Wood is a very hygroscopic material that easily changes its humidity. The moisture content of wood is the percentage of water (moisture) in it. The moisture content of wood does not depend on the type of wood. Wood moisture content is a quantitative indicator of the moisture content in it
Wood moisture content
Moisture exchange occurs all the time between wood and air. Therefore, wood moisture content is a very unstable value that changes along with humidity environment. If the humidity of the wood is greater than the humidity of the surrounding air, the wood will dry out. If it's the other way around, it's hydration. And if the humidity and temperature of the environment (air) remain constant for a long time, then the humidity of the firewood will also stabilize and will correspond to the humidity of the surrounding air.
The moisture content of wood, at which the exchange of moisture between it and the environment stops, is called “equilibrium”
In nature, equilibrium moisture content for wood is an extremely unstable state. Because in nature it is impossible to find air with constant temperature and humidity parameters for a long enough time. However, the state of equilibrium humidity is easily achieved for wood located in an artificial microclimate, for example, in a drying chamber or simply in any other room with constant temperature and humidity.
Distinguish between absolute and relative humidity of wood
Absolute humidity of wood
Absolute humidity is the ratio of the mass of moisture that a wood sample contains to the mass of absolutely dry wood of the same sample. According to , the value of absolute humidity (W) is calculated after examining (drying) the sample, according to the formula:
W = (m - m 0) / m 0 x 100,
where (m) and (m 0) are the mass of the sample before and after drying.
The concept of the value “absolute humidity”, according to GOST 17231-78, is interpreted simply as “humidity”. Like everything “absolute”, the value of “absolute humidity” is divorced from the real world and is an extremely indigestible form for thermotechnical calculations. For example, at an absolute humidity of 25%, a kilogram of wood will contain 200 grams of water. This discrepancy in numbers confuses calculations.
More convenient and practical is the value relative humidity
Relative humidity of wood
Relative (working) humidity of wood is the ratio of the mass of moisture that a wood sample contains to its total mass. According to GOST 17231-78, the value of relative humidity (W rel.) is calculated from the value of absolute humidity (W) of the sample, according to the formula:
W rel. = 100W / (100+W)
or more simply,
W rel. = m water / m sample x 100
Relative humidity is a very simple and convenient form for taking into account evaporated water in wood-burning heat engineering calculations. The value of relative humidity directly indicates the quantitative water content in wood. For example, one kilogram of wood with a moisture content of 20% will contain 200 grams of water and 800 grams of dry wood matter.
For comparison, let’s put a “live” example into a table. This is a table for the same our sample. Let us determine and compare the values of its absolute and relative humidity:
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Absolute humidity = 25%, |
Relative humidity = 20%, |
| absolute humidity will be 25%, | - if one kilogram of wood contains 800 grams of dry wood matter and 200 grams of water, then its value relative humidity will be 20%, |
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Formula for determining W = (m - m 0) / m 0 x 100 W = (1000 - 800) / 800 x 100 = 25% |
Formula for determining W rel. = 100W / (100+W) W rel. = 100 x 25 / (100+25) = 20% |
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Conclusion Despite the fact that the value of absolute humidity is the primary source for determining the value of relative humidity, it is the value of relative humidity that has a greater practical use. Because it (the relative humidity value) more realistically reflects the water content in the sample and does not confuse the numbers with discrepancies |
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Wood moisture level
According to humidity, all wood is divided into three groups: wet (humidity more than 35%), semi-dry (humidity from 25 to 35%) and dry (humidity less than 25%). Initially, the humidity of freshly cut trees is 50-60%. Then, when natural drying under a canopy in the air, the wood loses up to 20-30% of moisture within one and a half to two years and comes to a condition of conditional humidity. After this, the moisture content of the wood no longer changes significantly, and its value is ≈25%. Such wood is called air-dry. To reduce the moisture content of wood to a room-dry state (7...18%), it must be dried forcibly in drying chambers, or moved for a long time to an artificial microclimate with specified conditions (for example, moved to a room or other room).
There are the following degrees of wood moisture content:
- Splavnaya(humidity 60% or more)
This could be a tree that has been in water for a long time. For example, driftwood, or wood after sorting in a water basin, or simply a well-wetted (damp) log. - Freshly cut(humidity 45...50%)
This is wood that has retained the moisture of a growing tree. - Air dry(humidity 20...30%)
This is wood that has been aged for a long time outdoors, with good ventilation. - Room dry(humidity 7...18%)
This is wood that has been in a living room or in another heated and ventilated room for a long time. - Absolutely dry(humidity 0%)
This is wood dried at a temperature of t=103±2°C to constant weight.
Calorific value of wet wood
The calorific value of wood is directly dependent on its moisture content. The moisture content of firewood is a determining indicator of its quality. That dry wood burns better than wet wood is known to many, if not everyone. And everyone knows that wet firewood can always be dried, and dry firewood, on the contrary, can be wetted. Accordingly, the quality of the fuel will change - improve or deteriorate. But is this really important for modern heating equipment? For example, wood-burning pyrolysis boilers allow you to burn wood with a humidity of up to 50%, and even up to 70%!
The table shows generalized indicators of the calorific value of wood for each degree of its moisture content.
The table shows that the lower the moisture content of the wood, the higher its calorific value. For example, air-dried wood has a working calorific value almost twice as high as freshly cut wood, not to mention wet wood.
Wood with a humidity of 70% or higher practically does not burn.
