The equilibrium moisture content (EMC) of Russian larch wood, Sugi wood, and Hinoki wood was measured under vacuum conditions at temperatures of 45, 50, and 60°C and ambient pressures of 13.3, 53.3, and 101.3 kPa. The results show that the EMC of each species increased with a decrease in ambient pressure. The effect of temperature and RH on EMC under vacuum conditions showed a similar tendency. Wet-bulb temperature needed to be controlled to measure EMC, even under vacuum, because pressure was not maintained only by water vapor pressure because of the presence of air in the vessel. There were obvious differences between the EMC values obtained in this experiment and previous experimental EMC values in which the wet-bulb temperature was not controlled

Cai, Yingchun

Hayashi, Kazuo

Liu, Honghai

Sugimori, Masatoshi

Yang, Lin

English

Wood and Fiber Science (E-Journal)

EFFECT OF AMBIENT PRESSURE ON EQUILIBRIUM MOISTURECONTENT OF WOODHonghai LiuDoctoral StudentLin YangDoctoral StudentDepartment of Forest ResourcesFaculty of Agriculture, Ehime UniversityMatsuyama 790-8566, JapanYingchun CaiProfessorKey Laboratory of Bio-based Material Science and TechnologyNortheast Forestry UniversityHarbin 150040, P.R. ChinaMasatoshi SugimoriAssociate ProfessorKazuo Hayashi*ProfessorDepartment of Forest ResourcesFaculty of Agriculture, Ehime UniversityMatsuyama 790-8566, Japan(Received February 2010)Abstract. The equilibrium moisture content (EMC) of Russian larch wood, Sugi wood, and Hinokiwood was measured under vacuum conditions at temperatures of 45, 50, and 60C and ambient pressuresof 13.3, 53.3, and 101.3 kPa. The results show that the EMC of each species increased with a decrease inambient pressure. The effect of temperature and RH on EMC under vacuum conditions showed a similartendency. Wet-bulb temperature needed to be controlled to measure EMC, even under vacuum, becausepressure was not maintained only by water vapor pressure because of the presence of air in the vessel.There were obvious differences between the EMC values obtained in this experiment and previousexperimental EMC values in which the wet-bulb temperature was not controlled.Keywords: Equilibrium moisture content (EMC), ambient pressure, vacuum drying, wet-bulbtemperature.INTRODUCTIONThe moisture content of wood in equilibriumunder constant temperature and RH is termedequilibrium moisture content (EMC). AlthoughRH and temperature are the principal factorsthat determine EMC, it is also affected by sorp-tion processes, mechanical stress, species, andextractive content (Skaar 1988). EMC underatmospheric pressure has been presented in aunified description by Kollmann (1968) andserves well in wood research and wood manu-facturing, especially in conventional wood dry-ing. In the last decade, vacuum drying as a rapiddrying method (Simpson 1987) has beensteadily increasing for valuable species and* Corresponding author: hayashi@agr.ehime-u.ac.jpWood and Fiber Science, 42(3), 2010, pp. 346-351# 2010 by the Society of Wood Science and Technologythose prone to collapse from the relativelyhigher temperatures in conventional wood dry-ing and is likely to continue increasing in thefuture. For vacuum drying, EMC conditions stillneed to be controlled to perform the drying run(Chen and Fred 2002). However, few data onEMC for vacuum drying have been reported inthe literature. Although EMC under vacuum wasreported by Chen and Fred (2002) and Yi et al(2008) based on the assumption that there is noair present in the chamber during vacuum dry-ing, the results obtained showed obvious differ-ences from Kollmann’s EMC as well as eachother. Furthermore, the graph they derived wasthe relationship among temperature, ambientpressure, and EMC rather than the relationshipamong temperature, ambient