METHOD FOR DRYING OF WOOD The present invention relates to a method for dry- ing of wood, comprising the steps of drying the wood with hot air at a predetermined drying temperature in a drying chamber to a desired average moisture ratio, and subse- quently cooling the wood to a temperature below the dry- ing temperature. After this cooling phase, the tempera- ture can again be raised and lowered once more.

Background Art With a view to shortening drying times and improv- ing the quality of wood the temperature in drying plants has been gradually raised in the 20th century. Nowadays the temperature in drying plants is often about 80°C or higher. When the temperature is raised above 100°C, there is a dramatic change in the drying process owing to the fact that the wood temperature will be above the boil- ing point of water. At these temperatures, the method is referred to as high-temperature drying in contrast to conventional drying which is now often referred to as low temperature drying.

In low temperature drying, the surface layer of the wood is dried by means of a hot current of air and the moisture is conveyed from the interior of the wood to the surface essentially by capillary forces at the beginning of the drying and after that mainly by diffu- sion. Low temperature drying is relatively slow, and it is difficult to produce an even moisture ratio profile in the wood, which causes deformations and cracking.

High temperature drying is significantly quicker among other things because the high temperature makes the water evaporate inside the wood and leave the wood in the form of water vapour, under the action of the vapour pressure. At high temperatures, the wood becomes soft and, consequently, the drying stress decreases during drying. This results in reduced deformations as well as

reduced cracking. In several aspects, high temperature drying is therefore preferable to low temperature drying.

In both low temperature and high temperature drying it is often advantageous to let the wood cool down after termination of the drying phase. This is applicable, for example, if the wood is to be treated further in a so-called conditioning step to reduce deformations, cracking and stress. It is also applicable, for example, if the wood is to be additionally treated, such as by planing or sawing, or be used immediately after drying.

In low temperature drying at a sufficiently low tempera- ture, it has been possible to proceed immediately from the drying phase to the conditioning phase without inter- mediate cooling. In recent years, there has, however, been a tendency to increasing the drying temperature, and many low temperature drying plants are today operated at temperatures that are so high that the temperature in the conditioning phase cannot be raised. Conditioning is then carried out merely by moistening the wood surface without any increase in temperature. This is a drawback since the levelling of stress increases dramatically with a simul- taneous change in moisture and change in temperature.

There is thus a need for cooling also in low tempera- ture drying so as to enable in the conditioning phase an increase in temperature preferably to just below 100°C.

Cooling of wood has previously occurred by using outdoor air, simply by opening the doors of the drying plant, opening dampers in the walls and top of the drying chamber, or alternatively discharging the wood from the drying plant. Such cooling results in the surfaces of the wood being subjected to extreme drying since the air is cold and dry and, thus, cracks arise in the surfaces of the wood.

FR 2,685,247 discloses a device and a method for burning of charcoal. The raw material is dried initially, which is necessary for the wood to be treated at the high temperatures as required. Otherwise, boiling/evaporation

of the water would prevent the temperature from rising sufficiently, and at atmospheric pressure the temperature of the material would be close to 100°C owing to boiling.

When the charcoal is ready, it is cooled with water with a view to lowering the temperature below the ignition point so as to prevent ignition of the charcoal when dis- charging it from the furnace.

FR 2,720,969 discloses a method and a device for pyrolysis/hydrolysis of wood, i. e. high temperature treatment by partial carbonisation of at least the wood surfaces in order to provide improved moisture resistance and rot resistance. Also this patent specification thus does not relate in the first place to the drying of wood, but in the same way as in the former patent specifica- tion, the wood is initially dried to enable treatment at the high temperatures as required. In the drying phase the wood is repeatedly sprayed with water to cool the wood surfaces. This is done to accelerate the drying pro- cess by using the reverse temperature gradient which thus arises in the wood, i. e. that the interior of the wood is warmer than the surface, which thus accelerates the dry- ing owing to the fact that moisture strives to pass from hot to colder areas. In this case, drying therefore occurs in alternating heating phases during which thermal energy is conducted into the wood, and cooling phases during which the moisture in the wood is driven towards the surface. The process is graphically illustrated in Fig. 3, and the described drying procedure occurs essen- tially within the marked areas a and bl along the x axis.

