GB2099870A - Process for drying wood - Google Patents

Abstract

The present invention relates to a process for drying wood and other organic material in which the drying is carried out by heating the wood in an oil with a selected boiling point and under vacuum. The feature of the process is that the heating wholly or partly is carried out by dielectric energy.

Description

SPECIFICATION Process for drying wood The present invention relates to a process for drying wood and other organic products.

According to my United Kingdom Patent No.

1 402 191 a very fast and mild drying of wood can be carried out in warm oil under vacuum.

The most difficult problem using this process is to avoid the presence of residual drying medium - the oil - in the surface layer of the wood. Too large a residue increase costs, creates a fire risk and environmental problems, and causes difficulties in, for instance, gluing and painting the dried wood.

In order to solve these problems a drying medium is chosen with a specially-adjusted boiling point, and the process is carried out at a suitable vacuum and temperature. Furthermore the process can be performed in separate operations to facilitate the evaporation of the drying medium from the wood surface.

Investigations have made it clear that this oil drying process under vacuum (ODV) can with a definite advantage be combined with an electrical process (ED).

ED has the advantage that the heat can be applied directly within the wood at the place where it is needed. Thus the heat does not, as by usual convection, slowly move from the surface to the inner parts of the wood in order to be utilized there for the evaporation of the moisture.

ED-heat can be supplied dielectrically within a large frequency range. From conventional and technical (apparatus) reasons it is above both of the two ranges that are of interest. The higher lies in the GHz region (1000 million cycles per second). The lower frequency rangeusually called high-frequency -- lies in a region of some tens of MHz (million cycles per second). The energy producers for the higher frequency are generally called microwave generators. The energy producers for the lower frequency range are called HF - or high-frequency generators.

Here the notation EDA is used for heatingdrying that takes place within the higher frequency range, preferably the GHz region, whilst the notation EDB is used for heating/drying that takes place within the lower frequency range, preferably in the region of some tens of MHz. The given limits shall not be interpreted strictly since they fluctuate depending on different methods of construction and other circumstances.

The EDA model must have electrically-closed rooms (cavities) with wave guides as connections to the microwave-generator, while EDB uses capacitive field activators with related electrodes and connecting wires.

EDA seems definitely to be preferred, but EDB is not to be excluded as different operational methods and constructions can give different results. The disadvantage of EDB is that the wood must be fixed between electrodes, so that the realization of ODV can be more difficult or result in a decrease in capacity.

In ODV it is possible to introduce heat wholly or partly by the ED. The ED-energy is, however, rather expensive and thus when ODV is suitable from the point of view of capacity and heat technology for the evaporation of the large quantities of water that are present comparatively near the surface, this indicates that a combination of the usual ODV and ED should give the best result. The procedure will then be that the process is started with the usual ODV. In this way the main portion of the water is evaporated from the wood.

It may here be a question of, for instance, 80 per cent of the water that eventually has to be evaporated from the wood.

When the outer and more voluminous parts of the wood in this way have been essentially dried and when the inner parts still are moist the EDdrying starts. The problem then is to evaporate away the lesser portion of the water in the wood.

When this water is present in the inner parts of the wood, the evaporation of it represents decidedly the most difficult part of the drying process. The ED heat will, however, be directed just to this moist region within the wood and therefore represents an excellent way to solve this problem and complete the drying process.

Another reason for adopting the described dividing-up of the drying process is that the heat generated by EDA can be introduced only a short distance in to very wet wood and not to the inner parts, since a high moisture content is electrically screening. In certain circumstances this can be useful.

From above it is clear that the ED-heating in an effective way can be used in ODV. There exist other interesting circumstances which show how well ODV is suited for ED-heating. The drying medium has suitable electric properties for the task which mean that the ED heat is essentially introduced not to the drying medium but to the wood. If a part of that heat is transferred to the drying medium this part is not wholly lost, but results in less heat from other sources being needed for the warming-up of the medium. The wood is enclosed in a metallic cavity which forms a suitable room for the EDA heat. The cavity can be made in different ways. Very often its main part is made of iron in form of a cylinder and is further improved by covering the inside of the cylinder with a suitable metal as copper or aluminium.By creating a vacuum in the cylinder ionization effects and arcs are more easily kept under control. When the last part of the drying is carried out accordingly with ED the possibility of solving the most difficult problem in connection with ODV is facilitated, that is to remove the residue of the drying medium from the surface of the wood. This means also that the constraints on the performance of the ODV can be relaxed. Thus it is not necessary to demand the same high quality of the drying mediums. For example, mediums with somewhat higher boiling point - up to 300C higher than given in U.K. Patent No. 1 402 1 91 can be used which enlarges the scope of the operation of ODV.

Above it has been described how ED can be introduced into the later part of the ODV. As ODV together with ED has such improving qualities, the ED can, in addition, be introduced earlier in the ODV. Thus ED can very well be applied from the beginning of the drying. In this case a ED-heated ODV process is obtained where the drying in the beginning occurs comparatively similarly to a usual ODV-process especially when adapting EDA - and without real action on the inner parts of the.

wood. Gradually the process changes character and the ED heat is more and more concentrated on the inner parts of the wood. The heat would in such a process be more expensive to begin with than in a normal ODV process corresponding to the above given description, but here an important saving is made because no heating unit for a normal ODV process is needed.

When the heating is carried out dielectrically with drying medium left in the cylinder sooner or later a higher temperature is obtained in the inner parts of the wood than in the outer parts and in the drying medium. In this situation, it is possible to arrange the steam evaporating from the wood to be condensed in the drying medium which then is heated, i.e. it takes care of the condensing heat of the steam. The condensed water is separated from the drying medium in the bottom region in the drying cylinder. Usually the condensation heat is taken care of in condensers outside the drying cylinder.

Claims (7)

1. A process for drying wood and other organic products under vacuum in a warm drying medium (oil) with a selected boiling point, sufficiently high for fulfilling the drying process in a rational way and sufficiently low for its possible evaporation from the wood surface after the carrying out of the drying process, wherein, as a heating source wholly or partly, dielectric energy is used within a higher frequency range for example within the GHz region.

2. A process as claimed in Claim 1 wherein the dielectric energy is only introduced during the later part of the drying process.

3. A process as claimed in Claim 1 or Claim 2 wherein the dielectric heating is started or is accentuated at about the time when the drying medium is taken away from the drying cavity (cylinder).

4. A process as claimed in Claim 3 wherein the drying process is carried out wholly with use of dielectric energy.

5. A process as claimed in any one of the preceding claims wherein the inner wall of the drying vessel is covered with a material such as copper or aluminium in order to facilitate the transfer of the dielectric energy to the wood.

6. A process as claimed in any one of the preceding claims wherein the drying medium which is used has a boiling point of up to about 2300C.

7. A process as claimed in Claim 1-substantially as hereinbefore described.