A METHOD FOR DRYING WOODEN PRODUCTS

Abstract: The invention relates to a method for drying wooden products and solves the problem of providing such a method which is very economical and allows continuous control of the drying process. According to the invention the products are dried inside a closed chamber and there exposed to microwave energy which heats the interior of the products thus causing the moisture to migrate to the external surfaces of the products where it is absorbed by the air which is then dehumidified. The temperature and moisture content of the chamber atmosphere are continuously controlled.

Description: A method for drying wooden products Background of the invention The oldest and still dominating method for drying timber and other wooden products is to place the pro¬ ducts in a chamber through which heated air does conti¬ nuously pass. The warm air flows past the external sur- faces of the products and absorbs therefrom moisture which then leaves the chamber together with the air. Accordingly, that method can be defined as a continuous process in an open circuit. It suffers from several disadvantages and limitations, the most important of which are the following ones. The wooden products are dried by heat conduc¬ tion in the way that the surface layer is first dried. The drying process does then, at a gradually lower ■rate, proceed towards the center of each product. This is disadvantageous for several reasons. The dry¬ ing occurs very slowly, primarily because wood is a very bad heat conductor. In addition thereto, the slow removal of moisture is accentuated by the direc¬ tion of the moisture gradient. The reason for this is that the dryin* -* of the surface layers results in a shrinking, a reduction of the distance between the wood fibres and, hence, in a corresponding restriction of the passages through which the moisture can migrate outwards. This effect is differently pronounced in different sorts of wood and, in many cases, it is accompanied by crack¬ ing in the surface layers. For that reason sor.e sorts of wood cannot at all be dried by forced air circula¬ tion; instead they require that the drying takes place during a very extended period of time, in extreme cases several years. - Due to the fact that the drying occurs in a completely open system the content of ex- cess heat in the air leaving the chamber-cannot be used or, stated in other words, the efficiency of the method is very low. In an effort to avoid that the drying will start at the external surface and then progress inwardly and require long time there has, during the latest years, also been applied another method. According to that method the article to be dried is placed between a pair of electrodes connected to a high-frequency generator. As is well-known, disregarding the edge effects which in this context are completely negli¬ gible, the useful electrical field is only present in the space between the electrodes. This means that, for practical and economical reasons, the usefulness of that method is limited to articles the shapes of which are suitable for high-frequency drying and the dimensions of which are relatively small. A third condition is that the articles must be manufactured in long series. This applies to e.g. components used in the furniture industry. Another factual circumstance, also limiting the usefulness of that prior art method, is the low capacity of wood-to absorb HF energy. This often results in electrical flash-overs caused by the high electrode voltage necessary. ^ A still further re- quirement implies that practically no variations in the cross-sectional area of the products can be per¬ mitted since, otherwise, they are not dried homogenous- ly and subject to damages by cracking and the like. Accordingly, it is difficult to control such a drying process and it is obvious that it cannot be_used for drying logs and the like. Summary of the Invention The object of the present invention is to provide a method for drying wooden products which shall not suffer from the above-mentioned disadvantages and limi¬ tations. The invention is based on the foiloving OMPI realizations. In order to satisfy the requirement for economy it is necessary to abandon the continuous process in favour of a discontinuous one carried out inside a closed chamber. Second, in order to shorten the dura¬ tion of the process, the direction of the drying must not be from the external surfaces of the pro¬ ducts to their centers but opposite, so that the mois¬ ture is forced to migrate from the centers to the surface layers. Third, the heat shall be supplied via electromagnetic waves but, in contrast to the prior art high-frequency (HF) method, the effect of the electromagnetic energy shall not be restricted to a comparatively small space between a pair of electrodes but useful within a much bigger volume, more parti¬ cularly inside all of a drying chamber. Another object of the invention is to provide a drying method which may conveniently be controlled. The latter requirement . is twofold. One requirement is for good control during the drying.of a given batch of products. Another requirement is that the method shall render itself for flexible matching to different types of products, especially as far as differences in respect of wood sorts, .moisture