Process and equipment for drying wood

Abstract: Disclosed is a drying process which comprises primary drying in which many pieces of wood (logs with bark (1)) in a drying chamber (A) equipped with an exhaust system (5) for maintaining a chamber pressure at a normal pressure or in a state close thereto, are heated up with only live steam fed into the drying chamber (A) to heat a chamber interior from a normal temperature up to 90 to 100°C, thereby heating the whole inner part ranging from a surface layer to a central layer of the wood; secondary drying that the chamber interior is heated quickly up to a temperature of 100 to 200°C by the use of a heater (9), thereby boiling moisture in the wood for the purpose of high-temperature treatment up to a fiber saturation point in order to obtain a target percentage of moisture content; and finally, tertiary drying that the dry-bulb temperature and the wet-bulb temperature in the drying chamber are changed according to the dried condition of the wood to thereby treat the wood at low temperatures and moisture down to the target percentage of moisture content under the temperature conditions. According to this process, it is possible to form drying conditions in the drying chamber under which drying speeds (a rate of decrease in the percentage of moisture content) on the surface layer side and the central layer side of the wood can be equalized or brought as close to each other as possible, thereby enabling to dry logs without a crack.

Description: BACKGROUND OF THE INVENTION The present invention relates to a process and equipment for drying wood and, more particularly, to a process for holding raw wood such as sawn wood including logs, timbers and lumber, and boards in a drying chamber, in which they will be artificially dried by increasing a drying chamber temperature to a specific degree. As a generally known artificial process for wood drying, a drying chamber holding raw wood is heated up to the drying temperature of around 40 to 90°C. According to this prior art process, it took 20 days or so to dry wood containing 140% moisture for example to a target percentage of moisture content of 17% (a finish percentage of moisture content). Therefore, the prior art process requires a long period of time for completion of wood drying, resulting in a poor production efficiency. In addition, since the drying process mainly resorts to evaporation of moisture from the surface layer of wood, only the surface layer side is dried; the drying effect will not reach the central layer of wood (the core of wood). It is, therefore, impossible to uniformly dry the whole body of wood from the surface layer to the central layer. That is, the target percentage of moisture content is not uniformized, and no complete drying that largely affects various strengths of wood can be expected. Also, because drying starts at the surface layer side where moisture evaporates, a crack occurs in the surface layer with the unisotropy of shrinkage caused by drying, resulting in substantial deterioration of utility value due to a large decrease of a material usable for building and fittings, or in a large increase in a useless material. According to the prior art drying technology, therefore, it is impossible to perform wood drying without cracking, especially to dry logs and lumber having a core. Logs and lumber having a heart are, from their nature, liable to crack from the surface layer towards the central layer in an artificial drying process. To describe more particularly, wood shrinkage in each direction caused by drying (a decrease in the moisture content) is greater in a radial direction advancing from the surface layer towards the central layer than in a circumferential direction (a tangential direction relative to annual rings) along the surface layer; and therefore if the surface layer is dried earlier than the central layer side, the wood is liable to crack in the surface layer containing much moisture where coarse annual rings are formed as compared with those on the central layer side, and a crack tends to gradually advance radially towards the central layer. SUMMARY OF THE INVENTION In view of the above-described problem, noticing, as a result of researches carried out on the basis of years of experiences and actual results, that the most important for purpose of crack prevention was to produce in a drying chamber a drying condition (a drying atmosphere) under which a drying speed (a rate of decrease in the moisture content) would be equalized between the surface layer side and the central layer side during artificial drying of wood, or the drying speed on the central layer side would be brought as close to the surface layer side as possible, the inventor et al. made a variety of researches in an attempt to realize the above-described drying conditions, developing the present invention, which has as an object the provision of an epoch-making process and equipment capable of completely drying at a lower cost the whole part of wood from the surface layer to the central layer without cracking. In order to accomplish the object stated above, a technical means pertaining to the present invention presents a drying process which includes a primary drying process that many pieces of wood stacked in multiple stages at spacings are held in a drying chamber equipped with an exhaust system for maintaining a chamber pressure at a normal pressure or in a state close thereto so that heat may pass among the pieces of wood, and then only live steam is fed into the drying chamber to heat the chamber interior from the normal temperature up to 90 to 100°C, thereby heating the whole interior ranging from the surface layer to the central layer of the wood; a secondary drying process that, after the central layer of the wood is heated up to 90 to 100°C, the chamber interior is heated up to the temperature