RU2182291C2 - Method for drying of wood blanks and installation for its realization - Google Patents

Abstract

FIELD: wood working, in particular, process of conveyer drying of wood blanks in a radio-frequency (r.f.) field, drying of prepacked wood blanks inclusive. SUBSTANCE: the method of conveyer drying of wood blanks consists in their heating in a radio-frequency field between horizontally oriented plate electrodes 4,5 of the working capacitor in drying chamber 2 with a simultaneous feed of air to chamber 2 and exhaust of it from the upper part of chamber 2; in accordance with the invention blanks 13 are installed with their end surfaces perpendicularly to the vector of the electric field strength, blanks 13 are so arranged that their lower end surfaces would be positioned above the level of the surface of conveyer belt 3, forced air blowing of blanks 13 is effected by heated air fed under the lower end faces of blanks 13. The installation for realization of the method has radio-frequency generator 1, drying chamber 2 accommodating upper and lower plate electrodes 4,5 of the working capacitor, conveyer belt 3 for movement of wood blanks 13 through drying chamber 2 between electrodes 4,5 of the systems of air feed to the drying chamber, and at least single hole 11 for air exhaust located in the upper part of drying chamber 2, the installation has rectangular-shaped containers 12 positioned on conveyer belt 3 for installation of blanks 13 in them in the vertical position, the containers have side walls and a row of rectangular partitions 14, the partitions and the side walls of the container parallel to them are provided with horizontal side bosses positioned above the bearing platforms under the edge sections of the lower end faces of blanks 13, one of the walls of container 12 that is perpendicular to the vertical partitions has slotted opening 16 in the lower part made lengthwise the wall to a height corresponding to the level of arrangement of the horizontal side bosses, the installation system of air feed is made in the form of air heater 7 with fan 8 connected through pipe-line 9 to tubular distributors 10 installed lengthwise conveyer belt 3 with the outlets inside chamber 2 at the level providing air passage through slotted openings 16 in the walls of containers 12, containers 12 are positioned on conveyer belt 3 so that their slotted openings 16 would face the outlets of the tubes of air distributors 10. EFFECT: process of radio-frequency drying fitted to wood blanks with a various shape of cross- section, to prepacked blanks inclusive. 3 cl, 3 dwg positioned above the bearing surface of container 12 and forming the bearing platforms under the edge sections of the lower end faces of blanks 13, one of the walls of container 12 that is perpendicular to the vertical partitions has slotted opening 16 in the lower part made lengthwise the wall to a height corresponding to the level of arrangement of the horizontal side bosses, the installation system of air feed is made in the form of air heater 7 with fan 8 connected through pipe-line 9 to tubular distributors 10 installed lengthwise conveyer belt 3 with the outlets inside chamber 2 at the level providing air passage through slotted

Description

 The invention relates to the field of woodworking, and in particular to the technology of conveyor drying of wood billets in a high-frequency (HF) electric field, including the drying of pre-compacted wood billets.

 Currently known methods of drying wood, based on the heating of wood billets in an HF electric field in conveyor drying chambers. High-frequency heating provides intensification of the wood drying process in comparison with the widely used technology for drying wood billets in the heat carrier flow.

 A known method of conveyor high-frequency drying of coil billets of wood and installation for its implementation, in which they control the drying process by changing the interelectrode gap of the working capacitor, while ensuring the same drying conditions for all billets by organizing their laying in one layer in the conveyor groove using automatic feeder [a.s. USSR N 100924, class 82 a, 8].

 A known method and installation of high-frequency drying of wood billets with simultaneous air circulation through a conveyor drying chamber and automatic adjustment of the feed rate of the dried material depending on its properties [J. Woodworking industry, N 3, 1963, with. 33]. The specified method and installation allows you to dry wood billets having significant linear dimensions in the transverse and longitudinal directions.

 As the closest analogues of the claimed method and device, the authors chose the method and installation of high-frequency drying [Sat. Metal-cutting and woodworking machines, automatic lines, cutting and instrumentation, 1965, v.3, NIIMASH, p. 21-23].

 According to the specified method, RF heating of wood billets, in particular boards, is carried out according to a special program between horizontally oriented plate electrodes of the working capacitor in the drying chamber, for which they sequentially act on the workpieces placed on the conveyor belt with several RF fields providing different temperature parameters. In this case, the microclimate in the drying chamber is regulated by supplying air to the space under the lower electrode and venting the air together with water vapor from the upper part of the chamber. The dried workpieces are laid horizontally on the conveyor belt so that their end surfaces are oriented along the electric field vector of the working capacitor.

