• Journal of the Korean Wood Science and Technology
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• v.41 no.5
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• pp.456-465
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• 2013

Log cross sections of yellow poplar were dried in a radio frequency vacuum (RFV) dryer under alternating vacuum and release (AVR) process. The average moisture content (MC), temperature and vapor pressure at the volumetric center were monitored as functions of time. Three different log thicknesses (33, 60 and 75mm) were tested. The results show that the AVR process caused an increase in the drying rate when the moisture content was above fiber saturation point (FSP, about 30% MC) but that it had an inverse effect on the drying rate when the MC was below FSP. The effect of the AVR process on the drying rate decreased, and the severity of heart checks increased, with the increase in the thickness of the specimens.

• Journal of the Korean Wood Science and Technology
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• v.26 no.3
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• pp.1-8
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• 1998

Western redcedar timber, 26 by 26cm in cross section and by 200cm long, was dried in a laboratory radio-frequency/vacuum kiln under 65torr of ambient pressure and a fixed frequency of 6.78MHz for the potential rapidly dry large timber. All process data were collected and saved in a computer through a data acquisition system. The temperature in the middle of timber was higher than temperature at the quarter point of timber length and thickness. Temperature gradients developed in the longitudinal and transverse direction of timber. The pressure in the middle of timber was higher than pressure at the quarter point of timber length. The pressure in the middle of timber was lower in the early stage of drying, and higher in the latter stage of drying than pressure at the quarter point of timber thickness. Power density was very highest during heating period and then gradually decreased. The drying curve was approximately linear and the total drying time was 27 hours from an initial moisture content(MC) of 48.6 percent to a final Me of 19.2 percent with only a few mild internal checks in the middle location of timber.

• Journal of the Korea Furniture Society
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• v.10 no.2
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• pp.29-36
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• 1999

During a radio-frequency/vacuum plus press drying process of some hardwood veneers for decoration we investigated the effect of a grain and thickness of a veneer sheet on drying rates, variations of final moisture content within a bundle of veneer sheets, and formation of checking, end waving, and burning mark. About thirty three hundreds sheet of veneer could be dried in sixty five hours from green to in0-use moisture content, and a final moisture content was significantly effected by initial moisture content of veneer. There were nearly variations in a final moisture content among the veneer sheets in the same bundle. A checking was never found in a edge-sliced veneer, and very slight in a flat-sliced veneer of ash and red oak. There were no end waving and no burning mark in all veneer sheets.

• Journal of the Korea Furniture Society
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• v.17 no.1
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• pp.33-46
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• 2006

This study was carried out to investigate a radio-frequency/vacuum (RF/V) drying ability and physical properties of the green boards and the pre-kiln dried boards with 40 mm thickness, and the 70 mm-thick green board of Iroko (Milicia excelsa). The major results were summarized as follows; The drying time from initial moisture content (MC) of 110% to approximate 6% MC for a 40mm-thick green board was 192 hours, and about 200 hours for the 70 mm-thick green board, respectively and so the RF/V drying times were dramatically shortened compared to conventional kiln drying time. The case hardenings at the RF/V drying completion stage test were very negligible, thus represented almost no existence of the residual stress. The checks were very slightly formed on all of the boards during the RF/V drying test, but crook appeared quite severely. During the accelerating test, the water-resistant treated specimens had not experienced any signs of checking occurred, whereas the control boards had encountered very frequent occurrences of end checking and slight surface checking. There were no observations of warping and discoloring regardless of the treatment.

• Journal of the Korean Wood Science and Technology
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• v.39 no.2
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• pp.125-131
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• 2011

The characteristics of radio-frequency/vacuum drying Japanese larch boxed heart round logs with 150 mm and 210 mm in diameter and 2,500 mm in length, subjected to compressive loading, after the pretreatment of kerf were investigated. The results of this study were as the follows: The drying time of about 120 hours~130 hours was needed from green to about 15 percent of moisture content. The gradient of final moisture content for all specimens was very gentle in both longitudinal and transverse directions owing to dielectric heating. The surface checks seriously occurred although the occurrence extent of surface check for the kerfed specimens was slight compared with that for the control specimens because drying stress was relieved by kerf. The occurrence of surface checks for the L-specimen was more serious than that for the S-specimen.

• Journal of the Korea Furniture Society
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• v.17 no.1
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• pp.1-10
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• 2006

Movement behavior, shrinkage and equilibrium moisture content (EMC), in this experiment reflected a change of hygroscopicity mainly affected by continuously compressive load during radio-frequency/vacuum (RF/V) drying and humidity changes during equilibrating. As a result of interaction of the compressive load and moisture content changing under the RF/V condition, the shrinkages in loading direction were significantly increased while those perpendicular to loading direction were decreased. The shrinkages were affected most in tangential, and least in longitudinal direction. The shrinkages showed higher values in continuous drying than in intermittent drying. In the direction of increased shrinkage, all the movements were also increased, for example, the tangential movement for the loaded-RS and the radial movement for loaded-TS; in the direction of decreased shrinkage, all the movements except the tangential movement for the loaded-TS were decreased such as the tangential and radial movements for the loaded-ES, and the radial movement for the loaded-RS, comparing with those of the load-free. EMCs of the loaded specimens were all higher than that of the load-free specimen, and the highest for the loaded-TS, the lowest for the loaded-ES. The transverse hygroscopicity of specimen was reduced for the loaded-ES, but increased for the loaded-TS.

