Vacuum drying– this is processing various materials, which has now become very popular in many directions. This type of processing is used in food, industrial and similar industries that require equipment to carry out such processes.
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If we talk about the industry where vacuum drying has become an integral part, then, of course, this is the furniture industry, where without carrying out such a process, creation cannot take place. quality furniture. This type of processing is used in the furniture industry during primary wood processing. After going through the process of vacuum drying, the tree finds its characteristic properties. Firstly, it becomes of higher quality, secondly, more practical, and most importantly, furniture made from such wood looks simply incredibly beautiful. In other industries, vacuum drying has a completely different operating principle. In addition to the furniture industry, this process is also very popular in the food industry, but the operating principle there is completely different. Vacuum drying is labor-intensive process, which has a huge number of nuances that should never be forgotten:
- The first point is the availability of a large amount of high-quality equipment, which is an integral part of such a process.
- The second point is high equipment performance, without which it is impossible to achieve high results efficiency will be quite problematic.
- The third point is the place where the vacuum drying process will take place. In order for everything to happen efficiently, you need to have a place that will meet the standards of humidity, air, light and similar criteria that play a huge role in this matter.
The cost of vacuum drying is a rather controversial issue, and giving an exact answer to it is very problematic. The price of such a process directly depends on many factors, ranging from the material that will be processed and ending with the vacuum equipment in which this process will take place. An equally important aspect is also the time in which you need to complete the vacuum drying process. All this plays a certain role, which is why the price of such a process can be completely different. All these aspects play a big role, which means that they must be taken into account in any case if you want to get a specific answer to what the result of vacuum drying will subsequently be.
Vacuum drying technology
Vacuum drying technology also boasts a lot of interesting aspects. Now we will try to analyze the principle of operation of vacuum drying, taking wood processing as an example, since it is this material that most often lends itself to vacuum drying.
We have already said earlier that before creating furniture, the wood must undergo a vacuum drying process, which will allow all excess liquid to be extracted from it and the wood to be significantly reduced in size. Main processes of vacuum drying:
- Removing excess liquid from wood using evaporation
- Circulation of water through wood
First, circulation occurs in the system, since this process takes much longer. The speed of processing wood using the vacuum drying method directly depends on the speed of water circulation in the wood. The main part of the vacuum drying process is the complete drying of the wood, and this is done so that the wood loses its extra dimensions and has only its own mass, which will later be used in the creation of furniture. It's no secret that in this industry there is a rule that states that before creating wood products, its surface must be completely free of wood. Since only in this case will it be possible to extract moisture from the core of the tree, which will come out through circulation. But it is worth noting that in addition to vacuum drying of wood, this technology is also actively used in other areas, which we will now tell you about:
- Vacuum drying of meat
- Vacuum drying of cheese
- Vacuum drying of powders
- Vacuum drying of liquid-viscous products
- Vacuum drying of milk
This is only part of the list of areas where the vacuum drying process is actively used. This technology has already gained enormous popularity, which is only growing every day. This technology is truly unique, which is why it began to be used in many enterprises that now simply cannot function fully without this process. This means that there is a possibility of further spread of vacuum drying. It is possible that this process will soon be applied in all areas without exception, giving the opportunity to increase productivity for many enterprises.
Vacuum drying chambers
Vacuum drying chambers are mechanisms that are designed to carry out a huge number of labor-intensive tasks. We have previously talked about how beneficial the vacuum drying process is. As for chambers for vacuum drying, this is something without which such a process simply cannot be carried out. Vacuum drying chambers play a huge role in this regard, and without them, you can completely forget about some performance indicators.
Price segment vacuum chambers for drying at the moment it is far from the lowest. If you wish, you can find already supported installations that will be quite average in terms of price. But it is best to buy new installations, since only they can provide you with all possible quality guarantees. On modern market You can see a huge number of types of chambers for vacuum drying, which can have completely different purposes. All these cameras have their own functional features, as they are used in a specific industry. Now we will look at the main types of drying chambers:
- Vacuum drying cabinet
- Vacuum chamber for drying fruits
- Drying chamber for drying food
- Drying chamber for wood
The operating principle of each camera is individual, as it is designed to work in a specific industry. But in any case, we can say that such equipment at the moment is really very effective.
Vacuum freeze drying
Freeze drying is a process that is based on the constant sublimation of ice crystals of frozen products. This process, in a short period of time, produces high-quality dehydration of products, and does it with truly high quality.
The important point is that dehydration does not in any way affect Chemical properties products that remain just as useful in the future. In addition, vitamin activity also remains in the same form, chemical composition and anatomical structure. Vacuum freeze drying consists of three key steps:
- Primary freezing of the product
- Sublimation of ice without heat inside the system
- Final drying in special heated chamber
Based on this, we can conclude that the efficiency of freeze drying is at a high level and this process is really worth the money invested.