pressure, RH, andEMC. Hence, only Kollmann’s EMC can beused for moisture content monitoring underradiofrequency/vacuum drying based on a newconcept (Cai and Hayashi 2007) using detectedtemperature and pressure in wood. Therefore,with the development of vacuum drying proce-dures, EMC data with sufficient accuracy undervacuum condition are more important for woodvacuum drying in theory and practice.Although it was assumed that there was no air inthe chamber during vacuum drying in theresearch by Chen and Fred and Yi et al, someair does, in fact, enter the chamber, resulting intotal pressure differing from partial vapor pres-sure. The condition of the chamber under vac-uum is also governed by temperature and RH asatmospheric conditions. The definition of RHunder vacuum is the same as that under atmo-spheric conditions, which is the ratio of the par-tial vapor pressure to the saturated pressure for agiven temperature (Siau 1995). Xiao and Cai(2009) investigated the factors affecting RH dur-ing wood vacuum drying and found that RH wasaffected slightly by ambient pressure. Effects onRH by ambient pressure can also be found inequations for RH calculations. Hence, to main-tain correct RH, ambient pressure must be con-sidered as indicated by the equations. Generallyspeaking, from the isothermal process, RHdecreases with decreasing pressure based on thepremise that pressure is maintained only bywater vapor. In fact, there is air in the chamberas explained previously in this article. For acertain ambient pressure of the chamber, RHdepends on the extent of water vapor pressureand not on total pressure. Because the evapora-tion of water in wood is governed by tempera-ture and humidity, RH as well as temperaturemust be considered to determine EMC of woodeven under vacuum conditions. Therefore, inthis study, the objective was to investigatethe effect on EMC of ambient pressure whilemaintaining a constant RH throughout the testby adjusting the wet-bulb temperature and toestablish the relationship among EMC, tem-perature, RH, and ambient pressure based onthree softwoods under various ambient pressureconditions.MATERIALS AND METHODSMaterialsMaterials used were Russian larch (Larixgmelinii, 510 kg/m3 basic density, 48% initialMC), Hinoki (Chamecyparis obtusa, 400 kg/m3basic density, 37% initial MC), and Sugi (Cryp-tomeria japonica, 320 kg/m3 basic density, 90%initial MC). Each species was processed into 25end-matched EMC test samples with dimensionsof 5 (L)  30 (T)  30 (R) mm.MethodsAs shown in Table 1, the temperature ranges andambient pressure ranges were 45, 50, and 60Cand 13.3, 53.3, and 101.3 kPa, respectively. Foreach condition, the temperature and RH wereheld constant throughout the test. Intermediatemass within a 3-da interval was measured inevery pressure condition and the specimens wereconsidered at EMC while the mass differencewas within 2 mg between the last two intervals.First, five samples of each species were subjectedto an ambient pressure of 13.3 kPa until all thesamples reached EMC at this pressure. After thesamples were weighed, the ambient pressure waschanged to and held at 53.3 kPa until all theLiu et al—EQUILIBRIUM MOISTURE CONTENT OF WOOD UNDER VACUUM 347samples again reached EMC. After the sampleswere reweighed, the pressure was changed to andheld at 101.3 kPa until all the samples reachedEMC under atmospheric pressure. Finally, afterweighing, all the samples were dried in the ovenat 103  2C for 24 h.The pressure inside the chamber was measuredwith a diaphragm pressure gauge. The pressurewas maintained to within 0.26 kPa of the con-trolling value by the vacuum pump. The internalRH was controlled using dry- and wet-bulb tem-peratures. To control the wet-bulb temperature,the chamber was equipped with a steam genera-tor, condenser, and