 The installation for placing this method includes an RF generator, a drying chamber with lower grounded and upper high voltage, divided into several sections, plate electrodes, between which there is a conveyor belt for moving workpieces. Sections of the upper electrode are installed relative to the lower electrode each at a certain height in accordance with the drying program. An air distribution sheet is placed under the lower electrode, under which an air intake device is installed. In the upper part of the chamber there is a ventilation hole for the release of air and moisture vapor released from the wood.

 The specified method and device provide high-quality drying for a limited range of blanks.

 Placing wood blanks on a conveyor belt so that their end surfaces are oriented along the electric field vector of the working capacitor leads to the creation of unequal heating conditions for blanks, the height of which in the cross section varies along the width of the workpiece, for example, in the case of drying workpieces having in the transverse sectional shape of a circle, hexagon. In this case, due to the fact that the thickness of the layer of wood material in the direction of the electric field vector is not the same, the field strength in the material is different for different sections of the workpiece. This leads to uneven heating of the material in the volume of the workpiece, and, as a result, to warping, cracking of wood, deterioration in the quality of drying.

 The objective of the proposed method and device is the creation of high-frequency drying technology suitable for wood blanks with various cross-sectional shapes, including pre-compacted blanks.

 This problem is solved by the fact that in the method of conveyor drying of wood billets by heating them in an RF electric field between horizontally oriented plate electrodes of a working capacitor in a drying chamber while supplying air to the chamber and extracting it from the upper part of the chamber according to the invention, the workpieces are installed with end surfaces perpendicular to the vector electric field intensity, and the workpieces are placed in such a way that their lower end surfaces are located above the level nya surface of the conveyor belt, which perform forced air cooling preforms heated air supplied under the lower ends of the preforms.

The most effective is the supply of air heated to a temperature of from 80 ^o C to 110 ^o C.

 This problem is also solved by the fact that the installation for implementing the method, comprising an RF generator, a drying chamber with the upper and lower plate electrodes of the working capacitor placed in it, a conveyor belt for moving wood billets through the drying chamber between the electrodes, an air supply system to the drying chamber and at least one hole for extracting air, located in the upper part of the drying chamber, according to the invention contains rectangular containers placed on a conveyor belt on the day of installation of the blanks in them in an upright position, having side walls and a number of rectangular partitions, partitions and parallel side walls of the container are provided with horizontal side protrusions located above the supporting surface of the container and forming supporting platforms for edge portions of the lower ends of the blanks, one of the walls of the container perpendicular to the vertical partitions, has a slotted opening in the lower part, made along the length of the wall to a height corresponding to the horizontal level lateral protrusions, the air supply system of the installation is made in the form of a heater with a fan, connected via a pipe to pipe distributors installed along the length of the conveyor belt with outlet openings inside the chamber at a level that allows air to pass through the slotted openings in the walls of the containers, while the containers are placed on the conveyor tape so that their slotted openings are facing the outlet openings of the tubes of the air distributors.

 A feature of the proposed method is the impact on the dried blanks of the RF field in combination with forced blowing them with heated air, with the position of the blanks in the drying process relative to the plates of the working capacitor is important.

 With the vertical position of the workpieces between the horizontally oriented plate electrodes of the working capacitor, the same thickness of the layer of the heated material is ensured in the direction along the electric field vector of the capacitor for any shape of the cross section of the workpiece. This achieves the same magnitude of the electric field in the workpiece material, and, accordingly, the power released in a unit volume of the material, which ensures uniform heating of the workpieces and high quality drying.

 The installation of blanks with a gap relative to the surface of the conveyor belt allows you to organize the supply of heated air under the lower ends of the blanks, while due to blowing the blanks from the bottom with an upward flow of heated air, the drying process is accelerated.

 At the same time, the supply of heated air to the specified area ensures the heating of the structural elements of the installation located in close proximity to the lower ends of the workpieces, in particular the conveyor belt, which prevents the formation of condensate on the surfaces of these structural elements of the installation.

 When drying wood, especially when drying pre-compacted preforms, it is possible to release moisture droplets from the wood material and drip them down, which can lead to sparking and electrical breakdowns in the interelectrode space when moisture accumulates. By supplying a stream of heated air to the lower ends of the workpieces, the evaporation of moisture drops is accelerated and their accumulation is prevented.

 Due to the extraction of heated air from the upper part of the drying chamber, along with blowing the workpieces with an upward flow of air, the heating of the upper electrode is ensured, thereby preventing the formation of condensate on its surface.

It was experimentally established that the greatest efficiency of the method is achieved by supplying air heated to a temperature of from 80 ^o C to 110 ^o C.

 The inventive installation provides the implementation of the described technology of wood drying.

