The present invention relates to a system for drying articles, especially wood or lumber and a method for drying such.

Although the system and the method according to the invention have been developed in particular in view of drying lumber, they can also be used for drying many other articles.

Wood is a hygroscopic material and it may contain different amounts of water. Wood properties, such as volume, durability, abrasion resistance, workability and others, change along with the change of the water content. Therefore, multiple studies and practical experiments have been performed which resulted in determination of the moisture content required for specific wood properties depending on the contemplated use of the articles made therefrom. The moisture content required e.g. in the case of the wood used for furniture is different to that for producing decking. However, there is always a need for drying wood before using it for the production of any articles because the humidity of rough wood immediately after cutting a tree is too high and makes it impossible to be used for any purpose. Thus, methods and devices have been developed for reducing the humidity of articles, for example of wood, to a desired level.

Generally, the process of drying materials, e.g. wood, aims to reduce their water content in order to impart properties and characteristics enabling their use in the manufacture of usable objects. The wood with a low water content has a very high durability and machining workability, as well as good thermal insulation properties, and it also allows to obtain articles having an aesthetic and durable surface. Due to these characteristics wood is widely used in many domains as a material for building, construction, furniture, flooring, tools, decorative elements and others. The widespread use of wood results in a continuing high demand for the drying systems enabling to prepare it for further processing.

Typically, wood is dried in drying chambers. Reduction of the humidity of the air within the drying chambers, leading to lowering of the moisture content of the materials that are being dried, results from the moisture capacity difference between the cold air introduced into the drying chamber and the hot air expelled from the chamber. The ability of the air to absorb moisture varies with the changes of the air temperature. For example, at 0°C, a 1 m ³ of the air contains 4.8 g of water, while at 100°C it contains as much as 597.30 g of water. Thus, exchanging the hot air from the inside of the dryer, having a higher content of water, with the cold outside air having a lower content of water leads to a moisture content decrease in the air inside the dryer, and thereby causes the drying of the articles inside the dryer. The moisture of the articles, especially the water vapour, is discharged from their surface by convection. In conventional wood dryers the discharge of the water vapour from the surface of wood is ensured by the air circulating around the wood, the air being driven by ventilators installed in the dryer. Dried materials may deform or crack while drying, however, appropriate management of the drying process allows to avoid these negative effects.

Wood is composed of fine cells the membranes of which form a porous structure. A freshly cut tree left in the open air loses its moisture and its weight is reduced over time due to evaporation. As there is no visible water outflow from the wood, it evaporates into the air by convection and the air constitutes a medium for the moisture discharge. On the basis of tests it was found that the ability of the air to absorb moisture depends on its temperature and moisture content. The higher the temperature and the lower the water content of the air, the more moisture (water vapour) it may absorb. After many trials and observations basic drying factors were discovered. Physical properties of the air as the drying medium, rate of water evaporation from the wood (from its surface), hygroscopic nature of the wood, hygroscopic balance of the wood, changes in the wood due to the evaporation of bound water are of particular importance for the drying process.

Three main types of the wood drying systems are known - convection, vacuum and condensation drying systems.

In the conventional convection drying systems, the process of drying is performed in a manner shown schematically in Fig. 1 .