cooling pipe, and an aircirculation fan was used to equalize the temper-ature and humidity. A variable-frequency motorwas used to ensure the airflow velocity over thewet-bulb probe was over 1 m/s at various ambi-ent pressure conditions.RH Calculation for Vacuum ConditionsThree equations were used to calculate RHunder vacuum conditions (JSME 1959; Carpen-ter 1982; Yan and Wang 2004). The calculatedvalues using these equations were almost thesame, therefore, the simplest, Eq 3, was chosenfor the RH calculation. The equations show thatRH is affected by ambient pressure, therefore, tocontrol the designed test conditions, the wet-bulb temperature was adjusted as shown inTable 1. For example, to obtain the condition of45C and 60% RH under pressures of 13.3 kPa,53.3 kPa, and 101.3 kPa, the wet-bulb tempera-ture should be adjusted from 35.6 to 36.3C andthen to 37.0C, respectively.f1¼0:001Pw  k 0:001P755ðt twÞ0:001Ps 100% ð1Þf2 ¼ 100P Aw  Dð ÞðAP ðA Aw þ DÞPsÞ  100% ð2ÞA ¼ 1555:6 1:151tþ 0:00013t2 2:604twÞ PsP PsAw ¼ 1555:6 1:453twþ 0:00013t2w  Ps twð ÞPPs twð ÞD ¼ 1:002 t twð Þ þ 0:00005 t2  t2w Table 1. Equilibrium moisture content test conditions,controlling parameters, and calculated RH under variousambient pressures.Test conditions Controlling parametersCalculatedRHt (C) RH (%) Pressure (kPa) tw (C) Ps (kPa) Pw (kPa) F (%)45 60 13.3 35.6 9.535 5.779 60.153.3 36.3 9.535 6.009 60.1101.3 37.0 9.535 6.247 60.145 50 13.3 32.4 9.535 4.827 49.953.3 33.5 9.535 5.135 50.0101.3 34.5 9.535 5.433 49.950 60 13.3 40.1 12.221 7.398 60.253.3 40.7 12.221 7.639 60.2101.3 41.3 12.221 7.885 60.150 40 13.3 33.0 12.221 4.993 40.153.3 34.4 12.221 5.402 40.1101.3 35.7 12.221 5.811 40.160 50 13.3 45.9 19.841 9.969 50.153.3 46.5 19.841 10.324 50.1101.3 47.3 19.841 10.723 50.1t, dry-bulb temperature; RH, designed RH; tw, wet-bulb temperature; Ps,saturated vapor pressure at the dry-bulb temperature; Pw, saturated vaporpressure at the wet-bulb temperature; F, calculated RH by Eq 3.Psðt0cÞ ¼ 0:022064 exp7:2148þ 3:9564 0:745 tþ 273:15647:14 2þ 1:3487 0:745 tþ 273:15647:14 3:17782666437775 1647:14tþ 273:15 8>><>>:9>>=>>;348 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)f3 ¼Pw  ðP PwÞðt twÞ1546 1:44twPs 100% ð3Þwhere j is RH (%), Pw is saturated vapor pres-sure at the wet-bulb temperature (kPa), P isambient pressure (kPa), t is dry-bulb tempera-ture (C), tw is wet-bulb temperature (C), Ps issaturated vapor pressure at the dry-bulb temper-ature (kPa), and k is the air coefficient with avalue of 0.6 under vacuum.ApparatusThe vacuum chamber with an inside dimensionof 0.8  0.8 1.1 m (Yashijima Co, Ltd) shownin Fig 1 was used for the EMC test under vac-uum conditions and consists mainly of a pres-sure control system, steam generator, and onlinemonitoring system for dry- and wet-bulb tem-perature measurement. The performance of thisvacuum chamber such as precise dry- and wet-bulb temperature measurement, wind velocityadjustment under different vacuum conditions,etc, was tested by Myojin et al (2006) and fullymet the requirements of the experiment. An elec-tronic balance (METTLER AJ100) with a preci-sion of 0.1 mg was used to measure the weight ofEMC samples.RESULTS AND DISCUSSIONEMC of Wood Under Vacuum ConditionsThe EMC values of Larch, Hinoki, and Sugiunder various vacuum conditions are presentedin Table 2 and Fig 2 to show the observedtrends. Figure 2 clearly shows that the effect ofambient pressure on EMC for the three specieswas similar under the same temperature andRH conditions. That is, EMC increased with adecrease in ambient pressure. Table 2 showsthat the difference in EMC between 13.3 and53.3 kPa was between 1.2 and 1.9%, while thedifference in EMC between 53.3 and 101.3 kPawas only 0.1-0.4%. This shows that the extent ofthe effects was