of 100 to 200°C by the use of a heater, thereby boiling the moisture content in the wood for the purpose of high-temperature treatment up to a fiber saturation point in order to obtain a target percentage of moisture content; and a tertiary drying process that, when the moisture content has dropped to the fiber saturation point, drying chamber temperature conditions such as the dry-bulb temperature and the wet-bulb temperature are changed to treat the wood at low temperatures and moisture down to the target percentage of moisture content under the temperature conditions. Also, the present invention provides a drying process that at a point of time of the drying treatment of wood to the target percentage of moisture content by the tertiary drying process, the chamber temperature condition of only the wet-bulb temperature is changed to remove a residual drying stress and moisture content inclination of wood under the condition. A drying equipment disclosed in the present invention comprises a drying chamber of an approximately rectangular form as viewed in a plan view which has a wood entrance hermetically closed with a door on one side, and a boiler room disposed beside the drying chamber; in the upper part of the drying chamber are routed, from the boiler room, steam injection pipes for heating to raise the chamber temperature by supplying live steam into the chamber and a heater for heating to raise the chamber temperature by means of heat exchange; also in proper places inside the drying chamber are set an exhaust system for natural exhaust during heating to raise the chamber temperature to thereby maintain the chamber pressure at a normal value or close thereto, the steam injection pipes, and a circulating apparatus for circulating the heat from the heater to the entire area of the chamber interior during the heat-up; and furthermore on the floor of the drying chamber are laid rails for bringing in a truck loaded with multi-stage stacks of a number of timbers and for taking the truck out after the completion of drying. Furthermore, the circulating apparatus of the drying equipment is provided with a propeller shaft horizontally supported on a heat-resisting bearing at the lower end of a supporting member suspended from the ceiling, with one end of the propeller shaft protruding out of the drying chamber through a through hole which has a heat-resisting sealing member at the edge and opens in the side wall of the chamber. And a motor is disposed outside of the side wall on the axial center of the shaft, being connected with one protruding end of the shaft, and is mounted with a propeller fan on the other end of the shaft in the drying chamber. Furthermore the drying equipment has an opening-closing damper disposed in an exhaust passage provided in a proper place in the drying chamber and so designed as to automatically exhaust unidirectionally by utilizing the internal pressure of the drying chamber. According to claim 1 of the present invention, the drying chamber is heated from the normal temperature up to 90 to 100°C with the live steam alone; with this condition maintained, the heat is supplied throughout the chamber interior to heat multi-stage stacks of wood arranged at spacings through which the heat passes. At this time, the chamber humidity gradually rises with the rise of temperature, reaching 100% at a point of time when the temperature has reached 90 to 100°C. The heat, therefore, is gradually transmitted to the central layer of wood while the drying of the surface layer part of wood is being restrained by the atmosphere of the 100% humidity, thereby ensuring uniform and balanced heating of the entire inner part of the wood at temperatures near 90 to 100°C immediately before boiling of the moisture content while preventing the surface layer part of the wood from drying. That is, the primary drying is accomplished by the combination of balanced temperature and humidity for uniformized and balanced heating of the whole inner part of wood ranging from the surface layer to the central layer in accordance with a so-called drying schedule. Next, after the wood is heated sufficiently up to 90 to 100°C, the chamber interior is quickly heated up to 100 to 200°C by the heater, and by maintaining this state, the inner part of the wood is rapidly heated up to 100 to 200°C to thereby boil the moisture content in the wood and at the same time an increased pressure is applied to let the boiled moisture out of both ends of the wood, thus accomplishing the secondary drying to the fiber saturation point. At this time, the drying chamber pressure that has increased with the rise of the chamber temperature is discharged out of the drying chamber from by means of the exhaust system, thereby maintaining the chamber interior at the normal temperature or a temperature close thereto. Accordingly, the boiled moisture in the wood is blown off quickly, thereby enabling to equalize the drying speed (a rate of decrease in the moisture content) between the surface layer side and the central layer side, or to bring the drying speeds on both the surface layer side and the central layer side as close to each other as possible in order to realize the uniform and balanced drying of the entire inner part of the wood. Then, when the moisture content has reached the fiber saturation point, the drying conditions such as the dry-bulb temperature and the wet-bulb temperature within the drying chamber are changed according to the drying conditions of the wood and, by keeping the drying conditions for a specific period of time, the moisture content in the inner part of the wood is dried to the target moisture content, to thereby accomplish the tertiary drying. According to claim 2, when the wood has been dried up to the target percentage of moisture content, only the wet-bulb temperature as the drying conditions in the tertiary