Factors affecting relative humidity during wood vacuum drying

Effects of pressure and temperature in the chamber during vacuum drying on the relative humidity and evaporation of wood surface were investigated by using the vacuum chamber. The setting temperature during vacuum drying included dry-bulb temperature td, the wet-bulb temperature tw and the temperature difference between the air in the vacuum chamber and the cooling water in the condenser. Results indicated that relative humidity during vacuum drying was affected by the dry-bulb temperature td, the wet-bulb temperature tw and the temperature difference between the air in the vacuum chamber and the water in the condenser. Relative humidity of wood decreased with the increase in temperature at the given temperature of the water in the condenser. The relative humidity was affected slightly by pressure in the vacuum chamber pA, and it decreased from 70% to 65% with pA increased from 50 kPa to 101 kPa. Moreover, there was nearly no evaporation under the vacuum without external heating.     

Effect of steam pretreatment on wood moisture content and characteristics of vacuum drying

The effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in this study. Poplar and manchurian walnut woods were pretreated with high temperature steam at 100°C and 140°C, prior to vacuum drying. A comparison of the characteristics of vacuum drying between steam pretreated wood and untreated wood was carried out. The results show that during steam pretreatment, the MC of wood decreased within a few hours. The reduction of MC varied with the temperature; the higher the temperature, the faster the MC dropped. During the vacuum drying stage, the rates of drying of pretreated samples were higher than those of untreated samples when MC was below the fiber saturation point. Furthermore, the total drying time of samples treated at a steam temperature of 140°C was lower than that of untreated samples. Therefore, a vacuum procedure after steam pretreatment can effectively shorten the drying time when drying wood.     

Effect of steam explosion treatments on drying rates and moisture distributions during radio-frequency/vacuum drying of larch pillar combined with a longitudinal kerf

This study was carried out to investigate the effects of steam explosion treatment on drying rates, moisture distribution, shrinking, and checking during radio-frequency/vacuum drying of Korean larch pillar combined with a longitudinal kerf. All the pillars except the nonkerf/control with high initial moisture content (MC) could be dried within 89 h from the green condition to about 15% MC. In pillars with high initial moistures or in the early stage of drying, the drying rates were sharply accelerated by steam explosion treatments. The final moisture gradients along the transverse direction were gentler for the steamexploded pillars and for pillars with a longitudinal kerf, respectively, than for the unexploded pillars and for pillars without a longitudinal kerf. The moisture gradients along the longitudinal direction on all the layers were gentler for the pillars with a longitudinal kerf than for those without the kerf. Formation of checking was significantly controlled in the pillars with a longitudinal kerf. All the steam-exploded pillars except the kerf/1-cycle, however, were more severely damaged by checks than the unexploded pillars. A prong test revealed an extremely low level of residual stress for all the dried pillars.     

木材含水率测定方法的比较研究

为修订国家标准“CB1931—80木材含水率测定方法”提供依据,本文对含树脂和挥发性物质较多的马尾松等5种木材,采用烘干法、真空干燥法和蒸馏法进行含水率测定的比较。结果表明,烘干法简便实用,但所测得的含水率结果比真空干燥法和蒸馏法为高,如果试样含有较多的挥发性物质,应用烘干法测定含水率误差偏大时,可采用真空干燥法。     

Effect of EMC and air in wood on the new in-process moisture content monitoring concept under radiofrequency/vacuum (RF/V) drying

To improve the accuracy of the new in-process moisture content (MC) monitoring concept under radiofrequency/vacuum (RF/V) drying, equilibrium moisture content (EMC) tests were carried out under various ambient pressures, and pressure curves in wood were analyzed during chamber evacuation and heating phases. The results showed that EMC increased with a decrease in ambient pressure regardless of temperature, relative humidity (RH), and species. The accuracy of MC estimation for Hinoki under RF/V drying was improved from 1.5% maximum absolute errors to 0.6% after EMC modification. The pressure curves for Hinoki and Sugi under RF/V drying showed similar tendencies to an idealized process. Russian larch showed different curves, indicating that the pressure in the wood did not reach the ambient pressure because of its low permeability. Therefore, MC could not be estimated using this monitoring concept because of the presence of much air in the wood of Russian larch.     

