Journal Article•
Influence of Vacuum Medium Condition on Drying Rate and Drying Defect
TL;DR: In this article, a correlation between drying rate and drying defects under different absolute pressures of 0.02MPa, 0.04MPa and 0.08MPa was found.
Abstract: Larch lumbers with the dimension of 200mm×100mm×20mm are used as the experimental materials to find a correlation between drying rate and drying defects under the different absolute pressures of 0.02MPa, 0.04MPa, 0.06MPa and 0.08MPa and different temperatures of 60℃ and 80℃. Following conclusions are drawn: under the same absolute pressure, the drying rate of samples dried under 80℃ is 1.77～3.85 times as fast as that of samples under 60℃, and as the absolute pressure decreases,the drying rate increases; when the drying temperature increases, the increment speed of drying rate under the condition which the EMC is above FSP (fiber saturation point) is smaller than the condition which the EMC is under FSP, and the drying rate increases as the absolute pressure decreases; when the drying temperature is 60℃,the absolute pressure exerts relatively small influence on the drying rate and the increment speed of drying rate; the drying defect increases as the absolute pressure decreases or the drying temperature increases.
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TL;DR: In this paper, the influence of ultrasonic pretreatment prior to vacuum drying of Chinese fir specimens was examined in order to check micro-cracks, the loss of extractives from the cell walls and other micro-structural changes on the wood.
Abstract: The influence of ultrasonic pretreatment prior to vacuum drying of Chinese fir specimens was examined in this work. In the pretreatments, wood samples were immerged in a distilled water bath and were treated using two wave frequencies for four different elapsed times to investigate effects of ultrasonic frequency and treatment duration. Then the wood samples were vacuum-dried at 80°C and absolute pressure of 0,05 MPa. After the pretreatment, microscopic analysis was carried out on the wood samples to check micro-cracks, the loss of extractives from the cell walls and other micro-structural changes on the wood. Results show that the ultrasonic treatment prior to vacuum drying significantly shortened the wood drying time. The drying time decreased with increase in the wave frequency and the treatment time. Furthermore, ultrasound pretreatment tended to reduce the content of extractives in the wood cell walls and cause cell-wall micro-cracking.
12 citations
Cites background from "Influence of Vacuum Medium Conditio..."
...However, vacuum drying methods are not suitable for timber with high initial moisture contents (Welling 1994), and surface checking and internal checking can be significant problems with wood vacuum drying when the drying temperature is high (Li et al. 2009); This is because of insufficient moisture movement from the center of the wood samples to the surface during the vacuum drying process which can cause steep moisture gradients from the core to wood surface layers; such gradients can lead to checking (Avramidis et al....
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...…(Welling 1994), and surface checking and internal checking can be significant problems with wood vacuum drying when the drying temperature is high (Li et al. 2009); This is because of insufficient moisture movement from the center of the wood samples to the surface during the vacuum drying…...
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TL;DR: In this article, fast growing poplar (Populus tomentosa) was used as the specimen in this work and the water migration rates and the mass transfer coefficients were studied at temperatures of 35 and 50 °C, absolute pressures of 0.03, 0.06, and 0.1 MPa.
Abstract: Ultrasound was applied to enhance mass transfer within the boundary layer during wood vacuum drying. Fast growing poplar (Populus tomentosa) was used as the specimen in this work. The water migration rates and the mass transfer coefficients were studied at temperatures of 35 and 50 °C, absolute pressures of 0.03, 0.06, and 0.1 MPa, and ultrasound power-frequency groups of 60 W-28 kHz, 100 W-28 kHz, and 100 W-20 kHz, respectively. The results indicated that ultrasound could markedly increase the water migration rates within the boundary layer. The water migration rates increased with increasing ultrasound power and frequency. The mass transfer coefficients within the boundary layer for specimens treated with ultrasound were much higher than those of the control group, and the mass transfer coefficients increased with decreasing absolute pressure. Ultrasound could be applied in the wood drying industry as a means of saving time and energy.
9 citations
Cites result from "Influence of Vacuum Medium Conditio..."
...Additionally, the water migration rates increased with increasing temperature and decreasing absolute pressures; these results are consistent with previous reports by other researchers (Davidovic et al. 2006; Li et al. 2009; He et al. 2013)....
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TL;DR: In this paper, the effects of high temperature steam pretreatment on the change in wood moisture content (MC) and characteristics of vacuum drying were investigated in a comparison of the characteristics of steam pretreated wood and untreated wood.
Abstract: 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.
2 citations
26 Mar 2011
TL;DR: In this article, a Chinese fir was taken as a specimen and the results indicated that drying rates increased along with the decrease of the absolute pressure and the drying rates were 2.19, 2.39 and 2.46% when the absolute pressures were 0.06, 0.04 and 0.02MPa respectively.
Abstract: Chinese fir was taken as specimen in this paper. Firstly, specimens were preheated to 80°C, and then the pressure in the drying box was vacuumized to the needed vacuum degree (absolute pressures were 0.06, 0.04 and 0.02MPa respectively), finally, pressure in the drying box was recovered to the atmospheric pressure by vapor from wood during drying period. The results indicated that the drying rates increased along with the decrease of the absolute pressure and the drying rates were 2.19, 2.39, 2.46 (%/h) when the absolute pressures were 0.06, 0.04, 0.02MPa respectively. Then, the drying rates equations under the conditions of temperature being 80°C and absolute pressures being 0.06, 0.04, 0.02MPa respectively were regressed and the results also showed that it was better to use low pressure when wood moisture content was high while better to use high pressure during drying wood with low moisture content.
Cites background from "Influence of Vacuum Medium Conditio..."
...Though the vacuum condition in the drying box indeed accelerated the inner water migration rate, it also sped up the water evaporation rate at wood surface [5]....
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