drying is changed and the changed temperature is kept for a specific period of time to perform drying, whereby the drying stress remaining in the wood and moisture content inclination of the wood thus dried to the target moisture content (a finish percentage of moisture content) are removed. According to claim 3, wood loaded in multiple stages on a truck on rails is brought into the drying chamber at the entrance with the door opened, and after the wood is unloaded in the drying chamber and the entrance door is closed, the boiler is operated to send live steam from the boiler room to the steam injection pipe routed in the upper part of the drying chamber. The live steam is then injected into the drying chamber from above in the chamber to thereby heat the chamber interior from the normal temperature up to 90 to 100°C. Then, the live steam heat is uniformly applied by the circulating apparatus to the whole wood stacked in multiple stages at spacings on the truck through which the heat passes, thus heating up each piece of the wood. When the internal pressure of the drying chamber is increased by heating to raise the chamber temperature with the live steam, the heated air is exhausted out of the drying chamber through the exhaust system, thereby maintaining the normal pressure or a pressure close thereto in the drying chamber. Then, when the drying chamber temperature is raised gradually to 90 to 100°C with the live steam, the humidity also gradually increases with the rise of the chamber temperature, reaching 100%. Therefore, under the condition that the drying of the surface layer part of the wood is held in the atmosphere of the 100% humidity, the heat is gradually transmitted to the central layer until the wood is heated up uniformly and in a balanced manner throughout its inner part at the temperature of 90 to 100°C or a temperature close thereto immediately before the boiling of moisture. That is, the wood can be heated up uniformly and equally through the entire inner part ranging from the surface layer to the central layer according to a so-called drying schedule made for ensuring a balanced relationship between temperature and humidity by the use of a combination of temperature and humidity. After the entire inner part of the wood has been heated up to the temperature of 90 to 100°C or close thereto immediately before boiling, a hot steam is subsequently fed from the boiler room to the heater disposed in the upper part in the drying chamber, thereby heating up the chamber interior quickly to 100 to 200°C by heat radiated from the heater. Then, the wood inner part is rapidly heated up to 100 to 200°C, whereby the moisture in the wood quickly starts boiling, being blown off at both ends of the wood. Thus the wood is dried up to the fiber saturation point. At this time, as the internal pressure of the drying chamber is naturally exhausted from the exhaust system, being held at the normal pressure or close thereto, the boiled moisture in the wood is blown off quickly at both ends of the wood to thereby balance the drying speed (a rate of decrease in the percentage of moisture content) between the surface layer side and the central layer side, or to bring the drying speed on the central layer side as close to that on the surface layer side as possible in order to ensure uniformized and balanced drying of the whole inner part of the wood. After the moisture content in the inner part of the wood has reached the fiber saturation point, the drying conditions including the dry-bulb temperature and the wet-bulb temperature in the drying chamber are changed in accordance with the dried condition of the wood; the dried condition is then kept for a specific period of time until the completion of drying, thereby drying the wood to the target percentage of moisture content in the wood. According to the technical means stated in claim 4, the motor is mounted outside to drive the proper fan in the drying chamber from outside, whereby the motor can be protected from the heat in the drying chamber where the drying temperature reaches 100°C or over. According to claim 5, the internal pressure in the drying chamber increases over the normal pressure when the drying chamber is heated up with the live steam emitted from the steam injection pipe disposed in the upper part of the drying chamber and the heat radiated from the heater, thus forcing the opening-closing damper of the exhaust passage to open. The internal pressure of the drying chamber exceeding the normal pressure then goes through the exhaust passage for natural exhaust out of the drying chamber, thereby maintaining the condition to force the boiling moisture out from both ends of the wood, that is, the internal pressure of the drying chamber at the normal pressure or close thereto. Other features and advantages of the present invention will become apparent from the following description of embodiment of the invention, together with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a front view, partly cutaway in section, showing an example of a drying equipment for executing a drying process of the present invention; Fig. 2 is a plan view, partly cutaway in section, of the example; Fig. 3 is an exploded perspective view showing upper and lower units constituting a drying chamber; Fig. 4 is a longitudinal sectional side view taken along line IV-IV of Fig. 1; Fig. 5 is an enlarged sectional view taken along line V-V of Fig. 1; Fig. 6 is an enlarged sectional view taken along line VI-VI of Fig. 2; Fig. 7 is an enlarged sectional view showing an exhaust fan connected to a through hole of an exhaust system; Fig. 8 is an enlarged sectional view taken along line VIII-VIII of Fig. 1; and Fig. 9 is a graph showing an example of experiments of drying conducted by the use of conifer logs with bark which have a top end diameter of 20 cm or less and contain 140% moisture. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter one example of a process and equipment embodying the present invention will be explained with reference to the accompanying drawings. Figs. 1 to 5 show one example of the drying equipment for performing the drying process (hereinafter referred to as the present process) for drying a raw log 1 with bark (hereinafter referred to only as the log); A denotes a drying chamber which is constituted, in an approximately rectangular form in a plane, of a floor A-1, side walls A-2 and a ceiling A-3 by filling a heat-insulating material 2-3 between an external wall material 2-1 produced of a waved steel plate and an internal wall material 2-2 produced of a ceramic-coated (surface-treated) stainless plate. The drying chamber is so constituted that many logs stacked in multiple stages in several horizontal rows at spacings through which the heat passes can be brought into the drying chamber by a truck 4 which travels on stainless rails 3 laid from the floor A-1 in the drying chamber towards the outside of the chamber through the entrance provided on one side thereof. On the side wall A-2 of the drying chamber A is installed an exhaust system 5 to prevent the chamber pressure from increasing with the rise of the chamber temperature. Natural exhaust through this exhaust system 5 keeps the pressure in the drying chamber A at the normal pressure or at a pressure close thereto. The drying chamber A of the present embodiment comprises a lower unit A' and an upper unit A''; the lower unit A' being factory-built by integrally assembling the side walls A-2 rising from the periphery of the floor A-1 to the entire floor A-1 to a halfway height and the upper unit A'' being also factory-built by integrally assembling remaining side walls A-2 to the entire ceiling A-3. The lower unit A' and the upper unit A'' thus built are carried to the site, where the upper edge of the side wall A-2 of the lower unit A' and the lower edge of the side wall A-2 of the upper unit A'' are connected by welding or other as shown in Fig. 3. The exhaust system 5 functions to maintain the internal pressure of the drying chamber at the normal pressure or at a pressure close thereto by releasing by natural exhaust the pressure out of the drying chamber A when the internal pressure has exceeded the normal pressure with the heat-up of the interior of the drying chamber A. As shown in Fig. 6, there is provided an opening-closing damper 5-2 in an exhaust passage 5-1 which is open in the side wall A-2 of the drying chamber A; the damper 5-2 is pushed to automatically open only in a direction of exhaust with the internal pressure. The opening-closing damper 5-2 is formed of a light-weight material so as to be opened with the internal pressure of the drying chamber A, and is axially supported at the upper edge to operably hang in the exhaust passage. Then, with the opening-closing damper 5-2 vertically hanging (in a closed position), a seal frame 5-3 for stopping the damper from opening on the drying chamber A side is mounted on the peripheral wall of the exhaust passage 51 so that the damper will open only in the direction of exhaust. Furthermore, in the exhaust passage 5-1 of the exhaust system 5, an exhaust fan 6 is inserted and connected for forcing the damper 5-2 to open when the drying equipment of the present invention is used in low-temperature drying, thus discharging the internal pressure as shown in Fig. 7. In the drawing, a reference numeral 7 refers to a drain hole provided in a form of passage in the floor A-1 of the drying chamber A, the passage being directed outwardly in order to rapidly discharge the moisture blown off of the log 1 outside without remaining on the floor A-1. This drain hole 7 functions to discharge the moisture forced out of the log 1, to the outside of the chamber, also serving as an exhaust port for discharging the chamber pressure outside by natural exhaust in order to restrain the increase of the chamber pressure with the rise of the chamber temperature. That is, the drain hole 7 keeps the normal pressure or a pressure close thereto within the exhaust system 5 as well as the drying chamber A. In the vicinity of the ceiling A-3 in the upper part of the drying chamber A are longitudinally set a heater 9 and a steam injection pipe 8 routed from the boiler room B beside the drying chamber A, so that a primary drying will be performed by heating the chamber interior up to 90 to 100°C with live steam injected from the steam injection pipe 8, and subsequently a secondary drying will be performed by heating the chamber interior up to 100 to 200°C quickly with the heat supplied from the heater 9, thereby drying the logs 1 containing 140% moisture up to the 25 to 28% fiber saturation point at which the moisture content of the logs 1 reaches the target value of 17%. The boiler used in the present embodiment has a maximum capacity to increase the chamber temperature up to the saturation temperature of about 164 to 170°C at the normal pressure of 7 to 8 kg/cm 2 and up to the saturation temperature of about 204°C at the normal pressure of 16 kg/cm 2 . A plurality of injection pipes 8 are installed on the right and left sides in the vicinity of the ceiling A-3 of the drying chamber A, for injecting the live steam from the boiler room B into the drying chamber at the injection port to thereby heat up the chamber interior from the normal temperature to 90 to 100°C. The heater 9 is a heat exchanger for heating the drying chamber A by heat exchange with the heating pipes 9-1 having a number of fins and routed in a vertical parallel form on the right and left on the ceiling A-3 of the drying chamber A. Steam is passed through each of the heating pipes 9-1 to exchange heat with the interior of the drying chamber A, thereby heating the drying chamber A interior up to 100 to 200°C