- Is your lumber dry or not?

However, all the answers to these questions are on the pages of the site, you just need to study carefully detailed information provided by the supplier about the material, rather than looking at the price and calling immediately.

Now about the essence of the question posed:

- All edged lumber GOST 8486 natural humidity(from 80-100%). It was not in the drying chamber (it was not subjected to heat treatment), so the exact percentage of humidity in in this case not measured.

All planed dry products (planed dry lumber) have passed chamber drying(heat treatment) and correspond to . Because This is an integral link in the production chain of any planed lumber. The exact % humidity depends on the cross-sectional size of the finished product and is regulated by the above GOST from 8 to 20%. See the link above.

- All profiled products for interior decoration. Either batten, lining, imitation, block house certainly undergo chamber drying. They are dried in one dryer to the same moisture content, manufactured on one machine. The division into classes (varieties) occurs at the sorting stage already finished products. The criteria for sorting are wood defects and defects that appeared during the production process. The basis for assigning a product to one class or another is the Standards for sorting finished products and their cutting. Optimal humidity should be taken into account:

Optimal humidity of profiled products for finishing?

The humidity that a profiled board should have is determined on the one hand by the features technological process manufacturing, and on the other hand, those consumer properties that a profiled board must have.

When producing profiled boards, it is necessary that the humidity of the workpiece is no higher than 20% and no lower than 10%. The parameters of the thickness and width of the future product should be taken into account. When humidity is more than 20%, the quality of surface planing deteriorates and there is a risk that the product will “bloom,” that is, a blue color will appear on it. When the humidity is less than 10%, a lot of fallen knots appear in the product (due to the difference in the shrinkage coefficients of the wood itself and the knots). Considering that during the drying process the spread of humidity in one batch is 2-3%, at the exit from the dryer the moisture content of the workpiece should be in the range of 12-18%. During the planing process, humidity decreases by another 2%. In total, the final moisture content of the finished product should be 10-16%.

The consumer should be completely satisfied with this humidity range, since on the one hand, humidity above 10% will avoid swelling of the profiled board (due to absorption of moisture from the air) and, as a result, “bulging out” of the nailed board, and on the other hand, humidity below 16% is guaranteed allows you to avoid shrinkage of the profiled board (due to evaporation excess moisture) and, as a consequence, the appearance of cracks.

Thus, the humidity of the lining (and likewise of other profiled products for finishing) in the range of 10 - 16% can be considered optimal.

And lastly, it is very important to choose a trusted supplier and be sure to check the declared humidity yourself

Determination of moisture content and drying of wood

1. DETERMINATION OF WOOD HUMIDITY.

There are several ways to determine wood moisture content. To determine humidity, you can use a special device - an electric moisture meter. The operation of the device is based on changes in the electrical conductivity of wood depending on its humidity. Electric moisture meter needles with electrical wires connected to them are inserted into the wood and passed through them electricity, while the moisture content of the wood is immediately marked on the instrument scale in the place where the needles are inserted. Electric moisture meters EVA-2M are widely used, determining humidity in the range of 7 - 60%.
Many experienced carpenters Determine the moisture content of wood by eye. Knowing the types of wood, its density and others physical properties, you can determine the moisture content of wood by weight (weighing several identical pieces of the same species in turn), by the presence of cracks at the end or along the wood fibers, by warping and other signs.
With the weight method, a section of moisture 10-12 mm thick is sawed off from a board (control sample) at a distance of 300 - 500 mm from the end, thoroughly cleaned of burrs and sawdust and weighed, the result is recorded in a journal, and the section is placed in a drying cabinet with a temperature of up to 103 °C. After 6 hours of drying, the section is weighed and the mass is recorded in a journal, then dried again and weighed every 2 hours after drying. If after repeated weighings the mass of the section does not change, this means that the section is dried to an absolutely dry state with humidity W0 = 0% and mass P.

The initial moisture content of the sample wood is determined by the formula:

W = (P n - P s) : P s * 100%,

where W is the initial humidity, %;

Р n and Р с - initial mass and mass in an absolutely dry state of the sample.

