EMPIRICAL DRYING OLD IDEAS ABOUT THE PRACTICE OF DRYING

The wood can only be reasonably used after eight years of use.

Only use absolutely dry wood in any carpentry work and we were not far from thinking that the longer the wood had remained on the job before use, the drier it would be.

We have long adopted a formula that has the merit of simplicity: hardwood dries one centimeter a year.

It was said that soft, white, resinous woods dry faster.

Therefore, we count one year per centimeter of thickness for hardwoods and six months for softwoods. This rule was applied almost generally at the beginning of the century. Therefore, there was always wood in stock that had been drying for a long time.

The war of 1914-1918 evidently absorbed the populations and the conditions that followed for several years, as well as the impoverishment resulting from the destruction of important forest regions in the north and east of France, posed a problem around 1920 in 1925.

We could no longer find sufficiently dry wood. We missed the deadlines and, on the other hand, we thought about artificial drying, but considering it as a temporary resource, since artificially dried wood was considered much inferior to wood dried in the open air. It must also be said that certain disappointing results justified this criticism. But since it was impossible to supply the market with wood dried in situ, we had to resort to using wood dried in artificial dryers. Some, convinced that it was necessary to dry and age the wood at the same time, imagined “smoking” or ozonation processes, which were intended to be achieved in a few weeks; “aging” analogous to the natural aging that occurs during outdoor drying. In fact, whenever possible, the carpenter or cabinetmaker user stated his preference for air-dried wood. In this first lesson on wood drying, we propose to explain what this drying in the open air was, then to study the phenomenon of drying itself, methodically, to deduce the rules of drying, rational use of the material and then show how they are applied these rules. They can find their application in construction sites or in drying rooms.

A – THE PROBLEM OF NATURAL DRYING CONSIDERED BEFORE THE RESEARCH ON ARTIFICIAL DRYING.

The principle of drying in the open air was as follows: wet wood in order to dry and avoid “heating” had to be in contact with the air on a maximum surface area on all sides. To do this, taking into account an always expensive location, they are stacked under special conditions on land called construction sites. First of all, we will choose the highest part of the land to establish the piles.

In this chosen place, pieces of wood, also called construction works, will be placed on the ground at intervals. They are the length of the beams that rest on them and their purpose is to isolate the first row of beams that will form the pile from the wet soil. In these places, a first row of wood is placed transversely, separated by a space equal to approximately 2/3 of the width of each one. Then, on these woods, obviously all of the same thickness, we place transverse slats at a distance of 0.80 to 1 meter, on which the second row of wood will be weighed, therefore in the same direction as the first row. Thus we continue the construction of the pile.

The plumb line allows the separation slats to be perfectly aligned vertically. When the pile has reached 6 or 7 meters in height, the upper part is covered with boards, forming a cantilevered roof, to protect the upper part of the pile from rain. This roof is usually made from scrap boards. It is slightly inclined. This is the pile on sheets suitable for imperfectly dry wood. We can also make piles without slats or wood piles on wood. Once the first row of wood is established as indicated above, the second row is placed directly on top, arranging the wood in a perpendicular direction. It will be necessary to move the wood from time to time so as not to always leave the same portions of surface in contact, which would cause overheating. Wood-on-wood piles are obviously square and to establish them it is necessary to have sufficient quantities of wood of the same length. These two stacking systems are very suitable for wood with sharp edges intended for carpentry or hidden construction work. But for the species used as exposed wood, it is advisable that the pieces that make up the entire piece of furniture have the same color and grain. To do this, we keep the thicknesses cut in a log in the same order in which they were cut. The trunk is reconstituted by stacking, separating the different trays using slats, close enough to each other to compensate for any possible deformation. This stacking system is called block stacking.

The so-called Honfleur stacking is suitable for already dried spruce boards. It is a stacking of wood on wood but in the same sense and not in crossed layers as previously stated. The woods leave spaces between them that correspond to the fullness of the top row. The so-called scalded stacking is a temporary system that is practiced immediately after sawing white wood and whose objective is to dry its surfaces sufficiently to be able to later use wood-on-wood stacking without fear of damaging the surfaces. This stacking is done for 9 to 10 weeks in summer. Finally, when the almost dry wood is ready for use in the workshops, it is returned to the attics or construction sites or under sheds closest to the workshop. They are then supported vertically against the walls or against crossbars arranged to support them, in the natural direction of the fibers, that is, supported on the floor by the part that touches the stump. But if it is important to store thick sawn lumber in a perfectly dry place, veneers should be stored in fairly humid basements to maintain flexibility suitable for veneer operations during excavations. This is a brief summary of drying patterns observed in the past. The rest of this presentation will show us what is valid in the relevance of these directives. But now we have to see how artificial drying was originally conceived.

