Dzurenda , Deliiski : Convective Drying of Beech Lumber without Color Changes of Wood

This paper presents the results of analyzing the infl uence of suggested regimes of chamber drying of beech lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm from the initial moisture content Wp = 70 % to the fi nal moisture content Wk = 8 %. The regimes ensure the preservation of the natural color of the wood. The study results show that the drying of beech lumber according to the suggested regimes causes no changes in the chromophoric compounds of the wood and the beech lumber preserves its original white-yellow color. Quality parameters of the dried lumber, such as: difference between the average fi nal moisture content and the required fi nal moisture content, and the range of the fi nal moisture and moisture gradient in dried beech lumber, classify this dried lumber into the fi rst quality class. The suggested regimes for drying beech lumber using hot air in the temperature range ts = 37 ÷ 65 °C has a negative impact on the duration of the drying process. In comparison with the regimes of beech lumber drying according to ON 49 0651, carried out at temperatures t = 60 ÷ 80 °C, the duration of drying beech lumber with the thickness h = 40 mm according to the suggested regime is 1.9 times longer, the duration of drying beech lumber with the thickness h = 60 mm is 2.3 times longer and the duration of drying beech lumber with the thickness h = 80 mm is 2.9 times longer. The specifi c heat energy consumption of beech lumber with the initial moisture content Wp = 70 % to the fi nal moisture content Wk = 8 % in a chamber type KWC 121, according to the suggested regimes, is equals to QTZN = 514.46 kWh⋅m-3 for the thickness h = 40 mm, QTZN = 557.62 kWh⋅m-3 for lumber with the thickness h = 60 mm, and QTZN = 643.16 kWh⋅m-3 for lumber with the thickness h = 80 mm. The specifi c heat energy consumption needed for the drying of 1 m3 of beech lumber with the thickness h = 60 mm according to the suggested regime is by 26.89% higher than the specifi c heat energy consumption used to dry beech lumber of the same thickness according to the regime of beech lumber drying by Vzduchotechnika a.s. which does not preserve the natural color of the wood.

Ključne riječi: bukove piljenice, konvektivno sušenje, režim sušenja, boja drva Wood drying belongs to the basic technological operations of wood processing.This technological process is based on considerably complicated hydro-thermal process and despite the noticeable effort of scientists and technologists, it is not yet fully clarifi ed.
The drying of beech lumber carried out in chambers according to the regimes of the companies: Hildebrandt GmbH, Incomac S.R.L., Vzduchotechnika a.s., and Mühlböck GmbH is commonly realized at temperatures t = 60 -80 °C.In beech wood, the hydro-thermal process at these temperatures not only removes water from the wood, but also develops some chemical reactions such as partial hydrolysis of the hemicelluloses and the extraction of water-soluble substances (Bučko, 1995;Trebula and Bučko, 1996;Halaj, 1999;Dzurenda and Deliiski, 2000;Kačík, 2001;Laurova et al, 2004), which cause irreversible color changes of wood.One such change is from the original white-yellow color of the beech wood to a dull brown-pink shade.The extent of the change of wood color during the hydro-thermal treatment of wood, Deliiski (1991) is defi ned by the criterion of color homogenization of wood S FH , which is equal to the integral area of the function of temperature change during the hydro-thermal treatment of wood.The unit of this criterion is a thermosecond (K⋅s).The change of the color shade of beech wood from the original white-yellow to brown-pink shade occurs after surpassing the limit for the color homogenization criterion S FH ≤ 1.2 x 10 7 K⋅s (Deliiski, 2003).
To eliminate this imperfection, special regimes were created for convective drying of beech wood in chambers, which preserve the natural color of the beech wood.The goal of this work is to present the suggested regimes for drying beech lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm, from W p = 70 % to W k = 8 %, in chambers and their evaluation from the aspects of quality of dried lumber, extent of color change (Deliiski and Dzurenda, 2003) and their specifi c heat consumption.