Vacuum drying of wood
In previous sections we took this type of drying as an example. As a result, we came to the conclusion that this type of processing is very effective, but also has a certain number of nuances that are also worth paying attention to.
The vacuum drying process can only be effective if the environment in which it occurs fully complies with all standards of humidity, temperature, light and similar aspects. The cost of vacuum drying currently fluctuates within the average price segment. If you wish, you can find a place where such a process will cost you just a few pennies, but in this case, you will not receive any guarantees of quality. It is best to overpay a little, but in the end get a high-quality result of work, which can be used in the future in the production of reliable and beautiful furniture.
There is not a single woodworking enterprise that can do without the wood drying procedure. To prevent the occurrence of various defects, it is customary to use special technology drying wood in a drying chamber. If you want to produce wood products yourself, you will also need a drying chamber for drying wood. Today we will talk about how to do it correctly.
The need to dry wood
How to dry a board efficiently and quickly? This question has interested every carpenter since ancient times. People have long been engaged in storing wood for many years in order to have time to dry it evenly. The grandfather prepared wood for his grandson, using the very material that his grandfather left him.
The importance of properly dried wood is colossal! For example, if wooden furniture, which is in the room, is made of too wet wood that has just been cut down, then it will dry out over time, because wood can dry out and decrease in size, which means it will deteriorate!
If the door to the house is made of excessively dry wood, it will swell over time and will not be able to close! If the door panel is assembled from blanks that are unevenly dried in volume, then it may burst or warp! Therefore, it is recommended to dry all wood blanks. In addition, drying protects the material from damage by wood-destroying fungi, prevents the size and shape of wood, and improves the physical and mechanical properties of wood.
Drying wood is a lengthy, complex and expensive procedure. Using traditional technologies, wood is heated with superheated steam or hot air. Dried wood can be transported and stored longer. In addition, during operation it does not deform. The boards are dried in steam chambers, where the possibility of internal damage is excluded.
Wood moisture concept
To fully understand the essence of the drying process, it’s worth diving a little into the theory. The procedure for removing moisture from wood is not entirely simple, because there are two types of moisture in the material itself. Wood consists of elongated plant cells. Moisture can be found in the cell walls and in their cavities, filling the microcapillary system. The moisture that is present in the spaces between cells and in their cavities is called free intercellular, and moisture in the cell walls is called bound intracellular.
Content in wood bound moisture limited. The state when the cell walls are characterized by maximum humidity when in contact with liquid moisture is called their saturation limit. It is generally accepted that the moisture content of the saturation limit does not depend on the rock and is on average 30%. If the wood's humidity is above 30%, then it contains free intercellular moisture. The wood of a freshly cut or growing tree has a moisture content above the saturation limit, that is, it is raw.
Depending on the purpose of wood blanks, wood is usually dried in different ways. The wood is dried to a moisture content of 6 - 8%, when the material is needed for mechanical processing and assembly of products for high-precision critical connections that affect performance (the production of skis, parquet or musical instruments).
Transport humidity is 18 - 22%. It is with this water content that lumber is suitable for transportation over long distances in warm seasons. Wood dried to such a moisture content is used mainly in standard house construction, in the production of ordinary containers and when there is no need for interchangeability during assembly.
Carpentry dampness is divided into several subspecies. Molded products (deck boards, sheathing, floor boards, casing) must have a moisture content of 15 ± 2%. Wood products (windows, doors, stairs and interior elements) made from solid or laminated wood can withstand humidity fluctuations from 8 to 15%.
Furniture humidity, depending on the level of the product and the use of solid or laminated wood, is 8 ± 2%, because it is at this humidity that wood exhibits the most optimal characteristics for processing, gluing and subsequent use. But it is usually customary to reduce the humidity to 7-10%, performing partial sterilization of the wood and taking into account the uniformity of humidity throughout the tree, preserving mechanical properties material, absence of surface and internal cracks.
Wood drying modes
Depending on the requirements for the quality of wood, lumber can be dried in different modes, which differ in temperature level. In the mini drying chamber for wood, during the drying process, the air temperature gradually increases in steps and decreases relative humidity agent. Drying modes are selected taking into account the thickness of the lumber, the type of wood, the final moisture content, the quality category of the wood being dried and the design of the chamber.
There are low- and high-temperature process modes. The first modes involve the use of moist air as a drying agent, the temperature of which in the initial stage is less than 100 degrees. There are three categories of these modes:
- The soft mode is capable of ensuring defect-free drying of the material while maintaining the natural physical and mechanical properties of the wood, including color and strength, which is important for drying wood to the transport moisture content of export lumber.
- The normal mode guarantees defect-free drying of wood with almost complete preservation of the strength of the material with minor color changes, which is suitable for drying lumber to its final moisture content.
- The forced mode maintains strength for static bending, compression and tension, but there may be a slight decrease in splitting or spalling strength with darkening of the wood, which is intended for drying wood to operational humidity.