B – PRINCIPLES OBSERVED IN THE FIRST ACHIEVEMENTS OF ARTIFICIAL DRYING

By grouping together a series of observations, such as those made on driftwood and on the wood studied, simply trying to accelerate the drying process, or trying to obtain a kind of aging at the same time as the release of water, rules can be established for artificial treatment. . Drying was established at the beginning of the 19th century, without being satisfied with the results obtained (both abroad and in France). We will limit ourselves to a brief presentation of what has been done without dwelling on a detailed description of absolutely obsolete processes or materials. The question was taken very seriously after 1930. Then we were able to unravel what was logical and essential. We had to face the fact that certain practices were useless although expensive (pre-drying, smoking, etc.) 

II – ELEMENTS THAT COME INTO PLAY IN WOOD DRYING 

A – THE STRUCTURE I advise you to refer to the first lessons of this course to delve deeper into the study of the structure of wood, of which we will only remember the essentials here. Wood is made up of a set of cellular tissues. The fundamental fabric is made up of fibers. Other important tissues are the rays and vessels. Each species is characterized by a certain proportion and arrangement of these tissues, constituting the woody plant of the species. Within a forest like the oak, we see in each annual zone 

a) the internal region, formed in spring, very porous 

b) the outer region of the layer with discrete vessels 

c) woody rays that form the links. Within a wood like beech we find annual layers with a much more uniform structure. Finally, resinous woods without vessels present marked annual layers in which the summer wood is noticeably more compact and dense. 

There are actually 3 categories of structure: heterogeneous hardwood structure (oak) homogeneous hardwood structure (beech) softwood structure. There is a relationship between the amplitude of the annual increases and drying. 

In conifers, the broad annual layers contain a high proportion of spring wood. 

The tree with wide layers (for example a common pine) is soft, not very dense, its wood will be more difficult to dry and will require less time. A Norwegian Scots pine, on the other hand, with thin layers is denser, more nervous and requires a longer drying time. An oak with extensive annual layers, unlike a conifer, contains a high proportion of summer wood. 

It is a heavy, hard wood, which will be subject to cracking during drying and will require a fairly long drying time. On the contrary, a fine-grown oak is a soft and relatively light oak, not very nervous and not prone to deformation during drying. The density, which depends on the structure, in particular the ratio between summer wood and spring wood, gives us a first indication of the ease or difficulty of drying. 

B – AIR TEMPERATURE In outdoor drying the air temperature factor is essentially variable and completely independent of our control. In artificial drying we have the possibility of using hot air. The problem of measuring air temperature is easy to solve: all types of thermometers are used

 a) ordinary mercury or alcohol thermometers 

b) probe thermometers, allowing the temperature in a dryer to be known without having to enter it.

 The probe is in the dryer and the dial is outside. These are vapor pressure thermometers. Conventional alcohol thermometers can only give valid readings up to approximately 65°. Mercury thermometers can give readings up to 200°. Vapor pressure thermometers can give indications of up to 400°, but in fact, even in the most advanced dryers we do not exceed 150°. Electrical resistance thermometers, like vapor pressure thermometers, can be recorders. We prefer recording devices that indicate all temperature variations with a curve. But thermometer registration can fail. Therefore, it is necessary to plan periodic checks. 

C – RELATIVE HUMIDITY OR HYGROMETRIC DEGREE OF THE AIR This concept is important and needs to be clearly specified. It must be understood that the degree of humidity of the air, or hygrometric state, does not depend on the absolute amount of water contained in the air, but on the pressure (or tension) of this water vapor. We enter an unheated room, closed for some time and whose walls are covered in steam. We say: this apartment is damp. Without opening windows or doors, let’s heat this apartment for 24 hours. The condensation has disappeared from the walls. We have the sensation of breathing dry air. We say: the humidity is gone. Now, since the apartment has remained closed, there is exactly the same amount of water in its atmosphere, but by increasing the temperature we have pushed back the saturation point of the vapor. (We say that vapor is at its saturation point when it is capable of depositing in the form of mist on the surface of objects. If we have understood the above correctly, we will easily accept the presentation of facts that always astonish laymen:

1°) Hot, dry air can contain more water vapor per cubic meter than cold, humid air. In summer, we often have greater amounts of water vapor per cubic meter than in winter.

2°) We were able to observe a fairly high hygrometric state in desert regions where it had not rained for years.

Therefore, we must define exactly what we mean by the hygrometric state of the air.