The spraying with water thus occurs repeatedly during the actual drying procedure and, as is evident form the curve in Fig. 3, no considerable cooling of the wood occurs except temporarily, but the average temperature of the wood increases practically continuously up to a tempe- rature between 180°C and 280°C. Such a method for drying of wood should, if anything, be expensive since the con- sumption of energy must increase drastically. At about

140-150°C the wood reaches the so-called ignition point and the pyrolysis/heat treatment of the wood occurs in the period which is marked b2. When the heat treatment is finished, the wood must be cooled before being dis- charged from the furnace so as to prevent ignition. In this second cooling phase, which in the diagram is desig- nated c, the wood is once more sprayed with water. As illustrated in the diagram, the cooling is, however, ter- minated at about 110°C. The temperature of the wood has then been lowered below the critical ignition point and the wood can be discharged from the furnace. The object of this second cooling with water is to ensure that the wood can be discharged from the furnace with no risk of ignition.

Description of the Invention The present invention aims at obviating the above- mentioned problems and drawbacks in drying of wood. More specifically, the invention aims at a method, which shortens the total time of the drying process, which reduces deformations and cracks in the wood, and which reduces inherent stress in the wood. At least these objects are achieved by a method according to claim 1.

The invention is thus based on the knowledge that the wood can be cooled quickly, efficiently and easily, at a reduced risk of deformations, cracks and remaining stress in the wood, by cooling the wood in a controlled climate by injecting water into the drying chamber and, thus, cooling both the atmosphere of the drying plant and the surface of the wood. Thus the cooling occurs on the one hand by water absorbing heat from the environment and, on the other hand, by water being evaporated. The method gives maximum advantages in high temperature dry- ing but also gives great advantages in low temperature drying. Experiments have shown that by using the method the entire drying process can be shortened by about 6 hours or about 25% in high temperature drying.

It is preferred to inject water under high pressure, preferably a pressure between 15 and 120 bar, through special nozzles so that the water is decomposed in drop- lets, practically as a mist. As a result, the water will have a maximum cooling effect and be capable of penetrat- ing everywhere in, for example, a pile of wood. In a pre- ferred embodiment, the water nozzles are capable of injecting at least 5 kg of water/h and m3 of wood. More- over the wood is cooled with a quantity of water of at least 4 l/m3 of wood.

In the following detailed description, a drying pro- cess is described, which comprises five more or less delimited phases, viz. heating phase, drying phase, first cooling phase, conditioning phase and a second, terminat- ing cooling phase. It is preferred for both the first and the second cooling phase to be carried out according to the present invention. However, the invention is not limited to precisely this. For certain applications of the wood, the invention could comprise only three phases, viz. heating phase, drying phase and cooling phase, after which the wood could be used or treated further, e. g. be planed. In fact it has been found that the inventive cooling process gives a"reverse"conditioning of the wood with levelling of stress and levelling of moisture ratio while lowering the temperature, so that the wood can be used immediately after drying and subsequent cooling. In applications involving high demands on wood quality, it is preferred, however, that a conditioning step be introduced after the first cooling phase, in which conditioning step the temperature is again raised under controlled moisture conditions, and that the wood then be cooled once more in a second cooling phase. In a preferred embodiment, the wood temperature is increased in the conditioning phase to a temperature exceeding 75°C during simultaneous remoistening.

The inventive method makes it possible to control the cooling speed and the relative air moisture in the

cooling phase. As a result, the drying of the wood sur- face decreases while at the same time compressive and tensile stress can be balanced against each other, which prevents the wood from cracking while at the same time temperature distribution and moisture ratio profile in the wood can be affected. Normally it is advantageous to use cold water, which gives a maximum cooling effect. For controlling the ratio of cooling speed to air moisture, it may however in certain cases be advantageous to use heated water. In a preferred embodiment, the average tem- perature of the wood is lowered to a temperature between 40°C and 75°C.