For low-temperature modes, a three-stage change in the parameters of the drying agent is assumed, and from each stage to the next one the transition can be made only after the material reaches a certain level of humidity, which is provided for by the mode.
High-temperature modes provide for a two-stage change in the parameters of the drying agent, and you can move from the first stage to the second after the wood reaches a transitional moisture content of 20%. The high temperature regime is determined depending on the thickness and type of lumber. High-temperature conditions can be used to dry wood, which is used for the manufacture of non-load-bearing elements of buildings and structures in which darkening of the wood and a decrease in strength are allowed.
Drying chamber concept
Chamber drying is the main method of drying wood. Drying chambers are required to dry softwood and hardwood to different quality categories. One of the most popular and economical methods of artificial dehydration of lumber is drying, when bound and free moisture is removed from the tree using a water supply. wet wood heat with hot air and removal of evaporated excess moisture humidified and partially cooled air.
The drying chamber is a completely finished installation that is equipped with all the equipment necessary for drying wood. According to their design, wood drying chambers are divided into prefabricated metal and made of building materials. The latter are constructed directly in workshops or as separate buildings from materials that are widely used in industry. The camera can be made entirely from monolithic reinforced concrete. Its walls can be made of solid red brick, and the ceiling can be made of monolithic reinforced concrete.
If several dryers are used, they are often combined into a single block, constructing a common control corridor where the heat supply distribution and automatic control system for all chambers are located. Depending on the volume of wood loaded into the chamber, there may be horizontal or vertical transverse air circulation.
Loading of lumber into the chamber can be carried out in the following ways: on trolleys in the form of stacks along a rail track, like packages with a forklift. Heat transfer to wood can be carried out: by air, combustion products or superheated steam; radiant heat that comes from special emitters; solid body, if you organize contact with a heated surface; current that passes through wet wood; high-frequency electromagnetic field that penetrates wet wood.
Equipment for a wood drying chamber is divided into basic and additional. The main ones include a fan system, a heat supply system, supply and exhaust ventilation and humidification; additional ones include an insulated door and psychrometric unit, stacking trolleys, and an electric motor for the fan drive.
The process of controlling wood drying in a chamber can be automated. Automation is capable of maintaining the humidity and temperature of the environment in the dryer at a given level. The temperature is regulated by supplying coolant to heaters or by turning an electric heater on and off, and humidity by using supply and exhaust ventilation and a humidification system.
The wood drying control system may have the ability to remote control humidity and temperature in the chamber. When drying lumber in a drying chamber, it becomes necessary to control the moisture content of the wood, for which a remote moisture meter is used, which allows you to check the moisture content of the wood at several points without entering the chamber. In the absence of external sources of heat supply, autonomous heating modules can be used for the dryer and gas, coal, wood waste, electricity and diesel fuel.
Types of drying chambers
In real life it is common to use the following types drying chambers. The necessary energy in convective drying chambers is transported into the material using the air cycle, and heat transfer to the wood occurs through convection. There are two types of convection chambers - tunnel and chamber.
Tunnel convection dryers are deep chambers where stacks of stacks are pushed from the wet end to the drier end. These chambers must be filled at one end and emptied at the other. Pushing the stacks (the process of filling the chambers and emptying them) is done one stack at a time with an interval of 4 - 12 hours. These chambers are designed for large sawmills and allow exclusively transport drying of wood.
Chamber convection drying chambers are shorter than tunnel and vacuum drying chambers for wood; during operation, the same parameters are maintained throughout the chamber. When the blowing depth is more than 2 meters, the technique of reversing the direction of ventilation is used to equalize the drying conditions for the wood. The emptying and filling of the chamber occurs on one side if it has one door. Other loading systems are also known that are similar to the procedure for loading tunnel chambers. Any lumber can be dried to any final moisture content, which is why 90% of wood in Europe and Russia is dried in chamber dryers.
The condensation drying chamber differs from the previous ones in that the humidity that arises in the air condenses on special coolers and water comes out of the drying process. Coefficient useful action This process is large, but the cycle is long, because the devices do not operate at high temperatures, and the total heat losses are also significant. The condensation chamber is primarily suitable for drying small volumes of wood, or for drying dense wood species - oak, beech or ash. Big advantage The advantage of such chambers is that a boiler room is not needed, the price of a drying chamber for wood and the cost of drying are lower.
Drying kilns are also classified according to the method of circulation and the nature of the drying agent used, the type of enclosure and the principle of operation. Batch drying chambers are characterized by the fact that they can be fully loaded to simultaneously dry all the material, and the wood drying mode changes over time, currently remaining the same for the entire chamber.
According to the method of circulation, there are chambers with incentive and natural circulation. Dryers with natural circulation are outdated, low-performance, the drying mode in them is almost uncontrollable, and the uniformity of wood drying is unsatisfactory. For modern construction such devices are not recommended, and existing ones must be modernized. Depending on the nature of the drying agent, the chambers are divided into gas, air and high-temperature chambers that operate in a superheated steam environment.
