The optimization of your drying result is the core aspect of our work. We have summarized the knowledge Mahild has gained over the years in short technical newsletters, which are available for download below. If you are interested in one of the topics and would like to learn more about the technical details of lumber drying, please contact us and we will send you the necessary access data.
Internal cracks are separations in the fibre of the wood, usually following wood rays which act as levels of failure. One of the causes of cracks is the use of a extremely high drying potential once the centre of the wood has reached the SFP (28%- 30%). Hence, these cracks are formed at the end of the drying process. At this stage of the process, the centre of the wood is submitted to great stress and if these efforts are greater than wood strength, then fibres separate
Cracks are breaks or separations of wood along the grain or fiber. Wood cracks are caused by to two reasons: 1. A difference of radial and tangential shrinkage. (If the mechanical stress generated by the difference of shrinkage is greater than the resistance of the wood) 2. A difference in the shrinkage of the wood tissues and development efforts of different magnitudes caused by moisture gradients
SUPERHEATING STEAM, IS IT USEFUL FOR DRYING WOOD?
WHAT IS PSYCHROMETRY?
Psychrometry is the term used to describe the field of ingenieering concerned with the thermodynamic properties and processes of water vapour and air. It studies the behavior of water vapour and air and its effect on thermal comfort, processes involving heat and moisture transfer. Psychrometry is present in the drying process through heat sensitive, sensible cooling, adiabatic mixing, wetting, adiabatic saturation, etc. Dry air is composed mainly of oxygen (O2) and Nitrogen (N2) in addition to Argon (A), Carbon Dioxide (CO2), hydrogen, and so on.
GROSS CALORIFIC VALUE (GCV) AND NET CALORIFIC VALUE (NCV)
RHEOLOGICAL BEHAVIOR OF WOOD MATERIAL (VISCOELASTIC)
A modern definition states that rheology is the branch of physics that studies the relationship between stress and strain on materials that are capable of flowing. The rheological behaviour of a material is present when the material is pressed exceeding its elastic limit (see figure), and the relation stress and deformation is time dependent.
WHAT IS THE HEAT TRANSFER IN TRANSIENT CONDUCTION?
Heat transfer problems are usually classified as transient (or unsteady) and permanent (or steady). Steady state transfer implies that there is no change with respect to time at any point within the medium, while transient implies changes in time or dependence on time. Therefore, the steady temperature or heat fluid remains unchanged in time; during the transient state temperature and heat fluid are variable with respect to time and space.
SENSIBLE HEATING AIR
Psychorometric is a term used to analyze variables and psychrometric processes concerned with the air. The psychrometric variables are: dry bulb temperature (DB), wet bulb temperature (WB), point dew temperature, relative humidity (HR), specific humidity (W), saturation pressure, partial pressure, enthalpy, density, etc. The most relevant psychrometric processes are: sensible heating, sensible cooling, humectation, adiabatic mixture.
STEAM QUALITY OR DRYNESS FRACTIONS
Saturated steam in reality always has some quantity of water. The existent humidity in the saturated water steam may be due to several causes. When steam circulates through the pipes some loss of heat for convection and radiation may be experienced, so vapour loses part of the enthalpy of evaporation (hfg) and as a consequence water particles are formed in the vapor. Another cause may be the use of an inadequate steam formation method due to lack of regulations of the boiler which may result in an abnormally high boiling point or faulty feeding boiler. The proportion of moisture determines the steam quality or dryness fraction, this is expressed as "x". Normally, boilers that produce saturated vapor contain an humidity or water of 5%-8%, that is to say the steam is not totally dry and therefore the quality is "x" between 95% to 92%.
Delta T or TDAL (Temperature Drop Across Loads). The temperature drop across the load corresponds to the difference of dry bulb temperature between the entrance and exit. Delta T, is a measure of the quantity of heat transferred at both sides of the load, the temperature of the load is lower than the atmospheric. This difference of temperature depends on several factors, among them: the speed of the air, thickness fillets, wide of the packages, number of packages in the width, thickness of the wood, drying schedules, turn changes, bypass.