 Necessary according to the method, the location of the workpieces between the plates of the working capacitor is ensured by placing the workpieces in containers of a special design, in which there are a number of vertical partitions. Available on the partitions and parallel to the walls of the container, the lateral horizontal protrusions form supporting platforms under the edge sections of the lower ends of the workpieces. Moreover, due to the fact that these lateral protrusions are located above the supporting surface of the container, the lower ends of the workpieces are higher than the surface level of the conveyor belt.

 For forced supply of heated air to the drying chamber, tubular distributors are placed along the conveyor belt, connected by a pipeline to heaters and a fan. At the same time, for supplying heated air under the lower ends of the workpieces, the containers have a slot opening made in the lower part of one of the walls. For free passage of air under the ends of all the workpieces installed in rows between adjacent vertical partitions, the slotted opening is made in the wall located perpendicular to the specified partitions. At the same time, the height of the slotted opening corresponds to the distance by which the supporting platforms for the blanks are raised above the level of the supporting surface of the container, and the outlet openings of the tubular distributors facing the slotted openings of the containers are set at the level of the location of the indicated slotted openings.

 By placing a ventilation hole in the upper part of the chamber, an upward flow of air is supplied in it, supplied under the lower ends of the workpieces.

 The method is as follows.

 Drying of wood billets is carried out in a drying conveyor chamber connected to an RF generator that supplies voltage to the horizontally oriented plate electrodes of the working capacitor.

 Wet loose or pre-compacted wood preforms are fixed in a certain way on the conveyor belt so that the end surfaces of the preforms are oriented perpendicular to the capacitor electric field vector, and the lower ends of the preforms are located above the surface level of the conveyor belt.

 The workpieces fixed in this way are moved between the plate electrodes in the drying chamber, and the workpieces are heated in the RF electric field of the capacitor. Simultaneously with the movement of the workpieces, heated air is pumped into the drying chamber and drawn from the upper part of the chamber, while air is supplied under the lower ends of the workpieces.

Air should be supplied, heated to a temperature of (80-110) ^o C.

 The parameters of the RF field, which determine the heating temperature of the workpieces during the drying process, and the conveyor belt speed, which determines the duration of the drying process, are set depending on the size of the workpieces, their initial moisture content, and also the degree of compaction in the case of drying the compacted workpieces.

 Examples of the method.

 Example 1

 The birch billets, which passed the preliminary compaction stage, were dried. The degree of compaction of the workpieces was 1.25. The initial moisture content of the blanks was 31%. The blanks were in the form of regular prisms with a hexagonal base. The length of the blanks is 400 mm, the distance between opposite sides is 70 mm.

 The blanks were fixed vertically on the conveyor belt with a gap of 30 mm between the lower ends of the blanks and the surface of the belt.

The workpieces fixed on the conveyor belt were moved between the capacitor plates installed at a distance of 520 mm from each other. The operating frequency of the generator and the voltage at its output were 13.56 MHz and 7.2 kV, respectively. The speed of the conveyor belt was 0.9 m / h, the temperature of the air pumped into the drying chamber was maintained in the range from 80 ^o C to 110 ^o C.

 Drying time was 6 hours 40 minutes.

 The moisture content of the blanks at the end of drying was equal to an average of 9.8%.

 The dried billets retained their original shape, did not contain cracks. The side surfaces of the workpieces had an even whitish color, which testified to the uniformity of drying of the workpieces along their length.

 Example 2

 The pre-compacted alder blanks were dried. The degree of compaction of the blanks is 1.25, the initial moisture content of the blanks is 29%. The blanks had the shape of a cylinder. The length of the blanks is 400 mm, the diameter of the blanks is 100 mm.

 The blanks were fixed on the conveyor belt, as described in example 1. The distance between the plates of the condenser, the values of the operating frequency of the generator and the voltage at its output, as well as the temperature of the air pumped into the drying chamber were the same as in example 1. The speed of the conveyor belt was 0 82 mph

 Drying time was 7 hours 19 minutes.

 The moisture content of the workpieces after drying was an average of 11%.

 The dried billets retained their original shape, did not contain cracks. The lateral surfaces of the workpieces had an even slightly yellowish color, which testified to the uniform drying of the workpieces along the length.

Figure 1 presents a General view of the installation, designed to implement the proposed method (front view); figure 2 is the same (view aa in figure 1); figure 3 is the same (view BB in figure 2 with a rotation of 90 ^o ).

 The installation includes an RF generator 1, a drying chamber 2, equipped with a conveyor, including a conveyor belt 3, which moves between the upper sectioned high-voltage electrode 4 and the lower grounded electrode 5. The open side sections of the chamber 2 are shielded using rectangular waveguide filters 6 . The sections of the electrode 4 are equipped with mechanisms of vertical and inclined movement (not shown in the drawing).