Perfect option For wood heating- this is to use firewood in a state of room-dry humidity. They provide such firewood maximum amount heat. But, since drying firewood to such a state is associated with additional energy costs, the most the best option For heating, air-dried wood will be used. Bringing firewood to an air-dry state is relatively easy. To do this, it is enough to prepare them for future use and store them in a dry, ventilated area.
Finally, I would like to note that the moisture contained in firewood not only worsens its calorific value. Increased moisture content in fuel negatively affects the combustion process itself. Excess water vapor serves as the basis for creating an aggressive environment, which causes premature wear of the heating unit and chimneys.
Manufacturers of modern heating equipment recommend using air-dry wood as fuel, with a humidity of no more than 30-35%
What is drying? Drying lumber is one of the most important and integral operations in technological processes woodworking, and largely determining quality and competitiveness finished products. Wood containing a large number of water, is easily affected by fungi, as a result of which it rots. Dry wood is more durable. A decrease in humidity leads to a decrease in wood mass and an increase in its strength. Dry wood, unlike raw wood, is easy to trim, process and glue. It does not change its size and shape, which is important during the manufacturing and operation of products.
As a result of drying, wood is transformed from natural raw material into industrial material, meeting a wide variety of requirements that are placed on it in different production and living conditions. MUCH MORE EXPENSIVE THAN RAW! They include the cost of drying, which is quite high, but all this is paid off by the quality of the product and its demand on the market.
Wood moisture content is the ratio of the mass of water to the mass of dry wood, expressed as a percentage, and is used to estimate the amount of water contained in wood.
After cutting down a tree and sawing it into boards, the wood tissue turns out to be more or less porous, depending on the type of wood, and more or less saturated with lymph - water, which precisely represents what in technical jargon is called “wood moisture”.
A freshly felled tree has a maximum moisture content, which for different species can even exceed 100%. Usually they deal with a lower humidity value (30 - 70%), since after cutting some time passes before sawing and placing it in the dryer, and it loses a certain amount of water.
The initial moisture content is taken to be the value that the wood has before being sent to dry.
The final humidity is the humidity we want to achieve.
Humidity of 20-22% is called transport, and the humidity at which the product is operated is called operational.
Operating humidity values for lumber and wooden products:
Table operating humidity lumber
So how do you get dry wood? How is it dried?
Drying lumber and drying chambers.
Drying wood is a long and energy-intensive process. Thermal energy for dryers is generated in boiler rooms. The heat carrier here is steam or hot water. Environmental parameters in drying chambers are usually measured with a psychometer. Management and regulation is carried out automatically. These are dryers classic type: convective with various systems supply and exhaust ventilation and types of coolant. Their advantages: low capital costs, simple process, convenience Maintenance, high quality drying.
Along with traditional convection chambers, vacuum, condensation, microwave and other dryers have become widespread, but their use does not always achieve the desired result.
IN Lately There have been significant changes in the organization, technique and technology of drying. If previously the bulk of drying took place in large enterprises where large drying shops were built, now the bulk of wood is processed in small enterprises, the needs of which can be met by one or two small-capacity chambers. Many small firms are trying to create homemade protozoa drying devices, which cannot provide high-quality drying of the material. At the same time, The market is placing increasingly stringent demands on the quality of wood products.
Low quality of drying, due to the unsatisfactory technical condition of dryers and poor technological training of personnel, leads to hidden defects - uneven distribution of final moisture, which for a long time may go unnoticed and affect when the product is already in use.
Modern convective forest drying chambers, both domestic and foreign, make it possible to achieve High Quality drying. They are equipped with a system automatic control process and are a complex set of equipment requiring qualified maintenance.
Now that we have cleared up the questions regarding what exactly is dry lumber, You can safely begin to survey the market, draw up estimates for construction or repairs, and will no longer become a victim of unscrupulous lumber sellers.
In our opinion, the most controversial issue on the Internet. Let's answer this question in detail, based on GOST. Also, based on experience and practical examples, let’s try to figure it out and give logical answers to all the above questions.
Wood moisture content - This is the ratio of the mass of moisture contained in the volume of wood to the mass of absolutely dry wood.
The moisture content of lumber is measured with a moisture meter.
A tree is a living material that grows, sleeps, breathes. Because of this, most indicators in wood change from year to year. And such an indicator as the moisture content in wood, as well as the moisture content of dry lumber, changes EVEN throughout the year. This indicator depends on the time of year, the region, and the place of growth.
There are two main indicators on which the natural moisture content of wood depends.
The moisture content of wood is also influenced by the region and place of growth.
When the wood arrives less wet, it dries faster, and the drying process is softer and tears it less.
Dry lumber
Transport humidity and furniture humidity obtained by drying.
Wood moisture content is:
- Natural humidity (40-60%)
- Transport humidity (18+/-2%)
- Furniture humidity (8+/-2%).
Wood moisture content depends on the use of lumber.
- Natural humidity of 40-60% is used for formwork, in rafter system, for lathing, etc.
- Furniture wood with a moisture content of 8 +/-2% is used, the name already gives a hint, first and foremost in furniture production, as well as for the production of laminated veneer lumber.
- In all other cases, wood with a transport humidity of 18 +/-2% is used and is used for any construction, for the production of lumber, for example, block house, sheet piling, etc.
Sometimes a client comes and says: “I want the moisture content of the timber to be 8%.”
You ask: “For what?”