It is the relationship between the amount of water vapor contained in a cubic meter of air and the amount of water vapor that would saturate that same cubic meter, at the same temperature.

Instead of relating the weights, we can relate the tension of water vapor to the tension of water vapor at the same temperature if the air were saturated.

The hygrometric state is measured using devices called hygrometers. There are hair hygrometers: hair shrinks and lengthens depending on its humidity. Its variations are transmitted to a dial from which the humidity level of the air can be read directly. But outdoor hair hygrometers do not work properly due to the high temperatures of dryers. Then we use psychrometers. A psychrometer is a set of two identical thermometers, the reservoir of which is wrapped in muslin that is kept constantly moist. Under the action of more or less dry air, the water that permeates the muslin evaporates. As it evaporates, it absorbs heat taken from the thermometer reservoir. Low temperature.

In absolutely dry air the 2 thermometers will make a maximum difference if the outside air is saturated there is no evaporation and the 2 thermometers show the same temperature: a pcrometer, to function normally, must be placed in a fairly high velocity air current (around 2 meters per second). It should also be supplied with very clean water (distilled water for example).

The muslin should be changed frequently. The scales give the value of the hygrometric state based on the temperature provided by the dry thermometer and the difference with that given by the wet thermometer. As an example we extract it from the table written in the CTB (Centre Technique du Bois) notebook dedicated to drying. the few lines relating to temperatures ranging between 52° and 60°; Temperature indicated by the dry bulb.

 E – SHRINK We study this shrinkage phenomenon in the lesson on the properties of wood. It will be useful to review this chapter. Let us remember here the essentials to know that 1°) Green wood soaked in water begins to dry without changing its dimensions. 2°) When it reaches a level of humidity corresponding to the release of all free water (25 to 30% depending on the species), its dimensions progressively decrease as the water that impregnates the cell walls leaves. 3°) Dry wood that recovers moisture undergoes an increase in volume. 4°) Wood always enters a state of moisture balance with the environment that surrounds it. 5°) Shrinkage varies depending on the nature of the wood; Dense hardwood always has relatively high shrinkage, unlike softwoods which have low shrinkage. 6°) The shrinkage or swelling values ​​in the different directions of the wood are variable. The attached sketch (Fig.6) recalls what we have already indicated in this regard and it is from this inequality of contraction that deformations (warping) and cracks or fissures come. Furthermore, it is necessary to take into account the importance of shrinkage so that the pieces have, after drying, the desired dimensions (for example cut to 32 mm by 30 mm). 

III – LIMITED MOISTURE OF WOODS 

We will conclude this study of the elements involved in drying by specifying the notion of hygroscopic balance of wood with air (external or from the dryer). For air at a given temperature and for a given hygrometric state, the moisture level of the wood stabilizes after a more or less long time. It is said that the wood has then reached its moisture limit. So it is quite obvious that if a room is perfectly conditioned by air at a constant temperature and a constant hygrometric degree, all the wooden objects found there will be in equilibrium with the atmosphere of that room. 

For example, if the room is air conditioned at 20° and 60% humidity, the wood will balance at 11%. Limit humidity values ​​have been quantified. The results can be recorded in a table or graph. 

 RATIONAL NATURAL DRYING – ARTIFICIAL DRYING 

 INTRODUCTION In the previous lesson we have studied separately all the elements involved in the drying of wood: its structure, its state of felling, the hygrometric state of the air that surrounds it, the precise measurement of temperatures and hygrometric degrees, the balance that It is established between a certain moisture content in the air and the water content of the wood. We also studied how air drying was done in the past. In this lesson and the next we will try to identify what is essential, or simply useful, to rationally dry the wood, not only in the air, but in a kiln. 

II – AIR DRYING 

A – RATIONAL CONSTRUCTION OF A PILE 

Properly carried out drying should allow the wood to dry quickly enough, without deforming or changing its shape. First of all, it is necessary to take a series of elementary precautions, obvious but often unnoticed: maintenance of the dryer floor (no holes where water can accumulate, no weeds that wither and rot in place, no pieces of wood wandering everywhere, especially if there is still bark or sapwood attached, with the risk of increasing insect and fungal attacks). But above all, batteries must be built rationally. We will not return to waiting piles or piles of dry wood waiting to be used. A pile is made up of layers or beds of boards between which pins (or sticks or wedges) are placed. These dowels should allow air to circulate between the board bases, but should also hold the wood pieces in place to prevent any warping. 