 The installation includes a system for pumping heated air into the chamber 2, which contains an electric air heater 7 with a fan 8, a pipe 9 connected to tubular distributors 10 installed inside the chamber 2 along the length of the conveyor belt 3. In the upper part of the chamber 2 there are ventilation holes 11 connected to the system exhaust ventilation.

 The installation also contains containers 12 of a rectangular shape for fixing the dried blanks 13 on the conveyor belt 3.

 The containers 12 are made of radiolucent material, for example fluoroplastic. On two opposite walls of the containers 12, vertical partitions 14 are fixed, having a cross section in the shape of an inverted letter "T". Partitions 14 have a height less than the height of the walls of the container 12, due to which there is a gap between the partitions 14 and the supporting surface of the container 12. On the walls of the container 12, parallel to the partitions 14, there are lateral horizontal strips 15 placed at a height consistent with the level of arrangement of the horizontal protrusions of the partitions 14.

 One of the walls of the container 12, perpendicular to the partitions 14, has a slotted opening 16 in the lower part for air to pass under the ends of the workpieces 13. The containers 12 are mounted on the tape 3 so that their slotted openings 16 are located opposite the outlet nozzles of the tubular distributors 10, and the tubular ones distributors 10 are installed in the chamber 2 at a level corresponding to half the height of the slotted opening 16 of the containers 12.

 The installation is equipped with a drive control circuit 17 of the conveyor. The specified circuit includes moisture meters 18 and 19, respectively installed at the entrance to the chamber 2 and at the exit from it, and connected to the signal processing device 20, the output signal of which controls the speed of the drive 17.

 Installation works as follows.

 When the RF generator 1 is turned on, a high voltage is applied to the high-voltage electrode 4 of the chamber 2.

 The blanks 13 are placed in containers 12 in rows between the partitions 14, placing them vertically on the supporting platforms formed by the lateral protrusions of the partitions 14 and the straps 15. The containers 12 are mounted on the tape 3 so that their slotted openings 16 are facing the nozzles of the tubular distributors 10.

 They include an exhaust ventilation system, as well as a heated air injection system.

 Turn on the drive 17, setting the initial speed of the tape 3 in manual mode.

 Further, during the drying process, the speed of the tape 3 is automatically adjusted according to the readings of the moisture meters 18 and 19, and the signal from the moisture meter 18 sets the speed of the tape 3 corresponding to the initial moisture content of the material of the batch of billets 13 being dried, and the speed of the tape 3 is adjusted according to the signal from the moisture meter 19 according to the drying results.

 The blanks 13 in containers 12 are moved in chamber 2 between the electrodes 4 and 5, being exposed to an RF field, as well as the effect of an upward flow of heated air pumped into chamber 2. At the end of drying, containers 12 passing through chamber 2 are transferred to a receiving table ( in the drawing is not shown), where upon cooling the workpieces, the containers 12 are unloaded.

Claims (3)

 1. The method of conveyor drying of wood billets by heating them in a high-frequency electric field between horizontally oriented plate electrodes of the working capacitor in a drying chamber with simultaneous air supply to the chamber and extracting it from the upper part of the chamber, characterized in that the workpieces are installed with end surfaces perpendicular to the electric tension vector fields, and the blanks are placed so that their lower end surfaces are located above the level of the surface of the conveyor belt you, at the same time, carry out forced blowing of the workpieces with heated air supplied under the lower ends of the workpieces. 2. The method according to p. 1, characterized in that the air is supplied, heated to a temperature of 80 - 110 ^o C.  3. Installation for implementing the method according to claim 1 or 2, including an RF generator, a drying chamber with upper and lower plate electrodes of the working capacitor placed in it, a conveyor belt for moving wood billets through the drying chamber between the electrodes, an air supply system to the drying chamber, and at least one hole for extracting air located in the upper part of the drying chamber, characterized in that the installation comprises rectangular containers placed on a conveyor belt for installation in them, blanks in an upright position, having side walls and a series of rectangular partitions, partitions and parallel side walls of the container, are provided with horizontal side protrusions located above the supporting surface of the container and forming supporting platforms for edge portions of the lower ends of the blanks, one of the container walls perpendicular to the vertical partitions, has a slotted opening in the lower part, made along the length of the wall to a height corresponding to the level of horizontal lateral protrusions Then, the air supply system is made in the form of a heater with a fan, connected via a pipe to pipe distributors installed along the length of the conveyor belt with outlet openings inside the chamber at a level that allows air to pass through the slotted openings in the walls of the containers, while the containers are placed on the conveyor belt so that their slotted openings are facing the outlet openings of the air distributor tubes.

Images