Answer: “They told me (I read) it would be better.”
Based on GOST 8486-86 and experience, transport humidity is the most optimal humidity for construction. Because at a humidity of 18 +/-2%, lumber does not warp, does not twist, does not turn blue, and is not susceptible to fungal infection. Lumber transport moisture completely justifies its physical and mechanical characteristics in construction.
Also, the belief that timber can be dried to 8% is absolutely false, and no one has ever seen such timber.It is impossible to dry the timber to less than 20%, and no one argues that the top layers can be dried to a moisture content of less than 20%, but what about the core? The moisture content of the timber in the core reaches 20%, which corresponds to GOST and DIN. At this humidity, timber and boards do not warp, do not twist, do not turn blue, and are not susceptible to fungal infection.
There is also interesting data that is included in the table below.
Based on the data in the table, the equilibrium moisture content of wood is 17-18.5%, based on average statistical data (air humidity 80-85% and temperature +10 C). It is logical that to build a house from timber, a humidity level of less than 20% is simply not needed. The construction site will not “gain” anything from this.
You can, of course, hear the argument about laminated veneer lumber; it is dried to a moisture content of 8%.
- Firstly, it is not the timber that is dried, but the lamellas (board).
- Secondly, manufacturers of laminated veneer lumber need to glue the lamellas together in the future so that they fit together tightly and do not come apart or dry out over time.
In principle, this is where the poor quality laminated timber came from. They dried it poorly, because it’s not easy to dry a board, not to mention timber, to a moisture content of 8 +/-2%, they didn’t dry it completely, they cheated, and over time the timber can dry out and the lamellas fall off.
Clients also come who say that we demolished my grandmother’s house, and we were never able to disassemble it. The roof “moved off”, but the frame remained rooted in place.
And the client sums it up with an exclamation: “They built it!”
Of course, before no one was chasing to build as quickly and cheaply as possible, no one was chasing “new technologies”. And they cut down the tree, debarked it, gave the log time to mature, and then just collected it.
How is it working out now? Everything is done exactly the opposite. The client wants faster and cheaper, the manufacturer gives what the client is willing to pay for. Here is the overall result.
The desire to obviously save money spoils the opinion about the best natural construction material. Wood, we repeat, is a living material; it “survives” only in the hands of professionals.
Wood is a capillary-porous material (heterocapillary system), which consists mainly of hydrophilic components, and therefore it constantly contains more or less water. A living tree needs water to ensure its life. Water content is characterized wood moisture. Humidity is one of the main characteristics of wood.
Wood moisture content is the amount of water it contains. Wood moisture content s affects the properties of wood and on the suitability of wood for construction purposes. Under humidity wood is understood as a percentage ratio of the mass of water to the dry mass of wood. Wood moisture content- the ratio of the mass of moisture contained in wood to the mass of absolutely dry wood, expressed as a percentage.
Wood moisture content
and the interaction of wood and its components with water has important for mechanical and chemical technology of wood, for example, for impregnation of wood with solutions of chemical reagents, antiseptics, fire retardants, etc., when rafting and storing timber in water.
Water plays a role in activating cellulose before chemical reactions occur. The interaction of cellulose with water in the paper pulp during grinding and the subsequent removal of water during the formation of a paper sheet causes the formation of strong interfiber bonds in the paper.
The properties of wood directly determine the properties of wooden products. When there is excess or insufficient humidity, wood usually absorbs or releases moisture, increasing or decreasing in volume accordingly. At high humidity indoors, wood can swell, and if there is a lack of moisture, it usually dries out, so all wooden products, including floor coverings, require careful care. To prevent deformation flooring the room must be maintained at a constant temperature and humidity.
There are two concepts - relative humidity wood and absolute humidity wood
- mass fraction of water, expressed as a percentage relative to the mass of wet wood.
Absolute humidity of wood
(moisture content) - mass fraction of water, expressed as a percentage relative to the mass of absolutely dry wood. Absolute humidity wood is the ratio of the mass of moisture contained in a given volume of wood to the mass of absolutely dry wood. According to GOST, the absolute humidity of parquet should be 9% (+/- 3%).
Absolutely dry wood conventionally refers to wood dried to constant weight at a temperature of (104±2)°C. Values of relative humidity of wood are needed for analyzing wood when calculating the mass fractions of its components as a percentage relative to absolutely dry wood. The absolute humidity of wood (moisture content) is used to quantitatively characterize wood samples when comparing them by water content.
According to the degree of moisture content, wood is divided into the following types:
Wet wood. Its humidity is more than 100%. This is only possible if the wood has been in water for a long time.
Freshly cut. Its humidity ranges from 50 to 100%.
Air dry. Such wood is usually stored in air for a long time. Its humidity can be 15-20%, depending on climatic conditions and time of year.
Room-dried wood. Its humidity is usually 8-10%.
Absolutely dry. Its humidity is 0%.
Wood moisture schedule: 1 – hot water; 2 – saturated steam; 3 – cold water
Water in a tree is distributed unevenly: roots and branches contain more water than the trunk; butt and top - larger than the middle part of the trunk; sapwood of coniferous species - more than sound and mature wood. In wood hardwood water is distributed more evenly across the cross-section of the trunk, and in some tree species (for example, oak) the core moisture is much higher than in coniferous species. In the bark, the moisture content of the bast is significantly (7...10 times or more) higher than that of the crust.
