The problem of choice of the type of kilns is well-known to engineer technician employees of wood processing enterprises. Its solution is a whole set of tasks related to production matters of the enterprise, market environment, as well as effectiveness of capital investments and their quickest return. Everything should be taken into account to make an optimal choice.

Let’s try to summarize general provisions reflecting modern approaches to evaluation of the drying technique.

One should start with determination of the required yearly, quarterly, monthly volumes of dried timber. At the same time it is required to determine specification of timber that shall be dried, namely: wood species, size of cross-section (thickness, width); length or spread in lengths for batches that are not of equal length; purpose of timber or quality category and related final humidity.

Choice of the type of kilns to great extent depends on this specification. Currently in the market kilns of periodic and continuous activity (tunnels) are available. The latter are used for mass drying of great volume of homogenous material to transport humidity (18-22%). Mounting of such type of kilns shall be justified only if it is envisaged to dry timber for export with use of mild regimes to prevent change in colour. For drying timber to the exploitation humidity (6-10%) and in particular of hard-drying species (oak, beech, larch, etc.) it is better not to use the continuous activity kilns. Due to some construction peculiarities (lack of reverse circulation, difficulties with maintaining the drying regime along the length of the kiln, etc.) the material drying quality in these kilns will not exceed the category III. Besides practice shows that the lower the final humidity is, the higher is the percentage of defects.

Therefore the conclusion is obvious: for high-quality timber drying to the exploitation humidity it is required to use chambers of periodic use. First of all it is true for timber with big cross-sections (beams) and hard-drying species. It is required to dry such assortment in convection chambers with low drying speed maintaining a comparatively mild regime. Otherwise high share of spoilage is inevitable.

The low-production process as a rule is not efficient and requires a big number of kilns at the great drying amount. In a number of cases it will be more efficient to use other types of drying (not convectional): these are different types of vacuum as well as dielectric type of drying with use of high frequency current and microwave energy as well as their combination. Application of vacuum drying allows reducing the oak drying duration (50 mm thick) five times with retaining the quality. In the course of microwave frequency drying the time economy is even bigger – almost twenty times. However, dielectric heating appliances are expensive, have high electricity consumption of the microwave frequency chamber and their exploitation costs are quite significant.

Today we would like to tell about the modern technology of vacuum desiccation.

The vacuum desiccation technology is attractive first of all with a real chance for significant reduction of drying duration at the same time retaining high quality of the dried timber and in a number of cases – increasing it. It is known that the convectional drying possibilities in this sense are exhausted. Today several technologies of vacuum drying exist. But first of all – let’s discuss the drying process in a vacuum from the physics point of view.

Formation of a vacuum in the environment, in which materials are dried, significantly modifies the physical character of the course of heat and mass exchange processes when drying. The principle parameter characterizing the vacuum depth is pressure in the drying environment, this parameter in particular determines conditions and processes taking place there. The most significant effect of the environment pressure decrease consists of reducing water boiling temperature and intense water steam. Principle processes that determine the timber drying kinetics are related to movement of humidity in a liquid and vaporous state inside the material with evaporation from the material surface and steam diffusion in the environment.

With reducing pressure when removing part of air-steam mixture from the chamber the number of molecules reduces and their free distance run increases. This value significantly exceeds in the vacuum environment the average size of microcapillaries. In this case the character of molecular steam movement in the wood pores significantly changes and the effusion effect appears (the Knudsen flow). Density of flow increases tens of times. Besides when the wood temperature is higher than the boiling temperature at the given environment pressure (twood>tboil.), the excessive steam pressure within the material appears, which essentially increases speed of the humidity movement from the centre to the wood surface. The twelvefold environment pressure decrease increases the speed of the humidity movement in the wood 4.7 times.

The mentioned effects within the vacuum drying allow getting a particular effect within the process of drying timber compared with traditional convectional drying. The energetic efficiency of the process however depends on the heat delivery to the material and optimal drying regime. In particular thanks to this condition quite a wide range of vacuum drying methods of wood materials have been developed, as well as a great variety of vacuum technologies and equipment for their realization.