with the live steam injected from the steam injection pipe 8. Furthermore, mounted in the vicinity of the ceiling A-3 in the upper part of the drying chamber A are several steam circulating apparatus 10 for circulating the live steam injected from the steam injection pipes 8 and the heat given by heat exchange with the heating pipes 9-1 throughout the chamber interior, so that all the logs loaded on each of several trucks 4 which have been brought into the drying chamber A will be heated uniformly. The circulating apparatus 10, as shown in Fig. 5, is of such a constitution that a propeller shaft 10-1 is axially supported horizontally on the lower end of a supporting member 11 hanging from the ceiling A-3, through a heat-resisting bearing 12, with its one end protruding out of the drying chamber A through a through hole 13 provided in the side wall A-2, and that a motor 10-2 is mounted outside of the side wall A-2 on the axial center of the propeller shaft 10-1; the motor 10 being directly coupled with the protruding one end of the shaft 10-1 by means of a coupling 14. Then, a propeller fan 10-3 is mounted on the other end of the propeller shaft 10-1 disposed in the drying chamber A. The propeller shaft 10-1 is produced of a heat-resisting stainless steel, with its one end side protruding out from the through hole provided in the side wall A-2 and with the other end side being axially supported through a heat-resisting bearing 12 to the lower end of the supporting member 11 suspended from the ceiling A-3 between both steam injection pipes 8 and the heater 9; the one end side protruded outside is axially supported on a bearing 16 mounted on a base 15 on which the motor 10-2 is mounted, whereby one end side of the propeller shaft protruding outside is directly coupled by the coupling 14 to the motor 10-2, while the other end disposed between the steam injection pipes 8 arranged on both sides in the upper part of the drying chamber A and the heater 9 is mounted with a propeller fan 10-3. The through hole 13 in which a stainless steel pipe is inserted to support the propeller shaft 10-1 protruding out of the drying chamber A is fitted with a heat-resisting seal member 17 on the outside open end thereof, thereby preventing the outflow (discharge) of the heat from inside the drying chamber. The heat-resisting bearing 12 is composed of a stainless steel bearing case, a shaft supporting case, and ceramic balls interposed between these cases, and is so designed as to smoothly support the propeller shaft 10-1 without being affected by the heat of the chamber interior. Therefore the motor 10-2 of the circulating apparatus for driving the propeller fan 10-2 through the propeller shaft 10-1, being disposed outside of the drying chamber, is designed to be free from the direct effect of the chamber heat exceeding 100°C. That is, the motor liable to a thermal effect is protected from the chamber heat, thus enabling to extend its lifetime. Next, log drying according to the present process using the drying equipment of the above-described constitution will be explained. First, the live steam is supplied from the boiler room B into the steam injection pipes 8 arranged in the upper part of the drying chamber A. The live steam alone is then injected from the steam injection pipes 8 to heat the drying chamber A up to a temperature of 90 to 100°C, so that 90 to 100°C heat is uniformly circulated by the circulating apparatus 10 throughout the chamber interior to heat up the logs 1 which are stacked in multiple stages at spacings on square lumbers 18 used as spacer blocks, through which the heat passes, with a weight 19 placed on the uppermost stage of the logs 1 brought by the truck 4 into the drying chamber A for the purpose of warp prevention of the logs 1 during drying, thus accomplishing the primary drying. Subsequently, the steam is supplied from the boiler room B to the heater 9 located in the upper part of the drying chamber A to heat the chamber interior rapidly up to 100 to 200°C by heat exchange with the heat radiated from the heating pipe 9-1 of the heater 9, thus heating up the logs 1 quickly for the purpose of high-temperature treatment at 100 to 200°C as the secondary drying process in which the logs 1 containing 140% moisture for example are dried to the target percentage of moisture content of 17% without cracking which is a vital problem of log drying. When the logs 1 have been dried to the target percentage of moisture content of 17%, only the wet-bulb temperature of the temperature conditions in the chamber is changed to remove a residual drying stress and moisture inclination of the wood under the temperature conditions. As the drying conditions of the drying chamber A in the present process, the chamber temperature is held within the range of from 90°C to 100°C in the primary drying. In the secondary drying, the chamber temperature is held within the range of from 100°C to 200°C. In the tertiary drying, it is necessary to hold the dry-bulb temperature to 70°C, and the wet-bulb temperature to the range of 64 to 69°C (a relative moisture of around 76 to 96%) in accordance with the dried condition of the logs 1 for the following reason. In the case of the primary drying, it is impossible to heat the inner part of the logs 1 fully and uniformly to the central layer thereof until the temperature reaches a temperature range immediately before the moisture in the logs boils, if the dry-bulb temperature in the drying chamber A is under 90°C. That is, in the secondary drying, when the drying temperature is quickly raised to 100 to 200°C, the moisture in the logs 1 must also quickly begin to boil almost simultaneously for rapid drying of the log inner part; if not, the surface layer part of the logs 1 which has a higher moisture content and is more liable to crack than the central layer side begins to dry first with