Causes of color changes in wood during drying

The forest industry operates in a dynamic and global market where change and competition are the rule rather than the     

In situ measurement of wood moisture content in high-temperature steam

Changes in moisture content of sugi (Cryptomeria japonica D. Don) wood during high-temperature saturated and superheated steam treatments were investigated. A system for in situ weighing of specimens was used, and the reduction of wood substance by heating was taken into consideration. At 160°C the loss of wood substance due to heating was significant and influenced the moisture content values, but it was almost negligible at 120°C. Treatment time and temperature affected the moisture content in saturated steam but not in superheated steam. Excess water in a saturated closed system appears to promote the decomposition of wood and condensation in or on specimens.Parts of this work was presented at the 49th, 50th, and 52nd Annual Meetings of the Japan Wood Research Society in Tokyo, April 1999; Kyoto, April 2000; and Gifu, April 2002     

日本柳杉高频真空干燥及防止开裂的研究

通过对日本人工林木材柳杉髓心方材进行系列高频真空干燥试验 ,掌握柳杉髓心方材的干燥特性及相应的高频真空干燥条件。结果表明 :高频真空干燥前在常压下进行合理的蒸汽和过热蒸汽预处理 ,柳杉髓心方材可有效地提高干燥质量 ,尤其可防止开裂产生     

Evaluation of moisture content distribution in wood by soft X-ray imaging

A technique for nondestructive evaluation of moisture content distribution in Japanese cedar (sugi) during drying using a newly developed soft X-ray digital microscope was investigated. Radial, tangential, and cross-sectional samples measuring 100 × 100 × 10 mm were cut from green sugi wood. Each sample was dried in several steps in an oven and upon completion of each step, the mass was recorded and a soft X-ray image was taken. The relationship between moisture content and the average grayscale value of the soft X-ray image at each step was linear. In addition, the linear regressions overlapped each other regardless of the sample sections. These results showed that soft X-ray images could accurately estimate the moisture content. Applying this relationship to a small section of each sample, the moisture content distribution was estimated from the image differential between the soft X-ray pictures obtained from the sample in question and the same sample in the oven-dried condition. Moisture content profi les for 10-mm-wide parts at the centers of the samples were also obtained. The shapes of the profiles supported the evaluation method used in this study. Part of this work was presented at the 57th Annual Meeting of the Japan Wood Research Society, Hiroshima, August 2007     

Hygroexpansion of wood during moisture adsorption and desorption processes

In order to investigate the shrinking and swelling behavior of wood at a non-equilibrium state, the moisture sorptlon processes of wood under constant and changing conditions were studied. For the static sorption experiment, Chinese fir （Cunninghamia lanceolata） specimens were subjected to the adsorption processes at 25℃, 10 different relative humidity environments and the moisture contents were measured at distinct time intervals of adsorption processes. For the dynamic sorption experiment, the specimens were exposed to periodically and linearly varying relative humidity between 45% and 75% at 25℃. Moisture content as well as radial and tangential dimensional changes in response to the changing relative humidity were measured. The main results from the experiments indicated that： the moisture sorption isotherms of Chinese fir at equilibrium state and different stages of adsorption processes could be characterized by S-shape curves. From the non-equilibrium state to the equilibrium state, the sigmoid moisture sorption isotherms changed from smooth, gradually increasing values to a steep rise at 100% humidity. Furthermore, under dynamic conditions with a constant temperature and a linearly and periodically varying relative humidity, the moisture content as well as radial and tangential dimensional changes of the specimens generally waved but lagged behind the relative humidity change.     

Effect of external compressive load on tangential strain behavior in Japanese larch log cross sections during radio-frequency/vacuum drying

This study investigated the effect of a compressive load of 0.092 MPa on the history of the tangential strains in Japanese larch (Larix leptolepis Gordon) log cross sections subjected to external compressive load during radio-frequency/vacuum drying. The external compressive load of 0.092 MPa played a role in inducing cracking in the outer zone of the log cross section, combining with other tensile strains. However, it also played a role in reducing the heart checks in the core of log cross section when exerted on the cross section of log cross section.     

