Measurements of surface emission factors in wood drying

D)l

D

Anders Samuelsson, Ove Söderström

Measurements of

Surface Emission Factors

in Wood Drying

Paper presented at 4th lUFRO International

Wood Drying Conference ^Improving Wood

Drying Technology^\ Rotorua, New Zealand,

August 9-13,1994

Trätek

Anders Samuelsson, Ove Söderström

MEASUREMENTS OF SURFACE EMISSION FACTORS IN WOOD DRYING Trätek, Rapport I 9409046 ISSN 1102- 1071 ISRN TRÄTEK - R - - 94/046 - - SE Nyckelord drying sorption surface emissions Stockholm september 1994

Rapporter från Trätek — Institutet för träteknisk forskning — är kompletta sammanställningar av forskningsresultat eller översikter, utvecklingar och studier. Publicerade rapporter betecknas med I eller P och numreras tillsammans med alla ut-gåvor från Trätek i löpande följd.

Citat tillätes om källan anges.

Reports issued by the Swedish Institute for Wood Technology Research comprise complete accounts for research results, or summaries, surveys and

studies. Published reports bear the designation I or P and are numbered in consecutive order together with all the other publications from the Institute. Extracts from the text may be reproduced provided the source is acknowledged.

Trätek — Institutet för träteknisk forskning — be-tjänar de fem industrigrenarna sågverk, trämanu-faktur (snickeri-, trähus-, möbel- och övrig träför-ädlande industri), träfiberskivor, spånskivor och ply-wood. Ett avtal om forskning och utveckling mellan industrin och Nutek utgör grunden för verksamheten som utförs med egna, samverkande och externa re-surser. Trätek har forskningsenheter i Stockholm, Jönköping och Skellefteå.

The Swedish Institute for Wood Technology Re-search serves the five branches of the industry: sawmills, manufacturing (joinery, wooden hous-es, furniture and other woodworking plants), fibre board, particle board and plywood. A research and development agreement between the industry and the Swedish National Board for Industrial and Technical Development forms the basis for the Institute's activities. The Institute utilises its own resources as well as those of its collaborators and other outside bodies. Our research units are located in Stockholm, Jönköping and Skellefteå.

SAMMANFATTNING

En ny experimentell tork har byggts för att mäta diffusionskonstanten för fukt i trä och ytövergångsmotståndet mellan en träyta och omgivande luft. I torken detalj studeras en enda bit under förhållanden som liknar de som finns i en industriell virkestork. Lufthastigheten, temperaturen och luftfuktigheten kan varieras godtyckligt.

Mätningarna utfördes som sorptionsexperiment, där en konstant luftfuktighet sänks hastigt till en lägre konstant nivå. En MDF-skivas fuktavgång studerades som funktion av tiden. Analysen gjordes med en diffusionsmodell inkluderande ett ytövergångsmotstånd med en ny metod, som har publicerats tidigare.

Mätningarna visar att fuktavgången har en eftersläpning i början, som tolkas som en effekt av ett ytövergångsmotstånd. Vidare visar mätningarna att tillnärmelsen till jämvikt sker långsammare än förväntat teoretiskt sett.

MEASUREMENTS OF SURFACE EMISSION FACTORS IN

WOOD DRYING

A. SAMUELSSON and O.SÖDERSTRÖM

Trätek, Swedish Institute for Wood Technology Research, P.O. Box 5609 S-114 86 Stockholm, Sweden, and Royal institute of Technology,

Building Material, S-100 44 Stockholm, Sweden

ABSTRACT

An experimental kiln has been built to render measurements of surface emission factors and diffusion coefficients possible during wood drying. The kiln is a one-piece kiln for measurements on one specimen in each experiment. The design of the kiln has been made in a way that the air stream in the measuring zone is similar to the conditions in a real kiln. The air velocity, the temperature and the humidity are infinitely variable in time.

The measurements were performed as sorption experiments, a constant humidity abruptly being changed isothermally to a lower constant humidity. The moisture content response of an MDF-board is measured over time. The data sets are interpreted as a simple diffusion model with a surface emission factor included. The theoretical analysis follows a method that has been published earlier.

The experimental data show that the relative changes of the moisture content have an initial time lag that can be interpreted as an effect of a surface emission factor. There are also indications that the approach to equilibrium is significantly slower than can be expected from a simple diffusion model.

INTRODUCTION

The main problem for wood drying is to choose a suitable drying schedule that gives a short drying time and a high drying quality. In Sweden fairly low temperatures are used in general and surface checking is the primary cause for quality loss. Surface checking is caused by the moisture content gradient when the surface moisture content goes below the fibre saturation point and the wood starts to shrink. Therefore, to obtain a high quality yield in the wood drying process, it is necessary to control the development of the surface moisture content. For that purpose it is necessary to study the interaction between the surface and the ambient air. Below the fibre saturation point the internal moisture transport can be well described with a simple diffusion model with a constant diffusion coefficient and a constant surface emission factor. The latter are also called the mass transfer coefficient and couples the moisture flux from the surface and the difference between the surface moisture content and the equilibrium moisture content of the ambient air.