Wood drying procedure
Before drying according to the selected mode, the wood is heated with steam, which is supplied through humidifying pipes, with fans running, heating devices turned on and exhaust ducts closed. First you need to calculate the drying chamber for wood. The temperature of the agent at the beginning of heating the wood should be 5 degrees higher than the first stage of the regime, but not more than 100 degrees Celsius. The level of saturation of the environment should be 0.98 - 1 for material with an initial humidity of more than 25%, and 0.9 - 0.92 for wood with a humidity of less than 25%.
The duration of the initial heating depends on the type of wood and for coniferous species (pine, spruce, fir and cedar) is 1 - 1.5 hours for each centimeter of thickness. The duration of heating of soft deciduous trees (aspen, birch, linden, poplar and alder) increases by 25%, and for hard deciduous species (maple, oak, ash, hornbeam, beech) - by 50% compared to the duration of heating of coniferous species.
After preheating, it is customary to bring the parameters of the drying agent to the first stage of the regime. Then you can begin drying the lumber, subject to the established regime. Humidity and temperature are regulated by valves on the steam lines and gates of the sugar-exhaust channels.
During the operation of an infrared drying chamber for wood, residual stresses arise in the wood, which can be eliminated by intermediate and final moisture-heat treatment in an environment of increased temperature and humidity. It is customary to subject lumber to processing, which is dried to operational humidity and is subject to further mechanical processing.
Intermediate moisture-heat treatment is carried out during the transition from the second stage to the third or from the first to the second at high temperature conditions. Coniferous species with a thickness of 60 millimeters or more and deciduous species with a thickness of 30 millimeters or more are subjected to moisture and heat treatment. The temperature of the environment during the heat and moisture treatment process should be 8 degrees higher than the temperature of the second stage, but not higher than 100 degrees, with a saturation level of 0.95 - 0.97.
When the wood reaches its final average moisture content, final moisture-heat treatment can be carried out. In this process, the temperature of the medium is maintained 8 degrees above the last stage, but not higher than 100 degrees. At the end of the final moisture-heat treatment, the wood that has undergone drying must be kept in the chambers for 2 - 3 hours at the parameters provided for in the last stage of the regime. Then the drying chamber is stopped.
Making a drying chamber
If you decide to make wood products yourself, then you simply need a wood drying chamber. However, when constructing a dryer, comply with all required standards. You will need a chamber, a fan, insulation and a heating device.
Build a drying room or select a separate room, one wall and ceiling of which will be made of concrete, and the other walls will be made of wood, which need to be insulated. To do this, it is customary to create several layers: the first of them is polystyrene foam, the second is wooden boards, which are usually wrapped in foil in advance.
After this, you should install a heating element, which can be made in the form of batteries. Water must be supplied to the batteries from the stove, in which it will be heated to 60-95 degrees Celsius. It is advisable to continuously circulate water using water pumps in the heating element. Also, a fan should be placed in a homemade wood drying chamber, which helps distribute warm air throughout the room.
Consider how the wood will be loaded into the drying chamber. One loading option could be a rail cart. To regulate the humidity and temperature in the drying chamber, you need to use work area corresponding thermometers - wet and dry. Provide shelves inside the dryer to increase work space.
During the drying process of lumber, sudden changes in temperature in the working room are not allowed, otherwise this will cause the wood to warp or cracks to appear in it. When constructing a drying chamber, it is extremely important to comply with fire safety requirements. Therefore, in the immediate vicinity of the dryer in mandatory install fire extinguishers.
And finally, remember that instead of heating element At home, you can use a two-burner electric stove. You can insulate the walls of the drying chamber yourself using wood shavings. Instead of foil in the chamber, you can use penofol, which can provide good reflection of heat from the surface. In such a dryer, the wood is pre-dried 1-2 weeks in advance.
Freshly cut wood is not used in production and construction, as it contains a large amount of moisture. This type of wood is called wet. To improve its mechanical and physical properties, a lumber drying chamber is used. In the process, biological resistance increases, the strength index increases, and other qualities of wood improve.
Wood moisture concept
The percentage ratio of the weight of the liquid contained to the weight of completely dry wood of a certain volume is called absolute humidity. The percentage of the mass of water removed (determined by two weighings) to the original weight of the wood is called relative humidity.
The degree of suitability for use is determined taking into account the relative humidity indicator. The value indicates the readiness of the material for gluing and drying; with a value above 30%, there is a danger of developing a fungal infection.
Depending on the indicator, wood is divided into categories:
- wet - with a relative humidity of more than 23%;
- semi-dry – within the range from 18 to 23%;
- dry – with a humidity value from 6 to 18%.
Drying wood in natural conditions
With this method of removing moisture, a drying chamber for lumber is not used; the liquid evaporates under the influence of atmospheric air. Dry the material under a canopy located in a draft. Sun rays the external heating is uneven and inner layer wood, which leads to deformations and cracks.
