Humidity of wood species: how much is natural, norms. Determination of moisture content and drying of wood Production moisture for boards

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 lumber conifers and sets technical requirements lumber intended for use in national economy and 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 sizes of lumber and maximum deviations from nominal sizes- 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), digital designation cross-section (for unedged lumber - thickness) and the designation 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 .

The purpose of sawn timber of various grades is given in the mandatory appendix.

(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 to 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 of 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 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 ships sea ​​navigation, gliders, high-speed lake and river boats and sports vessels of the 1st class, must comply with 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. Decking lumber sea ​​vessels shall comply with the requirements of Class I and Class I for exterior decks and Class I and II for interior decks, with the following additions:

on the best layers of lumber with a width of up to 100 mm inclusive, intended for external decks, the sapwood part is allowed with a width of not more than 30 mm, 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 area of ​​the edges of sawn timber, 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, sawn timber 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)

APPENDIX
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"

Wood is a rather porous material containing a large number of capillaries filled with moisture. In practice, the moisture content of wood 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 which is contained directly in the cells of the tree.

When dried, the tree shrinks - it decreases in size (volume). At the same time, there is practically no decrease in size along the fibers (along the length of the board), but in the direction transverse to the course of the fibers, there is a significant change in dimensions (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 changing 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 engagement with each other. In this case, to remove the gaps, you will have to tear off all the boards from the log and lay them again, driving them back to back.

“What humidity should the board have?”, You ask. It's simple - any wooden product, in the process of its operation, tends to the so-called "equilibrium humidity". "Equilibrium humidity" is determined by the temperature and humidity of the environment where the board will be located. You can see the values ​​of this humidity in the table. For residential premises it averages 8-10%, for the street it averages 12-14%. It logically follows from this that a wet board will dry out indoors, losing in its width, on the other hand, a dry board will be moistened outdoors, expanding.

Natural moisture, final wood moisture

natural humidity- this is the moisture content inherent in wood in a growing or freshly sawn (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 seasons. Thus, the natural moisture content of freshly cut trees in a "winter" forest is traditionally less than that of freshly cut trees in a "summer" forest.

Initial humidity- the same as natural moisture. A freshly felled tree has a maximum moisture content, which for various species can even exceed 100%. A balsa tree can have moisture levels as high as 600% when freshly cut. In practice, we are dealing with lower values ​​(30-70%), because after felling, it takes some time until the tree is sawn and placed in the dryer, and it, of course, loses some water. We take as the initial moisture content the moisture content of the wood, which it has before being sent to the drying chamber.

End Moisture- this is the humidity that we want to get after full cycle drying. 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 evaporating it.

Wood drying is one of the most important operations in the wood processing process. The wood is dried after sawmilling, but before woodworking. The wood is dried in order to protect it from damage by wood-staining and wood-destroying fungi during its further storage and transportation. Drying prevents wood from changing shape and size during the manufacture and operation of products from it, improves the quality of wood finishing, gluing. The humidity to which the wood is dried depends on the scope of its further application. The whole point is to bring the humidity of the board to the same value that the product from this board would have reached over time during operation under these conditions. This value of humidity is called "equilibrium humidity", it depends on the humidity and temperature of the surrounding air. For example, a board from which parquet and other indoor products will be made must have a moisture content of 6-8%, since it is this humidity that will be equilibrium. For products that will be used in contact with the atmosphere, (for example: wooden windows, outer skin of the house) the equilibrium humidity will be 11-12%.

You ask: "What will happen otherwise?" We answer: Otherwise, there will be something that is found all the time in Russia, namely, the consumer will face problems. Imagine that you bought a lining in order to sheathe the walls inside your country house or cottages. If you buy from a negligent manufacturer a lining made of raw board and cover the walls of your house with it, then it will slowly begin to dry naturally in an already installed 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 the bases and markets, lining very often can have a moisture content of 14% or more (we also met 30%). Next, imagine that your lining begins to dry out, losing water from its pores. Drying is a process called "shrinkage" and is expressed in a decrease in the size of a wooden product. The amount of shrinkage depends on the type of wood, the direction of the fibers in the product, etc. The main shrinkage goes across the fibers (according to the thickness and width of your lining). When your lining dries in the installed state to the equilibrium humidity, in the worst case, you risk not only seeing that the lining has come apart in places, but getting gaps between the boards, almost a finger wide.