The lumber 2 disposed in a drying chamber 1 is subjected to heating up to a temperature specific to a given species of the wood through the air supplied by an inlet chimney 5, circulating in the dryer and passing through the lumber, wherein the air in the dryer is heated by a heater 3, and driven to circulate by fans 4. The air, having absorbed the heat from the heater 3, is passed through the pile of the lumber 2 on its way back to the fans. The heated air passes through the gaps between the layers of the lumber, the height of the gaps depending on the thickness of the spacers between the layers. There is an outlet chimney 6 between the fan 4 and the heater. During the heating of the lumber, the inlet and the outlet chimneys 5, 6 are kept closed and the humidity of the air is maintained at a level equivalent the initial moisture of the lumber by means of sprinklers 1 1. Upon completion of the heating stage, the proper drying process starts during which the inlet and the outlet chimneys 5, 6 are kept opened. The inlet chimney 5 is arranged in the fans' air inlet zone and due to a negative pressure resulting from the operation of the fans 4, the air is drawn into the interior of the dryer. The incoming outside air passing through the fans 4 is mixed on its way to the outlet chimney 6 by the fan blades with the damp hot air coming from the inside of the dryer. In the area between the fan 4 and the outlet chimney 6 the operation of the fan blades causes a significant turbulence of the pumped air being a mixture of the air drawn from the outside and the hot air from the inside of the dryer. Therefore, an air mixture is formed occupying a volume delimited by the inlet chimney 5, the outlet chimney 6, the ceiling (or the wall of the dryer) 1 and the pile of lumber. Water vapour condensation occurs in this mixture due to the cooling by the sucked air and the moisture capacity of the air contained in the mixture decreases. A part of the air mixture is discharged to the outside through the open outlet chimneys 6 due to the overpressure at the outlet of the mixing fans. The air discharged to the outside constitutes just a small part of the air mixture formed by the outside air and the humid inside air that passed through the dried lumber. The remaining part of this mixture is being heated again by the heaters and driven by the fans, and it flows on through the dried lumber, where it grabs water molecules by diffusion from the surface of the wood, and it continues towards the fans. The air exchange process, i.e. suction, mixing, discharge and heating including the flow through the pile of lumber is repeated many times throughout the drying cycle. The speed of the drying process depends on the amount of the air exchanged within the dryer which is the function of the degree of opening of the inlet and outlet chimneys.

A disadvantage of this method is that the outside air that entered by the chimneys 5 is mixed with the entire flow of the hot and humid internal air arriving at the fan 4 to form a mixture which enters the space between the fan 4 and the heater 3. In fact only a small portion of this mixture of the internal air and the external air is expelled from this space through the outlet chimney. Most of this mixture gets back into circulation within the dryer and flows through the heater, the lumber, the fan and so on. The fact that a significant portion to the mixture of the air with a high content of the condensed water vapour is not removed from the drying chamber, but is re-heated and included in the circulation instead, constitutes an evident disadvantage of the known dryers. Leaving the strongly condensed water vapour within the circulating flow of air contributes to the loss of energy having a negative impact on the efficiency of the dryer because in order to discharge the required amount of moisture from that type of a dryer, it is necessary to exchange a disproportionately large amount of the air.

It can be unambiguously stated that due to the large amount of water in the lumber that is being dried and its physical properties, the drying process is costly and time-consuming. It should be noted that 2260 J is needed for evaporation of 1 g of water at a temperature of e.g. ~ 00°C. The evaporation rate depends on the difference between the water vapour pressure on the surface of the dried material and that in the surrounding air. The pressure and the air flow velocity also constitute significant factors determining the rate of evaporation.

Another convective dryer is known from the document EP 0746736.

On the other hand, condensation dryers are known, where the process of drying is carried out as described e.g. in documents DE3206348 and EP 0170648. The air inside the dryer, flowing around the piles of lumber is heated to a high temperature in an analogous manner as in the convective dryers. As a result of the water vapour diffusion from the surface of the dried material, the air is strongly saturated with the water vapour. The moisture discharge is carried out by way of condensing the water vapour contained in the hot air on the surface of cold condensation elements in the form of plates or pipes that are being cooled by means of a refrigeration system to a temperature close to 0°C. The hot air driven in circulation by means of fans is rapidly cooled when it passes by the cold condensation elements and its moisture capacity decreases. In consequence, the excess moisture, resulting from the decrease of the moisture capacity of the air, condenses on the surface of these elements and is discharged outside the dryer in the form of water by a gutter system. Due to the continuity of this process, the air humidity as well as the moisture content of the dried material decreases within the dryer.

Another method of drying lumber which is quite frequently used involves the use of so called vacuum chambers, in which the lumber to be dried is placed. Through a substantial reduction of the pressure within the chamber achieved by means of vacuum pumps, the water contained within the lumber is squeezed out of it by the vacuum and it flows under the pile and the lumber dries gradually but fairly quickly to a desired moisture level. The aim of the invention was to provide a method of drying and a drying system, in particular of drying wood or lumber, the design of which would be simple, and the efficiency would be significantly higher than that of the known solutions of this type, which in turn would allow for the greatest possible energy and drying time savings while maintaining a good drying performance.