In the drying process, the term bypass corresponds to the airflow (m3 /hr) which does not go throughout the timber stack, therefore it is a loss of air and a loss of drying capability. When assessing this loss it is surprisingly to find values that generally range from 40% to 60%, which means an airflow loss of 50%. This fact should be considered as a great opportunity to improve our process and reduce these indexes.
Flash steam is the partial vapor that occurs when a saturated stream undergoes a reduction in pressure. How it is formed?: When water equals the environmental pressure surrounding it; water boiling point at atmospheric pressure is 100 °C, the higher temperature that water reaches at this pressure. Additional supply of heat does not increase temperature , it simply transforms water into vapor.
WOOD WITH FUNGI AND BACTERIAS EFFECT IN DRYING
Wood affected with the attack of fungus or bacteria causes problems in the drying process. General requirements for the development of fungus Humidity (water): in general fungi are not able to retain the water they contain. That explains why fungal attack can occur only when moisture content of wood is greater 28%-30%.
It is a longitudinal separation of wood fibers in the tangential direction; this is a rupture or separation of the rings of growth, specifically between the spring wood and summer. Some of these shaken failures could be very fine and not perceptible in green wood but later on during the drying process they become evident.
The drying rate indicates the velocity at which the water evaporates from wood and it depends on many factors, but mainly on air velocity, drying bulb temperature (DB) and wet bulb temperature (WB). Hot air enters into the wood cell with a DB1, then the air cools down as it passes through the cells (it releases sensitive heat) and finally leaves the cell with a DB2 (DB1>DB2). Equally, air enters with a specific humidity of W1 and goes increasing humidity until it leaves approaching a W2 (W2>W1). Similar behavior presents the relative humidity, that is to say a HR1 enters and leaves a greater HR2.
FIBER SATURATION POINT (FSP), Is it really 28%-30%?
SURFACE CHECKING or CRACKS
Surface checking is a rupture or split of the wood along the grain or fiber. Surface Checks are caused mainly by two facts: • Due to a difference in the radial and tangential shrinking, with a resulting stress of such a magnitude that the wood breaks or separates • and due to the difference in the shrinking of the layers of the wood and stress forces of different degrees caused by gradients of content of humidity.
Quantification of thermal energy consumptions during the wood drying process is highly valuable to optimize schedules and consumptions. Generally a meter is available in main lines (steam, hot water, thermal oil), but it is more relevant to quantify consumptions in each kiln. The market offers a great variety of meters that differ in sizes and operation ranges as well as in their performance principles.
WHAT IS PT100 or RTD ?
In Drying area it is very common to speak about RTD or PT100. In this newsletter we will offer you an approach to that issue. Basically, a Resistance Temperature Detector is a sensor to measure temperature. (In drying it is used to measure dry bulb temperature, wet bulb and wood temperature for phytosanitary purposes). This sensor bases its operation on the electrical resistance variation caused by changes in ambient temperatures. If temperature increases electrical resistance also increases.
A new era of sustainable wood drying and process
WHAT IS THE CONTRIBUTION OF VARIABLE SPEED DRIVES?
The variable speed drive (VSD) is a piece of equipment very useful in operational cost reduction, quality improvement and production increment. Usually, drying schedules are based on temperature dry bulb (TDB) and temperature wet bulb (TWB) (or ugl) while speed remains unchanged. The use of variable speed drives involves a range of possibilities (greater flexibility). During the first drying hours (greater capillarity) greater speed is required for water evacuation. As wood dries off, air speed may be lowered since diffusion prevails (which depends mostly on temperature).