Also, checking the current humidity during the drying process can be carried out by weighing control samples with a length of at least 1000 mm, which are also cut from the boards to be dried at a distance of 300 - 500 mm from the end, cleaned of bark, burrs, sawdust, after which the ends are painted. . The sample is weighed to the nearest 5 g.
When processing lumber with a planer, its thin shavings clenched hand, easily wrinkles - 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.
The complete saturation of wood with water is called the hygroscopic limit. This stage of humidity, depending on the type of wood, is 25-35%.
In practice, wood is distinguished: room-dry (with a humidity of 8-12%), air-dry artificially dried (12-18%), atmospheric-dry wood (18-23%) and damp (humidity exceeds 23%).
Wood from a tree that has just been cut down or has been for a long time in water is called wet, its humidity is up to 200%. There is also a distinction between operational humidity, which corresponds to the equilibrium moisture content of wood under specific conditions.

Requirements for wood moisture content in products

Table 1.

Product name	GOST	Humidity, %
external and vestibule door frames
boxes internal doors
door leaves
sashes, vent valves, blinds
strips, layouts
Profile details:
floor boards and bars, plinth, window sill
internal trim
platbands and external cladding
handrails, external cladding
handrails, external cladding
Wooden floor beams:
solid wood
laminated wood

The moisture content of freshly cut wood (which has the moisture content of a growing tree) depends on the species and the location of sampling along the cross section of the trunk. U coniferous species The moisture content of wood in the peripheral part of the trunk (sapwood) is greater than the moisture content of wood in the central part of the trunk (core). hardwood The humidity throughout the entire section of the trunk is approximately the same.
The moisture content of driftwood is usually higher than that of wood delivered overland, and the moisture content of driftwood is higher than that of freshly cut wood. Thus, the moisture content of the sapwood part of pine logs after rafting increases to 150%, and the core part of the logs - to 50%.
As you know, wood has a cellular structure. Moisture in wood can fill cell cavities, intercellular spaces and permeate cell walls. Moisture that fills cell cavities and intercellular space is called free, and permeating the cell walls - related, or hygroscopic.
Freshly cut wood has both free and bound moisture. When drying wood, free moisture is first removed, and then bound moisture.

Moisture content of freshly cut wood

Table 2

Wood type	Humidity, %
	Kernels or mature wood	Sapwood	Average
Larch

2.DRYING WOOD.

When making any type of joinery, the wood must be dry. Dry wood has high strength, warps less, is not susceptible to rotting, is easy to glue, is better finished, is more durable, and finished products do not crack. 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. Therefore, for joinery products, wood must be dried to the degree of humidity that is expected in the future during their use. In the room, the wood humidity is sufficient up to 10%, and under open air- no more than 18%.
Drying is the process of removing moisture from wood by evaporation. Drying of lumber can be natural or artificial.

NATURAL DRYING

Natural drying occurs under the influence of atmospheric circulating air, which evaporates moisture from the wood. Natural drying of lumber is combined with storage. It is necessary to dry the wood 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. Wet lumber is dried immediately after sawing. This prevents the appearance of wormholes and rot.
Stacked materials dry worse in spring than in summer. This process occurs more intensively in June. The time for drying coniferous lumber under natural conditions to 18 - 22% humidity is given in the table.
Time required to dry lumber stacked with spacers to 18-22% moisture content:

Table3

A month of laying lumber for drying	Climate zone number	Drying time in days for lumber thickness, mm
March April May
June July
Aug. Sept

Note: For larch, drying time increases by 60%.

Climate zones

1st - Arkhangelsk, Murmansk, Vologda, Kuibyshev, Perm, Sverdlovsk, Sakhalin, Kamchatka, Magadan regions, the northern half of Western and Eastern Siberia and Komi, the northern part of the Khabarovsk Territory and the eastern part of the Primorsky Territory.

2nd - Karelia, Leningrad, Novgorod, Pskov regions, the southern part of the Khabarovsk Territory and the western part of the Primorsky Territory.

3rd - Smolensk, Kaliningrad, Moscow, Tver, Orel, Tula, Ryazan, Ivanovo, Yaroslavl, Nizhny Novgorod, Bryansk, Chelyabinsk, Vladimir, Kaluga, Kostroma, Amur regions, the southern part of Western and Eastern Siberia, the Republic of Chuvashia, Mari El, Mordovia, Tatarstan, Bashkotorstan, Udmurtia.

4th - Kursk, Astrakhan, Samara, Saratov, Volgograd, Orenburg, Voronezh, Penza, Tambov, Rostov, Ulyanovsk regions, North Caucasus.