The industry uses various technologies for drying wood, which differ both in the equipment used and in the features 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, which simultaneously use different kinds heat transfer (for example, convective-dielectric) or other signs are combined various technologies wood drying.

Independent drying technologies

Chamber drying

Chamber drying. This is the main industrial wood drying technology carried out in the forest. drying chambers various designs, where lumber is loaded in stacks. Drying takes place in gaseous environment(air, flue gases, superheated steam), which transfers heat to the wood by convection. To heat and circulate the drying agent, the drying chambers are provided with heating and circulating devices.

With chamber wood drying technology, the drying time of sawn timber 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 controlled.

Atmospheric drying

The second most important and widespread at sawmills is the method of industrial drying of wood, carried out in stacks located in a special open area (warehouses), washed atmospheric air without heating. The advantage of atmospheric wood drying technology is the relatively low cost. In addition, this method is the most gentle. Disadvantages: seasonality (drying practically stops in winter); long duration; high end moisture. Atmospheric wood drying technology is used mainly for drying lumber at sawmills to transport moisture and at some woodworking enterprises for pre-drying and leveling the initial moisture content of lumber before drying in wood drying kilns.

Drying in liquids

Drying in liquids is carried out in baths filled with a hydrophobic liquid (petrolatum, oil) heated to 105-120 °C. Intensive transfer of heat from the liquid to the wood makes it possible to reduce the drying time in comparison with the chamber one by 3-4 times, all other things being equal. This method is used in wood conservation technology to reduce its moisture content before impregnation. Attempts to apply drying of lumber in petrolatum at woodworking enterprises did not give positive results due to the fact that lumber after such drying does not meet the requirements for wood for furniture and joinery and building products.

Conductive drying technology

The conductive (contact) wood drying technology is carried out by transferring heat to the material through thermal conduction in 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 efficiency of radiation drying is determined by the flux density of infrared rays and their permeability in wet solids. The intensity of the radiant energy flux weakens as it goes deeper into the material. Wood is one of the most permeable infrared radiation materials (penetration depth 3-7 mm), so 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. coatings. Electric stoves, electric heating elements, gas (flameless) burners, incandescent lighting electric lamps with a power of 500 W and more are used as emitters.

Rotary dryer

Rotary drying of wood is based on the use of centrifugal effect, due to which free moisture is removed from the wood when it is rotated in centrifuges. Mechanical removal free moisture is achieved with a value of centripetal acceleration of at least 100-500g (g is the acceleration of free fall). Due to the difficulty of precise balancing of a centrifuge with a stack, such accelerations have not yet been achieved in practice, only experimental development of appropriate devices is underway. In known industrial rotary dryers centripetal acceleration does not exceed 12g. Under these conditions, mechanical dehydration occurs to a small extent. However, the intensification of the drying process in the range of humidity above the limit of hygroscopicity is observed.

When installing a carousel in a drying chamber, the lumber drying technology is the same as in conventional chambers periodical action. The duration of drying at the first stage (from the initial moisture content to the limit of hygroscopicity) is reduced by 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 drying quality, the rotary method has not yet found industrial use for drying lumber.

vacuum drying

Vacuum drying under reduced pressure in special sealed drying chambers. Due to the complexity of the equipment and the impossibility of obtaining a low final wood moisture vacuum drying has no independent significance. It is used in combination with other drying methods and as an auxiliary operation in preparing wood for impregnation.

Dielectric drying

Dielectric drying - drying of wood in an electromagnetic field of high-frequency currents, in which wood is heated due to dielectric losses. Due to the uniform heating of wood throughout its volume, the appearance of a positive temperature gradient and excess pressure inside it, the duration of dielectric drying is ten times less than convective. Due to the complexity of the equipment, the high consumption of electricity and the insufficiently high quality of drying, dielectric drying itself is not widely used.

