Effects Of Pre-Steaming On The Characteristics And Vacuum Drying Rate Of Poplar

In order to investigate the internal relations between pre-steaming and vacuum drying, which can provide a theoretical foundation for fast drying method of fast-growing wood, poplar was taken as specimen and was treated with100℃,120℃,140℃steam respectively in this paper. Then the process of heat transfer during pre-steaming and the dimensional stability and permeability of wood after pre-steaming was studied. In addition, the vacuum drying rates of wood after pre-steaming was discussed. The main results are as follows:(1)Research on the temperature field during pre-steaming shows that the heat is transferred from outside to inside of wood under the temperature gradient when pre-steaming. The difficulty of heat transfer is inversely proportional to the thick of wood and proportional to the temperature of steam. Also the heating time in the centre of wood was calculated. Take140℃pre-steaming as example, the heating time of theoretical value and measured value of2cm,4cm and6cm thick specimens when the center of the wood reach to100℃was66min and85min,135min and150min,194min and170min respectively, and the correlation coefficients are above0.95.(2)Research on dimensional stability and transverse permeability of wood after pre-steaming shows that both the dimensional stability and the transverse permeability were increased after pre-steaming. Take120℃as example, the air-dried and water absorption rate of volume expansion were4.07%and14.60%while the control are4.88%and14.90%. The radial and tangential permeability after140℃pre-steaming were0.44darcy and0.37darcy, while the control group were0.29darcy and0.24darcy respectively.(3)Research on the vacuum drying rates of wood after pre-steaming shows that the pre-steaming could shorten15%to60%drying time which required by un-steamed wood. The drying rate of wood after steaming is higher than that of un-steamed wood. For example, the radial and transverse drying rates of wood under0.02MPa after140℃pre-steaming were1.34%/h and0.89%/h, while the control group were0.83%/h and0.73%/h. Lower absolute pressure and higher steam temperature could obviously promote the drying rate. The effective water diffusivity increases as the MC decreases when MC above FSP and reaches maximum value below FSP.