significant at low ambient pres-sure (13.3 kPa) compared with the effects atmedium ambient pressure (53.3 kPa), and ambi-ent pressure is one of the factors affecting EMCin addition to temperature and RH. On the otherhand, EMC of the three species in Table 2 atatmospheric pressure (101.3 kPa) for varioustemperatures and RH was different from thevalue in Kollmann’s EMC chart for Sitkaspruce, which shows EMC is affected by speciesas mentioned before.EMC at atmospheric pressure increases withincreasing RH and decreases with increasingFigure 1. Schematic diagram of vacuum drying testchamber for equilibrium moisture content test. (1) Leakvalve; (2) manometer; (3) condenser; (4) hot water tank;(5) heating pipe; (6) circulating fan; (7) vacuum pump; (8)steam generator (9); samples; (10) dry- and wet-bulb tem-perature sensors; (11) dehumidifier; (12) water collector;(13) control cabinet.Table 2. Equilibrium moisture content of Russian larch,Hinoki, and Sugi under various conditions.aSpecies Pressure (kPa)Temperature (C)/RH (%)45/60 45/50 50/60 50/40 60/50Russian Larch 13.3 11.6 10.3 10.8 8.6 9.253.3 9.7 8.7 9.3 6.8 7.5101.3 9.6 8.6 9.0 6.4 7.1Hinoki 13.3 10.5 9.9 10.2 7.2 8.753.3 8.9 8.2 8.7 6.0 7.0101.3 8.8 8.0 8.6 5.7 6.5Sugi 13.3 10.1 9.2 9.8 7.5 8.553.3 8.9 8.0 8.6 6.2 7.1101.3 8.6 7.6 8.5 5.8 6.5Kollmann 101.3 9.7 8.2 9.5 6.5 7.3a Kollmann’s equilibrium moisture content was for Sitka spruce.Liu et al—EQUILIBRIUM MOISTURE CONTENT OF WOOD UNDER VACUUM 349temperature (Skaar 1988). Figure 2 and Table 2show that, under vacuum and at any ambientpressure, EMC has the same tendency for allthree species. That is, at the same temperaturefor a given ambient pressure, EMC increasedwith an increase in RH, and at the same RH for agiven ambient pressure, EMC decreased with anincrease in temperature. The degree of effect wasnot significant at the three ambient pressures.Comparison of EMC to Previous ResearchA comparison of the EMC obtained by Yi et al(2008) to the EMC determined for Russian larchwood, Hinoki, and Sugi in this experimentshows differences between the two sets of pres-sure values (see Fig 3). The EMC obtained byYi et al increased with an increase in ambientpressure—even EMC in this experiment dif-fered between species at any given ambientpressure—while the EMC in this experimentdecreased with an increase in ambient pressurefor all three species at a constant temperatureand RH. The discrepancies at low ambient pres-sure (13.3 kPa) were not significant for two rea-sons: species and actual RH. The material usedby Yi et al was a wafer (a cellulose pad) which isdifferent from wood fibers but resulted in simi-lar EMC to wood species, for which the differ-ences can be attributed to species effects. InYi et al’s test, the RH inside the chamber wasnot controlled, and in fact, the RH at this pres-sure level was close to the RH controlled in thepresent experiment, which resulted in the EMCin their test being similar to the EMC of thethree species in this experiment. The discrepan-cies at medium ambient pressure (53.3 kPa)were significant, which can be attributed mainlyto RH effects and not species because no signif-icant effects result from differences in species.These differences can be explained by the dif-ferences in RH at this pressure level in the twoexperiments. For example, for 60C/50% RH(see Fig 3) in this experiment, the temperatureand RH were kept constant at 60C/50% RH atall ambient pressures; in Yi et al’s test, tempera-ture was maintained at 60C but the RH was notcontrolled at the corresponding ambient pres-sures because their test was based on theassumption that pressure in a vacuum wasmaintained only by water vapor. At 53.3 kPa,the RH in Yi et al’s test was 100%, but theactual RH was lower than that because of airbeing present in the