Freshly cut wood has a moisture content of 80 - 100%, while the moisture content of driftwood reaches up to 200%. In conifers, the moisture content of the core is 2-3 times lower than the moisture content of the sapwood.
In construction practice, wood is usually classified according to its moisture content:
- air-dried wood is wood dried in air until its moisture content is in equilibrium with the relative humidity of the air; the absolute humidity of such wood depends on the relative humidity of the air and is usually 16...21%;
- room-dry wood - wood kept in a heated room and having an absolute humidity of 9...13%; wet wood, resulting from prolonged exposure to water, with an absolute humidity above 100% (up to 200% or more).
freshly cut wood with an average absolute humidity of 50 to 100% depending on the time of felling (water content is significantly higher in spring and lowest in winter period), and tree species and growing conditions;
There are two forms of water found in wood - bound (hygroscopic) and free (capillary). These add up to the total amount of moisture in the wood. Bound (or hygroscopic) moisture is contained in the cell walls of wood, and free moisture occupies the interior of the cells and intercellular spaces. Free water is removed more easily than bound water and has less effect on the properties of wood.
Free (capillary) moisture is contained in the cavities of the cells, and bound moisture is contained in the walls of the wood cells. The gradual saturation of dry wood with water initially occurs due to bound moisture, and only when the cell walls are completely filled does a further increase in moisture occur due to free moisture. Therefore, it is obvious that it is the change in bound moisture that affects the processes of shrinkage and warping of wood, as well as its strength and elastic properties. An increase in free moisture has practically no effect on the properties of wood.
Water absorption of wood- the ability of wood to absorb water in direct contact with it.The tree is natural material, susceptible to fluctuations in temperature and humidity.Its main properties include hygroscopicity, that is, the ability to change humidity in accordance with environmental conditions.
They say that wood “breathes,” that is, it absorbs air vapor (sorption) or releases it (desorption), reacting to changes in the microclimate of the room. The absorption or release of vapors occurs due to the cell walls. Given a constant state of the environment, the moisture level of wood will tend to a constant value, which is called equilibrium (or stable) moisture.
In wood, moisture is contained in wood cells, in the intercellular space, in the channels of blood vessels and it is called free moisture. IN Lag contained in cell membranes is called hygroscopic (bound) moisture.
Hygroscopicity of wood- the ability of wood to change humidity depending on changes in the temperature and humidity state of the surrounding air. Hygroscopicity for most breeds is 30% at 20°C.
The maximum amount of bound moisture is called hygroscopic limit or the fiber saturation limit. At a temperature of 20 o C the hygroscopicity limit is 30%. As the temperature rises, part of the bound moisture turns into free moisture and vice versa.
Free and hygroscopic moisture is removed from wood by drying. Moisture can be contained in wood in the form of chemically bound moisture in the form of substances that make up wood; this type of moisture can be removed during chemical processing of wood.
The maximum amount of hygroscopic moisture is almost independent of the type of wood. The percentage of the weight of water to the weight of absolutely dry wood is usually 30% at a temperature of 20°. Such wood moisture, is called the saturation point of cell membranes, or the saturation point of fibers. A further increase in humidity occurs due to free moisture filling the voids in the wood.
When humidity changes from zero to the point of saturation of the cell membranes, the volume of wood changes and it swells. When humidity decreases, wood dries out.
Dimensional changes are always observed in the transverse direction and almost do not appear in the longitudinal direction; denser wood has a larger volume weight, therefore, more shrinkage and swelling. Late wood is denser.
Wood contains free (in cell cavities and intercellular spaces) and bound (in cell walls) water. The saturation limit of cell walls Wn,H is on average 30%. Content reduction bound water causes shrinkage wood
The ability to absorb moisture is affected not only by the microclimate of the room, but also by the type of wood. The most hygroscopic species include beech, pear, and kempas.
They respond most quickly to changes in humidity levels.
In contrast, there are stable species, for example, oak, merbau, etc. These include the bamboo stem, which is very resistant to unfavorable climatic conditions. It can even be installed in the bathroom.
Different types of wood have different moisture levels. For example, birch, hornbeam, maple, and ash have low humidity (up to 15%) and, when dry, tend to form cracks. The moisture content of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and dry less quickly. Alder is one of the most drying-resistant species. Its humidity is 30%.
When testing wood to determine its physical and mechanical properties, it is brought to normalized humidity (on average 12%) by conditioning at a temperature of (20±2)°C and relative air humidity<= (65±5)%.
DETERMINATION OF WOOD MOISTURE
There are several ways to determine wood moisture content. At home, they use a special device called an electric moisture meter. The operation of the device is based on changes in the electrical conductivity of wood depending on its humidity. The needles of an electric moisture meter with electrical wires connected to them are inserted into the wood and an electric current is passed through them, while the moisture content of the wood is immediately marked on the scale of the device in the place where the needles are inserted.
Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by mass, by the presence of cracks at the end or along the wood fibers, by warping and other signs. By the color of the bark, its size and the color of the wood, you can recognize ripe or freshly cut wood and the degree of its moisture content. When processing a semi-finished plane with a plane, its thin shavings, compressed by hand, are easily crushed, which means the material is wet. If the chips break and crumble, this indicates that the material is dry enough. When making transverse cuts with sharp chisels, also pay attention to the shavings. If they crumble or the wood of the workpiece itself crumbles, this means that the material is too dry. Very wet wood is easy to cut, and a wet mark from the chisel is noticeable at the cutting site. But it is unlikely that cracking, warping and other deformations will be avoided.