the high-temperature heat of 100 to 200°C, producing a radial crack which starts in the surface layer part, developing towards the central layer. To describe more particularly, if the moisture inside of the logs takes a time to boil in the secondary drying, moisture evaporation from the surface layer part of the logs 1 will advance first, to thereby dry the surface layer part before the moisture boiling in the inner part begins, thus allowing occurrence of a crack in the surface layer part owing to circumferential shrinkage of the logs 1. In the meantime, if the chamber temperature exceeds 100°C, the moisture evaporation inside the logs 1 starts halfway, failing to rapidly blow off moisture from the ends of each log 1. At the same time, the moisture evaporation in the surface layer part proceeds, resulting in a failure in balancing the drying speed ( a rate of decrease in the percentage of moisture content) between the surface layer side and the central layer side, or in developing the drying condition (the drying atmosphere) under which the drying speed of the central layer side can be brought as closer to that of the surface layer side as possible. The drying temperature in the primary drying, therefore, is set to the range of from 90 to 100°C. By thus setting the drying temperature it becomes possible to allow the moisture to be quickly blown off at both ends of the log 1 instantly after the commencement of the secondary drying process, thereby presenting the drying conditions in the drying chamber A where the drying time can be equalized between the central layer side and the surface layer side or brought closer to each other. In the case of the secondary drying, if the drying temperature in the drying chamber A is 100°C or lower, there will take place the same trouble as in the case of the primary drying where the drying temperature exceeds 100°C; that is, quick moisture boiling in and expelling from the inner part of the logs 1 can not be expected. In the meantime, when the drying temperature exceeds 200°C, quick moisture boiling and expelling can be expected; reversely, however, moisture evaporation in the surface layer part of the logs 1 proceeds quicker than in the inner part, resulting in complete evaporation of moisture in, and accordingly carbonization of, the surface layer part before the moisture content decreases to a 25 to 28% fiber saturation point at which the moisture content in the inner part of the logs 1 reaches the target value. In the secondary drying, the drying temperature is set to the range of from 100 to 200°C, whereby the inner part of the logs 1 that has been heated, in the primary drying, to a point immediately before the boiling of moisture may quickly be heated up, so that the moisture may quickly be blown off for rapid drying of the inner part before the drying of the surface layer part advances. That is, it becomes possible to form the most desirable drying conditions (the drying atmosphere) in the drying chamber A in which the surface layer side and the central layer side of the logs 1 can be dried at an equal drying speed (a rate of decrease in the percentage of moisture content), or the drying speeds at these sides can be brought as close to each other as possible. Hereinafter an experimental example of the present process conducted under the above-described drying conditions will be explained. Chamber temperature Primary drying 90 to 100°C Secondary drying 100 to 200°C Tertiary drying Dry-bulb temperature, 70°C Wet-bulb temperature, 64 to 69°C (Relative humidity, approx. 76 to 96%) Logs used Kind Conifer with bark (Logs) Shape Top end diameter grade, 20 cm or less; length, 3 m A number of logs 1 are loaded in multiple stages on several trucks 4, with a spacing provided by interposing space blocks 16 between the logs 1 through which the heat passes, and are brought into the drying chamber 4. Then, with the door A-4 and an inspection door A-5 closed, the live steam is sent from the steam injection pipes 8 into the drying chamber 4, to thereby heat the drying chamber A interior up to 90 to 100°C. From a point of time when the chamber temperature has reached this temperature range, the primary drying was carried out while holding the temperature for a period of six to eight hours. It takes about two to three hours to heat the chamber interior from the normal temperature up to 90 to 100°C. During this period, the steam circulating apparatus 10 is started to send the live steam to the whole chamber interior until the heat reaches all of the logs 1 stacked at spacings through which the heat flows to heat up the logs 1. At the chamber temperature of 90 to 100°C, the humidity in the chamber interior gradually rises to 100% with the rise of the chamber temperature; the drying of the surface layer part of the logs is held by the atmosphere of the 100% humidity and in this state the heat is gradually transmitted to the central layer of the logs 1. With the surface layer being held from drying under this condition, the logs 1 are heated uniformly and equally up to 90 to 100°C or to a temperature close thereto immediately before the moisture in the entire inner part begins to boil. After the inner part of the logs 1 has been heated fully to the central layer up to about 100°C in six to eight hours, the heat is circulated by the circulating apparatus 10 throughout the chamber interior. By thus continuing heat circulation in the drying chamber, the chamber temperature is quickly raised by the heater 9 from the temperature of 90 to 100°C up to the temperature of 100 to 200°C. The secondary drying is performed at the chamber temperature of 100 to 200°C kept for about 24 hours. It takes about three hours to heat the chamber interior from 90 to 100°C up to 100 to 200°C. When the chamber temperature has reached the temperature of 100 to 200°C, the inner part of the log 1 is heated