Combined wood drying technologies

It is more efficient to use combined wood drying technologies, 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, responsible lumber and blanks from difficult-to-dry wood species, these methods can be used.

Convective-dielectric drying

With the combined convective-dielectric technology of wood drying, a stack loaded into a chamber equipped with thermal and fan devices is also supplied with high-frequency energy from a special HDTV generator through electrodes located near the stack.
The heat consumption for drying in the drying chamber is mainly compensated by the thermal energy of the steam supplied to the heaters, and high-frequency energy is supplied to create a positive temperature difference across the material cross section. This difference, depending on the characteristics of the material and the rigidity of the specified mode, is 2-5°C. The quality of convective-dielectric drying of sawn timber 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 to dry wood using HDTV energy. This technology uses the advantages of both vacuum and dielectric drying. By heating the wood in the HDTV field at reduced pressure, the boiling of water in the wood is achieved at low wood temperatures, which contributes to the preservation of its quality. The movement of moisture in wood during vacuum-dielectric drying of wood is provided by all the main driving forces of moisture transfer: moisture content gradient, temperature, overpressure which shortens the drying time.

With vacuum - dielectric drying, a stack of lumber is placed in an autoclave or sealed chamber, where a vacuum pump creates a reduced pressure of the medium (1-20 kPa). The lower the pressure of the medium, the lower the evaporation temperature of moisture and wood during drying. The heat consumption for drying is provided by the supply of high-frequency energy to the wood. When using this technology for drying wood, operational difficulties also arise - the complexity of the equipment, especially the adjustment and operation of high-frequency generators, and the high consumption of electricity for drying. Therefore, when deciding on the use of vacuum-dielectric chambers, it is first necessary to develop a feasibility study according to the conditions of a particular enterprise.

Induction or electromagnetic wood drying

The method is based on the transfer of heat to the material from ferromagnetic elements (steel meshes) 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 the drying chamber. Steel elements (grids) 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 by convection from circulating air, also heated by meshes.

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 any natural material, wood 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.

Wood is a very hygroscopic material that easily changes its moisture content. 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. Moisture content of wood is a quantitative indicator of the moisture content in it.

Wood moisture

Moisture exchange takes place between wood and air all the time. Therefore, the moisture content of wood is a very unstable value, which changes along with the moisture content. environment. If the moisture content of the wood is greater than the humidity of the surrounding air, the wood will dry out. Otherwise, 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, the equilibrium moisture content for wood is an extremely unstable state. Because, in nature it is impossible to find air with long enough constant parameters of temperature and humidity. However, the state of equilibrium humidity is easily achieved for wood that is in an artificial microclimate, for example, in a drying chamber or simply in any other room with a constant temperature and humidity.

Distinguish between absolute and relative humidity of wood

Absolute moisture content of wood

Absolute humidity is the ratio of the mass of moisture that a sample of wood contains to the mass of absolutely dry wood of the same sample. According to , the value of absolute humidity (W) is calculated after the study (drying) of the sample, according to the formula:

W \u003d (m - m 0) / m 0 x 100,

where, (m) and (m 0) - 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 real world and is an extremely indigestible form when thermotechnical calculations. For example, with an absolute humidity of 25%, a kilogram of wood will contain 200 grams of water. Such a discrepancy between the numbers is confusing in the calculations.

More convenient and practical is the value of relative humidity

Relative humidity of wood

Relative (working) moisture content of wood is the ratio of the mass of moisture that a sample of wood 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 easier,

W rel. = m water / m sample x 100

Relative humidity is a very simple and convenient form for accounting for evaporated water in wood-fired heat engineering calculations. The value of relative humidity directly indicates the quantitative content of water in wood. For example, one kilogram of wood at 20% moisture content will contain 200 grams of water and 800 grams of dry wood matter.

