Humidity- one of the main characteristics of wood. Moisture is the ratio of the mass of water to the dry mass of wood, expressed as a percentage.

The absolute moisture content of wood is the ratio of the mass of moisture in a given volume of wood to the mass of absolutely dry wood.

The relative humidity of wood is the ratio of the mass of moisture contained in wood to the mass of wood in a wet state.

Moisture in wood can be bound (hygroscopic) and free. They add up to the total amount of moisture in the wood. Bound moisture is located in the walls of woody cells, while free moisture occupies cell cavities and intercellular spaces. Free water is removed more easily than bound water and affects the properties of wood to a lesser extent.

Wood moisture levels

According to the degree of moisture, wood is:

• wet wood (humidity over 100%, this happens if the wood has been in water for a long time),
• freshly cut (humidity from 50 to 100%),
• wet (from 23 to 50%),
• atmospheric dry (18-22%),
• air-dry artificial drying (12-18%),
• room dry (humidity 8-10%),
• absolutely dry (humidity 0%).

Wood moisture determination

To determine the moisture content of wood, you can use a special device - an electric moisture meter. Its action is based on the change in the electrical conductivity of wood depending on its moisture content. The needles of the electric moisture meter with electrical wires connected to them are stuck into the wood sample under study in the direction along the fibers and passed through them. electricity. At the same time, the moisture content of the wood is immediately noted on the scale of the device in the place where the needles are inserted.

The sensor for measuring the moisture content of chips is a detachable cup, where a certain weight portion of the material under study is placed between two disk electrodes, which is compacted with a press. A four-needle probe is used to control the moisture content of particle boards. This method of measuring wood moisture is simple, but has its drawbacks. The absolute measurement error is in the range from 7 to 12% to ±2%; in the range from 12 to 30% - ± 3%, with a sample moisture content of more than 30%, it increases several times.

Wood moisture requirements in products

The moisture content of freshly cut wood depends on the species and the location of sampling along the trunk section. At conifers wood moisture in the peripheral part of the trunk (sapwood) is greater than wood moisture in the central part of the trunk (core). At hardwood humidity throughout the cross section of the trunk is approximately the same.

Moisture content of freshly cut wood

Our production

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

Humidity: the concept of free and bound moisture

In wood, the main part of the water is in the cell cavities, intercellular spaces, in 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 the tree is retained by 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, it is the amount of free moisture that is meant.
Bound (microcapillary) moisture is retained in the tree by complex physical and chemical processes, and its removal is associated with huge energy costs. In nature, it evaporates from wood during combustion or natural aging, that is, when the cells are completely destroyed.

The percentage of moisture in lumber is one of the most important specifications affecting the quality and scope of products. In addition, the moisture indicator can translate a board or beam into one of five grades. So, GOST 8486-86 states that the 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.

Separate absolute and relative humidity. In construction practice, attention is mainly paid only to the absolute, 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 (floating) - 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.

There are also the concepts of "semi-dry" lumber, wood products "transport moisture" - about 22%.

Why you need to know the percentage of moisture in lumber

Wood moisture content is unstable. Wood is hygroscopic, it always strives to enter into equilibrium with the environment. Moisture exchange occurs constantly, with an increase in air humidity, lumber absorbs water from it, and when it decreases, they give it back. This interaction causes a number of processes to change the structure and shape of the material, such as:

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

During shrinkage, 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 up different directions unequally, least of all along the fibers (by 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 the right dimensions, when sawing logs, for example, into a beam or a board, allowances are made. At the same time, it is taken into account that shrinkage depends on the density of wood - the higher it is, the more it dries out. By the way, different breeds react differently to changes in the humidity regime, distinguish among them more or less resistant, and some are characterized by carpenters and joiners as "nervous".

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

The property of wood to swell and shrink when fundamental change humidity is considered negative. However, in some cases, for example, swelling is useful - it ensures the tightness of the connection of parts, for example, in the manufacture wooden boats, barrels, etc.

street wooden structures processes of swelling and shrinkage are subjected periodically, with the change of seasons. Therefore, they are particularly susceptible to cracking and warping as a result of constantly changing humidity.

