The present invention relates to a process for drying wood in a drying tunnel through which the wood is gradually fed and there permeated by drying air which flows in the longi¬ tudinal direction of the tunnel. In particular, the present invention relates to such a process in which the drying tun¬ nel is divided into two sections separated by an intervening space and with the drying medium divided into two circulating substreams. By means of the process improved quality of the dried wood is obtained with unchanged drying time or/ ^• alter¬ natively, a shorter drying time with unchanged quality level.

Sawn wood should be dried to a moisture content of approx. 15-22 %, calculated on the dry weight of the wood, in order that the wood can be stored without biological attack in the form of mould etc. For drying wood at sawmills two main types of drying kiln are employed, so-called compartment kilns and progressive kilns (tunnel kilns) , whereas timber- yard drying has practically ceased.

In the compartment kiln the entire quantity of wood which is to be dried is loaded into the kiln at one time, stacked in piles in known fashion. In principle any drying schedule whatsoever can be achieved in a kiln of this type. By drying schedule is meant how the temperature and moisture content of the drying air and its speed of flow through the wood pile are caused to vary during the drying period. It is therefore possible in this type of kiln to employ what is, by some criterium, the optimum drying schedule. This is the principal advantage of this kiln. The disadvantages include a relatively high energy consumption and that these kilns cannot, be made especially large because otherwise the drying climate would vary too much in different parts of the wood load.

In a conventional single-stage progressive kiln the piles of wood move gradually through the tunnel while new piles are loaded at regular intervals and at the same time dried piles are taken out from the other end of the tunnel. The drying air flows along the length of the tunnel in a coun¬ ter-current direction through the piles. As the drying air flows through the piles it is cooled at the same time as its moisture content rises. Once the condition of the drying air which is fed into the tunnel and its speed have been chosen the changes in the temperature and moisture content of the air (i.e. the drying schedule) can no longer be controlled but depend only on the interaction with the wood through which the air flows. Thus unlike the compartment kiln, in a single-stage progressive kiln it is not possible to achieve any optimum drying schedule. Against this the pro¬ gressive kiln has the advantage that the energy consumption is appreciably lower since the air which leaves the kiln is almost saturated and also heat recovery can readily be ob¬ tained. Further, the progressive kiln can advantageously be constructed for high capacities, 10,000 - 20,000 m /annum.

A division of the progressive kiln into two stages has been proposed and has also come into use at some sawmills. In such a two-stage kiln the drying air is introduced into the tunnel oetween the stages so that part flows in a counter- current direction in the first stage of the kiln and part in a concurrent direction in the second drying stage. Compared with the single-stage progressive kiln, this two-stage pro¬ gressive kiln has advantages primarily in regard to control technology as it has a number of self-regulating properties.

In the choice of the drying schedule for a compartment kiln or of the condition of the inlet air for a progressive kiln there are two main requirements which should be satisfied. On the one hand the final moisture content of the wood after the desired drying time should be that which is aimed for, and on the other hand the quality loss of the wood in drying

should be as little as possible or at least acceptable. In general the speed of drying increases as the difference be¬ tween the dry-bulb and wet-bulb temperatures of the air in¬ creases. The magnitude of the change in the quality of the wood is a more complicated function of the drying procedure, but roughly it can be said that the faster drying is carried out the greater are the quality losses. Thus in general it is a question of a compromise between slow drying with low throughput but good quality and fast drying with reduced quality. The compartment kiln has thereby achieved an in¬ creased importance for drying with preservation of quality since in such a kiln the drying schedule can be chosen in an optimum fashion. Drying can namely be carried out rela¬ tively fast without accentuating the quality loss.

Against the background of the aforesaid circumstances there has been a clear effort to try to construct progressive kilns with the characteristic advantages of this type but so that the disadvantage of the non-optimum drying schedule can be circumvented.