If the site does not have a drying chamber for lumber, it is well suited for drying attic space, ventilated barn, equipped shed. The material is stored in a stack, the first layer must be placed on stands with a height of at least 50 cm from any durable material. Rows of lumber are laid with dried slats, all subsequent boards and logs are placed above the previous blanks to create vertical air wells.
Logs sawn lengthwise and ready-made boards put inside up to reduce the size of the deformation. For the same purpose, a stack of wood is pressed from above with a heavy load. Due to the formation of cracking at the ends of the workpiece when drying the material, select the length of the workpiece to be 20-25 cm longer than the intended part.
The ends of the lumber are carefully treated with oil-based paint, drying oil or hot bitumen to prevent cracks. Before stacking, the log trunks are cleared of bark to reduce the likelihood of wood beetles breeding. Removing moisture from wood naturally is considered an economical method.
Solar wood dryer
The second method, the cost of which quickly pays off, is drying chambers for lumber. The manufacturing drawings are quite simple; you just need to understand the operating principle of such a device. The chamber is an assembled plywood or metal container, the roof of which is made of transparent materials.
The size of the glazed roof surface is calculated depending on the total horizontal area of all lumber laid for drying. The area of the transparent coating should be one tenth of the total surface of the boards. The roof of the building is made pitched, the amount of slope depends on geographical location terrain. In cold northern regions, where the sun does not rise high above the horizon, the roof slope is made steep. The southern sun warms up the gently sloping surfaces well.
How to make a drying chamber for lumber?
The frame of the building is made of metal or timber treated with an antiseptic under pressure. The lining of the walls and floor of the chamber is made of moisture-resistant materials, the fences are insulated mineral wool or hard foam boards. The internal surfaces of the walls are treated with water-repellent compounds, aluminum powder is applied to them, and then painted black.
As part of superchargers fresh air There should be no blades made of plastic, fusible materials. If the drying chamber for lumber is not used constantly, then the room is used for drying herbs, vegetables, berries or a seasonal greenhouse. After packing everyone wooden blanks For drying, there should be a distance of about 30-40 cm between the stack and the wall on all sides.
Drying wood in artificially created conditions
When removing moisture in a natural way Relative humidity readings of about 18% are obtained. To improve the value, lumber is dried in drying chambers, where temperature and speed are regulated forced submission air and its humidity.
Basic equipment for dryers
Whatever type of forced wood drying chamber is used, standard groups of equipment are allocated for all.
Transportation equipment is designed for loading and unloading logs or boards into the drying room. Includes cars and mechanical devices for storing workpieces in a stack or package, lifting and lowering lumber.
The thermal equipment of the chamber serves to raise the temperature of the internal air in the chamber and consists of many systems that determine the interconnected work of producing and transferring heat. These include heat exchange tanks, heaters, pipes for the passage of steam or hot water, devices for removing condensate, shut-off valves and control devices.
The fuel is gas and liquid fuel. For small volumes of work, a drying chamber for wood-burning lumber is equipped. The coolant is saturated steam, water, gas obtained from combustion of the furnace, organic fillers of the system, having high temperature boiling. Electric heaters are widely used, where current energy is converted into a thermal component.
Circulation equipment is designed for organized movement air masses in the drying chamber. The elements of the system are fans, injectors and joint installations of these elements. To increase the efficiency of wood drying, automation of lumber drying chambers is used.
Drying chamber fence
To isolate wood from action environment install a chamber fence, which consists of a floor, ceiling, walls and intermediate partitions. Requirements for partitions:
- should not allow steam to pass through;
- fences must have low thermal conductivity;
- must have a long service life.
Fences are made separately from various building materials or can be prefabricated with a set of standard metal elements.
The first type of cameras has a longer operating life, but differs more for a long time commissioning, which is not always justified. Prefabricated metal frames They are installed quickly, they are equipped with control and thermal devices, but the steel is subject to the destructive effects of wet and thermal conditions.
Working principle of vacuum drying
After stacking the wood, close the chamber door hermetically and begin the drying process. By using automatic devices Some of the air is removed from the chamber until a pressure of 8-10 bar is created inside. Thanks to this scientific approach, the moisture released from the wood moves faster from the center to the outer fences of the chamber, thereby ensuring uniform and high-quality drying. This is how vacuum drying chambers for lumber work.
Making a drying chamber yourself
Private developers dry wood in their yard; for this purpose, they install a drying chamber for lumber with their own hands. Its installation will require a large room, a heat source and a device for distributing air between drying packages of wooden blanks.
You can, of course, purchase used lumber drying chambers, but the degree of wear cannot always be determined correctly; it is much more profitable to arrange a room for drying wood yourself. This is an opportunity to get excellent results for little money.