During shrinkage, an uneven distribution of water in the layers of wood can form, as a result of which stress arises in different directions, leading to the formation of cracks. Due to shrinkage, a curvature of the product is often observed - warpage. It can be longitudinal or transverse, appears unevenly in different directions. Curved lumber complicates installation and may lose load-bearing capacity. They even single out (GOST 2140 81) a specific visible defect of the board - “wingedness”, that is, a helical longitudinal bend.

Choosing lumber according to the percentage of moisture

Lumber of a certain moisture content is intended for certain purposes. It is allowed to use a board / beam with a moisture content of 20-30 percent for the construction of any street structures - fences, gazebos, sheds, animal fences, and the like. In addition, timber and board natural humidity suitable for arranging some structural elements when building houses and repair work. For example, to arrange truss system or lag under the floor covering. In this case, the reliability of the fastening will not allow cracks and warpage to appear during drying. And to protect against fungus, products are treated with special antiseptic compounds.

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

Woodworkers have such a thing as "transport moisture". Its indicator should not be more than 20-22 percent. If the moisture content of lumber is higher, then it is not permissible to transport them over long distances, since transportation will take a long time, during which the wood may rot.

Absolutely dry wood with an indicator of 0 percent does not occur in practice. This concept is used only when using one of the methods for determining moisture - weight.

Basic methods for determining moisture

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 bar 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 on 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 (NMP).

Next, the sample is dried in a special oven at 100-105 degrees. After five hours, it is taken out and weighed, fixing the weight, dried again, checking the indicators every 1-2 hours. When the weight stops changing, absolutely dry wood is obtained - the final mass of the sample (CMP) is recorded. Further, 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 index is obtained.

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

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

Using a moisture meter

A moisture meter is an electrical device specially designed to measure 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.

The needle moisture meter has two sharp metal needles that plunge into the wood. Then the button is pressed, closing the circuit. The device measures the electrical resistance, which changes depending on the level of moisture in the material. Further, according to a special formula embedded in the memory of the moisture meter, the percentage of moisture is calculated. Measurements in this case 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 traces of 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 in the material.

The main advantages of using moisture meters are ease of use and the ability to quickly get results. Non-contact instruments are also excellent for measuring the moisture content of finished products. The main disadvantage is that the indicators of moisture meters are not very accurate, the error can be from 2 to 7 percent.

The main methods of drying lumber

Wood drying is the most important operation aimed at improving its technological and consumer properties. Even if the product has become waterlogged, the former dry lumber is much less likely to warp and crack, it is easier to process and install. A dry tree perfectly resists infection with harmful fungi. The weight of the products is less, while the strength and hardness increase, and the thermal insulation qualities are also noticeably improved.

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

Natural drying

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

The drying agent in this case is air, although, unlike forced way, it is impossible 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, the higher in them relative humidity and lower temperatures, which means that the wood will dry out more slowly.

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

The main advantage is the relative cheapness of the process. It does not require special equipment and costs for air heating. In addition, residual stresses during shrinkage will not be so strong, cracks will not form - this is especially true when drying wood with initially high humidity.

Minus - the lumber dries for a long time, it is impossible to control the temperature and humidity. With a prolonged increase in ambient humidity, there is a risk of fungal infection.

Forced drying

Most technological highly efficient method, which allows you to get 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 cameras in which the necessary 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 both natural and forced.

6. The limitation of the period of validity was removed according to protocol N 3-93 of the Mechgosudarstvenny Council for Standardization, Metrology and Certification (IUS 5-6-93)

7. EDITION (September 2007) with Amendments No. 1, 2, 3, approved in December 1987, in September 1988, February 1990 (IUS 3-88.1-89, 5-90)


THE AMENDMENT IS MADE, published in IUS N 12, 2013

Amended by database manufacturer

This standard applies to sawn softwood and specifies technical requirements to lumber intended for use in the national economy and for export.

The standard does not apply to resonant and aviation lumber.

(Changed edition, Rev. N 3)

1. MAIN PARAMETERS AND DIMENSIONS

1.1. Lumber is divided into edged, unedged, boards, bars and beams.

Terms and definitions - according to GOST 18288.

1.2. Nominal dimensions of sawn timber and maximum deviations from nominal dimensions - according to GOST 24454.

By agreement with the consumer, lumber is allowed for the domestic market with a gradation in length, dimensions and tolerances established in GOST 9302 and GOST 26002.

(Changed edition, Rev. N 2).