MATERIJAL I METODE
The parameters of the regimes for drying beech lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm, from W p = 70 % to W k = 8 %, which cause no change of the natural color of the wood, are shown in Table 1.
At the end of lumber drying, when the wood moisture content reaches a value of approximately 20%, conditioning is included for partial decreasing of the moisture gradient.Conditioning is carried out by an increase of the relative humidity.
The drying of beech lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm, according to individual regimes, was carried out in chamber type KWC 121 manufactured by Vzduchotechnika a.s.Nové Mesto nad Váhom.
The quality of the drying process of beech lumber is evaluated by drying gradient U defi ned as the quotient of the average moisture content of the lumber subjected to drying and equilibrium moisture content throughout the process.
After the completion of the drying process, a quality check of the dried lumber was performed on 8 samples with the determination of the following parameters: difference between the average fi nal moisture content and the required fi nal moisture content, fl uctuation of the fi nal moisture content, moisture gradient, and change of wood color.
The difference w 0 between the average fi nal moisture content and the required fi nal moisture content (in %) was quantifi ed using the following equation: where: n -number of samples / broj uzoraka W ik -average fi nal moisture content of samples / prosječni konačni sadržaj vode u uzorcima, %; W k -required fi nal moisture content of samples / potrebni konačni sadržaj vode u uzorcima, %.
The range of fi nal moisture content of the dried lumber was calculated as the difference between maximum and minimum values of the moisture content in dried samples according to the equation: where: W max -maximum moisture content in samples / najveći sadržaj vode u uzorcima, %; W min -minimum moisture content in samples / najmanji sadržaj vode u uzorcima, %.
The moisture gradient ∆W (in %) was calculated as the difference between the moisture content of the middle layer of the samples and the average moisture content of both surface layers: where: W c -moisture content of the sample middle layer / sadržaj vode u srednjem sloju uzoraka, %; W pov -moisture content of the sample surface layers / sadržaj vode na površini uzoraka, %.
The color of wood on the surface of beech lumber was measured by a colorimeter Color Reader CR-10, which determines the color by a set of values in the color coordinate system CIE -L*a*b.The principle of measuring with this colorimeter is based on the determination of the following parameters: L*, which quantifi es the lightness of the color from 100 for white color to 0 for black; a*, which quantifi es the chromatic coordinate of color shade between red and green; b*, which quantifi es the chromatic coordinate of color shade between yellow and blue.
The change of the color of beech wood subjected to drying was determined by the size of changes of the individual coordinates in the color coordinate system before drying and also on a planed surface of wood after drying as well as by the size of color deviation ∆E a.b according to CIE (1986), presented by the equation:  (Dzurenda and Deliiski, 2009).The normative specifi c heat energy consumption (in kWh⋅m -3 ) for lumber drying in chambers is presented by the equation: where: Q W -heat energy, necessary for the heating of lumber in chambers / toplinska energija za zagrijavanje piljenica, kWh; Q V -heat energy, necessary for the heating of wet air in the drying chamber and its moistening during the phase of heating and fi nal treatment / toplinska energija za zagrijavanje vlažnog zraka u sušionici tijekom faze inicijalnog zagrijavanja, kondicioniranja i obrade, kWh; Q A -heat energy, necessary for the heating of the construction of drying chambers / toplinska energija za zagrijavanje konstrukcije sušionice, kWh; Q L -heat energy, necessary for the heating of air in the drying chamber for the purpose of water evaporation from wood / toplinska energija za zagrijavanje zraka u sušionici radi isparavanja vode iz drva, kWh; Q HG -heat energy, necessary for the release of hygroscopically bounded water from wood / toplinska energija za oslobađanje higroskopski vezane vode iz drva, kWh; Q S -heat energy, necessary for the covering of heat losses of the drying chamber / toplinska energija za nadoknadu gubitaka topline sušionice, kWh; V D -volume of wood subjected to drying / obujam drva koje se suši, m 3 .

REZULTATI
The change in the average moisture content of the lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm during the suggested drying regimes is shown in Fig. 1 The change in the drying gradient of the suggested regimes depending on the phase of the regime and on lumber thickness shown in Fig. 2.
The initial and fi nal moisture content of the dried samples, the values of the parameters of color coordinates L*, a*, and b* of dried samples before and after drying, as well as the size of color deviations of beech wood ∆E ab caused by the drying process are shown in Tables 2, 3 and 4. Table 5 shows the results of the analysis, by which the quality was evaluated of dried beech lumber based on the difference between the average fi nal moisture content and the required fi nal moisture content, fl uctuation in the fi nal moisture content and moisture gradient of lumber.
Based on the comparison of the measured values of deviations of the average fi nal moisture content from the required fi nal moisture content, the range of the final moisture content of the dried beech lumber and the moisture gradient in the dried lumber with the values of quality characteristics of individual quality classes, it can be concluded that the dried beech lumber with the thickness h = 40 mm fulfi lls the criteria of the fi rst quality class according to the analysis of the following quality characteristics: Analogously, the quality evaluation of the dried beech lumber with the thickness h = 60 mm with the qua lity  characteristics W 0 = 0.4% ≤ 0.5 %, W k0 = 1.5 % ≤ 2.0 % a ∆W = 0.6 ÷ 2.3 % ≤ 2. 5 %, as well as the dried beech lumber with the thickness h = 80 mm with the quality characteristics W 0 = 0.4 % ≤ 0.5 %, W k0 = 1.3 % ≤ 2.0 % a ∆W = 1.8 ÷ 2.3 % ≤ 2.4 %, fulfi ll the criteria for placing the dried beech lumber into the fi rst quality class.
During the drying process, the color coordinates of dried samples of beech lumber with the thickness h = 40 mm, as well as h = 60 mm and h = 80 mm have shown slight differences in the form of increasing the whiteness of the beech wood by ∆L* = 0.4 ÷ 1.2, decreasing of coordinate a*, which describes the range of color shades between red and green color by ∆a* = 0.3 ÷ 1.3 and an increasing of coordinate b*, which describes the range of color shades between yellow and blue color by ∆b* = 0.1 ÷ 0.9.
The total changes of color differences of beech wood in the drying process ∆E ab = 0.8 ÷ 1.6 belong to the interval of color difference ∆E ab ≤ 2, also marked as "inconsiderable changes of the color".
Table 7 illustrates the values of the total specifi c heat energy consumptions and the heat energy consumptions of individual items of the balance of the drying process of lumber with the thickness h = 60 mm, according to the suggested regime and the regime for drying beech lumber of Vzduchotechnika a.s. in chamber type KWC 121 from moisture content W p = 70 % to W k = 8 %, at initial temperature of the wood t d = 10 °C and the average air temperature near the drying chamber t o = 10 °C.