chamber. However, the RHwas estimated to be much greater than 50%,resulting in a higher EMC compared with thisexperiment. For other conditions, the significantFigure 3. Comparison of equilibrium moisture content ofRussian larch, Hinoki, and Sugi wood to previous researchunder vacuum conditions.Figure 2. Equilibrium moisture content of Russian larch,Hinoki, and Sugi wood for various conditions under vacuum.350 WOOD AND FIBER SCIENCE, JULY 2010, V. 42(3)discrepancies at 53.3 kPa were also because ofthe same reason for the differences in RH atthe same temperature and ambient pressure. InYi et al’s test, the actual RH increased with anincrease in ambient pressure. Therefore, RHshould be controlled to determine EMC evenunder vacuum conditions.CONCLUSIONSEMC of Russian larch, Hinoki, and Sugi atthree temperature levels was measured undervarious vacuum conditions with the followingconclusions:1. Ambient pressure was one factor affectingEMC. EMC increased more than at atmo-spheric pressure for the three species with adecreasing ambient pressure. It became clearthat wet-bulb temperature needs to be con-trolled to measure EMC, even under vacuum,because pressure was not only maintained bywater vapor pressure, but also from air enter-ing the vessel.2. The effect of temperature and RH on EMCunder vacuum conditions showed a similartendency to the effect of temperature onEMC under atmospheric pressure in thatEMC increased with a decrease in tempera-ture and increased with an increase in RH atany ambient pressure.3. There were obvious differences betweenEMC values based on the assumption thatpressure under vacuum is maintained onlyby water vapor and the EMC values in thisexperiment, because EMC was obtained fordifferent RH under the same ambient pressure.ACKNOWLEDGMENTSSpecial acknowledgment is extended to theYasujima Company for their assistance.REFERENCESCai YC, Hayashi KZ (2007) New monitoring concept ofmoisture content distribution in wood during RF/vacuumdrying. J Wood Sci 53:1-4.Carpenter JH (1982) Fundamentals of psychometrics.Carrier Corp., Syracuse, NY. 57 pp.Chen ZJ, Fred ML (2002) Theoretical equilibrium mois-ture content of wood under vacuum. Wood Fiber Sci34(4):553-559.JSME (1959) Humidity–moisture measurement and envi-ronment monitoring [in Japanese]. The Japan Society ofMechanical Engineers. GIHODO SHUPPAN Co., Ltd.Pages 43-45.Kollmann FFP, Cote WA Jr, Wilfred ACJ (1968) Principlesof wood science and technology. I. Solid wood. Springer,Berlin, Heidelberg, New York. Pages 189-197.Myojin MY, Yasujima M, Hayashi KZ (2006) Develop-ment of hot blow vacuum drying kiln (1). Full size drykiln suitable for production of high quality hardwoodlumber [in Japanese]. Mokuzai Kogyo 61(3):105-109.Siau JF (1995) Wood: Influence of moisture on physicalproperties. Department of Wood Science and WoodProducts, Virginia Polytechnic Institute and State Univer-sity, Blacksburg, VA. Pages 1-9.Simpson WT (1987) Vacuum drying northern red oak.Forest Prod J 37(1):35-38.Skaar C (1988) Wood-water relations Springer, Berlin,Heidelberg, New York. Pages 22-31.Xiao H, Cai YC (2009) Factors affecting relative humidityduring wood vacuum drying. J Forest Res 20(2):165-167Yan JL, Wang YQ (2004) Engineering thermodynamics [inChinese]. China Electric Power Press, Beijing. Pages221-223.Yi SL, Zhou YD, Liu YR, Zhang BG, Feng XJ (2008)Experimental equilibrium moisture content of woodunder vacuum. Wood Fiber Sci 40(3):321-324.Liu et al—EQUILIBRIUM MOISTURE CONTENT OF WOOD UNDER VACUUM 351

Effect of Ambient Pressure on Equilibrium Moisture Content of Wood

https://wfs.swst.org/index.php/wfs/article/download/543/543

Effect of Ambient Pressure on Equilibrium Moisture Content of Wood

Abstract

Similar works

Full text

Available Versions

Wood and Fiber Science (E-Journal)