PARADIGM OF 1.20 METERS PACKAGE WIDTH
In difficult and hard times it is necessary to look for any alternative which allows competitive advantages. Pack width is certainly one those alternatives. Many countries have adopted, though without convincing arguments, the 1.20 meter width. Some plants use packages of 2.40 m width (New Zealand), in Europe it is very common to use packages of 1.8 to 2.0 m width
REACTION WOOD (TENSION AND COMPRESSION)
Reaction wood is an abnormal defect which is present in all type of trees. Reaction wood forms when part of the tree is subjected to stress. This stress may be the result of trees growing in a sloping ground location, excess of snow and wind exposure can also create these stresses. In coniferous timber, this wood is called compression wood , it is formed by pressure compression (it forms below the bent part; see figure 1 and 2,) Compression wood is rich in lignin. In hardwood this wood is called tension wood. Tension wood forms above the affected part of the plant (see figure 3 and 4). It is rich in cellulose.
DRYING SCHEDULE BASED ON PSYCHROMETRIC DIFFERENCE OR EQUILIBRIUM MOISTURE CONTENT?
In Europe drying schedules are normally better undestood or known as drying schedule based on DBT (dry bulb temperature) and UGL-EMC (equilibrium moisture content) as well as on drying gradient (DG). In South America, New Zealand and Australia, drying schedules are mostly related to DBT, WBT (wet bulb temperature) or psychrometric difference. In other areas, a mixture of both concepts is applied. The issue may be discussed for a long time, but trying to concentrate everything in a newletter would be impossible. We will try to offer you some important points for consideration. Basic definitions of these variables are given in Newsletter 01, 02, 13 and 16.
HEAT TRANSFER IN DRYING PROCESS
Heat transfer has direction as well as magnitude. In general, heat transfer in the drying process (wooden pieces) is three-dimensional (for face of the thickness, face of the width and longitudinal sense) and it depends on time and temperature. Kiln sticks or laths, for example, is closely related to this topic. When pieces are closed together heat transfer occurs mainly by the faces (surface) of the width and worthless for the thickness. On the other hand if pieces or some pieces are separated, they dry off quicker because of the heat transfer occuring in the face of the thickness (which is more considerable at higher thickness). This process also explains why boundary pieces dry off quicker.
AIR VELOCITY IN DRYING (Generalities)
Air velocity is one of the fundamental variables in the drying process, in addition to temperature and humidity (temperature wet bulb): these elements are the drying potential. Currently, Mahild kiln schedules , depending on species, are based on air velocity which range goes from 2 m/s up to 12 m/s. Generally, higher speed means higher work temperature. For example, to dry off at 140°C with 3 m/s is not recommended due to the high dispersion of moisture generated in the wood.
WHAT IS THE MEANING OF HYGROSCOPY IN WOOD?
WHAT IS THE MEANING OF HYGROSCOPY IN WOOD? Hygroscopic is the ability to attract water molecules through either adsorption or absorption and to release water from or through a surface (desorption). This phenomenon is related to the cell walls of wood and its components (cellulose, hemicelluloses and lignin). If dry wood is placed in a wet environment it will absorb water (humidity) from the environment and if the environment is drier it will release water. This property of wood is responsible for the changes in wood dimensions.
CAPILLARITY IN THE DRYING PROCESS
Capillarity is a natural mechanism based on the spontaneous rising or falling of a liquid in a small diameter tube (capillary). This movement is due to two different forces: adhesion and cohesion. Cohesion Force : the attractive force between the molecules of a liquid. In the case of water it is the attraction of the molecules in water to each other This force in water correspond to the Bridge to Hydrogen.
A BASIC TOPIC CONCERNING DRYING
To Achieve high quality of the drying process mostly depends on load formation and sticker positioning in the pack of wood. In newsletter N°15 we referred to the most appropriate sticker thickness; In newsletter N° 29 we emphasized several topics that are helpful in the optimization process. In this issue we will concentrate on the vertical alignment of the stickers.
ECOVENT III MAHILD
It is a reality that either electricity or thermal energy is both continually experiencing price increments which increase drying operational costs. In this sense ECOVENT III is an alternative that allows saving of thermal energy or steam. With the use of ECOVENT III savings until 25% are achieved in relation to the total amount of energy required. The saving magnitude depends on the type of dryer (conventional, accelerated, high temperature) and the environmental temperature average.