1°) The ground. A pile will be established on healthy, permeable, well-drained, strong and perfectly clean soil. The weight of a pile is considerable and must be supported by solid and durable foundations. Planks that rest on flattened or even cemented parts of the floor, or masonry blocks, are suitable. 

2°) Batteries. It is essential to reserve a large space under the pile (40 to 50 centimeters) to allow the evacuation of humid air that accumulates in the pile. It is also essential to only dry wood of the same species in the same pile. It is not essential, but it is recommended that the boards have the same thickness and the same length. The piles must be sufficiently spaced (not less than 0.50 m between two piles, and more if the height of the piles is high enough). In a large construction site, it is necessary to provide corridors for the transit of trucks and spaces for their handling. These large spaces form firebreaks in case of fire. Batteries must be absolutely protected from inclement weather and sunlight. Light roofs are ideal. It is necessary to avoid scrap boards, as well as scrap beams, on the floor, to eliminate the causes of deterioration. 

3°) Stacking. We may have to stack wood in blocks or edged sawn wood, but in all cases the pieces that form the same bed will never be joined together. It is not necessary to strictly adhere to the old rule of spacing equal to 2/3 of the width, which probably generates cracks in certain regions, but it is necessary to leave a few centimeters (2 or 3 at least) between the boards, and, sometimes, For resinous woods or white woods, it will be good to install chimneys inside the stacks to speed up drying: 

Pins are extremely important. 

We must reject the simplification that consists of taking the same boards from the batch to be dried as spacer strips. 

The chopsticks should be made of poplar heartwood. They must be dry, of the same thickness along their entire length, and we must have a stock of them, of varying thicknesses because these thicknesses will have to be chosen according to the species that make up the pile and the season of the year in question. .which we carry out the stacking. 

The sticks must, as we have said, hold the pieces during drying to prevent their deformation. 

To do this, they must be perfectly aligned vertically. 

You cannot think of another stacking system for a long period of time. 

Therefore, vertical piles do not hold the wood in place well during drying and this results in significant warping. Therefore, we only practice this vertical stacking during a period of drying of the newly sawn wood, or on the contrary, with the wood already dried, in a workshop, waiting for its use. 

B – NATURAL DRYING MECHANISM In reality there is no natural drying mechanism and no artificial drying mechanism. Drying, whatever the method, is made up of two phenomena: the circulation of water from the center of the room to the surfaces and evaporation at the surface. We specified in the previous lesson that the first of these phenomena was controlled mainly by the structure, so it was particularly linked to the essence considered, while evaporation depends mainly on the temperature, the hygrometric state of the air and the circulation of this air. . In the case of natural drying, these characteristics are highly variable from one season to another, and even from one day to the next. 

C. and the average temperature for the month of August is 20 -22° C. For the same periods the humidity level ranges from 80 to 90% to 50 to 50%. The daily variations are little accentuated during the winter season and much greater in summer. In June we easily go from 30° to 15° at night and from 50% to 90% humidity. Under these conditions, the wood practically recovers the moisture lost during the day at night. But, obviously, in general wood will dry much more in summer due to 2 factors: higher temperature and lower air humidity. We cannot take any action on this data. The only thing we can do is regulate the circulation of this more or less hot and humid air depending on the thickness of the rods. In fact, an essential condition for good drying is good air circulation around and inside the pile. The air in contact with a piece of wet wood becomes saturated. The continuous evaporation of water in the wood produces a drop in temperature and therefore makes saturation easier. If this saturated air is not removed and replaced with drier air, evaporation stops. The renewal of air in natural drying occurs first thanks to the wind. We can use the direction of the prevailing winds in a specific region. Air circulation in the pile is also established. Upon contact with wet wood and under the action of evaporation, the air cools and its density increases. A vertical circulation is created from top to bottom, all the easier if chimneys have been provided inside the pile. The cold air must be able to be expelled from under the pile (which is why it is important that it is elevated) through natural air currents. 

C – POSSIBLE INCIDENTS ALTERATIONS Unforeseeable variations in the temperature and hygrometric state of the air, the violence of the wind can determine the appearance of defects in the critical period that occurs around 30% humidity in the wood. These defects are: deformations, surface cracks and end cracks, overheating and rot. Buckling. We know that warping is the result of wood shrinkage. We limit them to a minimum by rationally building the battery (No. 3 of paragraph A above). Surface grooves. Superficial cracks occur in summer, in circumstances that are very favorable to rapid drying, when the hygrometric state of the air is low. Therefore, its water absorption capacity is very high. But the internal circulation of water from the heart of the piece to the surface cannot follow the same rhythm. Therefore, the wood on the surface shrinks faster than that on the inside and the wrapper becomes too small for the container. It cracks. There is still a palliative of using thinner sticks during the driest time of the year, especially if the pile is built during this dry season and if it is a pile of particularly nervous wood. Example: we stack 27 mm oak and do the stacking in winter, we take 25 mm sticks. We do the same operation in summer. We will take 15 mm rods for the same thickness of 27 mm. Finally, it should be noted that softwoods and poplar are the least likely to suffer from superficial cracks. On the other hand, woods such as eucalyptus and cork oak are almost unusable due to their great propensity. Openings at the end. The circulation of water in wood occurs easily in the longitudinal direction, much more than in the radial and tangential direction. 