up quickly to 100°C and over to boil the moisture therein, and at the same time the boiled moisture is quickly blown off at both ends of the log with an increased internal pressure in the log 1. From the moisture content in the inner part of the log 1 after the lapse of 24 hours of drying, it is understood, as is clear from Fig. 9, that the log 1 containing 140% moisture has been dried to about 25 to 28% fiber saturation point in about four days. It also has been confirmed that in the secondary drying process, even when the chamber temperature has risen to 100 to 200°C, the opening-closing damper 5-3 of the exhaust system 5 is automatically opened to allow the internal pressure of the drying chamber A out through the exhaust passage 5-1, and therefore the chamber interior is constantly kept at the normal pressure or a pressure close thereto; the boiling moisture in the inner part of the log 1 is forced to be quickly blown off from both ends of the log 1, so that the surface layer side and the central layer side will be dried at an equal drying speed (a rate of decrease in the percentage of moisture content), or the drying speeds on both sides will be as close to each other as possible, thereby enabling uniform and equalized drying of the entire inner part of the log 1 without a crack while lowering the moisture content in the inner part of the log 1. After the moisture content has decreased to the fiber saturation point of 25 to 28%, the dry-bulb temperature in the chamber is changed to 70°C and the wet-bulb temperature to 68°C (relative humidity, approximately 91%) according to the dried condition of the logs 1. Then, after this condition is held for about one day, only the wet-bulb temperature is further changed to 66°C (relative humidity, approximately 83%), which is held for about one day to accomplish the tertiary drying. In this case, the humidity also is gradually lowered with the lowering of the chamber temperature, to enable to dry the inner part of the logs 1 to the target moisture content of 17% without cracking. Finally, the wet-bulb temperature alone is changed to the drying condition of 69°C (relative humidity, approximately 96%) as a process for removing the residual drying stress and moisture inclination (a difference in the moisture content between the surface layer and the central layer) of the dried logs 1. After the drying of the logs 1 is performed under the above-described drying conditions which are kept for about one day, the door A-4 is opened to bring the logs 1 out of the drying chamber A. According to the drying process of the present invention, it is possible to dry not only the logs 1 with bark stated above and also square timbers and plates made by sawing unbarked logs. That is, the entire inner part of square timers and plates can be fully dried to a target percentage of moisture content without cracking as in the case of drying of the logs 1 with bark stated above. It is to be noticed that wood to be dried includes conifers such as Japanese larch, spruce, red pine, etc. and broad-leaved trees such as oak, Japanese judas tree, shioji, etc. but is not limited thereto; that is, the wood to be dried by the present invention is wood to be used for structures (building materials), fixtures, and furniture. The drying process and equipment of the present invention, having the above-described constitution, have the following effect of operation. (1) In the primary drying, only the live steam is supplied to heat up the chamber interior to 90 to 100°C to thereby keep an atmosphere of 100% humidity. Since, in this atmosphere, the logs are heated up to 90 to 100°C, the drying of the surface layer part is restrained, so that the entire inner part of the logs including the central layer is uniformly and equally heated up to a temperature close to 90 to 100°C immediately before the boiling of moisture. Then, in the secondary drying, the chamber interior is quickly heated up to the temperature of 100 to 200°C and accordingly the inner part of the logs that has been heated up to the temperature immediately before the boiling of moisture in the primary drying is heated quickly up to 100 to 200°C, whereby the moisture in the inner part of the logs will be rapidly boiled to be blown out from the ends of the logs. Consequently the drying speed (a rate of decrease in the moisture content) of the surface layer side and that of the central layer side of the logs are balanced or the drying speed on the central layer side of the logs is brought as close to that on the surface layer side as possible, thereby drying the whole inner part of the logs uniformly and equally to the fiber saturation point. Finally, in the tertiary drying, drying conditions such as the dry-bulb temperature and the wet-bulb temperature are changed according to the dried condition of the logs; holding the drying conditions for a specific period of time, the logs are dried until the moisture content of logs containing 140% moisture reaches a target percentage of moisture content, for example a target percentage of 17% moisture content, thus completing drying. Therefore, according to the present invention, it is possible to dry wood under such drying conditions (in a drying atmosphere) that the drying speed (a rate of decrease in the percentage of moisture content), which is important to prevent cracking of wood during artificial drying, on both the surface layer side and the central layer side is equalized or the drying speed on the surface layer side is brought as close to that on the central layer side as possible, thereby enabling to dry the logs fully to the target percentage of moisture content without cracking. The drying process of this invention is an epoch-making process capable of realizing prosperity of the construction industry by preventing cracking of wood likely to occur in the prior art drying process, thus