Construction stages
You will need material for the frame, usually metal racks from a corner or channel; a wooden beam is used after careful treatment with an antiseptic. Metal sheets and panels are used as wall coverings moisture resistant plywood, profiled rolled products. Thermal insulation is carried out using mineral moisture-resistant wool and polystyrene foam.
Before construction begins, the location of one dryer or several is determined, which serves as a plan for constructing a concrete foundation. The foundation is made for the stability of the structure and uniform distribution of the load on the ground. If a ready-made railway container is taken for the camera, then four are made columnar foundation under the corners of the car.
The metal frame is assembled using welding or bolted connections. When installing, check verticality and horizontality building level, trying to strictly observe the geometric dimensions. After securing the frame in the installation position, they begin to cover the outer walls, simultaneously inserting doors and ventilation windows.
The thermal insulation layer of the floor, walls and ceiling must be at least 12-15 cm, the base is insulated from moisture roll material. After this, the chamber is checked for leaks. To lay the first layer, stationary supports made of metal or wood are installed. Install a heat source, usually a powerful fan heater, position it so that the direction of the hot air is parallel to the lying boards.
Drying wood is a necessary condition to obtain quality raw materials. Building a house or making fillings for openings from damp lumber is fraught with distortions and damage to integrity. In order to carry out work with wood without problems, you need to take seriously the removal of excess moisture from the material.
February 19, 2017 
Among Russian woodworking specialists, the method of drying lumber in a vacuum has been discussed for quite some time. Interest in the problem arose after the appearance of media reports about Italian-made installations, and then about WDE Maspell products in our market. After some time, a number of domestic companies mastered the production of similar drying chambers: Energia-Stavropol, MV-Impulse, etc.
The increased attention to such installations is explained by the fact that their manufacturers announce drying of lumber in an unprecedentedly short time: within 1-4 days, depending on the type of wood and the thickness of the lumber - and at the same time guarantee high quality resulting boards or blanks. Such drying times caused distrust among those who did not have the opportunity to practically check the quality of products dried in such chambers. The extremely scanty information from manufacturers of vacuum drying equipment about the essence of the process does not allow us to dispel these doubts. Let's try to figure it out.
Drying lumber at reduced pressure (usually p abs = 0.15-0.4 bar absolute pressure or p dis = 0.85–0.6 bar vacuum, which corresponds to the saturation temperature t sat = 54.0–75.9 ° C ) refers to the so-called. high temperature drying process. This type of process occurs when the wood temperature t dr exceeds the saturation temperature t us of water vapor at a given pressure. The high-temperature drying process is more intense compared to the low-temperature process, when the wood temperature is below the saturation temperature (t sat = t kip, t kip is the boiling point). The speed of defect-free vacuum drying in comparison with the standard GO ST convective mode chamber drying 4–5 times higher. So, for example, for a group of hardwood species (beech, maple, ash, elm, etc.), the standard drying time for a low-temperature convective mode with a board thickness of 50 mm is 12–14 days, while the drying time in press-vacuum installations for the same assortments - three to four days. The process of high-temperature drying was described in domestic literature back in 1957. Below is information on wood science and the theory of the high temperature drying process.
“Water can be found in two main structural elements of wood: in the cavities of cells and vessels - free moisture, and in the walls of cell membranes - hygroscopic, or bound moisture. When drying wet wood, first of all, free moisture within the cell is completely removed and only then, below the hygroscopicity limit (w pg), which is also the drying limit, bound moisture begins to evaporate from its shell. When the content of bound moisture in wood decreases, the wood dries out.”
“Consider the case of drying raw (w n > w pg) wood in the form of an unlimited plate in a gaseous environment with a temperature t c > 100 °C. At some intermediate stage of the process, all free moisture is removed from the outer zones of a plate of thickness X. The humidity of these zones varies from equilibrium on the surface to the saturation limit inside and has a certain average value wper. The inner zone of thickness (S – 2x) at this stage remains damp, its humidity is close to the initial one. The temperature of the internal zone is maintained at the boiling point of water t kip, and in the surface zones and boundary layer it gradually increases to t c. At the boundary of the zones, free moisture evaporates, due to which this boundary gradually deepens.”
The possibility of maintaining impeccable drying quality at such a significant intensity of a high-temperature (not even necessarily using a vacuum) process can be explained by taking into account the fact that when the saturation temperature t us is reached, intensive evaporation of free water (pseudo-boiling) occurs first on the surface and then in the thickness of the lumber. and the movement of the resulting water vapor outward. In the pseudo-boiling zone of the steam medium, the relative humidity φ steam = 100%, and the wood moisture content w tends to the equilibrium moisture content w р = 10.6 (φ/100) (3.27-0.015t), %, corresponding to the hygroscopic limit w р = w pg (w pg = 26.1% at t = 54 °C and w pg = 22.6% at t = 75.9 °C). Humidity w p g, %, is a function of temperature only: w p g = (34.66-0.159t) - and is characterized by the fact that it is the boundary below which there is no free moisture either in the cavities or in the walls of wood cells. Provided w
The first is that lumber is loaded into the chamber layer by layer with flat heaters - heating plates, which ensures uniform and intense heat transfer.