Likewise, evaporation on the testa surfaces is active. This results in an especially rapid drying of the end, accompanied by a contraction that causes cracks that tend to progress along the board. We act by delaying evaporation in the final wood. To do this, you can nail small wooden slats at the ends. But you must be very careful, when removing the slats, not to leave nails that would damage the cutting saws. It is also possible, and much more preferable, to cover the head with more or less waterproof paints or varnishes: (paint: bituminous products). For certain species, it may be beneficial to accelerate drying initially to quickly overcome the critical deformation period. This period is the one during which the wood meets the optimal conditions for a cryptogamic attack. Between 20 and 30% humidity in the wood and a temperature of 20 to 35° Celsius, the conditions for these alterations called overheating occur; which can turn into characteristic wood rots. But woods prone to overheating, such as beech, may not be split too much. They then favor the acceleration of drying during the first period. One way to stop incipient alterations whose damage is not yet visible to the naked eye is to steam the wood before starting to dry it in the open air. Steaming stops fungal development, kills larvae, if any, but does not protect the wood against further attacks. Steaming should not be confused with preparation for drying. It is a precise operation to destroy germs that destroy wood.

D – DURATION AND LIMITATIONS OF NATURAL DRYING

In addition to the factors already studied: temperature, hygrometric state, good air renewal, it is necessary to mention the influence of: species: hardwoods dry slowly compared to softwoods and resinous woods. the thickness the cutting method: cutting into slabs makes the wood dry more quickly than wood cut into quarters. stacking season. It is important, for the duration and quality of drying, to stack hardwoods before winter and softwoods at the beginning of summer.

The size of the piles: when we exceed 2 meters in width we noticeably slow down the drying. the empty space under the stack. We said that it was advisable to leave a space of 40 or 50 centimeters under the batteries. If all these conditions are met, natural drying takes much less time than previously thought. However, it can be estimated that hardwoods 27 to 30 mm thick stacked in the fall will be dry the following spring and that hardwoods 50 mm thick will be dry the following fall. If it is made of soft wood (poplar) or resinous, one summer and early autumn will be enough: 3 to 5 months depending on the thickness. But it is worth remembering that it can only be dried in the air, that is, with a humidity of 15 to 20%. 

III – ARTIFICIAL DRYING

 A – ANALOGIES AND DIFFERENCES WITH NATURAL DRYING The opposition of the two terms “natural” and “artificial” gave rise to the idea of ​​two absolutely different phenomena. This was enough for irrational opposition to arise. In fact, the drying principles are the same in all cases. But they are more or less easy to apply and, above all, the results are more or less limited. We have seen that air drying can provide 15-20% dry hardwoods in a year. Artificial drying (at the cost of a certain amount of energy and work) can give us wood of the same nature that is 8 or 10% dry in about a month. 

B – GENERAL CONDITIONS The results of artificial drying as practiced at the beginning were absolutely disappointing. Sending hot air without any precautions resulted in unusable wood for the following reasons. 

a) Too rapid evaporation at the surface produced surface cracks that became more pronounced and propagated deeper. 

b) the beginning of internal splits in the weak parts (medullary rays in particular) also amplified when the heart reaches below the saturation point. 

c) we ended up having accentuated internal cracks, while the surface cracks tended to become invisible because they were partially closed. The reason for these failures was discovered at the beginning of the 20th century and in 1930 a rational method was developed. The initial mistake was using hot, dry air. Hot air favors the circulation of water from the interior to the surface of the wood, but also, if it is dry, surface evaporation. If, on the other hand, we use hot and humid air we will always have adequate circulation inside the piece of wood, towards the surface, but we will reduce surface evaporation which will thus align with this internal circulation. We can say that if we manage to find the match between the air temperature and the humidity level we can achieve quick and perfect drying. Thus, despite how paradoxical this conclusion may seem, wood must be dried with hot, humid air. Let’s look at the table at the end of the previous lesson. A temperature of 60° and an air humidity of 70% determine a wood humidity of 14%. Let’s assume this rate is the one I want to get. But I have wood that is 30% saturated. If I blow air at 60° I cause cracks. Therefore, I will start with air at 20° and a hygrometric index of 95%, which will bring the wood to 28%. Then I will gradually increase the temperature and never go below 70% humidity. 