The introduction of the surface emission factor in connection with wood drying was made long ago. A complete list of references of earlier works is found elsewhere (Söderström and Salin 1993). The problem has also been studied in a recent experimental work (Morén et al

1992, Wadsö 1993). The concept of surface emission factor has also been questioned in a letter (Kayihan 1993).

The main object of the present work is to perform sorption experiments, to analyse the observations in a mathematically correct manner and, finally, to interpret the results with a simplest possible model. The practical impact for industrial purposes in the future is to see if it is possible to control the surface moisture content during a complete drying process.

T H E O R Y

The moisture transport in wood below the fibre saturation point can be described with a simple difflision model with the moisture content as the potential. The diffusion equation can be written:

^ = (1)

where u is the moisture content, t the time, D the diffusion constant and x the space coordinate. The initial condition is given by:

u = , V X e[0,a] w h e n t = 0 (2)

where a is half the material thickness. The boundary condition is given by:

= S(u. - (3)

where u^^is the equilibrium moisture content for wood in the ambient air. The relative mean moisture content is defined by:

(4)

where u is the mean moisture content. The quantity F is determined by continuous registration of the weight over time. By plotting F vs. \ft an initial straight line is expected if the process is solely diffusion. An initial time lag is interpreted as the existence of a surface emission factor. F starts from zero and approaches 1.0 in time. The time when F=0.5 is denoted to 5 and this quantity is connected to D and S by:

^0.5 0.2 0.7

(Söderström and Salin 1993). This means that some sorption experiments can be performed with different thicknesses. The quantity to.j/a^ is plotted vs. 1/a. The intercept is 0.2/D and the inclination 0.7/S. Eq.(5) is an approximative formula, but can be used as a first attempt. Further mathematical details can be found elsewhere (Söderström and Salin 1993).

E X P E R I M E N T Kiln design

The experiments have been performed in a recently built kiln. The kiln was built for experiments with just one specimen in each experiment. The intention is that the air velocity and climate around the specimen should be similar to the conditions in a real kiln at a sawmill. Figure 1 shows the construction of the kiln. The climate in the kiln is created with an electric heater, a steam generator and vents. Before the air stream gets to the specimen it travels through a channel with a height of 25 mm and a length of 3000mm. That height is similar to the thickness of common stickers in industrial kilns in Sweden. The specimen is hanging in a construction that is connected to a balance which in turn is connected to the control system of the kiln. The climate probes are placed close to the surface of the specimen. The probes consist of wet- and dry-bulb temperature gauges which are used for control of the climate in the kiln.

measuring zone connection to balance

air stream specimen

71

Figure 1. Principle sketch of the experimental kiln.

Test material

The inside width of the measure zone in the kiln is 1015 mm. That gives a maximum length of the specimen of 1005 mm. Four different thicknesses were used for the specimens, 27, 31, 35 and 46 mm, all with a length of 940 mm and width 150 mm excluding insulation. All specimens where taken from one MDF (Medium Density Fibre) board with an average density of670 kg/m^. To ensure that the MDF had a homogeneous density the test material was scanned with a gamma-ray density meter. The result showed that the surface layer had a much greater density than the interior of the specimen. Those layers were cut off which resulted in a material with a homogeneous density.

Ail specimens were insulated on the four edges against moisture and heat so that the moisture will evaporate from the same areas, i.e. the flat sides, as the heat is transferred into the MDF piece. The insulation material against heat was cellular plastic with a thickness of 30 mm and the insulation against moisture was a layer of silicone.

cellular plastic silicone

MDF

Figure 2. The design of the specimen.

Measurements

The measurements were performed as sorption experiments. The first phase was conditioning in a climate chamber at 56 °C dry-bulb temperature and 55 "C wet-bulb temperature for at least three weeks. After the conditioning the specimen was quickly moved from the climate chamber to the kiln. The climate in the kiln was 56.0 °C dry-bulb temperature and 47.0 °C wet-bulb temperature. All temperatures were controlled within ± 0.2 "C. The average time for moving the specimens from climate to climate was less than one minute. The equilibrium moisture contents for MDF in the used climates were tested with thin samples of MDF with the dimensions 100 x 80 x 2 mm. Those samples were kept in the climates for at least one week. During all experiments the average air velocity was 3.0 m/s, which is normal in industrial kilns. Immediately after the specimen had been placed in the kiln the weight signal from the balance was logged every 10 seconds within the first 15 minutes. After the first 15 minutes the logging interval was gradually increased to 15 minutes after 12 hours.

RESULTS

The measured parameter is the weight of the four specimens as a ftinction of time. With the measured equilibrium moisture content for MDF in the two used climates, the relative change in moisture content F can be calculated as described in Eq. (4). The F-value for all four specimens are plotted in Figures 3 and 4vs./t.