RASPRAVA
According to the specifi cation of the temperatures and psychrometric differences of the drying air in the suggested regimes of convective drying of beech lumber in chambers, according to the categorization of drying regimes (Kollmann, 1955;Trebula, 1989) these regimes are classifi ed to the so-called regimes with rising temperature and decreasing relative humidity of the drying environment.
From the range of values of the drying gradient U = 4.6 ÷ 3.3, during the drying of beech lumber with the thickness h = 40 mm and h = 60 mm, it can be concluded that from the beginning of the drying to reaching the average moisture content W = 35 %, despite low temperature of the drying air, the drying regime can be considered as hard with the probability of creating a moisture gradient along the lumber section.A similar statement also applies to the drying gradient U = 4.6 ÷ 3.1 during the drying of beech lumber with the thickness h = 80 mm.63 (2) 95-103 (2012) From the average moisture content W = 35 % up to the end of the drying process, the parameters of the drying medium ensure a fi ner drying of beech lumber.Despite this, the goal of the authors to suggest drying regimes that cause no change in the color of beech lumber while reaching the topmost quality of dried material is validated by the results of the qualitative checks made at the end of drying.According to the results of these checks, the beech lumber dried in chamber type KWC 121 had the natural white-yellow color and the moisture gradient ∆W ≤ 0.8 ÷ 1.3 % for lumber with the thickness h = 40 mm, ∆W ≤ 0.6 ÷ 2.3 % for lumber with h = 60 mm, and ∆W ≤ 1.8 ÷ 2.4 % for lumber with h = 80 mm.

DRVNA INDUSTRIJA
The difference between the suggested regimes and the classical regimes for convective beech lumber drying in chambers developed by the companies Hildebrandt GmbH, Incomac S.R.L., Vzduchotechnika a.s., Mühlböck GmbH (carried out at temperatures t = 60 ÷ 80 °C and allowing color change of beech lumber to brown-pink) is that the process of drying beech lumber according to the suggested regimes is carried out at temperatures within the lower temperature range of the classical regimes.
During the removal of free water located in the lumens of beech wood cells, the drying is carried out by hot air with temperature lower than t = 50 °C and psychrometric difference ∆t = 5 ÷ 10 °C so that the temperature of the cell walls and free water in the lumens do not exceed t m ≤ 40 °C.At these temperatures, the conditions for the process of deacetylation of beech wood, and for subsequent hydrolytical solution of polysaccharidic and extractive substances of beech lumber, which cause changes in the chromophoric structure of wood, cannot be fulfi lled (Trebula and Bučko, 1996).The increase of the temperature of the drying medium to t = 53 ÷ 65 °C and the temperature of the wet wood over t m ≥ 40 °C are achieved when the average moisture content W ≤ 20 %.Then due to the absence of free water in the wood, the increased temperature of the drying air does not cause a change in the chromophoric structure of beech wood.This statement is validated by the results of the measuring of changes of beech wood color caused by drying, as well as by the results of works by Klement and Smilek (2010) and Klement et al. (2011), who analyzed the infl uence of the temperature and the method of drying of beech wood on the color shade of the dried beech wood.
The drying process of beech lumber at the lower temperatures compared to the classical regimes for convective drying of beech lumber is characterized by a lon- ger time needed for lumber drying.In comparison with the drying regimes of the company Vzduchotechnika a.s.realized at temperatures t = 60 ÷ 80 °C, the duration of drying beech lumber is 1.9 times longer for the thickness h = 40 mm while preserving the natural color of wood, 2.3 times longer for the thickness h = 60 mm and 2.9 times longer for the thickness h = 80 mm.This fact refl ects negatively on the use of the capacity of drying chambers, as shown in works by Drahoš andViktorin (1975), Pervan (2000), Trebula and Klement (2002), Videlov (2003), Dzurenda and Deliiski (2010).
The convective drying of beech lumber in chamber type KWC 121 according to the suggested regimes is characterized by the normative specifi c heat consumption of Q TZN = 514.46kWh⋅m -3 for lumber with the thickness h = 40 mm, Q TZN = 557.62kWh⋅m -3 for lumber with the thickness h = 60 mm and Q TZN = 643.16kWh⋅m -3 for lumber with the thickness h = 80 mm.
Comparing the normative specifi c heat consumption for lumber with the thickness h = 60 mm and the normative specifi c heat energy consumption for drying beech lumber of the same thickness in chamber type KWC 121 according to the regime of the company Vzduchotechnika a.s., whose value is Q TZN = 557.62kWh⋅m -3 , it has been determined that in the process of drying beech lumber according to the suggested regime, by which the natural color of beech wood is preserved, 26.89 % more heat is consumed.This amount of increased heat energy consumption is caused by the increased heat consumption for heating the air during water evaporation from wood and for covering the heat losses of the drying chamber, due to the prolonged duration of drying.
Comparable results -prolongation of the duration of drying due to the decrease of temperature of the drying medium and the increase of specifi c heat consumption for drying lumber in drying chambers, can be found in the works of Drahoš and Viktorin (1975), Glijer (1990), Dzurenda (1993), Dzurenda and Deliiski (2004).