Natural drying of lumber decreases sharply from mid-August. Spruce lumber dries faster than pine lumber. Thin-size materials dry faster than thick-size materials. Sawn softwood 16 mm thick loses half of the initial moisture content after 4 days of drying, then the drying intensity drops sharply. Lumber with a thickness of more than 20 mm evaporates most of the moisture after 20 - 30 days of drying.
Laying the stack begins with the construction of the base, the height together with the logs is at least 50 cm. The top of the base must be horizontal. The base supports are placed in increments of 1.5 m to prevent deflection of the lumber. The shape of the stacks is square or rectangle.
Stacks of lumber are protected by a roof that protects the material from precipitation and direct exposure sun rays and dust.
The lumber is laid on dry softwood pads measuring 25x40 mm. The outer spacers are laid flush with the ends of the boards, and the rest at a distance between them of no more than 70 cm. To create better ventilation Stacks of all gaskets are laid in a strictly vertical row along a plumb line. Between the boards or bars stacked in stacks, equal-width gaps (splits) are left, forming vertical channels along the entire height of the stack. The width of the spacing, depending on climatic conditions and the cross-section of the boards, is set for lumber with a thickness of up to 45 mm from 1/2 to 3/4 of the width of the lumber and for lumber with a thickness over 45 mm from 1/5 to 1/3 of the width of the lumber. To ensure uniform drying of lumber along the height of the stack, vents 150 mm high are installed at a distance of 1 and 2 m from the bottom row of boards. The boards are laid with the inner faces up to reduce their warping. To prevent cracking, it is recommended to carefully paint the ends of the boards oil paint or soak it several times with hot drying oil to protect the pores of the wood. The ends should be processed immediately after cross-cutting to size. If the tree has high humidity, then the end is dried blowtorch, and only then paint over it.

ROOM DRYING OF TIMBER

Chamber drying is the main method in which lumber is dried in drying chambers having the necessary equipment and devices. The chambers regulate temperature, humidity and the degree of air circulation.
Atmospheric drying is used for preliminary drying of lumber and, as a rule, is combined with a wood drying chamber.
Lumber can be stacked in pieces or in batches. When forming a stack individually, dry (with a moisture content of no more than 18%) calibrated softwood and hardwood pads with a cross-section of 25 x 40 mm and a length equal to the width of the stack are laid between the rows of boards. Spacers along the height of the stack must be laid perpendicular to the boards and strictly vertically one above the other.
The stack is formed from boards of the same type and thickness. The number of spacers laid along the length of the stack is given in the table:

Number of spacers laid along the length of the stack

Table4

Note: The numerator is the number of spacers for stacks made of softwood, the denominator is the number of spacers made of hardwood.

Methods for stacking lumber depend on the direction (circulation) of the drying agent. For drying chambers with countercurrent circulation, lumber is laid at intervals (splits), and for chambers with transverse reverse and countercurrent rectilinear circulation - densely.

Drying modes

Drying of lumber occurs under a certain temperature and humidity regime, which is understood as a natural alternation of processes of temperature and humidity effects on wood in accordance with its humidity and drying time.
During the drying process in the chamber, the air temperature gradually increases (in stages) and the relative humidity of the drying agent decreases. Drying modes are prescribed taking into account the type of wood, the thickness of the lumber, the final moisture content, the quality category of the materials being dried and the design (type) of the chambers.

Depending on the requirements for lumber, modes are divided into:

· soft M, with soft modes defect-free drying is obtained while preserving the physical and mechanical properties of wood and color;

· normal H, under normal conditions defect-free drying is obtained with a possible slight change in color of coniferous wood, but maintaining strength;

· forced F, with forced drying modes, wood is obtained with preservation of bending, tensile and compressive strength, but with a decrease in chipping and splitting strength by 15 - 20% and with possible darkening of the wood. These modes provide for a three-stage change in the parameters of the drying agent, and the transition from each stage of the mode to the next can be made only when the material reaches a certain humidity specified in the mode.

High-temperature drying process modes for batch kilns
provide for a two-stage change in the parameters of the drying agent, and the transition from the first stage to the second is made when the wood reaches a moisture content (transition) of 20%. The high temperature regime is determined depending on the species and thickness of the lumber.
High-temperature conditions can be used for drying wood used for the manufacture of non-load-bearing elements building structures, in which a decrease in strength and darkening of wood is allowed.