The quality loss of the wood in drying can be divided into two main components. One is that with high temperature levels and/or long drying times there is a flow of resin at knots etc. together with a darkening of the surface of the wood, the other is the occurrence of cracks in the wood. Of these two groups crack formation is, especially with thicker dimen¬ sions, clearly the more important... The cause of crack forma¬ tion can be explained in the following manner. In drying the surface of the timber dries faster than the inner parts of the piece of timber because of the resistance to the movement of moisture within the material. When the fibre saturation point is reached, i.e. when the free water has been removed and only water bound to the wood substance remains, the wood starts to shrink. This means that an internal mechanical ten¬ sile stress arises in the surface of the timber. This tensile stress produced by shrinkage is balanced by a corresponding

compressive stress in the inner parts of the timber. If the tensile stress in the surface layer exceeds the strength of the wood a rupture takes place, i.e. surface cracking occurs. Thus it is clear that if the difference in moisture content between the surface of the timber and its inner parts (the moisture profile) is pronounced the risk of crack formation increases, i.e. in rapid drying the risk increases. The mat¬ ter is complicated, however, by the fact that wood is not a purely elastic material but exhibits viscoelastic properties. This means e.g. that if the surface of the timber is sub¬ jected for a longer time to tensile stress then creep occurs, i.e. there is a permanent extension of the surface layer. When drying has progressed so far that also the inner parts have reached the fibre saturation point, the surface has ac¬ cordingly extended more than the inside and stress pattern is then reversed so that the outside is subjected t ^' o compressive stress and the inner parts to tensile stress. Hence during this latter phase of drying internal cracking of the timber can occur. Though these internal cracks cannot be seen they are of great importance in possible subsequent working of the timber.

Even though both the mechanisms of quality loss described above and the mechanisms of moisture transport have long been known at a qualitative level, the development of improved drying schedules has been almost exclusively empirical, i.e. based on direct experience concerning the final moisture con¬ tent and quality which is obtained with the drying schedule which is tested. It can also be stated that the continuous measurement of the moisture content and profile of the tim¬ ber during drying is admittedly possible but in practice very troublesome. On the other hand so far there exists no reli¬ able method for the continuous measurement of the stress con¬ ditions in the timber or even for registration of when cracks occur.

It has, however, now proved possible with the aid of physical

and mathematical methods of calculation to predict in a reli¬ able fashion on the one hand how the moisture content and moisture profile of the timber develops and varies in differ¬ ent drying climates, and on the other to predict on the basis of these profiles what stresses occur and thus the risk of crack formation. Similarly there are possibilities to esti¬ mate resin flow and the colour change of the timber surface. Thus the final moisture content of the timber with a given drying schedule can be calculated and also the quality loss can be predicted with such models. Figure 1 may be cited as an example. In the figure the measured loss of value in per cent is marked on the vertical axis for quality grades 1-3 of 75 x 150 mm redwood timbers with various drying schedules. The horizontal axis shows an index calculated for the respec¬ tive drying schedules which sets the maximum tensile stress in relation to the strength of the wood. Taking account of the experimental difficulties of such tests the correlation must be considered as entirely satisfactory.

When a conventional single-stage progressive kiln is analyzed with the aid of model calculations of this kind one obtains a picture which can be exemplified with the aid of Figure 2. The upper part of the figure shows on the vertical axis how the relative tensile stress in a 75 x 200 mm redwood timber, changes as a function of the drying time expressed in days when the timber is dried from the fresh state down to a final moisture content of 19 % in 6 days under normal conditions. The lower part of the figure shows how the pεychrometric dif¬ ference (the difference between the dry-bulb and wet-bulb temperatures) varies in the wood pile when the drying air flows countercurrent through the timber with a speed of 4 m/s. From the figure it can be stated that no stress occurs in the wood during the first 24 hours since then the surface of the wood has not yet dried to below the fibre saturation point. After that the stress rises rapidly to reach its maxi¬ mum towards the end of the second day. The stress then de¬ creases continuously during the entire remaining drying time.

