Drying and gas or vapor contact with solids – Process – With nondrying treating of material
Patent
1998-04-03
1999-11-09
Bennett, Henry
Drying and gas or vapor contact with solids
Process
With nondrying treating of material
34469, 34471, F26B 700
Patent
active
059790741
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a process and an apparatus for drying cut timber or other hygroscopical plate-shaped or bar-shaped goods in a vacuum, stacked with intermediate bars ("stickers") in a vacuum-solid chamber, which is equiped with ventilators (fans) whose effect in direction extends crosswise to the longitudinal axis of the drying chamber to revolve a gaseous drying medium, with one or several heating units (heating coils) that extend along the length of the drying chamber (kiln), and with a dehumidifying device (condenser) inside or outside the drying chamber.
The vacuum drying in a rough vacuum offers a possibility for considerably shorten drying time compared with conventional technical drying in atmospheric pressure. The mobility of water inside the wood rises generally in parallel to decreasing pressure, so that the drying process accordingly can be accelerated without resulting in any mechanical tensions (stresses) in the wood due to overdried surface with a wet core (so called "case hardening") that can lead to crack building or deformation.
To shorten the drying time it is required that the evaporation heat needed is transferred faster from the heating coils to the wood. This is not that easy to achieve in vacuum with convective heat transfer, since the heat capacity of the drying medium (the heat energy carrier) reduces proportionally to decreasing pressure. Consequently, compared to conventional drying a significant higher flow velocity of the drying medium has to be produced, in order to be able to transport sufficient energy per time unit.
In order to avoid too high an investment and operating costs as a consequence of fan capacity installed, increased abrasion and electrical energy consumption, the flow velocity is usually not extended up to the required maximum value. Therefore, the transfer of evaporation heat in particular to fast drying softwood in the beginning of drying with still a lot of light moving free water in the spaces between wood cells, in general shows a short supply which determines the drying progress more than the other parameters.
Keeping an important quality characteristic of the drying process (low dispersion of the final wood moisture content) causes problems in vacuum drying. In practice, especially very big volume kilns often are loaded with varied timber batches, with green and with predried ware, for example after storing under roof in the open air. The existing difference of the wood moisture of an individual stack or part of a stack at the beginning, whereat also differences in length direction of the boards can occur, remains nearly the same by homogeneous drying conditions. Low final moisture dispersion without additional steps in a long conditioning and equalizing stage, is achieved only if the initial dispersions were not too high.
This problem arises during conventional drying only to a small extent. This can be explained as follows: With atmospheric pressure and drying temperature below 100.degree. C., humidity removal occurs by evaporation at the wood surface and diffusion into the drying medium (steam-air-mixture) that supplies in its part the required evaporation energy. For wood moisture content below fibre saturation point, when the wood shows hygroscopical characteristics, the drying force at given temperature and air velocity is determined by the so called "drying gradient"=wood moisture content (MC)/equilibrium moisture content (EMC). At a climate held constant (this means constant EMC) the drying gradient for the most humid timber-batch is the highest one. This is drying accordingly quicker, so that initial existing moisture differences at homogeneous flow of drying medium through the stacks balance out automatically during the drying process without any special steps.
In the vacuum drying there exists this self-regulating mechanism under normal conditions only to a small extent. As long as the total pressure in the drying chamber is below the water steam saturation pressure (dependent on temperature; it has the same meaning as to exceed the bo
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patent: 4194296 (1980-03-01), Pagnozzi et al.
patent: 4862599 (1989-09-01), Brunner
patent: 5197201 (1993-03-01), Salin
patent: 5228209 (1993-07-01), Brunner
patent: 5425182 (1995-06-01), Brunner
Brunner Kal
Brunner Reinhard
Bennett Henry
Gravini Steve
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