IMPORTANCE OF CLEANING MAINTENANCE IN HEATERS AND FINS
Fin mainly purpose is to increase the area of heat transfer and hence the rate of heat transfer . For example, 1 meter of finned tube may be equal to 10 meters of tube without fins. Fin thermal efficiency depends on several factors, among them: high , thickness, material, amount of fins for inch or step, transfer coefficients, grade of dirt (fouling), etc.
SOME ADVICES ABOUT THE DRYING PROCESS
In hard times it is useful to have present some advices that may help the process: 1. – Narrow wood of same thickness dries off quicker . This could be explained by the action of the edge area; more narrow pieces are contained in the load width so dry rate by the edge it is no longer worthless. Loads which are built up of several packages in the width (or depth) and packages of different widht pieces (complete package of single width), should arrange packages containing narrower pieces at the critical areas (central packages) for a more uniform and quicker drying.
STALL PHENOMENON IN STEAM SYSTEMS
In the Newsletter issue N°28 we approached the topic of the Stall phenomenon in aerodynamic system (fans), now we will focus on the Stall phenomenon in thermal systems. Certainly, you are not familiarized with the term " Stall phenomenon ", however, many of you have probably faced some of the following problems in your own plants:
ADJUSTMENTS OF DEFORMATIONS TO DIFFERENT DIMENSIONS
It is not the same to have a 10 mm of bow in a 4 m board that a 5 mm of bow in 2 m board. Deformations are a common problem in dry wood. Sometimes, studies or control quality procedures have to consider a comparison of deformations for different length or width. Direct deformation comparisons are not possible when lengths or widths are not the same.
EXTRACTIVE SUBSTANCES FROM WOOD AND THE RELATIONSHIP WITH DRYING
Cellulose, hemicelluloses and lignin are tree compounds, in addition to other substances present in smaller proportion. These are the extractive substances, which are compounds of aliphatic hydrocarbons (which do not contain a benzene ring) and aromatic hydrocarbons, terpens, phenols, fatty and resinous acids, resins, fatty, esters, essential oils and others. The quantity and composition of them depends on the species, the part of the tree, the season of the year, the growth conditions and other factors
WARPING RESULTING FROM DRYING- CUPPING
Warping in boards occurred when tangential shrinking is greater than radial shrinking. Pieces taken from the most external areas of thick logs show relatively little curve in growth rings, offering less opportunities of cupping deformation than pieces taken from the inner parts or from thin logs, in which the curve of growth rings is more marked. The concave surface of the cupping affects the face which is closer to the bark.
WARPING RESULTING FROM DRYING- CROOK
It takes place when one edge shrinks lengthwise more than the other. The piece curves toward the more shrinkage edge forming an arch. The factor of mayor influence on crook is the origin of the piece regarding the log. Tangent pieces develop minor crook; on the other hand radial pieces are prone to crooking
DEFORMATION (WARPING) PRODUCT OF DRYING - TWIST
This type of deformation is caused by a combination of heterogeneous longitudinal shrinking and a deviation in fibers, resulting mainly by the presence of knots. The central wood has bigger longitudinal shrinking that the lateral wood. Another factor that affects is the unequal moisture distribution inside wood after drying. When a piece of wood gets dried faster than usual, an uneven dispersion of moisture takes place in thickness; the surface area is drier than the core.
DEFORMATION (WARPING) CAUSED BY DRYING - BOW
Warping takes place when a face shrinks lengthwise more than the other face. The piece then curves toward the most shrinking face forming an arch. It often affects the inner face as the internal wood shrinks lengthwise more than the external wood. Juvenile wood (located in the center) shrinks lengthwise more than mature wood. The same happens in compression or tension wood.
The cogeneration can be defined as the simultaneous and sequential production of steam and electricity, coming up from the same boiler. The high pressure steam goes through a turbine which rotates an axis connected to an electric generator. On the other hand the waste steam or low pressure steam coming out from the turbine is used to dry wood; afterwards, steam is accumulated in condenser tank to return to the boiler.