1.3. Symbol should consist of the name of the lumber (board, bar, beam), the number indicating the grade, the name of the wood species (coniferous or individual species - pine, spruce, larch, cedar, fir), the digital designation of the cross section (for unedged lumber - thickness) and the symbols of this standard.

Symbol examples:

Board - 2 - pine - 32x100 - GOST 8486-86

Board - 2 min. - 32 - GOST 8486-86

2. TECHNICAL REQUIREMENTS

2.1. Lumber must comply with the requirements of this standard and be made from wood of the following species: pine, spruce, fir, larch and cedar.

(Amendment. IUS N 12-2013).

2.2. According to the quality of wood and processing, boards and bars are divided into five grades (selective 1, 2, 3, 4th), and bars - into four grades (1, 2, 3, 4th) and must meet the requirements specified in the table .

Appointment of lumber different varieties given in the mandatory application.

(Changed edition, Rev. N 1, 3).

2.3. Lumber of selected, 1, 2, 3 grades is made dry (with a moisture content of not more than 22%), raw (with a moisture content of more than 22%) and raw antiseptic. In the period from May 1 to October 1, the production of raw antiseptic and raw lumber is allowed by agreement with the consumer (customer).

Humidity of lumber of the 4th grade is not standardized.

Antiseptic - according to GOST 10950.

2.4. The assessment of the quality of sawn timber, with the exception of deck timber, should be made on the basis of the worst face or edge for a given board, and square bars and beams - on the worst side.

2.5. The surface roughness parameter of lumber should not exceed 1250 microns for selected, 1, 2 and 3 grades, and for grade 4 - 1600 microns according to GOST 7016.

2.4, 2.5. (Changed edition, Rev. N 3).

2.6. The non-parallelism of the layers and edges in edged lumber, as well as layers in unedged lumber, is allowed within the limits of deviations from the nominal dimensions established by GOST 24454.

2.7. Additional requirements for lumber intended for special shipbuilding

Rules for limiting vices

Notes:

1. Knots less than half of the maximum allowable are not taken into account.

2. In sawn timber with a thickness of 40 mm or more (with the exception of the selected grade), oblong and sewn knots are allowed with a size along the minor axis up to 6 mm and a depth of up to 3 mm without limiting the size along the major axis.

3. A stepson is allowed according to the norms of non-united knots. Not allowed in selected variety.

4. The size of the knot is determined by the distance between the tangents to the contour of the knot, drawn parallel to the longitudinal axis of the lumber. For the size of an oblong and stitched knot on the faces of the lumber and on all sides of the bars and bars, take half the distance between the tangents drawn parallel to the longitudinal axis of the lumber.

5. In sawn timber with a length of more than 3 m, it is allowed to have one knot in the size provided for in the norms of the adjacent lower grade.

6. On a section of lumber with a length equal to its width, the largest sum of the sizes of knots lying on a straight line intersecting the knots in any direction should not exceed the maximum size of permissible knots.

Continuation

Notes:

1. Warping standards are set for lumber with a moisture content of not more than 22%. With higher humidity, these norms are halved.

2. Wood defects not mentioned in this standard are allowed.

2.7.1. Lumber for sheathing parts and connections of sea boats, boats of seagoing vessels, gliders, high-speed lake and river boats and sports vessels of the 1st class must meet the requirements of the selected grade with the following additions:

the core part in the middle of the length of the lumber should be on the inner face: in the longitudinal sheathing - at least 50%, in the diagonal - at least 25% of the width of the face;

the sizes of intergrown, partially intergrown and non-united knots taken into account should not exceed 10 mm;

the number of considered intergrown knots should not exceed 1 pc. on any one-meter section of the length of the lumber, and partially fused, not fused - 1 piece, per 2 m of the length of the lumber;

counted knots are allowed no closer than 10 mm from the edges of the lumber;

pockets on the outer face of lumber are not allowed.

