Most of parameters in conventional drying cannot explain a lot of phenomena appeared in vacuum condition, so it is necessary to further explore the moisture migration way and driving forces in wood under vacuum condition. For the water movement channel in wood is very narrow, the characteristic of heat and mass transfer in wood is similar to that in microscale. This article applies theory of liquid nucleate boiling in microscale, combines with wood structure, deduces the vessel’s critical radius of wood if the normal nucleate boiling of water in wood could occur under 60°C, 70°C, 80°C, 90°C, 100°C and 0.01MPa, 0.02MPa respectively, and finally comes to a conclusion that: Because the radius of vessels, tracheid, and pit which are main passageway for water getting out of wood when wood drying are much smaller than the smallest critical radius, therefore the microchannel in wood is smaller than the “evaporation space” and there is no normal nucleate boiling but fictitious boiling phenomenon exist when wood vacuum drying. There also is a certain degree of pressure pulse during the process of water nucleation throughout the drying, which not only prevent from the producing of larger cluster but also drives the moisture inside wood moving out; and the low pressure in the vacuum drying and small channel in wood lead water coming out in the form of bubble during vacuum drying, and the radius of bubble is very small, which make its water potential is higher than that of conventional drying, as a result, the drying rate in vacuum condition is much faster than that in conventional.

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