According to the invention a drying system is provided, in particular for drying wood, comprising at least one drying chamber having side walls and a ceiling, provided with at least one external air inlet and at least one internal air outlet, in which the wood articles to be dried are arranged in a pile comprising gaps between the layers of the articles, the drying chamber housing at least one heater for heating the internal air and at least one circulation means for enforcing a circulating flow of the air within the drying chamber in a chosen direction, so that the air is flows successively through the at least one circulation means, the at least one heater and the pile of the dried articles, out of which moisture, in particular water vapour, is being discharged.

The drying system according to the invention is characterized in that the at least one external air inlet and the at least one internal air outlet to/from the drying chamber are located in the path of the air circulated within the drying chamber downstream the dried articles and upstream the circulation means in the direction of the flowing air, each external air inlet being located upstream the internal air outlet, looking in the direction of the flowing air, and the at least one internal air outlet being provided with a means enabling to exhaust the air from the drying chamber in to the outside by negative pressure.

Preferably, the at least one circulation means is at least one fan.

The at least one means enabling exhaustion of the air from the drying chamber may be at least one exhaust fan, preferably located outside the drying chamber and communicating with the internal air outlet.

The at least one heater and the at least one circulation means are preferably arranged directly below the ceiling of the drying chamber, in the path of the circulating air and over the dried articles, the pile of the dried articles being covered with a barrier preventing the air from flowing in the vertical direction and over the dried articles.

Preferably, the at least one heater and the at least one circulation means are arranged in the path of the circulating air, between the pile of the dried articles and a side wall of the drying chamber, the pile of the dried articles extending substantially from the floor to the ceiling of the drying chamber.

According to an embodiment of the system, the at least one heater extends vertically, the height of the heater is close to that of the drying chamber and in the circulation means preferably comprises a series of fans arranged vertically at a length similar to the height of the drying chamber, the heater (3) being disposed in the vicinity of the pile of the dried articles, the fans being located between the heater and the side wall of the drying chamber which comprises the at least one external air inlet and the at least one internal air outlet.

Preferably, the at least one heater is supplied with a heating medium from the bottom.

The at least one external air inlet and the at least one air outlet may be located close to each other, and they may be separated by a baffle enabling mutual exchange of heat therebetween. Preferably, the at least one air inlet and the at least one air outlet have a form of a longitudinal slit each.

The at least one air inlet and the at least one air outlet may also have a form of a series of openings having a circular or rectangular cross section.

The air inlet preferably communicates with an internal inlet channel delimited by a side wall of the drying chamber and a water vapour condensation plate extending inside the drying chamber, preferably along said side wall.

Preferably, the drying chamber is provided with an additional condensation plate connected with the wall and located between said side wall of the drying chamber and an outside wall so that an external inlet channel and an external outlet channel are formed, the channel being delimited by the side wall of the drying chamber and the additional condensation plate, and the channel being delimited by the additional condensation plate and the outside wall.

The water vapour condensation plate is preferably provided at its lower end with a trough enabling drainage of condensed water vapour into the outside of the chamber.

Preferably, the water vapour condensation plate is arranged at an angle to the side wall of the drying chamber, the angle being adjustable depending on the speed of the air circulating in the drying chamber and the size of the drying chamber, the angle being preferably about 10 °.

The size of the at least one external air inlet and of the at least one internal air outlet preferably are adjustable.

The drying chamber may be a sealed enclosure, so that the exchange of air between the chamber and the outside is possible only via the inlet(s) and the outlet(s).