Wood moisture content determined in various ways: by drying samples of wood, wood chips or sawdust until completely dry; distillation of water in the form of an azeotropic mixture with non-polar solvents that are immiscible with water; chemical methods (titration with Fischer reagent); electrically.
The moisture content of lumber is determined by the formula
W = (m s - m o) / m s,
where m c and m o are the mass of the sample in the original and dried states, respectively.
In fact, wood moisture content is determined by control weighing or using an electric moisture meter.
Humidity of drift wood is 200%, freshly cut wood is 100%, air-dry is 15-20%.
DRYING WOOD
WITH
abalone wood- the process of removing moisture from wood to a certain percentage of humidity.
Bwood line- the ability of the wood surface to directionally reflect light rays.
Gloss depends on the type of wood, the degree of smoothness of the surface and the nature of the lighting. The radial surfaces of maple, sycamore, beech, elm, oak, dogwood, white acacia, ailanthus wood are distinguished by their shine. rocks in which a significant part of the surface is occupied by medullary rays consisting of small cells. The shine of wood is a decorative property and is taken into account when determining species.
Dielectric properties of wood- properties that are characterized by dielectric constant and dielectric loss tangent.
Wood swelling coefficient- average swelling of wood with an increase in the content of bound moisture by 1% humidity.
Wood shrinkage coefficient- average shrinkage of wood with a decrease in bound moisture content by 1% humidity.
Deformability of wood (warping)- the ability of wood to change its size and shape under external influences of load, humidity, and temperature.
Transverse warpage associated with various shrinkage (swelling) of wood in the radial and tangential directions. Its character depends on the location of the annual layers, determined by the shape cross section assortment, as well as the place where it is cut from the log.
Longitudinal warping associated with certain wood defects, such as large knots, leaning, and tilting of the fibers.
The consequence of warping is a wood defect - warping (transverse, longitudinal along the face and along the edge, wingedness).
Transverse and longitudinal warping also occurs due to an imbalance of residual stresses in dried lumber during machining: one-sided milling, edge division of thick boards into thin ones.
Longitudinal warping of the boards is observed during sawing; a change in the cross-sectional shape of samples cut from different parts of the log during drying.
Dry wood has high strength, warps less, is not susceptible to rotting, is easy to glue, is better finished, and is more durable. Any wood of various species reacts very sensitively to changes in environmental humidity.
This property is one of the disadvantages of timber. At high humidity, wood easily absorbs water and swells, but in heated rooms it dries out and warps.
Indoors, wood moisture content is sufficient up to 10%, and outdoors - no more than 18%. There are many ways to dry wood.
The simplest and most accessible - natural type of drying - atmospheric, airy. Wood should be dried in the shade, under a canopy and in a draft. When dried in the sun, the outer surface of the wood quickly heats up, but the inner surface remains damp.
Due to the difference in stress, cracks form and the wood quickly warps. Boards, timber, etc. are stacked on metal, wooden or other supports with a height of at least 50 cm. The boards are stacked with their inner layers facing up to reduce their warping. It is believed that boards placed on the edges dry faster, since they are better ventilated and moisture evaporates more intensely, but they also warp more, especially material with high humidity.
It is recommended to compact a stack of p/m, prepared from freshly cut and live trees, with a heavy load on top to reduce warping. During natural drying, cracks always form at the ends; to prevent cracking and preserve the quality of the boards, it is recommended to carefully paint the ends of the boards with oil paint or soak them in hot drying oil or bitumen to protect the pores of the wood. The ends must be processed immediately after cross-cutting into the cut.
If the wood is characterized by high humidity, then the end is dried with a blowtorch flame, and only then painted over. The trunks (ridges) must be debarked (cleared of bark), only small collars-muffs 20-25 cm wide are left at the ends to prevent cracking. The bark is cleaned so that the tree dries out faster and is not affected by beetles. A trunk left in the bark in relative heat with high humidity quickly rots and is affected by fungal diseases. After atmospheric drying in warm weather, the wood moisture content is 12-18%.
There are several other ways to dry wood.
Way evaporation or steaming has been used in Rus' since ancient times. The blanks are cut into pieces, taking into account the size of the future product, placed in ordinary cast iron, sawdust from the same blank is added, filled with water and placed in a heated and cooled Russian oven for several hours, “languishing” at t = 60-70 0 C.
In this case, “leaching” occurs - evaporation of wood; Natural juices come out of the workpiece, the wood is painted, acquiring a warm, thick chocolate color, with a pronounced natural texture pattern. Such a workpiece is easier to process, and after drying is less likely to crack and warp.
Way waxing. The blanks are dipped into melted paraffin and placed in an oven at t=40 0 C for several hours. Then the wood dries for a few more days and acquires the same properties as after steaming: it does not crack, does not warp, the surface becomes tinted with a distinct texture pattern.
Way steaming in linseed oil. Wooden utensils steamed in linseed oil are very waterproof and do not crack even with everyday use. This method is still acceptable today. The workpiece is placed in a container, filled with linseed oil and steamed over low heat.
Warping: 1 - transverse; 2 - longitudinal along the surface; 3 - longitudinal along the edge; 4 – wingedness of logs due to residual internal growth stresses.
Linear shrinkage of wood- reduction in the size of wood in one direction when bound water is removed from it. Linear swelling of wood is an increase in the size of wood in one direction with an increase in the content of bound water in it.