2.7.2. Sawn timber for decking of marine vessels shall comply with the requirements of premium and first grades for exterior decks and first and second grades for interior decks with the following additions:

on the best layers of lumber up to 100 mm wide inclusive, intended for external decks, the sapwood part is allowed no more than 30 mm wide, and the surfaces of the layers must be radial or close to it sawing (without wedge cuts of annual layers);

considered knots are allowed: intergrown - not closer than 10 mm, partially intergrown and not intergrown - not closer than 15 mm from the ribs of the outer layer;

on the worst face and the lower halves of the edge area of ​​the lumber, intergrown knots are allowed without restriction, and partially intergrown and unjoined - up to 1/3 of the width of the face;

cracks are allowed in lumber for external decks up to 1/4 of the thickness; for internal decks - 1/3 of lumber thickness. Cracks in deck lumber are not limited in length;

blunt wane is allowed in deck lumber with a size of not more than 5 mm;

cancer on the best faces and upper halves of the edge area, and pockets on the best face of lumber for outdoor decks are not allowed;

core within the bottom half of the deck lumber is allowed.

Note. The assessment of the quality of deck lumber is carried out by the best layer and the upper halves of the edge area.

(Changed edition, Rev. N 1).

2.8. Sawn timber should be sorted by type of processing into edged and unedged, by size and grade (each grade separately).

At the request of the consumer, lumber can be sorted into groups of grades in accordance with the destinations established in the mandatory annex to the standard.

Lumber for export must be sorted in accordance with the work order of the foreign trade organization.

2.9. The grade, nature of processing, dimensions and species of wood must be indicated in the customer's specification.

3. RULES OF ACCEPTANCE AND METHODS OF CONTROL

3.1. Acceptance rules and control methods - according to GOST 6564.

APPENDIX (mandatory)

APPLICATION
Mandatory

Electronic text of the document
prepared by Kodeks JSC and verified against:
official publication
Lumber. Specifications:

Sat. GOSTs. - M.: Standartinform, 2007

Revision of the document, taking into account
changes and additions prepared
JSC "Kodeks"

Natural moisture, final moisture, free moisture - all these terms characterize the quality of wood and lumber produced from it.

Wood has a porous structure, in the capillaries of which moisture accumulates. The moisture content of wood and lumber is defined as the ratio of the weight of water to the weight of dry material.

like anyone natural material, the tree is sensitive to temperature fluctuations and changes in humidity. No wonder they say that wood breathes - it absorbs and releases air vapor with any changes in the microclimate.

There is such a thing as equilibrium humidity - its indicator is constant, any type of wood tends to it if climatic conditions do not change.

Breed and humidity

Each type of tree reacts differently to changes in humidity. Beech and pear are considered hygroscopic species, so any temperature fluctuations are reflected in their wood.

Oak and bamboo are characterized as stable species, so they are often used in the construction and decoration of swimming pools, bathrooms, and other rooms with high humidity.

Hornbeam, birch, maple have low humidity, its indicator rarely exceeds 15 percent. During the drying process, cracks often form on such wood.

Walnut is a tree with moderate humidity, its upper indicator is 20 percent. This type of wood has relative resistance to drying and cracking.

The most resistant to drying is alder, its moisture content is 30 percent.

Absolute and relative humidity

Consumers often confuse these two concepts, so let's dwell on them in detail.

Absolute humidity is the ratio of the mass of moisture to the mass of dry wood. At parquet board this indicator should be 9 percent, a deviation of 3 percent in any direction is considered acceptable.

Relative humidity is the ratio of the mass of moisture to the mass of wet wood. That is, until the lumber has gone through the drying process. These indicators are given in the previous section.

Degrees of humidity

There are five degrees of wood moisture content:

1. Wet wood with a moisture content of 100 percent or more. This is rare, because such indicators are possible when the tree long time was in the water.
2. Freshly cut wood. Moisture indicators at this stage are 50 percent or more, depending on the type of tree.
3. Air dry. This degree of humidity occurs when the wood has lain in the air for a long time. The humidity index is, on average, 20 percent.
4. Room dry wood. This degree is characterized by humidity no more than 10 percent.
5. Absolutely dry wood - 0 percent moisture.

What influences humidity?

Excess and deficiency of moisture adversely affect the quality of lumber. With excess moisture, they swell, and with insufficient moisture, they dry out and crack. In both cases, the deformation of the board, beam, logs occurs.

How to determine humidity?

The moisture content of lumber is determined by an electric moisture meter. This device measures the level of moisture based on changes in the electrical conductivity of the wood.

Experienced carpenters determine the percentage of moisture by eye. The presence of cracks, the location of these cracks, the weight of the board, the color of the wood and other features are taken into account.

For example, shavings taken from a log and easily crushed by hand indicate that the wood is wet. And brittle chips - that the wood is dry.