木材微波干燥缺陷产生原因及其预防措施分析

从木材介电特性、微波穿透深度、微波设备、干燥工艺等方面分析了木材微波干燥缺陷产生的主要原因,并从优化干燥器设计,改善微波场分布均匀性,控制微波辐射功率和辐射时间,改变微波辐射方式等方面探讨了预防干燥缺陷产生的主要措施,以期为木材微波干燥技术的研究和应用提供参考。     

Determination of wood moisture properties using a CT-scanner in a controlled low-temperature environment

Abstract

The aim of the present work was to examine an advanced image-processing algorithm for moisture content (mc) calculation and also to use this algorithm to analyse moisture loss data for low temperature drying. Since wood starts to shrink below the fibre saturation point during drying, the geometrical shape of the wood piece will change. The dry wood image was thoroughly transformed to the shape of the wet wood image prior to calculating the dry weight mc. The results show that the algorithm for the dry weight mc on density data from the CT-scanning during low-temperature drying in the climate chamber is a powerful tool for analysing the moisture loss inside the wood piece. This method can make it possible to get a higher quality on the product.     

STUDY ON THE EFFECT OF WOOD MOISTURE CONTENT ON PERMEABILITY IN THE HYGROSCOPIC RANGE

Woodpermeabilityhascloserrelationshipwiththemanyfiledsofwoodworkingandusing,woodfluidpermeabi1-ityisrelatedtoinprocessingsuchaswooddrying;fireretartenttreatmen;pup1ingandpaper;woodpreservation.Theflowoffluidinwoodincludestwokindsoftransportprocess.Oneispenetrationfromwoodoutsidetoinside,suchas'woodpreservationandfireretartenttreatment,anotherisdiffusionfromwoodinsidetooutside'suchaswooddrying,Infact,woodpermeabilityinfulenceddirectlythequalityofwoodwaterheattreatment,Forthisreasontheresearchin…     

Comparison of moisture distribution along radial direction in a log cross section of heartwood and mixed sapwood and heartwood during radio-frequency/vacuum drying

The purpose of this study was to compare the distribution of moisture contents (MCs) along the radial direction during radio-frequency/vacuum (RF/V) drying of log cross sections of heartwood (HLC) where sapwood was removed from a green log cross section and log cross section of mixed sapwood and heartwood (MLC) prepared with debarked logs of Japanese larch (Larix leptolepis) and locust (Robinia pseudoacacia). For Japanese larch, an even distribution of MC was observed over the entire cross section in HLC not only at the initial stage of drying but also up to the completion of drying. Furthermore, the moisture gradient between the outermost slice and the adjacent inner slice was more gradual compared with that in MLC. For locust, the moisture gradient between the outermost slice and the adjacent inner slice became severe as drying progressed. It decreased after reaching the maximum during the middle stage of drying but continued until the late stage of drying. Furthermore, despite the fact that the average initial MC of mixed slice within MLC was higher compared with that in heartwood slice, this trend reversed immediately after drying started. It was suggested that the possibility of formation of border checking would be high during drying the MLC, since it would be so complicate that the sapwood and heartwood reach fiber saturation point together because of differences in their green MCs and permeability between them.     

Effect of compressive load on the dimensional changes of the Japanese larch dried in a radio-frequency/vacuum drier

This study investigated the effect of a compressive load of 0.092 MPa on the dimensional changes of Japanese larch in a humidity chamber after continuous radio-frequency/vacuum drying. The dimensional changes in the loading directions were significantly increased while those perpendicular to the loading directions were decreased. The shrinkages showed higher values in continuous drying than in intermittent drying. For the specimens loaded on their tangential sections, the radial shrinkages were even higher than the tangential shrinkages; thus, the tangential/radial shrinkage ratio was decreased by 0.27. The transverse hygroscopicity was reduced for the specimens loaded on their cross sections, but increased for the specimens loaded on their tangential sections.     

Combined effect of wood moisture content,drill bit rotational speed and feed rate on drilling resistance measurements in Norway spruce (Picea abies (L.) Karst.)