content and dimen- sions are concerned. More specifically, .the latest re¬ quirement involves that it shall be possible, by programming the equipment governing the drying process, in each individual case to satisfy all conditions for optimized drying. The above-mentioned and other objects and advan¬ tages are achieved by a method according to the pre¬ sent invention the detailed characteristic of which appear from the claims. However, the basic inventive concept is based on the following realization. By use of microwave energy instead of HF energy it is possible to generate electromagnetic fields inside all O PI of a drying chamber. By control of the humidity and temperature of the air inside the chamber it is possible to govern the drying of wooden products in¬ side the chamber in such a way that the surface layers of the products are prevented from being dried first which may cause cracking and other disadvantageous effects as above explained. It is also a main charac¬ teristic of the invention that the chamber is closed and that the air inside it is recirculated. This does very significantly improve the economy of the process because, as a matter of principle, no heat is lost to the surrounding atmosphere. As will be explained below, the only input to the chamber is electrical microwave energy and the only output is water absorbed from the products by the air inside the chamber and then removed therefrom. Detailed.Description of the Invention The method according to the invention will now be described in greater detail. As has already been mentioned, the wooden pro¬ ducts to be dried are placed inside a closed chamber. In the interior of the chamber there are generated microwave energy fields produced by one or more gene¬ rators mounted outside the chamber and connected to waveguides opening into the chamber. It should be underlined here that the number of generators, i.e. primarily the total power, is in each case chosen with regard to the actual circumstances, above all the volume of the chamber and the operating frequency, of the generators. Thus, in some cases it could be both sufficient and most suitable to have one generator only, for instance a magnetron, whereas in other cases several generators are used. In the latter case the generators may operate on mutually different frequen- cies, protection against disturbing interactions being provided for by filters according to principles well- known in the art. As far as the selection of the magnetron frequencies is concerned a* first considera¬ tion is that they must fall within the so-called ISM bands, the only ones permitted for industrial pur¬ poses. Selection of the exact frequencies inside those bands is then governed by actual operational parameters, including especially the chamber volume, the dimensions of the wooden products, the sort of tree and the moisture content. Therefore, when the invention is worked in practice, the frequency selec¬ tion is generally a compromise between different con¬ siderations which may point in different directions. However, in order to prevent the wooden material from drying up, crack and get clogged before the water con¬ tained therein has been expelled, it is necessary that the heat-generation be concentrated to the water and not to the wood. This means that the frequency must not be close to the' HF range, as in that case the dominant energy absorption will be caused by the re¬ sistive losses in the wood which are relatively inde¬ pendent of its moisture content. On the other hand, use of too high a frequency limits the penetration depth because one will then approach the dipole relaxa- tion frequency of v.'ater (around 20 GHz) . For practical purposes the upper frequency limit is generally around 10 GHz.. •In this connection it could be mentioned that, in addition to water, the wood does also contain lignin, resin and other substances comprising OH radicals. Wi.th a proper frequency selection the dominating heat generation will be in the water and the second greatest in the substances just mentioned, whereas the heat amount absorbed in the wood will be insignificant. The positive result of the fact that the water will absorb the majority of the microwave energy supp- OMPI lied is not only that the wood itself will be heated very insignificantly but also that the heated water will tend to migrate towards the external surface of the product and that, due to the relatively low tem- perature of the wood, its "pores" or "capillaries" will be kept open so that the water may pass. As is understood, this condition is in sharp contrast to what applies in prior art methods where the heating occurs in the opposite direction, by conduction from the external surface of the product towards its cen¬ ter., meaning that the migration .passages originally existing in the humid material will be contracted. This is the reason why, according to the. prior art methods, it is necessary to accept either a very time-consuming drying process or wood cell bursting in the surface layer material. Such bursting often immediately results in cracks and flaws but it does also frequently happen that the result is a build-up of internal tensions which do riot " damage the material until the product is to be machined long after the termination of the drying process. While