ZAKLJUČAK
Based on the studies performed, it can be stated that the drying of beech lumber with the thickness h = 40 mm, h = 60 mm and h = 80 mm from moisture content W p = 70 % to W k = 8 % in chambers according to the suggested regimes with temperature range of the drying medium t s = 37 ÷ 65 °C causes no changes in the chromophoric structure of wood and that beech wood maintains its natural white-yellow color.The addition of the conditioning phase -partial elimination of moisture gradient during the drying process (∆W = 25 ÷ 20%) refl ects positively on the spreading of moisture content along the section of beech lumber at the end of drying, which enables the grading of this dried lumber into the fi rst quality class.
The negative effect of drying beech wood in the lower part of the temperature range of the classical convective drying regimes is the prolonged drying process.In comparison with the drying regimes of beech lumber of the company Vzduchotechnika a.s.carried out at temperatures t = 60 ÷ 80 °C, the duration of drying beech lumber with the thickness h = 40 mm while maintaining the natural color of wood is 2.1 ti-mes longer and with the thickness h = 80 mm it is 2.9 times longer.
The specifi c heat consumption of beech lumber in chamber type KWC 121, according to the suggested regimes for drying beech lumber is Q TZN = 514.46kWh⋅m -3 for the thickness h = 40 mm, Q TZN = 557.62kWh⋅m -3 for the thickness h = 60 mm and Q TZN = 643.16kWh⋅m -3 for the thickness h = 80 mm.The normative heat consumption used for drying beech lumber with the thickness h = 60 mm according to the suggested regime is by 26.89 % higher than the specifi c heat energy consumption used for drying beech lumber with the same thickness according to the regime of the company Vzduchotechnika a.s.

Figure 1 Figure 2
Figure 1 Change in the average moisture content of the lumber during the suggested regimes, depending on the thickness h Slika 1. Promjena prosječnog sadržaja vode u piljenicama tijekom sušenja predloženim režimima, ovisno o debljini piljenica

Table 2
Changes in moisture content, color coordinates L*, a*, and b* of beech lumber with the thickness h = 40 mm at the beginning and at the end of drying, and the size of color deviation of wood ∆E ab during the drying process Tablica 2. Promjene sadržaja vode, koordinata boje L*, a* i b* za bukove piljenice debljine h = 40 mm na početku i na kraju sušenja te veličina promjene boje ∆E ab tijekom procesa sušenja

Table 3
Changes in moisture content, color coordinates L*, a*, and b* of beech lumber with the thickness h = 60 mm at the beginning and at the end of drying, and the size of color deviation of wood ∆E ab during the drying process Tablica 3. Promjene sadržaja vode, koordinata boje L*, a* i b* za bukove piljenice debljine h = 60 mm na početku i na kraju sušenja te veličina promjene boje ∆E ab tijekom procesa sušenja

Table 4
Changes in moisture content, color coordinates L*, a*, and b* of beech lumber with the thickness h = 80 mm at the beginning and at the end of drying, and the size of color deviation of wood ∆E ab during the drying process Tablica 4. Promjene sadržaja vode, koordinata boje L*, a* i b* za bukove piljenice debljine h = 80 mm na početku i na kraju sušenja te veličina promjene boje ∆E ab tijekom procesa sušenja