Normalized wood moisture content— equilibrium wood moisture content acquired at a temperature of 20 ± 2°C and a relative humidity of 65 ± 5%.
Volumetric shrinkage of wood- reduction in the volume of wood when bound water is removed from it.
Volumetric swelling of wood— an increase in the volume of wood with an increase in the content of bound water in it.
Relative humidity of wood- the ratio of the mass of moisture contained in wood to the mass of wood in a wet state, expressed as a percentage. Wood is a hygroscopic material, and the humidity to which it tends under given temperature and humidity conditions is called equilibrium. For example, at a temperature of 20 o C and air humidity of 100%, the equilibrium moisture content of wood is W = 30%.
A rapid change in bound moisture and uneven drying in different directions leads to warping or, conversely, swelling of the wood.
In massive elements, due to uneven drying, shrinkage cracks form. Therefore, in the production of lumber, great importance should be attached to the organization of drying, and when operating wooden structures, large and sudden changes in temperature and humidity should be excluded. Wood is characterized by a certain inertia of moisture exchange processes.
Wood shrinkage: 1 – shrinkage; 2 – cracking; 3 – transverse warping; 4 – the same, longitudinal
The amount of shrinkage is different in different directions: it is greater in the tangential (6 - 12%) and less in the radial (3 - 6%) direction of the cross section of the trunk. Due to such uneven shrinkage, warping of the boards appears when drying. When humidity increases above the saturation point of the fibers, further swelling does not occur.
With a sharp change in the temperature and humidity conditions in a room, internal stresses arise in the wood, which lead to cracks and deformations. The optimal temperature in a room with parquet flooring should be approximately 20 0 C, and the optimal air humidity should be 40-60%. Hydrometers are used to control indoor temperature, and relative humidity in the room is maintained using humidifiers.
Deformation of wood during drying
Wood for building parts (windows, doors, floors, etc.), especially for glued structures, should contain no more than 8-15% moisture. Hence the need to dry the wood. Natural drying takes a long time; for example, to dry a board 50 mm thick in the summer in central Russia to a humidity of 20%, it takes 30 - 40 days. Artificial drying in conventional dryers reduces the drying time of such boards to 5 - 6 days, and drying at elevated temperatures (>100°) can be carried out in 3 - 4 hours.
Ultimate wood moisture must correspond to the humidity under operating conditions.
During prolonged drying, water evaporates from the wood, which can lead to significant deformation of the material. The process of moisture loss continues until the moisture level in the wood reaches a certain limit, which directly depends on the temperature and humidity of the surrounding air. A similar process occurs during sorption, that is, absorption of moisture. A decrease in the linear volume of wood when bound moisture is removed from it is called shrinkage. Removing free moisture does not cause shrinkage.
Shrinkage is not the same in different directions.On average, complete linear shrinkage in the tangential direction is 6-10%, and in the radial direction - 3.5%.
With complete drying (that is, one in which all bound moisture is removed), the moisture content of the wood is reduced to the limit of hygroscopicity, that is, to 0%. If moisture is unevenly distributed when drying wood, internal stresses can form in it, that is, stresses that arise without the participation of external forces. Internal stresses can cause changes in the size and shape of parts during mechanical processing of wood.
Schemes for the development of deformations during convective drying
The process of convective drying of wood is accompanied by an uneven distribution of moisture throughout its volume. This causes its uneven shrinkage, which in turn causes the formation of internal stresses.
Let us consider how internal stresses arise and develop in wood, without yet taking into account its anisotropic structure, i.e., assuming the shrinkage in the tangential and radial directions to be the same. For simplicity, we will also assume that the movement of moisture in the material occurs only along its thickness. This will allow us to depict moisture distribution curves on a cross-sectional drawing of the dried assortment.
Let's consider the moisture distribution curves along the thickness for the most characteristic moments of the process: 0 - the moment of drying begins; 1 - the moment when the moisture content of the surface layers has dropped below the saturation limit of the cell walls Wn, and there is still free water inside the assortment; 2 - the moment when the humidity across the entire section became below WH, but a significant difference in humidity across the thickness was still observed; 3 - the moment of the end of the process, when the humidity has become approximately the same throughout the entire cross-section, close to stable humidity.
At the initial moment of the process there is still no shrinkage and tension is obviously absent. After some time, the moisture content of the surface layers will drop below Wn (moment) and they will tend to dry out. However, this desire cannot fully manifest itself due to opposition inner layers, the drying out of which has not yet begun. It is possible to identify the beginning of shrinkage by cutting out an end plate, the so-called section, from the dried assortment along its entire cross-section, and dividing it into a number of layers according to thickness.
Wood is a rather porous material containing a large number of capillaries filled with moisture. In practice, wood moisture content is defined as the ratio of the weight of water contained in the tree to the weight of absolutely dry wood. There is a concept of “free” and “bound” moisture. “Free” moisture is contained in the pores and capillaries of the tree. “Bound” moisture is that contained directly in the cells of the tree.
When drying, the tree shrinks - it decreases in size (volume). In this case, there is practically no decrease in size along the fibers (along the length of the board), but in the direction transverse to the grain, there is a significant change in size (along the thickness and width of the board). The magnitude of this change depends on the type of wood and the specific value of the change in wood moisture content. In life, the most unpleasant surprises are associated with changes in the width of the board.
