Field of the Invention

The present invention relates to a method for drying wood by circulating hot air around and through one or more piles of wood, arranged in batche's in one or more closed drying chambers in a chamber drier, the circulating hot air first being caused, in a heating phase, to heat the wood to the starting temperature for drying, subsequently in one or more drying phases accomplishing the actual drying, and finally, in a conditioning phase having a specially adapted air moisture content and air tempera¬ ture other than during drying, being caused to level, as completely as permitted by the method, the moisture ratio and stresses in the wood after drying. The invention also relates to an arrangement for carrying out the method according to the invention. This comprises a chamber drier having at least one drying chamber and intended for drying of wood in batches, means for moving one or more piles of wood into and out of the drying chamber, means for circulating hot air in the drying chamber, and means for conditioning the hot circulating air and for convey¬ ing it to and from the drying chamber.

Background of the Invention

In artificial chamber drying of wood, hot air is today used as heat- and moisture-conducting medium. This means that heat is transferred by convection to the wood sur¬ faces during the heating phase as well as the drying phase. Experiments have also been made with alternative heating methods, for example by supplying, by microwave energy, the necessary thermal energy for heating and evaporation. Today however, the air circulation principle is the predominant drying method in the sawmill industry, among other things because bark and sawmill refuse can be

used as inexpensive fuel in the sawmill's own central boiler plant.

Since more and more wood is dried to a lower mois¬ ture ratio, which is adapted to the requirements of the carpentry industry, it is also necessary to finish the drying with a so-called conditioning phase. This finish¬ ing phase of the drying ^' aims at levelling out differences in moisture ratio within as well as between "wood indi¬ viduals", and at eliminating the stresses in the wood that are built up during the drying phase. If this condi¬ tioning phase is left undone or is carried out in an in¬ correct manner, the wood will be deformed during process¬ ing and splitting.

The development that has been going on up to now in order to improve the chamber drying has mainly aimed at developing the programmed control of the climate during drying and conditioning, given certain basic performance characteristics in the form of maximum thermal effect, batch volume, wood dimensions etc. On the other hand, considerably less has been accomplished to study critical phases during drying in respect of incidence of loss and time expenditure/drying capacity for the purpose of optimising the drying process. In this context, it can be established that the heating phase and the conditioning phase have been especially neglected.

Technical Problem

In conjunction with the drying of wood, certain quality- reducing damage may be caused to the wood, that is re- lated to the drying technique. For example, the formation of cracks is a type of drying damage which frequently arises in wood of great thickness. The formation of cracks in wood is drying damage arising early in the drying process. This is a consequence of the fact that the mechanisms which are the basis of a crack arising are strongly associated with the moisture and stress condi¬ tions of the wood surface. A crack arises basically when

the tensile stress transversely of the direction of the fibres exceeds the tensile strength.

It is generally known that interaction between strength and modulus of elasticity, which together pro- duce the property break elongation, leads to the inclina¬ tion to cracks decreasing with an increasing temperature of the material. In addition, wood acts more and more plastically at higher temperatures, i.e. wood has the ap¬ titude for acting in a nonelastic manner. For example, wood may creep, especially under the action of stress and variation in moisture. Such creeping, which is frequently called mechanosorptive creeping, is in fact a condition of drying wood of great thickness, with no formation of cracks. When sawing the wood before drying, wood surfaces are uncovered, which will then emit water vapour to the environment, i.e. a drying of the surface begins. At this stage, this drying is uncontrolled, i.e. outer conditions in the form "wind and weather" determine the drying in- tensity. It has been found that wood that is left in a woodyard will already obtain surface checks after some day, probably earlier in hot and dry weather. These checks then grow during the continued artificial drying owing to the stress concentration arising in the initial cracks. In this case, it thus is not possible to dry such wood without the occurrence of considerable cracks.

However, it has been found that such surface checks may also arise during the heating phase in the drier. The purpose of this phase is to heat the batch of wood to the starting temperature of the actual drying phase. If heat¬ ing is carried out by convection by circulating air through the wood, this air will be dehumidified fairly immediately by condensing on the cold wood surfaces. Sub¬ sequently, an uncontrolled drying begins by the warmer and warmer air absorbing moisture from the wood surfaces, which thus will dry. At this stage, the surfaces will al¬ ready be so dry that tensile stress arises transversely

of the direction of fibres, especially if the surface contains heartwood of low water content, and a crack may form. The wood also has a low temperature at the initial stage of the heating, which is unfavourable in respect of the tendency to forming cracks.