According to the invention a method of drying is provided, in particular of drying wood, in a system comprising at least one drying chamber having side walls and a ceiling, provided with at least one external air inlet and at least one internal air outlet, the method including exchanging the internal air with the external air having a temperature lower than the temperature of the internal air, in which the wood articles to be dried are arranged in a pile comprising gaps between the layers of the articles, and the internal air is heated to a predetermined temperature in a closed drying chamber by means of at least one heater, and the air is forced to circulate in a chosen direction, so that the air flows successively through at least one circulation means for enforcing a circulating flow of the air within the drying chamber, at least one heater and the pile of the dried articles, out of which moisture, in particular water vapour, is being discharged.

The method according to the invention is characterized in that upon heating the internal air to said predetermined temperature, the at least one external air inlet and the at least one internal air outlet are opened and at least a portion of the air which left the pile of the dried articles is discharged from the drying chamber via the at least one outlet, the rest of said air flowing again through the circulation means, the heater and the dried articles, wherein each external air inlet is located in the path of the circulating air downstream the dried articles and upstream the internal air outlet, looking in the direction of the flowing air, and the internal air is discharged from the drying chamber to the outside by means of at least one the negative pressure means. Preferably, the air which left the pile of the dried articles and is saturated with water vapour, prior to being discharged via the outlet, is contacted with the air supplied via the external air inlet and is being cooled by it, which results in an intensive condensation and liquefaction of said water valour.

Said cooling of the air may preferably be enhanced by locating the at least one external air inlet and the at least one internal air outlet adjacent to each with a baffle therebetween in order to enable exchange of heat between the air being discharged and the air being supplied.

The outside air may preferably be introduced into the drying chamber via the external air inlet communicating with an inner inlet channel delimited by a side wall of the drying chamber and a water vapour condensation plate extending inside the drying chamber, preferably along said side wall and downwards from the inlet and the outlet.

Preferably, the liquefied water vapour is discharged from the condensation plate into the outside of the chamber.

The advantage of the system and the method of the invention is that due to the specific arrangement of the air inlets and air outlets (or channels) at appropriate locations in the path of the air circulating within the drying chamber, a combination and mutual complementing of the three above- mentioned drying methods is achieved and the efficiency of the process is greatly increased. The damp and warm air leaving the pile of wood is discharged from the drying chamber to the outside in the area of the chamber where the concentration of the water vapour is the highest, i.e. via the outlet(s) (or channel(s)) located in the path of the circulating air upstream the circulation means, e.g. a mixing fan. The fan may be adapted to operate in two directions, depending on the choice. The air is discharged from the drying chamber by the negative pressure produced by an exhaust fan. Also, the negative pressure caused by the action of the exhaust fan results in the external air being sucked into the drying chamber through the neighbouring inlet. The air movement is most orderly in the area nearby the inlet and the outlet, i.e. between the pile of wood being dried and the mixing fan (fans) which enables thorough mixing of the internal air saturated with the water vapour collected by diffusion from the surface of the dried material, with the cool air entering the drying chamber. This allows the mixture of the external air with the internal air having the highest saturation of the water vapour in the drying chamber, to be discharged to the outside. In the situation where the inlet and the outlet are adjacent, and preferably are in direct contact via their walls, the heat energy is automatically recuperated from the air being discharged. For example, a condensation plate separating the inlet from the outlet may be provided on the inner wall of the drying chamber and in the path of the air flowing to the outlet, so that an elongated inlet channel is formed between the plate and the wall of the drying chamber. In such an arrangement, as a result of the heat exchange between the air coming in via the inlet and the air going out via the outlet, the water vapour cooled with cold in-coming air condensates on the condensation plate. The heat recovery is thus enabled and significant savings of energy costs as well as an increase of the rate of moisture discharge may be achieved. The condensation of the water vapour on the condensation plate(s) occurs mainly in the initial phase of the drying process, when the dried material is very humid. In the following phases the intensity of the condensation decreases, while on the other hand the convective and negative pressure drying becomes very effective. The best economic effect of the drying may be achieved by additional sealing of the drying chamber, so that it becomes essentially a pressure chamber, with a simultaneous use of a medium or high pressure exhaust fan. The pressure within the chamber is regulated by the use of an appropriate exhaust fan and by adjusting the size of the external air inlet.