For example, if you lay a floor with a board that has natural moisture, then the decrease in its width over time can be so significant that two adjacent boards will lose their grip on each other. In this case, to remove the cracks, you will have to tear off all the boards from the joists and lay them again, fitting them end to end.
“What humidity should the board have?” you ask. It’s simple - any wooden product, during its operation, tends to the so-called “equilibrium humidity”. “Equilibrium humidity” is determined by the temperature and humidity of the air in the environment where the board will be located. You can see the values of this humidity in the table. For a residential premises it averages 8-10%, for a street it averages 12-14%. It logically follows from this that a damp board will dry out indoors, losing its width, on the other hand, a dry board will be moistened outdoors, expanding.
Natural moisture content, final wood moisture content
Natural humidity- this is the moisture inherent in wood in a growing or freshly cut (sawn) state, without additional drying. Natural humidity is not standardized and can range from 30% to 80%. The natural moisture content of wood varies depending on growing conditions and time of year. Thus, the natural humidity of freshly cut trees in a “winter” forest is traditionally less than the humidity of freshly cut trees in a “summer” forest.
Initial humidity- the same as natural humidity. A freshly felled tree has a maximum moisture content, which for different species can even exceed 100%. Balsa wood can have a freshly cut moisture content of up to 600%. In practice, we deal with smaller values (30-70%), because After cutting, some time passes before the tree is sawed and placed in the dryer, and it, of course, loses a certain amount of water. We take the initial moisture content to be the moisture content of the wood that it has before being sent to the drying chamber.
Final humidity- this is the humidity that we want to get after a full drying cycle. In this case, the purpose of the product made from dried wood is taken into account.
First of all, wood drying is the process of removing moisture from wood by evaporation.
Drying wood is one of the most important operations in the wood processing process. The wood is dried after sawing, but before wood processing. The wood is dried in order to protect it from damage by wood-staining and wood-decaying fungi during its further storage and transportation. Drying prevents wood from changing shape and size during the manufacturing and use of products made from it, improves the quality of wood finishing and gluing. The humidity to which wood is dried depends on the scope of its further use. The whole point is to bring the moisture content of the board to the same value that a product made from this board would reach over time during operation under these conditions. This humidity value is called “equilibrium humidity”; it depends on the humidity and temperature of the surrounding air. For example, the board from which parquet and other products used indoors will be made should have a humidity of 6-8%, since this is the humidity that will be equilibrium. For products that will be used in contact with the atmosphere (for example: wooden windows, external cladding of the house) the equilibrium humidity will be 11-12%.
You ask: “What will happen otherwise?” We answer: Otherwise, what happens all the time in Russia will happen, namely, the consumer will face problems. Imagine that you bought lining in order to sheathe the walls inside your country house or dachas. If you buy lining made from a careless manufacturer from raw boards and cover the walls of your house with it, it will begin to dry slowly naturally in an already established state. Let us turn to the table of equilibrium humidity and experience. If you heat a room in winter to 25 degrees Celsius, then with a typical indoor air humidity of 35% for winter, the equilibrium humidity value for a board in such a room will be 6.6%. At bases and markets, lining can very often have a humidity of 14% or higher (we have encountered 30%). Next, imagine that your lining begins to dry out, losing water from its pores. When drying, a process called “shrinkage” occurs and is expressed in a decrease in size. wooden product. The amount of shrinkage depends on the type of wood, the direction of the fibers in the product, etc. The main shrinkage occurs across the fibers (according to the thickness and width of your lining). When your lining dries in the installed state to equilibrium moisture, you, in the worst case, risk not only seeing that the lining has come apart in places, but getting gaps between the boards, almost the width of a finger.
The industry uses various technologies for drying wood, differing both in the equipment used and in the characteristics of heat transfer to the dried material.
The classification of types and methods of drying is usually based on heat transfer methods, according to which four wood drying technologies can be distinguished:
- convective drying technology;
- conductive drying technology;
- radiation drying technology;
- electric drying technology;
Each type of drying can also have several varieties depending on the type of drying agent and the characteristics of the equipment used for drying wood. There are also combined technologies for drying wood, in which they simultaneously use different kinds heat transfer (for example, convective-dielectric) or combine other features of various wood drying technologies.
Independent drying technologies
Chamber drying
Chamber drying. This is the main industrial technology for drying wood, carried out in wood drying chambers various designs, where lumber is loaded in stacks. Drying occurs in a gaseous medium (air, flue gases, superheated steam), which transfers heat to the wood by convection. To heat and circulate the drying agent, drying chambers are equipped with heating and circulation devices.
With chamber wood drying technology, the drying time for lumber is relatively short (from tens of hours to several days), the wood dries to any given final moisture content at the required quality, and the drying process can be reliably regulated.
Atmospheric drying
The second most important and widespread method at sawmills is the method of industrial drying of wood, carried out in stacks placed in a special open area (warehouses), washed atmospheric air without heating. The advantage of atmospheric wood drying technology is its relatively low cost. In addition, this method is the most gentle. Disadvantages: seasonality (drying practically stops in winter); long duration; high final humidity. Atmospheric wood drying technology is used mainly for drying lumber at sawmills to transport moisture and at some woodworking enterprises for drying and leveling the initial moisture content of lumber before drying in wood drying chambers.