Wood drying process

Before the drying process is carried out according to the selected mode, the wood is heated with steam supplied through humidifying pipes, with heating devices turned on, fans running and the sugar-exhaust ducts closed. At the beginning of heating, the temperature of the drying agent should be 5°C higher than the first stage of the mode, but not more than 100°C. The degree of saturation of the environment should be for wood with an initial moisture content of more than 25% in the range of 0.98 - 1, and for wood with a moisture content of less than 25% - 0.9 - 0.92.
The duration of the initial heating of wood depends on the type of wood and for lumber of coniferous species (pine, spruce, fir and cedar) at an outside temperature of more than 0°C is 1 - 1.5 hours at a temperature below 0°C - 1.5 - 2 hours for every centimeter of thickness. The duration of heating of lumber of soft deciduous species (aspen, birch, linden, poplar and alder) increases by 25%, and for lumber of hard deciduous species (maple, oak, ash, hornbeam, beech) increases by 50% compared to the duration of heating of coniferous wood breeds
After warming up, the parameters of the drying agent are adjusted to the first stage of the mode and then they begin to dry the lumber, observing the established mode. The temperature and humidity of the air are regulated by valves on the steam lines and gates of the sugar-exhaust channels.
During the drying process, residual internal stresses arise in the wood; to eliminate them, intermediate and final moisture-heat treatment is carried out in an environment of high temperature and humidity. In this case, lumber is processed, dried to operational humidity and subject to further mechanical processing.
Intermediate moisture-heat treatment is carried out during the transition from the second to the third stage or from the first to the second when drying at high temperatures. Lumber of coniferous species with a thickness of 60 mm and above and deciduous trees (depending on the species) with a thickness of 30 mm and above are subjected to moisture-heat treatment. In the process of heat and moisture treatment, the temperature of the environment should be 8°C higher than the temperature of the second stage, but not more than 100°C, with a degree of saturation of 0.95 - 0.97.
The final moisture-heat treatment is carried out only when the wood reaches the required final average moisture content. During the final heat and moisture treatment, the temperature of the environment is maintained 8°C above the last stage of the regime, but not more than 100°C. At the end of the final moisture-heat treatment, the dried lumber is kept in the chambers for 2 - 3 hours at the parameters provided for by the last stage of the regime, after which the chambers are stopped.

Wood is one of those materials that are sensitive to changes in the external environment, primarily to fluctuations in temperature and humidity. One of the key properties of wood is the ability to absorb atmospheric moisture, i.e. hygroscopicity.

What is the natural moisture content of wood?

By natural moisture content of wood we mean the moisture content that is present in the still growing state of the tree or after it has been sawed and sawn into individual elements without any additional drying. This figure varies very widely - on average from 30% to 80%, specific figures depend on the specific type of wood.

Coniferous species are characterized by the highest natural humidity:

• Spruce - 90%;
• Different types of pine trees - 88-92%;
• Fir - 90-92%;
• Larch - 80-82%

Soft hardwoods:

• Willow - 85%;
• Aspen, alder - 80-82%;
• Linden - on average 60%.

Hardwoods:

• Different varieties of birch trees - 68-78%;
• Beech - 65%;
• Elm - 75-78%;
• Hornbeam - 60%;
• Oak - 50%.

At the same time, wood felled in winter period has a lower level of humidity than summer.

How and why is wood dried?

After sawing into individual boards/beams, the wood is dried under atmospheric conditions or using chambers, hydrophobic liquids, and various heating elements.

Drying wood protects or at least reduces the likelihood of its rotting, prevents deformation of shape and size, improves the quality of finishing of the finished product, and increases the strength of adhesive joints. During the drying process, not only weight loss occurs wooden element due to water loss, but also a slight change in size - up to 5-7% in length, width or height.

The main purpose of drying is to bring the wood to the so-called. equilibrium humidity, i.e. one that she would acquire later certain time operation under specific conditions. If this is not done artificially, then the process will occur naturally - for example, doors will begin to become damp and swell, parquet or lining may dry out, and as a result, at the joints individual elements cracks will appear, etc.

Depending on where and under what conditions a product made from this wood is subsequently used, it is dried to a certain level of humidity. So for floor coverings the optimal humidity will be 6-8%, for those items that will come into contact with atmospheric air(those. window frames, doors) - 11-12% or even more in the case of a humid climate in the region.

Wood is a “living” material; structures made from it breathe and can change their humidity. This is its main difference from brick, concrete, metal... This feature causes some problems when using wood building materials, especially with regard to changes in the percentage of humidity.

Humidity: concept of free and bound moisture

In wood, the main part of the water is found in cell cavities, intercellular spaces, channels, voids, cracks - this is free moisture. A certain amount of water is present in the thickness of the cell membranes - bound moisture.