Preferably the inventive cooling process is carried out in the same chamber as the drying. This requires, however, that the drying chamber be equipped with amply dimensioned ventilation holes designed in such manner that they protect the drying chamber against negative pressure. Otherwise there is a great risk that the drying chamber is deformed since at the end of the drying phase it contains practically pure vapour at a high tempera- ture. As water is injected, a large amount of the vapour will be rapidly condensed and then the pressure in the drying chamber drops quickly. In a preferred embodiment, the free flow area of the air valves amounts to at least 0.04 m2/m3 of wood.

In the embodiment described below, the invention is applied to a so-called batched drier where a pile of wood is introduced in a drying chamber and removed after dry- ing, cooling and optionally conditioning. The invention could, however, also be used in so-called progressive dryers where piles of wood are introduced at one end of the dryer and undergo all phases of the drying process while being slowly moved through the dryer so as to be discharged at the other end of the dryer after termina- tion of the drying. The different drying phases can then be delimited by different sections in the dryer which are separated by movable partitions or the like. It would

also be possible, instead of drying the wood in piles, to dry the individual pieces of wood one by one along a "running belt"which runs in a loop in a continuous pro- gressive dryer. Such a solution would be feasible by the drying time in high temperature drying in combination with a cooling process according to the present invention being so short that the"running belt"would have a rea- sonable length.

Detailed Description of a Preferred Embodiment of the Invention Now follows a description of a drying method involv- ing high temperature drying in a so-called batched dryer.

The drying process comprises the following five phases: 1. heating phase 2. drying phase 3. first cooling phase 4. conditioning phase with steaming 5. second, final cooling phase.

In a first step, the wood is heated by supplying heat from the heating batteries or the heating batteries in combination with injection of hot water vapour into the dryer at the same time as fans in the drying chamber circulate the mixture of vapour and air. Dampers in the walls and top of the drying chamber are closed to prevent moisture from leaving the drying chamber. Water is sup- plied to the atmosphere in the drying chamber, if needed, to control the relative humidity. It is important to pre- vent drying of the wood surfaces in the heating phase so as to prevent cracking. Since the wood is cold, it is relatively brittle.

In a second step, the drying phase, the vapour is turned off and the air in the dryer is heated essentially by means of heating batteries. If needed, also superheat- ed vapour can be supplied. The dryer is operated accord- ing to a time-controlled schedule with preselected values of temperature and relative humidity. At the beginning of the drying phase the humidity is high to prevent too

rapid drying of the wood surfaces before the interior of the wood begins to dry, so as to become dryer and dryer later on in the drying, to allow expelling of moisture from the dryer wood. In the drying phase, the temperature is about 100-120°C, and the relative humidity at the end of the drying phase is about 30%.

In a third step, the first cooling phase is carried out. The temperature of the wood is then lowered in a time-controlled manner and regulated with predetermined values of temperature and relative humidity by injecting water into the dryer in the form of droplets.

The cold water results in the vapour in the dryer being cooled and condensing on the droplets of water.

In the beginning of the cooling phase, the atmosphere in the dryer is practically clean superheated vapour and the pressure therefore drops dramatically as the vapour con- denses. Safety valves open and let in outdoor air to pre- vent the dryer from being deformed. The injected water is evaporated partly even before it has landed, but a consi- derable portion lands on the wood surfaces or on the walls and top of the dryer.

In the cooling phase, cold water is pumped in and cools the surfaces of the wood while simultaneously mois- tening them. The heat from the wood results in all of or parts of the cooling water evaporating from the wood. To control the climate in the cooling phase, both the heat- ing batteries and the dampers can be used according to the same principle as in the drying phase, and the cool- ing water pumps are switched off if the cooling proceeds too quickly. Temperature, flow, pressure and size of droplets of the water can, if required, be regulated to control the effect on the climate in the drying chamber.

