Drying and pasteurisation of wood

Drying is transmission of water from a material to the surrounding medium, typically without reaching the boiling point. Free water in wood is contained in cell cavities, whereas the bound form is found in cell walls. When the moisture is 25-30 % (depending on the type of wood), the saturation point of fibers is reached, which means that the cavities contain no water, but the fibers are still saturated with water. With the loss of moisture from the saturation point, the wood loses its volume, it contracts and changes its shape.

CONVENTIONAL HEATING
In heating processes with traditional thermal sources, the increase of surface temperature of dried material produces overpressure of saturated steam on its surface. As a consequence of pressure gradient, the steam passes into surrounding flowing air. This results in higher capillary pressure, which moistens again the highest layers. The speed of evaporating is constant, if capillarity ensures moistening of the surface at a speed sufficient to compensate the evaporation. Otherwise, the highest layers dry out, the diffusion slows down, and the speed of drying is lower.
During the conventional heating, two mechanisms act against each other - thermal gradient (heat transmission) and moisture gradient (mass transmission). This often creates impermeable surface dry layer that works as thermal insulator and prohibits heat transmission and capillarity impact.

DIELECTRIC (DE) HEATING
High-frequency (HF) energy affects simultaneously the whole depth of the material. It produces thermal gradient operating from inside of product, that supports expulsion of water. By selective heating almost exclusively the moisture, it creates effective water pump displacing the water towards the surface. The surface remains wet also because of the fact, that surrounding air is not heated by HF energy. As a consequence of the selective heating, the drying is more uniform, the moisture differences are lower, and the basic material is significantly less affected. The process doesn't depend on thermal conductivity. The drying occurs already at relatively low temperatures sufficient for evaporating. Though increasing temperature accelerates evaporating, maximal speed of evaporating is limited by the properties of the dried material. For many materials, optimal temperatures are lower ones that don't allow evaporating by boiling at the normal pressure.
Dielectric (DE) heating includes both radio frequency (RF) heating and microwave (MW) heating. In contrast with traditional driers, the use of DE heating is very quick. Even though it ensures uniformity, it must be slow enough to prevent creation of internal stress, which affects the quality of the product. Besides standard use for timber drying, DE heating is suitable also for sophisticated applications. Even furniture parts, that require precise control of moisture, can be dried in a few minutes instead of days and weeks, and the contraction is well under control. As an example of coping with typically specific application, drying of glued golf clubs or tanoak wood can be mentioned. This wood is usually dried up to 2 years, whereas using the microwaves it can take only 4 hours and additional two weeks of final drying.

RADIO FREQUENCY (RF) HEATING
The depth of penetration with RF heating using i.e. the 27.12 MHz frequency is in the order of meters. The output of RF systems is usually from dozens of kW to 1500 kW. With output over 50 kW, the RF heating is usually more economical than MW heating. The most important area of using the RF heating in wood industry is drying of timber.
RF heating is massively used also in plywood production, where electrodes function simultaneously as press plates. The RF energy removes wet spots in veneer and equalizes the moisture without the creation of steam bubbles and interference of board lamination. The RF energy affects also the glue in plywood, fiberboard and chipboard.
The softening (plastification) time of wood, dried to 18-24 % with uniform moisture in the whole profile, is reduced by RF heating to a few minutes compared with standard steaming.

This basic information is in the remaining text intentionally focused only on timber drying.

TIMBER DRYING - GENERALLY
In hot air driers, stripped boards and balks are usually stacked in a chamber. The temperature is gradually increased to 45 oC at the moisture of 80 %, or to 80 oC at the moisture 40 %. Such process lasts 2-4 days with soft wood, whereas hard wood is dried in 4-7 weeks. There are also continuous driers with conventional thermal sources, in which the timber gradually passes through the drying chamber at a higher temperature.
The RF heating allows substantial acceleration of a drying process. Old variations of this method use the transport of individual boards between the electrodes of appropriate size and arrangement. Heat is created by the effect of electric field, converting moisture in boards into steam that passes towards the surface. Too fast heating nevertheless could cause splitting of wood. For that reason, the board remains only 1 minute in electric field and 4 minutes out of it. The process lasts approximately 1 hour. The stabilization takes another hour and then the whole cycle repeats. Profiles can be in stripped stack in this method, but the aggregate layer is limited by approximately 2 decimeters. Otherwise, the heating is uneven.
The systems, where the initial heating is in hot air driers and the DE heating is used only in final stages, produce usually considerable savings of operational costs, but their purchasing price may be higher than the price of conventional driers. Therefore the new technologies of RF drying, were developed, that will be described in following two chapters.