For comparison, let's collect a "live" example in a table. This is a table for the same our sample. Let us determine and compare the values ​​of its absolute and relative humidity:

Degree of wood moisture

According to moisture, all wood is divided into three groups: raw (moisture more than 35%), semi-dry (moisture from 25 to 35%) and dry (moisture less than 25%). Initially, the moisture content of freshly cut trees is 50-60%. Then, during 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 state of conditionally moisture. After that, the moisture content of the wood does not change significantly, and its value is ≈25%. Such wood is called air-dry. To reduce the moisture content of wood to the state of room-dry (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, transferred to a room or other room).

There are the following degrees of wood moisture:

• floating(humidity 60% or more)
  It may be a tree that has been in the water for a long time. For example, driftwood, or wood after sorting in a water basin, or just a well-soaked (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 in the open air, 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 humidity of firewood is one of the defining indicators of its quality. That dry firewood burns better than raw wood is known to many, if not all. And everyone knows that wet firewood can always be dried, and dry firewood, on the contrary, can be wetted. Accordingly, the quality of fuel will change - improve or deteriorate. But, is it so important for modern heating equipment? For example, wood-fired pyrolysis boilers allow you to burn firewood with a moisture content of up to 50%, and even up to 70%!

The table shows the generalized indicators of the calorific value of wood for each degree of its humidity.

The table shows that the lower the moisture content of wood, the higher its calorific value. For example, air-dry wood has a working calorific value almost twice that of freshly cut wood, not to mention wet wood.

Wood with a moisture content of 70% or more practically does not burn.
Perfect option for wood heating is to use wood in a state of room-dry degree of humidity. Such firewood gives 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-dry 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 their calorific value. The increased moisture content in the fuel adversely 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 moisture content of no more than 30-35%

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. It is said that wood "breathes", that is, it absorbs air vapor (sorption) or emits them (desorption), reacting to changes in the microclimate of the room. Absorption or release of vapors is carried out due to the cell walls. With a constant state of the environment, the moisture content of wood will tend to a constant value, which is called the equilibrium (or stable) humidity.

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 are beech, pear, kempas. They respond most quickly to changes in humidity levels. In contrast, there are stable species, such as oak, merbau, etc. These include the bamboo stalk, which is very resistant to adverse climatic conditions. It can be laid even in the bathroom. Different types of wood have different levels humidity. For example, birch, hornbeam, maple, ash have low moisture content (up to 15%) and tend to crack when dried. Humidity of oak and walnut is moderate (up to 20%). They are relatively resistant to cracking and do not dry out as quickly. Alder is one of the most desiccation-resistant species. Its humidity is 30%.

Humidity is one of the main characteristics of wood. Under humidity wood refers to the ratio of the mass of water to the dry mass of wood, expressed as a percentage.

absolute humidity wood is the ratio of the mass of moisture 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%).

Relative Humidity wood is the ratio of the mass of moisture contained in wood to the mass of wood in a wet state.

There are two forms of water in wood - bound and free. They 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 half of the cells and intercellular spaces. Free water is removed more easily than bound water and affects the properties of wood to a lesser extent.

According to the degree of moisture, wood is distinguished into the following types:

Wet wood. Its humidity is over 100%. This is possible only if the wood has been in the water for a long time.

Freshly cut. Its humidity ranges from 50 to 100%.

Air dry. Such wood is usually stored in the air for a long time. Its humidity can be 15-20%, depending on climatic conditions and time of year.

Room dry wood. Its humidity is usually 8-10%.

Absolutely dry. Its humidity is 0%.

With prolonged drying, water evaporates from the wood, which can lead to significant deformations of the material. The process of moisture loss continues until the level of moisture 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, the absorption of moisture. The decrease in the linear volumes of wood when bound moisture is removed from it is called shrinkage. Removal of 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 shrinkage (that is, one in which all bound moisture is removed), the moisture content of the wood decreases to the limit of hygroscopicity, that is, to 0%. With an uneven distribution of moisture during the drying of 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 the mechanical processing of wood.