contributing to the supply of dried wood at a low cost notwithstanding the recent circumstance of the construction industry which is suffering high costs resulting from a lack of resources, and particularly from insufficient supply of construction material. In addition, the process can dry wood by quickly blowing moisture out of both ends of wood, thus ensuring substantial curtailment of drying period as compared with the prior art drying technique which dries wood by moisture evaporation through gradual moisture evaporation from the surface layer part of wood, and consequently contributing to improvements in production efficiency and large reduction of drying cost. (2) A motor is of an external type mounted outside of the drying chamber that a propeller fan in the drying chamber is driven from outside, and accordingly can be protected from heat in the chamber where the drying temperature rises to 100°C or higher. Consequently, giving proper considerations to the protection of the motor from direct thermal effect in the drying chamber where the drying temperature exceeds 100°C can extend the life of the motor. (3) The drying chamber pressure increases over the normal pressure with the rise of the temperature in the drying chamber interior which is heated up with the live steam injected from steam injection pipes arranged in the upper part of the drying chamber and heat radiated from a heater. Then, an opening-closing damper disposed in an exhaust passage communicating with the outside is automatically opened with the internal pressure, thereby discharging the internal pressure out of the drying chamber through the exhaust passage and accordingly maintaining the condition under which the boiling moisture in the inner part of wood is quickly blown out at both ends of wood, thus constantly keeping the drying chamber pressure at the normal pressure or a state close thereto. Therefore it is possible to form in the drying chamber the drying conditions under which the boiling moisture in the inner part of wood is forced out rapidly at the ends of wood.

Claims: A process for drying wood, comprising primary drying in which many pieces of wood stacked in multiple stages at spacings are held in a drying chamber equipped with an exhaust system for maintaining an atmospheric pressure or in a state close thereto so that heat may pass among the pieces of said wood, and then only live steam is fed into said drying chamber to heat a chamber interior from an ambient temperature up to 90 to 100°C, thereby heating the whole interior ranging from a surface layer to a central layer of said wood; secondary drying in which, after said central layer of said wood is heated up to 90 to 100°C, said chamber interior is heated up to a temperature of 100 to 200°C by the use of a heater, thereby evaporating moisture in said wood for the purpose of high-temperature treatment up to a fiber saturation point in order to obtain a target percentage of moisture content; and tertiary drying in which, when the moisture content has dropped to said fiber saturation point, drying chamber temperature conditions such as the dry-bulb temperature and the wet-bulb temperature are changed to treat said wood at low temperatures and moisture down to said target percentage of moisture content under said temperature conditions. A process for drying wood according to claim 1, wherein only the wet-bulb temperature of said temperature conditions is changed at a point of time when said wood has been dried to said target percentage of moisture content, thereby performing drying treatment to remove a residual drying stress and moisture inclination of said wood. An equipment for drying wood comprising a drying chamber of an approximately rectangular form as viewed in a plan which has a wood entrance hermetically closed with a door on one side, and a boiler room disposed beside said drying chamber; in the upper part of said drying chamber are routed, from said boiler room, steam injection pipes for heating up a chamber by supplying live steam into said chamber and a heater for heating to raise the chamber temperature by means of heat exchange; also in proper places inside said drying chamber are set an exhaust system for natural exhaust during heat-up of the chamber temperature to thereby maintain the chamber pressure at an atmosphäric value or close thereto, said steam injection pipes, and a circulating apparatus for circulating the heat from said heater to the entire area of said chamber interior during the heat-up; and furthermore on the floor of said drying chamber are laid rails for bringing in a truck loaded with multi-stage stacks of a number of timbers and for taking said truck out after the completion of drying. An equipment for drying wood according to claim 3, wherein said circulating apparatus is provided with a propeller shaft horizontally supported on a heat-resisting bearing at the lower end of a supporting member suspended from a ceiling, with one end of said propeller shaft protruding out of said drying chamber through a through hole which has a heat-resisting sealing member at the edge and opens in a side wall of said chamber; and a motor is disposed outside of said side wall on the axial center of said shaft, being connected with one protruding end of said shaft, and is mounted with a propeller fan on the other end of said shaft in said drying chamber. An equipment for drying wood according to claim 3, wherein said exhaust system has an opening-closing damper disposed in an exhaust passage provided in a proper place in said drying chamber and is designed to automatically exhaust unidirectionally by utilizing the internal pressure of said drying chamber.

Inventors: NARA NAOYA, SATO JIRO, NARA, NAOYA, SATO, JIRO
Applicants: NARA NAOYA, SATO JIRO, NARA, NAOYA, SATO, JIRO
Application year: 1995
Main classification: F26B21
IPC codes: F26B9, F26B21