It is the summing up of all costs involved in the prevention of errors or failures, plus all costs needed to define the way work is being performed, in addition to those costs considered when the product does not meet customer expectations . Quality Cost is the summing up off all costs an organization has to face when an activity or a product is not well done at the first time. It is the cost of not fulfilling customer’s requirements, the cost of doing things in the wrong way. It is the profit-margin not obtained for doing something
Basic aspects of Drying
If wood is subjected to a quick change in its ambient temperature, there will be a time lapse before the wood temperature balances. This is because of the transfer of heat in wood. Increased timber thickness equals increased thermal gradients and therefore it is necessary to be more careful when cooling larger dimensions. Cooling time depends on many factors; some of them are:
The most probable thing is that you In more than an occasion has listened something related with the adjustment of the PID. Now we will try to explain what it is the topic. A controller PID (Proportional Integral Derivative) it is the mathematical part of the control system that, by means of an actuator (control valve, vents), it is able to maintain the value of a process variable in a wanted point (Sep Point). It is a method of very frequent control in the modern sy
What is DIFFUSION?
Diffusion is the term that explains how the hygroscopic or bound water exits the wood. Therefore is very important when drying softwoods below FSP (Fibre Saturation Point). In hardwood diffusion occurs over a larger range of humidity, especially if they present tylosis or they are very prone to collapse. Also heartwood drying generally is explained by diffusion
How much thermal energy to dry timber?
Every year world prices for thermal and electric energy increase, therefore what energy is required in the timber drying process? We define the specific consumption of thermal energy as the quantity of energy required to dry a m3 of wood. The consumption will vary depending: Species, initial Moisture Content (MC), density, final MC, external environmental conditions (DB, WB), temperature of the steam or thermal fluid, time of heating, time of conditioning, efficiency of the dryer, schedule of drying (DB, WB, airspeed), time of drying. The initial MC is the biggest factor in the specific consumption,
What function does the Partial Pressure of water vapor have in the air (Pv) during the drying process?
In timber drying, the true drying potential (the force that allows the water to evaporate) is a product of the difference between partial pressure of atmospheric air that circulates in the kiln, and the water pressure at the surface or interior of the wood. When timber drying we are interested in the partial pressure. Air is a mixture of gases; each one
Effect of airspeed in the wood drying process
The air circulation around the wood is very important. This is because the capacity of the air to absorb water is finite (limited). The air passing the wood absorbs water and begins to get saturated, this is the reason for which is important to replace humid air with dry air. However when an airflow exists it continues passing air on the wood, the layer of air that is immediately in direct contact with the wood moves more slowly and it has a greater vapor pressure the main flow. This is known as layer limit effect. When increasing the air speed the layer limit diminishes and therefore the evaporation rate increases from the wood, particularly when the airflow is more turbulent than laminar. Therefore an increment in air speed reduces the barrier of humidity that is near the surface of the wood.
Pressure Generated by an axial Fan
Static pressure: This is the pressure that the circulating air in the kiln exerts in the perpendicular plane to the kiln wall. This corresponds to the necessary pressure to conquer all load losses; these are: the air going across the wood, across the coils, colliding with the wall, etc. Dynamic pressure (pressure of speed): This is the equivalent pressure to the kinetic energy that the air possesses when moving from one point to another. It is the pressure coming from the movement of the air. The dynamic pressure is used to create and to maintain the speed of the air. Also it exerts a pressure on any obstacle that intervenes in its road. Total pressure (PT): it is the sum of the static and the dynamic pressures.
Kiln Brown Stain
Kiln Brown Stain or KBS as it is often known, is a chemical stain of the timber. It is brown color that is developed during kiln drying in the sapwood of radiata pine and not in the heartwood. KBS happens due to a thermo-chemistry reaction and it involves components present in the sapwood such as: sugars, amino acids, tannins, proteins, phenols. Fructose, glucose, sucrose and glutamic acid were the main components found in the stain of radiata pine. This chemical reaction obeys the Maillard reaction is intensified with the temperature. Some compounds that degrade from the lignin also contribute to the stain.