In summary: during a drying operation, each degree of wood moisture must correspond to a specific temperature and hygrometric state of the air inside the dryer. a) Temperatures generally vary from 45 to 80° depending on the species. Sometimes they can be significantly exceeded. b) The hygrometric state must always be maintained under the established conditions. It is always very high for most of the drying time (75 – 80%). All major setbacks are due to failure to meet this essential condition. Many dryers, if used incorrectly, have too low a hygrometric state. We must realize the difficulty of maintaining a high hygrometric state at high temperatures, at the same time that powerful ventilation will operate. And it only takes a short time to deteriorate wood. Therefore, it is absolutely necessary to have control devices in good condition that allow very precise monitoring of the essential maintenance of the humidity level. You shouldn’t want to dry too quickly either. 

In comparison, artificial drying is certainly very fast. But we must not recklessly reduce drying times, which, for wood of common thicknesses, range between a few days and 1 month. Let us remember here that the most common thing is that we seek to obtain 8 or 10% dry wood. 

C – VARIOUS PERIODS OF ARTIFICIAL DRYING We distinguish 3 periods in artificial drying 

1°) Preparatory period 

2°) Dry 

3°) Equilibrium period. 

1°) The preparatory period. Wet wood will only begin to really dry when the entire mass has reached dryer temperature. During this period: it is important to have a very humid environment.

We’re not trying to dry out yet;

Any intense evaporation on the surface should be avoided. Furthermore, the transmission of temperature from air to wood is much better in a humid atmosphere than in a dry atmosphere. This period is very important, although limited to a few hours. In fact, cracks can appear if the air is too dry in the drying cell. 

2°) Own drying.

The wood heats up. It will start to dry.

It will be carried out at the desired pace following the fundamental principle already stated: each degree of humidity in the wood must correspond to a well-determined temperature and hygrometric state of the air in the dryer. To correctly apply this principle, you must have before your eyes scales called drying tables. Observations and reference points made during previous operations are added to these scales. We know each dryer and “operate” it according to its own characteristics. 

3°) The equilibrium period. The equilibrium period is also important. Wood is considered dry when it has reached, on average, the desired degree of humidity. But this moisture is not distributed evenly throughout the thickness of the wood. To avoid harmful stresses, this wood should not be worked immediately. Therefore, a period of equilibrium is to be expected. 

This balance, of course, will occur on its own. Wood coming out of the dryer, properly stored in a covered room, will balance over a period of 10 to 15 days. This period is relatively long, often almost as long as the drying itself. We also usually prefer to do the balancing in the dryer itself. This is to allow the increased humidity in the heart of the room to be evenly distributed throughout the mass of this room. This result is obtained by continuing the normal heating of the dryer, but considerably increasing the humidity level of the air. We achieve the desired result within 10 to 48 hours. Wood is ready for use when it has the same final moisture content throughout its thickness. 

D – WHAT A WOOD DRYER ESSENTIALLY INCLUDES. 

A dryer is an installation capable of creating the temperature and hygrometry conditions necessary for drying. 

a) a cell capable of containing an optimal volume of wood to be dried (importance of the company, but also the shape and dimensions that must be respected to obtain good drying uniformity). 

b) in this cell an air conditioning system that necessarily includes a heating set, a humidification system and an air circulation system.

c) still in the cell, but with the possibility of external reading, control devices: temperature control, humidity control (which we have described above). 

1 The cell. Drying cells can be built in masonry, concrete; wood, sheet metal. Masonry cells are good and are often used in large installations. The cell must be waterproof and well insulated. (because heat and humidity losses are more expensive than the cost of good insulation). Evacuation doors and chimneys must be well studied from the point of view of watertightness. An inspection door is necessary to avoid disturbing the drying regime by opening the large entrance door too frequently. The location of the cell is of great importance: no cell will be installed “in the open”. Cells under hangars must be absolutely protected from inclement weather and sun. It is also very important to think about accessibility, load, proximity. , carts, stocks of green wood and soft wood, etc. We can introduce an average volume of wood equal to one third of its capacity into a drying chamber (10 m3 of wood for 30 m3 of the interior volume of the chamber). have several medium or relatively low capacity cells instead of one high capacity cell due to drying of species and several flow rates.’perform at the same time. 