Drying in liquids
Drying in liquids is carried out in baths filled with a hydrophobic liquid (petrolatum, oil) heated to 105-120 °C. Intensive heat transfer from liquid to wood allows the drying time to be reduced by 3-4 times compared to chamber drying, all other conditions being equal. This method is used in wood preservation technology to reduce its moisture content before impregnation. Attempts to dry lumber in petrolatum at woodworking enterprises have not yielded positive results due to the fact that lumber after such drying does not meet the requirements for wood for furniture and joinery and construction products.
Conductive drying technology
Conductive (contact) wood drying technology is carried out by transferring heat to the material through thermal conductivity upon contact with heated surfaces. It is used in small volumes for drying, thin wood materials- veneer, plywood.
Radiation drying
Radiation drying of wood occurs when heat is transferred to the material by radiation from heated bodies. The effectiveness of radiation drying is determined by the flux density of infrared rays and their permeability in solid wet bodies. The intensity of the radiant energy flow weakens as it goes deeper into the material. Wood is classified as low-permeable infrared radiation materials (penetration depth 3-7 mm), therefore this method is not used for drying lumber. It can be used for drying thin-sheet materials (veneer, plywood), in addition, this method is widely used in the technology of finishing wood products for drying paint coatings. Electric stoves, electric heating elements, gas (flameless) burners, and incandescent lighting lamps with a power of 500 W and above are used as emitters.
Rotary drying
Rotational drying of wood is based on the use of the centrifugal effect, due to which free moisture is removed from the wood when it is rotated in centrifuges. Mechanical removal of free moisture is achieved at a centripetal acceleration value of at least 100-500g (g is the acceleration of gravity). Such accelerations have not yet been achieved in practice due to the difficulty of accurately balancing a centrifuge with a stack; only experimental development of corresponding devices is underway. In known industrial rotary dryers, the centripetal acceleration does not exceed 12 g. Under these conditions, mechanical dehydration occurs to a small extent. However, intensification of the drying process in the humidity range above the hygroscopic limit is observed.
When installing a carousel in a drying chamber, the technology for drying lumber is the same as in conventional batch chambers. The duration of drying at the first stage (from the initial moisture content to the hygroscopic limit) is reduced several times depending on the thickness, species and initial moisture content of the wood compared to conventional convective drying under the same conditions. Although rotary dryers are economical and provide high quality drying, the rotary method has not yet found industrial use for drying lumber.
Vacuum drying
Vacuum drying at reduced pressure in special sealed drying chambers. Due to the complexity of the equipment and the impossibility of obtaining low final moisture content of wood vacuum drying has no independent meaning. It is used in combination with other drying methods and as an auxiliary operation in preparing wood for impregnation.
Dielectric drying
Dielectric drying - drying wood in an electromagnetic field of currents high frequency, in which wood is heated due to dielectric losses. Due to the uniform heating of wood throughout its entire volume, the emergence of a positive temperature gradient and excess pressure inside it, the duration of dielectric drying is tens of times less than convective drying. Due to the complexity of the equipment, high energy consumption and insufficient quality of drying, dielectric drying itself is not widely used.
Combined wood drying technologies
More effective application combined technologies wood drying, for example convective-dielectric and vacuum-dielectric. For mass drying, the use of these methods is uneconomical, but in some cases, especially when drying expensive, critical lumber and blanks made from hard-to-dry wood species, these methods can be used.
Convective-dielectric drying
With a combined convective-dielectric technology for drying wood, high-frequency energy from a special high-frequency generator is also supplied to a stack loaded into a chamber equipped with thermal and fan devices through electrodes located near the stack.
The heat consumption for drying in the drying chamber is mainly compensated by the thermal energy of steam supplied to the heaters, and high-frequency energy is supplied to create a positive temperature difference across the cross section of the material. This difference, depending on the characteristics of the material and the rigidity of the given mode, is 2-5°C. The quality of convective-dielectric drying of lumber is high, since drying is carried out with a small difference in humidity across the thickness of the material.
Vacuum dielectric drying
This is another way of drying wood using high-frequency energy. This technology uses the advantages of both vacuum and dielectric drying. By heating wood in a high-frequency field at reduced pressure, boiling of water in wood is achieved at low wood temperatures, which helps preserve its quality. The movement of moisture in wood during vacuum-dielectric drying of wood is ensured by all the main driving forces of moisture transfer: moisture content gradient, temperature, overpressure, which reduces drying time.
During vacuum-dielectric drying, a stack of lumber is placed in an autoclave or a sealed chamber, where a vacuum pump creates a reduced pressure of the environment (1-20 kPa). The lower the environmental pressure, the lower the evaporation temperature of moisture and wood during drying. Heat consumption for drying is provided by the supply of high-frequency energy to the wood. When using this wood drying technology, operational difficulties also arise - the complexity of the equipment, especially the setup and operation of high-frequency generators, and the high energy consumption for drying. Therefore, when deciding on the use of vacuum-dielectric chambers, it is necessary to first develop a feasibility study based on the conditions of a particular enterprise.
Induction or electromagnetic drying of wood
The method is based on the transfer of heat to the material from ferromagnetic elements (steel mesh) stacked between rows of boards. The stack, together with these elements, is in an alternating electromagnetic field of industrial frequency (50 Hz), formed by a solenoid mounted inside drying chamber. Steel elements (mesh) are heated in an electromagnetic field, transferring heat to wood and air. In this case, a combined transfer of heat to the material occurs: by conduction from the contact of heated meshes with wood and convection from circulating air, which is also heated by the meshes.