorsto the model in which moisture content was estimated.The analysis showed that surface temperature alone was not a strong estimating base (R2 = 0.68 for moisturecontent). For elapsed time alone, the R2 was 0. 84. An additive model combining surface temperature andelapsed time improved the R2 to 0. 89. By including surface temperature, elapsed time, and specific gravity inthe model, the R2 value was still further improved to 0. 95. Only by adding initial moisture content could thestrength of the estimating equation be further increased (table 1). The last model must be rejected, however,because it is impossible with the present state of technology to determine the initial moisture content beforedrying. Too, the model involving specific gravity must be discarded for the present, since specific gravity isdifficult to measure.The most practical model, then, includes only two variables--surface temperature and elapsed time. Thismodel is shown in figure 4 and a few of the estimated values are shown in table 2.Table 1.	 Independent variables and their combinations with corresponding R2 values.Variable R2Surface temperature (S) 0.68Elapsed time (E) . 84S	 E .89S + E + specific gravity (G) . 95S + E + initial moisture content (I) . 96E + I .96The strength of the relationship would probably have been weaker if the test material had not been so uni-form. Ponderosa pine was the only species studied, but other softwood species could be expected to behave in asimilar manner. Exploratory tests with Douglas-fir confirm this.Surface temperatures, however, can be measured fairly easily. It is difficult to visualize thermocouples,as used in this study, being used in commercial drying. But it has been demonstrated that infrared sensing in-struments are easy to use and are accurate for measuring surface temperatures. This greatly enhances the in-dustrial potential of the surface temperature-elapsed time method of estimating moisture content.A frequent but not always reproducible plateauing of surface temperature appeared to be associated with the-8-Surface temperaturei.F)**********44****,_— Moisture content( percent)•••••••• **00000000000000000000000000000I	 I	 I	I	 I 0	 50	 100	 150	 200 250	 300 350 400 453E lapsed time ( minutes )Figure 1. Typical graph from another study showing the relationbetween surface temperature-elapsed time and averagepercent moisture content-elapsed time. The flat-sawnponderosa pine specimen, 19 inches long, was driedbelow the fiber saturation point at 230° F and 300 feetper minute airspeed.Low	 High= 126.4 +10 514 (5-.01 E )" — .72581(5),50 s E S 450120 sS 180RI. .89, 0.70,5 OM%7•11Standard Gifford moan residual =13.—1501000HighElapsed timeFigure 3. Expected relation between moisture content and elapsedtime for three different levels of specific gravity.Elapsed timeFigure 2. Expected relation between moisture content and elapsedtime for three thiclaiesses.•a.20003s120	 140	 160	 180Surface Temperature (S) in °F.Figure 4. Three-dimensional model showing relation between sur-face temperature and elapsed time for a specific gravitylevel of 0. 450.Highconstant rate period (Figure 1). Investigation of this characteristic may establish a technique for deciding when toshift drying conditions so that optimum conditions can be employed for both constant-rate and falling-rate dryingperiods.Table 2. Predicted values for moisture content ' based on elapsed time, surface temperature, and specific gravity. 2SurfacetemperatureElapsed time (minutes)(°F) 50 100 150 200 250 300 350Percent120 120 101 84 68 53 40 28130 114 96 79 63 48 35 23140 109 91 74 57 43 29 18150 103 86 68 52 37 24 12160 99 80 63 47 32 19 7170 93 75 58 42 27 14 1180 88 70 52 36 22 8 01 Drying temperature was 180o F and airspeed was 150 feet per minute.2 The specific gravity value used was 0. 450.CONCLUSIONS A fairly reliable estimate of wood moisture content during drying can be made by using surface temperatureand elapsed time. It is important to be able to measure initial moisture content and specific gravity easily--andthere is need for further research on this problem. Although only ponderosa pine was used in this study, tentativetests with Douglas-fir indicated that the study results should have general application to other softwood species.A wood-drying monitoring system involving surface temperature appears promising.

Surface temperature as an indicator of wood moisture content during drying

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Surface temperature as an indicator of wood moisture content during drying

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