2 Atmosphere conditioning Includes: 

a) A heating system that can operate with hot water, high or low pressure steam, hot air or electricity.

Steam is used most frequently.

In any case, the heating coil must have sections that can be controlled separately, to achieve adequate temperature regulation. 

b) An air humidification system that can be either water vaporization or, more conveniently, a steam injection system. Of course this system is adjustable. In addition, correction can be obtained through fresh air inlet and humid air outlet chimneys whose openings are provided with registers that allow their regulation.

We emphasize again the importance of absolutely correct management of the humidification system that really controls the entire drying. 

c) A ventilation system. The ventilation system can be natural ventilation or artificial ventilation. In the first case we simply use the difference in density between hot air and cold air. Circulation is regulated by the corresponding opening of registers. The speed of the air passing through the batteries is then quite limited (20 cm per second, instead of 75 cm/second, as is considered desirable). In the second case, the fans allow you to obtain a better speed, but two things must be taken into account: an exaggerated speed runs the risk of rapidly changing the hygrometric state. From the dryer, with the risks of deformation and cracking that this entails. Furthermore, the “block” of air set in motion risks going around the stack instead of passing through it. The dryer operator must be able, through experience, to find the proper speed for his fans and may be forced to “block” certain spaces above or below the stack to force air through it.

3 Control devices. 

It is important to know how to place them, we must measure the air temperature and its hygrometric state before entering the wood pile, our measurement should not be influenced by openings, that is, doors and chimneys, or by heating elements. Therefore, the best location is halfway along the dryer and halfway to the air inlet side of the woodpile.

In the case where a periodic reversal of the direction of air circulation is expected, it is absolutely necessary to have a set of measuring devices on each side of the stack, the number of thermometers and psychrometers will depend on the length of the dryer and the number of heating batteries. The control panels, which in fact are also control devices, are always distributed along the stack and placed on the air outlet side, or on both sides if there is a traffic reversal. 

1 fresh air inlet 

2 Register 

3 Humid air outlet 

4 Register 

5 fan 

6 Heating 

7 humidification 

8 thermometer 

9 psychrometer 

10 wood 

11 car 

12 Shutters and elements to direct air over the wood pile.

 Figure 2 Arrangement of the control boards along the stack on the air outlet side. 

E – DIFFERENT TYPES OF DRYERS 

There are two categories of dryers. 

1°) Dryers in which the wood is immobilized during the entire drying period. They are called “stationary” dryers, box dryers, compartment dryers or even discontinuous dryers. 

2°) Dryers in which the wood circulates, the green wood entering at one end to come out dry at the other. 

These are progressive dryers or continuous dryers, or tunnel dryers. In a box dryer, the temperature and hygrometric state of the air vary from the beginning to the end of drying. In a tunnel dryer, along the dryer, a succession of temperature and humidity states occur that the wood will encounter successively. 

1 – Box dryers.

They understand : 

a) dryers with natural ventilation 

b) artificial ventilation dryers. 

a) Dryers with natural ventilation. 

Fresh air enters the base of the dryer. This cold air passes through the heating elements of the radiator coil. It heats up, rises in the dryer among the piles of firewood. It is then humidified, therefore cooled, and descends to the lower parts of the dryer from where it is evacuated through chimneys arranged on the side walls. Humidification tubes can be placed next to the heating tubes to condition the air delivered to the wood pile. The diagram in Figure 3 illustrates this general principle. In this general line of construction, the builders studied adaptations: reintroduction of part of the refrigerated air into the circuit, stacking firewood with internal chimneys to improve air circulation and even interior heating of said chimneys. Advantages: This type of dryer does not require driving force for ventilation. The air circulation speed is low. Therefore, there is practically no danger of wood deterioration, so control is reduced. Disadvantages:

Drying time is longer (due to reduced air circulation speed).

Lack of homogeneity in the results as long as the drying is a little long.

Installing chimneys improves drying, but reduces the volume of wood that can be stacked in the cell.

There is greater steam consumption than in other types of dryers.

Among dryers with natural ventilation, an interesting principle are the so-called condensation dryers.

The humid air is not evacuated through side chimneys, but is sent through tubes filled with cold water. 

The water vapor it contains condenses in these tubes. Thus freed of its humidity, the air returns to the closed circuit, that is, with greater regularity than if it were subjected to external atmospheric conditions. 

natural ventilation dryer 

1 fresh air inlet 

2 heating tubes 

3 Hot air entry into wood piles 

4 Cold, humid air 

5 piles of wood 

6 Humid air outlet 

7 Part of the cold air is recovered and returned to the circuit. 

8 Exhaust chimney 

9 Expel fresh air. 

B) Dryers with forced ventilation or mechanical ventilation. 