It should also be noted that if the system according to the invention includes the above- mentioned mixing fans adapted to operate in two directions, and the drying chamber is provided with two sets of the air inlets and outlets positioned respectively on the two opposite sides of the pile of the dried articles, then a bidirectional drying is possible which is advantageous in some cases because of the high efficiency and uniformity of such drying.

Exemplary embodiments of the invention will now be described in reference to the appended drawings, in which: fig. 2 shows a vertical section of a first embodiment of the drying chamber of the system according to the invention, fig. 3 shows a vertical section of the drying chamber according to a second embodiment; fig. 4 shows a horizontal section of a third embodiment the drying chamber; fig. 5 shows a horizontal section of a fourth embodiment of the drying chamber; fig. 6 shows a vertical section of the drying chamber shown in fig. 5; fig. 7 shows a partial vertical section of a fifth embodiment of the drying chamber.

In the described embodiments, the system is used for drying wood, but the invention is not limited to this application and may be used for drying other articles.

Fig. 2 shows a vertical section of a drying chamber 1 of the first embodiment of the system according to the invention. In an initial stage of the process, which consists in heating, an external air inlet 8 and an internal air outlet 9, located in a side wall of the drying chamber 1 are closed. The wood, for example a pile or stack 2 of lumber, placed in the drying chamber 1 with intervals between the individual blanks, is being flown around by the circulating air driven by a circulation means 4, for example a fan or fans 4, the air flowing through heaters 3, where it is heated and then through the pile 2 of wood, specifically through the gaps between the layers of lumber which are separated by spacers. The direction of the air circulation is shown in fig. 2 by arrows. The wood may be further covered with a barrier 12, a so-called dummy ceiling, which prevents the air from flowing up in the vertical direction over the dried wood.

During this stage of the process, i.e. during the heating stage, the temperature in the drying chamber 1 is raised to a temperature appropriate for the drying program adopted, depending on the type and parameters of the wood. In addition, in the case of wood, air humidity may be maintained at a level equivalent to the initial moisture content of the wood being dried by sprinklers 1 1.

After the heating stage, the proper drying stage starts. The external air inlet 9 and the internal air outlet 8 are opened. During this step, the hot internal air, highly saturated with water vapour is mixed with the cold external air having a low water vapour content which is being introduced into the chamber 1 via the inlet 9. This mixture is being discharged from the drying chamber 1. Due to this air exchange, relative humidity in the drying chamber l is gradually reduced and the drying process goes on. The air is discharged from the drying chamber 1 via the outlet 8, where a means 7, for example an exhaust fan, is provided enabling exhaustion of the air from the drying chamber 1 to the outside by negative pressure. The exhaust fan is preferably located outside the drying chamber 1 so that it communicates with the outlet 8. The air inlet 9 and the air outlet 8 provided in the side wall of the drying chamber 1 are located in the path of the air circulating in the drying chamber 1 , downstream the dried wood pile 2 and upstream the circulation means, i.e. the fan 4. In this area of the drying chamber 1 the air has the highest water vapour content. It is essential to the system according to the invention that the air inlet 9 is located upstream the air outlet 8, in the direction of the flowing air. The inlet(s) 9 and outlet(s) 8 may be formed by slits extending substantially horizontally or vertically, depending on the direction of the air circulating in the drying chamber, along a side wall of the drying chamber 1 or along the ceiling, and they may also have the form of a number of holes of different cross-sections, in particular rectangular or circular.

It should be noted that the exchange of air takes place in the area where the air movement is the most balanced and calm (upstream the fan 4). This arrangement ensures that the air coming in from the outside is thoroughly mixed with the internal air circulating in the air exchange zone of the drying chamber 1 - i.e. the area near the inlet and the outlet. Due to the operation of the fan 7, negative pressure prevails near the outlet 8 which draws the outside air inwards via the inlet 9.

