Technische Newsletter (engl.)

Learn from our experience in lumber drying

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.

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%.

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.

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.

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.

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.

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%.