The second condition: the surface temperature of the heaters must exceed the saturation (boiling) temperature at the pressure (vacuum) created in the installation, by definition.
The third condition (which is not mandatory for atmospheric installations): a reduced - relative to atmospheric - pressure is created in the chamber cavity. If the top cover of the chamber is made in the form of a flexible membrane (usually made of silicone rubber), then due to the difference in pressure values, a pressing force is created between the layers of lumber and the heaters, which is transmitted layer by layer to metal structure bottom of the chamber. This clamping force ensures ideal flat shape boards and a tight fit of the surface of the lumber to the heaters, which is very important if heat transfer between them is carried out by conduction. With this option, precise calibration according to the thickness of the lumber is necessary to avoid loose fit of the boards to the heaters.
To ensure uniform heat transfer from the heaters to the surface of the boards without calibrating them, it is reasonable to organize heating by thermal radiation through a small gap between the plane of the heaters and the lumber, specially created due to special protrusions of the heating plates (such protrusions exist, for example, in installations manufactured by the Energia company -Stavropol"). Heat transfer by radiation in a flat gap does not depend on its size and on the inevitable spread of lumber in thickness.
As mentioned above, vacuuming for a high-temperature drying process is not a prerequisite, however, press-vacuum drying chambers have undoubted advantages, for example, the possibility of reducing pressure, and, consequently, saturation temperature. Firstly, lowering the process temperature helps reduce heat losses in the installation and minimize the discoloration of the wood during drying. Secondly, the use of a membrane press helps to achieve ideal fixation of the plane of dried boards and workpieces. Thirdly, the tight pressure of the layers of heaters and layers of lumber ensures uniform heat transfer during the drying process.
To dry thick assortments of difficult-to-dry species (for example, oak), special modes are used at stages of wood moisture above and below the hygroscopic limit. The use of these modes ensures defect-free drying of 50 mm oak assortments within 6-8 days.
The process of press-vacuum drying of wood is implemented in installations with a single loading from 0.5 to 10 m 3, providing seven and a half chamber revolutions (drying cycles) per month for a 50 mm thick hardwood board (drying for four days), and in the case of drying softwood lumber (when drying for two days) - 15 revolutions of the chamber, for boards 30 mm thick of hardwood (when drying for two days) - 15 revolutions, coniferous species (when drying for 24 hours) - 30 revolutions cameras per month.
Water evaporating from wood (about 250 liters per 1 m 3 of lumber) condenses on the metal walls of the chamber, as well as in the heat exchanger-condenser (if provided for by the design). The condensate is drained into the sewer.
Briefly about some features design installations for press-vacuum drying of wood "Energia" (manufacturer - LLC "Energia-Stavropol", Russia), as well as WDE Maspell (manufacturer - WDE Maspell srl, Italy). These installations use water flat heaters. Cameras from other manufacturers, for example, MV-Impulse LLC and Voyager-Vostok LLC (both companies are located in Ufa), use heaters with electric ohmic heating elements. Chambers from WDE Maspell are equipped with electric water boilers, and the design of chambers from Energia-Stavropol companies allows the use of both electric and gas water heating boilers as heating sources.
The ratio of the cost of 1 MJ of thermal energy obtained using electricity, propane and natural gas is now 15:7:1, respectively, so it is most profitable to use boilers running on natural gas for drying wood. It is clear that the quality of dried lumber does not depend on the type of energy carrier used, but is determined by the technological drying modes and the correct operation of the automation.
Text: Sergey Bondar
Wet wood is practically not used in the manufacture of high-quality and durable wooden structures - furniture, exterior and interior decoration, as well as load-bearing elements of the building. Installation and operation of lumber is preceded by removing moisture from it. The long-known process of natural drying can last several years, which is unacceptable given the large volumes and pace of modern construction.
There are two main types of moisture in wood that affect the density value and technical parameters of the structure being built:
- intracellular moisture– is easily removable, but is also quickly absorbed back into the wood in a humid environment;
- intercellular water– found outside the cells of wood (also called hygroscopic). This type moisture is most difficult to remove and forms the basis of constant humidity (about 30%).
Drying occurs as a result of two processes - the evaporation of water and its movement from the middle of the material to the surface.
If the rate of evaporation is greater than the internal migration of moisture, then the surface dries faster. This causes uneven change linear dimensions and leads to the appearance of cracks and bends. The gradual occurrence of the process ensures the preservation of the structure and shape of the wood.
The basics of modern drying methods are based on several methods of influence that accelerate the evaporation of moisture from the surface of the wood:
- temperature increase;
- increasing the frequency of air circulation;
- decreased blood pressure;
- reducing the humidity of air circulating over the wood.