Summing up, it can be established that uncontrolled pre-drying and heating of wood by dry air causes initial cracking of the wood, and growing of these cracks during the continued drying. In air circulation drying, a gradient in moisture ratio always arises from the surface. The surface will then be drier than the interior parts of a piece of wood, and this difference (gradient) in moisture ratio corre¬ sponds to the driving power that leads to moisture flow- ing to the surface. The stress then arising causes creep¬ ing, which results in a stress condition at the end of the drying. The surface will then be in a condition of compression stress, while the interior parts of the wood will be in a condition of tensile stress. These differ- ences in stress lead to deformations, especially when splitting the wood.

In addition to the difference in stress, which is the consequence of air circulation drying, also a mois¬ ture ratio gradient will, as mentioned above, develop during the drying. The highest moisture ratio will be found in the centre of the wood, while the surfaces are significantly drier. When splitting the wood, an unde- sired deformation will arise also for this reason, since the moistest parts will be uncovered and dry further during shrinking and, thus, deformation. In both cases as related, it may be established that drying that is fin¬ ished without the moisture ratio gradient in the cross- section of the wood being levelled, leads to deformation problems in the further processing of the wood. It may also be established that levelling of the moisture ratio in the wood individual at low temperatures (room temperature) does not yield stress-free wood. This

is due to the fact that the necessary stress relaxation requires a high temperature of the material. Thus, a con¬ ditioning of the wood after the actual drying phase must be carried out at basically such a high temperature of the material as is possible in respect of production en¬ gineering, thereby making it possible to implement the goal which is "stress-free wood having a levelled mois¬ ture ratio".

Object of the Invention

As discussed above, it is thus important to heat the wood without initiating surface checks and to finish the dry¬ ing process with an efficient conditioning. Today, the programmed control of the heating is in many cases taken but little notice of, which can imply that the heating is carried out with air that is too dry. If, on the other hand, it is desired to keep the air moisture at a suffi¬ ciently high level so as to avoid the formation of cracks, this results in prolonged heating times, which reduces the capacity of the drying plant.

Today, the conditioning phase is carried out by in¬ creasing, at the final stage of the drying, the moisture of the air by supplying water through nozzles, or di¬ rectly as vapour from special evaporating boilers. The most common problem in this case is that water nozzles supply too small amounts of moisture, and that if vapour is supplied directly, the evaporating boiler of prior art designs is heavily underdimensioned. In these two cases, the supply of vapour aims at setting a balanced climate in the drying atmosphere, which results in a moistening of the wood surfaces corresponding to but a few per cent above the intended final moisture ratio (mean value) . In practice, this conditioning process requires at least 24 hours for plank dimensions. A radically different technique is applied in, above all, New Zealand, in which after high-temperature drying of Pinus radiata (radiata pine) , the wood is introduced

into special conditioning chambers in which vapour is generated from open water tanks. A considerable moisten¬ ing of the wood surfaces is accomplished, which levels the moisture ratio and prevents the formation of internal cracks in conjunction with the cooling. To implement this, the wood must, however, be cooled for some time to 70-80°C so as to cause condensation on the wood.

The object of the present invention is to provide such a method and such an arrangement for drying wood in a chamber drier

that the heating time before drying is shortened, at the same time as the initiation of cracks during heating is prevented, and

that the conditioning, which is carried out for levelling of the moisture ratio and stresses in the wood after the drying phase, is shortened and made more efficient.

Solution of the Problem

According to the present invention, this object is achieved by a method of the type mentioned by way of in¬ troduction and characterised in that, during the heating phase, water vapour is supplied at atmospheric pressure to the hot air circulating around the wood, in the drying chamber used for the heating. According to a preferred embodiment of this method, the supply of vapour to the drying chamber occurs immediately from the start of the heating phase and is then continued preferably continu¬ ously without interruption and preferably up to the end of the heating phase.

Optimum heating of the wood before drying aims at avoiding the initiation of cracks in the wood, at the same time as the heating should be effected in as short a time as possible. According to the invention, this is ac¬ complished, as mentioned above, by supplying water vapour

at atmospheric pressure to the drying chamber during the heating phase. The water vapour will then condense on the colder wood surfaces. As a result, these are protected against uncontrolled drying and, resulting therefrom, the formation of cracks. Moreover, condensation heat is transferred as the water vapour condenses on the wood, which yields a significantly greater heat transfer as compared with heating by convection only. In other words, this means an essentially quicker warming of the wood be- fore the drying.

Although the object of the method according to the invention is to dry the wood, this drying process is sur¬ prisingly begun by strongly wetting the wood with con¬ densing vapour during the heating phase. If vapour is supplied immediately from the start of the heating phase, a quick heating by the hot circulation air and, besides, a complete wetting of the wood surfaces will be obtained even from the beginning by transferred condensation heat in addition to heating by convection. As a result, the initiation of cracks in the wood is avoided, which per¬ mits a drying process according to the invention for pro¬ ducing wood which, to the greatest possible extent, is free of cracks.

As mentioned above, it is not possible, however, to completely avoid the formation of cracks, if, after the sawing, which is carried out before the "artificial" dry¬ ing, the wood is stored in such a manner in, for in¬ stance, a woodyard that already at this stage, small sur¬ face checks arise, which then will grow. If the wood is correctly stored after sawing, ready-dried wood of very high quality can, according to the invention, be produced in a method which is time-effective and, thus, cost- effective.

The heating rate in degrees per unit of time is ul- timately limited by the thermal conductivity of the wood, so that only such an amount of heat can be supplied to the surfaces as the wood is capable of conducting to the

interior parts. In practice, this means that the heating rate is highly dependent on dimensions and ultimately de¬ pends on the surface/volume ratio of the wood. However, a condition is that sufficient enthalpy of vaporisation is available.

For a drying chamber having a batch volume of 150 m^, a power of about 2MW transferred to enthalpy of vaporisa¬ tion is sufficient to produce heating times of about 2 hours. This corresponds to a vapour generating capacity of about 13 kW/rr-3 batch volume of the wood. The vapour generating capacity should, according to the invention, be kept in the range of 6-28 kW/m^ batch volume, preferably 10-15 kW/rn-^ batch volume. The duration of the heating phase is generally between 1.5 and 2.5 hours, and at the end of the heating phase, the wood has obtained a temperature of between 50°C and 65°C. This heating time prevails under normal conditions, and it is understood that in winter, when the wood can even be frozen, the heating time will be considerably longer. The batch volume may vary between different driers, today's driers in many cases having a batch volume of 150 π-3, whereas older driers generally have a lower batch volume, for example 60-70 m-^.

After the heating phase, the drying of the wood is carried out in one or more drying phases, during which hot drying air is caused to circulate through the piles of wood. The drying phase may last between 2 and 14 days. During this period, dry air is supplied to the drying chamber, and moist air is removed. After the drying phase, when the wood has been given a final temperature of between 50°C and 90°C, the conditioning phase is initiated.

A convenient embodiment of the inventive method is characterised in that during the conditioning phase, water vapour is supplied at atmospheric pressure to the hot air circulating around the wood in the drying chamber used for the conditioning phase, and that this supply of

vapour is begun immediately when starting the condition¬ ing phase and essentially directly after completion of the drying phase/s. Preferably, the same drying chamber should be used for the heating phase, the drying phase/s and the conditioning phase.

Efficient conditioning of wood after drying should be carried out such that the temperature of the wood is as high as possible. Then the stresses relax quickly, and variations in moisture are levelled. By supplying water vapour to the drying chamber directly after completion of the drying, vapour will condense on the wood surfaces. Since heat is then released, the temperature of the wood rises, which is positive for the stress relaxation, at the same time as the dry wood surface is moistened and the moisture gradient is levelled.

By analogy with the conditions in the heating phase, the vapour generating capacity in the conditioning is dimensioned on the basis of the heating rate. It may be proved that the initial heating requires the highest ca- pacity/kg of wood, and so it may be established that a vapour generating capacity of about 2 MW/150 m^ of wood during the conditioning phase is sufficient to level out the moisture ratio and stresses of wood in 1-2 hours. The water vapour supplied during this phase should be sup- plied at a capacity of 6-28 kW/m^, preferably 10-15 kW/m-3 batch volume of the wood. During the conditioning phase, a final temperature of between 70°C and 98°C is imparted to the wood.

One embodiment of the inventive method is character- ised in that the heating phase and/or the conditioning phase is controlled by the temperature increase which during the conditioning phase, can be measured by means of the ordinary temperature transducer of the drying chamber used, being followed and converted into medium moisture ratio increase of the wood. The connection between temperature increase and amount of condensate renders this possible. Since the heat transmission is

efficient, the temperature of the wood will thus be close to the temperature recorded by the ordinary temperature transducer of the drier. By following this development of temperature, it is possible to convert the temperature increase into a medium moisture ratio increase of the wood, which permits an advantageous control of the condi¬ tioning process. Thus, the drier need not be equipped with special transducer systems for this purpose.