Psychrometric difference (DB-WB) of the First Step of Drying
On many occasions you may have noticed that for a given product/ thickness the kiln achieves the DB/ WB setpoints. On other occasions for different thickness (smaller), running the same schedule or drying step the kiln may not reach the same conditions. This doesn't necessarily mean that there is an error. This is dependant on the installed heating power of the coils, air speed, and vents fitted. Some kilns can achieve conditions for certain psychrometrics, others cannot. In general the thicker the timber, the easier it is to achieve larger psychrometric differences, in other words quicker the setpoints can be achieved. The second and third drying steps, demand less total energy, due to the diffusion part of drying the wood. At this point we have an opportunity to increase the temperature and to reduce the speed of the air.
Fillet (Stickers) and Bearers; An alternative of optimization
Fillets & bearers (the separators between packages) play a very important role in drying wood. The fillets are responsible for generating the drying channel, the area where circulating hot air transfers heat to the wood in order to evaporate the water in the wood (convection humidity transfer). The fillets should be of a good quality (at least sized) since they are the major contributor to the aerodynamics of the dryer. They should be aligned to avoid vortexes of air that create irregular drying rates, also to minimize warp.
Dry Air or Wet Air; it’s Impact on Electrical power & Drying
The atmospheric air is a compound that consists mainly of Oxygen, Nitrogen & water in a gaseous state. When a kiln or RECO chamber’s water bath is activated, it generates a vapor which displaces the dry air (O2 and N2). The dry air is heavier than the steam; therefore it falls to lowest parts of the chamber. Why does contains molecules has more mass. The dry air is, basically, a mixture of O2 and N2 whose molecular masses are 32 and 28, respectively. In saturated air, we do not see the O2 and N2 because they are displaced by vapor of water H2O whose molecular mass is of 18. Therefore, the wet air will be lighter. Mahild’s OXIVENT system evacuates the dry air so that the steam comple fills the chamber, and gives effective steaming to the entire load.
GRADIENT OF DRYING AND GRADIENT OF MOISTURE
These concepts are very different. The first is a variable that allows the operator to drive the drying schedule and the second is the consequence seen in the product. Drying gradient (DG) The DG is the relationship between the real moisture content (MC) of the wood and the equilibrium moisture content (EMC) of the internal atmosphere of the dryer. DG = MC real wood / EMC. EMC is function of the DB, WB, HR.
INITIAL VAPORIZED (STEAMING) IN HARD OR REFRACTORY WOOD (Dual Kiln)
The process vaporized or initial steaming is applied before beginning to dry. It is a common practice in the drying of hard wood or of difficult drying wood (refractory wood) and their main objective is to improve the distribution of color of the product. The wood by nature has color changes and it is intensified with the formation of the heartwood
LONGITUDINAL AND TRANSVERSE STRESS
Longitudinal Stress; calls growth stress are also inherent to the trees (compression in the center and tension in the periphery). They are related with the mechanical balance of the tree to remain of foot and for the process of maturation of the cells. The maturation tensions are bigger to those of the weight characteristic of the tree. Their origin this in the growth of the cells that produces the cambium and that during their aging process they spread to expand laterally and to contract lengthwise, that that when being impeded by the cells formed in previous years, it generates a group of tensions. They are generally manifested when cutting or to saw the tree. They are very relating with deformations (crook and splits). The tensions of growth can explain the bow and crook partly, these deformations are generally the result of severe tensions of growth, forms of sawing, and other properties of the wood that interaction with the tensions of growth.
KILN LOADS LATERAL, A PARADIGM?
For a lot time has the paradigm it has existed regarding the kiln of lateral load that are of less efficiency than another type of drying and they are not adapted to dry wood of softwood or of high permeability with good standards of quality and productivity. However, this paradigm do not exists at the present time the advances technological and bigger knowledge of the drying process have worried about demolishing this paradigm.
DISPERSION OF MOISTURE CONTENT, BETWEEN PIECES, WITHIN PIECES AND TOTAL
When an operator enters to checking a kiln dryer usually obtains a moisture average and a standard deviation. This deviation (dispersion) it corresponds to the total standard deviation the one which this made up of the standard deviation between piece and the dispersion characteristic of the pieces.