1,0 -1 0.9-1,0 0.9 0,8- 0,8 0.7- 0,7 0,6- 0,6 fa 0,5- ta. 0,5 0,4- 0,4 0,3- 0,3 0,2 • 0,2 0.1 • 0.1 0.0-^ 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0,0 -Figure 4. 200 400 600 200 400 600

Figure 3. F vs. v// for MDF. Left: thickness 27 mm. Right: thickness 31 mm.

1,0-0,9 1 1.0-r 0,9--0,8 \ 0,8--0,7 • 0,7 •-0,6 - 0,6--fa 0,5 - fa 0,5 • • 0,4- 0,4 • • 0,3- 0,3--0.2- 0,2 •• 0,1 • _{0.1 •} 0,0-0,0 + 0 200 400 600 0 200 400

F vs. i for MDF. Left thickness 35 mm. Right: thickness 46 mm.

600

The time to 5 is evaluated with the help of the four diagrams in Figures. 3 and 4. These four to.5-values are used to plot to.j/a^ vs. 1/a which makes it possible to calculate D and S with the help of Eq. (5). This has been described eariier (Söderström and Salin 1993). In Figure 5 to.5 /a^ is plotted vs. 1/a for all four specimens. A linear regression analysis of these values results in Eq. (6).

— = 506300 • - * 73130000 1^=0.99 ₍₆₎

With the intercept (1/a = 0) for Eq. (6) the diffusion coefficient is calculated with the help of Eq. (5). This gives the diffusion coefficient for MDF to 2 .7 • 10'^ mVs. The inclination of Eq. (6) gives the surface emission factor S according to Eq. (5). The S-value for MDF was calculated to 1.4- lO'^m/s.

1.20E+08 1.10E+08 4-E 1,0O4-E+08 • • 03 9,00E+07 8,00E+07 4-7,00E+07 20 40 60 1/a [m 80 100

Figure 5. to j/a^ vs. I/a for MDF with four thickness: 27, 31, 35 and 46 mm.

With the measured values for D and S the relative moisture change F is calculated as described by Söderström and Salin (1993). The calculation of F is made for the specimen with a thickness of 27 mm. Those calculated values are in Figure 6 compared with the measured values.

0,9 + 0,8 +

0,6 +

200 400 600

Figure 6. F vs. /t for MDF board specimen with a thickness of 27 mm. The smooth line is calculated values based upon the measured diffusion D and surface emission factor S. The dotted line is measured values.

DISCUSSION

The main sources of errors in a sorption experiment are the climate control and the step-wise change of climate. The climate is controlled with the wet and dry bulb thermometer method which is a reliable method. The equilibrium moisture contents of the initial and final climates are registered by way of thin sheets that will reach to equilibrium fast.The step-wise change of climate is realized manually within less than one minute, which is shorter than the times involved in the sorption process. The determination of S and D with Eq. (5) from Figure 5 is approximate, because L=Sa/D is between 6.8 and 11.6 and the used equation is not completely linear. (Söderström and Salin 1993). The determination of the diffusion constant with the intercept is only influenced with less than 2% and can consequently be neglected. However, the whole method presupposes that the diffusion constant is really constant and, consequently, independent of the moisture content. Figure 6 shows that the wood sample approaches equilibrium more slowly than could be expected from the diffusion theory. The determination of the surface emission factor with the inclination gives a value with an accuracy of about 10%. This means that the S-value is

1.3-1.5-10-^ m/s.

CONCLUSION

In Figure 6 the initial time lag is observed and the simplest interpretation of that is the existence of a surface emission factor. However, the conception of a surface emission factor has been subject of a discussion in the scientific press, but no alternative theory has been presented so far (Kayihan 1993). With the present value of the surface emission factor the practical consequence is that the surface moisture content deviates considerably from the equilibrium moisture content of the ambient air and this will have an impact on the choice of drying schedule. The value of the surface emission factor that is obtained in this work is much less than the value obtained with the boundary layer theory. Further research, e.g. with different air velocities, will hopefiilly clarify this question, and such research activities are underway.

R E F E R E N C E S

KAYIHAN,F. 1993: Letter to the Editor. Drying Technology J 1(6): 1467-1469. MORÉN,T.; SALIN,J.-G.; SÖDERSTRÖM,0. 1992: Determination of the Surface

Emission Factors in Wood Sorption Experiments. 3rd lUFRO International Wood Drying Conference,"Understanding the Wood Drying Process", Vienna,

August 18-21,1992: 69-73.

SÖDERSTRÖM,0.; SALIN,J.-G. 1993: On Determination of Surface Emissions Factors in Wood Drying. Holzforschung 47: 391-397.

WADSÖ,L. 1993: Studies of Water Vapour Transport and Sorption in Wood. Doctoral Dissertation, Report TVBM-1013 Building Materials, Lund University.

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