RF HEATING WITH RELATIVE MOVEMENT OF NON-STRIPPED TIMBER STACK
One of the progressive methods of drying stacked non-stripped boards by RF heating is based on the fact, that material moves straight between two electrodes. Electrodes are connected to a generator and they are alternately grounded to achieve uniformity of heating. The length of electrodes, up to 180 cm, corresponds to the width of a stack. This method can be used for almost any height of a board stack. It is, however, practically important for stacks higher than 30 cm. The typical height of a stack is below 120 cm. The soft wood, for example, can be dried by this method in 2-3 hours, which is one third of the time needed for previous methods of RF drying. Besides, there is no need of breaking action of RF field as in previous methods.
Conventional heating would last several days.
The critical stage of any quick timber drying is the moistening of its surface. Otherwise, the surface dries up sooner than the core, which causes its disruption, cracking of ends and hardening of surface. The most frequent is fine water spraying or moistening by wet roller. The method of RF heating with relative movement of stripped timber stack uses the fact that the surfaces of adjoining boards are kept wet. The leakage of steam from boards is thus lowered, which reduces the danger of cracks. A reusable upper and lower board, between which the dried timber is placed, is impregnated by a liquid that is not affected by RF energy. Non-polar liquids, such as paraffin and glycol, are suitable for this reason. These boards remain wet by the effect of non-polar liquid and owing to the moisture absorbed from the surface of adjoining dried boards.
Due to high intensity of the RF field, the wood is exposed to extensive heat shock, which creates overheated steam. The soft wood can be exposed to the temperature of 105 oC and the overpressure of steam about 35 kPa, whereas the values for the hard wood are 150 - 160 oC and 350 - 415 kPa. The table shows three illustrative examples for various types of wood.

RF HEATING IN VACUUM APPARATUS
The stack of timber is first closed in an elastic permeable cover (net) and then in an elastic impermeable layer. The electrode boards placed above and under the stack heat the timber dielectrically. The stack is affected by sub atmospheric pressure; the steam is taken away from the stack and condensed. This reduces the moisture from 75 - 200 % to less than 15 %, usually to 5 %, suitable for furniture industry. This method leads to very accurate drying during relatively short time (48 - 72 hours).
The upper electrode forms vacuum container. The sidewalls are coated with a hermetic layer, e.g. of PVC. The lower electrode has holes for the passage of moisture evaporated from wood, and adjoins to a lower part of vacuum container, into which a vacuum pump, passage of cooling water into condenser, and outlet of precipitated water are connected.
The wood is heated to 38 - 93 oC at the 2 - 3.3 kPa underpressure. Under certain circumstances, the value of underpressure can be raised up to 20 kPa with corresponding rise of the temperature. This can be used, however, only for some types of wood. The stack of timber can be e.g. 2.4 m wide, 3.6 m long and 0.75 m high, which represents about 6.5 m3.

MICROWAVE (MW) HEATING
The MW equipment in Europe is usually constructed, with regard to industry applications rules, on the worldwide used 2450 MHz frequency. The depth of penetration is directly proportional to the wavelength, therefore the MW heating is suitable either for relatively small pieces or products with small profile. If the requirement for the output density is very high, microwaves are preferred to RF system, because they are less susceptible to arcing and burning of material.

30 minutes 56 oC drying process - the fast and safe way

MICROWAVE DRYING PRINCIPLES
Dielectric heating is applied whenever material with low thermal and electrical conductivity is being considered and where it is profitable to use fast and deep-penetration heating. The drying process in fact is considered as the result of three factors:

• kinetics of the heat transfer caused by radiation or by induction
• kinetics of migration or diffusion of constituents (water)
• kinetics of phase transformation such as evaporation.

The speed of the process is controlled by the slowest of these three factors.

Because of the uniformity of heat release the acceleration of the process can be obtained without the constrains and deformations that can always occur with other processes, nevertheless as to internal constraints it is not possible to run the process faster than a certain limit, set by the nature of the product.

The different possible applications are as follows:

• Preheating before drying
  In this process we make use of speed and uniformity
• Initial drying
  this process is carried out based on the selectivity and uniformity of heating by radiation and to the extent that it is more a migration of water from the interior towards the surface of the product than drying itself. It is feeding up the drying (pumping water from inside) and suppression of crust formation.
• Finish drying
  In case of removing the last percentage of water conventional processes are less and less efficient when the water content decreases below a certain amount.
  

SELF-REGULATION AND LIMITING OF OVERHEATING
If the load in chamber changes quickly or the dielectric constant is substantially variable during the heating cycle, the RF heating is more favourable than MW systems, owing to its possibility of immediate automatic change.
If the tangent of load loss angle is higher than 1, the ohmic nature of the load prevails the capacity one. This phenomenon, however, disappears in many cases during drying of the material or its treatment. RF energy heats up resistive elements, which results in self-regulation in these areas, preventing thus overheating or hard drying.
If the material has the tangent of loss angle lower than 0.5, it has mainly capacitive nature and allows better self-regulation on MW frequencies. With some materials, the RF heating may have small balancing effect on the moisture content below 5 %, whereas the microwaves can dry up the same material up to zero moisture content.

RELATING INFORMATION
Before the start of drying, moisture content of wood can be verified at chosen locations of the stack or charge. Then, the speed and temperature of drying is determined on the basis of experience.
To estimate the speed of drying and adjust the operating mode to achieve uniform moisture of the charge, some methods use monitoring of the inlet dry air and outlet wet air temperatures, or it is possible to monitor continuously the overall weight of the charge and the speed of its change.
The disruption of wood structure during the conventional heating causes lower yielding - 60-80 % by coniferous wood, and 50-60 % by leaf wood. Therefore the drying is frequently finished before achieving the required moisture.
During another method, the wood is dried to 12 - 20 % at 56 oC to balance and stabilize the moisture content. Then the temperature is raised until the moisture drops to approximately 6 %. Finally, the dry surface layers are exposed to a high moisture (steam). The treated wood has the quality sufficient even for precise fitting of joints in the furniture production.