TAGUCHI METHOD , Powerful tool of operational improvement
Let us suppose that you wants to optimize their drying plant (or any other industrial process) for that which has decided to testing 4 variables (temperature Dry bulb, Temperature wet bulb, speed of the air, turn changes) and for each variable you has decided to prove three levels. For example for TDB (80°C, 90°C; 100°C). Now the objective is to look for the good combination that allows optimizing the drying. This simple problem is translated in proving 81 loads to be able to prove all the combinations. This means a lot of work, time, many costs and high risks.
ANALYSIS OF CAPACITY OF PROCESS (Cp, Cpk). Application in Drying
The capacity of a process refers to the ability that has the process of to produce quality or to generate inside the specified limits. A process is said that "it is capable" if it produces 100% approximately inside the specified limits. The limits are fixed by the clients, engineers or administrators. These limits are sometimes called requirements, goals, objectives, or standard. These limits should not be confused with the control limits, which are on the base of the data obtained by a system and they are used in the chart control. It should have two principles before calculating the capacity of the process.
Wood treated thermally
The thermal modification consists on treat the wood with temperatures between 160° C and 260°C in absence of oxygenate. This produced a permanent change in the chemical composition of the wood, giving better properties of durability and it is translated in: color change, reduction of the equilibrium moisture content, improves the dimensional stability; it improves resistance to the fungus and insects. In Europe this process is a reality for several years, in the Pine radiate it is very new. The investigation has shown that the product is more stable and with t
Heat of Wetting - Heat of Sorption
The heat of wetting is defined as the heat generated by the dry wood when it is moistened and adsorbed water (moisture) until arriving to moisture more than PSF. This heat depends on the content of moisture of the wood before being moistened, for example anhydrous dry wood produces more heat than wood with moisture of 15%. The sorption heat is the difference among wetting heat from the anhydrous condition with regard to another value of moisture.
Evolution wood moisture content after the drying
One of the properties of the wood is to be highly hygroscopic, that is it has the capacity to capture or to liberate molecules of water from the atmosphere until achieving an equilibrium state with the atmosphere called equilibrium moisture. The results of two loads in which the evolution of the moisture was measured after drying for wood of Pine radiata lateral 40 mm filleted and under warehouse is shown next:
Heating or Vaporized Initial
The difference among heating and initial vaporized it resides in: Heating is used when the wood is fresh and homogeneous in moisture. The procedure consists on only using the heaters (coils) for energy contribution, it is key that the vents stay closed. If one has steam in excess you can use heaters and water bath. The one vaporized initial is for wood with high dispersion of moisture or aired wood and the objective more than to heat is to homogenize the initial moisture. Without initial vaporized it becomes bigger the severity of the superficial checks.
Specific humidity, Relative Humidity, Moisture content wood, Equilibrium moisture content
SPECIFIC HUMIDITY OF THE AIR (W): It is defined as the mass of steam water in the humid air with regard to the mass of dry air. Why a vaporized at 100/99°C (TDB/TWB) does it consume much more steam that a vaporized at 95/94°C? Does it mean that if they notice a condition of 100/99°C 17.1 kg of water they are needed (vapor that it gives the water bath) for each kg of dry air to reach the TWB 99°C. However for the condition of 95/94°C 2.5 kg of water/kg dry air is needed only. This is the reason for which the valves open 100% are always when they notice conditions 100/100°C or similar. The UGL or EMC that it is achieved with the previous strategies are very similar: 100/99°C UGL = 17.2%, 95/94°C UGL = 17.6%.
Temperature Dry Bulb (TDB), Temperature Wet Bulb (TWB) and Psychrometrics Difference
Temperature Dry Bulb (TDB) is the temperature that corresponds to the air measured with a thermometer of normal bulb. In drying (Pine radiata) conventional it is used among 60 at 90°C, in accelerated drying it goes from 80 to 110°C and in high temperature from 120 at 160°C. In thermal treatment of wood it is used temperatures from 180 to 220°C