Fig. 3 shows a vertical section of the drying chamber 1 according to a second embodiment of the invention. In this embodiment, the inlet 8 and the outlet 9 are adjacent to each other and are separated by a baffle, for example a common wall 10. Alternatively, the inlet 8 and the outlet 9 may be placed close to each other but not in a direct contact. In addition, a water vapour condensation plate 5 may be used. As shown in fig. 2, directly adjacent to the outlet 8 the inlet 9 is located communicating with the internal inlet channel 9a, the latter being delimited by the inner wall of the drying chamber 1 and the condensation plate 5. The condensation plate 5 extends inside the drying chamber 1 , generally between the pile 2 of the dried wood and the fan 4. In this embodiment, the condensation plate 5 constitutes an extension of the wall 10 separating the inlet 9 from the outlet 8 and it extends along the side wall of the drying chamber 1 downwards from the inlet 9 and the outlet 8. The negative pressure and the relatively rapid circulation of the air result in that the mixture of the air, which is highly saturated with water vapour, is very effectively discharged from the surface of the condensation plate 5 into the outside via the outlet 8. The air discharge is enhanced by the action of the exhaust fan 7. The heat exchange between the air flowing out of the drying chamber 1 and the incoming air occurs through the wall separating the outlet 8 from the inlet 9 and through the condensation plate 5 where the water vapour condensates. Such a system of discharging moisture from the interior of the drying chamber 1 , i.e. from the area where the water vapour condensation is the highest, greatly reduces the total volume of air exchange during the drying process, thereby increasing energetic efficiency of the process. This efficiency is particularly high if there is a direct contact between the inlet 8 and the outlet 9; it is also increased when smaller diameters of the outlet and the inlet are used. In terms of the heat exchange efficiency it is advisable to provide a ribbed wall 10 separating the inlet from the outlet. The plate 5 is preferably provided at its lower end with a trough 13 enabling drainage of the condensed water vapour to the outside of the drying chamber 1 . Moreover, the plate 5 may be arranged at an angle a to the side wall of the drying chamber 1 , angle a being adjustable depending on the speed of the air circulating in the drying chamber and the size of the drying chamber. The angle a is preferably about 10 °.

It is also possible to use a fan 7 with a variable output which may be adjusted according to the needs of the process. The air coming into the drying chamber 1 via the inlet channel 9 flows by the inlet channel 9a and on just above the condensation plate 5 from where it flows in a very stable way along the shortest path to the outlet (channel) 8. In this area (over the plate 5) the external air is mixed with the internal air, which is cooler. Therefore intensive condensation of water vapour takes place in said mixture . The condensation plate 5 may be in particular flat, ribbed or slightly arched in order to increase the efficiency of the air mixture flow. In the case of a high initial moisture content of the wood and a low external air temperature, the condensate which can be drained to the outside through the drain trough 13 is gathered on the plate 5.

Fig. 4 shows a horizontal section of the drying chamber 1 of a third embodiment of the system according to the invention. In this case, the piles 2 of the dried articles, e.g. wooden timber occupy substantially the entire space between the floor and the ceiling of the drying chamber 1 since the heaters 3 and the fans 4 are disposed between the side wall of the drying chamber 1 and the pile of the dried wood. The inlets 9 and outlets 8 are located in the side wall of the dryer opposite the fans 4 and the heaters 3. In this case, the inlets 9 and the outlets 8 have the form of elongated vertical slits, each outlet 8 being adjacent to two inlets 9, located on the opposite sides of the outlet. Both the fans 4 and the heaters 3 preferably extend along the entire height of the drying chamber 1. Similarly to the examples shown in figs. 2 and 3, the outlets 8 and the inlets 9 may also have the form of the openings of different cross-sections, in particular rectangular or circular extending e.g. substantially vertically along the wall of the drying chamber 1 . The outlets 8 may communicate with the outlet channel 8a and may have a common exhaust fan 7 as shown in fig. 4. It should be noted that the air circulating in the drying chamber 1 flows through the gaps between the layers of the lumber, along the arrows shown in fig. 4 so that the inlets 9 are again located upstream the outlets 8, and both the inlets 8 and the outlets 9 are located between the pile of the dried wood and the mixing fan 4, looking in the direction of the flowing air.