also accor¬ ding to the present invention the humidity in the ex¬ ternal layers will first leave the products, for the reason just mentioned this will result in a reduced . heat generation in those layers so that the total heat absorption there * will be less than in the central portions. Since wood is a bad heat-conductor, practi¬ cally no heat equalization due to conduction will occur or, stated otherwise, there will successively build up a higher temperature in the central portions of the products. It has been explained above that, in order to achieve that the drying cf the products will start in their central regions and successively move to the outer layers, in contrast to the prior art methods OMPI where the heat is by conduction transported from the external layers to the central portions, it is necess¬ ary to control the process so that the conversion of the electromagnetic energy to heat energy be concen- trated to the water in the material. However, such a control is not sufficient to realize the technical advantages of the invention. More particularly, two further conditions must be satisfied, both relating to the"climate" inside the chamber. One of those con- ditions relates to the humidity and the other to the temperature of the air in the chamber. Therefore, it is a characteristic feature of the invention that, during the initial stage of the drying process, the moisture content of the air is kept high so that the surface layers of the products are not dried by delivery of moisture to the ambient air..In order to achieve this it may be necessary during the initial stage to raise the relative humi¬ dity of the air by introducing water in atomized form. As far as the temperature of the chamber air is concerne it should be pointed out that, in contrast to what applies according to conventional methods, the temperature of the air should always be lower than the temperature inside the products. In this way the products cannot receive heat from the air which, as explained above, would result in conditions counter¬ acting the desired moisture migration in the outward direction. When microwave energy is absorbed by the water and by the other substances making up the wooden material, the air temperature will of course rise but it should always be maintained at a lower value than the surface temperature of the products. The major importance of this difference relatively the prior art, where the air temperature is instead so e- what higher than the surface temperature of the pro¬ ducts, is not the prevention of a certain drying of O PI the surface layers under the influence of* the air. In¬ stead the decisive factor is that the lower air tempe¬ rature contributes to the maintenance of a temperature gradient inside the products directed opposite that prevailing according to the prior art, v/hereby the moisture migration is facilitated. To the extent practical it is suitable to create homogenous conditions in the chamber both as far as the temperature and the humidity content of the air is concerned and in respect of the heat absorption inside the products. Homogenous air conditions can be obtained by means of fans circulating the air in the chamber and, especially, effectively distributing atomized _ water supplied during the initial stage- Such fans may also perform a second funtion, namely circulate the chamber air through a special space housing a condenser on which the moisture in the air is condensed, whereup¬ on the air is fed back to the drying chamber proper. The partition between the chamber and the dehumidifying space suitably consists of a sheet of perforated alumi¬ num, the openings of which are dimensioned so that the wall becomes impermeable to microwave energy whereas air can freely flow therethrough. In some applications one could also rely on metal propellers which improve the distribution of the micro¬ wave energy. The number of such propellers and their -locations is to be determined in each ' individual case taking into consideration inter alia the number of magnetrons, the shape of the waveguides etcetera. In order still further to enhance a homogenous microwave energy absorption by the products one can place the latter on a table rotating slowly so that no shadow effects will arise. The use of fans and the air circulation caused by the fans differ in two essential respects from the prior art. These differences do both stem from the fact that OMP the method according to the present invention is carried out discontinuously in a closed chamber where¬ as conventional warm air drying is carried out conti¬ nuously in an open system. The one difference concerns the power consumption of the fans. As they are not used for the purpose of continuously changing the air in the chamber but only in order to perform a "stir¬ ring" action in one and the same air volume for the purpose of homogenizing the air as far as its tempera- ture and humidity * content is concerned,, the power con¬ sumption of the fans will amount to a fraction only of that required in conventional installations. The second difference, that the air is recirculated, where¬ by the major portion of its heat energy content is preserved, does also result in a most substantial improvement of the economy of the method. Further advantages .are achieved