Free (capillary) moisture in wood is retained due to simple physical and mechanical bonds; it easily evaporates during normal drying. This is the water that wood can absorb and release. When it comes to the moisture content of lumber, we mean the amount of free moisture.
Bound (microcapillary) moisture is retained in wood by complex physicochemical processes; its removal involves enormous energy costs. In nature, it evaporates from wood during combustion or natural aging, that is, when the cells are completely destroyed.

The moisture content of lumber is one of the most important technical characteristics, affecting the quality and scope of products. In addition, the humidity indicator can convert a board or timber into one of five grades. Thus, GOST 8486-86 states that selected and first-third grades include lumber with a moisture content of up to 22 percent (dry) or from 22% (raw, treated with an antiseptic), and only the 4th grade is not standardized for this indicator.

Absolute and relative humidity are distinguished. In construction practice, attention is paid mainly only to the absolute value, which is defined as the ratio of the mass of moisture contained in the tree to the mass of dry wood.
It is customary to distinguish several types of humidity:

• wet wood (floated) - 100 percent or more;
• freshly sawn - from 50 to one hundred percent;
• air-dry - up to 20 percent;
• room-dry - 7-10 percent;
• absolutely dry - 0 percent.

The concepts of “semi-dry” lumber and wood products with “transport humidity” are also distinguished - about 22%.

Why do you need to know the moisture content of lumber?

The moisture content of wood is an unstable value. Wood is hygroscopic; it always tends to enter with environment into balance. Moisture exchange occurs constantly; when air humidity increases, lumber absorbs water from it, and when it decreases, it releases it. This interaction causes a number of processes to change the structure and shape of the material, such as:

• swelling;
• shrinkage;
• warping;
• cracking.

When drying, lumber decreases in volume due to the evaporation of moisture from the wood. Moreover, the volume decreases in direct proportion to the decrease in humidity. The tree dries out different directions unequally, least along the fibers (0.1-0.3%), in the radial direction - 4-8%, in the tangential direction - 6-10%. Volumetric shrinkage can average 12-15 percent. So that after drying the manufactured lumber has required sizes, when sawing logs, for example, into timber or boards, allowances are made. It is taken into account that shrinkage depends on the density of the wood - the higher it is, the more it shrinks. By the way, different breeds react differently to changes in humidity conditions, some of them are more or less resistant, and some are characterized by carpenters and joiners as “nervous”.

Swelling - increase wooden products in volume, that is, the process opposite to shrinkage. It occurs when wood is exposed to high humidity conditions. An increase in volume in itself does not affect the strength of wood as such, but can lead to a violation of the shape/integrity of structures assembled from it.

The property of wood to swell and shrink with a radical change in humidity is considered negative. However, in some cases, for example, swelling can be useful - it ensures the tightness of the connection of parts, for example, during manufacturing wooden boats, barrels, etc.

Street wooden structures They undergo swelling and shrinkage processes periodically, with the changing seasons. Therefore, they are particularly susceptible to processes such as cracking and warping, which are the result of constantly changing humidity.

During drying, uneven distribution of water in the layers of wood can occur, resulting in stress in different directions, leading to the formation of cracks. Due to shrinkage, curvature of the product - warping - is often observed. It can be longitudinal or transverse and appears unevenly in different directions. Curved lumber complicates installation and may lose load-bearing capacity. They even highlight (GOST 2140 81) a specific visible defect of the board - “winging”, that is, a helical longitudinal bend.

Selecting lumber based on moisture content

Lumber of a certain moisture content is intended for specific purposes. It is allowed to use boards/timbers with a moisture content of 20-30 percent for the construction of any street structures - fences, gazebos, canopies, fences for animals, and the like. In addition, timber and boards with natural moisture are suitable for arranging some structural elements during the construction of houses and repair work. For example, to arrange rafter system or lag under the flooring. In this case, the reliability of the fastening will not allow cracks and warping to appear during drying. And to protect against fungus, products are treated with special antiseptic compounds.

Planed small and large moldings (lining, block house, skirting boards, corners, casing) external and internal are made from dry lumber (room-dry humidity at the level of 7-10 percent). door leaves, window transoms and frames, parquet, furniture.

Woodworkers have such a concept as “transport humidity”. Its figure should not be more than 20-22 percent. If the moisture content of the lumber is higher, then transporting it over long distances is not permissible, since transportation will take a long time, during which the wood may rot.

Absolutely dry wood with an indicator of 0 percent is not found in practice. This concept applies only when using one of the methods for determining humidity - gravimetric.

Basic methods for determining humidity

To determine the moisture content of lumber, two main methods are used today - by weight and using a moisture meter.