ABSTRACT

An IML-RESI PD 400 drilling tool and a standard spade drill bit (IML System GmbH, Wiesloch, Germany) were used to study the combined effect of wood moisture content (MC), drill bit rotational speed and feed rate on drilling resistance (DR) and feeding force (FF). Tests were made with Norway spruce (Picea abies (L.) Karst.) conditioned in a normal climate (20°C/65% RH), at 20°C/95% RH, vacuum-pressure impregnated in water, and oven-dried. Rotational speeds and feed rates had an impact on feed rate per cutting edge for the major cutting edge of the drill bit which was used for correlation with DR and FF for various MC. Impact of MC on DR and FF depended on rotational speeds and feed rates of the drill bit. For feed rates per cutting edge less than 0.09?mm, DR was higher for water saturated (WS) specimens. Negligible differences between DR for various MC were found for feed rates per cutting edge between 0.09 and 0.15?mm. DR was higher at low MC for feed rates per cutting edge which were higher than 0.15?mm. FF extremely increased in conditioned (20°C/95% RH) and WS specimens at feed rates per cutting edge less than 0.1?mm.     

Characterization of major,unused, and unvalued Indonesian wood species I. Dependencies of mechanical properties in transverse direction on the changes of moisture content and/or temperature

Mechanical property changes due to the moisture content (MC) and/or temperature changes were examined for 15 Indonesian wood species. A static bending test was carried out at 20°C, 65% relative humidity (air-dry), and water-saturated at 20°C (wet-20) and 80°C (wet-80). For individual test conditions, modulus of elasticity (MOE) and modulus of rupture (MOR) increased linearly with specific gravity regardless of wood species; however, maximum deflection did not correlate with specific gravity for any MC or temperature conditions. The relative values of MOE and MOR measured in wet-20 to air-dry conditions were variously affected from slightly to strongly depending on the wood species. However, the relative values always decreased markedly when saturated in water at 80°C, regardless of wood species. The relative MOE, MOR, and maximum deflection values due to the change in MC or MC and temperature combined were independent of specific gravity but may be dependent on wood type: softwood or hardwood.     

Effect of end-taping and removal of sapwood on radial distribution of moisture content and tangential strains during radio-frequency/vacuum drying of <Emphasis Type="Italic">Cedrela sinsensis</Emphasis> log cross sections

This study was carried out to investigate the effects of end-taping of sapwood (ETS) and removal of sapwood (ROS) treatments of log cross sections on the radial distributions of moisture content (MC) and tangential strain, according to the circumferential slicing method (CSM), during radio-frequency/vacuum (RF/V) drying of log cross sections. The MC of control and ETS samples showed an even distribution around the center but decreased closer to the periphery side. In ROS samples, the MC decreased closer to the periphery side when above the hygroscopic range but showed an even distribution at the late stage of drying. Tangential strain in ETS samples showed slight compressive stress or tensile stress through the entire area of the cross section during the initial stage of drying, although the periphery side was in tensile stress from an area starting at a relative radius of 0.7. ROS samples showed tensile stress in the heartwood around the center and the heartwood of the periphery side and showed no stress or slight compressive stress in the middle area at the initial stage of drying, after which it showed an even distribution of tensile stress throughout the entire end surface. About 90% of the total checks contained in ETS and ROS samples were observed on the periphery side starting from a relative radius of 0.7.     

Chemical changes in terpenes of sugi (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) wood during steam drying in kiln at high temperature

Sawdusts of sugi (Cryptomeria japonica) wood prepared before and after steam drying at 120°C in a kiln were extracted with n-hexane and ethyl acetate to give n-hexane extracts and ethyl acetate extracts. From gas chromatography-mass spectrometry analysis of the ethyl acetate extracts from woods before and after steam drying, the components of 4-epi-cubebol, cubebol, and 2,7(14),10-bis-abolatrien-1-ol-4-one, which existed in the raw sugi wood, were proved to disappear in the steam-dried wood. These components were also absent in the ethyl acetate extract of the steam-condensed solution of waste steam from the kiln outlet. When these three components were treated with 0.2% (v/v) acetic acid solution at 120°C, δ-cadinene was produced as a major product from both 4-epi-cubebol and cubebol by dehydration and cleavage of the cyclopropane ring, and cryptomerone from 2,7(14),10-bisabolatrien-1-ol-4-one by hydration. The chemical changes of the three components presumably occur during steam drying of the sugi wood. This study was presented in part at the 85th Spring Meeting of the Chemical Society of Japan, Kanagawa, Japan, March 26–29, 2005