The properties of wood directly determine the properties of wooden products. With excess or insufficient moisture, wood usually absorbs or releases moisture, respectively increasing or decreasing in volume. At high humidity indoors, wood can swell, and with a lack of moisture, it usually dries out, so everything wooden crafts, including floor coverings, require careful maintenance. To prevent deformation of the flooring in the room, it is necessary to maintain a constant temperature and humidity. This has a positive effect not only on quality and durability. floor coverings and wooden furniture but also on people's health. With a sharp change in the temperature and humidity conditions in the room, internal stresses arise in the wood, which lead to cracks and deformations. Optimum temperature in a room with a parquet floor should be approximately 20 0 C, and optimal humidity air - 40-60%. Hydrometers are used to control the room temperature, and the relative humidity in the room is maintained with the help of air humidifiers.

WOOD MOISTURE DETERMINATION

There are several ways to determine the moisture content of wood. AT living conditions use a special device-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 passed through them. electricity, while on the scale of the device, the moisture content of the wood is immediately noted in the place where the needles are inserted. Many experienced carvers determine the moisture content of wood by eye. Knowing the types of wood, its density and other physical properties, it is possible to determine the moisture content of wood by weight, by the presence of cracks at the end or along the fibers of the wood, by warping and other signs. By the color of the bark, its size and the color of the wood, one can recognize ripe or freshly cut wood and its degree of moisture. When processing a p / m planer, its thin chips, clenched by hand, easily crumpled - it means 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. Very wet wood is easily cut, and a wet mark is noticeable at the cutting site from the chisel. But it is unlikely that it will be possible to obtain a high-quality thread as a result, since cracking, warping and other deformations cannot be avoided.

DRYING WOOD

Drying wood - the process of removing moisture from wood to a certain percentage of moisture.

Dry wood has high strength, warps less, does not rot, sticks easily, finishes better, is more durable. Any wood of various species is very sensitive to changes in environmental humidity. 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. Indoors, wood moisture content up to 10% is sufficient, and outdoors - no more than 18%. There are many ways to dry wood. The simplest and most affordable - natural type of drying - atmospheric, air . It is necessary to dry the wood in the shade, under a canopy and in a draft. When drying in the sun outer surface wood heats up quickly, and the inside remains damp. Due to the difference in stresses, cracks form, the tree quickly warps. Boards, beams, etc. p / m are stacked on metal, wooden or other supports with a height of at least 50 cm. Boards are stacked with inner layers up to reduce their warping. It is believed that the drying of boards placed on the edges is faster, as they are better ventilated and moisture evaporates more intensively, but they also warp more, especially material with high humidity. A stack of p / m, harvested from freshly cut and live trees, is recommended to be compacted from above with a heavy load to reduce warpage. During natural drying, cracks always form at the ends, to prevent their cracking and preserve the p / m, it is recommended to carefully paint the ends of the boards oil paint or impregnate with hot drying oil or bitumen to protect the pores of the wood. It is necessary to process the ends immediately after the transverse cuts into the cut. If the tree is different high humidity, then the end is dried with a blowtorch flame, and only then painted over. Trunks (ridges) must be debarked (cleaned of bark), only at the ends they leave small belts-couplings 20-25 cm wide to prevent cracking. The bark is cleaned so that the tree dries out faster and is not affected by beetles. The trunk, left in the bark, in relative heat with high humidity quickly rots, is affected by fungal diseases. After atmospheric drying in warm weather, the moisture content of wood is 12-18%.

There are several other ways to dry wood.

Way evaporation Or steaming has been used in Russia since ancient times. The workpieces are sawn into pieces, taking into account the size of the future product, laid in ordinary cast iron, sawdust from the same workpiece is poured, poured with water and placed for several hours in a heated and cooling Russian furnace “languishes” at t = 60-70 0 C. In this case, “leaching” - 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 it cracks and warps less.

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 out 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. Tableware made of wood steamed in linseed oil is very water-resistant and does not crack even with everyday use. This method is still acceptable today. The workpiece is placed in the container, poured linseed oil and steamed over low heat.