Weight method

Humidity is determined as follows: a small sample (control sample) 20-25 mm wide is cut out of a timber or board. It is very important to take it not from the very edge, but from the middle, since the end parts always have less moisture. The sample is cleaned of sawdust and weighed technical scales, capable of giving ultra-precise indicators (up to one hundredth of a gram). The resulting weight is recorded - this will be the initial mass of the sample (ISM).

Next, the sample is dried in a special drying cabinet at 100-105 degrees. After five hours, it is taken out and weighed, recording the weight, dried again, checking the indicators every 1-2 hours. When the weight stops changing, absolutely dry wood is obtained - the final sample mass (FSM) is recorded. Next, the moisture content of lumber is determined as follows: the difference between the NMP and the KMP is divided by the KMP index, the resulting figure is multiplied by 100 - the initial moisture content is obtained.

The main advantage of the method is that it gives very accurate indicators (error no more than 1 percent). Minuses:

• analysis may take a long time;
• you will need to cut out a sample of the material, which is unacceptable for finished products.

Using a Moisture Meter

A moisture meter is an electrical device specially designed for measuring humidity. There are two varieties of it:

• contact (needle) - the work is based on the conductometric method;
• non-contact - the work is based on the dielcometric method.

A needle moisture meter has two sharp metal needles that plunge into the wood. Then the button is pressed, completing the circuit. The device measures electrical resistance, which changes depending on the moisture level in the material. Next, using a special formula stored in the moisture meter’s memory, the percentage of humidity is calculated. In this case, measurements are carried out locally, so the procedure must be carried out in several places on the product to obtain accurate results.

The main working element of a non-contact moisture meter is a radio frequency generator. Measurements are carried out using built-in contact pads, therefore, unlike the needle model, the device does not leave marks on the product. The work is based on measuring the dielectric constant of wood - water itself is characterized by a high dielectric constant, which makes it possible to obtain accurate indicators of the percentage of moisture content of the material.

The main advantages of using moisture meters are ease of use and the ability to quickly obtain results. Non-contact devices are also ideal for measuring the moisture content of finished products. The main disadvantage is that moisture meters are not very accurate; the error can range from 2 to 7 percent.

Basic methods of drying lumber

Drying wood is the most important operation aimed at improving its technological and consumer properties. Even if the product is over-moistened, the lumber that was dry is much less likely to warp and crack, and is easier to process and install. Dry wood Excellent resistance to infection by harmful fungi. The weight of the products is less, while the strength and hardness increase, and the thermal insulation qualities also noticeably improve.

Today, the woodworking industry uses two main drying methods - natural (atmospheric) and forced (chamber).

Natural drying

During atmospheric drying, lumber is stacked under a canopy on outdoors. When forming a stack between rows of boards, beams, etc. put gaskets. The stacks are installed with gaps for air circulation. The ends of the products, in order to avoid the appearance of cracks, are treated special compounds. In addition, hardwood lumber must be subjected to antiseptic treatment before stacking.

The drying agent in this case is air, although, unlike the forced method, it is not possible to control its parameters (temperature, humidity). It all depends on the weather and time of year. An important point is how tightly the stacks are stacked. The denser it is, the higher the relative humidity and lower the temperature, which means the wood will dry out more slowly.

Atmospheric drying allows you to obtain products with a moisture content of 18-20 percent. The speed of the process will depend on the initial moisture content of the tree, time of year, type of wood and cross-section of lumber.

The main advantage is the relative cheapness of the process. No special equipment or costs for air heating are required. In addition, residual stresses during drying will not be so strong, cracks will not form - this is especially true when drying wood with initially high humidity.

The downside is that lumber takes a long time to dry, and it is impossible to control temperature and humidity. With prolonged increases in ambient humidity, there is a risk of fungal infection.

Forced drying

The most technologically advanced, highly effective method that allows you to obtain wood with a moisture content of 7-12 percent in a short period of time. Its essence lies in the fact that specially formed stacks of lumber are placed in special chambers in which the required temperature and humidity conditions are maintained for a certain time. The drying agent can be steam, heated air or flue gases; their movement can be either natural or forced.

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, the moisture content of wood is a very unstable value, which changes along with the humidity of the 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 contained in a wood sample 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 when 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:

Absolute humidity = 25%, sample weight: before drying = 1kg (1000g), after drying = 0.8kg (800g)	Relative humidity = 20%, sample weight = 1kg (1000g)
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%,
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%
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

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 to long time into an artificial microclimate with specified conditions (for example, move it to a room or other premises).

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 kept outdoors for a long time, 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%

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