The fans guarantee the circulation of indoor air, keeping the rest of the conditions exactly the same, namely: heating and humidification of the air. Ventilation can come from a fan outside the dryer or, conversely, from an inside fan. So, external ventilation or internal ventilation. 

In the first case, the fan can blow the air in the longitudinal direction of the dryer or in the transverse direction. The most modern dryers have adopted this last system. 

In any case, whatever the system, the fan blows the air over the heaters. The air cools on the wood and returns to the heaters.

The “outlets” allow excess moisture to be evacuated. The “inlets” allow the arrival of fresh air. Air circulation speeds of 0.75 meters per second to 1.50 meters per second can be achieved between the plates. The conditioned air is mixed and distributed very evenly in the batteries. 

Advantages: Better use of heat than in dryers with natural ventilation. Better air homogenization. Much more intense traffic. 

More uniform and faster drying. 

Disadvantages: The lower cost of heat has the counterpart of a higher cost of driving force. 

Careful monitoring is necessary to avoid walking incidents that produce dry spaces. 

 Artificial ventilation dryer 

1 fresh air inlet

2 Admission registration 

3 heating tubes 

4 fan 

5 Hot air on 

6 Cold, humid air on, 

7 Evacuation record 

8 Humid air outlet 

9 pile of wood. 

2 tunnel dryers. 

All of them are mechanically ventilated, either through a single fan placed on the outside of the dryer, or through multiple fans placed inside. There are tunnel dryers not only for the wood industry, but also for drying paints, varnishes and drying ceramic products (tiles, bricks, ceramics, plates, bowls, cups) before firing. 

In a tunnel dryer the air circulates in the opposite direction to the circulation of the wood. 

As you walk through the tunnel, you come across different climate-controlled zones, each time warmer and less and less humid. 

The air current, in the opposite direction to the movement of the cars, is intense. The humidified air can be recirculated or exhausted outside. 

Find a tunnel 

1 fresh air 

2 radiators 

3 fan 

4 Hot air on 

5 Entrance to the forest 

6 Walk in the forest 

7 Out of danger 

8 Humid air outlet 

F – WHICH DRYER TO CHOOSE Box dryers and tunnel dryers respond to clearly different user needs. 

Tunnel dryers are suitable for drying large quantities of bowls of the same type and thickness in good condition. Example: they can cover the requirements of a regular and continuous demand for a cube of wood per day. 

A tunnel dryer will easily be used by a manufacturer of 27 mm thick oak parquet friezes.

 They are not suitable for drying wood of various types and thicknesses.

 In this case it would be necessary to: o operate the tunnel with a partial load that impairs good air circulation, which was normally designed for a complete tunnel. 

Or slow down the operation of the dryer by drying the wood more slowly, which of course is a contradiction from an economic point of view. For this reason, box dryers are used more frequently, especially because the number of drying cells can be proportional to the evolution of the company’s needs. 

Then we can hesitate between: the use of a cell with natural ventilation or the use of a drying fan. If we believe that large users (USA, England, Germany, Scandinavian countries), dryers with high air circulation speed are the most economical from the point of view of efficiency. 

Industrial conditions in France are perhaps more nuanced than in these countries. 

Such an industrialist buys directly from the cuts and receives “sawfall” wood in his factory, that is, very moist. He uses a lot of soft, white wood in his manufacture. 

In fact there is a lot of water to eliminate. You must use boxes with mechanical ventilation. 

On the contrary, such a manufacturer uses clearly recycled wood. 

It can produce steam at a low price. It does not carry out intensive production and, in particular, it is not subject to imperative deadlines. You may be able to settle for a dryer with natural ventilation. The dimensions of the cells to choose depend on the length of the pieces of wood used, and the monthly performance desired. It is always a miscalculation to choose a box that is too big. The best dimension is 6 meters. 

Beyond 10 meters the drying is no longer uniform. 

These dimensions are for length. The width can correspond to one stack or two stacks next to each other. 

If we take a maximum stack width of 1.80 m, we obtain approximately 4.50 m width for a two-row dryer. 

The height of the piles is generally 2 meters. 

The tunnels can be about thirty meters long. 

The performance depends on the volume of the dryer (useful volume equal to 1/3 of the capacity), the species to be dried, the thickness of the sawn wood and the type of dryer. In a medium and, above all, varied sector, a dryer with a capacity that is too large may not have satisfactory performance because it does not usually operate at full capacity: 

Once the dryer has been chosen, drying must be carried out to obtain the best results.