If pieces of wood crumble during processing, it means that it is too dry. If the saw slides like butter in it, the wood is very wet.

Determination of moisture content and drying of wood

1. DETERMINATION OF MOISTURE OF WOOD.

There are several ways to determine the moisture content of wood. To determine the humidity, you can use a special device - an electric moisture meter. The operation of the device is based on the change in the electrical conductivity of wood depending on its moisture content. The needles of the electric moisture meter with electrical wires connected to them are inserted into the tree and an electric current is passed through them, while the moisture content of the wood is immediately noted on the scale of the device in the place where the needles are inserted. Electrohygrometers EVA-2M, which determine humidity in the range of 7 - 60%, are widely used.
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 blanks of the same species in turn), by the presence of cracks at the end or along the fibers of the wood, by warping and other signs.
With the weight method, from the board (control sample) at a distance of 300 - 500 mm from the end, a moisture section 10 - 12 mm thick is sawn off, thoroughly cleaned of burrs, sawdust and weighed, the result is recorded in a journal, and the section is placed in drying cabinet with temperatures up to 103 °C. After 6 hours of drying, the section is weighed and the mass is recorded in a log, 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 a moisture content W 0 = 0% and a mass P.

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

W \u003d (P n - P s): P s * 100%,

where W - initial humidity, %;

P n and P s - the 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 sawn out of 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 with paint . The sample is weighed to the nearest 5 g.
When processing lumber with a planer, its thin shavings, compressed by hand, are easily crushed - which means that the material is wet. If the chips break and crumble, this indicates that the material is dry enough. When transverse cutting with sharp chisels, they also pay attention to chips. 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 moisture content of 8-12%), air-dry artificial drying (12-18%), atmospheric-dry wood (18-23%) and wet (humidity exceeds 23%).
The wood of a tree that has just been felled or has been in water for a long time is called wet, its moisture content is up to 200%. There are also operating humidity corresponding to the equilibrium moisture content of wood in specific conditions.

Wood moisture requirements in products

Table 1.

The moisture content of freshly cut wood (having the moisture of a growing tree) depends on the species and the location of sampling along the trunk section. In 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). In hardwood species, the moisture content throughout the entire section of the trunk is approximately the same.
The moisture content of driftwood is generally higher than that of overland delivered wood, with the moisture content of driftwood being higher than that of freshly cut wood. So, the moisture content of the sapwood part of pine logs after the alloy increases to 150%, the core part of the logs - up to 50%.
As you know, wood has a cellular structure. Moisture in wood can fill cell cavities, intercellular space and impregnate cell walls. Moisture that fills cell cavities and intercellular space is called free, and impregnating 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 content of freshly cut wood

Table 2

2. DRYING THE WOOD.

When making any kind of joinery, the wood must be dry. Dry wood has high strength, warps less, does not rot, easily glues, finishes better, is more durable, finished goods do not crack. Any wood of various species is very sensitive to changes in humidity. environment. This property is one of the disadvantages of timber. At high humidity, wood easily absorbs water and swells, and in heated rooms it dries out and warps. Therefore, for joinery, wood must be dried to the degree of moisture that is assumed in the future during their operation. Indoors, wood moisture content up to 10% is sufficient, and outdoors - no more than 18%.
Drying is the process of removing moisture from wood by evaporation. Drying 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 heats up quickly, while the inner remains damp. Due to the difference in stresses, cracks form, the tree quickly warps. Wet lumber is dried immediately after sawing. This prevents the appearance of wormholes and rot.
Stacked materials dry less well in spring than in summer. This process is more intense in June. The drying time of softwood lumber in natural conditions up to 18 - 22% humidity is given in the table.
The time required to dry to 18-22% moisture content of lumber stacked with spacers:

Table3

Note: For larch, the drying time is increased 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 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 republics of Chuvashia, Mari El, Mordovia, Tatarstan, Bashkotorstan, Udmurtia.