What happens to wood when drying, besides removing moisture
The main processes observed in the structure of wood during drying are shrinkage and shrinkage. Shrinkage refers to the inevitable companions of moisture removal and represents a reduction in the dimensions of lumber in all directions after the removal of hygroscopic moisture begins.
An increase in wood size with an increase in free moisture content is called swelling. Shrinkage observed when quick removal moisture when the outside of the tree is much drier than the inside. This phenomenon often occurs when drying thick wooden beams and logs. The occurrence of shrinkage and shrinkage is taken into account when planning future structures, as well as swelling during operation in humid environments.
When wood overheats, which sometimes occurs during (chamber drying), a process similar to dry distillation occurs. It is the decomposition of wood fibers without access to air, resulting in the release of gaseous, liquid and solid ( charcoal) products. This process is irreversible, so when heating it is important to maintain optimal temperature conditions.
More information about the drying process itself:
Technologies and methods for drying wood
Vacuum (chamber)
Drying wood in vacuum drying chambers involves creating reduced pressure in a chamber loaded with stacks of lumber. Moisture, which is in the form saturated steam above the surface of the wood, removed together with the drying agent. The role of the latter is played by air, which is supplied in small quantities into the chamber.
Changing the degree of vacuum and air supply allows you to adjust the rate of water removal. For material various shapes and sizes, strict dehydration conditions are applied to maintain constant moisture content throughout the volume.
Depending on the type and size of wood, drying time lasts from several days to a month. Light conifers (pine, spruce) are the easiest to dry, while heavy oak boards have to be kept for 3-4 weeks until moisture is completely removed.
Photos of wood drying chambers
Example 1 Example 2 Example 3
Condensation
It is based on constant blowing of the drying chamber with a stream of dry heated air. The exhausted wet air mixture is sent to a cooling heat exchanger, in which water condenses after the air reaches its dew point.
This method most fully imitates natural drying wood If the temperature does not rise above 40-60? C, then it does not undergo significant shrinkage.
Condensation removal of moisture is a development of classical chamber drying, in which the wood is kept in a stream of hot air. The conditions of chamber drying are regulated by regulatory documentation and include soft, normal, forced and high-temperature modes. The process can occur continuously in chambers of large volume and length, which provides increased productivity.
Natural
It is carried out until the wood reaches an air-dry state (about 25-30%) and does not ensure the removal of hygroscopic moisture. The lumber obtained in this way is usually used for construction purposes in the construction of structural frames that have undergone special processing from and supporting structures.
Drying under natural conditions is carried out in rooms that are protected from precipitation and have good ventilation. When laying, the material is placed in stacks, in which distances are left between the boards to ensure ventilation.
The video will tell you how to properly dry wood in an atmospheric (natural) way:
Drying chambers and their structure
Drying equipment (chambers) are cylindrical or box-shaped, lined on the outside metal sheets. Raw wood material can initially be stacked inside or placed on special shelves.
Convection is ensured compressor units, and the air is heated in radiator heat exchangers. The air is supplied from above, since after contact with the wood load and evaporation of moisture, the temperature of the air mixture decreases. This causes an increase in its density and the descent of cold, humidified air into the lower part of the chamber.
Drying devices can be intermittent or continuous. In batch systems, work is interrupted while the dried batch is unloaded and a new block of wood is loaded. Continuous dryers ensure constant movement of stacks from the “wet” part of the chamber to the “dry” part, and loading is carried out as free space becomes available inside.
This video describes in detail a device for vacuum drying wood with your own hands:
Wood harvesting rules
The procedure and conditions for lumber procurement are regulated in accordance with the Forestry Code of the Russian Federation.
The basic rules that must be followed during logging are as follows:
- felling of trees is carried out after submission and approval of a declaration, which justifies and describes the volume of felling, territory, type of wood, etc.;
- the first on the list for cutting down are trees affected by natural causes or as a result of human activity (fire, storm, flood, disease);
- Only trees that meet age standards can be felled;
- during logging, it is necessary to remove the sawn material in a timely manner, prevent clogging of the area and demolish temporary buildings used in the work;
- It is prohibited to leave undercuts - individual trees in cleared areas.
Drying wood yourself
If you need to prepare wood at home, it is recommended to proceed in this order:
- choose a place for drying. It is best to provide not a canopy with a roof, but a building with dense walls, which prevent the penetration of precipitation;
- equip a base for future installation, under which air will flow freely;
- lay the lumber crosswise in several rows, as shown in the figure;
- close the resulting stacks in the upper part from random drops of water, dust and debris;
- fasten wooden boards or beams together. Best to use for this polymer materials– rubber puffs or nylon cords;
- keep the stacks for the time recommended for a specific climate zone (usually several months).
You can prepare lumber yourself if you have time and the need to prepare a large amount of wood. The resulting material is suitable for use for construction purposes, but is poorly suited for the manufacture of furniture. To obtain dry wood for finishing and decorative works, a set of wood preparation methods should be used, such as,.
DIY wood dryer: