Drying and gas or vapor contact with solids – Apparatus – With means to treat gas or vapor
Patent
1982-07-06
1984-12-04
Schwartz, Larry I.
Drying and gas or vapor contact with solids
Apparatus
With means to treat gas or vapor
34 68, 34 138, F26B 334
Patent
active
044855648
DESCRIPTION:
BRIEF SUMMARY
The present invention is related to a method of drying wood in the shape of boards, planks, etcetera.
A well-known method of drying timber is carried out by exposing the timber to dry hot-air currents which suck out the moisture of the wood. During such a procedure the surfaces of the wooden objects will dry first so that the thin capillaries in the wood which keep the moisture will shrink at the surface and obstruct further transfer of moisture out of the wood. In such a well-known procedure (e.g. Swedish patent publication No. SE 319 129) the hot-air drying is completed with a high frequency treatment of the wood. By such high frequency heating it is possible to achieve a temperature gradient in the load which is positive in the direction from the surface to the centre of the load. As the heat is then generated in the wood it is necessary to provide air to cool the wood in order to create a temperature balance. The moisture is then pressed out instead of being sucked out as in the case of conventional hot-air drying when the temperature gradient is oppositely directed. The high frequency treatment according to the well-known procedure provides, however, just a partial advantage to the conventional drying. Some improvements of the procedure are achieved by the use of microwave heating for the drying. (1) The saving of time by microwave heating can be essential; the drying period is often reduced to 20% of the corresponding period of hot-air drying. (2) The microwaves are self-adjusting i.e. the drier parts of the load absorb less effect than the moister ones. (3) The microwave drying gives a higher quality due to the fact that the occurrence of cracking and wrappedness is practically eliminated as both the temperature and the mositure gradients are essentially less than in the case of other drying methods. (4) The treatment costs are considerably reduced by microwave drying due to the fact that a laying of bed laths is eliminated and the improvement of quality allows a simplification of the subsequent manufacturing procedures. (5) Much less energy consumption is a consequence of the use of microwaves, primarily owing to the fact that no need exists for heating air separately, that the air temperature in the drying room is lower and thus the heat dissipation less, that the drying period is shorter and that the power of ventilators is reduced as the air circulation is used just for removing the moisture, not for blowing air on the timber to suck out the mositure as in the case of hot-air drying.
In carrying out the method according to the invention a closed microwave-tight room with dimensions in all directions exceeding some four microwave wave lengths and a microwave generator with feeding devices are needed. The load is positioned in the room in the form of a packeted, heat insulated wooden product on a stand or a carriage. The insulation is made of a moisture- and microwave-transparent material and the cross-section areas of the load inside the insulation in any direction may be of a value of some penetration depths at most of the used microwave frequency. When the frequency is for instance 915 MHz this measure will be about 1 meter. The microwaves penetrate the insulation without attenuation and are absorbed in the load. Thanks to this an even drying is achieved which is lenient and bring about great technical advantages owing to reduced occurrence of cracking and warping in the wood.
A preferred embodiment of a method according to the invention is described in the following as carried out in a treatment room which is shown in a vertial cross-section on the drawing.
FIG. 1 shows a room in which the load is the said insulated wooden product and
FIG. 2 a room in which a stack of timber is treated.
The physical quantities to be used in carrying out a method here referredto are microwave effect, temperature and time, in addition condensation of moisture is assumed to be provided. Microwave effect is supplied from a microwave generator which works in an established frequency range. Condensation is achieved
REFERENCES:
patent: 2617202 (1952-11-01), Reedy
patent: 3721013 (1973-03-01), Miller
patent: 3845270 (1974-10-01), Widugris, Jr.
Iverlund Bertil G.
Risman Per Olov G.
Uddborn Hans G.
Aktiebolaget Edane Komponenter
Miller Alfred E.
Schwartz Larry I.
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