• Journal of the Korean Wood Science and Technology
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• v.30 no.2
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• pp.108-114
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• 2002

Red pine(Pinus densiflora) square timbers with 14.0 cm and 16.5 cm of face size and 24 m long were dried in a vacuum-press kiln and in a radio-frequency/vacuum(RF/V) kiln to compare drying rate, moisture content(MC) distribution and specific energy. RF/V drying rate was higher than vacuum-press drying rate. The effect of size of cross section on the RF/V drying rates were more pronounced than those of vacuum-press drying. The longitudinal- and the transverse MC distribution of dried square timber showed convex profile for the vacuum-press drying and concave profile for the RF/V drying. Moisture gradient of width direction was similar to the thickness direction in vacuum-press dried square timber and was more slight than that of the thickness direction in the RF/V dried large square timber. The specific energy consumption curve increased as MC decreased. Specific energy(kWh/kg of water evaporated) of the vacuum-press process required more than that of the RF/V process.

• Journal of the Korean Wood Science and Technology
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• v.39 no.2
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• pp.132-139
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• 2011

This study examined the characteristics of radio-frequency/vacuum dried Korean red pine ($Pinus$ $densoflora$) heavy timbers with 250 ${\times}$ 250 mm (S), 300 ${\times}$ 300 mm (L) in cross section and 300 mm in diameter, and 3,600 mm in length, which were subjected to compressive loading after a kerf pretreatment. The following results were obtained : The drying time was short and the drying rate was high in spite of the large cross section of specimens. The moisture gradient inall specimens was gentle in both longitudinal and transverse directions owing to dielectric heating. The shrinkage of the width in the direction perpendicular to was 21 percent ~ 76 percent of that of the thickness of square timbers in the direction parallel to the mechanical pressure. The casehardening for all specimens was very slight because of significantly reduced ratio of the tangential to radial shrinkage of specimens and kerfing. The surface checks somewhat severely occurred although the occurrence extent of the surface checks on the kerfed specimens was slight compared withthat on the control specimen.

• Journal of the Korean Wood Science and Technology
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• v.37 no.3
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• pp.192-199
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• 2009

This study was carried out to investigate the performance of a radio-frequency/vacuum dryer combined with a mechanical compressive load (RF/VC) with a scale of about $3m^3$ during drying board of Azobe (Lophira alata) and Korean red pine (Pinus densiflora). The degree of vacuum of the RF/VC system was maintained at 80~105 torr, and wood temperature was increased from $40^{\circ}C$ at the beginning of drying to $60^{\circ}C$ at the end of drying and the radiofrequency generator was operated with schedule of 7 minute-ON and 3 minute-OFF. The wood temperatures near charge plate always remained higher than those of the control during all of the drying stage, whereas the wood temperature near ground plate always remained lower than those of the controlled. As drying time proceeding, the temperature of the wood near ground plate presented lower than those of the control. Whereas the temperature of the wood near charge plate presented higher than those of the controlled. The final average moisture contents of the Azobe boards stacked near the input side of the RF generator showed slightly lower than those near the opposite side. Those of the wood stacked in the layers near the charge plate were lower than those of near the ground plate. The average length of surface checks of the Azobe boards stacked near the charge plate was very slight, whereas that toward the ground plate represented high values. The efficiency of input energy was simliar with the commercial systems.

• Journal of the Korean Wood Science and Technology
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• v.36 no.6
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• pp.59-68
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• 2008

This study was carried out to investigate the effects of the Inside end-taping (Inside-ET) and Outside end-taping (Outside-ET) treatments on prevention of drying defects during radiofrequency/vacuum (RF/V) drying domestic yellow poplar log cross section with size of 20~75 mm in thickness and 120~470 mm in diameter. The treatments of Inside-ET and Outside-ET were quite effective for producing sound log cross sections with thinner than 33 mm and smaller than 270 mm in diameter. Even the 60 mm- and 75 mm-thick log cross sections, if the diameter was smaller than 30 cm, it might expect preventing formation of border checks by appropriate end-taping treatments. The positions of the observed border checks differed in diameter ranging at a relative radius about 0.50~0.63 diameter, and it was estimated that it was necessary to select the border position of end-taping treatment properly according to the diameter of log cross sections. It was observed that the radius of 0.3 diameter was the critical location for pretreatment for preventing formation of border checks in the log cross section with middle diameter. There were severe border checks and V-shaped cracks in the log cross section with large diameter. This was because of the severe variation of moisture content along radial direction of natural characteristic of domestic yellow poplar.