The important thing is that the surface of the wood is cooled according to the predetermined schedule, i. e. the surface of the wood is to be kept moist and the relative humidity is to be kept at the correct level.

After the cooling phase, the wood surfaces are cold and moist. The heat from the interior of the wood results, however, in the surfaces partly drying after the cooling phase. The interior of the wood should be cooled to a level considerably below the boiling point and pre- ferably about 70°C. Moisture from the interior of the wood has been driven in the form of vapour closer to the surfaces but has to a great extent stopped a distance therefrom, where it has condensed. The temperature in the dryer is usually about 60°C and the relative humidity about 95°C.

In a fourth step, the wood undergoes a conditioning phase. The surfaces of the wood are now cooled to about 60°C and the wood can be remoistened and heated again.

Vapour is injected into the dryer and the fans are ope- rated to distribute the vapour in the pile of wood. The vapour condenses on the surfaces of the wood and moistens and heats them simultaneously.

In a preferred embodiment, the wood is dried to an average moisture ratio of about 4 percentage units below the desired moisture ratio so as to be once more moisten- ed in the conditioning phase. Wood that is to be deliver- ed with 16% moisture ratio is thus dried to 12% so as then to be remoistened to 16% in the conditioning phase.

This is done to reduce the distribution of moisture ratio between different pieces of wood in the pile. The tempe- rature is raised during the conditioning from 60°C to about 100°C.

In a fifth step, the wood is subjected to a second, final cooling phase. The surface temperature of the wood is now again about 100°C and its outermost surfaces have been moistened to practically fibre saturation, i. e.

25-30%. Before the wood can be removed from the dryer, it must be cooled. Otherwise it cracks when it reaches dry outdoor air, and moreover the wood needs to cool down while being subjected to a load in order to be stabilised in its straight state.

The final cooling phase resembles the first cooling phase by water being injected as droplets and the wood being cooled in a time-controlled manner according to a predetermined schedule for temperature and relative humi- dity. Heating batteries and dampers can be used for this control of climate in the same way as in the first cool- ing phase. A crucial difference is, however, that the wood surface was dry as the first cooling phase was ini- tiated whereas it is wet as the second cooling phase is initiated. Moreover the wood surface has swollen in the conditioning phase and the distribution of stress in the wood has been affected. This means that it is possible and desirable to subject the wood to a more powerful final cooling phase. By cooling more quickly with a smaller supply of moisture, the outermost layer is dried and any compressive stress is eliminated.

When the temperature of the wood is below 40°C, it is ready to be discharged.

By cooling the wood in the drying chamber in a con- trolled climate with cold water which is pumped in under high pressure instead of cooling the wood in air or in separate conditioning chambers, the temperature will be lowered at a controlled speed to a sufficient extent for the water to be able to condense on the wood surfaces. At the same time the climate is controlled to keep the humi- dity in the dryer at a maximum theoretical level during cooling. By controlling the cooling speed and also the relative humidity in the cooling phase, the drying of the wood surface can be reduced at the same time as compres- sive and tensile stress are balanced against each other.

This means that the wood does not crack. The controlled cooling phase results in an efficient first conditioning.

After the cooling phase, the wood will be able to efficiently absorb moisture from the vapour or the water that is injected to condition the wood while at the same time the wood has a sufficiently low temperature to pro-

vide space for a sufficient increase in temperature in the conditioning phase.

Thus, a double conditioning effect is achieved; first through the great decrease in temperature of the wood surfaces, which results in relaxation as well as a quick decrease in the moisture gradient by the great tem- perature gradient driving moisture from the interior of the wood to the surfaces, and then by a conventional con- ditioning by an increase in temperature together with moistening of the surfaces. The effect will be that the moisture ratio gradient in the wood as well as residual stress are minimised.

By controlling the cooling speed and the supply of water as well as the temperature level where the cool- ing phase is terminated, it is possible to determine the moisture profile, the temperature profile and the stress profile of the wood before the subsequent conditioning during heating and moistening. As a result, both the cooling phase and the subsequent conditioning phase are shortened compared with prior art.