when, according ■ to a preferred ' embodiment of the invention mentioned above, the air is dehumidified in a separate space which from a microwave point of view is insulated from the chamber housing the wooden products but in communi¬ cation therewith as far as the air flow is concerned. One such advantage is elimination of the difficulty of mounting conventional .temperature and humidity signal transmitters in places where they are subjected to microwave energy. However, there is no problem at all in installing such transmitters inside the separate space housing the condenser. On the other hand, one should try to mount them at a maximum distance from the condenser whereby the transmitted signals will be re¬ presentative of the conditions in the drying chamber. Generally, it is possible jus- to carry out a few ex¬ periments in order to determine the relevant correc¬ tions and then correspondingly to calibrate the instru- ment equipment.- The signals supplied by the transmitters form a direct basis for the air and humidity control. OMPI However, they may also indirectly supply .an empirical information which can be relied upon for variation of the supplied microwave energy during the course of a drying process. One reason why such a variation may be needed is the* following one. As the humidity con¬ tent of the products decreases, there are inside the products formed dried cavities which may generate multi- resonance cavity effects tending to increase the field intensity inside the material. If the wooden material is not homogenous, for example due to the presence of local areas having a high resin content, the heating may be inhomogenous. However, by successively decreas¬ ing the microwave power input it is possible to com¬ pensate for that effect so that the field intensity can all the time be kept at an optimal level. It has above been repeatedly underlined that, when the method according to the invention is carried out in practice, one has to take -into account a plura¬ lity of basic input parameters, the geometry of the chamber, the power of the magnetrons, their operating frequencies, number and locations as well as the sort of wood, the moisture content and the shapes of the products. This means that it is impossible to give working instructions in the form of absolute numbers. Instead, during an introductional stage, it will as a rule be necessary by experiments to determine which operational parameters correspond to the input para¬ meters. Therefore, the invention is utilized in any instance where wooden products are heated by icro- wave energy inside a closed chamber the atmosphere of which is controlled in such a way that the drying occurs by a moisture migration as above described. On the other hand, when some experiments have been carried out and the optimal operational values for different products have been established, it is possible to compile programs which, when a certain process is to be repeated, can be used for automatically con¬ trolling it. As appears from what has been said above, such programs will generally differ substantially from each other, above all programs relating to different sorts of wood.

Claims: Claims * 1. A method for drying wooden products " , characterized by the steps of introducing a batch of said products into a closed chamber, the walls of which are imper¬ meable to microwave energy, generating inside the chamber microwave energy fields, and controlling the temperature and moisture content of the air in the chamber, namely by initially ke'eping the moisture content at a high level and reducing it when, in res¬ ponse to heat generation inside the products caused by the microwave energy, migration of moisture from the interior of the products towards their external surfaces has been initiated, and by keeping the air temperature at a value slightly below the temperature of said surfaces, allowing it successively to rise in pace with the rising of the surface temperature, and removing from the air the moisture it has re¬ ceived from the products. 2. A method as claimed in Claim 1, characterized in that the air is, in a manner knov/n per se, dehumidified by being brought into contact with a condenser. 3. A method as claimed in Claims 1 or 2, charac¬ terized in dehumidifying the air in a space separated from the chamber by a partition, e.g. a perforated metal wall, which permits air flow communication but insulates the space from microwave energy. 4. A method as claimed in Claims 1, 2 or 3, charac¬ terized in that, during the initial portion of the drying process, water in atomized form is introduced into the chamber in order then to maintain a high moisture content therein. 5. A method as claimed in any of the preceding Claims, characterized in that the microwave energy power input is controlled during the prosecution of the drying process. 6. A method as claimed in any of the preceding Claims, characterized by the use of a plurality of microv/ave generators for feeding microv/ave energy into the cham¬ ber. 7. A method as claimed in Claim 6, characterized by the use of microwave generators operating at frequen¬ cies significantly spaced from one another.

Inventors: LOEOEF NILS O
Applicants: LOEOEF NILS O
Application year: 1981
Main classification: F26B3
IPC codes: F26B3, F26B21, F26B25, H05B6