Since it is the maximum stress level which determines the risk of cracking (Figure 1) , it can be seen that it is the psychrometric difference around the end of the second day that determines the quality loss of the wood. Both before and after this critical period the psychrometric difference could be greater than the levels given in Figure 2. This, however, can naturally not be chariged in a conventional pro¬ gressive kiln.

It has now unexpectedly been discovered that these negative properties associated with the conventional progressive kiln can to a large extent be eliminated if the kiln is divided into two drying stages in an appropriate fashion.

The principal characteristics of the invention are apparent from the accompanying Claim 1.

The present invention is accordingly based on the discovery that if the direction of flow of the drying air during the first stage of drying is countercurrent and during the lat¬ ter stage is concurrent in relation to the wood, then a low psychrometric difference is obtained during the period which is critical for the quality of the wood with a increasing psychrometic difference on either side of this point. As an example of an embodiment of the invention Figure 3 is pre¬ sented. By the reversed direction of flow of the drying air during the first stage, the psychrometric difference here decreases with time, which leads to a rapid drying at in the beginning, so that the fibre saturation point is reached al¬ ready after 12 hours, whereas the stress level does not now rise as high as in Figure 2. During the second drying stage the conditions differ from those in a conventional kiln only in that the speed of the drying air can now advantageously be kept somewhat lower (for example 2.6 m/s) which gives a milder drying atmosphere during the critical period. The ex¬ ternal conditions are unchanged both in the example concern¬ ing a conventional kiln (Figure 2) and in the example con-

units that can be made common are the heating unit for the drying air, fans for transport of the air through the timber, and the ventilation unit for maintenance of the desired air humidity.

As was apparent from Figure 3 it is advantageous to maintain a higher air speed in the first stage than in the second dry¬ ing stage. Since, however, the number of timber piles in the first stage (tunnel length) is less than in the second stage this means that the flow losses are almost equal in the two stages despite the different speeds. Accordingly the air cir¬ culation in the two stages can be maintained by a single fan unit without the division of the air between the two stages departing significantly from that desired. Similarly it is apparent from Figure 3 that the air flows which are led into each of one of the drying stages at the ends of ^" the tunnel do not differ very much as to their condition. This shows that the quality advantages of the invention can be retained even if air with the same condition, i.e. from the same heating unit, is fed into both the drying stages.

Figure 4 shows an example comparable with Figures 2 and 3 and in which the same air (psychrometric difference 9.5°C) is fed into both drying stages and in which the air speeds (3.12 and 2.24 m/s respectively) are matched so that the pressure drops are the same, i.e. a situation which can be obtained using only a single common heating unit and a single fan unit. It is apparent from the figure that the stress peaks are practi¬ cally identical with the peaks in Figure 3. Further it is found that the drying air from each of the stages has almost the same condition (psychrometric difference approx. 4°C) . Thus it is of no major consequence for the energy consumption whether the exhaust of moist air from the kiln takes place from the fist or the second stage or after mixing of the air from these stages, i.e. a common unit for exhaust of moist air and input of fresh air can be employed. An embodiment of the invention represented by Figure 4 shows that this two-

stages. If the wood, after passing through the first stage but before being fed into the second drying stage, is stored in essentially stationary air of approximately the same tem¬ perature as in the drying stage for a period of e.g. a few hours, a rapid equalization of the moisture profile of the wood in the thickness direction takes place, upon entry into the second drying stage because .of the increased moisture content in the surface layer more rapid drying is obtained, whereby the total drying time is not affected to a decisive degree despite the dead time which the relaxation stage re¬ presents. It is, however, earlier known that the visco-elas- tic properties (creep) of wood are accentuated by moisture changes. Because of these so-called mechano-sorptive effects the relaxation stage has a positive effect with respect to the development of stresses in the second drying stage. This can be exploited in the form of improved quality of the wood after drying or alternatively higher kiln capacity.