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

Natural drying of lumber has been sharply reduced since mid-August. Spruce lumber dries faster than pine lumber. Thin materials dry faster than thick materials. Softwood lumber 16 mm thick after 4 days of drying lose half of the initial moisture, then the drying intensity drops sharply. Sawn timber with a thickness of more than 20 mm most moisture evaporate after 20 - 30 days of drying.
The stacking begins with the device of the base, with a height of at least 50 cm together with the lags. 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 a square or a rectangle.
Stacks of lumber are protected by a roof that protects the material from atmospheric precipitation, direct impact sun rays and dust.
Timber is laid on dry softwood pads 25x40 mm in size. The extreme gaskets 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 of the stack, all gaskets are laid in a strictly vertical row along a plumb line. Between stacked boards or bars, gaps (spaces) of the same width are left, forming vertical channels along the entire height of the stack. The width of the spacing, depending on climatic conditions and the section of the boards, is set for lumber up to 45 mm thick from 1/2 to 3/4 of the lumber width and for lumber with a thickness of over 45 mm from 1/5 to 1/3 of the lumber width. For uniform drying of lumber along the height of the stack, at a distance of 1 and 2 m from the bottom row of boards, air vents 150 mm high are arranged. The boards are laid with the inner layers up to reduce their warping. To prevent cracking, it is recommended that the ends of the boards be carefully painted over. oil paint or soak several times with hot drying oil to protect the pores of the wood .. It is necessary to process the ends immediately after the cross cuts to size. If the tree is different high humidity, then the end is dried blowtorch, and then painted over.

CHAMBER DRYING OF Lumber

Chamber drying is the main method in which sawn timber is dried in drying chambers ah, having the right equipment and appliances. Temperature, humidity and air circulation are controlled in the chambers.
Atmospheric drying serves to pre-dry lumber and is usually combined with a wood drying chamber.
Sawn timber can be stacked in stacks in piece or batch way. When forming a stack in a piecewise way, dry (with a moisture content of not more than 18%) calibrated coniferous and hardwood strips with a section of 25 x 40 mm and a length equal to the width of the stack are laid between the rows of boards. Stack height spacers must be laid perpendicular to the boards and strictly vertically one above the other.
The stack is formed from boards of the same breed and thickness. The number of spacers stacked along the length of the stack is given in the table:

Number of spacers stacked along the length of the stack

Table4

Note: In the numerator - the number of pads for stacks of softwood, in the denominator - of hardwood.

Ways of laying lumber in piles depend on the direction (circulation) of the drying agent. For drying chambers with countercurrent circulation, lumber is laid at intervals (spaces), and for chambers with transverse reverse and countercurrent rectilinear circulation - tightly.

Drying modes

Drying of lumber takes place under a certain temperature and humidity regime, which is understood as a regular alternation of the processes of temperature and humidity effects on wood in accordance with its moisture content and drying time.
During the drying process in the chamber, the air temperature gradually rises (in steps) 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 structures (type) of the chambers.

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

· soft M, with soft modes, defect-free drying is obtained with the preservation of 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 at softwood, but with the preservation of strength;

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

Modes of high-temperature drying process for batch chambers
provide for a two-stage change in the parameters of the drying agent, and the transition from the first stage to the second is carried out when the wood reaches a moisture content (transitional) of 20%. The high-temperature regime is determined depending on the breed and thickness of the lumber.
High-temperature regimes 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 the dampening pipes, with the heaters turned on, the fans running and the sugar-exhaust channels 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 within 0.98 - 1 for wood with an initial moisture content of more than 25%, and 0.9 - 0.92 for wood with a moisture content of less than 25%.
The duration of the initial heating of wood depends on the type of wood and for sawn softwood (pine, spruce, fir and cedar) at an outdoor temperature of more than 0 ° C is 1 - 1.5 hours at a temperature of less than 0 ° C - 1.5 - 2 hours for every centimeter of thickness. The duration of warming up of soft hardwood lumber (aspen, birch, linden, poplar and alder) is increased by 25%, and for hard hardwood lumber (maple, oak, ash, hornbeam, beech) it is increased by 50% compared to the duration of warming up of softwood breeds.
After warming up, the parameters of the drying agent are adjusted to the first stage of the mode and then they start drying the lumber, observing the established mode. The temperature and humidity of the air are regulated by valves on the steam pipelines and dampers 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. At the same time, 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 in high-temperature conditions. Softwood sawn timber with a thickness of 60 mm or more and hardwood (depending on the species) with a thickness of 30 mm or more are subjected to moisture heat treatment. In the process of heat and moisture treatment, the temperature of the medium 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 medium is maintained at 8°C above the last stage of the mode, 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.