A preferred embodiment of the inventive method is characterised in that in a steam generator, high-quality vapour is generated and kept pressurised in an accumula¬ tor, and that the vapour from said accumulator is dis¬ tributed to connected drying chambers by the actuating of valves. According to another preferred embodiment of the in¬ ventive method, the primary energy for generating the used vapour is taken from the sawmill's own solid fuel central boiler plant. This means that sawmill refuse, mainly bark, can be utilised for production of high- quality vapour.

The hot water is conducted from the solid fuel cen¬ tral boiler plant to a central steam generator. In this steam generator, the hot water is conducted through tubes in a cylindrical tank, partly filled water. The heat is transferred to the water which is caused to boil, whereby a vapour volume is generated in the upper part of the cylinder. The system is kept pressurised, such that the vapour can be quickly discharged in the mains made of stainless steel, which serve to distribute the vapour to the connected drying chambers. The distribution of the vapour to each chamber is finally carried out by the ac¬ tuating of valves.

The advantage of such an arrangement is that a cen¬ tral vapour generator can serve a plurality of chambers sequentially for both the heating phase and the condi¬ tioning phase. Moreover, the method implies that the

change-over times will be short since vapour is supplied to the chamber as soon as the valve opens.

Vapour is supplied to the drying chamber in the saturated state or slightly superheated. If the vapour is supplied via a tube system, a certain superheating (and thus excess pressure) of the vapour will become necessary to produce the requisite pressure through the dis¬ tribution system from the vapour generator. The excess pressure can be between 0 and 1 atm. gauge. However, the vapour is not allowed to be too superheated, since too much dry heat will then be supplied. Decisive of the in- ^■ vention is that the dew point of the vapour both during the heating phase and during the conditioning phase is above the dew point of the wood, thus ensuring condensa- tion on the wood surfaces. In case of the vapour being generated directly in the drying chamber in an open va¬ porisation system, the vapour is supplied in the satu¬ rated state.

An embodiment of the invention will be described be- low, reference being made to the accompanying drawings in which:

Fig. 1 is a schematic vertical section of a chamber drier as seen from the side, and Fig. 2 is a schematic horizontal section of the chamber drier as seen from above.

Fig. 1 illustrates a chamber drier 1 having a drying chamber 2 intended for drying of wood in batches, the wood being in the drying chamber in the form of one or more piles 3. The piles of wood are arranged on a car¬ riage 4 or the like, and means (not shown) are available to move one or more piles 3 of wood into and out of the drying chamber 2 through the openable door 5. The drying chamber further contains means 6 (schematically shown) for circulating hot air in the drying chamber for carry¬ ing out all or at least one of the steps heating, drying and conditioning of the wood, and means (not shown) for

supplying dry air and discharging moist air which is transported to and from the drying chamber through ducts 7, 8. The arrangement according to the invention is characterised in that the drying chamber further com- prises means 9 for introducing and spreading of water vapour at atmospheric pressure in the drying chamber, and that a vapour generator 10 is arranged to supply water vapour to these means through conduits 11.

The means 9 may consist of adjustable valves with nozzles and can be arranged in one or more positions in the drying chamber for introducing the water vapour di¬ rectly into the free air volume of the drying chamber. Alternatively, they can be arranged to introduce the water vapour into the hot circulation air in connection with the circulating means 6. They can also be arranged to supply water vapour both directly to the drying cham¬ ber and to the circulating means.

Fig. 2 illustrates an embodiment having a plurality of drying chambers 2 a-f, provided with means 9 a-f for supplying vapour to the respective chamber. The vapour generator 10 serves all drying chambers sequentially for both the heating phase and the conditioning phase. The vapour generator should be provided with an accumulator (not shown) for vapour and be designed to be kept pres- surised to supply vapour, via the means 9 a-f, to the re¬ spective drying chamber. The means 9 a-f suitably consist of nozzles with valves which can be remote-controlled. Thanks to the vapour accumulator, it can be ensured that sufficient vapour capacity is available for each drying chamber both during the heating phase and during the con¬ ditioning phase.

The vapour generator 10 can be arranged to be sup¬ plied, preferably via a hot water conduit 12, with energy from a solid fuel central boiler plant, schematically shown as 13, which works by burning sawmill refuse, mainly bark.