The heaters may be supplied with the heating medium at the bottom, which is the opposite to the conventional way. Such arrangement allows to obtain a more uniform temperature within the drying chamber 1 which significantly influences the uniformity of the moisture discharge and thereby shortens the drying time. This is particularly advantageous in the third embodiment of the system shown in fig. 4, in which the heaters 3 extend along the height of the pile 2 of the wood. When the vertical system of the heaters is supplied at the bottom, the air that is drawn in the direction of the pile 2 of the dried wood is warmer at the bottom of the drying chamber 1 . This warmer air at the bottom of the drying chamber 1 is naturally being drawn upward very quickly due to its increased volume and hence the temperature throughout the drying chamber 1 becomes more uniform. In the case of the conventional heaters supplied with the heating medium at the top, it was impossible to obtain such a uniformity of temperature.

Fig. 5 shows a horizontal section of a fourth embodiment of the drying chamber 1 which is analogous to that shown in fig. 4, i.e. with the fans 4 and the heaters 3 placed between the pile 2 of the dried wood and the side wall of the drying chamber 1 . The only difference between these two embodiments is that in the example of fig. 5 in which the inlets 9 and the outlets 8 have a form of the elongated vertical slits, additional condensation plates 5 were used extending vertically along the inlets 9 and the outlets 8. As shown in fig. 5, the walls 10 between the inlets 9 and the outlets 8 may be extended so as to form the inlet channels outside of the drying chamber.

Fig. 6 shows a vertical section of the drying chamber 1 of fig. 5. In this example, a common exhaust fan 7 is arranged at the bottom of the drying chamber 1 .

Fig. 7 shows a vertical section of a fifth embodiment of the drying chamber. In this embodiment, an additional condensation plate 5' is provided which is connected to the wall 10 between the inlet 9 and the outlet 8. The condensation plate 5' extends along the condensation plate 5 but on the opposite side of the side wall of the drying chamber 1 . Externally form the condensation plate 5' an additional outside wall 14 is provided so that two external channels are formed: an inlet channel 9b and an outlet channel 8a. In the arrangement shown in fig. 7, the inlet 9 and the outlet 8 may have the form of horizontal slits and the external channels 8a and 9b also have the form of horizontal slits located below the inlet 9 and the outlet 8. The outside wall 14 is preferably made of a material having good thermal insulation properties because its function is to promote condensation of the water vapour from the hot internal air leaving the drying chamber 1 and hence to additionally recuperate its heat energy by means of collecting the warm condensed water vapour. The above described additional condensation plate 5' may be used both in the drying chamber 1 with the heaters 3 located between its celling and the pile 2 of the lumber (fig. 3) and in the drying chamber 1 with heaters 3 located between its side wall and the pile 2 of the lumber (figs. 4, 5, 6). If the heaters 3 are located between the side wall and the pile 2 of the lumber, the inlet 9 and the outlet 8 may have the form of vertical slits and the external channels 8a and 9b also have the form of vertical slits located offset laterally with respect to the inlet 9 and the outlet 8. It should be noted that in order to enhance the heat energy recuperation, the condensation plate(s) 5' may extend along substantially the whole height of the side wall.

The system according to the invention enables to achieve a surprisingly high energetic efficiency of the drying process and - provided that the wood hygroscopic properties allow it - maximum shortening of the drying time may be achieved. At the same time, it is possible to reduce thermal and electrical energy consumption by 28÷34% on average. As it clearly results from the experiments performed, the amount of the exchanged air is reduced by 20÷40% on average in comparison with the existing methods due to the considerably higher content of the water vapour in the air discharged from the drying chamber. The drying time is also drastically reduced. In the system according to the invention it is possible e.g. to reduce the time of drying wet rough cut oak lumber in long planks having a thickness of 30 mm down to the moisture content of 6÷9%, to 140÷170 hours. In the case of wet rough cut pine lumber in long planks having a thickness of 32 mm, the time of drying down to the moisture content of 6÷9%, can be reduced to 48÷54 hours. These results are very satisfactory and they are comparable to the theoretical results obtained with the assumption of the maximum permissible drying regimes, while the wood maintains excellent parameters and does not present any micro-cracks and unusual internal tensions after drying.